Bias settings and tube life for 6V6's

[Mr Songwriter]

I've just been trying out different bias settings on my DRRI (it has two Groove Tubes 6V6's) and based on what I've read, the maximum plate dissipation for them is 12W, and the general recommendation for Class AB amps is 70% of that number, i.e. about 9W, my question is, if you were to run those tubes at or around 12W, how much would it shorten the life of the tube by? would it halve it? quarter or it or what?

[tommythelurker]

At 100% you've got worse things to worry about than tube life It wouldn't even be class AB anymore, that's class A and probably drawing too much current for the transformers to handle unless they're significantly over-spec'd.

[Mr Songwriter]

At 100% you've got worse things to worry about than tube life It wouldn't even be class AB anymore, that's class A and probably drawing too much current for the transformers to handle unless they're significantly over-spec'd.

 

Yes, I've read around the subject a bit more now and it doesn't seem like a very good idea - there may even be a risk of melting some of the wires in the tubes. I didn't know the bit about the transformers though, what would be the risk there? could you end up overheating the windings and damaging them?

 

Having said that, there's a video on Youtube with someone showing how to rebias a DRRI, and he uses a chart (at 3.30 in the video) that says 6V6's have a max diss of 14W (and not the 12W figure that I've read elsewhere) he even says that he likes to bias it 'towards Class A' and ended up setting it to 27mA, which for a Plate Voltage of 410 - 430V could mean he was running it at 82%. I'll have to see and find what figure Groove Tubes quote for Max diss.

 

[YOUTUBE]JB1pZa0Vj4I[/YOUTUBE]

[msormune]

You can bias up to 85% without much shortening the tube life by cooking them. But 75% is the usual and generally accepted number. You could probably try values between 65% - 80% and listen if you hear any difference. If not, go with lower value.

[Mr Songwriter]

 

You can bias up to 85% without much shortening the tube life by cooking them. But 75% is the usual and generally accepted number. You could probably try values between 65% - 80% and listen if you hear any difference. If not, go with lower value.

 

 

Well I've set the bias to 22.4 now (which is either 68% or 80% depending on what number you choose to believe for Max dissipation, I've been told they're Sovteks, but I can't find a figure) and they sounded better than the stock bias (about 19mA) so I'll probably just leave them at that.

[Duesentrieb]

Bias them to 10 Watts.

 

10Watt/plate voltage = current in Amp

[Duesentrieb]

btw - which brand? With high plate voltage 'd use JJs 6V6S !

[BluesMan60]

Be careful with Sovteks. These are known as "firecrackers" by amp techs across the US. I'd keep them at or below 20mA in a DR.

 

70% is really too high in most cases for ANY power tube but it DR's it seems almost unavoidable. I usually recommend low 20s.

[PlinytheWelder]

This is what the Russian tubes look like when they go bad...

Not my video...

 

Reflektor brand 6V6 tubes did that in my Crate Stealth at 415 plate volts.

They don't seem to like the high voltage.

 

I thought that I had broken the amp, but after inserting some NOS tubes

the fireworks were gone and the amp was OK. Since I don't have a full set of NOS tubes,

I got some JJ's and I'm going to put them in today and bias them up.

 

The JJ 6V6 data sheet specs max wattage at 14W. Most 6V6 's spec out at

12W. Then there's the Bendix Red Bank 5992 which can probably do even more.

 

When in doubt, check with your vendor or ask around here.

 

[YOUTUBE][/YOUTUBE]

[Baron55]

In fixed biased amps 70% is usually considered the max bias at idle.

 

Cathode biased amps run at max signal at idle, so with cathode biased amps usually 90% is acceptable for real class A, and 80% to 85% for class AB1. In most common guitar amp circuits, cathode biased amps will self regulate plate current, in many cases plate current will drop when the amp is at full power. In fixed biased amps usually the opposite happens and plate current rises with output. So fixed bias amps are biased about 70% max so when the amp is cranked the maximum plate dissipation isn't exceeded.

