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Post subject: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 2:05 pm
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I just got my new Biaspro plug and test thingy (for lack of a better word).

I plug in using my handy dandy digital multimeter from craftsman (spared no expense) and here are my readings on this supposedly "matched" quad of tubes.
left to right from the back of the amp.
31.9 37.5 32.6 36.2 m/a

And i noticed that the second I hit the standby and add full power to the tubes the readings jumped to over 40 ma on every tube then setteled back down rather rapidly.

Cheap chinese e-bay tubes !!!

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 2:18 pm
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You usually get what you pay for.

Now chuck them chicom bottles into a convenient dumpster then find yourself a true matched quad from JJ, Ruby, or EHX and be done with it.

:mrgreen:

Arjay

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 3:29 pm
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Do you know that most amps have what they call are push-pull "pairs?" And if you know which position each pair is located, you can put the 31.9mA and the 32.6mA as pairs. And the 37.5mA and 36.2mA as pairs. These two sets match up pretty well. If you look the the schematic for a Fender Twin Reverb:

http://www.ampwares.com/schematics/twin ... _aa769.pdf

V7 and V8 are push-pull pairs. And V9 and V10 are push-pull pairs. So, if you put the 31.9mA and 32.6mA in V7 and V8 and 37.5mA and 36.2mA in V9 and V10-- you are good to go. Tubes change bias points as they break-in. Sometimes, a lot the first few weeks. So you prolly want to check them, at least once a week for the few few weeks.

The little tang that helps hold the tube in the socket (shaped like a Vee), sometimes can interfere with the bias tool's socket adapter. You can splead them slightly, to help seat the tool. Them pinch them again, when you replace the tool with the tube.

HTH. Good luck!


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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 3:46 pm
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How are you getting that much current draw on the tubes with the amp in standby? Something isn't right about that as the standby switch should eliminate the Plate voltage, so you should only read a few mA of grid current in standby. Are you sure that you weren't reading closer to 3 mA in standby instead of 30 something mA? The bias should be set when the amp is not in standby. :idea:

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 3:56 pm
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Shimmy, Maybe he's getting "flyback" from the output tranny---when he activates the standby switch. Some amps with big OPT's can have several amps of flyback. Like Ampeg SVT's and Hiwatts. They use avalanche sets of diodes to keep the back current from destroying the power tubes and accessories attached to them.

Just a WAG...


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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 4:30 pm
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BMW2002Ti wrote:
V7 and V8 are push-pull pairs. And V9 and V10 are push-pull pairs.


All well and good.

Except......the summed output of the V7/V8 pair are pushing/pulling against the summed output of the V9/V10 pair. If all four tubes do not share similar transconductance factors, guess what?

The output stage will not operate symmetrically.

Some folks like the tone this condition produces but eventually, an unbalanced final drive will cause premature output tube failure. Having a balanced phase inverter is another example of why the principle of equilibrium is so important in a class AB amp.

JMO, YMMV

Arjay

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 6:07 pm
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Sorry for the confusion shimmilou, I placed the subjects in the wrong order in that post.

little or no reading with the standby on then the big flash over where the meter jumps to 40 or so, then it settles back to the low to mid 30's for a final reading.

I'm new to bias settings on my own and so I'm just trying to get a feel for what to expect, the tubes were $30 for a quad on evil-bay so what can you expect !

Being able to see a needed repair and do it is one thing, knowing the theory behind this stuff is a whole other world, I only wish i had the theory down so that i could diagnose the problems instead of begging insight from fellow (knowledgable) Members.

Thanks to you guys who took the time to learn the hard way !!!!!

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 6:25 pm
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Cool blues bondsman, that sounds better! Although I just read your response when I clicked submit, it took me 15 minutes to type my response, so I'll post it anyway. :)

BMW2002Ti wrote:
Shimmy, Maybe he's getting "flyback" from the output tranny---when he activates the standby switch. Some amps with big OPT's can have several amps of flyback. Like Ampeg SVT's and Hiwatts. They use avalanche sets of diodes to keep the back current from destroying the power tubes and accessories attached to them.

Just a WAG...


At idle, the flyback can only occur momentarily if the voltage supply (B+) to the transformer is applied and then removed, resulting in the expanding and collapsing of the magnetic field of the transformer generating the reverse current. If the tubes were conducting over 30 mA with no Plate voltage applied, any flyback would be quickly discharged and then a current draw of a few mA for the grid would be all of the remaining current. No B+, no Plate current.

