• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

tube amp being weird -- any ideas

soundbadger

Member
2007-09-24 3:22 pm
just finished building a single ended kt88 stereo amp and it works fine for a while then one channel goes very load and the other very quiet. it is consistent i doing this i can turn it off leave it for a while then turn it back on and get perfect stereo sound for a while. i am confused.
any ideas
 
Hey soundbadger, what are you driving the KT88's with? A schematic and/or layout would be helpful. It is possible that the problem lies in the driver stage. Also go ahead and double check all your voltages and currents. Make sure nothing is shorted. I'm no expert, but I know how frustrating it can be to troubleshoot an amp.....
 
if both channels use a single supply, especially if tube rectified,
it might be that one of the channels`s tubes and/or bias gets runaway.
what biasing method is used?
what value grid bias resistors are used?
resistor possibly going open when getting warm?
but just guessing now..

do you have and will post a schematic?
 
I believe he's building Mikael's amplifier, is that right soundbadger?
I've built this amp myself, as well as many others, and it's a very simple design.
I'd do as Ray_moth suggested and take some voltage readings. I have voltage measurements on my page if this helps. It's got to be something simple.
Glenn

Mikael's KT88 SE amp

Glenn's KT-88 build
 
Beware using a 470k grid resistor on the KT88. I've had modern KT88's draw enough grid current that 470k is too high. The result is that the bias point goes weird, sometimes even oscillates.

Just for kicks solder a 100k in parallel (it may hurt LF response) and see if the problem goes away.

If that fixes it you might need bigger coupling caps to restor LF response.

Pete
 

soundbadger

Member
2007-09-24 3:22 pm
THE BIAS VOLTAGE APPEARS TO DROP ON ONE CHANNEL WHEN THE PROBLEM IS occurring.
(sorry caps off now)
the bias resistors are also pretty hot, i measured them about 70 degrees centigrade , they are 12W resistors which i believe should be fine.
i am guessing that either one of the bias resistors or one of the bypass capacitors is not good.

the pre-amp stage seems fine even when the problem is occurring.
 
soundbadger said:
THE BIAS VOLTAGE APPEARS TO DROP ON ONE CHANNEL WHEN THE PROBLEM IS occurring.

Most likely is that either the coupling caps are leaking or the grid resistor is too large (see above). Both cause the grid voltage to drift above 0V.


the bias resistors are also pretty hot, i measured them about 70 degrees centigrade

70C is not hot for a power resistor. that's it's job - to turn electric energy into heat. Many WW resistors are designed to run at at 150C or higher when operated at rated power.

Pete