PYE P75 radio conversion to guitar amplifier

[Gabe001]

If you're going to start a new valve amp build, I would start a new thread in the "vintage audio" section with the appropriate title, for maximum exposure.

[RogerLLL]

OK, I'll do that as and when I start.

[RogerLLL]

Just thought I'd send my final schematic for the amp. I installed the tonestack but it really did reduce the gain too much, so I re-modeled it as in the attached to keep the Fender tone curve but with higher gain, which worked much better. Still reduced gain as you'd expect but much less so. I also included a tone bypass switch just so I can revert to the raw higher gain tone when the mood takes me!
I also swapped the 2uF V2 screen filter cap for a 32uF as I was still getting some instability at higher gain settings, now sorted.
I'll send pics and maybe some soundclips when I get it all mounted in the case.

[RogerLLL]

Slight version control issue there, showed the previous negative feedback wiring. Updated here:

[trobbins]

Did you make any measurements from post #63 - they are somewhat base level for checking the output stage is operating nominally.

[RogerLLL]

No I didn't, but see what you mean. I'll do that when I get the chance.

[RogerLLL]

I've mounted the amp in the case with the speaker, and the fact that C2 & V3 are so microphonic is an issue. At higher volumes the speaker vibrations create all sorts of weird distortions due to those two tubes. With the amp on the bench & speaker on the floor no problem. The tone stack has vastly improved the overall tone, and it plays beautifully at just below where the microphony kicks in, but volume is significantly limited. I might try tube shields, or some other damping device, I doubt there's much more I can do about this, but any suggestions welcomed!

[Gabe001]

You could try some rubber damping between chassis and case eg some rubber washers around the bolts. Same with the speaker

[RogerLLL]

I suppose that's a similar principle to how a reverb tank is mounted. I'll definitely give that a try. I have an old reverb tank so may use the rubber mounts on that.

[trobbins]

Some more insight in vibration isolation is in link
https://dalmura.com.au/static/Microp...n%20valves.pdf

[Gabe001]

Thank you. Ideally you damp the valve bases, as the article mentioned, but that's pretty invasive. This is what I used in my 5-10, which it probably didn't need, but just to show you that dampened valve bases do exist for the problem you're experiencing.

I have no experience with tube rings but I suppose they won't do any harm and easy and cheap enough to try.

[RogerLLL]

That article covers this issue very nicely, and certainly advocates for decoupling the chassis from the speaker/ box. I've ordered some of those silicone rubber damping rings to fit over the tubes and also some rubber grommets to fit in place of the cage nuts on the chassis mounts, both very cheap fixes. Might also order some neoprene tape to use as a speaker gasket as a next option. Hoping that all those together will at least lessen the effect of the microphonic tubes. BTW, I checked all the tubes by tapping to see which were the most microphonic, and contrary to my earlier post it's the EF41 (V2) and the EL41 (V4) that are the worst offenders. The EBC41 (V3) is actually pretty quiet. Thinking the EL41 might just be due to old age/ use, as this isn't known for being microphonic, unlike the EF41 which definitely is!

[Robert Gribnau]

A note caused by this thread: https://www.vintage-radio.net/forum/...d.php?t=198821

The EL41 in the Pye 75 is running very modestly (at 4.6 Watt, so practically at half of its maximum plate dissipation of 9 Watt). So for more output power, lowering the value of the cathode resistor (220 Ohm) could give more output power. But (of course) there's a hinge. Lowering that value will make the current through the EL41 go up, something that the power transformer perhaps can't handle. But since you are not using the ECH42, you have close to 6 mA extra to spare.

There is a second hinge though. For power pentodes, the maximum power output occurs when the load is Va / Ia. So if the primary impedance of the output transformer of the Pye 75 is optimal for maximum output power, than highering the anode current could get you away from that optimum.

But still, maybe something to try out.

Addition: You could also try to decouple the cathode resistor of the EL41 (so add a capacitor of something like 100 uF parallel to the cathode resistor). The hinge there is that this will lower the output impedance of the EL41, so maybe it will (again) get you away from the optimal load. But again, still maybe something to try out.

[RogerLLL]

Interesting. Like you say, I should have spare capacity through removing the ECH41, so should be reasonably safe to try. I'll go 50 ohms lower to a 170 ohm as a test, and see what that does. A bit more volume would definitely help the clean tones which atm are a little quiet. I had considered trying an EL84 as in that other thread, but tbh the EL41 seems to be working well.

[Radio Wrangler]

Originally Posted by Robert Gribnau

So for more output power, lowering the value of the cathode resistor (220 Ohm) could give more output power. But (of course) there's a hinge. Lowering that value will make the current through the EL41 go up, something that the power transformer perhaps can't handle. But since you are not using the ECH42, you have close to 6 mA extra to spare.

This also increases the DC bias current AND the peak current in the output transformer, moving it further into saturation.

The inductance of the transformer diminishes as the current increases, spoiling low frequency response, increasing distortion.

A single-ended amplifier has the output valve and power supply drive the speaker in only one direction. While it's doing this, the magnetic flux in the output transformer is also being driven up. During the other half-cycle of the signal, it is the stored energy in the output transformer which powers everything as its magnetic flux is run down.

So the output transformer in a single-ended amplifier is most often the limiting factor.

Rather than making huge transformers, people go towards push-pull amplifier architectures, so higher power transformers for single-ended amplifiers are rarities and it's hard to find one if you want to go that way.

David

[ms660]

The output transformer looks to be a reasonable size, it looks bigger than the one say in the Dansette Major (EL84) Could be good to 40mA if so it would give you quite a few mA's to play with providing the mains transformer can take it, the ra for the EL41 and EL84 aren't too far apart in the valve data so the hum cancelling properties of the transformer should still be ok.

Lawrence.

[RogerLLL]

I went down to 150 ohms for the cathode resistor. The existing one, supposedly 220 ohms had drifted in value to 260 ohms, so clearly wasn't helping. Definitely more volume which is nice like I say for the clean tones, and helps to counteract the reduction in output due to the tonestack, which is working perfectly. Is that a safe value given the increased current draw? I gather the general understanding is to aim for 70% of maximum plate dissipation on the power tube, but that said I realise this amp was designed for around 50%, though we only have 3/4 main valves.

[Gabe001]

For cathode bias you can theoretically go up to 100% anode dissipation. I think 70% is for fixed bias.

90% or thereabouts should be fine, 85% if you want to be conservative.

Try bypassing the 150ohm resistor with an electrolytic capacitor (100uf or so, 25v) and see what happens

[RogerLLL]

What's the purpose of that capacitor? I normally see that setup for power stages and was wondering why that hasn't been used for this amp?

[ms660]

https://www.valvewizard.co.uk/se.html

....Scroll down.

https://www.valvewizard.co.uk/Common_Gain_Stage.pdf

...Section 1.18

Lawrence.