Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Russ Fuquay on September 19, 2018, 02:28:37 pm

Title: Blues jr Conversion, upgrades, bias
Post by: Russ Fuquay on September 19, 2018, 02:28:37 pm

Greetings.
My first amp build is a Hoffman blues jr Conversion. I've installed the Allen TP 24 power transformer and OT20 output transformer. Why, you may ask? Well, let's say The Originals had to be replaced out of necessity and leave it at that. I figure
as long as I'm replacing them I would like to get a little more headroom and clean power if possible, but still keep the el84's at least for now.

My question is about the correct bias. Given that I'm using the OT 20 and PT 24 what should the correct bias be? Also, do any of the parts that came in the conversion kit need to be changed due to the different Transformers?

Many thanks.

Russ

Title: Re: Blues jr Conversion, upgrades, bias
Post by: pompeiisneaks on September 19, 2018, 02:58:04 pm

The bias point isn't based upon the OT, but upon the tubes, and EL84's depend on the plate voltage coming into them.  I don't know the Blues Jr voltages, but if you look at the weber bias calculator you can get a quick check, after measuring plate voltages:

http://www.tedweber.com/webervst/tubes1/calcbias.htm

example for 350 V for EL84 class ab, it's about 24mA for 70%. 

Play with the calculator, it's a great tool.

~Phil

Title: Re: Blues jr Conversion, upgrades, bias
Post by: Russ Fuquay on September 19, 2018, 03:27:46 pm

Good to know. Many thanks!

Title: Re: Blues jr Conversion, upgrades, bias
Post by: ginger on October 01, 2018, 02:03:05 pm

..... I do this on Stock Blues Juniors

Title: Re: Blues jr Conversion, upgrades, bias
Post by: sluckey on October 01, 2018, 02:14:39 pm

Quote from: ginger on October 01, 2018, 02:03:05 pm

I do this on Stock Blues Juniors

That video won't help Russ. His bj no longer resembles an original or a bm modded original.