b+ Delay Arrangement for Power Amp

[commstech]

I have individual filament transformers for my pair power output DHTs, and a separate power transformer for the high voltage. Rectification for HV is by means of SS diodes.

I am planning to have the powering up sequence in such a manner :-
Upon turning on the amp, the filament transformers will be powered up and light up the filaments of the output DHTs. 20 seconds later, the AC input to the primary of the HV transformer will be turned on by means of a delay relay and thus giving the HV. And since this is SS diode rectified, full high tension DC will immediately hit the anode of the DHTs and also channels to supply the driver section of the amp, without the slow start benefit of tube rectifiers.

Will there be any potential problems with this kind of arrangement? Eg. any noise or pops when the maximum HV dc hits the tubes?

[oshifis]

Originally Posted by commstech

I have individual filament transformers for my pair power output DHTs, and a separate power transformer for the high voltage. Rectification for HV is by means of SS diodes.

I am planning to have the powering up sequence in such a manner :-
Upon turning on the amp, the filament transformers will be powered up and light up the filaments of the output DHTs. 20 seconds later, the AC input to the primary of the HV transformer will be turned on by means of a delay relay and thus giving the HV. And since this is SS diode rectified, full high tension DC will immediately hit the anode of the DHTs and also channels to supply the driver section of the amp, without the slow start benefit of tube rectifiers.

Will there be any potential problems with this kind of arrangement? Eg. any noise or pops when the maximum HV dc hits the tubes?

I use a similar arrangement, but separate manual switches for the filament and for the HV. Actually the filament is permanently on, except being away from home for several days. When I turn on the HV, there is no any pops heard. Take into account that the mains transformer has some stray inductance, and together with the filter capacitor it makes a low-pass filter, so reaching the full HV takes some (however little) time. At least the capacitors do not charge with infinite speed. The HV switch should pass high surge current, and should withstand at least 2x the amount of HV. I switch the AC at the secondary of the transformer before the rectifier diodes, not the filtered HV after the capacitor.

[Osvaldo de Banfield]

I think that would be better to add a NTC surge protector at each AC line of the rectifier, to protect them from inrush currents, and reduce surges in the power line.

[garbage]

i use something similar, but it is because of the MV rectifiers (ok, taboo; i know). the amps are mono blocks and when b+ kicks in on the first mono, no pop or clicks. but at the second one, i get a pop on the speaker.

the speakers are coral beta 8 in BR box. ~100db sensitivity.

i tried various NTC values, but they did not help.

[commstech]

strange that it only happens for one channel. could be due to other reasons besides the turn on surge. Have u managed to solve the problem?

[garbage]

i did not manage to solve the problem. the pop occurs at the speaker with the mono that is already turned on. i think it is because the b+ in rush on the 2nd mono caused some spike in the electrical outlet. both monos are connected to the same electrical circuit.

the circuit to my audio room is a dedicated ring that comes from my main circuit breaker.

i guess the only way to really solve this is to build a soft start circuit like the one at ESP, or implement some muting at the output transformer secondary (but i was told that output transformers should ideally not be unloaded for too long).

[Wavebourn]

Here is DIY delay relay that can be more effective:



But if to speak seriously, all depends on which tubes you use, on which voltages, and how is bias organized.

[12E1]

Originally Posted by garbage

i guess the only way to really solve this is to build a soft start circuit like the one at ESP, or implement some muting at the output transformer secondary (but i was told that output transformers should ideally not be unloaded for too long).

You should not mute by removing the load from an output transformer, since large current transients on the primary side while the secondary is unconnected can induce very high voltages across the primary which can result in arcing and insulation breakdown. Bye-bye transformer.

In my view, if you really need to disconnect the speaker, then you should first switch an alternative dummy load across the secondary and then remove the speaker from the circuit. It's more complex but safer. An alternative would be to simply place an additional temporary load across the speaker terminals with a relay contact which will absorb most of the unwanted noise (this could be less than one ohm, possibly almost a short circuit). If there is no real signal going through the amplifier at this time then that should be a pretty safe option.

[commstech]

Originally Posted by Wavebourn

But if to speak seriously, all depends on which tubes you use, on which voltages, and how is bias organized.

Let's just say large transmitter tubes in general working in the 1kV range, in self-bias design. Any disastrous effect on the powering up arrangement mentioned?

[flatheadmurre]

Should work fine.

Always keep the relays on the AC side if possible.

Preferable on primary low voltage.

If trying to switch DC the chance of arcing is high.

Only thing i found working on the HV DC (>1kV) is high vakuum reed relays and they cost an arm and a leg !