Just had another look at that schematic. I mis-read the cathode voltage for the EL34s (pin 8 to ground). It should be +0.3VDC (not 3vdc). That'll be a function of the Output Balance control (R2) and the Bias control (R3). The grid voltage should be -35VDC. On these old amps the coupling capacitor between the Driver and the Output tubes (in this case C6 & C7 - 0.15mfd) often become leaky leading to damage to the output transformer. They should be replaced first thing (this is unless it hasn't already, the perf board in the photo makes me suspicious - I think this amp has been gotten to and is not original). Output tubes should be a matched pair. Careful with the voltages, 425VDC won't let you let go.
Tube amp bring up after 20 years in storage
- Thread starter WOPL Sniffer
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Can someone explain to me, in a way that does not violate Ohm's Law, why a load is needed to bring up a tube amp?
The amp has an output transformer. So there is no DC offset at the speaker terminals to be concerned. To me the load only reflects back on the primary of the output transformer if there is some signal on the primary of the transformer. There shouldn't be any signal if there is no input signal. If the amp requires a load on the speaker terminals for stability that suggests to me the design is not stable in the first place and should be corrected.
The amp has an output transformer. So there is no DC offset at the speaker terminals to be concerned. To me the load only reflects back on the primary of the output transformer if there is some signal on the primary of the transformer. There shouldn't be any signal if there is no input signal. If the amp requires a load on the speaker terminals for stability that suggests to me the design is not stable in the first place and should be corrected.
Further to Don's note, I don't recall any warnings as such for the Dynaco Stereo 70 that I build eons ago about needing an output load to bring it up successfully (I went back and read the bring up instructions). I had an Eico tube amp before that and I don't recollect any warnings there either. When you have zero output voltage on the output windings of the output transformer, I am not sure what a resistor of any value would contribute.
You are both right - for me, it's a matter of best practice, just in case a signal source is there. Under quiescent conditions and no stability issues no harm should be done. It's kind of belt and suspenders thing. Many of the ones I worked on had a tuner (or it was actually a radio). Even with a signal source, a momentary "no load" condition shouldn't blow anything - just keep it momentary.
Hi wattsabundant;
Did a web search for this question:
"should a load be connected to tube amps?"
There is much information and explanations.
Example:
https://www.head-fi.org/threads/why-do-tube-amps-must-have-a-load-connected.484643/
As for myself, if a debatable potential exist,why take a chance?
With that said, I leave this debate for others.
Sincerely;
MB
Did a web search for this question:
"should a load be connected to tube amps?"
There is much information and explanations.
Example:
https://www.head-fi.org/threads/why-do-tube-amps-must-have-a-load-connected.484643/
As for myself, if a debatable potential exist,why take a chance?
With that said, I leave this debate for others.
Sincerely;
MB
