[baronbeehive]

From what I have learned about those amps, is their clarity and detailing is mostly attributing to the tubes and bias points, once the amp is sufficiently moded, as the circuit design is very good.

Especially the issue of soundstage and detailing is dependent on bias.
In Your specific case, the driver tube bias need to be boosted and you will be shocked how much more it can go....

Right! I won't hijack this thread but you are right I haven't tested, or finished all the mods completely yet so I look forward to confirming that. But the speed of the these SET amps is second to none. I'm glad though that the APPJ's sound is rich as well as fast because I think if it goes too fast and transparent without any contribution from the tube sound then the sound is going dangerously close to SS sound which I don't want. That would put me off going for a decware amp for example although they are outstanding amps by all account.

Anyway I'm very happy with both my headphone amp, and the little APPJ of course! The thought just crossed my mind that the 2 APPJ's side by side would be a great
talking point and it would be interesting to compare their sounds although comparing headphones with speakers would make that difficult it would be interesting to hear from someone who had them both, or maybe I'm just fantasizing, probably!

 

[Maxx134]

going for a decware amp for example although they are outstanding amps by all account

From learning about apps and the moding threads,
You can get a feel of an amp just by looking at the insides...

I am hoping ZMF creator Zack gets his suggestions implemented on the designs Decware is doing for him.

Many designs have same issue of having to rely on a coupling device between stages, and most time it is a coupling cap over an interstage transformer...
So the quality of this device is the pivot point of the whole amp.
That's why, for this amp, I specifically suggest best caps you can get...

My suggestion will always be a copper foil as best for that specific scenario...

2 APPJ's side by side would be a great
talking point

I think the short period of the mass drop sale may be over...
It was just posted a few page back..

 

[benjisun]

This topic is getting very interesting;

For me it's end of the mod game, (after trying 1000 uf mundorf)
Stock 220 uf/220V replaced by Nichicon FG fine gold 330 uf/50v
Stock 0,1 uf ceramic replaced by Audyn Qs6 0,47 uf

Did some of you tried to replace the fuze ? It"s by default a poor 1,6 amp model. I added a ceramic 2,5 a instead.

I'm very hapy with the sound of this little amp.

 

[Maxx134]

This non-traditional switching PSU, probably have different sensitivities of normal PSU when it comes to noise or fuse or current capabilities.

Another thing to consider is that too large a coupling cap may introduce unwanted low frequency oscillation or changes to the global feedback.

I am currently using .68uf wich I do not recommend, as I encountered detectable oscillation with use of the "42" tube with adapter,
but only at zero volume...

So adverse effects can happen and with splice or oscilloscope would have known if we had all the detailed circuit values.

Another point about the coupling cap in this circuit,
is that when I tried .2uf it was not optimal,
yet once I went up to .3uf, it sounded optimal,
as no benefit/gains was observed going bigger.

So my recommended max value for the coupling cap here, would be .47uf, with around .33uf being optimal.

Yet on another note,
The quality" or type of the coupling cap,
did have effect with treble tonality and soundstage.

Same observation with my big amp.

Edit**
Had a zero by mistake in my cap values.

Only in my additional bypass cap, ( that is paralleled to the main electrolytic cathode cap) ,
Did I use a small value of .047uf,
And recommend no higher than .1uf additional bypass value is necessary to help that area.

 

[baronbeehive]

.......

Many designs have same issue of having to rely on a coupling device between stages, and most time it is a coupling cap over an interstage transformer...
So the quality of this device is the pivot point of the whole amp.
That's why, for this amp, I specifically suggest best caps you can get...

My suggestion will always be a copper foil as best for that specific scenario...

I really don't want to go on about this but the Jupiter caps have transformed this amp from a good one to a great one, and I really don't know how this has happened, it is after all just a coupling cap and therefore cannot change the nature of the circuit, only optimize what is already there!

This topic is getting very interesting;
....

Did some of you tried to replace the fuze ? It"s by default a poor 1,6 amp model. I added a ceramic 2,5 a instead.

Yes I replaced it with a similar one to yours, not because I'm hopeful of it transformig sound quality but just it was easy to do, and you ever know.....

..........

Another thing to consider is that too large a coupling cap may introduce unwanted low frequency oscillation or changes to the global feedback.

I am currently using .068uf wich I do not recommend, as I encountered detectable oscillation with use of the "42" tube with adapter,
but only at zero volume...

So adverse effects can happen and with splice or oscilloscope would have known if we had all the detailed circuit values.

......

Yes I would not have gone higher than 1uF for that reason, also it's debateable that anything higher could be heard anyway.

What did these oscillations sound like?

Edit:
Silly me, obviously you can't hear them at zero volume, I assume you picked them up using an oscilloscope.

