Bias mod issue

[Markorock37]

Recently I modded a Peavey VTM 60 head for adjustable bias. Its running too hot even with the pot all the way down. I get one of the tubes just faintly glowing red. I have a 15K resistor feeding a 25K pot then returning to the power board with a 33K resistor. Plate voltage is 475V running 6L6 power tubes. I was following someone else's mod when I did this and it originally called for a 10K pot. What value resistors should I be running to make this work? I rock with a solder gun but not so much on theory.

[Blue Strat]

Assuming it's a standard circuit (listing resistor values doesn't confirm that) increasing the resistor between the pot and ground (33K?, I don't know) is what you need to do.

Hopefully you're taking a measurement to set the bias as opposed to looking at the plates which is worthless.

[Markorock37]

Right. I have a bias probe and an oscilloscope. Guess I will play with a few resistors when I get outta work.

[Blue Strat]

Originally Posted by Markorock37

Right. I have a bias probe and an oscilloscope. Guess I will play with a few resistors when I get outta work.

Forget the scope. The bias probe is all you need. Whichever resistor goes to ground is the one to increase.

BTW, what bias current are you shooting for? The "burning red" comment concerns me.

[dualhumstrat]

I modded my VTM 60 for adjustable bias as well.
I find you need to squeeze every volt out the bias tap, and I still have some
tubes that run too hot to use at all. The bias tap is a bit low on voltage compared to
other PTs.

As I recall, I used something around 2 or 3k for the top resistor, a 47k trim pot
and a 10k resistor to ground. Most of my tubes will bias ok with this arrangement.

Good luck

[donnyjaguar]

There's always a way to get w-a-y more bias voltage than you'll ever need.

[RedRock]

Do you mean like this?

http://www.webphix.com/schematic%20h...1164_schem.pdf

The Princeton taps off of one of the secondary high voltage legs and rectifies
it for negative voltage for the bias circuit. It will give you all the voltage you
want.

[cap47]

If your bias resistor is 33K You should have at least that much resistance in your circuit since the amp design is at 33K. Solder a 15k resistor in series to a 25 K or 50K bias pot. Set the knob to read 33K across the pot and resistor. Solder it in and you are at factory spec before you even start to fine tune the bias.

[dualhumstrat]

Yes, you can always take the bias voltage off the HT, but why go to that much trouble when you can just optimize the existing circuit for maximum range, and then
just select your tubes a little more carefully. Most good tube suppliers can supply tubes that will bias up in whatever range you want.

I am someone who makes all kind of needless changes just for the fun of it, but I
still see no need to change the bias circuit any more than I have already.

I am running EL34s in my VTM, and I can just order 34/4.7 matched pairs and know
that they will work every time.

[Markorock37]

This is what I did right here:

http://www.triodeamplification.com/VTMBIAS.HTML

This amp has basicly been a platform to experiment with.

[dualhumstrat]

Yes, I thought that might be the mod you were referring to.
Those resistor and pot values would probably work well in a lot of amps, but
I don't think they work well in this particular amp because of the relatively low voltage
available in the stock circuit. 40V vs 45 to 55V in most other PTs I've seen, at least.

Give my setup a whirl, and see how that works for you. I find that about 80% of the
tubes I tried worked fine. If you need more bias voltage, then by all means follow the
other posters' suggestion and take the bias voltage off the high voltage winding.
There are lots of examples of this available. Check out a marshall 1987 schematic
for another example.

Good luck

[Markorock37]

__________________

Forget the scope. The bias probe is all you need. Whichever resistor goes to ground is the one to increase.

BTW, what bias current are you shooting for? The "burning red" comment concerns me. __________________

That means red plate. I am only shooting for just a little hotter bias, just want to see what the amp is capable, for all I know it might not be worthwhile but I'm having fun doing it and getting my feet wet.
I'll bias using both methods.

[Markorock37]

Thanks dualhumstrat, I think I can get what I need off the stock location, getting plenty of power just need to cut it down a bit. BTW I am using 6L6's instead of EL34's, would the resistor values be the same as 6L6's need more ma?

[Markorock37]

Can I just change the 33K to 15K (giving me a total w/ pot of 58K)? I'd hate to change out the $10 25K pot I drilled in.

[dualhumstrat]

It's really more about ratios than specific values - you have to look at the circuit as a whole.

You could either make the 33k resistor larger, or make the 15k smaller, or a
combination of both.

Either way, though, I think you'll find the range of adjustment rather narrow
with the 25K pot, but you should be able to find a spot that works for you.