Peavey Duel 212 problem

[DrF]

The previous owner had 5881 tubes in it and it was overheating big time! I installed the correct 6L6's... but now I get distorting sounds big time (very loud with out playing it!).
What happened? How do I fix it? Did the 5881 tubes burn out anything?
TX

[Amp Kat]

Do you know what the bias is set at. Take one of the tubes out and use a DMM and put the black lead on chassis and check pin 5 of the socket. It should be about -55 VDC. Be very careful as a few of the other pins will have high voltage that could be lethal. Most of the sockets are marked with the numbers on the outside and some of the tubes are too.

[Enzo]

5881s are just a variant of 6L6, especially in current model tubes. They will work fine in that amp. They like any other tube would expect proper bias levels. If the amp doesn;t work right, then it doesn't matter which tubes it gets.

Check the bias supply for excess ripple and for level like KB said. I'd be checking for burnt out screen resistors as well.

[DrF]

Duel 212 overheating

Thank you for you prompt and kind reply.
So if the 5881 is a variant of the 6L6's, I can use either. But, since the Peavey is fixed bias... why was the chassis and facelpate so very hot?? (there is no pot to adjust teh biasing) AND, how do I adjust the bias exactly on the amp? The tubes are 'attached' to a PCB. how is the resistor altered, and which way do I go with a resistor to cool it down? (larger or smaller)

The 6L6's produce a great deal of 'noise' sounds, with out any input.

Your help is VERY appreciated!!

[CharlieP]

Thank you for you prompt and kind reply.
So if the 5881 is a variant of the 6L6's, I can use either. But, since the Peavey is fixed bias... why was the chassis and facelpate so very hot?? (there is no pot to adjust teh biasing) AND, how do I adjust the bias exactly on the amp? The tubes are 'attached' to a PCB. how is the resistor altered, and which way do I go with a resistor to cool it down? (larger or smaller)

The 6L6's produce a great deal of 'noise' sounds, with out any input.

Your help is VERY appreciated!!

Increasing resistance cools it down. You could get a bias pot and maybe a 15k resistor. Solder resistor in series with the pot, set the resistance of the pot and resistor in series at the schematic bias level so you are somewhere in the ballpark. Then when you install in place of the bias resistor you can adjust where you want it. See my VK112 Bias Pot Mod in the Mods area of this forum for an example.

[Enzo]

Why is the amp hot? I suspect if you put 6L6s in there it would still be hot.

The thing to do is measure the current through the tubes and check the B+, then calculate the dissipation. Plenty of amps get darn hot as a normal part of operating.

You can;t just stick a pot in these, the top resistor in the voltage divider is only 470 ohms. R90. This supply is a voltage doubler and the spec'd -50v is pretty much its max. You'd have to remake the bias supply to increase that voltage any.

Power tubes don;t as a rule introduce much noise of their own. Are you sure it is not coming from a preamp tube? Noisy power tubes are usually bad power tubes.

The tubes are in sockets, the sockets are on circuit boards. The bias supply is on the main board anyway, the tubes are all on separate tube socket boards.

[DrF]

B+ & dissapation

Thank you. As I mentioned I am a 'newbie'... how do I go about checking the B+ and dissipation? And the remake the bias supply??? What does the bias supply on the PCB look like?? What am I to look for?

Thank you! Sorry I'm so new to all this!

[DrF]

Thanks! I looked and could not locate the schematic you posted in mods.

[CharlieP]

Thanks! I looked and could not locate the schematic you posted in mods.

It wasn't a schematic, it was pictures of what I had done to the VK112. According to Enzo it would not apply to your application. Something is different with your amps design. Your VDC should be set to -55 on pin 5. As to how that is done someone will have to help you. Maybe something is wrong if your VDC is not right.

[Enzo]

DrF, I think you might need to enlist th help from someone local to you. To do what you need, you need a good voltmeter and a "bias probe" of some sort, and the knowledge of how to use them.

You need to measure the B+ - the high voltage, about 450-500 volts - at the tubes, then measure the current through the tubes with the probe. Dissipated power is voltage times current, so we do the arithmetic.

If it is excessive, we first explore the circuit to determine if both tubes are equally overbiased or just one. We also determine if something like a leaky coupling cap is allowing unwanted DC on the tube grids.

To make tubes run cooler, we need to increase the negative voltage on the power tube grids. The negative power supply in there is already producing the most it can. SO if we need to increase that voltage, we then have to recreate the bias power supply in some new form.

The popwer supply is not a separate thing, it is just a circuit on the main board. On the end of the main board where the fuses are, there is a collection of capacitors - those cylindrical things. SOme of them are involved in the bias power supply.

SO I don;t mean it unkindly, but that is a whole lot of stuff to have to explain in a manner you can follow with limited experience.

Add to that that the voltages the amp runs on can KILL YOU, and I an hesitant to tell you to dig in where you don;t know what you are looking at.

Charlies conversion photos are worth looking at, very neat work.

[DrF]

thanks...still need help tho'

Hi! Thanks... there is no one locally who can assist me. I'm over 60yr, with a Phd. in Chemistry... and am very safety minded. I've already done 'cap jobs', changed resistors... just trying to learn more. When I retired from teaching at the University, I took up electronics a while ago. Love it.
I do appreciate all the help you guys give here.

[CharlieP]

Hi! Thanks... there is no one locally who can assist me. I'm over 60yr, with a Phd. in Chemistry... and am very safety minded. I've already done 'cap jobs', changed resistors... just trying to learn more. When I retired from teaching at the University, I took up electronics a while ago. Love it.
I do appreciate all the help you guys give here.

I'm going to be 62 in Oct if I don't electrocute myself. I have a PHD in hard knocks. I really enjoy tinkering with my amps and I am always careful to discharge my caps and also to be mindful if I am exploring live voltages. I think I enjoy modding and fixing almost as much as playing my guitar.

[NickS2319]

For those interested:

DrF is Dr. Fret, a man who poses to be a guitar and amp technician in Youngstown Ohio. The Duel 212 is mine, I gave it to him to fix. He put in the 5881's, it fried, and he tried to get me to pay for his mistake.

YOU CANNOT PUT 5881's IN THIS AMP.

The reason is, the plate voltage in the Peavey Duel 212 is 480 volts. The max voltage for a 5881 is 360. Hence why he fried it. A regular 6L6's max voltage is 500 volts. The voltage is so high because there are only 4 power tubes in it.

This was explained to me by a real Peavey technician, who I am now taking my amp to to be fixed.

And yes, I am pissed.

[Gtr_tech]

Bah....they were prolly Sovtek wafer base "5881"s....which are not a real 5881. Way heavier. http://www.pulseonline.com/Sovtek/5881-6L6WGC.jpg
Those guys can take whatever you throw at them plate voltage wise. But they still need to be biased to a safe operating current. Those seem to sound best in my power supply as pass regulators

Real 5881s look like this:
http://www.kcanostubes.com/pictures/...ched-Pair).jpg
You can't use those in there......

[NickS2319]

Bah....they were prolly Sovtek wafer base "5881"s....which are not a real 5881. Way heavier. http://www.pulseonline.com/Sovtek/5881-6L6WGC.jpg
Those guys can take whatever you throw at them plate voltage wise. But they still need to be biased to a safe operating current. Those seem to sound best in my power supply as pass regulators

Real 5881s look like this:
http://www.kcanostubes.com/pictures/...ched-Pair).jpg
You can't use those in there......

I think you're exactly right, it had Sovtek's in it beforehand, but they didn't have any markings on them.