Volume loss in a marshall jcm tsl 60 watt

[Twist]

Before you go buy new tubes we need to address your concern that the tubes glowed red when you jumpered the effects loop. Leave the tubes out and ground your meter and put the red lead in pin 5 and leave it. Turn the amp on and see if you have a negative voltage ( it should be around -50 volts ). Now plug your jumper in and see if it changes. After studying the schematic I see this amp taps into the plus\minus 15 volt power supply for the bias voltage. It's possible that you have an issue there. Just for a long shot plug one end of your jumper into the effects loop and see if you have DC voltage there. There should be no DC on the effects loop in and out.

[sdre27]

Pin 5 was reading right at -50 stayed consistent even with the loop patched.

I checked for dc off the loop in and out... all i was reading was .004 Not sure if that really matters.


The tubes where red hot when i had the volume (master and channel) all the way up. It just is lacking some output it seems... 60 watts should hurt the ears even a little bit cranked all the way shouldnt it?

When looking for cold solder joints what am I looking for?

[Alex R]

Let's try to eliminate ultrasonic oscillation as a cause of the redplating tubes. That might explain the loop issue. Could you try removing the preamp tube nearest to the power tubes, then try patching the loop again and see if the tubes still start to glow? Don't buy new tubes yet - they can put up with a few brief episodes of redplating, and the fact that they both glow at once makes me feel that they are probably ok.

Just a couple of things to check:

- when you talk about the tubes glowing, do you mean that the glow appears in an area of the grey metal that you can see inside the glass enclosing the rest of the workings?

- and you do have a speaker plugged in all the time?

And to answer a couple of your questions: the power transformer will hum a bit, but shouldn't hum a lot. And bad ('cold' or 'dry') solder joints are things you can hear easier than see, by moving components with a chopstick. What often happens is the solder cracks in a ring around the wire, making an intermittent connection. But if your bias is a steady minus 50v that's not a primary issue.

[sdre27]

I'm pretty sure one of the gtubes was faulty because only one, with the volume turned up, would pulse to the rhythm of my strumming. only at max volume. So I put the el34's that were in my bogner (recently bought) in it and the pulsing stopped. although both tubes pulse blue within the rhythm of my strumming. only at max volume. Still low volume even at max. on max it seems to only play as if it where only turned up a quarter of the way or half at best.

I tried taken the preamp tube closest to the power tubes out and both tubes still where glowing... but could not get any sound. Speaker cab attached at all times.

[Twist]

The blue glow pulsing while your playing is normal. Some tubes glow blue more than others.

[sdre27]

So im back to square one then? could it be a bad volume pot? or output transf.?

[rf7]

Yeah, it could definitely be a bad output transformer. Check out the tube amp debug info on the www.geofex.com site for info on how to troubleshoot this.

[Mars Amp Repair]

Hello,
it does seem like you have 2 separate problems as linking the loop together shouldn't have anything to do with the bias.
As far as the volume issue, it looks like the speaker out signal passes through the 8/16ohm jack before getting to the 16 ohm jack...I've seen on the dsl & tsl's where the switch in the 16ohm only jack gets dirty & discolored as a result & causes low or no output. Just a thought. glen