Tube Amps / Music Electronics
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|12/29/2005 5:54 PM|
||Ampeg Jet Trem Problem|
So Iíve been messing with this Ampeg Jet. The schematic inside the back says it is a ďTĒ tho the thing is just like a ďDĒ here
except for a tube rectifier instead of the solid state. It had the 7591 power tubes but I rewired the sockets and bias for 6V6ís. Since the 6BK11 tubes are in short supply, expensive, and one was bad I moved a ground on the 12 pin sockets to take 6C10 or 6AC10 tubes. The 6AC10 doesnít have the moxie to drive the trem so I have a 6C10 in there now and the 6AC10 as the preamp. I canít tell the difference between the two in the preamp slot anyway.
The problem is that the trem just really thumps. Fortunately it has an on/off switch on the pot so it is easy to take out of the circuit. I found the diode mod for killing the trem thump here
but the Ampeg trem is pretty different to my eye, I am at a loss as to just how the other two triode sections of the trem function, and I donít see where the diode would go so it would function like it would on a Fender trem. I replaced the caps in the trem loop with .047uFís, stock calls for .05uF, so Iím pretty sure that the caps are good. The resistors in the whole amp are some pretty zooty looking enameled kind that are just too pretty looking to figure they are bad and so I have not bothered with them hahaha.
Anyway, I have no idea how to stuff the diode in the Ampeg circuit ot what else I might look for to squash the thump but I have to bet that it didnít sound that way when the amp was new 40 years ago or so. Otherwise the thing sounds just great with the stock CTS alnico still in there. If anyone has any suggestions about what to try, or how to get that diode in there, I would be crazy to hear about it.
Many thanks for any assistance on this
|12/30/2005 12:04 PM|
That's a different trem all right. The oscillator is pretty normal, but it drives the grids of the back to back triodes, which form a switch. Pins 3 & 5 of V1 lead to a .01uF-47K and a 470K-.1uF. Check these caps for leakage - particularly the .1uF. This could cause your thump. My guess is that this circuit may never have worked all that well. I see no high-pass filter to remove any residual thump. Pulsing the input stage will amplify any such resulting thump. There's also no intensity control so this circuit is running full-on trem. I don't think the "diode trick" will work in this circuit. You might consider converting it to drive the grids of the power tubes as a last resort, (then you could use the diode) maybe even adding an intensity control, an internal trim pot at least to allow some adjustment of intensity.
|12/30/2005 5:29 PM|
Actually, that trem should be very smooth. I don't completely under stand why they added that back to back triode buffer as the Jet trem was fine as it was.
There are two fixes that come to mind, and I've done both of them as an amp tech and an owner of all the Jet models A, B, D, Echo Jet, and even the reissue.
One fix is to eliminate the back to back triode section and wire it like the previous models. Another approach would be to punch a hole for a third tube and use some 6SL7 or 12AX7 tubes (with socket hole adapters). Or a combination of those tubes. Another thing I do, unless the trem is real strong, is just to shotgun the LFO. Those caps always leak eventually shutting down the trem. The resistors usually drift also. My personal preference is the 6SL7's.
Of course you CAN find those 6BK11 tubes if you search long enough.
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