[Baron55]

 

 

 

 

There are 2 types of 6V6 tubes.

 

6V6 = 12 watts

6V6GT = 14 watts

 

The JJ and Electro harmonix 6V6's are 14 watt tubes. The JJ's can handle 500v on the plates and 450v on the screens.

 

The Electro Harmonix can handle 450 on the plates and 350v on the screens.

 

 

NOS 6V6 tubes usually can't handle more than 300 volts on the plates. But due to the ruggedness and quality, most people got away with it.

 

In most AB1 push pull amps, the Screen voltage is is very close to the same value as the plate voltage, most tube failure is due to exceeding screen voltage and current ratings. Plates can handle higher voltages much better than the screens, screens are the most fragile part of the tube.

[BluesMan60]

 

There are 2 types of 6V6 tubes. 6V6 = 12 watts 6V6GT = 14 watts The JJ and Electro harmonix 6V6's are 14 watt tubes. The JJ's can handle 500v on the plates and 450v on the screens. The Electro Harmonix can handle 450 on the plates and 350v on the screens. NOS 6V6 tubes usually can't handle more than 300 volts on the plates. But due to the ruggedness and quality, most people got away with it. In most AB1 push pull amps, the Screen voltage is is very close to the same value as the plate voltage, most tube failure is due to exceeding screen voltage and current ratings. Plates can handle higher voltages much better than the screens, screens are the most fragile part of the tube.

 

 

Actually its like this:

6V6G, 6V6GT = 12 watts

6V6GTA = 14 watts

 

The EH tube specs are marketing hype. Besides, there are few or zero guitar amps that run the screens any lower than virtual plate voltage...maybe a volt or two tops.

 

EHs have been known to self destruct in some of the least abusive amps like Princetons and Princeton Reverbs.

[tommythelurker]

Cathode biased amps run at max signal at idle

 

I know what you're trying to say, but it came out completely wrong

[Baron55]

For 6V6 tone for higher voltage amps, the JJ's are the only way to go. They really aren't 6V6's but more of some smaller 6L6 hybrid. and they can take a beating.

[iansmitchell]

Tubes can take way more than people credit 'em.

I know a dude who put 6V6s, EL34s, 6L6s, and KT66s in his twin reverb. He's never rebaised it. He says he changes tubes about once every 8 months, and he cranks the thing daily. He's been doing it on a 1966 twin reverb since the day he bought it.

 

Same with speakers, he says he's tried V30s, alnico blues, greenbacks, and (currently) eminence patriots in it.

Not blown a single speaker, still has the originals.

 

The man is insane, but his amp doesn't seem worse for wear(other than coffe and nicotine(?) stains on the grill cloth).

 

Seriously, unless you seriously think they can't make amps or tubes liek they used to, don't worry so much, particularly when you really ARE rebiasing.

[jasonguitar]

 

There are 2 types of 6V6 tubes. 6V6 = 12 watts 6V6GT = 14 watts The JJ and Electro harmonix 6V6's are 14 watt tubes. The JJ's can handle 500v on the plates and 450v on the screens. The Electro Harmonix can handle 450 on the plates and 350v on the screens. NOS 6V6 tubes usually can't handle more than 300 volts on the plates. But due to the ruggedness and quality, most people got away with it. In most AB1 push pull amps, the Screen voltage is is very close to the same value as the plate voltage, most tube failure is due to exceeding screen voltage and current ratings. Plates can handle higher voltages much better than the screens, screens are the most fragile part of the tube.

 

 

Retubing a 6G2 Brown Princeton and had the amp sounding terrific with max headroom and about 12watts static dissipation for the EH6v6s but I noticed the screen voltage was over 360v so I changed the bias resistor and got it down below 350 but now the amp has lost the headroom and sounds good but not nearly as good. Any way to tame that screen voltage and get the bias up? Screen resistors?