Vibroverb 63 Reissue Bias Pot Mod

[Newguy]

I have a 63 Vibroverb RI and have been looking for the bias adjustment pot mod and have had no luck. Some forums mention it but no links.

In my reading I have seen that the bias is also effected by the trem pots (the way the vibroverb has the vibrato set up...I have read it has speed and intensity pots inside chassis??

If so, if setting bias, what would you set these two internal pots at?

Thanks for any information on the mod, internal vibrato settings if to bias, bias mod, and parts needed.

Thank you

[MWJB]

Bias, as in idle current, should not be set with the tremolo turned on at all. It should be physically turned off at the footswitch (not just turned down). Yes the trem affects the current, but you do not use it to set current.

The bias supply load resistor is mounted on the PCB on the front control panel, a PITA to get at...forget the pot, go somewhere with a good selection of tubes & keep subbing them until you find a pair that run at 30-35mA. With some bias probes, this will probably be the quickest method too. Or, you could record your plate, screen & negative grid voltages (6L6 pins 3, 4 & 5 respectively) and see if you can find a vendor that has some tubes that bias up at around 35mA.

Remember there is more than one way to set idle current, either mod the circuit to suit the tubes, or change the tubes to suit the circuit.

[Newguy]

There is no place that has a good selection of tubes that I know of. I was going to just order online.

They have a Guitar Center an hour away, but I think they have limited supply.

I know of no place that would allow me to bring in amp and try out their tubes? Wish there was though.

I had it apart a few years back as I needed to pull boards to get to one tone pot. It was a pain I guess, but not that big a deal (I am used to working on computers).

Why do you say forget the pot?

It is not that the amp sounds bad. But I just adjusted the bias idle current on a Deluxe Reverb RI last night. That pot is like a tone pot to me...I increased plate voltage a small amount (10vdc), and it made a big difference (warmer, more touch sensitive, at lower volumes which is what I want). More then 440 on the Deluxe started getting distortion way too soon, and just did not sound good.

[Jazz P Bass]

The colder the bias, the higher the B+.
At 440V what you probably "heard" was crossover distortion.
Which does sound bad.

[Newguy]

The colder the bias, the higher the B+.
At 440V what you probably "heard" was crossover distortion.
Which does sound bad.

Yea probably so Bass. I have only read about cross over distortion, but it was distorting, and something did not sound right. Now I know what cross over distortion sounds like..thats for that post Jazz.

[Newguy]

You guys know of any other forums that might have or link to the Vibroverb bias idle current pot addition.

Amp is bone stock. I hate to mess with it, but feel that a bias adjustment pot is not a tone mod....I think it should just have one.

Not sure why they would leave it off? I think it is a tool to sweeten the tone a bit.

[MWJB]

They didn't "leave it off", 6G15 never had one, neither did any Fender until the AA763 BF Fenders.

Ring some tube vendors, tell them what voltages you have, if they can match those voltages to a decent idle current, buy some tubes off them. I say forget the pot because it's a PITA to install & run flying leads to a trim pot mounted internally. When you install tubes at the market rate you learn to make circumstances work in your favour, you don't need a pot, you just need tubes to try, or voltages to give someone who can match tubes to them. Fitting a trim pot is unnecessary & probably the least practical & cost effective route to biasing your amp. Do not buy any tubes from Guitar Centre...unless you just want to fit them & live with them. 440vdc is not high for 6G15 plate voltage, it does not automatically mean that your idle current is cold, or that you have crossover distortion.

The 6G15 RI is still a rare, desirable amp, they hold their value well, apart from the speakers which are crap just like the 60's gold frame Oxfords, I wouldn't mess with it too much.

[Newguy]

OK

What voltages does the tube vendor need?

I take it:

1. Pin 3 (plate Voltage)


Any other voltages they need to know?

Do I need to put 1 ohm resister from pin 8 to chassis ground to get mv (ma) readings?

BTW what vendors do you guys like here?

[MWJB]

They need power tube pin 3 (plate), pin 4 (screen), pin 5 (grid) voltages. You want 30-35mA per tube, tubes matched within 5mA of each other.

Where is "here"? Not that it matters, you will know within 2 minutes of conversation if they can do what you ask..if they can't, move on to someone who can.

[Billy R aka DynaFreak]

There is no reason not to put a bias adj pot in there if thats what you want. Trying to modify the little pcb that holds the speed and intensity pots to hold a trimmer is a possibility, I've done that but its kinda PITA and the traces are very thin and prone to lifting. What works better is to just mount a pot on the chassis and use a standard Fender bias circuit on a little eyelet board and tie in to the downstream end of the bias intensity pot and it works fine.

[Newguy]

Here, I should have said, I live in East Tennessee so the lower 48. By here I meant guys here, where do most deal with from the US (IE tubestore, lord valve, ebay, tube depot)??

Billy, I am having second thoughts about the mod. I would not be able to follow those directions you gave, I would need to know exact locations and part numbers...thus, why I have been looking for a link to (hope with pic even), of someone who has done this mod and documented it. Maybe just easier to do what MWJB says as I may want to sell the amp some day. I think I would have second thoughts about buying any amp with any modification.

If any one happens to see a link to such information, please post. I have been on Google for hours and I am beginning to think there is no such link.

If no one posts a link I will just do what MWJB said and call a tube place with my plate voltages when ready to re-tube.

BTW MWJB, should I take these readings with tubes in or out or both?

[MWJB]

Take the readings with the tubes installed, if you take the tubes out the voltages will skyrocket & not make any sense.

[Newguy]

Take the readings with the tubes installed, if you take the tubes out the voltages will skyrocket & not make any sense.

Thank you

[vavlehead]

The colder the bias, the higher the B+.
At 440V what you probably "heard" was crossover distortion.
Which does sound bad.

IS not B+ constant??? Coming right off the PT...

[MWJB]

No, it fluctuates with current draw, the more current drawn, then the lower the voltage. After you set the idle current, that's the highest that the voltage will be, as you play, the tubes draw more current & pull down the voltage.

For example, I have an amp that runs 535vdc on the plates (to ground), it's cathode biased and at max output, plate current doubles and B+ drops by over 80vdc.