[Zachik]

I think finding Landfall back in 2012 has turned out to be the most meaningful contribution I've made to all of this. Being able to specify any X-Y-Z dimension is such a boon compared to the Chinese chassis products that are fixed dimension.

I have used their great services for my new amp that Glenn has just finished building for me... A+ customer service, and Dave is the nicest guy

 

[2359glenn]

Just to see what all of the hype is about, this morning I did another experiment.

You might recall that I left the driver tube cathodes unbypassed in the 6A5 amp given the CCS load. I had some leftover electrolytics from the build, some non-polar Audio Note Kaisei, I thought I would clip in the caps, bypass the cathodes, and see how it affected the sound. Many tube DIYers go to great lengths to remove these electrolytics from the signal path, but how bad can it be really?

The alligator clips I'm using don't have teeth, so the caps can be very quickly removed for listening comparisons.



The difference is pretty BIG! With the cathodes bypassed, there is a collpase in the soundstage, instrument separation suffers, there is less definition in the bass, and the high end sounds more harsh. It isn't subtle whatsoever. The amp doesn't sound bad with the cathodes bypassed, but once you've heard them unbypassed, no way you could go back. Audio quality is all relative after all. I did do some FFT measurements, bypassing the cathode resistors doesn't seem to affect distortion in a measurable way.

So the hype is real, keeping electrolytics out of the signal path is the way to go. Luckily in the 45 parafeed amp, I will be leaving the 6J5 cathode resistors unbypassed, and I should be able to get away with leaving the 45 cathode resistors unbypassed as well using the "Western Electric" parafeed wiring configuration.



This setup could potentially inject some power supply noise onto the cathode, but seeing as I am back to using a MOSFET cascode CCS on the output tube (rather than the pentode CCS), I think I will get away with it. That would leave only two capacitors in the signal path, the interstage coupling caps and the parafeed caps, which will both be high quality film types.

Since I am waiting around for parts again, the gears have started turning on an all DHT two-stage amplifier. This is probably a 2021 project, so I won't talk about the details. Might be collecting some tubes for it, but I'll certainly be looking into ways to avoid bypass caps in this setup too. To be continued...

Another thing I did yesterday, which you might be able to see in my photo above, was rerouting the B+ wiring from the mains transformer to the rectifier. A made a novice mistake routing this wiring along the periphery of the chassis originally. Keeping this loop area small should help with power supply noise, and seems to have made a measurable difference. Hindsight is 20/20.

TTFN!

In my normal EL3N amp I use LED bias on the driver EL3Ns I feel this is a improvement in SQ
Usually you will lose gain with a unbypassed cathode resistor. It puts feedback into the cathode reducing gain.

 

[L0rdGwyn]

I think finding Landfall back in 2012 has turned out to be the most meaningful contribution I've made to all of this. Being able to specify any X-Y-Z dimension is such a boon compared to the Chinese chassis products that are fixed dimension.

Well thank you then for discovering them, I really like the work they do. I am expecting the 45 parafeed chassis this Thursday, looking forward to seeing how it turned out. Then the long wait for transformers, again.

In my normal EL3N amp I use LED bias on the driver EL3Ns I feel this is a improvement in SQ
Usually you will lose gain with a unbypassed cathode resistor. It puts feedback into the cathode reducing gain.

I haven't given LED bias a try yet, some people seem to think it has a "sound", maybe I will experiment in the 45 amp. I've left the cathode unbypassed on the MH4 drivers since they are CCS loaded.

 

[L0rdGwyn]

45 parafeed chassis arrived today.





Cut outs on the back for heatsinking the power tube CCS (top device).



Here is how it will look with the heat sinks.



Going to switch the hardware to black, forgot to mention it to Dave. But very happy with the look! Got some other goodies today too, sockets, resistors, etc. but I'll save it for later

 

[A2029]

I haven't given LED bias a try yet, some people seem to think it has a "sound", maybe I will experiment in the 45 amp. I've left the cathode unbypassed on the MH4 drivers since they are CCS loaded.

Another experiment you can try is to use an LED, but put a resistor of your choice in parallel with the LED. You can play around with the value of the resistor, but try a resistor value such that 70-90% of the current is going through the LED and 10-30% through the resistor. The LED sets the voltage and gives low dynamic impedance. The parallel resistor reduces dynamic impedance of the combo even further and decreases the parallel inductance of the pair. There are also some factors such as noise correlation that are mitigated, but I don't know how much difference those factors would make.

45 parafeed chassis arrived today.





Cut outs on the back for heatsinking the power tube CCS (top device).



Here is how it will look with the heat sinks.



