[baronbeehive]

Sounds like alotta effort for unkown performance gains.
I'm not bothering with PSU, because as coin stated, the balance design cancels PSU ripple, plus I already have film caps as decoupling..
I'm only looking to change up my cathode caps and WCF caps.

I was thinking.. again.. must stop that . Don't know why that hadn't occurred to me before but we might be missing something there. We already have transformers there plus transistors and diodes, so why create what might be an unnecessary bottleneck by putting in large, slow lytics when we could have smaller, faster films. OK so we have the capacity but what might be missing is the speed? I think Copperfox got it about right with the 400uF Wimas, so I think that is definitely worth a try.

 

[Maxx134]

I was thinking.. again.. must stop that . Don't know why that hadn't occurred to me before but we might be missing something there. We already have transformers there plus transistors and diodes, so why create what might be an unnecessary bottleneck by putting in large, slow lytics when we could have smaller, faster films. OK so we have the capacity but what might be missing is the speed? I think Copperfox got it about right with the 400uF Wimas, so I think that is definitely worth a try.

We already placed better lyctics after the transistors, and already placed film caps as decoupling caps by the tubes.
I dunno why we running in circles here.

Copperfox was investigating films in the Cathode position so there you have your area of improvement.
Also, We already have film caps in WCF stage, so I am only changing my brand for a better film.

So this is it. There is nothing left to do, without adding in solid state and changing the design.
So to evlove from this type design would be to implement the better stages that Coinmaster has designed, or part from little dot to another design.
The problem is that there is nothing I would recommend or move to except @SonicTrance amps, or DIY a totally new design from coinmaster.

 

[baronbeehive]

How large a chassis extension are you planning on?

I can probably answer your question now, most likely to accommodate those 400uF Wimas at 45mm-65mm size so it will have to take account of this, any extra space left over will leave room for the cathode bypasses easily. I really think the superfast wimas will do well in PSU area in place of the slower lytics. I will post pics when I'm ready.

 

[CopperFox]

So in the last few weeks I've been trying out some of the different resistor brands for the cathode resistor postition... and I'd say they've had unexpectedly large differences in sound.

Perhaps I should first list some other preferences and choices I have in the system. And also for starters I haven't really understood the statements that the mkVI has a bass weakness as I've never felt that way and now it even has my favourite bass of all my amplifiers for my bassy planars.

My preferences for sound signature are that I like the Harman curve and I care most about bass, low treble/mids and high treble in this order and want them all to be good.

For the driver tubes I've been using the black and gold Shuguang 6SL7-T tubes nearly exclusively for almost 12 months. I've found these to have the best sub-bass extension, a very well balanced, detailed and smooth sound in all areas of the FR spectrum. The other tubes I've tried in addition to the Soviet stock tubes are the Tung-Sols in both the reissue and original forms and Full Music 6SL7 tubes and some apparently quite rare Siemens tubes that came with my second hand mkVI. My experience with the various tube brands here is not extensive so I'm naturally open for suggestions too but really like these 2012-released Shuguangs.

For the power tubes I have a quite wide selection and I've found my favourites in a combination of the Tung-Sol 6080s (in the "preferred construction") and white top Shuguang 6N5PJ. The reason why the Tung-Sols are good is that they have the best sub-bass of all tubes I tried. And the way in which the Shuguang 6NP5Js are unusual is that they have both the second strongest bass and also some of that big soundstage thing like the Svetlanas do. There are (at least?) three different variants of the Shuguang 6N5PJ: (1) One with clear top and metal base, (2) one with clear top and black base and (3) one with white top and black base.

Of these, the white top has some emphasis on midbass punch and low treble, the black base clear top has emphasis on sub bass and higher treble sharpness and the metal base is most neutral of them. The white top combines well with the Tung-Sols as the midbass and low treble foci add dynamics to the sub-bass emphasized Tung-Sol sound. In some combinations I've also used a combination of two variants in the black base clear top and black base white top, using the clear top variant as a kind of substitute for the Tung-Sols if they didn't work well in some configuration of rest of the amp. The metal base variant I use in my other OTL amp that runs two output tubes.

