Power tube matching question

msk011

Member
Messages
72
Hi folks,

I just picked up a deluxe reverb reissue. I have one of the bias testers that goes between the tube and the amp and I read the milliamps with my digital meter. So I stick the tester under the first tube and it reads about 17.5 mA. I've read that these amps like about 25 mA so I turn the pot. Only problem is that the pot bottoms out and the tube is still only about 21.5 mA.
So I switch it over and the other powertube is running about 24.5 mA which is closer to what wanted but I didn't think I would have to push the pot all the way to get there.
questions:
Should I have bottomed out the bias pot just to get them to these readings?
and
How close should power tubes be matched?
and
could I have a problem with my amp? It's new and has the Groove tubes (relabled electroharmonix) 6V6s still in it.

The amp sounds fine by the way I should probably just stop messing with it but I'm curious about the tubes and bias pot now.

Thanks,

Mike.
 

rockon1

Platinum Supporting Member
Messages
12,980
To know what they should be set at you need to know the plate voltage. Divide the max dissipation in watts of the 6V6 by the plate voltage. The 6V6 is a 12 watt tube. 12 /plate voltage = ma max dissipation. Then you want to set it at roughly 60% of max dissapation. ma max dissapation X .6 = idle dissipation ma you set it at.

As far as the discrepancy between the two tubes you can swap the positions and see if the readings follow the tube. If they do then its just a not closely matched pair of tubes. Bob
 

slider313

Gold Supporting Member
Messages
8,210
21.5 and 24.5 are a close enough match and are within range for that amp. I would leave it there if it sounds good.
 

rockon1

Platinum Supporting Member
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12,980
21.5 and 24.5 are a close enough match and are within range for that amp. I would leave it there if it sounds good.

I didnt see it was 21.5 and 24.5 which isnt bad.(thought it was 17.5 and 24.5) I wouldnt worry about that amount of difference. Still its hard to say whether the average between them of 23.ma is correct without knowing the plate voltage. If the plate voltage is around 300 volts is at around 60% which is nice. If its 350V then its around 70% max dissipation-probaly hotter than necessary. If its 400+ volts on the plates than its running at 80+% max and is way hot. Bob
 

slider313

Gold Supporting Member
Messages
8,210
I believe most decent 6V6 tubes being produced today are actually up to spec and on par with a 6V6GTA, which has a max plate dissapation of 14 watts. 70% of 14 watts at 410 volts is 23.9 ma
 

rockon1

Platinum Supporting Member
Messages
12,980
I believe most decent 6V6 tubes being produced today are actually up to spec and on par with a 6V6GTA, which has a max plate dissapation of 14 watts. 70% of 14 watts at 410 volts is 23.9 ma

Well I dont know about that. IMO its better to err on the side of caution. Theres no sense running them hotter than needed either. Ive done a lot of reading on the subject and come to the conclusion 70% isnt a magic number and that lower is (if it sounds good) is a better bet. I guess the discussion is moot since we dont know the PV.:) Bob
 

msk011

Member
Messages
72
To know what they should be set at you need to know the plate voltage. Divide the max dissipation in watts of the 6V6 by the plate voltage. The 6V6 is a 12 watt tube. 12 /plate voltage = ma max dissipation. Then you want to set it at roughly 60% of max dissapation. ma max dissapation X .6 = idle dissipation ma you set it at.

As far as the discrepancy between the two tubes you can swap the positions and see if the readings follow the tube. If they do then its just a not closely matched pair of tubes. Bob
Hi Bob thanks for the reply.

Will a not closely matched set of tubes damage the amp? I decided to try this bassed on a video I watched online of a well know tube seller (JJs) biasing a DRRI. He didn't seem to have any problem getting both tubes right up to 25 mA so I thought I would try mine there. I was suprised when I couldn't turn the pot any longer and I was "only" at 21.5 mA. Just the difference between JJ 6V6s and Electro harmonix?
 

msk011

Member
Messages
72
Overall it sounds like I should probably back the thing off a little from the ~24 mA I have on the one tube.

Thanks for the replies, very helpfull.

Mike.
 

rockon1

Platinum Supporting Member
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12,980
Hi Bob thanks for the reply.

Will a not closely matched set of tubes damage the amp? I decided to try this bassed on a video I watched online of a well know tube seller (JJs) biasing a DRRI. He didn't seem to have any problem getting both tubes right up to 25 mA so I thought I would try mine there. I was suprised when I couldn't turn the pot any longer and I was "only" at 21.5 mA. Just the difference between JJ 6V6s and Electro harmonix?
Slightly mismatched wont hurt anything. JJ's from what Ive read are about the stoutest 6V6 out there. If there was a 6V6 I'd push it would be that one. They may draw more current than the EH typically but I dont know for sure. If you measured the plate voltage you can set them for whatever idle dissapation you want. I'd say back them down to an average between the two of 20-21 ma if your not going to calculate the actual idle bias. Bob
 

Blue Strat

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30,152
Anything within 5mA is more than close enough. Once you get past 10mA you're getting into seriously mismatched territory though you won't damage the amp unless one tube idles much higher than where you are now and the tube shorts out.