Going to switch the hardware to black, forgot to mention it to Dave. But very happy with the look! Got some other goodies today too, sockets, resistors, etc. but I'll save it for later

Looks great! I like the brushed non-anodized aluminum look.

 

[L0rdGwyn]

Another experiment you can try is to use an LED, but put a resistor of your choice in parallel with the LED. You can play around with the value of the resistor, but try a resistor value such that 70-90% of the current is going through the LED and 10-30% through the resistor. The LED sets the voltage and gives low dynamic impedance. The parallel resistor reduces dynamic impedance of the combo even further and decreases the parallel inductance of the pair. There are also some factors such as noise correlation that are mitigated, but I don't know how much difference those factors would make.

Interesting! I'll keep it in mind, that kind of tinkering is right up my alley. Quasimodo and friends will be arriving from JLCPCB today as well

 

[L0rdGwyn]

Oh one more thing - I got a fitted perforated sheet to replace the bottom plate of the 6A5 amp. This is going to be the approach going forward when ordering from Landfall. Nice and neat, great for ventilation. Will be adding some hidden vents to the top plate of the 6A5 amp some time next week, beneath the transformer shields.

 

[Tjj226 Angel]

Well thank you then for discovering them, I really like the work they do. I am expecting the 45 parafeed chassis this Thursday, looking forward to seeing how it turned out. Then the long wait for transformers, again.



I haven't given LED bias a try yet, some people seem to think it has a "sound", maybe I will experiment in the 45 amp. I've left the cathode unbypassed on the MH4 drivers since they are CCS loaded.

Everything has a sound. However audiowize told me that there are some things you have to consider with LEDS.

1: LEDs are considered fixed bias rather than cathode auto bias, so you need to make sure your grid leak resistor is the appropriate size.

2: LEDs do cause a small amount of positive feedback into the tube. If your tube is prone to oscilations, then it might be a bad idea to use LEDS.

3: Not all LEDs are equal. Different quality LEDs will have slightly different dynamic impedance and other issues. Buying a higher quality / better build LED is important.

3.5: The other thing to point out is LED colors. Different LED colors will produce different bias voltage. It is important to buy a color that has the right voltage AND low dynamic impedance.

 

[A2029]

Also another thing to look for when selecting LEDs is that the inefficient LEDs are often best as they often have lower dynamic impedance. I often go with red or infrared LEDs as they tend to have lower dynamic impedance than other colors. I usually use HLMP-6000 LEDs (note: you want the old HLMP-6000, and not the newer HLMP-6000-E/G that is higher efficiency, and much higher dynamic impedance). An HLMP-6000 gives you around 1.6-1.7V per LED, and can take up to 10ma or so current. They can also be matched and paralleled for higher currents, and put in series to increase bias voltage. Around 5-6ohm dynamic impedance each I believe. You want their ForwardCurrent X ForwardVoltage to be as vertical as possible such as the STD RED below.

 

[L0rdGwyn]

Great information here, thank you both. If memory serves, the HLMP-6000 is used in a few of Bottlehead's designs. I'll give some thought to trying LED bias on the 6J5 of the 45 amp. The plan as of right now is to use a 8mA / -5.5Vg bias point, could stack a few HLMP-6000, but I'll take a look at my other LED options as well.

 

[L0rdGwyn]

Doing some checks on my planned layout this morning, here is sort of where I am going with it:



The Rod Coleman raw DC boards will be mounted to the side panels, filament regulators will be underneath, heat-sinked to the top plate. I've run the numbers here, should be good to go without dedicated heat-sinking. I am plus/minus on scotch-taping the raw DC boards to the side panel, we'll see how I feel about it later...

6J5 CCS boards will sit behind the octal sockets, likely will mount them with some pretty long standoffs to take advantage of the height of the chassis, tag strips will be mounted behind/underneath for the rectifier diodes, snubber resistor/capacitor, etc. The power tube CCS will be mounted on the back panel, up and out of the way, leaving the entire area between the UX4 sockets and rear vents for the parafeed caps. Once the value of the caps are determined (using cheap Solen stand-ins), we'll see what I can fit in there. The pot will be another Goldpoint V47, which I haven't picked up yet. Cramming a lot in this chassis, but it's gonna work.

Here is a little preview of how the amp will look, if you can imagine some Sowter transformers sitting behind the tubes and a big Goldpoint knob on the front.



Was going for a sort of compact, "strong" look for the amp (whatever that means). It's going to be a dense little guy, can't wait to get to work on the build! Tempted to get started, but think I better wait until the transformers are here, just in case the dimensions aren't exactly as listed on Sowter's site.

 

[whirlwind]

Doing some checks on my planned layout this morning, here is sort of where I am going with it:



The Rod Coleman raw DC boards will be mounted to the side panels, filament regulators will be underdeath, heat-sinked to the top plate. I've run the numbers here, should be good to go without dedicated heat-sinking. I am plus/minus on scotch-taping the raw DC boards to the side panel, we'll see how I feel about it later...

