Supro inspired amp to build soon just finished 1st schematic

[Chuck H]

I don't know what it sounds like but the waveform is horrible when the bypass capacitor is omitted on a class A/B amp.

Hmm... That just looks like a slightly compressed wave form to me. And without crossover distortion. How did you get the center point at 0V

I'll bet that in actual practice the wave form would look much worse than that. But it probably wouldn't sound bad either. I don't actually know because I've never tried it. No excuse really. I do more cathode biasing than fixed biasing because most of my work has been with EL84's. I guess I just never wanted an amp THAT squishy.

[Jazz P Bass]

That wave form looks good to me, for an unbypassed cathode.
(At one frequency)
Compressed is a good word.

A bypass capacitor of the correct value will pass all of the signal.

[Dave H]

Hmm... That just looks like a slightly compressed wave form to me. And without crossover distortion. How did you get the center point at 0V

It's the output of the OT. I said it was horrible because I thought it looked like a salt cellar (straight sides, rounded top) rather than the curved sides and flatter top I'm used to with a bypass cap. I think there's no crossover distortion because both tubes are on for the whole of the flat sides. The domed top is when one tube is off.

Waveform distortion like that is 'dirty' compression. The normal circuit with a bypass cap (I haven't measured it without) also does 'clean' compression. If you set the output just below clipping so it looks like a sinewave and then halve the input signal (-6dB) the output only drops 3dB not 6dB, i.e. from about 14V peak to 10V peak on my amp.

[EFK]

Well fwiw I'm one of those sickies that thinks these sound a whole lot better without the bypass cap. More compressed yes, but the onset of distortion is smoother an much less abrupt and the big plus of no horrid crossover distortion.

[swingarm]

I see you're committed now. In case you find the need to alter anything this is how "I" might implement a switchable cathode/-V grid bias w/ PR mod, measurement resistors and individual bias control. No disrespect to KOC but I think it's an eminently simpler and more eloquent circuit.

Cool drawing Chuck! Thanks for sharing. BTW, you say no disrespect then you called him KOC, Thats not very nice!

Since you mentioned before Chuck there was not a huge difference tonal difference between fixed and cathode bias using the cap, I thought it might be interesting to play with this. I put a 560 ohm in there for now. (i have not had lift off yet) its a 10 watt. Does the larger value need to have such a large wattage ratting? I have more variance in 5 watt resistors in my tickle trunk. furthermore, maybe I could use a temporary pot pot to find the sweet spot, maybe a 2 watt pot? Efk what values have you used? Any recommendations?

[Jazz P Bass]

Well fwiw I'm one of those sickies that thinks these sound a whole lot better without the bypass cap. More compressed yes, but the onset of distortion is smoother an much less abrupt and the big plus of no horrid crossover distortion.

The unbypassed cathode is known for degenerative feedback.
More even gain.

[swingarm]

Well fwiw I'm one of those sickies that thinks these sound a whole lot better without the bypass cap. More compressed yes, but the onset of distortion is smoother an much less abrupt and the big plus of no horrid crossover distortion.

Now Im wondering about this buzz fix. Maybe I dont need it? Or at least I could make it switchable if it does not need to used in cathode mode, No?

I could put the diodes on a switch to hear if they do anything by lifting the ground-end of the diode strings with a single-pole switch ,Ya? or maybe not... *shrugs like a noob*

Also I looked at the 18 watt schematic used by paul ruby and realized that the 470k resistors in pauls schematic are grid leaks from the 18watter and should not be included for my purposes. right? revised power amp schematic attached.supro power amp davis 106.pdf

Also, should the grid resistors (pin 2) be changed to a different value? opinions? Thanks

The davis 106's power transformer is 280-0-280 and uses an ez81 rectifier .

[swingarm]

The unbypassed cathode is known for degenerative feedback.
More even gain.

Cool, Jazz have you used this in an el84 based amp before?

[swingarm]

Searching this out, turned up this.

Cathode-Bias/Fixed-Bias Switching in Tube Power Amplifiers

[Dave H]

The unbypassed cathode is known for degenerative feedback.
More even gain.

Would that be better with separate unbypassed cathode resistors? It would stop the tubes interacting at the cathodes when they were both conducting.

[Chuck H]

"KOC" has become an internet moniker for the big cootie he has become. I doubt he'd be offended

I think it's important to compare apples to apples. A 560 ohm cathode resistor is cold as hell. So, not a good reference point for comparison. There would be A LOT more voltage rise with current so the bias shift would be exaggerated too. I'm using 130 ohms @ 355Vp with decent tube life (about 500 hours). Cooler actually works alright too. I started with 180 ohms, then 150 and stopped when I got to 130. It just sounded and felt right. I haven't actually tested at very high resistances but that's because I wouldn't expect good results. So, if you're going to compare fixed vs. cathode bias I'd say set current for fixed at about 75% dissipation and set cathode for about 90%. This as it applies to EL84's IMHE.

[swingarm]

Ok I dug through a surplus box of pots and found a 500 ohm 4 watt pot. I will use that to explore the best sound when I'm ready.

I doubt he'd be offended

KOC would not be offended. I will say this about him, He answers his phone at london power press and has helped me with very informative info concerning the project amps in his books. Very nice guy.

[Dave H]

I think patching sections of different amplifiers together has resulted in a design which is overly complicated with redundant components. For example that PI was designed for cathode bias and has extra coupling caps to make it work with fixed bias.

Step back, what do you really want? I agree with Chuck I can't hear much if any difference between cathode and fixed bias with EL84s. Try it yourself with his simple zener fixed/cathode switch and if you agree why not stick with cathode bias and dump the bias pots? “But then I can't use unmatched tubes” you are thinking. OK, separate the EL84 cathodes and use separate 270R cathode resistors with bypass caps. It will then balance well enough with mismatched tubes. I've measured it. If a pair of EL84s are say 12mA different in cathode bias with a common cathode resistor they will only be about 3mA different with separate cathode resistors (and about 6mA different in fixed bias).

Get that 560R cathode resistor out of there It's is far too big. I've used 150R when the B+ was 275V. The EL84s were at nearly 14W (it had a small fan) but I think it sounds better at 335V B+ with a 135R cathode resistor and the EL84s are then running at just over 11W.

[swingarm]

Ok so it turns out both of you have in fact experimented/run an amp with no bypass cap. Please excuse me I am a blasphemer and a heretic. Thank you for bringing me back to reality. What the hell was I thinking? I will get that 560r out of there AT ONCE! Oh and the bias pots too. I will cover the holes with flower magnets, ah shit! I will glue them seeing as copper is not magnetic to macth my persona, LOL. I have taken a step back...I have lost control of my senses! I would like to ask the community to please accept my apologies. I have gone through several years now living in a world of woo woo. I want to make it my intention to manifest an amp that contains the properties that will please the community, I would never want to stray from those goals. Thank you for stopping me from making a huge mistake Wait...do any of these phenomena apply here? I think the guy running the experiment below may be chuck before he took on his Methuselah like features, LOL just kidding Chuck, you look marvelous! OH and would you all please validate me by liking this post? I have not received as many likes as I think I deserve in this thread and feel there may be a conspiracy at play here

Instead of lines, substitute resistance values LOL

Or maybe I'm going for the forbidden banana ?

https://www.youtube.com/watch?v=0344qRfAOtA

[Dave H]

Whoops! Sorry, I didn't mean to sound condescending.