I'm interested in working on a pretty fun project, but want to know what's doable.
A good friend of mine who has inherited a lot of the projects I've made over the years wants to start playing out again, and asked me if I could build a stereo amp with two 12" speakers. First thing was, I told him it'd be really heavy. He was fine with it even if it weighed 100 pounds. So I've been thinking about it.
A year or two ago I restored a Gibson GA-79RVT, which is a stereo amp with trem and reverb. It uses two pairs of cathode biased EL84s for the two outputs, and has options for summing the two channels to mono, running in stereo, etc. However each channel is the same, and while I might borrow from it here and there I want to make some pretty substantial changes and am wondering if they are do-able.
He wants a dirty old school amp on one channel and a clean, AB763 style amp on the other. There will be no effects like trem or reverb.
Ideally, what I would like to do is build something with octal preamp and PI tubes, like an early Fender Deluxe or Gibson GA15, in the first channel. These are cathode-biased circuits with no choke, and a 5Y3 or such rectifier. However, the AB763 I would use in channel 2 uses a choke and I will probably use diodes to rectify. The choke and SS rectifier will be problematic for the old school channel, plus the voltages in this channel should ideally be a good bit lower than they should be for the AB763. So what to do about it?
As far as I can see it, I have five options:
1. Ditch the idea entirely.
2. Suck it up and build an old school circuit with higher voltages than I'd want, with a solid state rectifier and a choke. These will all change the character tremendously and I worry it will kill the grungy old school tone.
3. Use one transformer, and rectify the HV with diodes. Following the rectifier, I would split the HV into two branches .... one off to the old school side of the circuit, the other to a choke and then on to the rest of the AB763 circuit. If this is even doable, it would make the old school side voltages a good bit higher than I'd want, I think.
4. Another idea: Instead of using one giant transformer, with HV probably rated for 260ma or so, install TWO power transformers, one maybe 120ma and the other maybe 140 or so. In either case, I would be putting the power transformer(s) in a second chassis at the bottom of the cabinet, center, with a wiring harness running into the main chassis where the output transformers and the rest of the circuit lives.
However, if I do this, would there be any way to bridge the two channels together, from what will actually be two separate and distinct amps? It's important in this build, for him, to be able to use one channel, or the other, or both. A bonus would be if I could build in an option to run them in mono mode, mixing the signals before they hit the speakers.
5. The only other option I could see is having Heyboer or Musical Power Supplies build a custom transformer, with two separate high voltage windings and enough 6V current capacity for four 6V6s, the rest of the tubes and a 2a 5V winding for the old school rectified channel. This seems like it would be quite expensive.
Sorry, a bit long-winded I know, but I'm in the early thinking stages and just would appreciate some feedback on what's doable here. I can build the amps themselves no problem, but joining them together is what I need to do more thinking on. Thanks!
Topic: Is this idea crazy? What would you do? (Read 2275 times)