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Post subject: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 12:21 pm
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Hi I just dug out my old vibrolux and now that i am getting better at understanding tube amps..i decided to have a look at the bias settings.. seems to be 24MA on one tube and 19 Ma other tube...when i try and adjust the smaller number up .. the larger number gets bigger so if i put the 19ma up to 24 then check the other one now its at 34? is it set up to be a bias balance? it looks like a black face setup (bias adjust)? now i am confused again? argh..seems like some of you guys would know about this..thanks!! Jimitrader

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 12:30 pm
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If the amp was built in May of 1968 or later, it's the AB568 circuit. That revision has a balance control. Look inside the chassis to locate a date ink-stamped near the bias board and power tranny. If it's week 24 or earlier it's the original AA964 layout, with a standard idle-bias layout.

Arjay

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 12:57 pm
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T351868 stamped on chassis = week 35 day 18 year 1968?

transformer stamps:
606815 = ot 1968 15th week?
022723 606720 = pt 1967 20th week?


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 2:10 pm
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Regardless of what bias control it has, I'd say it is time for new tubes as the ones in there now are no where near matched. :idea:

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 2:29 pm
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I ordered these tubes from Gerald Weber website..they where supposed to be NOS but he sent jj tubes 6l6gc ?I have played maybe two hours on them total? is there a way to set the amp up with true idle-bias?


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 2:32 pm
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Jimi, that's a blackface bias adjust set-up. So, some matched tubes are needed. Art

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 2:41 pm
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Your photo clearly shows a bias control, not a bias balance control. See the schematic and layout:

http://www.ampwares.com/schematics/vibr ... _aa964.pdf

Given that there is a 5 to 10 mA difference between tubes, either the two 220k bias resistors (2nd photo, upper left between the two blue caps) are way out of tolerance or the tubes were very poorly balanced to begin with. I buy my JJs from Eurotubes and they have always been balanced within 3 mA of each other, often within 1 mA.

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 2:57 pm
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Thanks bluesky636 the 2 resistors check in at 233 and 243 while still in the circuit..


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 3:34 pm
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jimitrader wrote:
Thanks bluesky636 the 2 resistors check in at 233 and 243 while still in the circuit..


You really should pull one leg to accurately measure them but as they stand now, they are both out of tolerance. 220K, 5% tolerance = 209K to 231K.

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 4:24 pm
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aclempoppi wrote:
Jimi, that's a blackface bias adjust set-up. So, some matched tubes are needed. Art


+1

Built in the 18th week of 1968. It's the original AA964 circuit.

As Art and Bill (above) have mentioned, you need a matched set of output bottles and a balanced 12AT7 for the V6 phase-inverter stage.

Looks like a nice amp!

Arjay

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 6:26 pm
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Totally agree with the guys!!! Bill makes a very good point about the(2) 220K-5% resistors. If you could check those tubes in another amp, that might save you some bucks. Like Arjay said, very nice amp!!! Art

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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 7:21 pm
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Also check the 1500-ohm grid stopper. These should also be within 5% tolerance to keep the output balanced.

FWIW .... the PI section of this amp seems slightly imbalanced (see the 82k-ohm and 100k-ohm resistors feeding the anodes of the 12AT7?). But, this is compensate for the imbalanced coupling onto the grids of the 12AT7. You get bigger swing with less instability, using this topology with long-tail AC-driven PI's.

Anyhoo... good luck! Keeps us informed. :D :mrgreen: :D


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 8:52 pm
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I guess what is concerning me is when i adjust the bias it only balances the bias ? as everyone said it seems to have a BF setup..i did notice on the schematic it has a white wire going to the bias board but on mine it isn't? everything else seems the same to me. :-) but i am still learning.. I am humbled by everyone's smarts..great site right here!!


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 10:15 pm
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Sounds like the white wire is the tap to the PT. Have a photo of the bias board setup?


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Post subject: Re: Vibrolux Reverb 1968 drip edge bias set-up question
Posted: Fri Oct 11, 2013 10:30 pm
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just going off this schematic

http://www.ampwares.com/schematics/vibr ... _aa964.pdf


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