diytube ST70 oscillates without a speaker load

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diytube ST70 oscillates without a speaker load

Postby davygrvy » Sat Aug 08, 2009 11:32 am

It's been doing this since I built it three years ago. Not a big deal only when swapping speakers when the amp is on. The schematic doesn't have an idle load across the outputs. Thinking 50 Ohms or similar. Anyone find a good value that prevents the amp oscillating without a speaker connected? I could spend time experimenting or just ask.
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Postby TomMcNally » Sat Aug 08, 2009 1:12 pm

Running any tube amplifier without a load on the outputs
is inviting disaster. The transformers can burn up.
Turn the amp off when you are swapping speakers, no
matter how fast, even if you are using a switch.
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Postby Geek » Sat Aug 08, 2009 3:40 pm

=:o

You *might* be able to get away doing that with a SET, but never have a tube amp on (or off standby) without speakers in general for Tom's stated reasons.... especially pentodes.

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Postby davygrvy » Sun Aug 09, 2009 10:36 am

Looks like its experimentation time to to find a suitably high value that prevents damaging oscillation then
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Postby TomMcNally » Sun Aug 09, 2009 1:14 pm

The transformer should always be loaded with the same value
as the secondary - 4, 8, 16 ohms ... a high value resistor may
keep it from oscillating, but you can still easily blow up the
transformer, especially if for some reason the volume gets
cranked up ...
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Re: diytube ST70 oscillates without a speaker load

Postby kheper » Sun Aug 09, 2009 2:09 pm

davygrvy wrote:It's been doing this since I built it three years ago. Not a big deal only when swapping speakers when the amp is on. The schematic doesn't have an idle load across the outputs. Thinking 50 Ohms or similar. Anyone find a good value that prevents the amp oscillating without a speaker connected? I could spend time experimenting or just ask.


I third the opinion that you should not run the amp without a load connected across the terminals. Just a few seconds disconnected can make the outputs really hot.

If the amp is oscillating, check for ground faults. Especially make sure that the driver board has an adequate ground.
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Postby davygrvy » Sun Aug 09, 2009 2:33 pm

All that is fine. Just unloaded when swapping cables with input muted. Yes, I should not drive signal when unloaded, and don't, but this osc thing I should manage before I hurt something (even if accidental). Murphy's law!

I don't have a good collection of 1W resistors below 100ohms.. will acquire.
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Postby crispycircuit » Thu Aug 20, 2009 8:54 am

Guitar amp heads usually have a 50 ohm resistor to ground in case the owner forgets to plug in the speaker cab.... Most the time the resistor is connected to a switching speaker jack. So when the cab is plugged in the resistor disconnected. They wouldn't spend the extra money to do this if there wasn't a reason......
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Postby dcgillespie » Sat Aug 22, 2009 7:42 am

While keeping a load on the amplifier may prevent the oscillation, it would be even better to find the cause of the problem and eliminate it at the source. The nature of the oscillation can be a very helpful tool in tracking it down. It would surely seem however, that the root of the problem lies in some form of inappropriate feedback, as the oscillation goes away when a load is connected. Assuming a good layout and proper build etc., the feedback network would be a good place to start.

As a reference, the original ST-70 as designed is a highly stable feedback amplifier -- that is, it will not oscillate under any conditions of use, whether no load, shorted load, or even with just capacitance connected across the output.

Assuming the circuit and components you are using is a known stable design, try disconnecting the NFB circuit and see if that eliminates the problem. If so, you at least know where to start. If the oscillation is of a very low frequency nature, coupling cap values, decoupling caps and the like would be a good place to start. If it is ultrasonic or near ultrasonic, then the HF stability is at issue. The feedback cap and any HF shunting networks between stages control this aspect of stability. Even lead dress can play a roll in it.

Since it is load related, the power supply is not likely to be the cause. If both channels do it identically and are built so, that tends to indicate a duplicated build or part issue, or a basic design issue.

Some where in all of this you will likely find the cause. An external safety load may be a solution, but finding the cause would be far better. Good luck in your search!

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Postby davygrvy » Tue Aug 25, 2009 12:28 am

From one DaveG to another, you're absolutely right pointing to the feedback network. I don't really consider this amp worth my time to redesign. I should ask separks instead to look into it as it's his circuit. For now, an idle load low enough for it not to oscillate would make me happy.
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How do you know it's oscillating without speakers?

Postby danf » Tue Aug 25, 2009 6:16 am

What tells you that the amp is oscillating if you don't have any speakers connected to hear the oscillation? Do you have an oscilloscope connected? At what frequency does the amp oscillate? Sometimes a powerful low frequency signal will trigger ultrasonic oscillation if there's too much feedback. My usual test for setting feedback resistors is to run the amp into overload at 20 Hz and look for bursts of oscillation on an oscilloscope.

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Postby davygrvy » Tue Aug 25, 2009 10:40 am

With the input shorted and setting bias current, it draws too much until a speaker is hooked-up. When I plug in the speaker, I hear a slight pop. Also, the EL34s dissipate a noticeable amount of extra heat.
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Postby TomMcNally » Tue Aug 25, 2009 11:41 am

With the input shorted and setting bias current, it draws too much until a speaker is hooked-up. When I plug in the speaker, I hear a slight pop. Also, the EL34s dissipate a noticeable amount of extra heat.


No comment. (sick)
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Postby mesherm » Tue Aug 25, 2009 11:46 am

Wire a 0.05uf cap in series with a 100 ohm 1 watt resistor across the speaker taps where the feedback is taken from.
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Postby JimB » Fri Sep 04, 2009 3:08 pm

It is interesting that the ST-70 and Mark III manuals do not mention having speakers or other load connected to the outputs when setting the bias initially. I wonder why, if this is so deadly to the output transformers?
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