In the stock configuration of the mkVI, however, they did not sound very usable as despite their good bass they had an overly soft sound and they came alive only after installing the decoupling caps and film caps for the PSU output.


But back to the different resistor brands. I had had the Kiwame brand resistors for cathode and anode resistors for a year and was looking to try some other options for the cathode resistor so I got Ohmite Audio Gold ceramics, TKD metal films, Shinkoh tantalum, Audio Note tantalum non-magnetic and Audio Note Niobium non-magnetic to try out.

(For the anode resistors I have the Kiwames at 68k and most other resistors are either stock or Mills MRA.)

I had read over various forums and sources that the Kiwames have both very low tolerances (0,5%) and a "tube-like" warm, thick and bassy sound so that's probably what I had gotten used to. The first replacement I tried were the TKD metal films. I was quite surprised over their difference. The TKDs have a very crisp and clear sound that even sounds a bit artifical and they also have the lowest bass extension and quantity of these all. For my tastes I'd even say that they have a bass deficit. But they do have a certain character too that may be preferred for other people. Next I tried the Ohmite Audio Gold. It had just a bit more bass and less clarity and crispness than the TKDs. They ended up sounding a bit generic and bland. Probably not the favourite for many?

At this point I was wishing for a sound that would have the bass of the Kiwames but more clarity like the TKDs. I still had the AN tantalums and niobiums and Shinkoh tantalums. Of these I had read in advance that the Shinkoh would have been considered the best resistor at some point in the past and that they would have great mids but possibly lean-ish bass. And what I had read about the AN tantalums was that their sound was overly "tubey". The AN niobiums were the newest product of these, released only in the last year or so and there was not much information or impressions of those, but one that I remember had said they were the best of the three. So I tested the AN Niobium next and they were pretty much exactly what I had hoped for - almost exactly the kind of bass like the Kiwames except cleaner and also very good clarity and detail over the FR, a very refined, detailed and smooth high end kind of sound. Over a few days I began to notice though that they had slight recess in the mid to high treble - so I decided it might be a good time to do the input/output cable upgrade mod for the amp to see if it would fix that... and that's what it did. Afterwards I still tried the AN tantalums, but they were clearly worse than the niobiums with soft bass and muddied low treble.

So as they now worked in my system I'd rank them as follows: First, Audio Note Niobium non-magnetic - these are worth their price here and they are in a different league to the rest. Second, the Kiwames - they have good punchy and deep bass and are relatively cheap too even if they don't have the very best clarity. Third, the AN tantalums which had many things in common with the Kiwames but were worse. Fourth the TKD metal films which have a crisp character but are low on bass. Fifth and last the Ohmite Audio Golds which didn't really stand out with their characteristics.

The Shinkoh I still haven't tried but will probably do in the future. I might also re-test the Kiwames at some point as I haven't done so after replacing the wires. It would be interesting to try the Mills MRA here as well.

In any case these comparisons again show that there are many, many different possibilities with which to configure the amp to match various preferences.


(Edit: The tested resistors were in the 2W power rating except the Ohmite which was 5W.)

 

[CopperFox]

Following on from the work of CopperFox, and... brain farts excepting... I have a plan. I'm thinking of relaxing the restrictions that a compact version of the amp has and go with a chassis extension using heatsink plates, easily done and that would allow some of the huge film caps that according to members here should give the amp a boost in speed, and with that sharper bass, a weak point with the amp, and improving soundstage as well. So probably the 400uF Wimas for PSU which makes a ratio of around 3:1 film to lytic cap, seems reasonable. And up the power decoupling caps to about 100uF Wima. CopperFox already verified this. Which leaves cathode and WCF caps.

