PYE P75 radio conversion to guitar amplifier

[RogerLLL]

Well that made quite a difference! Lots more volume! With the resistor alone the difference was much less. The EL41 definitely seems to be running hotter though, and I'm sure the filament is glowing brighter, which I imagine is due to a greater potential difference drawing more electrons, but might be wrong on that!

[Gabe001]

Just calculate the anode dissipation to be sure. I don't think it should have changed but you never know

[RogerLLL]

Originally Posted by ms660

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

....Scroll down.

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

...Section 1.18

Lawrence.

Really useful! Explains it all, thanks for that Lawrence! I notice the Rk value quoted for the EL41 is 170 ohms. Thinking that might be a safer value for the cathode resistor.

[Robert Gribnau]

Just measure the cathode voltage of the EL41. Than with Ohms Law (I = V/R) you will know its cathode current. Note that B+ is relatively low, so the cathode current has to be relatively high to reach the maximum anode dissipation of 9 Watt of the EL41.

I don't think that the filament can glow brighter because of lowering the value of the cathode resistor since the brightness of the filament only depends on the heater voltage.

[RogerLLL]

Here's my calculation:
Max plate dissipation 9 W
Va 198 V
Vrk 5.47 V
Rk 150 ohms

Ik=Vrk/Rk 0.036466667 A
Is=5.5% Ik 0.002005667 A
Ia=Ik-Is 0.034461 A
W=Va*Ia 6.823278 W
% plate dissipation 75.81%

Seems about right for this amp, and is sounding much better. I always have the option to go higher, but the addition of the cathode bypass capacitor has had a big impact on volume and sound quality, giving a much more even sound and a smoother overdrive.
Here's an updated schematic with revised voltages added in blue:

[Gabe001]

Looks fine.

Have you sorted the microphony?

[RogerLLL]

Originally Posted by Gabe001

Looks fine.

Have you sorted the microphony?

I had! Mostly at least, but with this extra volume there's also more microphony. I have so far installed a "Vibra-sorb" damping ring on the EL41 and EF41, which produced an immediate reduction in microphony - was pretty impressed with how effective they were. They're just over 5mm dia silicon rubber so reasonably heavyweight. I've ordered 2 more and also some rubber open ended wiring grommets to replace the chassis cage nut mounts which should provide some extra damping of speaker vibration.

[trobbins]

A few comments regarding post #125.

Did you measure Rk at 150 ? I note you realised the original resistor had drifted high. At 150 ohm then the cathode current is 36-37mA, and the anode current will be somewhat less depending on the actual screen current.

Where was the 226V measured at (ie. was it the incoming B+ feed to the output transformer) ? If it was the outgoing tap to the preamp/screen feed than that indicates 12mA is passing through the 1k, based on your blue voltage measurements (assuming the 1k has not drifted significantly). The first stage is taking about 1.9mA, and the driver stage and first stage screen feed through the 47k are likely taking about 1-2mA, which may mean the output stage screen is drawing at least 8mA (which is circa 1/4-1/5 the anode current).

Having a good estimate of both anode and screen dissipation at idle is a worthy exercise in measurement, as that is directly relatable to datasheet limits.

It can be a bit disconcerting to see values reported to 9 decimal places - you may want to try and round off any measurement values as that then indicates a better level of awareness of what the values mean and how they will be used.

[RogerLLL]

Yes Rk was measured at 150 ohms. I used a 5.5% reduction in Ik to estimate screen current and so calculate anode current. If the screen is drawing 8mA then my 5.5% estimate is wrong! More like 20%.
The 226v was measured between OT and the 1k resistor, so it's what is going into the circuit out of the OT. I haven't checked the 1k but will.
I get your point about the decimal places/ significant figures, I just copy /pasted from a spreadsheet. I think we can realistically go with 6.8W as a more meaningful figure!

[Gabe001]

I suspect your multimeter is not the most accurate of devices, so I wouldn't read too much into your readings other than to get a fairly broad idea of the valve's operating points.

Are you planning your next amplifier build yet?

[RogerLLL]

Starting to formulate some ideas in my head, but haven't actually started. Thinking some sort of 12AX7 & EL84 combo in a push-pull setup. I'll start in earnest once I get this one fully finished, with new faceplate etc.

[Gabe001]

I'd recommend cloning an established design, like a fender 6v6 push pull amp, as radiowrangler also suggested. The schematics and layout are all available online so the hard work is done for you. I have some spares you may find useful (namely an output transformer I won't use, and a 3mm aluminium plate that was sent to me in error in the wrong size), so pm me when you decide to get going

[RogerLLL]

That makes sense going for an established design. I prefer the Fender tones to those of other amps such as Marshals, Vox etc so a Fender 6v6 push pull sounds like it would make good sense.

[RogerLLL]

Was finding the EF41 tube microphonics pretty difficult to work with, so I tried using the ECH42 on the other socket, with 3 jumper wires from the EF41 socket to make the triode connection. It is still a microphonic tube but not as bad as the EF41, so does work better, and sounds pretty much the same tonally. As far as I can tell the operating characteristics are close enough to not need to change anything, so I think I'll just go with it.

[G4XWDJim]

Roger,

We’re not far apart geographically so if you’ like to collect it from me FOC I have a p/p 6V6, 2 x ECC83 amplifier I rebuilt a few years ago you can have. It’s minus its valves, they got used for something else but it should still be ok.

It has 2 inputs one more sensitive than the other but I can’t quote any figures. I think I should be able to find a couple of metal 6v6s that should be ok you can have with it.

Let me know

Jim

[RogerLLL]

That's amazing Jim! I'll very gladly take you up on your kind offer. If you let me know your address we can work out a suitable time for me to head over 👍

[G4XWDJim]

You have a PM coming your way

[RogerLLL]

Although the ECH42 was less microphonic than the EF41, realistically it was still too bad for use. So I bought a second hand EBC41 (basically a second V3) and installed that as V2. Huge difference, no microphony at all, or at least nowhere near enough to pick up speaker vibrations, so to all intents and purposes, the amp has found its final iteration!

[Gabe001]

Excellent,glad you've got it sorted. Any chance we could see the final product? Did you say you were getting faceplate done?

[RogerLLL]

Here's some pics of the final product, with its super simple faceplate! The Tone switch isolates the tone stack giving the raw tone of the amp, which certainly has its place! I've included a soundclip featuring 3 tone settings, the first is the usual one I use, Bass Mid & Treble all at 12 o'clock, clean with a bit of breakup. Then I switched the tonestack out of the circuit, and then ended with a more overdriven sound with the tone back in the circuit.






2023-03-25-16_16_14.zip