PYE P75 radio conversion to guitar amplifier

[RogerLLL]

Hi, I'm new here and not 100% sure whether my request fits with this forum, as it concerns conversion of a radio to an amplifier. That said, I'm hoping that members' familiarity with these circuits will enable good advice for my project!

As the title suggests, I'm really looking to cut out the radio section of the unit to just leave the audio amplifier portion. I've included an extract of the schematic from this which I have marked up with what I think might be the modifications needed to achieve my aim. I have tested it using the input point I have shown on the schematic and it works really nicely, albeit with quite low volume. I've marked up suggested cuts in red and have highlighted two components in green which I think value changes might be possible to modify tone and gain, but I might be wrong here. I've also included a negative feedback cut switch, again, not sure about this. I added an earth connection too, just to represent the ground rail.
Any suggestions would be greatly appreciated!

[peter_sol]

You can't increase the gain enough without having a preamp stage.
If you cut the NFB there the bottom of the volume pot needs connecting to the chassis.
The whole thing is not a good idea.
It's been suggested a few times before.

[RogerLLL]

Thanks for the reply! Do all my suggested cuts look feasible?

[Paul_RK]

This thread might be helpful - plenty of to and fro reflecting the various perspectives of forum members, but a number of useful points are made on the way to the successful conversion of a popular 1950-ish set to a practice amp.. Wouldn't dream of doing it myself, but I'm funny that way.

https://www.vintage-radio.net/forum/...d.php?t=158825

Paul

[Gabe001]

You will have nowhere near the gain required for a guitar amplifier with that circuit. It may be acceptable for playing music through your phone or a small Bluetooth adapter, but not great.

[AC/HL]

Facebook marketplace usually has a selection of these amplifiers. By chance there's one in your town: https://www.facebook.com/marketplace...3-169e49fb5c1b

[ms660]

A trick used by some radio manufactures was to configure the oscillator triode or the IF pentode as an extra amplification stage for a pickup/gram input.

Lawrence.

[RogerLLL]

Many thanks for all the responses!
The reason for this project is to create a small bedroom practice amp which uses only vacuum tubes for the amplification. The tone produced is much richer than a solid state amp, with more harmonics and a sweeter sounding distortion when overdriven. I did find that although it produced a nice clean tube tone, it was definitely lower in volume that the sound produced by the radio itself, so the suggestion of a preamp makes perfect sense. I might build a single stage preamp based on a 12AX7 or similar. It produced a beautiful sounding overdriven distortion when using an overdrive pedal, wayyyyy better than what is produced by a solid state amp, so I feel that the project has legs for this reason, and a pre-amp stage would make sense. I only paid £15 for the chassis with all the tubes, input & output transformers, which is great value given the high price of even second hand tube amps. I'll give it some thought and update when I've got to the next stage.

[RogerLLL]

Originally Posted by ms660

A trick used by some radio manufactures was to configure the oscillator triode or the IF pentode as an extra amplification stage for a pickup/gram input.

Lawrence.

I did wonder if I could use the intermediate frequency amplifier as an additional pre-amplification stage, but this is based on an EF41 RF pentode, so not sure whether this would even be possible with this tube.

[Lucien Nunes]

Do all my suggested cuts look feasible?

As per @peter sol the 'NFB switch' as shown would leave V3 without a grid leak. If you want to break the feedback path, either break it at C24 instead or re-instate a DC path from the bottom of the volume pot to ground.

The gain is what it is, unless you tack on an extra stage. It sounds like you have auditioned it with some degree of satisfaction, but consider whether it is adequate for realistic use. FWIW even if you can drive the EL41 fully, the audio output must be somewhere in the region of 3 watts at 10% distortion or so.

Maybe leave the HT supply to the front end alive, and probably no need to disconnect the diode anodes of V3. Then a simple switch and an input jack on the back will convert between radio and guitar modes and you can have the satisfaction of gaining a practice amp without destroying a radio. And, if you decide to move forward and build a complete practice amp perhaps with a bit more power, you don't need to do anything to get the radio back.

[RogerLLL]

Pye_P75.pdf


Just realised I didn't insert the attachments so here's my marked up schematic & specification.

