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Post subject: Twin Reverb Bias issue with new 6L6's
Posted: Thu May 02, 2013 7:02 pm
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Hey guys, I got my matched quad of TADs today. My Silverface is fully blackfaced.
When I had the Groove Tube 6L6GE's in there, I was able to adjust the bias to through a good suitable range.
I have 1 ohm resistors from pin #8 to gnd on the output tubes. After adjusting to optimum, when I measured across these with my old tubes(not matched) I got readings ranging from 30.5 - 46.5mV. I can't remember how many volts on pin #3, but my static dissapation wattage was around 21W on the 46.5mV tube(Within spec, anyway). I still had room to go higher on the bias pot.
I swapped out the GT's for my new TAD 6L6GC-STR quad. Now, I have the bias pot maxed out and all I can get is 28.2mV.(All 4 tubes measure within 1.4mV of each other, MATCHED!). My voltage on pin #3 is 451 and static dissapation wattage is only 12W!
In the instructions from my blackface kit he talks about the silverface bias pot resistor being 15K(mine is), and a blackface having a 27K. He makes it sound like 15K is the be all end all to adjust for any tubes.
Can I swap out the 15K resistor for a higher or lower value to fix my problem? If so, what should I try? 10K, 27K, 33K?
Thanks a lot.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Thu May 02, 2013 7:53 pm
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Go lower for hotter bias. Try the 10K. However, I have several silverface amps with the bias balance pot and use GT 6L6GE. They alway bias around 26 ma with the bias pot fully counter-clockwise. I've also used Tung-Sol 6l6GC-STR with the same result. You should be within range with a 15K resistor. Are you TAD's rated for average distortion? I've had major problems with power tubes lately.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Thu May 02, 2013 9:18 pm
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Krankster wrote:
Go lower for hotter bias. Try the 10K.


That's what I was thinkin. Luckily, I had a 10K resistor available from my blackface conversion. I swapped it out for the 15K and I'm good to go now. I've got it set at about 34mV, dissapation is about 15W and it sounds GREAT!

These TAD tubes have a nice blue glow to them. They seem to be a little noisier at idle than my GT's. I have a pair of Telefunkens in V2 and V4. A Mullard 4024 in V3 and a JAN phillips 12AT7WC in V6. It sounds and plays unreal! Love it!

Thanks Krankster.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 03, 2013 2:49 am
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TAD are selected Chinese output tubes. Being Chinese of origin, plus your description of purple haze --- they may need burn-in. Pull the 12AT7 phase inverter tube. Turn the bias pot full CCW. And let the TADs idle until the purple glow dies down.

Be sure the speaker(s) are connected and the STANDBY is to the "amp on" position.

If you clip the DVM leads onto the 1-ohm resistor, you may see the bias change, as the impurities burn off.

You should see some improventment in current flow witnin an hour or so. If not, may have a bad batch of tubes. That tests well, but doesn't draw adequate current.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 03, 2013 7:21 am
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BMW2002Ti wrote:
TAD are selected Chinese output tubes. Being Chinese of origin, plus your description of purple haze --- they may need burn-in. Pull the 12AT7 phase inverter tube. Turn the bias pot full CCW. And let the TADs idle until the purple glow dies down.

Be sure the speaker(s) are connected and the STANDBY is to the "amp on" position.

If you clip the DVM leads onto the 1-ohm resistor, you may see the bias change, as the impurities burn off.

You should see some improventment in current flow witnin an hour or so. If not, may have a bad batch of tubes. That tests well, but doesn't draw adequate current.

These TAD tubes are cryo-treated and they claim to burn them in for 24 hours after the cryo-treatment. From what I've read, the purple-haze is normal in some tubes and not something to worry about. By turn the bias pot all the way CCW, do you mean all the way "hot" or "cold"? I'm new to the tube amp world, so any input is appreciated.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 03, 2013 10:15 am
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Purple haze is contaminates burning off. It won't hurt the amp --- but an excessive amount may effect the current flow through the tubes. Twenty-four hours isn't a whole lotta time for most Chinese made output tubes. Many take a few days or more under amp-like conditions.

I've heard of folks with new Shuguang made tubes (Chinese manufacturer of many TAD tubes) taking days, if not weeks to finally settle into a nice, consistent bias.

