Troubleshooting EL34 push pull amp

[psysjal]

Hello all,

After reading this forum for a while and reading a few books on the subject I have finally built my first tube amp.

The problem is that it is not an amp at the moment
Just a complicated hum making machine.

Basically I have got to the stage where when turned on there is a squeal until the tubes heat up, this is then replaced by a very loud low frequency hum. Nothing coming in through the input seems to make it out the other end.

So far I have checked the power supply (with a dummy load) and measured the plate voltage on one of the EL34s (at 400V when the tubes have heated).

What should I be checking next?

[SY]

Before you go any farther, put some voltage equalizing resistors across the series filter caps in the power supply! This is a serious safety issue.

If you're getting squealing from your speakers immediately upon turn-on, focus on the power supply. A scope will be useful in seeing what's oscillating. As a diagnostic, you might want to try disconnecting all that power conditioning circuitry on the primary side and making a more direct mains connection to see if the trouble lies there.

The squeal and later hum may be related, but keep your eyes open- they may be two separate issues.

[Frank Berry]

Remove the negative feedback network. If this stops the squeal and hum, you'll need to swap the connections to the OPT secondary.

[Bas Horneman]

Measure the voltages across the cathode resistors
r5, r6, r13,r26,r27 to see if there is current flowing through the tubes.

And like Frank says : Remove the negative feedback network for now.

[icebear]

Could it be oscillating? The output tranny could be mismarked, so what you think is the positive wire, is actualy the negative wire. You could try to rewire the feedbackloop to the other post on the output.

bjorn

[psysjal]

Thanks for the suggestions so far. I will try them out when I get home.

What value resistors would you recommend to equalise the voltage across the filter caps?

[SY]

Guys, if it's squealing before warmup, it's not the feedback or misbiasing (there may be some problems there, but that's not the first layer of the onion).

Psysjal, you want to choose a resistor that's a decade lower than the cap's leakage resistance. If those are electrolytics, you'd be safe with 470K, 2W resistors across each cap.

[SHiFTY]

Double and triple and quadruple check your wiring, especially tube connections. It is very easy to make mistakes soldering them, esp with octals.

I would also check your filter capacitors. Are they (too) old? Does the -ve connect solidly to ground?

Also try disconnecting the feedback loop from the output trans. See what happens if you just connect the secondary straight to a speaker.

Resistors across each B+ cap should be ~300K 1W.

[psysjal]

Thanks for the help guys, I know have something that works.

It was two different problems in the end.

The squeal seemed to be down to a couple of bad joints in the power supply.

The loud hum was caused by a incorrectly labelled output transformer. Swapping the leads silenced it completely.

Going back to the value of the resistors across the filter caps. I am guessing that the value is chosen to allow the voltage even out across the caps without letting too much current bypass them. Is this a rule of thumb calculation?

Having never seen tubes in action before I have a few questions about how they should behave. They glow a nice orange colour at the moment and a slight blue can be seen in parts. Is this right or are they running a little hot?

Thanks,


James

[Bas Horneman]

The blue glow is harmless...

http://www.kcanostubes.com/faq/blue_glow.html