 

[CZ4A]

Yes, I just measured HT (B+) at 179V and the heater voltage at 11.7V (with the original tubes). I have updated the schematic accordingly. Here (Belgium), the mains voltage is 230V. If you're in the UK, it's more likely 240V which can explain the difference.

When I used the Tung Sol KT66 Reissue's, the right tube glowed noticeably brighter than the left one. Intrigued, I measured the voltages at the heater pins of both tubes: the left one had 5.50V and the right one 6.09V.

I will try to get some heater and HT measurements and see if there's any difference between your measurements on Belgian 230V mains and mine on US 120V mains. FWIW I haven't noticed a visual difference in the heater glow on any of my power tubes, but that doesn't meant they aren't running at different voltages of course.

I also still have the same reissue tubes, they look dam pretty.
I also have the big 6L6 and others I rolled into this amp...
I need to make a list..
The 6F6 sounds better still.

I've yet to try any 6F6s. My coworker is giving me the tuner/amp section of an old Admiral console radio that uses them. So once I have that radio's guts I can test the 6F6s and roll them in the APPJ if they're still good.

 

[Maxx134]

What did these oscillations sound like?

Nothing major, just a few audible tiny quick clicks while unit starts up, audible only on one side, with volume all the way down.(!)

Its not the tubes, as it happens to a few that not have it before,
And doesn't happen all the time on startup,
And also can be simulated with using two different output tubes at same time, NOT RECOMMEND, Lol.
Also I could not detect any low frequency oscillation on output but would need to test the transformer primary to but since it only sometimes at startup, I not worried about it.

I can test the 6F6s and roll them in the APPJ if they're still good

Looks like I will have to reevaluate all the tubes I tried, with notes about tonality of each tube,
Because now after upgrades done and burned in, the 6L6 is now performing nicely.
Specifically, It does not distort at max volume and I cannot even turn it up that loud.



The 6F6 does go a bit louder but a slight distorted encountered at max vol .
I can only briefly turn up because it just to loud.

So looks like this amp can now handle planars nicely without strain.

 

[Minbeo]

First i wanna thanks Max and other people for sharing. I've done replaced the amp's capacitors with these:
- 0.47uf/800v Jantzen superior z-cap
- 1000uf/50v Elna Silmic II
The result is generally good, fuller sound, better resolution, less sibilance, well worth the money and hassle(struggle with soldering ). But somehow i feel the amp sound quite a bit slower, the bass doesn't sound as snappy as before. Do you guys have any insight on this? caps value? better caps? thanks alot.

 

[pvico]

Just tested with the Russian military 6F6's and I must say it is great, much more balanced than the TS KT66's.
Current setup:

• Preamp tube: JJ 5751
• Power tubes: 6F6 (Ebay seller from Ukraine)
• Coupling caps: Mundorf MCAP EVO Silver Gold Oil 0.47µF
• Bypass caps: Nichicon KZ Muse 1000µF
• Beyer DT770 PRO 80 ohm

 

[pvico]

I will try to get some heater and HT measurements and see if there's any difference between your measurements on Belgian 230V mains and mine on US 120V mains. FWIW I haven't noticed a visual difference in the heater glow on any of my power tubes, but that doesn't meant they aren't running at different voltages of course.

Yes, the US switching PSU is obviously different from the EU one (230V with 10% tolerance, UK was traditionally 240V and my country was 220V. I measured 230V at the mains outlet).
With the KT66's, the glow was noticeably different on a steady state, not just on startup. Seems OK with the 6F6's. YMMV.

 

[Maxx134]

But somehow i feel the amp sound quite a bit slower, the bass doesn't sound as snappy as before.

Those Jantzen caps are actually supposed to be better than the Mundorf,
So It is quite possible a few things going on.
The caps need break in period and the power tubes are responding differently to the change in parameters.
So you can also be hearing true sound of your tube selection.

To offset any initial slowness or other effects of replacement cathode caps,
I added a Second small paralleled decoupling cap to the electrolytics (cathode bypass cap). ..
It is same or similar quality & brand as the coupling cap.
Look back in thread to note some others have done also
That should solve your issue.

Just tested with the Russian military 6F6's and I must say it is great, much more balanced than the TS KT66's.

Yes I find them very detailed and a touch brighter/closer than the other 6F6 I have.
So you have more options to match it with a preamp tube of your choice.

These russian tubes are low cost and old stock tubes, so they are just a dam great option.
I like them over my best juicy sounding Tung-sol because of the slight edge they have in treble air and detail.
Many thanks to member "Kryl007" for the suggestion.