As for cyro treatment... there has been a bit of controversy about doing this on output tubes. The heavier metal construction of these tubes, versus gain stage tubes, may make them more susceptible to unknown changes from very cold temps. Some ppl, on other boards, have commented on issues with treated power tubes. Verdict is still out on this.

Full CCW = coldest bias point of your amp. Set your amp this way, to avoid the possibility of arc-over, during early run-in with the largest burn-off of contaminates.

If the tube will not adjust to proper biasing range (iding too cold, even at max adjustment on the pot) --- they are weak or defective tubes. Even with certain selecting criteria by the seller, weak or bad tubes slip through. Reason most likely is these sellers really don't test the tubes under true amp conditions. Such as at amp plate voltages and idle dissipation.


Do not change the dropping resistors in the amp's bias supply. Unless you clearly mark your actions. For fear of frying another set of future, good tubes. And you need to measure the bias of those new TAD for a good month --- just in case they start kicking up their current flow.

Idle biasing of tubes under real amp conditions is the best way to test the strength of output tubes.


Good luck with your new tubes.


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 03, 2013 11:16 am
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Ok, BMW, I did change the 15K resistor on the bias pot to a 10K and I was able to get it to bias properly. Are you saying I should put the 15K back in, then do the burn in you suggested? Hoping to see the voltages come up?

Could having a 10K in there have any detrimental effects?


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 03, 2013 3:00 pm
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Well, be sure to mark the amp somewhere, in case later you replace the present tubes.

Go ahead and burn the new TADs in, with bias supply modified. See if the purple haze burns off and you get a somewhat stable bias reading.

Tell us how the tubes work out. Hopefully ok. :D


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Sat May 11, 2013 8:25 pm
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I let new tubes run in standby for a hour. With all controls on 1, turn the bias pot CCW to the lowest setting and let the tubes cook for a few hours, then set the bias and I'm good to go. Some folks pull the 12at7 and some use a variac to slowly increase voltage. Point is you don't want to run a signal (RF) throught the preamp until the tubes are burned-in and biased.

BTW, with all of the contributions I've made to this site, when am I going to get a star? Arjay?


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Sat May 11, 2013 8:25 pm
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Make that a pick. :wink:


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Sat May 11, 2013 10:33 pm
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Krankster wrote:
Make that a pick. :wink:


You know what they say, pardner......

"You can pick your friends and you can pick your nose, but you cain't wipe your friends off on the saddle."

Actually, I have no idea how the stars, picks, etc are "awarded". Mebbe it's one of those "Brad things" (most of which are considered corporate secrets much like the patented formula for Coca-Cola, the galactic origin of Unobtainium, or the Vatican's interpretation of the Third Secret Of Fatima).

:lol:

Arjay

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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Sun May 12, 2013 12:16 am
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Krankster wrote:
BTW, with all of the contributions I've made to this site, when am I going to get a star? Arjay?


Near as I can tell, you have to make 100 posts to get one "pick" under your username. To catch up to Arjay (6 picks), you'll need at least 9000 posts. Don't know about the other pick awards. No information in the FAQs of the board.

Jerry

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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 17, 2013 4:42 pm
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Wow, my thread got JACKED! lol!


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Fri May 17, 2013 5:39 pm
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Dingz, how's the new tubes?


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Post subject: Re: Twin Reverb Bias issue with new 6L6's
Posted: Mon May 20, 2013 2:08 pm
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BMW2002Ti wrote:
Dingz, how's the new tubes?

Thanks for asking. Well, I burned them in for another 48 hrs. The purple haze did not change one bit. The Bias did not change one bit. After a few weeks of playing it still apart, and checking the bias every few days, I finally said ok. I settled on a bias of 37mV. The dissipation is about 16W. They sound good. My only problem is that I find them to be a bit noisier at idle than I would like. But, simple fix, play loud, don't stop.

I've been mix 'n' matching all kinds of pre-amp tubes. I'm finding I really like the NOS Telefunkens 12AX7 in V2 and V4. I have Mullards, which are also great, but seem to prefer the Tele's. A NOS JAN Phillips 12AT7WA in V6.

I stuck a new JJ in V5, just cause it's new. I had a NOS Telefunken 12AT7 in V3 and liked it. But recently got some 1965 RCA 12AT7WA's and some 1968 Phillips 12AT7WA's. I really like the Phillips in V3.


Next time I need 6L6's, I won't likely go TAD's again. They're ok, But, I'd try something else.


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