Einstein bias problem

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dass101

dass101

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Dear Peter, I sent you an email regarding some problems I have when biasing a 50W Einstein, just trying to make sure you got it.
The problem is that the bias reading is not steady, and it ramps up to around 90mA, then slowly back to around 65 mA, only to ramp up again. This ramping up is associated with increased vibration in the PT. I couldn't trace if this is related to wall voltage variations.

Looking forward to any advice you may have.

regards,
Dragos
 
How old is the battery in your multimeter?

Also it could be caused by variation of your input voltage. Do you have a cheapo voltage regulator which indicates the mains input voltage?
 
I'm really not sure about the battery :)
The first thing I thought about was problems with the wall voltage. While checking the bias current I noticed that at exactly the same moment when the bias current rises there is a noise in the Power transformer (or is it the OT, I'm not sure, it's on the left while looking from the front of the amp) Also, I could swear that a few of those times I noticed the neon lights in the room flickering...

The next logical step was to measure the wall voltage, it wasn't steady, but I couldn't notice any voltage spikes or something like that...

I guess I should try to do that whole process again in a more controlled enviroment, the rehearsal room is in a former factory, God knows how are the power lines there.
 
Thats probably a good idea. Let us know about the results.
 
Ok, I have the amp at home right now, I get around 79 mA, with variations between 77 and 80 mA

I know this is higher than recommended (this is with the old tubes) but is the 3-4 mA variation normal?
 
Yes, that is a very small variation and ok. If you run EL34s bias them to 70mA. 77-80 is too high.

Kind regards,

Jens Kruse
 
Great, I lowered the current to around 65, but I'll do the final setting with the new tubes. So I take it that there's a problem with the power at the rehearsal space. How dangerous is this for the amp, and what are my options to avoid it (I hope something else than a very expensive stabilizer :( )
 
This is way above my budget, could I just use a good 200$ or so UPS unit?
 
Yes as long as the UPS unit supports your amps total wattage !

You can get the conditioners I mentioned on ebay used for little money.

Jens
 
I found one for ~$350. Furman AR-1215 I got it brand new for around that price. It's a 120v one, but its much cheaper than $850!
 
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