What bias ma should i set my amp forumla

Lake Placid Blues

Lake Placid Blues

New member
The formula is for Class AB amps with adjustable bias:

Max plate dissipation of tube type/plate voltage, multiplied by desired % of max plate dissipation.

For example, for a 400 volt plate voltage running EL34s, 70% would be 44ma: 25 watts/400 volts=62.5ma x .7 = 43.77ma

A 460 plate voltage running EL34s at 70% would be about 38ma

The % of max dissipation you think your amp sounds best is a matter of taste, but it will probably be between 65% and 72%

This ma figure is per tube. For some like 100watt DSLs it is per side with two tubes per side so it would double that per side.

Max Plate dissipation of typical octal power tube types:

6V6=14 watts
5881= 23 watts.
6L6wgb=23 watts
Kt-66 = 25 watts
EL34=25 watts
6CA7=25 watts
KT77=25 watts (30 watts absolute max)
E34L=25 watts ( on the JJ data sheet, but many say 30 watts in practice)
6L6GC=30 watts
7581=30 watts
7581A=35 watts
6550= 35 watts
6550A=40 watts
KT88=42 watts

Silver Jubilees tend to run higher plate voltage than typical for post 60s Marshalls. Some Plexi Marshalls from the 60's ran more than 500 volts. Late 70s JPM 50 watt Marshalls often run 400 volts or less. If you run a variac you will need to measure actual plate voltage and re-bias accordingly.
 
Re: What bias ma should i set my amp forumla

I used that formula when biasing my 4103 with 4 6550's, mostly because I had no prior experience with a 140W Marshall.

I'm curious how many mA you run on your Jubilee. I think mine is JJ E34L's at 38mA.
I think my 2204 is about the same.
 
Re: What bias ma should i set my amp forumla

I'm running it at 38ma, which is 68%. For years I ran it at 70%, but recently rebiased it to 38ma. I think it sounds a little sweeter. I'm still running the same set of 90s Svetlanas (same as Winged C) for about the last 15 years. Every time I think of changing them they still sound better than their new replacements, so back in they go. They really do sound fine in this amp.
 
Re: What bias ma should i set my amp forumla

Gearjoneser said:
I used that formula when biasing my 4103 with 4 6550's, mostly because I had no prior experience with a 140W Marshall.

I'm curious how many mA you run on your Jubilee. I think mine is JJ E34L's at 38mA.
I think my 2204 is about the same.
Click to expand...

Lake Placid Blues said:
I'm running it at 38ma, which is 68%. For years I ran it at 70%, but recently rebiased it to 38ma. I think it sounds a little sweeter. I'm still running the same set of 90s Svetlanas (same as Winged C) for about the last 15 years. Every time I think of changing them they still sound better than their new replacements, so back in they go. They really do sound fine in this amp.
Click to expand...

I typically bias my 2555 at 36ma with EHX EL-34s. Much hotter than that and it starts to get overly fat and feels rather stiff, while it gets BRIGHT in a hurry if I bias it much colder. I recently had my tech check it over after a couple years in storage and it came back @ 33ma; I've been meaning to change that for a while now.
 
Re: What bias ma should i set my amp forumla

As Ken linked to, just use an online calculator. I think the Weber site also has the max dissipation of the different tubes listed. At any rate it's a good post listing the different tubes and showing the basic math for biasing. It ain't rocket science ... and my math sucks.
 
You must log in or register to reply here.
Back
Top