high power supply voltage

[Hubie]

Hi, I am a newbie so be kind to amateurs! I am working on my 2nd build. I am using a power and output transformer pair from an old PA that I think originally had two 6V6s for the output section. I have wired the entire amp and am ready to test it. It has a solid state rectifier and the CT is fused and is wired as the standby switch. I have been trying to check the voltages through the stages of the power supply - without the tubes in their sockets yet. I want to set the bias voltage and make sure the plate voltages are acceptable before putting in the tubes. The trouble is...the supply voltage rises to over 700V before I just shut it off. It does not blow any fuses, which stands to reason, it really has no load. Should I put in the tubes and see if the load brings the voltage down? I have a new set of 6L6s that I thought I could use if the voltage is really closer to 450 or 500v under load.
I appreciate any/all help!

[Bruce / Mission Amps]

Just set the bias voltage as negative as possible if you are worried.

Here is a decent rule of thumb for starting a new amp up:
Most all good power tubes can be run for quite a few minutes with the bias voltage at about 12% to 15% of the plate voltage, if the plate voltage is not ridiculous.
If you are expecting about 500vdc under a load, then setting the bias voltage to -60vdc to -75vdc will surely keep the power tubes from running underbiased.

[Hubie]

Thanks for the advice! I will get my safety glasses and see where it will bias out with the tubes in.

[Hubie]

The amp powers up ok with the tubes in. I am not getting any bias voltage on pin 5 and the tubes are not blowing up either. The plates still have 650V and the screens around 638V. The filament voltage is rectified, filtered DC and is 6.0. With no input and connected to the speaker, the amp makes a barely audible hum with zero volume. I have not connected the guitar yet, nor tried the volume without the bias working yet. I will check out the bias suppy, which is off of one leg of the primary...a stolen adjustable supply. I use a current limiting light bulb, which reacted normally, and no blown fuses. No fizzing, popping, sizzling or anything like that either. What do you think? Any responses appreciated.

Scott

[Bruce / Mission Amps]

Very high voltages with no tubes is totaly normal as there is no load on the power supply yet so the main filter caps will charge up to peak ripple voltage or 1.414 X the AC going into your rectifier.
If the power tubes are not drawing idle current, the B+ rail will still be very high.

Off the top of my head... this scenario sounds like the cathodes of the power tubes are not grounded and there is no bias voltage being delivered to the tube sockets.
First thing is to figure out how to get the negative voltage from the power tube's grid load resistors over to the tube sockets... this is must to be sure when the tubes do turn on, there is real bias voltage so they don't blow up.
Then see if the cathodes of the power tubes are grounded and if not, why not.

[Hubie]

Hi, and thanks for your response;

Yes, I have found the bias supply to be wired improperly. More updates later...

[Hubie]

Does the standby switch have to be on to get bias supply current too? I am stealing a supply from a solid state diode rectifier leg, 180 ohms resistor, backward diode, two parallel 33/100v caps separated by 10k with the -ve on top. The bias tap is from between the diode and caps and goes to one tab of a 10k pot. the wiper serves the bias circuit. The other tab of the pot sees a 10k resistor to ground. It is the only place the circuit is grounded, except for another cap on the board at the bias entry point. Are the two parallel caps filtering what this cap on the board is for? I get -40 volts before the 10k pot, but no voltage after it? This is all of course with the standby switch off.
Help!


Scott Huber

[Hubie]

I have fixed the bias supply, and can obtain any amount now with a 100k pot. I will hard-wire the correct value when it all works. The best I could obtain with no guitar plugged in was 13ma with a bias voltage of -24. I originally set it up with a bias voltage of -50V but the current was only 8ma.
Will it destroy the 6L6 tubes like this? Should I plug in different tubes (rewiring if necessary to handle the model) for such high voltage? I still have not plugged in a guitar, either. This is from a Garnet Tripper Model Sound Console from about 1975, model G90PAR. I have not been able to find out the specs on this transformer, but a lot of these machines were 60Watters with 6L6s!

There is no 5v tap, to use a tube rectifier. Should I just go to a higher rated tube?

Help! and thanks,

Scott Huber

[Bruce / Mission Amps]

Assuming the power tubes are OK, is there any chance the power tubes are also cathode biased?
There is still something wrong... the power tube are WAY WAY too cold.
I can't see how a PA with +500v can be idling at a tiny 8ma to 13ma with -24 to -50v... that seems impossible to me. -50v at +500v should have those power tubes cooking.
So, there has to be something missing from this problem that is not evident in simple text trouble shooting.
What is the DC resistance to chassis ground of lugs 8 on the power tube sockets?

[Hubie]

There are one ohm resistors which I use to check the bias current in ma.

[Hubie]

Well I have checked the current draw of all tubes through the standby switch and it is only 38ma. The individual legs of the secondary of the power xfmr are 223 volts with the standby switch off. 445V at the input to the first cap, exactly what I want, but when the standby switch is closed the voltage rises to 640V (pretty close to 1.4 X RMS) . I have checked the pin connections and they are correct. I set it up with -50V bias at pin 5.
These tubes are brand new, maybe I have already ruined them without realizing.
I may have to phone Garnet amps in Winnipeg!
What should I be checking? I will pull the power tubes and check the current draw of the preamp stages...

Scott

[Hubie]

Current draw at the first stage=2ma
2nd stage=1.1ma
Post EQ clean stage=2.2ma
Post EQ crunch stage=2.2ma
Driver = 2.4ma
Total pre amp=9.9ma
Roughly 3.3ma per tube, total for the amp is still only about 38-40ma with 9ma at each cathode resistor, leaving about 10ma for the bias circuit I suppose.
I need to know what to look for.
Thanks for any help.
Scott