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Post subject: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 10:35 am
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Roadie
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I've worked on a couple of these UL Twins but the bias current is really low on this one. I've changed all the "normal stuff" already beforehand. It's averaging 21ma's of current which is low. I changed the 1k2 dropping resistor to 820 ohms to get it that high, before it was about 18ma's. PV is 485. I could go lower with that resistor, 470 or so, I don't like the idea of lowering the 33k on the back of the matching pot.

Any thoughts?

Thanks, UN


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 12:07 pm
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Location: Province de Québec, Canada
This the shematic you are taking about ?

http://el34world.com/charts/Schematics/ ... _schem.pdf

Power tubes milliamps show 21 ma at cathode right !
More negative bias voltage = low cathode current.
By dropping 1K2 to 820 you send more negative voltage :(

Put higher value at 1K2 or drop negative voltage with smaller value at the 2k2

Also;

If tubes are weak you'll read low current .Are tubes are good ?

Check 470 ohms and 1,500 ohms resistors at each Output tube
___________________

Check tremolo circuit

How many negative voltage do you read where schematic show -58 ?


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 2:00 pm
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Roadie
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Yes, that's the correct circuit. I changed out the screen and grid resistors when I replaced the caps. I'll take another reading at the 2.2k and return. Tubes test okay, Sovtek coin based 5881's.

UN


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 3:18 pm
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Roadie
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The two neg. voltage test points read as follows: -58.5 on one and -62.8 on the other.

With 21ma's of bias/plate voltage 485v that's only about 10 watts per tube. It'd be nice to at least get 60%. I've read people have added a trim pot - resistor off the matching pot in place of the 33k resistor. I also know there isn't much love for these Sovtek 5881's.

UN


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 3:35 pm
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upnorth2 wrote:
Tubes test okay, Sovtek coin based 5881's.

UN


Tube tester are not reliable for that , how do you test tubes ?

Only new ones will tell


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 4:08 pm
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Roadie
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I spoke with the customer and he was happy with the tone before he brought it to me. The amp came in with noise/hum and bad reverb osc. All of that is corrected and I see that many of these run pretty cold so I may call it good.

UN


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 4:53 pm
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Many Fender amp sound good with cold bias .
Sometimes my customer said " my amp sound better with the right bias , I discover my amp "


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Fri Feb 02, 2018 7:13 pm
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Roadie
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This amp definitely starts clipping earlier than other UL's I've heard. It has JBL D120F's (orange frame) which should be fairly efficient speakers. I played it at about 5 and 5 for 1/2 hour and could hold my hands on the 5881's. I've got some TAD 6L6's I may toss in there tomorrow for comparison.

UN


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Post subject: Re: '77 UL Twin Reverb Bias Current
Posted: Sat Feb 03, 2018 2:17 am
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upnorth2 wrote:
. I've got some TAD 6L6's I may toss in there tomorrow for comparison.

UN



That is only the right test for tube, put new one :lol:


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