• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Unstable monoblock mystery

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
It's been a few years since I've built a tube amplifier, and sadly I got rid of a lot of test equipment and my big stash of junk parts. Now I'm down to one plastic tube and a DMM! All part of moving into a smaller house in a nicer neighborhood.

Anyways, enough of my griping :violin:

For fun I built a prototype monoblock so I could play with plate-to-plate feedback. Design is rather simple - a 12HG7 (hat tip to Pete Millett) pentode driving a pair of parallel 1625s in pentode, single-ended. My thinking was I could burn the extra gain by using local feedback. After sorting out some oscillation issues, quelled by using 10ohm plate stoppers with wires wrapped around the 2W carbon composition resistors. The amp - mono - sounded good enough with my 86dB efficient speakers that I decided to built a matching partner.

Note - the feedback resistor isn't set in stone yet - I've played with different values, between 68k, 100k, 220k.

Well - the second amp seems to be cursed; clipped easily, making nasty distortion. I checked my wiring and layout multiple times without seen any differences between it and the first, successful amplifier. Suspecting a tube issue, I took the working tubes from the first amp. After this change the second amp oscillates - pumping noises and the OD3 gas tube beats along in tempo, turning off and on.

Now I'm ready to dump both of them into a nearby river. Or go for a "slower" circuit with more traditional loop feedback and a different driver. Before I do this, looking for pointers on where to start for troubleshooting.

Some oddness: The second amp has a higher B+, like 452VDC versus 420ish. The second amp also seems to draw slightly less current than the first as indicated by the output tube meters. This, of course, could be attributed to power transformer and output tube differences.

And yes, I wish I had a 'scope as I could suss out what is happening. And that may have to happen sooner than I think.
 

Attachments

  • 1625_signal.jpg
    1625_signal.jpg
    64.8 KB · Views: 162
  • 1625_ps.jpg
    1625_ps.jpg
    35.5 KB · Views: 162
The 12HG7 has a gm of 32ma/V, the slightest change in wiring length and or routing can change the parasitic properties and that thing will oscillate. Try a ferrite bead at the control grid.

Also do you have the 12HG7 really wired as a pentode? The high output impedance isn't going to want to drive the two 100k grid leaks AND the feedback resistor, they all will appear in parallel and will need a really low impedance driver.
 
I built two amps with this general topology, one of which also has a "hot" input tube like yours. I have no issues like you are experiencing. Since I have never used OC3/OD3 tubes I would be suspicious of that but; first I would try one quick and easy revision. Add one 10k resistor between Rfb and the coupling cap junction. Let us know how your doing. I like the potential for these amps.
 
The 12HG7 has a gm of 32ma/V, the slightest change in wiring length and or routing can change the parasitic properties and that thing will oscillate. Try a ferrite bead at the control grid.

Also do you have the 12HG7 really wired as a pentode? The high output impedance isn't going to want to drive the two 100k grid leaks AND the feedback resistor, they all will appear in parallel and will need a really low impedance driver.

Yeah - don't know what I was thinking when I put 100k grid leaks in! I must have been thinking fixed bias. That can be easily changed. Will also try the ferrite bead. Thanks!
 
Yeah - don't know what I was thinking when I put 100k grid leaks in! I must have been thinking fixed bias. That can be easily changed. Will also try the ferrite bead. Thanks!


For a pentode with high anode impedance that is a very tough load to drive but like you said easily fixed. Have you tried wiring the the 12HG7 as a triode?


Also before getting any ferrite beads try just putting a few turns of bare copper around the body of the 1k grid stop resistors, this makes them inductive.
 
I am sure taht this amp is stable without FB.
The question is:
why you have to use this type of FB instead of the standard way? (from secondary of OT to cathode of input tue)
There are some particular reasons to do this?
I used this type of FB but with cap to decouple the resistor from anode of the output tube then the connection to the cathode of the input tube.
It work fine


Walter
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.