6J5 CCS boards will sit behind the octal sockets, likely will mount them with some pretty long standoffs to take advantage of the height of the chassis, tag strips will be mounted behind/underdeath for the rectifier diodes, snubber resistor/capacitor, etc. The power tube CCS will be mounted on the back panel, up and out of the way, leaving the entire area between the UX4 sockets and rear vents for the parafeed caps. Once the value of the caps are determined (using cheap Solen stand-ins), we'll see what I can fit in there. The pot will be another Goldpoint V47, which I haven't picked up yet. Cramming a lot in this chassis, but it's gonna work.

Here is a little preview of how the amp will look, if you can imagine some Sowter transformers sitting behind the tubes and a big Goldpoint knob on the front.



Was going for a sort of compact, "strong" look for the amp (whatever that means). It's going to be a dense little guy, can't wait to get to work on the build! Tempted to get started, but think I better wait until the transformers are here, just in case the dimensions aren't exactly as listed on Sowter's site.

I love the look Kennan....simple look and elegant.

 

[L0rdGwyn]

I love the look Kennan....simple look and elegant.

Thanks, Joe! The look is very important to me, so I spend a lot of time planning the layout, dimensions, spacing of the tubes/components, etc., hope it shows in the final product.

 

[L0rdGwyn]

Time for another DIY rant...

I happen to have a week off starting Wednesday, not a vacation, just by way of scheduling (healthcare is weird like that). I've squared away all of the 45 parafeed plans, so I am going to start working on the build in earnest. I'm still at least a month away from having the transformers, likely two months, so I won't be doing any drilling on the top plate for fear of encroaching on transformer real estate, but I will begin working on the side panel components.

So, with all of those plans done, I am looking to the future. The next build is going to be a real doozy and there is no timeline. Might be crazy for planning it already, but helps keep me occupied from all of the stress at work!!! Disclaimer: this is a "can we do it", not a "should we do it" kind of project.

Here is the plan in a nutshell: it will be an all DHT, three-stage hybrid A2 design using all thoriated tungsten filament tubes. The gain will be supplied by an oddball DHT - the 841 / VT-51. It is a high-gain (mu of 30) version of the 10Y / 801A with tighter grid spacing. It will be CCS loaded and mu-output taken given its high internal resitance (62K). Current drive to the output tube grid will be supplied by a FET source follower (device TBD, although I have a short list). Output will be the 801A / VT-62, grid driven positive far into class A2 with a near 0V grid bias point.

Top choice for OPT right now is the Lundahl LL9202 at 50mA (100H). This transformer can be wired for 6.5K, 11K, and 23K primaries. My speakers are 6ohms nominal, so may end up using the 11K:8ohm wiring for something like a 8.25K:6ohm setup. With the grid driven positive to saturation, with ~625Vpp, that would give this amp an output of 6W into 6ohms. Would likely experiment with other turns ratios as well.

I had originally cooked up this idea using a 6BX7 cathode follower as the current gain stage. After snooping around diyAudio and PMing a fellow who is experienced using the 841, I am convinced that a source follower is the way to go. By happenstance, that same DIYer has built a very similar circuit to what I have in mind, so I will be using his schematic as a reference.

The major challenge of this design will be the power supply. The 841 and A2 biased 801A will take very different plate voltages. The most likely approach will be to use "stacked" power supplies, perhaps 300V + 300V, both Maida regulated, with the FET V- supply taken from the bottom. I haven't dived into the details here yet, there is much to learn. Given that all three stages will be taking DC only, one thought is to use a separate power supply chassis, such that I can run all of the DC power through an umbilical, keeping AC out of the amplifier chassis. Given the size of the caps that will be needed here along with the dedicated filament transformers, using a separate chassis is likely a necessity. Finding appropriate mains transformers will be another challenge, we'll see.

This will be a very challenging project, but taking on more complex designs is the way to learn. Particularly, this will be a great opportunity to learn more about SS devices, and I will also be using it to take my utilization of LTSpice a step further. I want to be very confident in my model before moving forward.

Okay, that's it, that's what I am working on in the background of the 45 amp. Again, no timeline and all subject to change, but I think it will be a very interesting amplifier if I pull it off.

 

[Xcalibur255]

Keenan, sometimes I feel like you were born with about 5 normal people's worth of ambition......

This is both a compliment to you and an admonishment of myself who is too lazy to actually try and do any of this stuff.

I really want to hear your impressions of this after you build it. The thoriated tubes are supposed to sound unique, people seem to struggle to describe it sometimes.