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 Post subject: Bias'ing help
PostPosted: Fri Mar 20, 2020 6:04 pm 
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Hey all!

I just got a Laney GH50L and i'm trying to check the bias, but something here seems amiss...

i was following this video...
https://www.youtube.com/watch?v=RIUm8_7hpMI

and following this schematic...
https://www.thetubestore.com/lib/thetub ... ematic.pdf

when i try to check the ohms, as show around the 36:13 mark...using the schematic, the black wire is point 42...but when i set the multi-meter to ohms, measure from the black wire to pin3 of the output tubes, i'm getting 50ohms...seems crazy high...
when i do the same measurement with the amp on, for the milliamps, i am getting 1.0

when i read the plate volts from pin3 to 8, i got 406V...
That means, if I've read this right, the tubes are working at 8.12W...for EL34s...
i've obvisously done something wrong...the only thing i didn't do that he did, was drain the caps...do you really need to do that?


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 Post subject: Re: Bias'ing help
PostPosted: Fri Mar 20, 2020 6:49 pm 
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update...drained filter caps...pin1 from V1 to ground...no change in the 50ohm


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 Post subject: Re: Bias'ing help
PostPosted: Fri Mar 20, 2020 10:04 pm 
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Joined: Wed Aug 26, 2009 11:27 pm
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Location: Ontario
The schemaic you poster is for the 100l

I asumme theres no 1R resistors in the amp to make life a little easyier?


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 Post subject: Re: Bias'ing help
PostPosted: Sat Mar 21, 2020 12:37 am 
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Interesting video. That's certainly one way to do it and it avoids needing special socket adapters or installing 1-ohm resistors on the cathodes.

I assume the 50L is the same as a 100L, just with two fewer EL34s for 50 watts.

Your math is right, but 50 ohms for a transformer winding does seem high. Are you sure you are measuring to the center tap of the output transformer's primary? Try removing the EL34s and measuring from pin 3 on one socket to pin 3 on the other socket. If you get 100 ohms, then your 50 ohm reading from before was OK. Otherwise just divide whatever you read by 2.

If you can find a place to measure the bias voltage, like at the junction of the two 220k resistors leading to the grids, what does it read? Is it in the -35 volt range? Also is the bias switch in the right position for EL34s?


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 Post subject: Re: Bias'ing help
PostPosted: Sat Mar 21, 2020 8:50 am 
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mitch m wrote:
Interesting video. That's certainly one way to do it and it avoids needing special socket adapters or installing 1-ohm resistors on the cathodes.

I assume the 50L is the same as a 100L, just with two fewer EL34s for 50 watts.

Your math is right, but 50 ohms for a transformer winding does seem high. Are you sure you are measuring to the center tap of the output transformer's primary? Try removing the EL34s and measuring from pin 3 on one socket to pin 3 on the other socket. If you get 100 ohms, then your 50 ohm reading from before was OK. Otherwise just divide whatever you read by 2.

If you can find a place to measure the bias voltage, like at the junction of the two 220k resistors leading to the grids, what does it read? Is it in the -35 volt range? Also is the bias switch in the right position for EL34s?


Thanks for the help Mitch!

The 50 is the same as the 100, just two less tubes, as i understand it...this schematic comes with the 50 model

I didn't take the tubes out...but measuring off R34 and R38, which at the time was escaping me as bias points, the amp was shooting 38v

I know the schematic says 35, but based on this chart, thats low, so i left it at 38...shoot the middle ground...i tried to say w eber chart but for some reason, once posted it changes to BBQ??
http://www.tedweber.com/webervst/tubes1/calcbias.htm

Also left a note inside so that the next time, i remember a little easier! Unless i dont have this right?
Thanks all!


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 Post subject: Re: Bias'ing help
PostPosted: Sat Mar 21, 2020 12:11 pm 
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Opened it back up...
Here is what im getting
Plate 417V
V5- 47.8ohm and voltage drop of 1.36V
V6- 49.7ohm and voltage drop of 2.68V

If i increase the bias, V6 is going to burn out, no?

Changing the voltage drop across V5 to 1.75V leaves 3.58V at V6

So...i took out V6 and put a spare EL34 in...
Now i get
Plate 416
V5 - 48.1ohm and 1.23V drop
V6 - 50.2ohm and 2.08v drop

So, i guess it stays there until i can get a matched set...well, better than the matched set it came with...unless im missing something?

I bumped the bias up a bit...the drop now is 1.68V and 2.55V for V5 an V6 respectively


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 Post subject: Re: Bias'ing help
PostPosted: Sat Mar 21, 2020 2:57 pm 
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To finalize..
I take the 416 x (1.23/48.1) = 10.63W....the miiliamps being 25.6, which is low for an EL34, but with the other tube reading 41.4 milliamps, thats a little hot...correct?
Will order a matched set after all of this...but i'm doing this right, correct?


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 Post subject: Re: Bias'ing help
PostPosted: Sun Mar 22, 2020 2:21 pm 
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Joined: Sat Dec 04, 2010 3:42 pm
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Location: Qualicum Beach, BC
You are doing it right. Sounds like your EL34s are just mismatched. Try swapping their positions and see if the readings are more equal.


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