distortion in one channel of tube amp

[fragman56]

Anyone care to take a shot at this: old Calrad 30B tube amp has scratchy sound in one channel (distortion). What's the cause?

Bad tubes? Transformer? Caps? All or any of the above?

Thanks from an aspiring diyer,
Rick

 

[SHiFTY]

Well, you should try all the obvious things first. Clean the pins of the tubes, swap the tubes around and see whether the problem moves with a tube, check all solder joints and connections, and check all resistors for correct values.

Process of elimination, its the best way!

 

[fragman56]

checking resistors

Hey Thanks!

I was told that the best way to check resistors is to de-solder and meter rather than meter in the circuit because of other influences from the circuit.

Is that right?

Aspiring diyer,
Rick

 

[SHiFTY]

Nah, just meaSure them in circuit... (with the amp off!) It works most of the time.

However, dodgy solder joints can be hard to spot... After eliminating all of the above, you might want to have the amp running and prod things with the *plastic* end of a screwdriver. You can quite often hear dodgy connections this way.

 

[Duo]

Depending on how old the amp is, I doubt you need to worry about resistor values drifting enough under normal circumstances to generate distortion. A more likely cause is bad solder/dirty pins/bad tube. There are a few resistors you may wish to check however, which are things like bias resistors in the cathode circuits and also check that any bias adjustment pots are clean and properly adjusted.

I hopw this helps

BTW: Just try to make sure you have the power supply cap discharged before you work, you'll get an awful slap if you touch a charged B+ cap. I should know, I've done it before!

 

[fragman56]

Hey, thanks for the advice!

Switched tubes around but problem remains in the same channel, ergo, not the tubes?

I'll check the resistors and bias pots. Will also check the solder joints. I will clean the pins since they need it.

Sounds like you suspect the bias for the tubes. So is this the sound of the distortion if the bias is off? Scratchy, static-y in one channel?

Thanks again.

Aspiring diyer,
Rick

 

[Duo]

Well, since I haven't heard your amp, I can only state bad bias as a possibility. If you eliminate the problems of resistors/solder joints/capacitors being bad, then it's most likely misadjusted bias.
However, if the bias pot is common for both channels (it shouldn't be) but if it is, then there is a problem specificly with the bad channel. I would next check caps if cleaning sockets didn't do it... Let me give you an idea to test...
Does the amp make static when you pass music through it, or all the time??? Cause if it's all the time, it's definately not misadjusted bias, but more likely a bad tube socket that has a carbon arc between pins or really bad dirt (which causes carbon arcs over time). However, if the static only occurs when you pass audio through the amp, then you must have a capacitor or bias problem. Check all your caps for DC resistance in this case cause they do dry out when they get old.


This should narrow your problem down significantly.