Dear forum members,
Here i would like to share my experience of re-BIASing my power amp.I tried to search the forum for this topic, but the only good piece of mind i could get was to find a good amp technician to do the job.
First of all about the BIAS current measuring tool.
I've recently got it from one ebay seller.The tool consists of a meter (basically it is a voltmeter with a channel switch for 4 channels).Each channel can be connected to a BIAS measuring probe, which has to be inserted into the amps tube slot. The probes have a high precision shunt resistor of 1 ohm in them in order to convert the voltmeter reading directly into mAmps.So you just insert a tube into the probe and switch on the amp. After heating up in stand-by mode you can switch the amp on and then the BIAS current reading will be displayed on the meter LCD csreen. By switching the channels you can measure each of the amp power tubes separately, and then adjust the BIAS current by the corresponding potentiometer.
My power amp is the VHT 2/50/2 stereo with EL34 power tubes.
Admittedly, the probes for this type of tubes have 8 holes (for the tube metal pins), however there are other probers which have 9 holes (for other type of tubes).
Here i would like to express my regrets. It was foolish from my side to choose to buy only 2 probes instead of having 4 probes, which makes re-biasing job WAY more easy to accomplish. I've spent numerous dozens of minutes for inserting and pulling out the probes back and forth before having the job done.
My first task was to check whether the old tubes were still good, so first i measured them (in 2 attempts because i had only 2 probes). The probe inserting procedure required to partly dismantle the tube support bracket, because the tubes are placed horizontally and there's just not enough space in the chassis for a tube together with the probe. But that was just a technical issue of the amp itself.
Then i read the biasing tool manual and measured each power tube pair bias currents for each of the channels of my stereo amp. What i came across was that the tube bias currents were a bit different for each tube, and this difference was out of the 2-3 mA range stated in the tool manual. My readings were showing about 6 mA difference between the tubes in channel A and about 10 in channel B.
Besides that i was unable to re-bias the tubes of channel B to keep the currents within the range stated in the manual for the EL34 tubes (as per manual should be 30-40 mA), bacause the biasing potentiometer of the amp was already set to the maximum current. So the tubes were supposed to be replaced anyway.
The next step was to measure the new tubes i got from the web superstore thomann.de
These were 2 matched pairs of SOVTEK tubes, but when i installed them and measured, it appeared that the bias current difference between the tubes was even greater than with the old tubes. However the re-biasing was possible with the potentiometer of the amp.
I marked the tubes and then tried to swap them with ones from the other channel, but the difference became even greater.
Well, here are my final re-biasing results:
for channel A one tube was reading 38.6 mA, the other one 25.9 mA
I choose to leave it this way, because if i tried to increase the lower value, the higher value would go out of the upper permissible value (beyond 40 mA) which is kinda more dangerous than just if it was lower than the lower value.
for channel B one tube was reading 36.2 mA, the other one 30.5 mA
That's basically how it is set now with the new tubes in.
My conclusion is that even the matched tubes in sets are sometimes not that well matched as they are meant to be.
I really don't know if it can be rectified somehow, maybe you can suggest some method?
Regarding the tool itself i have to admit, that it is easy to use, the manual is full of all the important info regarding the use of the tool and the recommended settings for different tube types. Very important aspect is that there are safety measures in it. Fit for dummies and newbies like me.
Any comments or opinions are appreciated!
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