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morfeas 23rd January 2004 05:07 PM

HV ON noise
In my 6C33CB SE I have a 3 minute delay on HV to heat up the power tube. On the power on I get a nasty boom on the speaker and a worst one on the power off. The contact is just before the OPT.

Any ideas welcome.

Thanks Morfeas

SY 23rd January 2004 06:23 PM

Why is the delay so long?

Am I correct in assuming you're using a solid-state power supply for the HV? Or do you actually use a relay to switch a fully-charged supply suddenly on and off of the output transformer?

SHiFTY 23rd January 2004 09:02 PM

Could be that your power supply is on the edge of oscillation. Please describe how much capacitance, the current draw, number of stages, what rectifier etc.

If you are using a centre tapped transformer, you can just use the relay to close the centre tap to ground, this works quite well. If you using a diode bridge, you should switch the circuit before the rectifier, on one (or both) of the power transformer leads.

richwalters 23rd January 2004 09:20 PM

Hi there......We need MORE circuit details......e.g are you using NFB from o/p tranny sec to input tube ? may not be phase shift stable with large voltage v.s gain conditions.
P-P circuits are more susp in this....Are you delaying B+ to o/p tube but not to input tube ?
Is circuit a home-made botch up ?


djmiddelkoop 23rd January 2004 09:40 PM

if you mean the HV switch is just before the Output Transformer, I suggest to move it to the HV AC tab of your power transformer.
This will work much better.


EC8010 24th January 2004 02:34 AM

Abruptly switching DC into inductors...
Well of course you get a nasty thump when switch on. You are attempting to suddenly magnetise the core of your output transformer, and Lenz's law objects. Even worse, by switching off suddenly, you turn your output transformer into an ignition coil, creating an even worse thump.

Move the switch so that it switches the AC to the HT rectifier. That way, the HT will come up rather slower, and certainly decay slower. If I have interpreted your post correctly, this should substantially reduce the thumps.

morfeas 24th January 2004 04:07 PM

Thanks for the replies
The power supply is like.

Solid State bridge, 4400mF, choke, 2200mF, 50mF MKP, relay, OPT. There is a 10k resistor in parallel of the relay to charge the OPT.

@SY: Yes, its fully charged.

@richwalters: Yes, the relay only switches on the HV of the output tube. The previous 2 stages get HV with no delay switch.

@EC8010: I think your explanation is what happens. But if I do move it on the AC point charging the caps would not give me a worst one?

The delay is there because I have some info that 6C33C-B has some kind of problem getting HV before heating up.

If you have some more info on the reliability of the tube that can take out the relay I would be more than happy. They cost just 10$.

EC8010 24th January 2004 07:16 PM

You may still get thumps, but they should be greatly tamed by the smoothing capacitors.

Sch3mat1c 24th January 2004 08:42 PM

Re: Thanks for the replies

Originally posted by morfeas
The power supply is like.

Solid State bridge, 4400mF, choke, 2200mF, 50mF MKP, relay, OPT. There is a 10k resistor in parallel of the relay to charge the OPT.

mF or F? :bigeyes: :bigeyes:

SY 24th January 2004 11:46 PM

OK, that's a little clearer. You'd do best to ramp up the voltages a bit more slowly. Maybe a lot more slowly.

One neat way to warm up without severe thumps is to put the timing relay on the primary side of the B+ supply; the contacts are put across a 100 ohm power resistor (10 watts or so) in series with the primary. After 10-15 seconds of filament warmup (that's all you need to prevent cathode stripping, not a full warmup), the relay closes and shorts the resistor. (this all assumes that the filament transformer is separate from the B+ transformer)

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