For cathode and WCF caps we need to take account of the cutoff on the rest of the frequency response, but also we need to have good power handling.

So for cathode caps, going from the cathode bypass calculator which is based on the mathematical formulae, probably starting at around 100uF to 500uF, figures for lytics. So for films around 50-200uF sounds about right. We will need to watch for oscillations for the higher values.

For WCF caps Maxx noted what Redge said on page 1 that half the current of the WCF push/pull design, comes from the cathode caps. Therefore the other half must come from the WCF caps? Anyway it looks like they could do with being uprated so maybe try around 1uF and possibly upwards to see the effect on the power drain, and taking account of oscillations of course here also. We may have overlooked the importance of the WCF caps before now as Maxx said.

So all of this should maximise the liveliness of the amp to a new level. I seem to remember coin, along with Redge, threw out the old chassis with his "alfresco workbench" version of the amp way back at the beginning of the thread!

How's that for a brain fart.. a huge one at that .

Do the WCF caps need to be "audio caps" or just have the best electrical properties for power delivery - (how much) does the audio signal pass through them?

Also, what about the eight small Wimas that are located between the power tubes and PSU section? At least some people have replaced those with something else, such as the Mundorfs here. Is their function power delivery or something else?

 

[CopperFox]

Haven't used the impedance adapter in a while as it was starting to look like the mkVI+ doesn't like having it attached to it. The vu meters were beginning to show imbalance with one channel rising in output and the other decreasing.

I've been looking into this too. This effect happens with the B10 impedance matcher connected regardless of the tubes. At startup the vu meters both show 60 ma, but the left then begins slowly approaching 40 ma or lower and the right side begins approaching 80 ma or higher. This doesn't fix itself (but instead gets worse) if I leave the amp running for a long time. It also happens only with the impedance matcher attached, and is reset (so that the readings are 60 and 60) if the amplifier is turned off and on again.

Would the opamps' biasing function somehow rely on feedback from the headphone output? And if yes, could it be that having the impedance matcher attached somehow cancels some of this?

 

[baronbeehive]

So in the last few weeks I've been trying out some of the different resistor brands for the cathode resistor postition... and I'd say they've had unexpectedly large differences in sound.

Perhaps I should first list some other preferences and choices I have in the system. And also for starters I haven't really understood the statements that the mkVI has a bass weakness as I've never felt that way and now it even has my favourite bass of all my amplifiers for my bassy planars.

To clarify that I do think that the bass is good and powerful enough, just that as you go down toward sub bass it gets looser which takes away a bit from the definition. My other amp has OT's and they improve the bass sharpness and definition over the LD. Interesting that you say it has the best bass of all your amps. I suspect that the film caps you have installed will make that difference, and that is why I intend to install them myself at some point later, more speed can only be beneficial.

For the driver tubes I've been using the black and gold Shuguang 6SL7-T tubes nearly exclusively for almost 12 months. I've found these to have the best sub-bass extension, a very well balanced, detailed and smooth sound in all areas of the FR spectrum. The other tubes I've tried in addition to the Soviet stock tubes are the Tung-Sols in both the reissue and original forms and Full Music 6SL7 tubes and some apparently quite rare Siemens tubes that came with my second hand mkVI. My experience with the various tube brands here is not extensive so I'm naturally open for suggestions too but really like these 2012-released Shuguangs.

For the power tubes I have a quite wide selection and I've found my favourites in a combination of the Tung-Sol 6080s (in the "preferred construction") and white top Shuguang 6N5PJ. The reason why the Tung-Sols are good is that they have the best sub-bass of all tubes I tried. And the way in which the Shuguang 6NP5Js are unusual is that they have both the second strongest bass and also some of that big soundstage thing like the Svetlanas do. There are (at least?) three different variants of the Shuguang 6N5PJ: (1) One with clear top and metal base, (2) one with clear top and black base and (3) one with white top and black base.

