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Vintage SVT (MTI) biasing procedure?

#41
SOB H.R.MC said:
My SVT has the stand-by light on the front of the chassis on the right hand side and is only lit when it's in stand-by mode
Click to expand...

I think mine has the same.
b-o-t: look at the one of the videos showing front panel chassis (with bias pots), it's right there.

BTW..

Today I installed all the tubes. First, the preamp, driver and phase inverter. As I don't have 12DW7s I replaced all them to 12ax7s (Penta Labs), 12ax7 (NOS Sylvania) and 12BH7A-EH's. The only thing is 6C4 which now I don't have but it's on the way. After replacing these tubes I noticed some loss of clean volume. But I... like it very much, it sounds way better on lower volumes now, not so sterile, but I think I will get the 12DW7s from JJ sometime..

Then I thought I will replace power tubes (have new sextet of 6550WE's). They were warming up about 20minutes, I set them to around 71.5mV/trio and then started to set up the balance (iPod, 40hZ wave file mono inserted through input), it was fairly easy to set it up (and I've done that 1st time!) but I ended around 3/4 scale on the pot (yy?). Then I got under 0.05 mismatch (ampeg recommend +/- 0.1 so it was even better) -- and it was under that values when the level was completely down AND when the 40hz sound was very loud and started to distort. SO GOOD. Then bias started to drift in some ways that on test point 1 i had 68mv, on the tp2 - 76. I started bringing them to the right levels around 72, then again I did the balance matching AND between that i played some bass etc.

I did this couple times in 2hours...

Bias was drifting every time and I get the mismatch around 0-8mV on the sides, is this is a lot for an SVT? Also, from the beginning, one of the power tubes does not have the blue light in it and it does not "flash" when I'm playing. Should I worry about it? I don't want to harm new set of tubes... :help:
 
#45
More than likely the 10 ohm 5 watt resistor in the plate circuit. If the cathode 1 ohm blew, none of the tube on that side would work.
There is an update for the MTI heads that replaces some parts for stability.
To me it sounds like you really should bring the amp to a tech because it gets dangerous inside once you open it up.

If you blew one resistor, you should replace all of them on all 6 power tubes to make sure they are fairly balanced.
 
#46
Given the drifting bias, I would inspect the solder joints of all the related components on the power tubes. Look at the plate resistor, connected at pin-3, the screen resistor at pin-4, and the 1 ohm cathode resistor on pin-8. There are only two 1 ohm resistors.

The cathode resistors are the ones that would concern me the most. A bad resistor (not necessarily burnt open) or a bad solder joint here can result in the drifting bias. If you change the cathode resistors, they should be matched as close as possible as was mentioned.

Normally before you set the bias, you need to warm the amp up, with the standby off (in playing mode) for at least 20 minutes. It isn't unusual to see a small amount of drift for an hour. If after two hours you are still seeing drift, there is a problem. If it eventually stabilizes, you still should consider changing the plate and cathode resistors or at least re-soldering the related connections. In an older amp, I just change them all for matched sets as part of what I call routine maintenance.
 
#47
OK.

Here's the final whistle: I took the amp to the tech's - two times since my last post. Amp went through the heavy check for everything. The original tube sockets were replaced for hq ceramic sockets. New set of tubes (6550WE) and also preamp (EH, JJ, PentaLabs + NOS 6CA4 from GE). Lots of bad resistors (or open ones) in the preamp that were fixed. The cathode resistor too. All the big caps are OK and have their specs at 100%.

I was writing last time about an issue with one power tube (looking from the back, it's first on the left side) that it wasn't flashing blue when played hard. The rest 5 tubes are flashing, this one was not and it's not a tube fault (I've swapped some in that place). The tech said TWO times (as the amp was two times at the doctor) that everything is great with power amp and it may not flash due to very little space to the big transformer and it may interference with it (?)... or he does not know, because all the tibes GIVE their power and all the 6 work OK. This tube dont flash also when you put the playing mode on (with switch on the back), the rest of them, are flashing. Amp is super quiet, it does not produce any hum or noise, even at maximum volume (could you believe?).

I had a 20+ tube amps and this is the first time I have that strange behavior of only one tube. Is it possible to be like this? I mean - I can't believe in it, but hey, if that tube would not work, the amp will be really hard to bias and will hum, right? So?...
 
#50
I'm not entirely clear on teh "flashing" you mention... If you mean the blue glow often seen on the tube, it can be there, or NOT be there, and the tube can still be good.

Usually if a tube used to show it, but now does not, it is time to take a look at it. but if it NEVER showed the glow, that's not necessarily an indicator.
 
#51
Jerrold, these tubes does not glow blue when are in playing mode (only a standard orange-red glow), they just "flash" blue when i play hard, e.g. when loud signal is present, which I think is quite normal for 6550's.

But one of them don't behave like the other five and this is that last one on the left side looking at the back. No matter which tube (of the rest) I put there, it's not flashing when playing, but remains standard orange&red glow (looks the same like the other when not playing).
 

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