twin reverb bias question

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davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

scratch that I was readin the wrong end of the damn thing, I have the same 330 v at the end of the two main filters
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martin manning
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Re: twin reverb bias question

Post by martin manning »

See if you can confirm the idle current on both sides using the primary DCR method. Don't forget to drain the caps. Do you know which circuit you have?
davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

ok i will do and report back. not sure if its aa 270 or 769, i would have to explore further. its a 71 model but knowing fender in those days i doubt thats an indication.
davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

ok, thats done .036 A/ .028 A ...of course we already knew it was mismatched
check my math:
1.27v/35.1 r
1.05v/37.3 r
I have also been probing each 12a-7 plate and cathode and everything is below specified voltages. going back to the power supply. I greatly appreciate the help BTW.
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martin manning
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Re: twin reverb bias question

Post by martin manning »

Looks good. With the amp in standby, you are getting 440V or so at the standby switch? Out of curiosity, what voltage is on the power tube grids (pin 5's)?

30 mA is very low for two 6L6's, but that would tend to make your plate voltage higher than expected. I'm running out of ideas here other than replacing the filters and the diodes too for good measure.
davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

yeah no doubt, everything in the power supply seems good, i thought a leaky cap could be pulling it down. i checked the choke for dcr in case it had an issue. its good, plus I stuck a mish mash of 6l6s I had laying around in there with almost Identical results. i will check the grids, if memory serves it was around negative 40 to get the 30 ma. I even did this on different meters in case my main one was screwy. at the end of the day the thing sounds really great so I wont lose too much sleep
davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

- 34 v on pin 5s. 330v on pin 3. I may do a last ditch main filter substitution to see if that bears any different results
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jjman
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Re: twin reverb bias question

Post by jjman »

How is the heater voltage during these tests. Does it change measurably?
If it says "Vintage" on it, -it isn't.
Stevem
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Re: twin reverb bias question

Post by Stevem »

You should have atleast 460 volts , with some 470 being normal at the standby switch with it in the open position!
Check out your PT.
Unhook the ends of the secondary's at the diodes and pump AC wall voltage into it , do you get the right voltage ratio out the primary side of the PT?
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Firestorm
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Re: twin reverb bias question

Post by Firestorm »

davebolden44 wrote: Sun Mar 19, 2017 9:15 pm scratch that I was readin the wrong end of the damn thing, I have the same 330 v at the end of the two main filters
A Twin has the first two caps wired in series. You should not see the same VDC at both anodes. Something is wrong with the main filter configuration.
davebolden44
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Re: twin reverb bias question

Post by davebolden44 »

no everything was right. I will make an admission, I was still plugged into my current limiting bulb when I was biasing and it was eating some of the current. Those whole issue was created by my error.
Stevem
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Re: twin reverb bias question

Post by Stevem »

Yup, that will do it!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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jjman
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Re: twin reverb bias question

Post by jjman »

I made that mistake once. Very frustrating. The internets tell us to use the bulb for initial startups but fail to remind us that we must stop using the bulb for any quantitative testing.
If it says "Vintage" on it, -it isn't.
tubeswell
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Re: twin reverb bias question

Post by tubeswell »

davebolden44 wrote: Sun Mar 19, 2017 11:52 pm - 34 v on pin 5s. 330v on pin 3. I may do a last ditch main filter substitution to see if that bears any different results

-34V is low for the bias voltage on these. You should have about -45. The hotter bias could be dragging the plate voltage down. Check the bias supply resistor values haven't drifted too much
He who dies with the most tubes... wins
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