Volume loss in a marshall jcm tsl 60 watt

[sdre27]

What could be causing this problem? I have recently changed the power tubes and had it biased. The amp sounds great it just doesn't reach the volume it use to.

Thanks

Jeremy

[Alex R]

Many things could cause this. If you want to have a go at fixing it you'll get plenty of support here. But if you have no equipment then you're limited in how far you can investigate it once you've tried swapping tubes. Have you tried putting a patch lead across the effects loop?

[sdre27]

This is what equipment I have and this is all I have done so far.

Bias rite.
Digital multi meter.

I'm fairly new at this, but I know the basics.

I have not tried the loop. What does that entail?

Thanks! Jeremy

[Twist]

The effects loop is a point in the signal path that can be broken to insert an effects pedal. When not used the pins of the return jack short together to complete the circuit. The jack can become corroded or dirty and cause an intermittent or complete break in the signal. Use a small 1/4" to 1/4" cable and plug it into the effects loop send and return. Also some amps have a pre amp out and power amp input that can cause problems too.
Is your volume loss major or slight?

[sdre27]

Its major. Ill have it all the way up and it wont even hurt the ears. The presence pot has little to no effect. You can hear a slight change when turned from 0 to 10.

[Twist]

Was the volume ok before you installed new tubes and rebiased? If so we need to look at the idle current. We need to rule out the effects loop first.

[sdre27]

The amp hasnt been working for over a year. It had the same problem before and after the tubes where replaced.
When I got it one tube was glowing way brighter than the other.
They where about 10.2 MA off from each other so I Replaced them with brand new groove tubes. they where only about 3 MA off from each other. They glow is even now. It was Biased at 70 MA when i first looked at it. Is that too high?

[Twist]

70 ma is too high for one tube unless you mean for the pair. Since you are reading some idle current then your high voltage supply must be ok. You should check the pre amp tubes to see if the filaments are lit. They are heated with a DC voltage instead of AC. There are a myriad of possible causes from broken traces on pots to cold solder joints.

[Alex R]

Try the effects jacks. Just sticking a jack in and out will make a difference if that's your problem.

Power tube glow: the inner bits of the tube will glow and that's fine of course. Any sign of patches glowing red on the outer metal is a bad thing, means there's a bias fault. But we'll come to that.

Let's make certain sure your power tubes are ok, as the glow thing is a concern. Pull out your power tubes and turn it on. Set the meter to DC volts, the highest scale, 1000v preferably or 500v if that's what you got. Ground one multimeter lead to the chassis and with the other check the voltages in the socket pin holes. You'll see the little lug in the centre hole - going (NB) counter clockwise from there, check pins 3 and 4 (should be 400-500v in those, be careful!), and pin 5 (about -50v).

- and get back to us, put our minds at rest...

[sdre27]

Plugged in the loop, send to return. Volume did come up some but the tubes started to glow red real red on the sides and started to smell nasty.

I checked the voltage and this is what I got.

First power tube closest to the trans. Pin 3= 495 pin 4= 493 and pin 5= 43
second power tube...... pin 3=498 pin 4= 494 and pin 5= 43.1

I really appreciate this guys!

Jeremy

[sdre27]

Oh and the preamp tubes are lit.

[Twist]

Since pin 5 has a negative voltage then your bias supply appears to be ok. If the tubes are turning red then you must have one or more bad tubes. One bad tube can load down your negative voltage and thus take out the other tube. You'll most likely need new tubes and a rebias adjustment. It also sounds like you may have had an issue with your effects loop too. Don't be surprised, many amps that come into my shop have multiple problems that aren't related to each other. I try to correct all problems on an amp even if the customer didn't request it. I don't want them coming back and saying I caused another issue with their amp after a repair. Let us know on here when you get new tubes and I'm sure someone will help you with the setup.

[sdre27]

hmmm. Well i just purchased the tubes last week. They are el34 gold series gt.
They only glow read when i patch the return and send on the loop. They may be my issue. What do you think?

[Twist]

Don't rule out new tubes going bad this soon. The effects loop has nothing to do with the output tubes or bias. The only way it could possibly have an effect would be if you have a cold solder joint in the bias section and by plugging something into the board it physically causes an intermittent connection elsewhere on the PC board. I'm looking at the schematic and I see that the cathodes of the output tubes are tied together with a single 1 ohm resistor to ground. So if you read 70 mv at some point , that would be a safe range for a pair of tubes. If the tubes got hot enough to smell then they are history.

[sdre27]

so now I need to get new tubes. Whats next?
That sucks though. ninety bucks down the drain.

should I check the resistor? Oh and should the power transformer be humming?