Tube Amp Archive

First off a kt77 is much more like a 5881 in terms of its 25 watt rating. Second, the tube data sheet for a kt77 with about 400 volts on its plate and screen calls for -31 volts with a 25% up or down range. I would want you to get at least a - 38 volts to show up on pin 5 of each output tube socket...

It is so mind boggling to me to have someone come to a forum asking for help to then go on being rude to people and acting like they know already how to resolve the problem. Your skill with electronic circuits in general doesn't directly easily apply to tubes, OR you're not as skilled as you think ...

You guys are harping on me about using KT77s instead of EL34s. That point is moot as far as the impact on bias...they are close. The bias without tubes is way too hot. I changed the 220k bias resistors all the way to 82k and the voltage is still only -34.

pdf64 wrote: ↑Sun Sep 12, 2021 5:35 pm Twiddle the 10k bias pot.

Twiddle??? What does that mean?

The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes! So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed. I al...

sluckey wrote: ↑Sun Sep 12, 2021 4:55 pm The ***first*** thing you should do is get the correct bias voltage on pin 5 of the output tube sockets ***BEFORE*** you ever plug in the tubes!

So, how is this accomplished...as I stated at the start, this was a complete kit from MoJo and I would assume that everything would work as designed.

I wouldn't be assuming as a new amp builder how to tell the experts what they should 'bounce around to' as they intimately know what's needing review and what isn't. Voltage drop with tubes in is normal. Once they start conducting, they create a voltage drop due to adding load to the circuit. How m...

We need to look at this from the start...not bounce around. First of all, why does my rectifier voltage drop so much when I install the power tubes...since this is the first thing the power goes through, voltage should not be affected. Why would the bias voltage also drop? I will look at the 100k fe...

OK so I followed the advice given...here are my findings and build details: 1. First off I am running KT77s (not EL34s) 2. In the attached image of voltage readings the new readings (without power tubes is in a BLACK BOX with WHITE LETTERING. 3. Diodes are in the correct direction. 4. All components...

FYI...pic shows what the PT should be putting out according to MoJo's documentation...I will remeasure with no power tubes...all components and wiring are correct. Will test diodes as well.

I have been working on this "project" for a couple months now...from study and prep to build. Last week the PT kept blowing fuses and I was sent a new one. Prior to the PT dumping, I was getting some really low voltage readings across the whole thig. Except for the local power being supplied...that ...

The schematic shows continuity to ground for those wires. OK I do see that...now I have some serious troubleshooting to do. Any easy way to test a PT with a VOM to rule that out as toasted? I have no idea where it went south. The only issue I had was the loud tone when I took the amp off standby. T...

Just swapped the wires on pins 3 (anodes)...nothing more.

Ok, here is where it stands. Switched the wires on pin 3 on the output tubes and now I am blowing main fuses??? Switched them back with the same results. Performed a continuity test and getting tones on VOM when black probe to chassis and red probe to red PT wires on the standby switch and all heate...

Also, I was able to clear up the no output issue but not in a good way. When I take the amp out of stand by...I get an extremely loud and deafening hi-pitch tone (squeal) and that is with all the knobs at 0. you need to swap anode wires on output tubes, pin 3 for the heater problem, remove ALL tube...