make a 6505 switchable from el34 and 6l6

[vscrl1]

ok so I did the bias mod, and the grill mod. the next thing I wanted to try doing was mod the amp to where I can use el34 or 6l6 tubes. I know a lot of people hate el34's in these amps, but I love el34's in general. I know the screen grid resistors would need to be changed and one of the pins needs to be grounded, just don't remember which one. also, all I need to do is convert the amp to take el34's and I can still use 6l6's by just rebiasing the amp, correct?

[KTB]

This mod is a little more tricky than the last because of the way the 6505 power board is layed out. If I'm not mistaken it involves cutting a trace and patching over one socket to the others. All the sockets are the same except for the last one which is where the problem is. Basically you need to tie pin 1 and 8 together on all sockets and ground them. The amp will get very loose on you with EL-34's to almost ice-pickey so just a warning.

[vscrl1]

oh crap I thought just v8 needed pin 1 and 8 tied together.

[vscrl1]

and then obviously the screen grid resistor upgrades

[Enzo]

Well you will also have to modify the amp for adjustable bias. The 6L6s want around -55v, while the EL34s want more like 42v. Yes, once you have rewired for the other type tubes, then all you have to do is rebias when you change types, but you need to be able to do that.

The other concern is that the EL34s draw considerably more heater current. Fortunately the 5150/6505 power transformer is up to it, but the ribbon cable that connects the heater power to the tube socket board may be marginal for it.

[vscrl1]

ive already performed the bias mod. so i just need to know what else needs to be changed out. do i just need to jumper pins 1 and 8 on v8? it looks to me like the other 3 tube sockets already have 1 and 8 connected?

[Enzo]

You most certainly have to strap pins 1 and 8 together on any power tube socket that isn't already that way.

[vscrl1]

im having trouble hunting down a diagram or schematic for the 6505 or 5150. are they the same or did they change them over the years? ive read that some traces on pin one also go to pin 3 and that the traces need to be cut? can anyone verify this? or can someone post a diagram with the necessary changes that need to be made? I need to also see if theres a way to pull the SGR's without pulling the board.

[Riffraff12571]

You can find a schematic of the stock amp here: http://blueguitar.org/schems.htm#Peavey

[Enzo]

5150 and 6505 are identical, and did not change. They did redo the tube socket board in 2001, the parts are positioned a little different. The pin 3 thing is a different model, this needs just the loose pin 1 connected.

If you look at the tube socket board, the solder side is facing you, while the screen resistors are "underneath". When I replace them or change them, I unsolder each, then poke its wires out of the hole so it drops off the board and down into the chassis, where I fetch it out. Now I install the new resistor on top, the solder side. The circuit doesn't care which side of the board the part is on. No board removal.


You never have to "track down" Peavey schematics, because customer service will send them out to anyone upon request.

[Enzo]

Here, this is the schematic for the 5150-2, but it does include a layout for the power tube board.

http://bmamps.com/Schematics/Peavey/Peavey_5150.pdf

[KTB]

Sorry I can't remember that mod but it was a long time ago however, I absolutely remember cutting a trace because of something and that sounds like it. If pin 1 on a socket is going to pin 3 for some crazy reason you need to isolate it or cut it or you are going to have fireworks much greater than those LED's.

Once you get all the connections made be sure to ohm out the board and make sure that pins 1 & 8 or tied to ground and nothing else on any of the other pins are. You'll need 1k 5 watt screen resistors also in place of the 100 ohm 5 watters.

[vscrl1]

so if the 5150 and 5150-ii share the same power section, then pin 1 on v8 is the only one that needs to be tied to pin 8. so can I just jumper them, or can I just run a bead of solder across from pin 1 to pin 8? and everyone keeps saying the tone will be muddy, but I ran el34's in a 5150 block head that I had a bias mod done on, and it didn't sound muddy at all. actually the exact opposite. what it did do was thin out the 5150's tone quite a bit. a lot of the lowend was gone. sounded almost like a stock jsx with el34's meets 5150 lol hard to explain. and thanks everyone for the help. you guys on the peavey forum are way way more helpful than the guys on the marshall forums. thanks again everyone.

[KTB]

After looking at the schemo I think you run either solder or a piece of wire (better) to pin 1 and 8 and be done. Pin 3 is not going to pin 1 but there is a protection diode going from pin 3 to ground that is connected to 1 & 8 so as long as that stays ok then no problem.

I don't think the amp gets muddy it just gets really loose and ice pickey but retains it's tone pretty well. Part of that is because EL-34's are true Pentodes and draw screen current so the tube is more active than a 6L6 and it's internal plate impedance is higher and it's damping factor is lower so it yields more distortion even at lower volumes. That's why 6L6's were used in this amp in the 1st place especially with the big amplitude signal hitting the grids that the 5150 preamp throws at it.

[vscrl1]

so v8 pin 1 to pin 8. pin 3 is the pin plate voltage pin, so im confused as to how that being shorted to ground would be ok...