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Hi,
I recently installed a diy driver board(rev.B) in my st-70, which already had an older sds capboard installed. Since this upgrade the bias drifts, even after a 2 hour period. For instance, if after 2 hrs with it biased at say 45ma.
I turn it off, the next time I turn it on from cold the bias will start at 54 ma.
and slowly drop. Is this normal? Also after zero-ing the voltage between tubes, this value drifts as well, not alot, but enough to make me think somethings not right.
power tubes are sed winged c
drivers are a pair of rca cleartop 12au7
phase splitter is a tele 12ax7 smoothplate, all test strong
meter is a fluke 75
wall voltage is 120v
Also the voltages at all tubes is a little low

thanks in advance for any suggestions,
scott

Scott --

Since the problem began with the installation of the new driver board, it would be obvious to start there. The only components on it that would generally cause a drifting problem would be the coupling caps, or the DC balancing circuit. It's very doubtful that the coupling caps are at issue here, as they would typically cause the bias current to drift the other way, starting low and gradually increasing if they were bad. On the other hand, if high temperatures are playing havoc with the dc balance control or associated components, that could be a very good possibility of what's causing your drift. I would monitor the actual negative bias voltage at the output tube end of the coupling caps, versus the voltage of the bias supply itself to use as a reference. If the voltage at the tubes changes, but the bias supply itself is basically stable, then you have a good clue of where to start.

Dave

Hi Scott ,

Are the output tubes new or the one's you already had , If they are new they could be the problem ,otherwise follow what Dave said .

Thanks for the replies,
The output tubes have a couple hundred hrs at most and bias was steady with the stock board. Temperature has been pretty cool with a small fan on the power trans. I've tried changing the stock mullard gz34 with a new sovtek with no change. I'll take some more readings tomorrow.
thanks much,
scott

I had some amplifiers that the bias started low and as the output tube warmed up the bias went to where I had set it hot. I have also had some that the bias was higher than the set value when it was first turned on. The bias always stabalized when the amplifier had warmed up completely. I believe that the bias shift runs with different brands of tubes. My rule is that if the bias returns to the set point an hour or so after it is turned on then there isn't a problem. If I set the bias to say 50mA hot then I expect to see a 50 reading when the amplifier is warmed up completely. If the bias is still varies after the amplifier has run an hour or so then I would start taking voltage readings or suspect the coupling caps as possible problems.

I have experienced similar findings...Identical in fact. See this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=4102

If you include the tubes in the equation, there is no doubt that different brands react differently to changes in temperature. In fact, this is also true on the other end of the scale. I use a power output tube tester for testing power tubes, and it is amazing how certain tubes from certain brands -- even when brand new -- will produce an initial power output indication, only to have the output steadily decline as the tube warms under the test. I have always felt it was due to a marginal cathode capability, as the best tubes will hold their initial power output reading for as long as the test is run.

In this case though, since the tubes are known to hold a steady static reading, there must be component drift causing the effect.

Dave

What wattage are the 10 ohm bias resistors supposed to be, as I bought the board assembled from e-prey and these were not included, I just used 4 closely matched 1/2 watt, 5% from ratshack?
Also, if I change the stock bias pots to 20 ohms will this give me more adjustment range as I would like to try some 6L6GTs but cannot get them to bias cool enough!
Thanks again,
scott

I've also noted that my bias will drop based on the number of window air conditioners that are running in my home while my ST70 is on. No, none of them share the same circuit as the amplifier, but all effect wall voltage as well. My home has 1-15,000 BTU unit and 3-5,000 BTU units. The 15k is on a dedicated 20 amp circuit. Generally speaking, only the 15k and no more of than 1 of the 5k's are on at once. The 15k will cause my bias to drop in both channels by about 12 mV. As an experiment, I fired them all up and noted bias dropped from 450mV per channel to about 405mV. It jumps right back up to 450mV as I shut the air conditioners off. Thankfully none of this interference has translated into any noticeable noise. Both my amp and preamp are plugged into an isolation transformer that may or may not reduce line noise.

Any difference in line voltage will cause a B+ difference which will cause a difference in the bias voltage which will either raise or lower the bias on the amplifier.

The 15k will cause my bias to drop in both channels by about 12 mV. As an experiment, I fired them all up and noted bias dropped from 450mV per channel to about 405mV

I can tell when my wife starts supper. The range, the A/C take my line down about 1v. and the bias swings with it. My line can swing 2.5v. during the day with everyone on the line at peak times.

Ah great point!

My amp is in my detached garage. A single 20 amp circuit feeds it via about
150' 12-2 uf cable. My voltage will vary depending on what is running in the garage. I'm sure when my 2 1/2 ton air conditioner cycles, my voltage also varies. I'll take some readings to see by how much.
As an electritian I've worked on homes where the neighbors air con. will make the lights dim momentarily.

scott