Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ACHIEVEIT on May 05, 2021, 11:20:18 am
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Hi all,
I have a rather unusual question - a good opportunity to learn something new. I have a stereo push-pull rack power amp and unfortunately one of the two output transformers stopped working properly. I had bought it used and channel 2 was always (suspiciously) quieter even after changing tubes, checking resistors/caps, biasing etc. As I was playing a few days ago, the volume dropped even more and there was an unpleasant smell coming out (without any smoke though). I stopped immediately and traced the smell to Ch.2 O.T. Nothing else failed, all voltages + bias are correct so as an ultimate test, I disconnected the O.T. of channel 1 and connected it to the circuit of channel 2 and for the first time channel 2 was loud and full.
I am unable to find a reasonably-priced replacement transformer and I am only using one side of the amp anyway. The two channels are configured differently (one channel has a pair of 6L6s and the other a pair of EL34s but they use identical O.Ts) so I'd like to be able to keep both (not used at the same time of course). So, my first thought was to switch the O.T. to whichever channel will be used.
So here's my question:
- What happens if in the unused channel there's no O.T. connected?
- Is it possible to put a switch to connect the O.T. to the anodes of either channel and at the same time disconnect the screens of the unused channel?
- In other words, would it be ok to leave the unused channel with disconnected anodes + screens (so having cathode at ground and control grid under ordinary biasing conditions)? Am I missing something fundamental here?
- If my thinking is right, any recommendations about switch rating?
Thanks!
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You could switch the plates and screens with a heavy duty 4PDT switch. I suggest turning the power off to operate the switch.
What is so special about your OT that makes finding a reasonably priced replacement difficult? Or are you too poor to afford an OT? There are lots of OTs available that I consider affordable. Can you post a pic of your OT?
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To implement this idea would seem to create the antithesis of good lead dress.
That’s not to say it will necessarily cause a problem.
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Since the channels won’t be identical anyway, you can get pretty much any OT you like that works for EL34 and/or 6L6 tubes. Should be plenty to chose from.
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What about to have a standby switch for each channel and act them in compliance with the plates and screens switch ?
Franco
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Thanks a lot for all the replies!
The issue with getting a replacement transformer is mostly the space constraints (has to be the exact dimensions to fit + ideally have a similar mounting pattern). Being in a small European country doesn't help with getting a custom one made either (it's almost exceeding the market value of the amp). I will definitely continue looking for a replacement transformer though - I'm just trying to establish a plan B.
So, if I were to use a 4PDT switch my thought was to use: 2 poles to connect to either channel's anodes, 1 pole to connect the screen voltage to either channel's point where the two screen resistors meet, 1 pole to pass the negative feedback from the jack to either channel's feedback circuit. Does that sound correct?
Regarding switch rating, what voltage rating should I go for? (screens + anodes are around 470V in this amp)
@kagliostro: You mean like on top of the solution above, implement a standby switch for each channel by disconnecting the cathodes from ground?
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I mean two switch to be used to disconnect B+, one swithc for each channel
this in junction with the plate + screen switch
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Act OFF the two switch as to don't have B+ on channels > act the plate + screen switch > act ON only the switch of the channel that is connected to the OT
Franco
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Don’t know if this was already suggested, but you could connect both pairs of output tubes to the same OT. Then use a switch to ground the input (grids) of one pair at a time.
You would only need a small DPDT switch and would not need to switch high voltages or sensitive OT connections.
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Great point, but I get the impression it’s a fixed bias amp, so connecting the control grids to 0V common wouldn’t be good :icon_biggrin:
Rather just use the switch to open the circuit between the coupling caps and the control grids.
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Couldn't you connect both pairs of tubes to the OT , and then lift the ground to the pair that is not used ?
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To clarify, yes it's a fixed bias amp
That's some good ideas - would the combination of them be the best solution?
In other words, simple 3PDT switch:
1 pole connecting either pair's cathodes to ground
2 poles connecting either pair's coupling caps to respective control grids (making sure the bias is supplied to the control grids at all times)
On second thought, should the NFB also be switched (that would make the above solution a 4PDT)?
I think the idea of not having to switch the O.T. (and respectively the high voltages) is quite appealing
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Ahh yes, the NFB, but then I can’t see the need to remove it from the inactive amp?
So 3 poles should do it.
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Great, thanks - I'll give it a try!
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Great point, but I get the impression it’s a fixed bias amp, so connecting the control grids to 0V common wouldn’t be good :icon_biggrin:
Rather just use the switch to open the circuit between the coupling caps and the control grids.
My bad! They should of course still be connected to the bias voltage, not ground.