Am I missing something?

blues bondsman,

You might want to try swapping your tubes around in a way that you can check to see if it is the components in the amp, or the tubes that are causing the imbalance. Maybe mark the tubes with a sharpie, 7, 8, 9, 10, or however you can track them, then put each tube into each socket and check the current draw to see if it changes much depending on which socket that it is plugged into. The grid voltage will largely determine how much each tube conducts, and if the resistors in each grid circuit are different values then the tube will conduct a little differently (there are tolerances). You might even find a combination that will be more balanced as far as current draw. You may need to put in better resistors to get a truly balanced set. Even a matched set of tubes can each idle at a different current if the circuit is unbalanced due to component tolerances. :idea:

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 7:35 pm
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shimmilou wrote:
Even a matched set of tubes can each idle at a different current if the circuit is unbalanced due to component tolerances. :idea:


+1!

Good call, shimmilou.

Arjay

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 8:20 pm
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I did just as you suggested and numbered the tubes 7 through 10

Tubes 8 & 10 were the low #'s during the first test, I changed all four tubes locations and retested, sure enough the tubes # 10 & 8 were again the lower #'s
So it is safe to say that the tubes are a close matched set and a close matched set, but not a matched quad.

So now tubes 8 & 10 are in V-7 and V-8
Tubes 9 & 7 are in v-9 and v-10

recap #10 was 31.9 ma, #8 was 32.6ma, #9 was 37.5, #7 was 36.2

So the two tubes in each pair are close but the quad is way off (bad tube matching if any at all from the seller).

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 8:23 pm
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Man,I couldn't get past the part of the readings on standby... :? but then saw the answer.
There's alot in this thread to chew on,I'm glad my amps only have two power tubes.
What kind of bias probe did you buy?


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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 8:41 pm
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Sorry about the whole cart before the horse deal :mrgreen:

Its a biaspro, just a tube socket with a resistor and multi meter leads built in.
works great and its relatively cheap too.

The guy makes them in Minnesota, he makes fancy models with a meter also

ken@asharpfretworks.com or asharpfretworks@gmail.com

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Wed May 18, 2011 9:40 pm
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Thanks for trying that blues bondsman. :) Yep, it seems that you do have two matched pairs instead of the matched quad, and you've pretty much verified that your amp circuits are closely balanced since the same tube draws the same current in different positions. You may be able to get your money back from the eBayer (can't hurt to send an email and ask), or as close as each pair seems to be, you could resell each set of two as matched pairs with a clear conscience, or try each pair in an amp with only two output tubes. I really like the sound of the newer generation Chinese tubes (not sure which type yours are). My HRDlx has them for preamp tubes and I noticed a new HRDlx III had Chinese output tubes stock. You could have done worse for $30 though, very cheap learning experience. Keep in mind that it isn't the tubes' fault that the seller didn't match them well, so the tubes might still be great in pairs, I wouldn't be too anxious to get rid of them. :idea:

Almost forgot, great job on your first bias check, thanks for sharing the details. :!:

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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Thu May 19, 2011 6:43 am
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The tubes are non branded Chinese with a brick red base, longer bottle too.

Thank all of you for your support and for those who have wanted to try to bias their own amps, i would say just do it !!! it is not at all difficult and a very small investment in $$$ gets you going.

The multi meter 19.95 at sears (seems real nice too)
The biaspro probe on evil-Bay around 35 shipped
http://www.asharpfretworks.com/index2.html

so for around $55 to 60.00 your able to do it yourself, thats about one trip to a tech to have them do it for you.
My local tech charged $25.00 however he was a 100 mile round trip and took us right past the wifes favorite shopping place and restaurant so a trip for a bias would cost more like $300 :lol:

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1968 Bandmaster with 2X12 cab C-rex speakers
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Post subject: Re: Matched tubes for my Twin ??? see what i found
Posted: Thu May 19, 2011 9:00 am
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blues bondsman wrote:
however he was a 100 mile round trip and took us right past the wifes favorite shopping place and restaurant so a trip for a bias would cost more like $300 :lol:


Lemme guess......Shoe-Mart and Red Lobster?

:mrgreen:

Arjay

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