Bias trouble.(very popular topic subject)

[garysanders]

'Lo Ampers!

I am building a 124 (50 watts) and all seems good except my bias pot just isnt working.Double checked EVERYTHING and my plates are getting red QUICK like 3 or 4 seconds.By using the 1 ohm resistor/mV method its only reading about .2 (!) and by the time I got the reading I had to kill it before the glass melted.SO...

I built this out of a salvaged Carvin XV112.It has a 100watt OT.The Carvin was switchable to 25/50/100% power but does that have anything to do with the OT?I wasnt sure.I figured it would STILL have to be since the only thing switchable on the Trans. are the 3 2ndary imps.(4/8/16)

This is the only thing I can think of.If this is the problem I'll just throw the other two 6L6's in but I needed it confirmed by experienced gooroos like yoodoods cause something else may be wrong,this is the first complete amp I have ever done from scratch.

Thanks!

Gary

[Structo]

Did you build a new board or did you modify what was in it?

If you can post some close up shots of the wiring somebody can help better.

Did you use the existing bias setup or is it all new?

I don't see where the Carvin had an adjustable bias.

[odourboy]

Check the bias pot function with the tubes OUT. You want to be able to get to a minimum of at least -55V on the bias pot (a very cold setting) to maybe -40V (a very hot setting). Typically you should find a properly biased 6L6 as at -45V to -50V. I haven't checked in a long time but I think I get a range of -35V to -60V on my builds.

To check the idle current, you measure the voltage across the 1 ohm resistor. I bias to about 35mA, so you should measure 35mV. Are you sure you're measuring .2mV and not 0.2V (red plate territory for sure!)?

[Structo]

Did you wire test jacks for bias test points?

Did you wire your bias pot with a 10K resistor or a 27K one?

Remember if you are only running two power tubes, that you need to have the resistor in the right range to bias.

[garysanders]

Did you wire test jacks for bias test points?

Did you wire your bias pot with a 10K resistor or a 27K one?

Remember if you are only running two power tubes, that you need to have the resistor in the right range to bias.

I didnt wire the test jacks,just a 1R resistor to the multi meter.I used the 27K version you see on the layouts and schematics with the diode/3.3K and 1000 uf cap.

Did you build a new board or did you modify what was in it?

If you can post some close up shots of the wiring somebody can help better.

Did you use the existing bias setup or is it all new?

I don't see where the Carvin had an adjustable bias.

No I started totally from scratch.I only used the chassis and PT/OTs.I dont have a camera that works at the moment or I'd show it to you.

And I am sure it was Milli-volts instead of Volts.

[garysanders]

you need to have the resistor in the right range to bias.

That is what I am unsure of also.What would be good for two 6L6?The 10K or 27K?My -supply line to the PT is reading 51.4 so I guess its good.Everything seems good.As I said this is my first total build and I really dont know the checklist for starting up.So thanks ahead for helping!

Now Its giving me a loud CHIRP when I kill the stand by and in a matter of seconds the plates are glowing.I believe I saved the tubes,I killed it as soon as I saw it happen.I will pull them and check the -V before I try them again.They're those good over priced,over rated GT's.

I wont bug you guys to death with it.If it doesnt work out soon I'll just scrap it and start over......but its so close to being done and I really like the Dumbos.

[John_P_WI]

You are reading negative voltage for the bias supply, right? The diode and cap are "backwards" to normal convention??? As Brian said, check your neg voltages first without any power tubes installed.

As far as the Carvin 25, 50, 100 power setting, Carvin ususally changed the gain of the PI tube by messing with the PI cathode resistors. I don't think this has anything to do with your bias problem.

John

[zimm]

I had a similar problem once. Power tubes were red plating and all components were wired correctly and measuring to spec. The culprit: I drilled some holes in the chassis after I had the amp together, the metal shavings got between the bias pot and the chassis and shorted the lugs to ground. This was weird but I found it by unbolting the bias trimmer from the chassis. I saw the filings, and the red plating stopped. If your trimmer is attached to the chassis, maybe unbolt the trimmer and see if that changes anything? Just a suggestion.

[BobW]

Gary,

Check the layouts from Structo's comments to insure you have pins 1 and 8 tied to ground via the 1 ohm resistors. Pull the output tubes, turn on the amp and measure the voltages at the grids, pins 5. If you don't see a negative voltage then your bias diode is in backwards, if the voltage is not approx. 50 volts, then the bias pot dropping resistor or other components in the bias circuit are not correct.

Call me, you know we only live about 5 miles from each other, and can stop by and check it out.

[garysanders]

Hey man,small world eh?I'll check that in a minute and post results.

[Structo]

Yes, the diode and bias filter cap must be backwards in polarity to develop a negative bias voltage.

It depends what your bias tap off of the PT is at voltage wise.
Measure that too without a load.
For example on my D'Lite the tap is 45v.

Also, the 1 ohm resistors from the cathodes must join pins 1 & 8 with the resistor going to ground.

[garysanders]

Now that I just did today and it was 52 +/- .5 Not fluctuating I just forgot the exact reading.

[DonMoose]

But was it MINUS 52.some?

(not to be pedantic about it)

[butwhatif]

if you haven't cooked the tubes into unreliable land already, never put them in until you verified a negative voltage on all pin 5s in the ballpark of at least - 40 volts dc. then verify the +plate(pin 3) and +screen(pin 4) voltages, then insert the tubes

[butwhatif]

Also, the 1 ohm resistors from the cathodes must join pins 1 & 8 with the resistor going to ground.

Pin 8 is the cathode for 6l6GC, pin 1 is not connected to anything in the tube. Pin 1 is used only for EL34 .