PRRI with Tung-Sol 6V6GT Reissues - Set Bias

[Pete'67]

Hello,

I swear I read through Zillions on Bias-threads. Still a bit confused by screen current, 6V6GT to be a 14 Watt or 12 Watt tube etc.

I just placed a matched pair of new Tung-Sol 6V6GT Reissues into my PRRI. I set the Multimeter to 200mv DC, attached the black cable to the chassis and the red tip to pin 8. Then I adjusted to 23mv from there.
No guitar plugged in, Volume, Reverb and Tremolo at zero.

Sound is good, Tremolo works great. No red-plating so far. I anyway only play at home at moderate volumes.

Is this an appropriate way to go?

Thanks a lot!

 

[dan40]

What is your plate voltage? You need this number to do the math for the proper dissipation. Does this amp come stock with 1 ohm bias resistors on pin 8?

 

[slider313]

The PRRI should run about 410v on the plates. With that in mind, the TS 6V6GT is a 12 watt tube. 70% mpd would be 20 ma. I wouldn't go any higher.

 

[theprofessor]

You probably know of this already, but... I've found Rob Robinette's (robrob) online bias calculator very handy. Just select your tube type, put in your plate voltage, and you have some numbers to use as guidelines. Folks who really know about amps (I'm not one) will say that it's not just about numbers, but about using your ears and eyes as well. But the numbers tell you when you're out of bounds. That said, I'd be surprised if you didn't like the way the tubes sound at about 70% max plate dissipation. If you use 420 plate volts as the number, this would yield 20mA as the max plate dissipation for 12 watt 6V6GT tubes. Or whatever. Depending on your plate voltage.

 

[robrob]

Does the Princeton Reverb Reissue have adjustable bias? The original PR has non-adjustable fixed bias so you can't adjust the bias and you can't measure the bias using voltage at the cathode unless someone has added adjustable bias and cathode resistors.

The original PR was made to just swap tubes and not change the bias. Fender did this because adjusting the bias on bias tremolo amps can keep the tremolo from working properly.

You can measure the bias current using either the output transformer resistance method or the output transformer shunt method, both are detailed here: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

 

[Commodore 64]

You can assume 5-10% of the cathode current you measured is screen current. Cathode current is a sum of screen and plate current. Screen current develops when stray electrons hit the screen on their way to the plate.

If you just bias to 70% using the full cathode current (which assumes all power being dissipated in the plate), then you've built yourself in a factor of safety of 5-10%.

 

[Joey]

Does the Princeton Reverb Reissue have adjustable bias? The original PR has non-adjustable fixed bias so you can't adjust the bias and you can't measure the bias using voltage at the cathode unless someone has added adjustable bias and cathode resistors.

The original PR was made to just swap tubes and not change the bias. Fender did this because adjusting the bias on bias tremolo amps can keep the tremolo from working properly.

You can measure the bias current using either the output transformer resistance method or the output transformer shunt method, both are detailed here: https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Yes the PRRI has adjustable bias which is located behind the Power tube next to the Rectifier tube.

 

[Pete'67]

Thanks a lot! Is there a safe way to measure plate current by myself?

 

[Jeru]

Watch uncle doug's video on YouTube re: biasing double-ended amps.
Stay safe.

 

[Pete'67]

Thanks, Jesu. Will do so.

 

[BobbyZ]

Actually I believe the real reason Princetons didn't get a bias pot is because they were the bottom of the line amp when they came out. Ok the Champs were the bottom. Just practice amps at any rate.
As they came the bias is set by swapping resisters. Leo probably figured no body would wear out the original tubes in the darn things so why waste money on a bias pot.
If you think about it if he thought anyone would really use them as much as we do now he probably wouldn't have used the Champ power transformer.
Don't get me wrong I love my Princeton Reverb. It's got a bias pot now though.