Of these, the white top has some emphasis on midbass punch and low treble, the black base clear top has emphasis on sub bass and higher treble sharpness and the metal base is most neutral of them. The white top combines well with the Tung-Sols as the midbass and low treble foci add dynamics to the sub-bass emphasized Tung-Sol sound. In some combinations I've also used a combination of two variants in the black base clear top and black base white top, using the clear top variant as a kind of substitute for the Tung-Sols if they didn't work well in some configuration of rest of the amp. The metal base variant I use in my other OTL amp that runs two output tubes.

In the stock configuration of the mkVI, however, they did not sound very usable as despite their good bass they had an overly soft sound and they came alive only after installing the decoupling caps and film caps for the PSU output.

I'm interested in what you say about you choice of tubes. AFAIK we haven't tried the Chinese tubes exhaustively due to some fairly negative comment initially. I've tried soviet tubes, (Sovtek), new issue Tungsols and Full Music tubes but only in my other amp, so they were 12AX7's, and although they were quite good they were not my preferred choice.

Re: your choice of power tubes I would be interested in getting hold of those Shuguang 6NP5J's if they indeed do have that Svetlana type soundstage, I steered clear of the Svetlana's also due to quite negative comments.

I'm assuming when you say you use Tungsols and Shuguangs in combination you mean having 2 Tungsols and 2 Shuguangs to make up the 4 power tubes? This is going beyond where I've gone, I haven't looked into this exhaustively, only tried a couple of combinations which weren't very promising.

You say that they did not sound very useable, do you mean the Shuguangs?

How did you manage to get hold of TS 6080's? I've been trying for ages and pretty much given up any hope of ever coming across any.

(For the anode resistors I have the Kiwames at 68k and most other resistors are either stock or Mills MRA.)

.

I had read over various forums and sources that the Kiwames have both very low tolerances (0,5%) and a "tube-like" warm, thick and bassy sound so that's probably what I had gotten used to. The first replacement I tried were the TKD metal films. I was quite surprised over their difference. The TKDs have a very crisp and clear sound that even sounds a bit artifical and they also have the lowest bass extension and quantity of these all. For my tastes I'd even say that they have a bass deficit. But they do have a certain character too that may be preferred for other people. Next I tried the Ohmite Audio Gold. It had just a bit more bass and less clarity and crispness than the TKDs. They ended up sounding a bit generic and bland. Probably not the favourite for many?

At this point I was wishing for a sound that would have the bass of the Kiwames but more clarity like the TKDs. I still had the AN tantalums and niobiums and Shinkoh tantalums. Of these I had read in advance that the Shinkoh would have been considered the best resistor at some point in the past and that they would have great mids but possibly lean-ish bass. And what I had read about the AN tantalums was that their sound was overly "tubey". The AN niobiums were the newest product of these, released only in the last year or so and there was not much information or impressions of those, but one that I remember had said they were the best of the three. So I tested the AN Niobium next and they were pretty much exactly what I had hoped for - almost exactly the kind of bass like the Kiwames except cleaner and also very good clarity and detail over the FR, a very refined, detailed and smooth high end kind of sound. Over a few days I began to notice though that they had slight recess in the mid to high treble - so I decided it might be a good time to do the input/output cable upgrade mod for the amp to see if it would fix that... and that's what it did. Afterwards I still tried the AN tantalums, but they were clearly worse than the niobiums with soft bass and muddied low treble.

So as they now worked in my system I'd rank them as follows: First, Audio Note Niobium non-magnetic - these are worth their price here and they are in a different league to the rest. Second, the Kiwames - they have good punchy and deep bass and are relatively cheap too even if they don't have the very best clarity. Third, the AN tantalums which had many things in common with the Kiwames but were worse. Fourth the TKD metal films which have a crisp character but are low on bass. Fifth and last the Ohmite Audio Golds which didn't really stand out with their characteristics.

The Shinkoh I still haven't tried but will probably do in the future. I might also re-test the Kiwames at some point as I haven't done so after replacing the wires. It would be interesting to try the Mills MRA here as well.

