B-52 at 112. 60 watt combo

[jd1961]

Hey...i would really appreciate if someone could help with this . I would like to run el34's in place of 6l6's in my b-52 at 112 60 watt combo. This is a class a amp. If i change the screen grid resistors to 1k ...is that all i have to do for this tube swap?

[The Dude]

This was recently discussed here.

http://music-electronics-forum.com/t44242/

[Jazz P Bass]

That amp is not Class A.

[jd1961]

Can you please explain?

[The Dude]

It's a typical push pull class AB amp. Here's the schematic.

http://music-electronics-forum.com/a...-9-06-2006.pdf

[jd1961]

Thank you much...if i may ask another question or 2 please..? If i swap out the grid resistors from 470 ohms to 1k ..can i use el34's in place of the 6l6's?

[The Dude]

On the basis of heater current alone, I wouldn't do it. We don't know the specs on the power transformer in the B52 amp, but it's heater winding must be capable of delivering an additional 1.2 amps to accommodate the EL34's. There's a chance you could burn up the transformer. There's also a chance that the transformer is over-spec'd enough to handle it. Not something I'd risk without knowing for sure.

[Enzo]

Also make sure the bias adjust has the range. EL34s generally have a lower bias voltage. The schematic says -45v at one end, but we don't know if it actually goes that low, and if that is low enough for the amp. We hadn't mentioned it, but the schematic shows pin 8 wired to pin 1 on teh sockets, which is necessary.

[jd1961]

Dude...man thank you so much..nice to be somewhat aquainted with somebody whos got their shit together on tube amps. I hate to keep asking what you would probly consider dumbass questions ...but...lol. Is there some way i can determine if this pt can handle the extra strain of el34's? I love the crunch they deliver! Im kinda determined to do this tube swap. I had sucsess doing this mod with a sovtek mig 50 long ago...but unfortunately it was stolen..( I guess somebody else liked it too) lol. I obtained this b52 for a little bit of nothing so a few bucks to make it sound better aint gonna bother me. However i am aware that pt's are pretty expensive and i dont really wanna go in blind just hoping it dont fail.

[Enzo]

Measure the heater voltage right at the tubes with the 6L6s in it. That is your baseline. Now stick EL34s in it instead. You don't even have to apply high voltage, measure the heater voltage now with the increased draw. How much has the heater voltage fallen? If it stayed close, you are probably OK. If it falls to some (arbitrarily) 5.9 or something, then it is straining.

[jd1961]

Enzo...thank you sir. Can you please explain to my dumbass how to measure the heater volts... Or is it possible to just operate the amp with the el's in it and feel the pt? I have been told ...i dont know how reliable this info is...that if i can hold my hand on the pt without it being like uncomfortably hot...then that is a good clue to weather or not it can take the extra stress...what do you think about that?

[Jazz P Bass]

You need a volt meter to measure the heater voltage.

[olddawg]

Enzo...thank you sir. Can you please explain to my dumbass how to measure the heater volts... Or is it possible to just operate the amp with the el's in it and feel the pt? I have been told ...i dont know how reliable this info is...that if i can hold my hand on the pt without it being like uncomfortably hot...then that is a good clue to weather or not it can take the extra stress...what do you think about that?

I would strongly suggest you read up on safety procedures and servicing tube amplifiers. There are voltages inside that can kill you. You also need to know how to use a multimeter, read schematics, etc. There are a lot of people here that are willing to help but no one wants to help you hurt yourself. Be careful and aware if it is beyond your skill set. No disrespect is intended. The danger is real.

[Enzo]

OK, the heaters are the same thing as the filaments, the glowing bits inside the tube. Pins 2 and 7 carry the heater current to the tubes in question, and you would set your meter to AC volts and see what ther is between 2 and 7.

BUT, the fact you have to ask that scares me. I am all about helping people learn, but as Dawg said, the voltages in these can KILL YOU. It ain't like working on a pedal or a little solid state. You need to have some basic shop skills and electrical awareness before traveling too far down the road into tube amp land. I want you to learn about this stuff, but I don;t want to read about you in the local obituaries. At my age I already know enough people in the obits.


Some PTs run very hot naturally, others less so, so that is not a very good test. And getting hot matters more in some transformers. SOme have good old fashioned enamel wires that will handle anything, others have this solder-through wire that will evaporate its insulation if it gets too hot.

[jd1961]

Im thanking you all for your advice...I have messed around with tube amps for years and know about the dangers and have been zapped a time or two years ago as you can probly tell...lol. I went ahead and changed the grid resistors to 5 watt 1k...installed the el34's ..
Turned the amp on and have been playing thru it for a couple hours with no problems so far...i havent did anything with the bias yet, the tubes look to be glowing normally and the pt is not even slightly warm...Im not gonna try and bias it myself because i dont have a multimeter and i wanna let somebody with a bit more experience in this area make sure its good to go. But as of rite now im way happy with the results and im thanking all you guys once again...rock on����