 

[Pete'67]

Ok, I measured plate voltage (Pin3 to ground) with 447 volts. Bias set to 23,0 mv.
This should be fine assuming the TS 6V6 have 14 watts max. output in class1 push-pull.

 

[Commodore 64]

TS 6V6 are a 12 watt tube, and you are running them 14W dissipation and over 400V plate voltage. Both of those numbers are too high for those tubes.

 

[Pete'67]

OK, but that would mean I have to set bias below 21mv which is not within the range of the bias pot.

So far tubes are doing fine without red plating at all. As I'm only playing at living room levels I'll give it a try.

 

[strat a various]

OK, but that would mean I have to set bias below 21mv which is not within the range of the bias pot.

So far tubes are doing fine without red plating at all. As I'm only playing at living room levels I'll give it a try.

How is your Vibrato working? If it sounds weak or faint, your power tubes are biased way too hot and you will probably burn out your output tranny before the tubes fail. The Vibrato should sound deep and three dimensional. If need be, compare it to a PRRI in a store, whatever it takes. If the Vibrato is at all iffy, you have a major power tube problem that will wreck some kind of havoc one way or the other.

 

[Pete'67]

Vibrato sounds fine. As usual only starts sounding deep after 7. But sounds similar to when the amp was brand-new.

The initial power tubes (Groove Tubes) were biased around 25mv.

 

[Wally]

IMHO, it would be of interest to be able to run a lower current draw so that you could observe how that would affect the trem function. If you do not change the circuit to accomplish this, then I would suggest that the next ime you buy some power tubes that you buy 'cooler' tubes so that you could adjust for a lower current draw, compare, and make your choice.
Redplating will not occur until the current draw is quite a bit higher than it is now, ime. You could do an investigation of how the biasing affects the trem by listening to the trem while you increase the current draw. I suspect that it will weaken. I actually suspect that it is weaker than it could be as it is...and as you bought it. I would have to do this experiment just to assuage my curiosity.

 

[Commodore 64]

I've got a regular gigger using an amp with TS RI 6V6. Biased at 90% (it's cathode biased) and 340V plate. It's going on 2 years. So I'm a fan of the TS 6V6 RI. Here's your data sheet: http://www.tungsol.com/specs/6v6gt-tung-sol.pdf

447V plate voltage on a 6V6GT. Wow. Design Center is 315V plate, though we typically run tubes in guitar amps past that. I consider 375 high, though I know Fender runs 'em hotter in the DRRI.

What's your screen voltage? Design center is 285V, again we typically exceed design centers so that number has plenty of wiggle room. Max dissipation on your screen is 2W according to datasheet. If your screen is anywhere near 447, you want your screen current <4.5 mA. Conservatively assume 10% of cathode current, and you are at 2.3mA. So I think you are OK there.

If I were in your shoes:

1. I wouldn't be unwilling to lower my plate voltage. You've got a 5AR4 in there. Try a 5V4. It'll handle the current draw and drop some more voltage, and still only draw 2A heater current.
2. But if I were unwilling to lower my plate voltage, I'd keep a set of JJs or EH on standby. And be prepared to replace my iron in the case of a catastrophic tube failure.

You are currently biased at 86% (a tad less if you assume 10% screen current).

 

[fender4life]

You might wanna use JJ 6V6's with that kind of plate voltage. They are good to 500v. They are said to sound more like a 6L6, but i find them to sound more like something in between the 2 and with a tighter low end than other 6V6. Very good tubes tho, and one of the few JJ's i really like.

 

[Bendyha]

Ok, I measured plate voltage (Pin3 to ground) with 447 volts. Bias set to 23,0 mv.
This should be fine assuming the TS 6V6 have 14 watts max. output in class1 push-pull.

No such thing as class 1................ AB1 I can live with.
I'd say as you have done it is fine ! Its sounding good, not red-plating, sure, your tubes might last 8 years instead of 12 at at a cooler bias, but hell, I wouldn't marry a plain girl purely for the advantage of knowing I will be safely stuck with her for the next 50 years.