How to install a bias pot in a Rectifier

[mikey383]

You may have popped a tube. Try biasing a little colder maybe?

 

[fluff191]

Yeah I got some replacement fuses and I will try it out tomorrow. I dont think I popped a tube. They are only about 4 hours old!

The reading at the power tubes before the mod was about 25ma. I bumped it up to about 32ma. Man I hope I didnt wreck something.

 

[fluff191]

Well I just got home and turned the amp around. Blown power tube. Which pisses me off cause they are brand new!!!

I went to go pull it out and realized the glass housing was perfectly split in 2!!!!!

Anyone ever seen this before?!

Should have stuck with the Sovtek WXT+'s!!!!!

 

[mikey383]

Wow. I've never seen anything like that. Good thing it wasn't anything to do with the amp though.

 

[Metaltastic]

Semi-random question on this topic - mikey, do you happen to know approximately how much plate dissipation on an average 6L6 comes from having the voltage at -51 volts? (stock setting, I guess) Just curious, thanks for the great tutorial!

 

[fluff191]

I think I have read somewhere its about 45% ball park. I could be wrong, but I remember it being very very cold.

 

[mikey383]

Man, that's a good question. I haven't ran 6L6s in my amp in years, so please bear with me...I'm dusting off my memory.

I took readings on a few different Mesa 6L6s several years ago. IIRC, they varied from about 20 mA to 30 mA. I think the average was around 23mA, so 40-45% would be correct.

Yes, that is very cold. I heard from someone that the reason Mesa, Bogner, and a few other high gain amp manufacturers bias their amps so cold is because of headroom. There is very little chance of pushing the power tubes into distortion when they are set so cold, so the amp stays cleaner longer. I don't find this to be beneficial at all, especially considering how little volume would be gained in that range. IMO, Rectifiers don't sound good with a lot of power tube distortion, but a little bit sounds great.

 

[Metaltastic]

Thanks for that Mikey - 23 mA is cold, but nowhere near as bad as a stock 5150 at least! And I'm pretty certain the reason all these companies bias their amps so cold is to minimize the risk of putting new power tubes in; simply a safety net!

 

[fluff191]

I took a reading on my Recto just before doing the Bias mod and the 6L6's were running at 23ma.....I didn't go nuts and just set everything to 33ma.

 

[domct203]

Anyone ID the bias resistor in a Roadster??

Dom

 

[mikey383]

Anyone ID the bias resistor in a Roadster??

Dom

If I could get some detailed gutshots and a schematic, I could ID it. But there aren't any schematics on the net yet.

 

[halfdriven]

[/quote] I then soldered a length of wire to the standoffs. These then went to the pot. One wire went to the middle lug on the pot, one went to an outside lug. [/quote]

When you solder the wires do you need to solder the third lug to the pot like a volume knob for grounding purposes?

 

[mikey383]

No. In a volume control, you bleed the signal to ground to control the amount of signal that passes. In this case, the pot is controlling voltage...it's purpose is to serve as a variable resistor. Hope that makes sense.

 

[halfdriven]

yup, thank you very much

 

[sfx70]

I want to know about the Roadster as well
The bias is super COLD - I've got 25mV per tube - it should be around 38-40mV...
Anybody?

Anyone ID the bias resistor in a Roadster??

Dom

If I could get some detailed gutshots and a schematic, I could ID it. But there aren't any schematics on the net yet.

 

[andylaughs]

Quick, related question: I have a series I Rectoverb (no bias switch). If I wanted to run EL-34s, would I need to install a bias pot, or could I just switch out a resistor and if so, which one? Thanks.

 

[skoora]

Mikey, were you saying that in the 6L6 setting you had enough sweep for 6L's and 34's. ie. not to bother with the EL34 setting anymore.

Also isn't the current draw setting on the tube dependent on the B+ Plate voltage. Do we need to verify what it is on the amp before setting the bias on the tube?

 

[Anonymous]

How does that 15k resistor work? Would replacing it with a solid wire make the EL34 - 6L6 switch on the back of the amp effectively do nothing?

 

[mikey383]

Quick, related question: I have a series I Rectoverb (no bias switch). If I wanted to run EL-34s, would I need to install a bias pot, or could I just switch out a resistor and if so, which one? Thanks.

I'm not 100% sure, but to run EL34s in a ROV (since they are set up for only 6L6s), you would need to change out the screen resistors also. EL34s take a larger screen resistor...1.5K if I recall, in addition to the bias resistor. 6L6s run something like 500 ohms or less (just off the top of my head). Series II ROVs and Dual/Triple Recs are setup to run either tubes, so they already have 1K5 resistors installed.

Installing a bias pot would be the better option, since you could always go back to 6L6s if you wanted. Whereas if you just change a resistor, you're pretty much stuck using tubes that fall within the bias rating that resistor can supply.

 

[mikey383]

Mikey, were you saying that in the 6L6 setting you had enough sweep for 6L's and 34's. ie. not to bother with the EL34 setting anymore.

Yes. I have plenty of adjustment on my amp in the 6L6 mode to cover most, if not all, types of tubes.

Also isn't the current draw setting on the tube dependent on the B+ Plate voltage. Do we need to verify what it is on the amp before setting the bias on the tube?

Again, yes. Bias is relative to the plate voltage. You'll need to measure that on your amp. Check from pin 3 to ground and use the link to the calculator in the first post. Rectifiers average about 475 volts DC when set on bold and diodes, but due to differences in the transformers (each one is different), you may end up above or below that. Mine puts out 486 plate volts on diodes and bold.

Keep in mind though, you want to check the voltage with whatever settings you use. If you tend to use spongy and tube rec settings, measure your voltage with those settings.