Egnater Vengeance - No sound (solved)

[glebert]

Posting this because there is very little information about fixing this amp on the internet. I got this non-working amp really cheap, even though I usually avoid Egnater amps because of the power transformer issues they had on some of the older models. Amp powered up but had no sound other than very low white noise and channel switching was not working. Pulling the tube covers I saw that the preamp tubes were not glowing (but PI tube was). I suspected that the preamp tube heaters were on a 12VDC supply and that had failed, which could also explain the channel switching not working. There are no schematics available for this amp, but I looked at other Egnater schematics and found the Vengeance seems to use a similar scheme as the Renegade 65 for gettng the 12VDC. There is a MJE3055T transistor that is mounted to the chassis that is the workhorse bit of the regulator circuit, and that was not passing current. Replacing that transistor got the amp back to working. That transistor gets quite hot so I made sure to clean off the old thermal paste and apply new to all mounting surfaces of the mica insulator and the small aluminum mounting block. I did note that there seemed to be an extra mica insulator on the failed transistor, I wondered if that was there by accident (easy to miss since they are clear and thin) and caused the failure since there probably wasn't thermal paste between the two insulators.

Pretty easy fix since the transistor is mounted on the chassis and wires to it have a molex-ish connector to the board.

Anyway, now I have this semi ridiculous 120W high gain tube head which I don't really have a use for but is pretty fun to rock out on.

Renegade schematic shared for reference on the 12VDC supply. Note they call it a 2N3055 transistor on the schematic (and the board) but the part really is the TO-220 package MJE3055T.

[nosaj]

Logical thinking saves the day no schematic needed.
+1
nosaj

[J M Fahey]

... I suspected that the preamp tube heaters were on a 12VDC supply and that had failed, which could also explain the channel switching not working. ..... for gettng the 12VDC. There is a MJE3055T transistor that is mounted to the chassis that is the workhorse bit of the regulator circuit, and that was not passing current. Replacing that transistor got the amp back to working. That transistor gets quite hot so I made sure to clean off the old thermal paste and apply new to all mounting surfaces of the mica insulator and the small aluminum mounting block...
Pretty easy fix since the transistor is mounted on the chassis and wires to it have a molex-ish connector to the board.


Renegade schematic shared for reference on the 12VDC supply. Note they call it a 2N3055 transistor on the schematic (and the board) but the part really is the TO-220 package MJE3055T.

Congratulations.

It is poor practice to use an *iron* chassis as heatsink: poor thermal conductivity and thin to boot.

I suggest to add a heat spreader between transistor and chassis, nothing more complex than a 2" by 2" square (larger if space is available) of aluminum sheet metal.
1.5mm/1/16" or thicker.
With grease and mica on the transistor side, and just grease on the chassis side.

You might also use a TO3P/TO218/TO247 transistor instead of marginal TO220

[Jon Snell]

There is little voltage drop across the transistor and only a maximum of 450mA. Ohms Law dictates a low dissipation therefore the steel chassis is adequate as a heat sink. That transistor normally goes short circuit, not open circuit. Odd.
Don't believe this schematic; Does V1 realy get its heater voltage through the switching relay? ... No it doesn't.

Good you got it going.

[glebert]

Congratulations.

It is poor practice to use an *iron* chassis as heatsink: poor thermal conductivity and thin to boot.

I suggest to add a heat spreader between transistor and chassis, nothing more complex than a 2" by 2" square (larger if space is available) of aluminum sheet metal.
1.5mm/1/16" or thicker.
With grease and mica on the transistor side, and just grease on the chassis side.

You might also use a TO3P/TO218/TO247 transistor instead of marginal TO220

There is an aluminum heat spreader, I just didn't mention it in detail. I had installed it in manner you suggested.

Jon, I would assume you are right, but did not trace out the V1 heater current path.