Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnwitt on June 30, 2010, 06:48:52 pm

Title: Proper bias voltage range for EL34s
Post by: bnwitt on June 30, 2010, 06:48:52 pm

Well I'm doing start up on this JMP50 and mias bias voltage maximum voltage is too low.  My circuit setup is a 220Kohm resistor between the HT leg and the diode, 2@ 10uf capacitors with a 15kohm resistor between them and then a 47kohm resistor and 25kohm pot between the bias output and ground.  Typical Marsahll JMP50 setup.  I have 257 VAC into the circuit, -39 VDC after the diode, -32 VDC max after the 15kohm resistor and I can dial down to -22vdc with the pot. I don't believe -22 to -32 is going to be a high enough range for most EL34 power tubes.  So my next step is to lower the value of the 220kohm dropping resistor.  Any feedback?

edit:  Ok if I go down to 160kohm on the dropping resistor and change the 25k pot to 50k I get -34 to -59   I'll put in some tubes and see what I get.

Title: Re: Proper bias voltage range for EL34s
Post by: plexi50 on June 30, 2010, 07:10:22 pm

I just did one last night. Funny how these run in cycles. My bias resistor was 56K with a 220K in parallel. The max range i could get was -44MA. It says -42 to -48 on the schematic. I settled on 435 VDC plate and -42 grid VDC. It will get  louder set at -34 but then the plate voltage drops to like 390+-. It sounds good to me set at -42

Some one definatley was in this amp before me

(http://i356.photobucket.com/albums/oo5/plexijtm45/BoardPrint.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/BiasResistorTrimmerPot.jpg)

Title: Re: Proper bias voltage range for EL34s
Post by: FYL on June 30, 2010, 07:18:58 pm

Quote

I don't believe -22 to -32 is going to be a high enough range for most EL34 power tubes.  So my next step is to lower the value of the 220kohm dropping resistor.  Any feedback?

257V AC seems strange, you should have around 350V AC and a -28V to -42V useful bias range using the stock circuit.
Cf. http://www.pentodepress.com/home/classic-circuits/JMP50-1987-grid-bias/

You may try a 150K dropping R.

Title: Re: Proper bias voltage range for EL34s
Post by: plexi50 on June 30, 2010, 07:29:04 pm

No tubes i got -58 grid VDC. When the tubes were in and there was a load i got -33 VDC

I have not seen a drop like that under load before. Maybe im going blind but thats what i saw

I tested and swapped out power tubes but all remained the same until trimming to -42(Zep?)

Nice PT/ Small OT

Title: Re: Proper bias voltage range for EL34s
Post by: bnwitt on June 30, 2010, 07:37:02 pm

FYL for some reason when I tested the voltage going into the 220k resistor the first time it read 257.  It reads 307 all day long now which is 1 leg of the 621VAC I have at the standby switch.  I don't know what happended the first time.