PYE P75 radio conversion to guitar amplifier

[ms660]

Originally Posted by RogerLLL

That's great, thanks, really useful link!

No problem.

Lawrence.

[ms660]

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've remembered one now that uses the oscillator triode, the Pye P53 radio receiver uses the triode section of the mixer/oscillator valve (ECH42) as an extra AF amplifier for the pickup function, grid bias for audio operation in the P75 shouldn't be any problem because the ECH42's cathode is connected to ground which means that grid current bias can be used if you wanted to which just needs an extra resistor fitting in the triode sections grid circuit, a 10 Meg resistor should do. (link to a source for a P53 schematic below, R4 is the 10 Meg resistor in that one that's used for grid current bias):

https://www.service-data.com/product...91/6649/t15391

Lawrence.

[RogerLLL]

Originally Posted by 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

Actually I've been playing around with the amazing and free Duncan Monroe Tone Stack Calculator software, and have modeled a Fender tone curve but based on the Marshall model which has higher gain, so that should help at least.

[RogerLLL]

Originally Posted by ms660

I've remembered one now that uses the oscillator triode, the Pye P53 radio receiver uses the triode section of the mixer/oscillator valve (ECH42) as an extra AF amplifier for the pickup function, grid bias for audio operation in the P75 shouldn't be any problem because the ECH42's cathode is connected to ground which means that grid current bias can be used if you wanted to which just needs an extra resistor fitting in the triode sections grid circuit, a 10 Meg resistor should do. (link to a source for a P53 schematic below, R4 is the 10 Meg resistor in that one that's used for grid current bias):

https://www.service-data.com/product...91/6649/t15391

Lawrence.

Thanks, I've ordered a copy of the schematic

[Boulevardier]

Originally Posted by ms660

I've remembered one now that uses the oscillator triode, the Pye P53 radio receiver uses the triode section of the mixer/oscillator valve (ECH42) as an extra AF amplifier for the pickup function,

Lawrence.

ISTR that the Fenman ll does the same thing with an ECH81
Mike

[ms660]

With an extra gain stage I would suggest some extra HT filtering for its anode feed supply.

Lawrence.

EDIT:

Originally Posted by Boulevardier

ISTR that the Fenman ll does the same thing with an ECH81

Yes Mike it does.

Lawrence.

[Uncle Bulgaria]

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.

What about a small solid state battery preamplifier wired to the back of a socket mounted on the radio's back panel (or run off a rectified heater supply)? Insert a switch (the equivalent of the radio having a 'Radio/Gram' switch) at the injection point on the grid of V3 marked on your initial post and you've done a minimum of fiddling and can remove the mod for something else easily at a later date. The solid state part would just provide enough clean gain to drive V3 properly, leaving you with the valve output stages for all the harmonic distortion you please.

EF41 might be designed as an RF amplifier, but it will of course amplify audio frequency signals too. Audio is about the least demanding task we ask amplifiers to do!

[RogerLLL]

Originally Posted by Uncle Bulgaria

What about a small solid state battery preamplifier wired to the back of a socket mounted on the radio's back panel (or run off a rectified heater supply)? Insert a switch (the equivalent of the radio having a 'Radio/Gram' switch) at the injection point on the grid of V3 marked on your initial post and you've done a minimum of fiddling and can remove the mod for something else easily at a later date. The solid state part would just provide enough clean gain to drive V3 properly, leaving you with the valve output stages for all the harmonic distortion you please.

EF41 might be designed as an RF amplifier, but it will of course amplify audio frequency signals too. Audio is about the least demanding task we ask amplifiers to do!

That's a good suggestion, and I have tried an external tube preamp, as well as an overdrive pedal. Both work well, but I like a challenge! I'd really like to give it a go with the EF41, it will help to develop my understanding of tube amps. I use a current limiter when testing any new mod, and am aware of the risk associated with the high voltages involved, but I'm still a newbie with tube amps. Hoping the end result will be worth it!

[Uncle Bulgaria]

Another alternative is to do what I do - a Ferrograph valve reel-to-reel with integrated loudspeaker serves as my practice amplifier and bedroom record player amplifier. I got the player on Freecycle, and it has the advantage of being able to record anything you're particularly pleased with.

[RogerLLL]

Originally Posted by Uncle Bulgaria

Another alternative is to do what I do - a Ferrograph valve reel-to-reel with integrated loudspeaker serves as my practice amplifier and bedroom record player amplifier. I got the player on Freecycle, and it has the advantage of being able to record anything you're particularly pleased with.

