Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AZJimC on March 04, 2012, 03:50:37 pm
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I picked up a dual EL84 amp, with the Crown logo (the word, not a crown) in chrome plastic on the front. It is a combo with a thin cabinet, and a 15" speaker. The tube lineup is as follows:
Pre: 12ax7 - first triode is preamp, second is slapping the reverb transformer
6av6 - grabs the return from reverb tank, and, best I can tell is also shaken by a second
6av6 - Trem oscillator
12ax7 - is the phase inverter, looks to be long tail, but I am not sure about that, hard to see what goes where for sure.
2XEL84 - Output. There is a 4K primary Output Trans tied to the output.
There is lots of room, and with the limited value of these 60's cheapos, I am interested in a small board, possibly using the same tubes, however I did think about replacing the 2 6av6's with 12ax7's, if the 6.3v will carry it...
Anyway, I am looking for suggestions about a way to proceed. I will have to be on a low budget, so much that I am thinking about building a hoffman type board in two sections, one output, then build a preamp section later, as funds permit.
I don't like the scrambled PTP they have going on, and primarily want to begin to clean up the wiring situation, while getting a bit more gain for a little grind. This amp is chimey and clean, and really sounds impressive, with the 15" speaker, but I want a little grind.
Have not been on the forum in a long time, like since 2010, but I am playing out a lot more these days, so I have to get my amps back in shape I guess. Cant even find my bias socket connectors I made long ago, so I will have to get that lined up to go over my super reverb, and bassman 50, both of which are a bit loud for the places I am playing, so I really want this more portable Crown to play well.
There is already another hole in the chassis that is 7 pin button size, and I could just add another one of those for some grind, while stopping the trem thump it now has.
Thanks everyone, and good to see you guys still kicking too!!
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One 12AX7 will replace two 6AV6s.
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Hi Steve, been a while.
I have been thinking that all along, and I see no real benefit to two 6av6's over a single 12ax7. As best I can tell they are the same triode all around. The more I think about it, the more I think I should just replace both of them with 12ax7's, I already have the sockets. They are going to use more 6.3v current, but as long as that doesn't overheat the trans it should be ok. I thought about a stout reverb, which is close to a marshall 18w with reverb. It amazes me the way they got the thing doing reverb, and trem with so few triodes, but I guess the sacrifice is gain stages. I hate it, but I best figure out this circuit, then I can figure out the cheapest way to expand on it. It is so hard for me to see every little connection in it, and I've already found one loose cap wire, and not positive where it came from, but I have a good idea. I think it is the cap transfering signa from the preamp triode to the reverb send triode. Reverb had quit, so I attached it to the grid, but have another bad connect on return side. Reverb worked for a while, but later just hums when increased.