 

[baronbeehive]

First i wanna thanks Max and other people for sharing. I've done replaced the amp's capacitors with these:
- 0.47uf/800v Jantzen superior z-cap
- 1000uf/50v Elna Silmic II
The result is generally good, fuller sound, better resolution, less sibilance, well worth the money and hassle(struggle with soldering ). But somehow i feel the amp sound quite a bit slower, the bass doesn't sound as snappy as before. Do you guys have any insight on this? caps value? better caps? thanks alot.

I'm not finding this with similar mods on my speaker amp. The frequencies on the transients are better, both low and high as far as I can ascertain leading to the greater detail and dynamics I mentioned before.
If it seems you are lacking the higher frequencies, it could be that you have a better frequency range now, low and high which is causing you to think that you have lost the higher frequencies you had before. The caps are good so no problems there.
What tubes are you using atm?
Edit: With 0.47 caps your amp should be bassier but any perceived loss of higher frequency response could be helped by crisper sounding tubes.
For example you could try JJ 12AX7's which are very quick and detailed sounding with snappy bass.
I might try some cathode bypass caps myself to see if this makes much difference but for the moment I'm happy with the sound.

Maxx, do you think a 470uF 25v cathode cap would work roughly the same as a 1000uF cap with a bypass cap linked?

 

[Minbeo]

Those Jantzen caps are actually supposed to be better than the Mundorf,
So It is quite possible a few things going on.
The caps need break in period and the power tubes are responding differently to the change in parameters.
So you can also be hearing true sound of your tube selection.

To offset any initial slowness or other effects of replacement cathode caps,
I added a Second small paralleled decoupling cap to the electrolytics (cathode bypass cap). ..
It is same or similar quality & brand as the coupling cap.
Look back in thread to note some others have done also
That should solve your issue.

Thanks Maxx , the sound does open up after a while, or maybe my ears are getting used to the fuller & more textured sound of the new caps. I like the caps upgrade enough to order a pair of 0.1uf Jantzen superior just for fun.

I'm not finding this with similar mods on my speaker amp. The frequencies on the transients are better, both low and high as far as I can ascertain leading to the greater detail and dynamics I mentioned before.
If it seems you are lacking the higher frequencies, it could be that you have a better frequency range now, low and high which is causing you to think that you have lost the higher frequencies you had before. The caps are good so no problems there.
What tubes are you using atm?
Edit: With 0.47 caps your amp should be bassier but any perceived loss of higher frequency response could be helped by crisper sounding tubes.
For example you could try JJ 12AX7's which are very quick and detailed sounding with snappy bass.
I might try some cathode bypass caps myself to see if this makes much difference but for the moment I'm happy with the sound.

Thanks Baron, actually i'm using a very similar amp from APPJ(with Voshkod 6j1p + Sylvania 6005 tubs) https://www.massdrop.com/buy/appj-mini2013-6j1-6p1
And yes, those upgrade caps add some serious slams to the amp's bass love it!. I also agree that better frequency range make you feel the treble is not as stand out as before, i'd bypass those huge 1000uf Silmic II(seriously they're the same size of Jantzen film caps) with 0.1uf Jantzen superior to see what happens.

 

[baronbeehive]

Thanks Maxx , the sound does open up after a while, or maybe my ears are getting used to the fuller & more textured sound of the new caps. I like the caps upgrade enough to order a pair of 0.1uf Jantzen superior just for fun.


Thanks Baron, actually i'm using a very similar amp from APPJ(with Voshkod 6j1p + Sylvania 6005 tubs) https://www.massdrop.com/buy/appj-mini2013-6j1-6p1
And yes, those upgrade caps add some serious slams to the amp's bass love it!. I also agree that better frequency range make you feel the treble is not as stand out as before, i'd bypass those huge 1000uf Silmic II(seriously they're the same size of Jantzen film caps) with 0.1uf Jantzen superior to see what happens.

Excellent! I would be very interested to hear what you think. I don't want to disturb my cathode caps yet, they will have to come out to put in bypass caps and I can't be bothered atm, besides I'm enjoying it as it is too much .

Edit: Those large capacitance caps should make the amp quicker but the values may need to be in a suitable range for the circuitry so as not to cause, for example, the oscillations that Maxx mentioned. Also the types of caps used can have certain properties such that, for example a bypass cap of a certain type helps out the main cap in certain areas, such as in treble.

 

[Minbeo]

Another point about the coupling cap in this circuit,
is that when I tried .02uf it was not optimal,
yet once I went up to .03uf, it sounded optimal,
as no benefit/gains was observed going bigger.
So my recommended max value for the coupling cap here, would be .047uf, with around .033uf being optimal.

Hi Maxx, i've just wanted to ask: is there any disadvantage of using 0.47uf coupling cap instead optimally 0.33uf, they're almost the same price. Is 0.33uf-0.47uf good values for bypassing cathode caps too? thankss