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Ampeg tube replacements?

12/6/1998 2:52 PM
Bob K.
Ampeg tube replacements?
My son has an Ampeg Gemini I that gives him the sound he after. The druver and power tubes on this amp are a 7199 and a pair of 7591's respectively. These tubes are not easy to find are are expensive when you do find them. The question is: Can I use the original output transformer if I rewire the sockets to accept a 12AT7 as the driver and 6L6's or 6V6's as the power tubes (the amp is cathode biased and is running a B+ of around 350v)?  
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12/7/1998 6:15 AM

Bob K; If you click on the Triode Ele. page scroll down just a little bit,you'll see the swap, and how to do it for the 7591 to 6L6,you could also use the 6V6 since the plate voltage is 350,you may have to do some figureing for the cathode resistor for the 6V6,but they tell you how to do it. As for the 7199,or 7591, Sovtek sells a replacement for both,and you can get the NOS tubes from Angela inst. and many others,there is also a pinout for this tube on the triode page,and if you do a search of the archive I think GW listed this too,and some advice with keeping the 7199,but you can rewire the socket for a 12ax7, But if he likes the sound of the amp right now, I would not mess with it untill it starts sounding crappy! It may be quite a while untill this happens,so you'll have plenty of time to research the swap for the tubes if you wanted to go ahead with it. [Richie]
12/7/1998 8:51 AM
Bob K.
Thanks Richie!
Thnks for the info. I'll check out the sources you mentioned.  
12/8/1998 3:14 PM
Re: Ampeg tube replacements?
I have done this conversion on my Gemeni I. The output tubes were changed to 6L6s (happen to be an old pair of Tung Sol 5881s), by rewiring the sockets and changing the cathode bias resistor. The driver / phase inverter tube was initially changed to a 12AT7 long tail pair configuration (like typical Fender amps), but subsequently replaced with an octal socket and a 6SL7 in a conventional PI configuration like some early Ampegs & Gibson amps.  
You can rewire the PI using a 12AX7, with a triode gain stage ahead of a split-load inverter, like a BF/SF Princeton. Just another way to go.  
If you opt to buy the sovtek rebased 5881/6L6 substitutes for the 7591, you can leave the output socket wiring as is, but you still must up the value of the cathode resistor from the stock 140 ohms to at least 250 ohms or you'll cook the new tubes and most likely the output transformer, too.  
Although 6CG7 tubes are not hard to get at this time, you may wish to rewire heater connection pins 4,5, & 9 of the reverb driver tube socket to use a 12AU7. That's what I did. The 6CG7 is a serious duty tube (actually a 9-pin miniature version of a 6SN7), but overkill for this amp's reverb circuit.  
12/8/1998 7:35 PM

Doc; I worked on one of these a few months back rewired the socket and put in the 6L6 tubes.I also used a 250 ohm cathode resistor with a new cap.It sounded great.I wanted to fix the tremolo,but finding the part took a while,and the guy said he didn't use it anyway.What is the difference of the tremolo in a fender compared to the Gemeni I. Why can't you use the fender type photo cell tremolo? [Richie]
12/9/1998 10:24 AM
John Martin

I used a 12BH7 for the reverb driver in one of my Ampegs with good results.  

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