You're well within proper operation. I'd leave them where they are, or lower, and start playing;)
 

rockon1

Platinum Supporting Member
Messages
12,980
Anything within 5mA is more than close enough. Once you get past 10mA you're getting into seriously mismatched territory though you won't damage the amp unless one tube idles much higher than where you are now and the tube shorts out.

You're well within proper operation. I'd leave them where they are, or lower, and start playing;)

Mike you know way more about this stuff than me . Is the 6V6 a 14 watt tube? Im asking because if its a 12 watt tube and the plate voltage is 400 then at 23 ma it would be running too hot right(over 70% )? Or is my math wrong? :confused:Bob
 

Blue Strat

Member
Messages
30,152
Mike you know way more about this stuff than me . Is the 6V6 a 14 watt tube? Im asking because if its a 12 watt tube and the plate voltage is 400 then at 23 ma it would be running too hot right(over 70% )? Or is my math wrong? :confused:Bob
NOS 6V6GTA's are 14 watt tubes. I would assume 12 watts for new production tubes because new tubes almost NEVER meet the spec's of the tubes they're trying to emulate.

You can't go wrong by being conservative. I use 60%. 70% is something out of the "more is better" (and usually wrong) book. ;)
 

rockon1

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12,980
Did a bit of digging ... The plate voltages seem to be around 400-410 volts on the reissue. Seems like most guys set them very hot. Bob
 

slider313

Gold Supporting Member
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8,210
I beg to differ and say the better made 6V6's are closer in spec or higher than a NOS 6V6GTA. The EH is good for 475 volts, so they say. I have run these at 25ma at 425 volts in a Deluxe Reverb with no problems of red plating or failure. The Tung Sol 6V6 is a stouter tube and is very happy at 25ma at 425 volts. The JJ is the most stout 6V6 I have ever seen and will run at 25-30ma in my Vibrolux Reverb at 440 volts.
 

Blue Strat

Member
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30,152
I beg to differ and say the better made 6V6's are closer in spec or higher than a NOS 6V6GTA. The EH is good for 475 volts, so they say. I have run these at 25ma at 425 volts in a Deluxe Reverb with no problems of red plating or failure. The Tung Sol 6V6 is a stouter tube and is very happy at 25ma at 425 volts. The JJ is the most stout 6V6 I have ever seen and will run at 25-30ma in my Vibrolux Reverb at 440 volts.
There are no BETTER 6V6GTs than NOS, period.

The EH MARKETING MATERIAL says they're ok to 475, does that make it true? I know techs all over the country who report numerous failures of EHs in low plate voltage amps like Princetons. Talk is cheap. It's all a matter of whether you believe the ads or real life empirical evidence.

Many professional amp techs call these tubes "fire crackers".

1, 10, or 1000 tubes is not indicative of the 100s of thousands that have been made.
 

rockon1

Platinum Supporting Member
Messages
12,980
I beg to differ and say the better made 6V6's are closer in spec or higher than a NOS 6V6GTA. The EH is good for 475 volts, so they say. I have run these at 25ma at 425 volts in a Deluxe Reverb with no problems of red plating or failure. The Tung Sol 6V6 is a stouter tube and is very happy at 25ma at 425 volts. The JJ is the most stout 6V6 I have ever seen and will run at 25-30ma in my Vibrolux Reverb at 440 volts.

Actually it seems you are agreeing with me when I stated that it seems most guys like to run the Deluxe hot. If your running them at 25 ma at 425 volts even assuming they are a 14 watt tube(which considering what Ive read about them is a bit scary)thats still 75% idle dissapation. If it works for you thats cool but still considered hot. I agree that from all acounts the JJ is amost a different breed-more like a 6L6 in 6V6 clothing.Bob
 

msk011

Member
Messages
72
Thanks for the replies. It is informative if not a little confusing. To tell you the truth I didn't really hear much difference between the old and new settings so I turned it down a bit and I'm just gona play it as Mike K said.

Thanks again

Mike
 

slider313

Gold Supporting Member
Messages
8,210
I find at 60% the amp sounds a bit anemic; the bass is not clear, the chords don't have the note seperation and sound a bit like mush and the highs are not snappy. At 70%-75% everything seems to be there. Is it running a bit hot for a 6V6GT(A)? Sure, but doesn't a champ run them at 40ma? And the 5E3 with 350+ volts runs them at 35ma. or so? I guess it depends what your after, good tone or longer tube life.
 

Blue Strat

Member
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30,152
Sure, but doesn't a champ run them at 40ma? And the 5E3 with 350+ volts runs them at 35ma. or so? I guess it depends what your after, good tone or longer tube life.
Both of those amps are cathode biased with lower plate to cathode voltage, so using them as a comparison is not valid.

Go with what sounds best to you, but be aware of the possible consequences (and don't hold your tube dealer responsible if you choose to abuse your tubes). ;)
 

Lonely Raven

Member
Messages
1,003
I played JJ 6V6 in my Super Reverbs for a few years. No issues. Perfectly fine tube life. I've also used the JJ 6V6 in my Peters Dual Channel, but not for long as I prefer the sound of the JJ KT-77.

The JJ 6V6 have been able to handle pretty much any 6L6 amp I've put them into. I can't say the same for other current production tubes.
 




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