Bias tube amp and my stupid mistake

  • Thread starter Thread starter thickwood
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thickwood

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Well it took me 30 minutes too find my stupid mistake .
First reading using a bias probe showed 327 plate volts DC , ah that's too low for my Rebel 20 and my Rebel 30 amp ?
The first thought was the probe was off a lot .
Conclusion was wrong , I forgot too check the AC volts coming into the house !
My DMM read 110VAC whoa that's 1950s voltages we used too get !

The variac I have got the voltage up too 120 VAC now how much plate volt increase will I get going up 10 volts ?

It went too 360VDC on the plates from 327 before humm.
It was 90+ degrees and AC units were going high speed , my Roommate said the Farm Fresh Cash registers were slow as to barely working and no one knew whats going on !

I told him what I found on the house volts being 110vac and he told me the problems at Farm fresh .

So if you bias your tubes make sure the VOLTS are up to about 120vac or your going to bias them very hot when your line volts come back up !
Complaints about red-plating power tubes is common , but it may well be our stupid mistake not checking the line voltage first !
10volts difference made 33VDC increase on the plates when the volts come back up, yeah I was stupid not to know this fact !
JUST SAYIN :aww:
 
You should be safe regardless (from red-plating), because as your input voltage rises from 110 to 120 so does your bias tap, and since your bias tap is rectified to negative it goes more negative and causes your tubes to conduct less. Actually I think it should even out within reason, since higher plate voltage would need more negative bias voltage, and you get higher plate voltage and more negative bias voltage as the primary voltage goes up on your PT.
 
What really thru me was how low my plate voltage was reading , so I checked the other Rebel 30 that has 370-365 plate voltage and it too was very low !
So if you want to get it right I guess you need to be a physic bias tech who knows what your AC voltage is going to be when you crank it up ?
Yeah I did my best , Those Gold Lion el84 pair do sound very good !
Talk is not to use them in a cathode bias amp , 15 minutes there awesome 25 minutes there fried at the cathode !
I may test bias again soon , practice I drop the VAC with my variac to 115vac to not push them at all except when it counts !

I do see your logic , no worries Crazynuts
 
The Gold Lion tubes do sound great. I have a quad set of GL KT77's. However they are not very durable from my experience, they developed rattle, become microphonic, and are way out match from one another. Then again the amp I build to put them in has earthshaking low end, so perhaps that was what did a number on them.
 
I have been reading reviews and the Gold Lion Tubes have gotten excellent if not the best tube made right now so I wanted too hear for myself.

I had a 6l6 amp and considered the KT77 but never tried them !

Not many guitar players like the prices of the El84 GL , if it sounds right you will play it right lol !
I just had to give in too my whim and now I need a chance to crank the Rebel 20 up abit when my roomys go out !

Yeah vibrations from that bass you have did cause microphonics and stuff as you say.
Perhaps a Cryoset pair would hold up much better ?
 
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