Hey there,
due to the idea of building a one-channel test setup for my amp, I made a design error I now have to cope with. Situation is as follows:
The PSU uses one custom-wound power transformer with separate windings for both channels, each channel gets its own GZ34 rectifier and filter chain.
Now, guess what does *not* exist twice on my carefully planned (and ordered/shipped) power transformer... Yes, right, the 5V winding for the rectifier heater. F**k!
As the GZ34 has its heater internally connected to the cathode, I have the following options:
a) Live with the fact that the rectifiers are paralleled. Stretching SOA to its absolute maximum, one GZ34 should be able to supply the whole B+ current of 250mA (at 480-0-480 into L-C-L-C) in case one of the rectifiers fails. So failure of one tube would lead to a detectable (protection circuit) sag in B+, not to the immediate destruction of the second rectifier.
b) Rewire the rectifiers with both anodes in parallel, so each of the GZ34 acts as a single diode in the full-wave rectifier. Now we're safely within current limits, and the failure of one rectifier leads to half-wave rectification with B+ sag and audible hum. BUT: Startup of the amp might be a little hum-my if both GZ34 don't come up exactly the same instant.
(c)) Not really an option: Use only GZ34 with the internal connection burned away intentionally
What do you guys think?
Andreas
due to the idea of building a one-channel test setup for my amp, I made a design error I now have to cope with. Situation is as follows:
The PSU uses one custom-wound power transformer with separate windings for both channels, each channel gets its own GZ34 rectifier and filter chain.
Now, guess what does *not* exist twice on my carefully planned (and ordered/shipped) power transformer... Yes, right, the 5V winding for the rectifier heater. F**k!
As the GZ34 has its heater internally connected to the cathode, I have the following options:
a) Live with the fact that the rectifiers are paralleled. Stretching SOA to its absolute maximum, one GZ34 should be able to supply the whole B+ current of 250mA (at 480-0-480 into L-C-L-C) in case one of the rectifiers fails. So failure of one tube would lead to a detectable (protection circuit) sag in B+, not to the immediate destruction of the second rectifier.
b) Rewire the rectifiers with both anodes in parallel, so each of the GZ34 acts as a single diode in the full-wave rectifier. Now we're safely within current limits, and the failure of one rectifier leads to half-wave rectification with B+ sag and audible hum. BUT: Startup of the amp might be a little hum-my if both GZ34 don't come up exactly the same instant.
(c)) Not really an option: Use only GZ34 with the internal connection burned away intentionally
What do you guys think?
Andreas
In my experience there are acoustic benefits in keeping the rectification and subsequent filters separate for the left and right channels. (I have that set up on my current amp.) It greatly helps channel separation.
The transformer that I have only had one 5V winding but it had a surplus 6.3V one, so I used that for the second rectifier with a resistor to drop the 1.3V (ideally 0.72ohms - I used two 1.5 ohm 5W wire-wound in parallel). The actual resistor dissipation is just over 2W in total.
If you don't have a spare 6.3V winding, don't be tempted to use one that feeds the other valves since it will be directly connected to +HT of course.
Other options are as already suggested.
The transformer that I have only had one 5V winding but it had a surplus 6.3V one, so I used that for the second rectifier with a resistor to drop the 1.3V (ideally 0.72ohms - I used two 1.5 ohm 5W wire-wound in parallel). The actual resistor dissipation is just over 2W in total.
If you don't have a spare 6.3V winding, don't be tempted to use one that feeds the other valves since it will be directly connected to +HT of course.
Other options are as already suggested.
Hi!
If your heater winding is specced for enough current for two GZ34, you might be able to use two 6BY5 instead. They only need 1,6A. If your heater winding delivers a bit over voltage due to the lower load, this might be an option. They have no connection beween cathodes and heater
Best regards
Thomas
If your heater winding is specced for enough current for two GZ34, you might be able to use two 6BY5 instead. They only need 1,6A. If your heater winding delivers a bit over voltage due to the lower load, this might be an option. They have no connection beween cathodes and heater
Best regards
Thomas
If your two HT secondaries are identical then parallel them and feed two GZ34 in parallel. This like your option (b) except the anodes are arranged differently. Avoids hum problem during warm-up, but at the expense of GZ34 overloading during warm-up during the few seconds when only one may be hot enough to function. No worse than (a) though, and only for a few seconds.
maybe you can make use of a 5V switching power supply to feed the 2nd rectifier. Or you could use TV set damper diodes, those have separate heaters and cathodes, and H-K voltage ratings that are pretty high. Problem with that is that damper diodes are just half wave.
Is the power transformer a toroid? If so You could add a winding easier than if it's a regular transformer.
Is the power transformer a toroid? If so You could add a winding easier than if it's a regular transformer.
Rather than putting the windings directly in parallel I'd put a resistor in series with each rectifier anode. This helps with load balancing between the windings if they are not quite equal in voltage and would possibly also help to ensure that the valve's requirement for a minimum source resistance is met (helps to limit the charging spikes into capacitive loads).
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