Bias mod Peavey VTM 60

[gbono]

Just put a VTM 60 back together and I'm reading plate voltages of 480V with a cathode current of about 44mA. If I'm not wrong this is over 70% of the maximum power dissipation for a 6L6. I need to modify the bias circuit on this amp and plan to replace R10 with a 25K trim pot along with a 33K ohm resistor then replace R9 with a 15K resistor. Any comments or better schemes?

[g1]

Are those component designations from the schematic you posted? Where you say R10, I think you mean R8?
And R9 is showing 470R, I don't think you would raise it to 15K.

[gbono]

Yes it should be R8 not R10. Leave R9 as 470 ohms?

[g1]

You are looking to increase the negative voltage, right? So if anything you would have to decrease the 470. But first I would just try increasing R8 (using pot in series with resistor). I think you would want to go higher than just 25K + 33K as it was 47K originally. Maybe try 33K and 50K pot.

[gbono]

Do I have this wrong but isn't there a voltage divider formed by R9/8?

[Enzo]

480v times 0.044 amps gets me 21 watts, which is exactly 70% of 30 watts for the 6L6.

Yes, R8 and R9 for a voltage divider. But look at it. You have 470 ohms over 47k, a 1/100 divider. That means you are already at 99% of the supply voltage. You can do whatever you want with the 470 ohm resistor, but you will never get the voltage more than that 1% higher. So if you have -50v now, you could dead short the 470 and still have maybe -50.5v. Not wrth the effort.

Most guys want to run the amp hotter, meaning lower bias voltage. Now for that you WOULD want to increase the 470 to 1500-3000 or whatever floats your boat. That way the upper leg of the divider has headroom to well, divide.

If you replace the 470 with 15k and the resistor/pot at 58k, then assuming even 60v at the supply, your 15k will right off the top take about 12v, leaving 48v. The pot would adjust LOWER from there. Hardly what you want.

Unless you want to add a voltage doubler or a new bias supply altogether, you are not going to increase your bias voltage.

If it were mine, I'd button it up and just enjoy the amp.

[gbono]

THX Enzo - I usually bias tube amps below 70% PD but I'll try the amp as it's biased.
BTW the amp came with Ruby's and I found one of the 6L6's with 72 ohms between control and screen grids - internal short.

[tedmich]

of course there's this guy...
https://www.flickr.com/photos/3rdof3...7627999774416/

[g1]

Thanks Enzo, I should have figured that there were no gains to be made as far as increasing the negative voltage but was too lazy to do the math.

Gbono, there is enough variation in power tubes that if you have other sets, you may find some that bias up colder.
Otherwise, next time you buy some, try to get them from a vendor that can choose ones that run colder than average.

[gbono]

I have some other 6L6's that I will try in the amp - the pair in there now are Sovtek. It looks like the mods that are out there, for this amp, will just give a lower grid voltage and run the tubes hotter - why?

[g1]

Some people like to run them beyond 70%, also it's possible that the "average" tubes out there are biasing up colder than yours. And on that note, it would be good to verify your bias cap is good, although I would expect hum if not. Maybe your amp should be running cooler and has a problem, as I'm more used to seeing Peavey's bias up cold.
Also, there is a general misconception out there that hotter bias always = better sound, and any hint of crossover distortion is a bad thing.

[gbono]

I had to replace the supply caps on this amp since I found several that had dried up and/or leaked. I'm not a fan of running "modern" tubes hotter than necessary - considering the lack of quality control of materials and construction.

[Enzo]

Most peavey amps are born with pretty cool bias levels. SOme sets of tubes naturally run hotter than average, but PV purposely designs the amps to run cool. That is why some people like to warm them up.

[Mick Bailey]

If you're measuring cathode current don't forget this includes screen current, so the plate dissipation is slightly lower anyhow.