Philco Stereo Radio - Transformation to SE Guitar Amp

[RectifiedAmps]

Yep, sounds like the coupling cap is leaking DC through from V2. Is the bass control scratchy? On a cathode biased amp the grid should be at 0V unless Rg1 is too large, but in your case it seems fine. Maybe measure resistance from g1 to ground, just to make sure the ground connection is intact? If the grid was floating it'd also create weird issues.

[sizzlingbadger]

Yep, sounds like the coupling cap is leaking DC through from V2. Is the bass control scratchy? On a cathode biased amp the grid should be at 0V unless Rg1 is too large, but in your case it seems fine. Maybe measure resistance from g1 to ground, just to make sure the ground connection is intact? If the grid was floating it'd also create weird issues.

and that is also why a pot in the grid leak position is not ideal. If you pull one end of the 50nF does pin 2 go to 0v dc ?

[Jay]

Yep, sounds like the coupling cap is leaking DC through from V2. Is the bass control scratchy? On a cathode biased amp the grid should be at 0V unless Rg1 is too large, but in your case it seems fine. Maybe measure resistance from g1 to ground, just to make sure the ground connection is intact? If the grid was floating it'd also create weird issues.

Thanks, with the amp off, I measure 330kohm on pin 2 to ground. So not floating.

[RectifiedAmps]

If you pull one end of the 50nF does pin 2 go to 0v dc ?

You could also just pull the EL84 and measure DCV on pin 2, with V2 installed. Any voltage and you know the 50nf is leaky.

[Jay]

Yep, sounds like the coupling cap is leaking DC through from V2. Is the bass control scratchy? On a cathode biased amp the grid should be at 0V unless Rg1 is too large, but in your case it seems fine. Maybe measure resistance from g1 to ground, just to make sure the ground connection is intact? If the grid was floating it'd also create weird issues.

and that is also why a pot in the grid leak position is not ideal. If you pull one end of the 50nF does pin 2 go to 0v dc ?

I have come to that conclusion as well and will stick with the Fender tone stack I put in between V1 and V2.

The leaky cap has been replaced and all is well A nice 0V reading on pin 2 and a steady bias voltage now.

[Jay]

Revision 3 of what was once a Philco amp section. Once the EL84 bias was sorted, I could play around with the pre amp section a bit more. The Fender tonestack works well between V1 and V2. Playing around with the 6AV6 bias is fun. The V2 valve will have a switchable bias setting, one for cold bias and one for warm. Flicking it to warm adds a subtle texture to the tone, a bit more grit. Very pleasant to hear.

Next I'll play around with the EL84 bias which is currently dissipating about 9W in its plate. Sounds great but I like to try a bias for plate dissipation of about 11W, slightly warmer.

The only major problem left is that the amp squeals at high volume. That might be because it is such a rat's nest though.

Next steps are:
- ordering parts,
- gut most of the resistor and capacitor components
- redo heater wiring and elevate heater voltage
- implement star based grounding
- rebuild it
- make cabinet


Leaky Suzuki caps:

20240923_091302.jpg (685.2 KiB) Viewed 1507 times

Circuit diagram to date:

20240923_113729.jpg (2.7 MiB) Viewed 1507 times

[RectifiedAmps]

I think having a master volume at R13 could still be really useful for tone shaping. You could use a 1M linear pot then put a 220k from wiper to ground, to simulate a log taper, plus it’d act as a failsafe ground reference in case the wiper ever lost contact. Just a suggestion though - at some stage you have to stop tinkering and start enjoying the thing!

[Jay]

Slowly getting there, tinkering is fun... trying to find that perfect tone

The RF radio section has now been gutted and new heater wiring in place with elevated voltage. Replaced all the old pots with 500KOhm audio pots and added some tinkering switches.

Tidied up the P2P wiring and replaced dodgy parts with new ones. The squealing pig has also disappeared as a result of the tidy up.

Haven't got to implement a star grounding system yet and might not bother. There is a slight hum but nothing to worry about when playing the amp.

I tried the master volume as suggested by RectifiedAmps but wasn't convinced it was worth leaving it in.

What I would like to try though is a NFB option. Anyone with a good suggestion for this single ended EL84 amp? I have seen one for a 6V6 feeding back to its bias capacitor earth connection side of the cap (after disconnecting the cap's earth of course).

Next is power wiring cleanup and adding at least one fuse.

Inside gutted & redone 20240930_163205.jpg (4.11 MiB) Viewed 738 times

Back with speaker jack 20240930_163652.jpg (4.42 MiB) Viewed 738 times

Front with new pots & switches 20240930_163559.jpg (3.02 MiB) Viewed 738 times

[Jay]

Revision 4 built and I must say, this amp pleases me very much

I have had a few EL84 single ended amps and played some borrowed ones as well and this one is good. Beautiful cleans with decent volume before it starts to break up. At full bore the EL84 adds a pleasing growl to it. The 6AV6 valves are adding a certain character me thinks, but then I am biased.

Need to test it a bit more with the P10R when the Mrs is out tomorrow. But first need to get some more 6mm bolts to tighten the baffle and speaker up.

Here the latest, and final, circuit diagram with the two switches for mid scoop and gain.

20241013_201629.jpg (3.96 MiB) Viewed 260 times

[StrummersOfThunder]

Very cool Jean !
Looking forward to hearing it