HT60 Problem with unwanted distortion and volume loss.

[Aphex triplet]

Hi all,

I Hope someone can help me with this on-going, seemingly insurmountable problem I am having with my HT stage 60. Let me give you some background first and indicate what I have done in an attempt to investigate and solve the issue.

It started when my HT slow blow fuse blew. I replaced the fuse and all seemed fine until I noticed that after 15-20 minutes of playing I was getting unwanted distortion on the clean channel and the volume was dropping. So I replaced the power tubes with a matched pair of JJ EL34s and bias to 55mV. The problem persisted. I swapped out all the preamp tubes and no change. I also used contact cleaner to clean all the cable and tube sockets and again no change. I also connected external speaker to all the outputs (impedance matched) and the problem was still there, indicating that the speakers weren’t the source of the problem. I then plugged my guitar directly into the power stage and the problem persisted, indicating that the issue was in the power stage. The time-delayed nature of the symptoms led me to the conclusion that this was a heat-induced problem i.e. a connection or component was close to failing and the increased temperature was pushing to the brink. I took the amp head out of the combo to look for any obvious signs of deterioration, cold solders joints etc. What I noticed was that the circuit board glue (that yellow stuff they put on boards) was browning around the phase inverter socket (see image). Is phase inverter part of the power stage? I assume it is. I assume that the browning is a sign of overheating because of a problem with the socket, a problem with one of the solder joints or a component closely associated with the phase inverter. The solder joints looked OK when I inspected them. BTW, the fuse hasn’t blown since.

Can anyone offer any advice please. It's starting to look increasing like I’ll have to take it to a tech. Thanks for reading.
Cheers,
AT

[Maxsys]

This may seem like really a basic question, but i will ask anyway.

Have you tried different guitars and cables to rule them out?

High capacitance cable can play havoc with guitar signal, i know from bitter experience with an issue on some pedal board leads that gave these exact symptoms.

[thephantum]

Failing the above (cable, pedal, guitar, etc), it probably needs to go to a tech. You said:

...my HT slow blow fuse blew. I replaced the fuse and all seemed fine until I noticed that after 15-20 minutes of playing...

That right there might have been your mistake. Fuses do not just blow. My guess (and it is just a guess) is that you had a bad power tube, which caused the fuse to blow. By replacing the fuse and continuing to play the amp, that bad tube forced excessive heat into one of the screen resistors, knocking one or both out of tolerance. You can check them with an ohm meter (make sure you drain the filter caps first). They'll be connected to pin 4 of the power tube sockets and should measure roughly +/-5% or so of their marked value.

Speaking of filter caps, it's also possible that they are the problem...although it's more likely if the amp is more than 10 years old. The fact is, since you already replaced the tubes, the symptoms you describe could be a lot of things: a bad cathode resistor, an open cathode cap, a failing bypass cap...the list goes on. It could be an issue with the phase inverter, but glue browning a little is not a symptom. Tubes get hot and so do the sockets, so that could be normal.

Like I said, it most likely needs to go on a bench and get diagnosed.

To answer your question, the phase inverter is considered part of the preamp...but it's really the bridge between the pre and power amps. It takes the signal from the preamp, duplicates it and inverts one of the signals. So one signal is out of phase with the other (hence it's name). Those signals are then sent to the power tubes for amplification....one gets the in phase signal, the other gets the out of phase signal...hence a "push-pull" pair.

[Aphex triplet]

Hi guys,

thanks for the reply, yes I have tried quite a few different cables and guitars to no avail. The strange thing is, the fuse hasn't blown since. Perhaps as you say, when it first blew it was related to the power tube(s) failing and in the process damaged a component(s) on the board? I've taken it as far as I am able to go. I'm not skilled enough to do further diagnose so it looks like tech time. Pity my arms are so short and pockets so deep
It’s a real pain, I'm using my back-up rig at the moment which is a solid state Peavey bandit I bought in 1983!.

Cheers,
A

[Aphex triplet]

Just as a quick update, I sent a request to a very reputable guitar tech in my area, outlining the issues as I indicated in the previous post. This was his response:

“Yours is the tenth email from customers with 60watt BlackStar problems. We have wasted hours trying to rectify all their circuit problems and will not be wasting anymore time on them. Your only hope is to return it to them as we have lost so much money trying to rectify customer’s problems.”

This guy has decades of experience and I know his excellent reputation from the testimonials of many others who have used over the years. For me, this is quite concerning re. Blackstar’s reliability.