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Hello,

I have recently finished a 18W Plexi MKII built. The Normal Channel works well and so does the power amp stage. Great Sound, the amp is punchy with beautiful variations in its overdrive sound.

Unfortunately the TMB channel has some problems and is very very quiet, i.e hardly audible. I can just about hear some amplification above the acoustic sound of the guitar.

Here are my observations:

The volume control works: when I turn the volume to higher levels, the amp starts to hiss audible.

The tone stack circuit also works, with full volume the hiss changes its characteristics when turning treble, middle or bass.
However, the gain control is not having any effect whatsoever. Just quiet.

I feel that the problem might be somewhere between V1 and V2. When carefully pocking around the powered-up amp the incoming signal at at both V1 and V2, pin 2 (TMB) exhibits hardly any crackly whereas pin 7 is clearly present.
I have swapped tubes V2 <-> V1 but to no avail.
I have also taken both HI and LOW inputs off the input resistors of V1, again with no positive result.

Here are the voltages. V2P3, V3P2 and V3P7 are too low compared to the voltage chart. Could this cause the problem?

V1
P1: 137
P2:
P3: 1.07
P4:
P5:
P6: 139
P7:
P8: 1.0
P9:

V2
P1: 171
P2:
P3: 0.77
P4:
P5:
P6: 271
P7:
P8: 176
P9:

V3
P1: 199
P2: 38
P3: 78
P4:
P5:
P6:197
P7: 38
P8: 77
P9:

V4
P1:
P2:
P3: 11
P4:
P5:
P6:
P7: 346
P8:
P9: 343

V5
P1:
P2:
P3: 11
P4:
P5:
P6:
P7: 351
P8:
P9: 342


Shall I take pictures of the inside?
Any suggestions and help is very much appreciated.

Best wishes

Lars

Looks like the B+ to the preamp tubes is OK. Don't worry about those low voltage readings, at least for now. They aren't low enough to cause your problem.

As you suspected, the problem is most likely in the V1 / V2 area of the TMB channel. Probably a bad connection. Recheck all your wiring and solder joints in the TMB channel between the input jacks and the tone stack. Start with the connection between V2-1 and V2-7.

You don't mention the voltage readings at the heater pins so I assume V2's filament lights up and the heater voltage is OK.

Check with an ohm meter to make sure everything that is supposed to be connected actually is connected. Check for accidental shorts to ground, too.

Most of the amp works so the problem should be a simple one.

Thanks Mitch,

I'll get on with the suggested tests. Will keep you posted.
Best wishes

Lars

Hello all,

here is an update. Some progress has been made, but the amp is still not working fully.

As before the normal channel and the power amp section are fine. Great sound.

I managed to get the TMB channel partially going, both the volume and gain control as well as the tone stack circuit works now. But it is still only working at approx. 20% of its loudness (as a guess). I.e. I can only get clear sound, but no distortion - even with gain and volume at full.
With gain and volume at full, the amp start to hiss audibly (as expected).

Here are the voltages. Some of them seem too low compared to the voltage chart. Could this cause the problem?

I rechecked the turret board and the connections. Found one mistake, corrected it but it didn’t make any difference in the above.
The boost switch does not have any effect at the moment.

AC mains voltage: 240V
B+ (all tubes installed): 341

V1
P1: 147
P2:
P3: 0.95
P4:
P5:
P6: 144
P7:
P8: 0.92
P9:

V2
P1: 171
P2:
P3: 0.82
P4:
P5:
P6: 278
P7:
P8: 175
P9:

V3
P1: 205
P2: 38
P3: 79
P4:
P5:
P6:199
P7: 38
P8: 79
P9:

V4
P1:
P2:
P3: 11
P4:
P5:
P6:
P7: 352
P8:
P9: 344

V5
P1:
P2:
P3: 11
P4:
P5:
P6:
P7: 352
P8:
P9: 344

V6
P1: 292
P2:
P3: 364
P4:
P5:
P6:
P7: 299
P8:
P9:

Shall I take pictures of the inside?
Any suggestions and help is very much appreciated.

Best wishes

Lars

pictures always help, someone will always help you here so don't give up

Hello all,

the problem has been solved: the shielded cable from the turret board to the gain pot was somehow wonky. I plugged a guitar in and carefully wobbled connections with pliers (wearing an insulated glove for protectection) and the TMB channel came into action. A replacement cable solved the problem.
Happy days and merry xmas to all.

Lars

problems with a new build can be a pain but boy when you find the problem and fix it, almost as good a feeling as a no issue build, great that you found your problem and can know enjoy that great sounding amp you built. Have fun and Happy Holidays