Philco Stereo Radio - Transformation to SE Guitar Amp

[Jay]

Thought I put this transformation in a dedicated thread.

I was visiting a friend who was loading up his trailer with stuff for the dump when I saw this discarded radio chassis. He said I could have it and he had the valves somewhere in a box. So I went home with a new project, not expecting too much. ( Boy, was I wrong).

Here a couple of photos of how it looked when I got it minus the big 5W resistor which I put in as the original resistor was snipped out of the circuit.

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[olegmcnoleg]

Stereo you say...

[Jay]

I found some info on the web about Philco radios and it looks this one is from the early sixties. Showing grid leak valve biasing for the pre-amps

The valve complement is as follows:
RF department - 2 x EF89 plus 1 x ECH81
Pre amp section - 2 x 6AV6 (single triode miniature valve), one for each channel
Power section - 2 x EL84, one for each channel. So a stereo radio setup Oleg
Rectifier - EZ81 I believe but markings gone, so just guessing but about 50V voltage drop over it.
All Mullard valves and they look like they are the original ones. Who knows but they certainly look old.

Power transformer provides 2 x 270VAC with center tap and 6.3VAC without tap it appears.

I powered it up gently with the EZ81, one EL84 and one 6AV6 and observed no smoke/smell and measured acceptable power supply and plate voltages.

Interesting observations:
- Ground loops galore
- Heater wiring not twisted, in fact one wire is just bare (grounded) and loops around all valve sockets close to the chassis; Good bit of hum though.
- The 6AV6 valves were grid leak biased with a 4.7M resistors. Apparently quite common for radios but not so great for guitar amps.
- With grid leak biasing the pre-amp valves cathodes go straight to ground

I drew up the circuit and will post that later as it will be interesting to see all the changes required to make it into a guitar amplifier.

Didn't get much audible sound out of this with the guitar wired into a single preamp. Also, having an 8 ohm temporary speaker hooked up didn't help. More on that later.

[Marshmallow]

Awesome

[RectifiedAmps]

6AV6 are awesome for guitar if you get them set up right! 7-pin tubes have extra special mojo…

[Jay]

6AV6 are awesome for guitar if you get them set up right! 7-pin tubes have extra special mojo…

There is certainly a vibe to this amp, two of them in 'series' is driving the EL84 super hard.

[Jay]

So here is the original circuit diagram for the Philco Radio amplifier section for one channel. Both channels are identical.

Note the grid leak biasing for the 6AV6, the peculiar powering of section B and C and the odd treble circuit. Lots of things have been changed since

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[RectifiedAmps]

That’s a wild treble control. Some kind of feedback arrangement?

Some tweed Fenders used grid leak biasing back in the 50’s to pretty good effect. The 5C1 Champ is a good example. Resistor drift and variance between tubes makes it inconsistent though, so cathode biasing became the norm.

[Jay]

Pseudo science here...

Total current in radio would have been something like this:

Screenshot 2024-09-06 140228.png (5.99 KiB) Viewed 1683 times

I know the rectifier is an EZ xx. The EZ80 is limited to 90mA and the EZ81 to 150mA.

Therefore, according to max calculated current, it must be an EZ81

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[Jay]

So I added one extra 6AV6 stage which means that two triodes are amplifying the guitar signal before it goes to the power stage. THERE IS TOO MUCH GAIN NOW.!! More on that later but the amp sounds rather interesting Those 6VA6 triodes are quite something.

I also increased headroom for each stage by about 30V via connecting the rectifier output straight to the OPT and then stepping down for each subsequent stage. Had too fix some nasty motorboating on the way.

Also worked out that the OPT was rated for a 2 Ohm speaker and with my 8 ohm speaker connected, the reflected load was over 20k ohm. (I had measured the turn ratio and then did the calcs). Not good so another OPT was required. Luckily the old BEACON Multimatch which I had lying around could be tapped for a perfect match. So the 8 ohm reflected load is now about 5000ohm.

So here is the current state of the rat's nest:

20240904_192949.jpg (4.49 MiB) Viewed 1297 times

[robthemac]

So I added one extra 6AV6 stage which means that two triodes are amplifying the guitar signal before it goes to the power stage. THERE IS TOO MUCH GAIN NOW.!! More on that later but the amp sounds rather interesting Those 6VA6 triodes are quite something.

I also increased headroom for each stage by about 30V via connecting the rectifier output straight to the OPT and then stepping down for each subsequent stage. Had too fix some nasty motorboating on the way.

Also worked out that the OPT was rated for a 2 Ohm speaker and with my 8 ohm speaker connected, the reflected load was over 20k ohm. (I had measured the turn ratio and then did the calcs). Not good so another OPT was required. Luckily the old BEACON Multimatch which I had lying around could be tapped for a perfect match. So the 8 ohm reflected load is now about 5000ohm.

So here is the current state of the rat's nest:


20240904_192949.jpg

This sounds great!

[StrummersOfThunder]

Ha this is wonderful Jean !
THose old suzuki caps are cool. Looking forward to hearing this beast

[Slowy]

So I added one extra 6AV6 stage which means that two triodes are amplifying the guitar signal before it goes to the power stage. THERE IS TOO MUCH GAIN NOW.!! More on that later but the amp sounds rather interesting Those 6VA6 triodes are quite something.

I also increased headroom for each stage by about 30V via connecting the rectifier output straight to the OPT and then stepping down for each subsequent stage. Had too fix some nasty motorboating on the way.

Also worked out that the OPT was rated for a 2 Ohm speaker and with my 8 ohm speaker connected, the reflected load was over 20k ohm. (I had measured the turn ratio and then did the calcs). Not good so another OPT was required. Luckily the old BEACON Multimatch which I had lying around could be tapped for a perfect match. So the 8 ohm reflected load is now about 5000ohm.

So here is the current state of the rat's nest:


20240904_192949.jpg

This sounds great!

Actually, it does. Treble is still a bit harsh but wound up provides a great fuzz tone.
It also brings every woman in the house bursting through the door and pleading for it to be turned down.

[jeremyb]

So here is the current state of the rat's nest:

That is utterly terrifying!

[MiniForklift]

That is utterly terrifying!

... but makes for compelling reading