60's projector amp - blowing fuses

[Stevem]

Please don't add any new parts to the amp at this point unless they are being put in to replace confirmed bad parts!
Resistors that have drifted in value even by 25% will not stop a circuit from working.

It looks to meat this point that the issue it has now was caused by you possibly by replace a fist full of parts and not testing out the amp with every new part installed ne at a time.

I too have screwed up at times when just shot gunning a amp with a ton of parts and then having to retrace my steps to find the issue!

[dayn]

Please don't add any new parts to the amp at this point unless they are being put in to replace confirmed bad parts! Resistors that have drifted in value even by 25% will not stop a circuit from working.

It looks to meat this point that the issue it has now was caused by you possibly by replace a fist full of parts and not testing out the amp with every new part installed ne at a time.

I too have screwed up at times when just shot gunning a amp with a ton of parts and then having to retrace my steps to find the issue!

Stevem, I hear you, and it is great advice to follow, being methodical and testing step by step. I appreciate your insight and assistance.

I can assure you though that I haven't been shot-gunning the troubleshooting. The amp started blowing fuses 6+ weeks ago after only replacing a couple select parts that tested bad or needed a value change to work as a guitar amp. Since then I have worked on the amp almost daily, inspecting with a magnifying glass, testing and tracing everything carefully, and asking for advice from generous people like you who have offered their tips on two different amp forums.

I'm learning a lot, even amidst the frustration of the amp not working. All the parts I originally replaced were tested before installing and have again been tested as part of the troubleshooting process. I do not believe I introduced the problem with my initial mods.

I did methodical testing, going one by one as you suggest, and trying the amp after every test and component test/replacement. I was not able to trace the issue to a component, tube, socket, or accidental ground/short. Having numerous times over traced every wire and component against the schematic, tested the replacement parts, tested the tubes, tested tube sockets, tested every component for shorts and value, tested transformers, and gathering input from several amp techs, the OT seems like the likely suspect having the problem appear only while under load.

I am getting intermittent resistance readings on the OT primary. I have a working replacement OT on the way to try out. In the meantime I am replacing back parts as I pulled some original resistors and caps to get accurate readings out of circuit, tested them all, and instead of resoldering them back in I'm going with new tested in spec components. It's good practice for me, and I'm being methodical, and I fully understand the calculated risk of introducing new problems which is not ideal in a troubleshooting scenario.

I'll post results of the new OT once I have it in hand and installed.

[R.G.]

I think you may be subconsciously sabotaging yourself by labeling the amp's problem "the short". It is an overcurrent, and it might be caused by a short, but as I mentioned, I'm leaning to a problem with the output tubes losing their bias somehow. Maybe funny stuff on the shared cathode resistor, maybe an open grid pull down,maybe something else. I have some experience with this process of my naming the problem keeping me from seeing the real issues.

[dayn]

I think you may be subconsciously sabotaging yourself by labeling the amp's problem "the short". It is an overcurrent, and it might be caused by a short, but as I mentioned, I'm leaning to a problem with the output tubes losing their bias somehow. Maybe funny stuff on the shared cathode resistor, maybe an open grid pull down, maybe something else. I have some experience with this process of my naming the problem keeping me from seeing the real issues.

Thank you R.G., that is good insight for me. Learning to dive deeper than the symptom to the problem causing it.

This is the resistor. I don't typically think of them going bad, and the connections look good and it tests at 220 ohms, but maybe there is a problem with it, or something it is connected to on the turret board?

[Harry1001]

dayn, could you disconnect the yellow wire from PT (pin 7) and try to switch amplifier via bulb limiter.

I'm sorry, I just now saw that a hole had burned out in the resistor. It could be seen at the beginning of the topic, but unfortunately the photo is very dark. But, if it shows 220 ohms, then it is not quite dead yet.
But how did you measure its resistance if you previously wrote that cathodes are short to ground?

[dayn]

I got the amp to stop blowing fuses by swapping the Power Transformer from another of the same amp. The original PT seemed to test fine, but under working conditions was problematic.

Now I'm working on biasing, and tweaking a few things, but it sounds great!

[Stevem]

!

[dayn]

!

Thanks for your help along the way. It was a sneaky problem, and I certainly learned a lot exploring all of the different aspects of the amp to try to sort out the problem.