self-oscillation in Dynaco ST-70

[kyle1]

Hi,

I'm troubleshooting a problem with a Dynaco ST-70 tube amp. There are two symptoms to the problem: self-oscillation of the output and a buzzing sound coming from the output tubes (literally coming from the tubes- I don't have speakers connected).

If I hook up an AC voltmeter to the 16-ohm output terminal and turn the amp on, the tubes will start buzzing after a few seconds. At the same time, a slowly rising AC voltage appears at the output. Within about 5 seconds it rises to around 90V RMS, at which point the 1k feedback resistor is smoking and I cut the power.

It does this even with the input grounded, so I guess there's some kind of self-oscillation driven by positive feedback from the output transformer. If I temporarily remove the feedback resistor from the circuit, the problem goes away, but then I can't bias the tubes. They run at a low current even with the bias pot all the way up.

If I take out the output tubes and test the plate voltage, control grid bias, etc. it all looks to be normal according to the test voltages in the Dynaco manual. Can anyone tell me what might be causing this? And what does it mean when a tube makes a buzzing sound? Could it be grid emission?

Thanks a bunch!
-Kyle

[DigitalJunkie]

(literally coming from the tubes- I don't have speakers connected).

Ohh,Tsk,tsk! Tube amps MUST be operated with a load! Never run one without speakers connected! That may be the problem.It may have also caused more problems,and fried parts.

[MellowTone]

Originally Posted by kyle1

I don't have speakers connected

I was about to say exactly what DigitalJunkie said, unlike solid state amps, which due to their low output impedance can deal with running loadless, tube amps cannot.

You'll need a more knowledgeable person than I to tell you what you will have fried (beyond the obvious) but there will be a few things that need replacing. How long has it been run without a load?

On a related not, also be careful of load mis-matching. That is, connecting a 16 ohm speaker to a 4 ohm output tap or things of that nature. This will also freak the amp out big time.

Don't turn the amp back on until you've replaced the parts. Replacing the tubes would also be a good idea, as you've probably done damage to them.

Nic.

[Geek]

Ah yes, the tube amp without a load scenario.

ST-70's oscillate at ~60KHz without a load and the buzzing you hear is the heavy saturated overload of the output tubes.

You will most likely now find your EL34's at about 5% emission (or lower) as the screens are shot.

Good luck with the repair and I hope you haven't cooked an OPT winding

Cheers!

[kyle1]

Oh, I was under the impression that you could run them without a load if there was no input? That's not true? Why does connecting a load stop the oscillation?

The thing is, when I get some questionable amp from a garage sale I usually want to check it out and make sure it works before I connect it to my expensive speakers! I guess I need to get some junk speakers to test with.

Thanks, everyone!
-Kyle

[Frank Berry]

Measure the actual values of all resistors. Some may have drifted from their original values. Replace all coupling capacitors as they may have become "leaky."
It's a good idea to replace the electrolytics as they may have dried out. This could cause the amplifier stages to oscillate through the power supply.
With no input, the amplifier should be stable ... even without a load on the output transformers.

[boywonder]

Originally Posted by kyle1

The thing is, when I get some questionable amp from a garage sale I usually want to check it out and make sure it works before I connect it to my expensive speakers! I guess I need to get some junk speakers to test with.

Thanks, everyone!
-Kyle

Or pick up a 25-50W 8 ohm load Resistor for around $7 or so...

Digi-Key - RHD-8.0-ND (Manufacturer - RH0508R000FC02)

[Yvesm]

Originally Posted by kyle1

Oh, I was under the impression that you could run them without a load if there was no input? That's not true? Why does connecting a load stop the oscillation?

Because the gain of a (pair of) penthode depends on how they are loaded.
Infinite load gives infinite gain ... well, almost ...
Even worst situation if negative feed back removed.

With no input, the amplifier should be stable ... even without a load on the output transformers.

Sure, we try hard to acheive that !

Yves.

[kevinkr]

It's pretty likely that the power supply and bias supply electrolytics are bad. Plan on replacing these regardless. The coupling capacitors and some resistors generally also need to be replaced in amplifiers that have not previously been serviced.

The loud noises you heard were most probably the output transformers buzzing. EL34 may make noise when the internal electro-static fields are powerful enough to damage them, otherwise they are generally pretty quiet.

As has been stated before you should never run a tube amplifier without loads because if the amplifier malfunctions and the tubes are driven to cut off the collapsing magnetic field in the OPT will result in voltages that can and most likely will punch through the wire insulation causing a short. Frankly given the scenario you describe I think you will be lucky if you do not also need to replace one or both of the output transformers. Always use a resistive load when testing an amplifier whether or not there is input signal present unless you know the thing was designed to run safely into an open circuit. (And most aren't)

I suspect you also don't know that old tube amplifiers should not be plugged straight into the wall after some period of disuse, but should be carefully checked out first and then provisions made for reforming the power supply capacitors.

I would do some reading on the newbie and safety threads here before proceeding any further.

[pointy]

or you can get 4 small cheap 8 ohm speakers

put them in series for 16 ohms

use one per channel for 8 ohms

and parallel for 4 ohms

this should be ok as long as they can handle the watts