Fender Twin 65 Reissue Preamp

[nickb]

I used another transformer and connected as Enzo suggested in #14. I could still hear the problem. So we should assume the Output Transformer is ok.

Here is how I am testing.....

Problem is occurring on both channels
Reverb tank disconnected
All controls on the Vibrato Channel are at "1"
All controls on the Normal Channel are at "5"
Testing is being done on the Normal Channel

I have 12mvac at grid of V1A.
I have 512mvac at the plate of V1A (a gain of ~43)
I have 26mvac at the grid of V1B
I have 662mvac at the plate of V1B

At the junction of C18 (.047uf) and R35 (220K), I am seeing 320mvac.
Even when turning up the volume control, there is no way I can get to 5.8V pk-pk at TP16.

Corrected: My reading of the test point set up conditions is that the Vibrato channel is used to get 5.8Vpp at TP16. The Normal is only used for the power output test.

[TomCarlos]

Thank you Nick....

I am able to get a signal through both channels. The amp does close to 90 watts rms before clipping. Using either channel, at a moderate volume level, the bottom end sounds "muddy" ... as if something is breaking up. I haven't used this amp much but I was thinking I'd get a nice clean sound. I will check the speakers again, cabinet rattle, etc.

[TomCarlos]

I've tested the amp during the week. I'm getting a full signal at the output, but did notice some oscillation at the top and lower ends of the signal. I realize this is not hi-fi. But I am wondering if this is normal. I'll begin "looking backwards" to see where this might be originating.

[TimmyP1955]

Make sure that the jack and pot nuts are OK, especially the outputs: Sometimes the little gray cardboard that they put around the jacks is between the jacks and the chassis, which can compromise the connection.

[cjenrick]

you going into a dummy load? try it thru some speaks and see what you get,

[TomCarlos]

CJ.... Going through speakers and watching on the scope shows a cleaner waveform. I'm getting close to 22vrms before clipping (@ 4 ohms). I'll take that! Thanks for the tip!!

My friend brought over a couple guitars, one with Humbuckers and the other was a Danelectro with single coils. The amp sounded great with both guitars. I think I can reassemble this amp and put it back in service.

Thanks everyone for your help on this fix!

Tom