The Amp Garage

Your screen be pulling mad current, I'd look at replacing tubes. If only one of the two 1K 5 watt resistors is getting hot, switch tube positions and see if the issue stays in the same socket or follows the tube, and then dig deeper if needed. Thank you for your help. Yes, I also think the problem ...

bepone wrote: ↑Sat Oct 14, 2023 8:11 pm dont even try to diagnostic this cheap pcb type of amp, throw all out, ,keep the transformers and build any decent amp

I hear you bro! I build my own amp all from scratch (minus the transformers) and I love the tone of it. But this amp - it seems that the problem is one of the tubes.

Hello. I was asked to diagnose 3 year old Marshall DSC 40C. Problem is that after a few seconds going off standby, the sound becomes crackly and distorted, and you can smell like something is getting hot (no smoke though). I checked all the tubes on my tube tester and they all checked ok. Next, I pu...

Thank you for in-depth replies. I measured my voltage between the plate and the cathode (pin 3 and pin eight) this time and got 352Vdc. I used milliamp tester in line with my tube and got 37 mA of the plate current. In order to calculate plate dissipation I have to multiply 352 by .037 and I got abo...

Thank you blackeye. Yes that’s what I was not sure about - what difference does it make. I do use 8ohm speaker And my voltage is at about 390 V DC. I’m thinking about installing a toggle switch so it would become an option and I will decide as I go by listening. Thank you very much for your answers ...

I have a simple question that puzzled me and after doing some research I am still not sure. I build a champ AA764 amp. I finally got a hold of a decent Hammond output transformer 1760CP, but it has two primary impedances 5000 and 8000. Which one should I use and why would they want to put two of the...

I ended up getting 1760K one. It has all the outputs I need.
Thanks for all your help. I will stay in touch.

Thank you!

Hello. I am getting ready to build from scratch Bassman 5F6a amp but I need to buy OT and the choke for it. The problem is that I’d like to get one with multiple speaker Impedance (4,8,16 ohms). I did some research and found the one the looks right, but I am not sure. On tubesandmore.com it is P-T17...

I would like to thank all of you who helped me with this problem. I just installed two capacitors instead of that old Dual can capacitor that was completely dead and the amp works wonderfully now. I learned my lessons and will pay forward one way or another.

Ok, in the light of that, my guess is that the vactrols may be fine, rather the issue is that the ecap on the 400V HT node is failing, ie excessive ESR. To my great shame, I did not check the filtering capacitors. The very first two ones that were part of the Can capacitor are completely dead... I ...

So when switched to Vo 1, do the Vol 2 gain & vol controls have any effect? And when switched to Vol 2, does the Vol 1 vol control have any effect? ...the V1 that is located on long narrow circuit board with potentiometers has no resistance between legs one and seven and three and five ( light sens...

Thank you for this much needed info. I identified both Vactrols. When the amp has no power, both LEDs between legs two and six show 51.7 ohms resistance. However the V1 that is located on long narrow circuit board with potentiometers has no resistance between legs one and seven and three and five ( ...

I am still not sure , what do vactrols do, but Would a failure of these components cause the channels to be shorted together? And that would explain the oscillation and hi squeel?

You need to verify whether my guess is correct, eg have both vactrol leds got Vdc across them? Or maybe a vactrol LDR element has failed such that it’s a constant lowish resistance? Dunno how likely that is, LDRs tend to fail high, but it's worth checking for. Or maybe there’s a low resistance path...