Build Your Own Clone Message Board

Hi All,

I ran into a problem with my Brit 50 that I could use some help on. It's been running for about 2 years with no issues. This week it started sounding mushy, tinny, more distorted and a loss of power. So I checked the bias which was initially set up as 33ma with a plate voltage of 488dcv. A little low based on what I've read about running at 70% utilization on the EL-34. When I tested the bias it was at 22ma running at 488dcv plate voltage. I was hoping a component or two drifted. When I tried to rebias the amp, I could only get the up to 29ma and I was at the end of the bias pot's travel. I also noticed the tubes had started to red plate when I got over 28ma. So I backed the bias off and started looking around. Maybe a bad tube, so I replaced the pair of tubes with another matched set. Same result. I'm thinking maybe the bias circuit (caps and diode)?? Any help would be appreciated. Thanks in advance.

Tubeamp

What DC voltage do you see at:

1. The junction of the two 220K resistors, and...
2. ...pin 5 of each of the EL34's?

...at the extremes of the bias trimpot sweep?

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Hi duhvoodooman,

Thanks so much for your help!!! In this case I hate to admit technician failure on my part which is much better than broken amp. I figured out why you needed the voltage readings. They came out like you would probably expect.

Here they are:

Bias pot full counterclockwise

Junction point of the 220K resistors = -53
Pin 5 on the EL34 = -51

Bias pot full clockwise

Junction point of the 220K resistors = -29
Pin 5 on the EL34 = -29

So bias circuit seems fine. I've been switching back and forth between the Eurotubes bias checker and testing via pin 1,8 and they are two different "animals". The bias checker tests ma and 1,8 is mv. My bad, so once the amp was opened I didn't bother with the bias probe and went direct to the tube pins with the meter set to ma instead of mv and you can see the problem.

The bottom line, a EL34 power tube went partially bad causing the bias to be off (dropped to 22ma from 33-34), bad sound etc.

Thank you!! All is good and bias correctly. I'm going to take some voltage readings throughout the amp and write them on the schematic so I have more than bias current and plate voltage numbers for the future. I'm really enjoying the amp, putting lots of hours on it.

When there are problems, its a learning experience. Thank you.

Tubeamp

Glad to hear you sussed it out!

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2