PV Classic 50 ate my JJs — EL84s need -18v to sound good ??

[g1]

New info: Remove V1 and/or V2 and the runaway problem with the power tubes is gone! I can have either V1 or V2 in and I don't get runaway. V3 is in during all this.

As to grounding pin 7 on v3, early on I noted that even with V3 removed there was red plating.

I don't see how these statements can both be true, that's why I asked you to ground pin7 of V3.

What you seem to be saying is idle current is fine when V1 or V2 is removed, but runs away even if V3 is removed? Can you recheck?

[Steve A.]

Check the power supply providing voltage for bias. I think it also provides voltage for OD relay coil but I don't have schematic handy...

What circuit was used for adjustable bias?

Random thoughts...

Steve A.

[Krwkka]

I don't see how these statements can both be true, that's why I asked you to ground pin7 of V3....

It was because removing V1 & V2 boosted the -V to the EL84 just enough to tame them down.

Not being educated, I was hesitant to change the voltage divider for the -V to allow more voltage because I didn't know if it would cause damage or something.

I just now finished making the change, boosting the -v and the amp is running great with new tubes. I didn't remeasure the negative voltage, but according to the math I should be around -19.7v. I guess that's OK?

Steve: Basically I did what you suggested on your mod page for this amp except I ran wires up to a pot that I mounted on the circuit board. I just had to use a lot more resistance than I expected. Left the 18K alone.

Thank you all for your help! I know I've been a bit of a pain with this one because of my ignorance.

[Krwkka]

Aaah. A good night's sleep and I took up the challenge again.

This voltage problem on the low voltage circuit is still a problem. The music-electronics website didn't come back up until I'm ready for bed, so I'll be back tomorrow with details.

... and I still need to know how much negative voltage is normal to bias EL84s in this scenario (400 volts on the plates). I'm considering building a separate supply, but that won't fix whatever is broken in this thing.

[Steve A.]

Steve: Basically I did what you suggested on your mod page for this amp except I ran wires up to a pot that I mounted on the circuit board. I just had to use a lot more resistance than I expected. Left the 18K alone.

I would hope that after 17 or 18 years someone has posted a better adjustable bias circuit than what I came up with back then experimenting with those Peavey amps... (hint, hint)
In any case you might want to monitor the current in the low voltage supply.

Steve A.

[Krwkka]

I would hope that after 17 or 18 years someone has posted a better adjustable bias circuit than what I came up with...
Steve A.

It's hard to improve on something simple that works!

Current? good idea. To measure I pulled the fuses and clipped leads onto each of the fuse holders, ran that through a couple of 1N5408 diodes, then through my meter and back to one of the on-board diodes on the amp.

Current | V | Bias V
175mA - 25v -17.9
192mA - 24v -16.8 -- with Overdrive on


Now with V1 & V2 removed

Current | V | Bias V
24mA - 26.8v -20
45mA - 25.6v -18.7 -- with Overdrive on

Is the transformer not supplying high enough voltage? I get 21.9 VAC per side unloaded, 21.5 VAC when connected to the circuit


~~

[Krwkka]

FIXED. The problem appears to have been an intermittent ground for the 16 ohm speaker jack OR an intermittent problem with the output transformer. In the end it became obvious that the transformer was bad.

New OT and a ground wire between the two jacks (instead of depending on the grounding of the jacks to the chassis) and she purrs!

In case anybody is interested, I used an output transformer that was originally for 2 6l6es. It is perfect for 4 EL84s.

[Steve A.]

FIXED. The problem appears to have been an intermittent ground for the 16 ohm speaker jack OR an intermittent problem with the output transformer. In the end it became obvious that the transformer was bad. New OT and a ground wire between the two jacks (instead of depending on the grounding of the jacks to the chassis) and she purrs!

In case anybody is interested, I used an output transformer that was originally for two 6L6's. It is perfect for four EL84s.

Thanks for the update!

For the benefit of future visitors can you give us details on OT you used? Brand, model # and wattage (or donor amp.) Any tricks to mouting it properly?

Steve Ahola

[Krwkka]

Hmmm. Well, the transformer was for a Reverend Hellhound or Kingsnake, so I don't guess that's much help. Any OT that's for 2 6L6 and has both 8 ohm and 16 ohm taps will do. I chose the Reverend from my pile of transformers because it's nice and small and oriented the same as the original.

To remove the old OT, two of the four screws that held it were accessible without removing the circuit board. The other two I just drilled into from the outside until they fell out. Shook 'em out of the amp before closing up, of course.

I then used the self-tapping screws that Doug Hoffman sells. Carefully predrill pilot holes and screw 'em into the chassis. Works good!

Go here: Hoffman Amps

Click on the menu item that says: Screws/Nuts/Washers

That's it.

[Steve A.]

To remove the old OT, two of the four screws that held it were accessible without removing the circuit board. The other two I just drilled into from the outside until they fell out. Shook 'em out of the amp before closing up, of course.

I then used the self-tapping screws that Doug Hoffman sells. Carefully predrill pilot holes and screw 'em into the chassis. Works good!

Thanks for the details. I checked out the pix of the self-tapping screws from Hoffman and they look fine! (The self-tapping sheet metal screws at hardware stores with a drill-like point can produce tiny metal shards so I would avoid using them blind in an electronic enclosure.)

I'll add the screws you mentioned to my next order from Hoffman. Thanks for the tip!

Steve Ahola