In any case these comparisons again show that there are many, many different possibilities with which to configure the amp to match various preferences.


(Edit: The tested resistors were in the 2W power rating except the Ohmite which was 5W.)

Again you've gone beyond where I have gone, and most others here! Incredibly interesting the differences you've found which I'm quite amazed by as I didn't think resistors would make that much differences and I think that goes for everyone here. Maybe the upgrades you have carried out have increased the sensitivity of the amp to other changes.

What wires did you put in the output position? These are the most important.

Very good work!
.

Edit: Do you intend to do any further experiments with different PSU, decoupling and cathode film cap values?

 

[baronbeehive]

Do the WCF caps need to be "audio caps" or just have the best electrical properties for power delivery - (how much) does the audio signal pass through them?

Also, what about the eight small Wimas that are located between the power tubes and PSU section? At least some people have replaced those with something else, such as the Mundorfs here. Is their function power delivery or something else?

This needs experimentation I think. The WCF caps are coupling caps and will have a direct sonic impact on half the power triodes. Together with their associated resistors they act to sense the current for the balanced push/pull operation of the 2 power triodes so that as one half of the circuit is pushing with a given current the other half is pulling with the same current in a balanced way.

So they need to be audio type caps but they also need to have the best possible power delivery. That is something I was thinking of looking into when I do my film cap upgrades. I was intending to try values up to 1uF here to see if a higher value improved power handling without the disadvantage of unpleasant resonances which are more likely the higher you go in values.

Not sure what the function of those Wimas is. Someone here replaced theirs with Mundorfs but only for cosmetic reasons as I remember.

 

[baronbeehive]

I've been looking into this too. This effect happens with the B10 impedance matcher connected regardless of the tubes. At startup the vu meters both show 60 ma, but the left then begins slowly approaching 40 ma or lower and the right side begins approaching 80 ma or higher. This doesn't fix itself (but instead gets worse) if I leave the amp running for a long time. It also happens only with the impedance matcher attached, and is reset (so that the readings are 60 and 60) if the amplifier is turned off and on again.

Would the opamps' biasing function somehow rely on feedback from the headphone output? And if yes, could it be that having the impedance matcher attached somehow cancels some of this?

I don't know, I can't account for this. Each channel provides a mirror image of the other channel so why one side only would increase while the other stays the same is beyond me. The opamps role is to amplify a weak signal as you probably know so I don't think that they could be involved in this. Not really sure I haven't any experience of using impedance matchers.

 

[CopperFox]

To clarify that I do think that the bass is good and powerful enough, just that as you go down toward sub bass it gets looser which takes away a bit from the definition. My other amp has OT's and they improve the bass sharpness and definition over the LD. Interesting that you say it has the best bass of all your amps. I suspect that the film caps you have installed will make that difference, and that is why I intend to install them myself at some point later, more speed can only be beneficial.

Yes the film caps as cathode bypass gave a significant improvement in sub-bass control and detail.

You say that they did not sound very useable, do you mean the Shuguangs?

The Shuguang 6NP5Js. Before adding decoupling caps and film PSU output caps they had nice bass but were otherwise too soft sounding. With the decoupling and psu output cap upgrades they improved a lot and got sharper so seems like they need that additional power available to sound their best.

What wires did you put in the output position? These are the most important.

I used this cheap SPC teflon coat wire from ebay. Bound/twisted pairs, meaning 4 wires in total for the positive and negative signal wires and a single wire for ground (should that be a pair of wires too?)

After installing them there was some interference or glitching on one channel so I routed the cables further away from the tube heater wires, shortened them and also added two different kinds of random car audio shielding (plastic tubing and nylon sheath on top of that) on the signal wires and they've worked great since.

Very good work!
.

Edit: Do you intend to do any further experiments with different PSU, decoupling and cathode film cap values?

