Mesa Subway Rocket Low Volume

[Pryde]

Hello gang.

I have a mesa subway rocket (non reverb) that has very low volume. Its a ~20 amp that sounds like a 1 watt amp. Everything functions as it should, just very low volume, both channels, fully cranked.

Schematic attached

I have swapped all preamp and power tubes with no change in volume.
I plugged in a boost pedal into the effects return and the volume was decently louder with the preamp bypassed so I suspected preamp issue but still not sure.
Signal traced through the preamp and power tubes and can't detect any noticeable volume drop anywhere with my tiny speaker and probe.
Plate voltage is 370 and bias is 40 m/a so its running pretty hot but I believe mesa designed it that way and that shouldn't make the volume low either.

I appreciate any advice on this. Thanks

[oc disorder]

Hi Pryde Have you checked the speaker ? I notice on page 6 of the schematic it has a recording out/headphone socket.
See what level is there. Well it might confirm or otherwise that the pre is working.
Just before that is V3 and preceding that.. is the FX loop in and out (page 4).
Have you another amp that you can plug the send into ? Have you tried plugging into the return to see if the power amp is healthy ?
An MP3 player with a volume control could be used with care here to drive the power amp.
I suppose I would check all the anode resistors for starters and make sure all the pre-tubes are getting their supply !

[Jazz P Bass]

I would send a 100mv signal through the amp with a suitable dummy load attached.

Max the controls.

Measure the AC voltage that the output grid pins are receiving.

Measure the AC voltage at the + - of the dummy load.

Please report results.

[Pryde]

I would send a 100mv signal through the amp with a suitable dummy load attached.

Max the controls.

Measure the AC voltage that the output grid pins are receiving.

Measure the AC voltage at the + - of the dummy load.

Please report results.

The speaker is fine. It has low volume on another cab as well.
OK I input a 2000 hz signal, put amp on dummy load and am getting 0.3vAC on pin 2 of both output tubes. The dummy load +/- gives me 1.3vAC so about 1 volt more.

Not sure if I did this correctly so please advise. Thank you.

[Pryde]

Hi Pryde Have you checked the speaker ? I notice on page 6 of the schematic it has a recording out/headphone socket.
See what level is there. Well it might confirm or otherwise that the pre is working.
Just before that is V3 and preceding that.. is the FX loop in and out (page 4).
Have you another amp that you can plug the send into ? Have you tried plugging into the return to see if the power amp is healthy ?
An MP3 player with a volume control could be used with care here to drive the power amp.
I suppose I would check all the anode resistors for starters and make sure all the pre-tubes are getting their supply !

Hello,
The speaker is fine. The headphone out has a pretty low volume as well. I can crank it full blast in headphones and volume is rather low. Effects send into another amp (solid state) also produces a low volume.
Running the same 2000hz signal through the preamp (input) is lower volume than going straight into the effects return so the problem must be in the pre somewhere.

Oddly when tracing a signal through the pre, everything checks out/gets amplified all the way to the output (master volumes after v3). Its as if the pre just doesn't have the juice to push the power amp enough.

[J M Fahey]

Well, it's a beginning

To make it somewhat more precise,
1) get a known value signal so numbers make sense.
For example this 1 kHz signal :
http://www.mediacollege.com/audio/to...6bit_30sec.mp3
or
http://www.mediacollege.com/audio/to...6bit_30sec.wav
can easily give you some 100mV continuous level from an MP3 player or computer/notebook/phone earphone out.

2) injecting that with the proper cable to the amp input, will give you an amplified signal at different points.
Measure after the coupling caps so as to avoid high voltage DC.

These are ballpark numbers, double or half are fine, 10% of expected value is not.

I expect some:

1) 2 to 5 V RMS on top of Lead Gain pot (you can set it to 0 ) , lead gain stage engaged (otherwise it receives no signal).

2) around 1V RMS on top of Rhy volume (which can also be set to 0) , with all tone controls set to 10 (T,M and B)
If you engage the lead gain stage, that value should easily reach 10 V RMS or more.

3) you should easily reach (raising proper volume and gain controls) 10 to 20V RMD at the plate of V3a , of course measure after the coupling cap and before the 680K resistor to avoid DC .

There's more but to avoid writingn a book, test and post testvresultsl.

We are checking health of the first part of the preamp, before it gets into the SS EQ .

[Jazz P Bass]

The speaker is fine. It has low volume on another cab as well.
OK I input a 2000 hz signal, put amp on dummy load and am getting 0.3vAC on pin 2 of both output tubes. The dummy load +/- gives me 1.3vAC so about 1 volt more.

Not sure if I did this correctly so please advise. Thank you.

You did not specify what the Vac level is of the input signal.

100 mv's will push pin 2 of the output tubes to at least 20 some volts.

[Pryde]

Using truRTA software program I am injecting a 1khz sine wave signal into the amp at -12 ampl. which gives me .105vAC (1.05 m/v) at the tip of the input jack to ground. maybe I am doing it wrong but not sure how else to see m/v.

With that I am getting .4vAC at pins 2 on output tubes and 1.3vAC on dummy load.
Not sure about this test but its clear I am not performing it correctly somehow.
Any other suggestions or ideas to resolve the issue? Thanks

[g1]

That measurement seems to back up your statement that the fault is in the preamp.
Put that same 100mV signal into the FX return and see what you get at pin2 of output tubes.

[Pryde]

That measurement seems to back up your statement that the fault is in the preamp.
Put that same 100mV signal into the FX return and see what you get at pin2 of output tubes.

I am getting 4.1vAC on pin 2 going into the FX return. Compared to 1v going into preamp.

[Pryde]

Tracing a signal through the EQ I found the coupling cap after Q4 (.1mf/60v on schematic) appears to be a 1mf in the amp. There is strong loud signal on the - side and low volume signal on the + side before the FX send. Did I find the problem? I also am getting .04vDC on the + side as well.
This is an original cap, not a replacement as far as I can tell. Factory error? The amp would have always been low volume if this was the case. Dont wan't to replace if going down the wrong rabbit hole. thoughts?

[chunkitup]

Hey Pryde
Boy this thread seems familiar.

Read this thread and it will make things easier for you. That cap after the eq was my problem. It is supposed to be 1uf

http://music-electronics-forum.com/t32652/

[Pryde]

Hey Pryde
Boy this thread seems familiar.

Read this thread and it will make things easier for you. That cap after the eq was my problem. It is supposed to be 1uf

http://music-electronics-forum.com/t32652/

Thanks I did just read the thread. My current cap is 1uf in there. It looks like you replaced yours with a 0.1uf correct? If so, were did you find an electrolytic that small?

[g1]

I think that there is a schematic error and the part should be 1uF like you had in yours. Chunkitup didn't specify what he replaced his with but I suspect he also used the 1uF he had originally found in the amp.

[J M Fahey]

1uF is fine and a proper value, that's not the problem.

But it might be dry/open so replace it with another, same type.

If any, there's a schematic error.

FWIW Mesa is known to sprinkle a few here and there, maybe to deter copiers.
cloner who never had the original one in his hands will follow the schematic (what else?) and build a poor performing copy.