Like it! I've been searching for a while, Freecycle, Gumtree, Friday-Ad, but nothing, then I searched on Ebay, which I thought would be too pricey, and found lots of old valve radios for pretty reasonable prices. This was chassis only but fully working, so perfect for this project. I have an amp housing with speaker, an old solid state 20w practice amp, so that's where this is going.

[ms660]

Out of interest, there's some Ia-Va curves for the EF41 in the link below (Vg2 @ 100 Volts) the reason for posting the link is that the curves aren't shown in the Philips valve data:

https://frank.pocnet.net/sheets/050/e/EF41.pdf

Lawrence.

[dave walsh]

My friends and I used domestic wooden radios in the sixties with electric guitars. We couldn't afford the "proper" amps [as advertised in Practical Wireless.] Using the pick up sockets we got a reasonable sound out of them on 2 or 3 watts. [pre Marshall amps proper bands got a great sound out of much less than 30 watts]. As the "woodies" were still valued in themselves back then and salvaged Loudspeakers were cheap, we connected up external speakers in home made cabinets... to avoid damaging the one in the set There were transistor pre-amps about but we didn't have any.

Dave W

I also had a relatively cheap tape recorder with a "PA" function switch that isolated the amp section to use on its own with a microphone [Garden Parties?]. Once again I hooked up an alternative spkr [utilising a cabinet from a wind up Gram]. With a donated "hum bucking" single pickup on a home made guitar [also free] everything overloaded beautifully. Sadly my playing expertise didn't quite match up to the Claton/Hendrix sound.

[Valvepower]

Hello,

Dave's post reminded me of a picture in the Mo foster book British Rock Guitar. I’ve attached photo of part of the relevant page in the book, I’ve kept the image closely relevant to the conversation, but if the mods feel it's infringing any copyright, then all means remove it.

I’m finding there is fine line between small lower power guitar amplifiers made by the likes of Fender and Rickenbacker [dating back to the 1950/60's] and the amplifiers used in 1940/50’s Table Radios and Radiograms.

Terry

[RogerLLL]

Originally Posted by ms660

Out of interest, there's some Ia-Va curves for the EF41 in the link below (Vg2 @ 100 Volts) the reason for posting the link is that the curves aren't shown in the Philips valve data:

https://frank.pocnet.net/sheets/050/e/EF41.pdf

Lawrence.

Looks like I'll have to be careful with the screen voltage which is specified as 100v nominal - not sure how critical this is though, but clearly needs to be around 100v max. In the P75 it's 47v, so aiming for somewhere between the two would seem like a sensible approach.

[RogerLLL]

That said, I might as well use the existing screen grid supply as is, no need to change it.

[ms660]

There's an article for small signal amplifying pentodes in the link below:

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

Lawrence.

[RogerLLL]

Hi again, I've put the attached schematic together, combination of extracts from various circuits, but it should be easy enough to interpret. Wasn't sure about whether C18, C19 & R9 were still needed, so any advice would be greatly appreciated. Also whether I've made any obvious errors with the first stage layout & component values.

[ms660]

I would include an coupling capacitor in series with the input to the 1 Meg pot and also an extra HT filter resistor say initially 10k connected in the HT rail just to the right of the 16uF in your schematic.

The other alternative using the same valve (EF41) is to connect the control grid (g1) to the anode and operate it as a triode, characteristics wise the EF41 isn't that far removed from the pentode section of the EAF42, typical resistor values for that one for triode connected are given in the link for either using cathode bias (page 44) or grid current bias (page 45) those values might need fiddling with as they are given for an HT supply voltage of 250 Volts and I think in yours the HT might be lower.

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

Good luck whicheverways.

Lawrence.

[RogerLLL]

Originally Posted by ms660

I would include an coupling capacitor in series with the input to the 1 Meg pot and also an extra HT filter resistor say initially 10k connected in the HT rail just to the right of the 16uF in your schematic.

The other alternative using the same valve (EF41) is to connect the control grid (g1) to the anode and operate it as a triode, characteristics wise the EF41 isn't that far removed from the pentode section of the EAF42, typical resistor values for that one for triode connected are given in the link for either using cathode bias (page 44) or grid current bias (page 45) those values might need fiddling with as they are given for an HT supply voltage of 250 Volts and I think in yours the HT might be lower.

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

Good luck whicheverways.

Lawrence.

Interesting - sounds like hum & distortion could be an issue if the full 120 gain is used. I've incorporated the HT filter resistor & coupling capacitor on the input.
Interesting also is the option of using the triode section of the ECH42 tube, which it seems can also be used as an audio amplifier. Nice to have a few options to play with.
http://www.r-type.org/pdfs/ech42.pdf

[ms660]

Yes, as in the Pye P53.

Lawrence.