I will experiment with the four big electrolytic PSU caps. The 200uf cathode bypass caps that I now have are working great and since they took a long time to burn in I probably won't try other values there. Probably not for the PSU ouput caps either. One thing that I had thought up about those is that it might be possible to install the 200uf value caps there with shorter wires, which might or might not have lower total resistance as the wiring is probably something like 1 mOhm per centimeter. The Wima DCL-MKP datasheet specifies the 400V 400uf version as having 1 mOhm ESR and 400V 200uf having 1,2 mOhm ESR so the wiring can have higher resistance than the caps themselves.

This needs experimentation I think. The WCF caps are coupling caps and will have a direct sonic impact on half the power triodes. Together with their associated resistors they act to sense the current for the balanced push/pull operation of the 2 power triodes so that as one half of the circuit is pushing with a given current the other half is pulling with the same current in a balanced way.

So they need to be audio type caps but they also need to have the best possible power delivery. That is something I was thinking of looking into when I do my film cap upgrades. I was intending to try values up to 1uF here to see if a higher value improved power handling without the disadvantage of unpleasant resonances which are more likely the higher you go in values.

It's unfortunate that the audio cap manufacturers don't bother listing the other electric properties of the different capacitance value models of the same capacitor product line.

With the datasheets of general (or other specific) purpose caps things like ESR and ESL are sometimes listed and their best values are not always consistently in the lower or higher capacitance value models. Sometimes lower capacitance is better and sometimes higher capacitance is better for those.

 

[CopperFox]

I don't know, I can't account for this. Each channel provides a mirror image of the other channel so why one side only would increase while the other stays the same is beyond me. The opamps role is to amplify a weak signal as you probably know so I don't think that they could be involved in this. Not really sure I haven't any experience of using impedance matchers.

I thought it was established early in the thread that the opamps are used for biasing here?

And that the blue trimpots are there to set some sort of starting point for the biasing, which will be affected by feedback from the output? (Is any of this correct? )

And if so, would the values from the trimpots somehow be multiplied or magnified in absence of normal/expected feedback from output?

In that case, some sort of imbalance in the trimpot adjustments could cause growing imbalance in the meters and also be redeemable by adjusting the trimpots.

But I don't know how to adjust the trimpots... how should that be done?

 

[baronbeehive]

I've been looking into this too. This effect happens with the B10 impedance matcher connected regardless of the tubes. At startup the vu meters both show 60 ma, but the left then begins slowly approaching 40 ma or lower and the right side begins approaching 80 ma or higher. This doesn't fix itself (but instead gets worse) if I leave the amp running for a long time. It also happens only with the impedance matcher attached, and is reset (so that the readings are 60 and 60) if the amplifier is turned off and on again.

Would the opamps' biasing function somehow rely on feedback from the headphone output? And if yes, could it be that having the impedance matcher attached somehow cancels some of this?

A further thought occurred to me while thinking about this in the night, couldn't get any sleep thinking about it! Perhaps the impedance matcher doesn't work with balanced amps, ie. that have 2 channels. It could be that it is connected to one channel only and that would explain why only one channel is changing once it is connected. You could check this be inputting a different value and see if the channel changes value correspondingly.

Edit: This may also explain why the Impedance matcher didn't work on the LD.

 

[baronbeehive]

I used this cheap SPC teflon coat wire from ebay. Bound/twisted pairs, meaning 4 wires in total for the left and right signal wires and a single wire for ground (should that be a pair of wires too?)

I think the one wire to ground should be alright so long as the right component is connected to ground.

I will experiment with the four big electrolytic PSU caps. The 200uf cathode bypass caps that I now have are working great and since they took a long time to burn in I probably won't try other values there. Probably not for the PSU ouput caps either. One thing that I had thought up about those is that it might be possible to install the 200uf value caps there with shorter wires, which might or might not have lower total resistance as the wiring is probably something like 1 mOhm per centimeter. The Wima DCL-MKP datasheet specifies the 400V 400uf version as having 1 mOhm ESR and 400V 200uf having 1,2 mOhm ESR so the wiring can have higher resistance than the caps themselves.