[RogerLLL]

Originally Posted by Lucien Nunes

As per @peter sol the 'NFB switch' as shown would leave V3 without a grid leak. If you want to break the feedback path, either break it at C24 instead or re-instate a DC path from the bottom of the volume pot to ground.

The gain is what it is, unless you tack on an extra stage. It sounds like you have auditioned it with some degree of satisfaction, but consider whether it is adequate for realistic use. FWIW even if you can drive the EL41 fully, the audio output must be somewhere in the region of 3 watts at 10% distortion or so.

Maybe leave the HT supply to the front end alive, and probably no need to disconnect the diode anodes of V3. Then a simple switch and an input jack on the back will convert between radio and guitar modes and you can have the satisfaction of gaining a practice amp without destroying a radio. And, if you decide to move forward and build a complete practice amp perhaps with a bit more power, you don't need to do anything to get the radio back.

A switchable amp/radio is definitely an option, with a separate pre-amp board. Not sure i can fit it in the amp/ speaker case I'm using, but will keep both options open.

[peter_sol]

Yes you can use the EF41 as a preamp.

[Valvepower]

Hello,

Just my two penn'orth…

One thing that springs to mind is you may need some HF lift (treble boost – or brightness) to just 'brighten' the sound a little as sometimes a guitar amplifier based on an amplifier such as found in the PYE could sound a tad dull.

Another thing it may need a bit of LF cut (which can also give the effect of HF lift) by using coupling capacitors to give a roll in the in the lower 100’s of Hz. This ‘tightens-up’ the sound by removing what could be a 'boomy' sound.

I concur you will need more amplification – off the top of my head – possibly a preamplifier with a gain of something like x10

Regards
Terry

[RogerLLL]

Good point, although it's a lovely rich sound, it does lack treble and is definitely a bit boomy in the bass. The tone control really doesn't do much, in fact it reduces the volume without altering the tone much. I'm thinking the 2nF tone capacitor may have an internal short so will change it anyway. That said, I'm thinking I may incorporate a Fender style tone stack which should provide that sound correction.

[RogerLLL]

Originally Posted by peter_sol

Yes you can use the EF41 as a preamp.

I've done a little research and although there's not much out there I did find one example where this tube was used as a pre-amp, though the tube is apparently a "remote-cutoff tube", also called variable-mu, though this shouldn't be an issue if its the first stage tube. I'll give it a try and see how it works, should be a fairly simple mod. The pinout is the same as for the EL41 power tube.

[RogerLLL]

One thing I haven't yet done is added an earth connection from the mains supply, just has a 2 lead power cord. Should this simply connect to the chassis?

[ms660]

Originally Posted by RogerLLL

I did wonder if I could use the intermediate frequency amplifier as an additional pre-amplification stage, but this is based on an EF41 RF pentode, so not sure whether this would even be possible with this tube.

It should be possible, the EAF42/EAF41 is also a variable-mu pentode that's mainly used as an IF amplifier in radio receivers but which can also be used as an AF amplifier, details and figures for that can be seen in the valve data here:

https://frank.pocnet.net/sheets/046/e/EAF42.pdf

Lawrence.

[RogerLLL]

Originally Posted by ms660

It should be possible, the EAF42/EAF41 is also a variable-mu pentode that's mainly used as an IF amplifier in radio receivers but which can also be used as an AF amplifier, details and figures for that can be seen in the valve data here:

https://frank.pocnet.net/sheets/046/e/EAF42.pdf

Lawrence.

That's great, thanks, really useful link!

[Valvepower]

Hello,

Using the Fender tone control network, either just bass and treble or the tone network with middle as well would be a sensible move. I figure use a single stage amp (EF41 as suggested – maybe triode connected?) then the Fender tone controls and then into the EBC41…

I suppose having the volume control a ’la Fender, after the tone control network, would do the trick, maybe fit a bright capacitor across the slider and track of the volume control.

You'll need some extra gain to overcome the insertion loss of the tone controls.

Terry

Originally Posted by RogerLLL

Good point, although it's a lovely rich sound, it does lack treble and is definitely a bit boomy in the bass. The tone control really doesn't do much, in fact it reduces the volume without altering the tone much. I'm thinking the 2nF tone capacitor may have an internal short so will change it anyway. That said, I'm thinking I may incorporate a Fender style tone stack which should provide that sound correction.