Cheers - my first thread in the tube-channel ( ...normally I hang out in the solid-state forum) ...I would need some advice from experienced tube-experts.
Anyway, picked up a beautiful Michaelson & Austin TVA10 amp for cheap, knowing it needed some love to get going. I have checked the circuit, reversed a strange mod and found a few broken wires. But then I measured the DC-resistance on the output transformers 😱
One channel is fine, but in the other transformer two of the feedback-windings had shortcircuitet to a much lower DC-resistance, which of course makes these windings useless.
However, the primary and the speaker-secondary windings are fine, so it would be a shame not to try to use them still (of course knowing it will not be quite as dry-sounding as with all these feedback-loops)
So my plan is outlined in the attached drawing.
1) First of all the cathode feedback winding is bust in one leg, so the plan is to just use some cathode-resistors instead.
Question: Can/should I keep the bootstrap 22uF/470ohm circuits?
2) Secondly the balanced feedback to the driver-pair also has to go since one leg of the winding is bad. Therefore I will make this into a standard diff-pair by the drawed modification. The open loop amplification will rise but if I just make sure that oscillation doesn't come in with it, then that should be dealt with by the global feedback going to the input diff-pair. That leg of the winding is seemingly okay, so I am going to let that save what the two other feedback loops can't achieve anymore.
Question: Does all this sounds completely off? All comments are much appreciated.
Nice to know:
- I haven't yet checked the power supplies, but I regard this as a minor issue as this would be easy to get going no matter what.
- I haven't yet checked if the output transformers are saturated ( how do one check that?)
- I am of course going to perform the same mod on the healthy channel to have the same sound in both channels.
- I bought new tubes....EL34 for the outputs
Thank you in advance
Anyway, picked up a beautiful Michaelson & Austin TVA10 amp for cheap, knowing it needed some love to get going. I have checked the circuit, reversed a strange mod and found a few broken wires. But then I measured the DC-resistance on the output transformers 😱
One channel is fine, but in the other transformer two of the feedback-windings had shortcircuitet to a much lower DC-resistance, which of course makes these windings useless.
However, the primary and the speaker-secondary windings are fine, so it would be a shame not to try to use them still (of course knowing it will not be quite as dry-sounding as with all these feedback-loops)
So my plan is outlined in the attached drawing.
1) First of all the cathode feedback winding is bust in one leg, so the plan is to just use some cathode-resistors instead.
Question: Can/should I keep the bootstrap 22uF/470ohm circuits?
2) Secondly the balanced feedback to the driver-pair also has to go since one leg of the winding is bad. Therefore I will make this into a standard diff-pair by the drawed modification. The open loop amplification will rise but if I just make sure that oscillation doesn't come in with it, then that should be dealt with by the global feedback going to the input diff-pair. That leg of the winding is seemingly okay, so I am going to let that save what the two other feedback loops can't achieve anymore.
Question: Does all this sounds completely off? All comments are much appreciated.
Nice to know:
- I haven't yet checked the power supplies, but I regard this as a minor issue as this would be easy to get going no matter what.
- I haven't yet checked if the output transformers are saturated ( how do one check that?)
- I am of course going to perform the same mod on the healthy channel to have the same sound in both channels.
- I bought new tubes....EL34 for the outputs
Thank you in advance
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Cathode degeneration has quite a different effect from transformer feedback to the cathode. Both reduce gain, but degeneration increases output impedance while the original circuit reduces it. You need to think again.
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It might be worth the $ to get the defective OPT re-wound, since the cathode feedback winding should be contained in a seperate section by itself. That way, you don't need to modify the original amp at all, especially since the OPT's were what made the TVA unique in the first place.
This design requires the degeneration to work. I would rewind the transformer. There are many transformer designers that can strip the transformer and rewind it as the original. Majestic Transformers here in the UK will be able to do that for you.
If you really have a shorted winding in the transformer then the entire transformer is no good. An internal short will throw off the impedance of all the other windings. Make sure by rechecking against the presumed good unit by using low voltage AC to measure voltage output. Or use an impedance bridge.
Okay thank you for that information. This is the kind of new knowledge I was hoping for.
Since the bias voltage is regulated form a potmeter, maybe one could then just use a very low ohm cathode degeneration resistor, or none at all, and also lose the bootstrapping in the same go.
Would that work then? (...I know that output watts and sound will be different)
Good point - but isn't that only if these shorted wirings are used?
If they are disconnected and therefore unloaded, then they shouldn't load the transformer at all, should they?
...anyway it seems that the majority thinks a replacement transformer or rewinding the defective one should be preferred.
Thank you everybody so far.
If you have shorted turns internally it will load the transformer down even if left unconnected. It's no different then if you shorted the two wires coming out. Only to a slightly lesser degree, but still fatal.
Hi,
It the winding measures low DCR it could have melted and
formed a shorted turn in the centre of the winding. This will
render the transformer useless on all windings used or not.
rgds, sreten.
Of course ...I see that now.
I am convinced already.
Now I am thinking to rewind the transformer myself.
Found a nice link
...this hobby just keeps leading one to interesting things 😀
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