We didn't do this so I will be interested in your findings as I also wondered if uprating them would be beneficial.

I would think your cathode caps should be ok, they are high value which will ensure a linear curve and so long as they don't cause any oscillation that should be that. I was thinking of maybe a compromise of 150uF if I don't go for the 200uF there. Same with the PSU output caps if they are working well the values seem ok there.

Did you notice any difference in sound characteristics such as harmonics with the film cathode caps? The lytics may output more harmonics I don't know.

Yes it can be important to get the wires as short as possible because of the rapid rise in resistance as the distance increases, especially as we are adding components to the circuit board with a different order of size entirely. I tried to make mine fairly short. Maybe not so important with components that do not rely on speed but if you have film caps there then it makes sense to maximize their potential effects.

 

[baronbeehive]

I thought it was established early in the thread that the opamps are used for biasing here?

And that the blue trimpots are there to set some sort of starting point for the biasing, which will be affected by feedback from the output? (Is any of this correct? )

And if so, would the values from the trimpots somehow be multiplied or magnified in absence of normal/expected feedback from output?

In that case, some sort of imbalance in the trimpot adjustments could cause growing imbalance in the meters and also be redeemable by adjusting the trimpots.

But I don't know how to adjust the trimpots... how should that be done?

As I understand it the trimpots are to set the signal starting point to zero resting volts as it can go off slightly signified by a popping when you plug in your headphones. If it is not at zero then one channel will predominate over the other. The signal needs to vary either side of zero in operation. If the popping noise does happen you can adjust the trimpots by turning the screw until there is no popping noise signifying a return to zero.

If you're not sure about it you could PM Maxx. He went into how to adjust the DC offset to zero but I can't find it. I haven't had to adjust it for myself.

This is Maxx's explanation: https://www.head-fi.org/threads/lit...e-on-first-page.782183/page-267#post-16641367

Edit: I managed to find Maxx's instructions on resetting the trimmers when the zero offset has gone off: " adjust the trimpots (trimmers) to get zero DC offset between pos &neg output of each channel.. Get DC meter and adjust. Both trimmers can be elevated or lowered equally to cause the corresponding meter for that channel to move up or down slightly by a few points. A final test for zero dc is having your headphones on and listening for any tiny click/pop sound as you insert into the balanced socket (not the single-ended as that by default always pops in any unit)."

 

[baronbeehive]

Do the WCF caps need to be "audio caps" or just have the best electrical properties for power delivery - (how much) does the audio signal pass through them?

A further point the push/pull phase/antiphase functioning of the feedback loop in the WCF circuit, between the T1 triode and the T2 triode of the power tube, is driven by the WCF cap and the plate load of the tube. Half of the power goes on the push function which then switches to the pull function due to the current sensing regulation by the plate load as I understand it. I have to say I don't fully understand the WCF functioning, especially the maths behind it and this appears to be critical especially for a high headphone load such as with low impedance headphones like you have got, and me as this is what directly drives the headphones in an OTL design.

(As a side note calculating the correct plate load is especially important according to Cavalli for 6AS7G power tubes which have an rp of 280ohms whereas, for example a 12AX7 has an rp of 80.000ohms which would swamp out any load. So given that we tried the impedance mod and most of us were unimpressed that is strange when it would seem so important to get Ra correct).

So half the power goes through the cathode bypass cap and I assume the other half also goes through it in the alternate phase but I'm not sure about this.

The coupling function of the WCF cap between the triodes is not the same as the coupling function between the coupling caps between driver stage and power stage so I'm not quite sure about what the power requirements are. We know that such requirements are critical for the cathode bypass caps but I'm assuming that the WCF caps values would be more similar to the coupling caps, especially given that they would affect the cutoff frequency in the same way so large values would not be right for the WCF caps.