Unknown amp conversion

[Rockn]

I would like to convert an amp that I just purchased and wanted to get some input and advice. The amp in it's present state has a tube compliment of a 5Y3GT rectifier, 2- 6b5Qs in PP, 12AX7 preamp and a 12AX7 for the reverb section. I measured the power section and got B+ voltages of 324V, 278V and 260V. I was thinking of a Matchless or 18w lite I saw mentioned here. There is also a design I wanted to try shown here:
http://www.retrovox.com.au/rh1254ga.pdf (can anyone suggest any mods to this circuit) or a VOX AC15 style amp less the reverb section.
using an EF86 in the preamp section and using the 12AX7 as the PI. I have an old Genalex EF86 that I want to try in it after I get it working properly with a less expensive EF86.

Question: I fired this thing up in it's stock condition and with the volume knob up to 10 it is really quiet. The input for the guitar is going through the input for the reverb which is probably why it is so quiet. It is a cool little amp with a lot of possibilities. I can post a pic if anyone would like.

[Chuck H]

It's probably "quiet" because what you have is likely a power amp and reverb only. So it's kind of like plugging your guitar into an effects return on an amp. Not much happening. It sounds like what you have came from a larger "console" or maybe an organ. If so, the PT should be able to handle adding a preamp tube. IME most old amps have enough spec on the filament winding to handle it. So you may need to add a tube, which it sounds like you wanted to do anyway with your EF86.

324 plate volts sounds good for EL84's (6bq5's). I don't know how your power rail is layed out, but if you have 278 volts on the screens that's not going to be very satisfying for punch and volume. If this is the screen voltage, it's probably just a couple of resistors to bring that up closer to 300.

Chances are whatever you build will have a certain layout. This can effect stability. Lots of "conversions" suffer instability because the original amp is fairly low gain compared to most guitar amps so they are layed out more for convenience than oscillation control. The new builder just tacks on or hacks in the circuit they want and, of course, now the amp is unstable. Whatever you decide to build, plan on following the layout for that design as well as the schematic. You may need to buy a new chassis to do this, but it may save you the trouble of putting many hours into a project only to end up with an amp you have to chase multiple problems on. It happens A LOT this way.

JM2C

Chuck

[Rockn]

amp build

There is quite a bit of room in this chassis to isolate stuff so getting the preamp away from the PT is not a problem. I think this was a reverb only amp from an organ so the gain is probably where it should be. The power section has a multi section cap at 40-40-40 with a 3.3K 2W resistor across the first stage, a 10.7K 5W resistor between the second stage and a 62K 7W resistor from the middle cap to ground. I am going to try and draw this thing up.