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FDP Forum / Amp Mods, Repairs, and Projects / bias pot resistor value



Bobby Roadrunner
Dec 17th, 2017 06:24 PM   Edit   Profile  

Hi I have a 72 silver face twin reverb, which i black faced a while ago and working great. I just installed some preferred series 6l6 tubes. My max bias range is only going to 24 milivolts with the pot maxed out and it has a 15k resistor on it. To get more bias range, to get to 34 milivolts, do I change the resistor down to 10K or go higher than the 15K that’s in there, thanks in advance, Bob

Contributing Member

Too Near Atlanta GA

Amp Tech Emeritus
Dec 18th, 2017 06:10 AM   Edit   Profile  

Increase the bias range resistor value to increase the current draw. Try a 27K as a test. I presume you are using a bias probe to measure your total bias current draw...




Dec 18th, 2017 08:38 AM   Edit   Profile  

Not to get too picky, but instead of "millivolts" I think you mean "milliamps".

If you are using some kind of a bias probe the reading using a DMM will be in voltage, but it's really an indication of current in the circuit, not voltage.



Dec 19th, 2017 01:56 AM   Edit   Profile  

Really you need to define exactly what you mean by 'blackfaced'.
As there isn't a 15k resistor in the TR AB763, we are forced to assume that you've left some areas of your amp as they were.
Is the linked schematic correct for the amp's bias arrangement?
If so I suggest to leave the 15k resistor between bias pot and 0V alone, rather increase the value of the 3k3 or 2k bias supply droppers, eg 3k3 to 3k9.
Use a tough stable 1 watt CF, MF or MO type, as the bias supply is mission critical.

Correct original schematic? Correct for current bias supply arrangement?



Bobby Roadrunner
Dec 23rd, 2017 08:11 AM   Edit   Profile  

Good Morning everyone: With the holidays amount us and I run amy own A/V business, I wasn’t able to respond to all your help suggestions. Basically, I had this 72 Twin I had done a full blackface on, including under the dog house as well as the phase inventor and tone stack, it was working great with some TAD 6l6GC in it. I let it sit for a year or so, before I decided to mess with it again. What I do normally, is to plug my amp into my short circuit 300 watt light bulb outlet to make sure something isn’t going to blow.

Whit that said, upon turning on the amp in that power configuration, i was able to get 24 miliamps max on the bias control, where previously i was able to get to 33 and beyond comfortably. I had bought some 6l6GC tubes from The Tube Store, in the preferred series line up.

I thought maybe they needed more bias. With all the great suggestions that came in from you guys and knowing you are a great techs, something else must be wrong. There just cannot be that much difference in tube brands.

Then it dawned on me, perhaps the short circuit protection system was limiting the supply from the wall. Well just now, I plugged it directly into the wall strip I have on my bench. Bingo, full till bias range.

What started me on bringing out the Twin once more, is I never had a pair of reissue Jensen’s. I have always read they are harsh and not good at all. so in fear of wasting a hundred bucks on them, I usually bought weber’s.

Because I respect Steve Dahlmans opinion, he has always commented on how he likes them. Well I bought 3 Jensen C12N’s. 2 8 ohm and 1 4 Ohm to try. I totally expected them to be harsh. I am very impressed and love their midrange on the Fender’ s I have tried them in.

Thank you all for your suggestions, perhaps other members of the forum will find this and know how to check for this issue

Happy Hollidays everyone and thanks again for you advice, Bob

(This message was last edited by avspecialist at 10:19 AM, Dec 23rd, 2017)

Contributing Member

Paris, France

It's just a guitar, not rocket science.
Dec 23rd, 2017 09:02 AM   Edit   Profile  

"What started me on bringing out the Twin once more, is I never had a pair of reissue Jensen’s. I have always read they are harsh and not good at all."

The current Jensen speakers are victim to a lot of unfair tone snobbery. True, they don't sound the same as the vintage speakers of the same model number, but the same is true of Celestion. Jensens, IMHO, are particularly well suited to rich, bright clean tones, though some models can get aggressive and harsh with high gain tones. I really like 'em.

There is no such things a'better' in tone, only what your ears like. Webers, or Celestions, or anythign else, are not better or worse, than Jensens. They are all just different flavours on the shelves of the big pantry of tone.



Bobby Roadrunner
Dec 23rd, 2017 01:19 PM   Edit   Profile  

DrKev: I agree with what you say. For me so far, I have 3 racks of 4 shelves of both new and vintage speakers. I love most of them. What I find certain amps light up and sound great with certain models.



Dec 24th, 2017 02:48 AM   Edit   Profile  

Haha, yes, a light bulb limiter is THE most secret weapon to identify/fault find big problems, but once any have been ruled out, get it out of there.

FDP Forum / Amp Mods, Repairs, and Projects / bias pot resistor value

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