Ampeg SVT Classic Bias Light issue

[tal35]

I have a couple of these that the red bias light will not go out even though the green bias light comes on. New matched tubes, all 220 ohm resistors are good, no bad sold joints on the tube sockets. Help?

[Chuck H]

According to the manual the amp is under biased. I've never used or repaired one so I just looked it up Link below is the owners manual and includes biasing instruction.

http://www.ampeg.com/pdf/SVTCL_OM.pdf

[The Dude]

What brand of matched tubes, where did you buy them, and do you own a bias probe?

[tal35]

Ampeg problems

What brand of matched tubes, where did you buy them, and do you own a bias probe?

The tubes are sovteks in one and mullard in the other The sovteks from Tube depot, don't remember on the Mullards. I do have a bias checker. I have not checked the bias on them as of yet. The problem started when I had a tube go bad in each. I have not checked the 10 ohm block resistors on the main board as of yet. What is the red/green light bias range?

[Chuck H]

If you haven't checked the bias why are you questioning the red/green light condition? Have you twiddled the bias adjustments and just not been able to make the red light go out? The manual states that a red light AND a green light indicate an under biased state. It also indicates that the bias controls are interactive. This may be like the old SVT's where a bias and balance had to be achieved. In this light it's entirely possible to have the two push/pull power banks cattywhompus. This could make it tricky to get to a "green only" state on both banks. I didn't read the whole instruction but I inferred that there is more than one bias adjust mechanism. Both need to be right AND in sync in order to illuminate the green LED only.

[tal35]

If you haven't checked the bias why are you questioning the red/green light condition? Have you twiddled the bias adjustments and just not been able to make the red light go out? The manual states that a red light AND a green light indicate an under biased state. It also indicates that the bias controls are interactive. This may be like the old SVT's where a bias and balance had to be achieved. In this light it's entirely possible to have the two push/pull power banks cattywhompus. This could make it tricky to get to a "green only" state on both banks. I didn't read the whole instruction but I inferred that there is more than one bias adjust mechanism. Both need to be right AND in sync in order to illuminate the green LED only.

Yes, I have tried to adjust the bias controls, both of them. Right side is able to adjust from Red to Green only and the left side which had the bad tube before, cannot adjust to only green, the red also stays on. Yes I did try to adjust both to get just green. On the other amp, the one with sovtek tubes, I did check and bias was the same withing 5 mils or so for each tube .

[The Dude]

I would adjust bias to just where the green light comes on and then check each tube with your bias probe. It's my guess that one or more tubes will be drawing much more current than the rest. In my experience, these amps seem to be picky about tube matching- too much so IMO. This will cause the LED problem you describe. It seems these days that matched tubes for some companies means that they are all packaged in the same style boxes, so I don't necessarily trust that they are indeed matched. If you find that this is the case, you may be able to put all of the "higher current" tubes in one side and "lower current tubes in the other and match the + and - sides with the two bias controls. I've had success doing this in the past.

[Enzo]

Having serviced many of these, I all too often find the small tubes in the power amp to be in the wrong sockets or even the wrong type. And when they are right, the symptom you report I often find is due to a bad 12AU7 driver tube.

There are three small tubes in the power amp. V1 is a 12AX7 and is the input stage and phase inverter. It is the tube closest to the SLAVE jack. V2 and V3 are 12AU7s, one is for the push side and the other for the pull side, as it were. They are located right behind the two bias controls. Make sure they are all the right types, and that they are stuck in the right holes.

[tal35]

How much difference in the tube bias would you consider mismatch for them?

[tal35]

Yes I did check the small tubes,usually first thing I do as I have found them mixed before, not sure how but both of these amps have it correct. I have also checked them with a checker. but the tube checkers don't always tell everything.

[The Dude]

As I understand it, if any cathode is under about 15mA - no green LED. If any cathode is over about 30mA, the red LED lights. Saying that, if you are mismatched more than 15mA you won't be able to get to "green light only" condition.

Another reason to check each tube with the bias probe is to make sure all of the tubes are conducting. If you had a tube failure, it's possible that a resistor or something burnt and one tube is not working at all.

Edit: I should add that I've run into a few where the supposed 15mA range is a bit tighter than that.

[tal35]

As I understand it, if any cathode is under about 15mA - no green LED. If any cathode is over about 30mA, the red LED lights. Saying that, if you are mismatched more than 15mA you won't be able to get to "green light only" condition.

Another reason to check each tube with the bias probe is to make sure all of the tubes are conducting. If you had a tube failure, it's possible that a resistor or something burnt and one tube is not working at all.

Thanks, I will check to see how much difference they have. It will be a week or so before I can get back down there to check though.

[nevetslab]

SVT biasing& bias lights

As Enzo had indicated, when you're NOT getting response in adjusting one side or the other with the bias pots, you could have a bad 12AU7. Or, possibly solder fractures on the bias pot terminals themselves. I service these amp often, and because the rear panel hardware is used to support that end of the main PCB in the chassis, I find solder fractures on the component leads....requiring de-solder and re-solder to restore them.

While the bias LED's are Very Handy, driven by the internal window-comparators, where GRN has it's window from 14.7mA thru 29.4mA, I'm normally setting my bias to around 22-24mA, by monitoring the current thru each tube's cathode resistors, found along the edge of the PCB on the Output Xfmr side, with two of the resistors parked 'inland' a bit, as seen in the photos below:



With the amp standing up on end, you're able to monitor the current thru all six power tubes, and it also facilitates juggling the tubes for best current balance on each half. In these photos, I had tack-soldered some eyelets on each of the cathode resistors so I could hang the probes in place for photo purposes...normally I'm just clipping the Gnd lead of the DMM on the bottom of the cathode resistors...just enough lead exposed on the component side to do that, and measuring on the solder pad for each of the cathodes, and jotting down the readings.

I've also had to repair the Window Comparator circuit before, finding bias adjustment was working fine, but NOT the LED circuit. Ive also got the amp powered thru a variac & power analyzer, so I can see change in AC Mains current/power as I adjust each side.

On other amps, I'll make use of my Compu-Bias Bias Probe meter & probes, but.....it's limited to 600V, which the SVT is beyond on it's plate voltage. Since the SVT-CL has discrete cathode resistors, I just make use of those.

[gui_tarzan]

I re-tubed one of these a few months ago. Make sure the ribbon cable connections are good. Bias voltage goes through that cable and if I remember correctly, I had to re-solder those connections because a few of them were funky.

[TimmyP1955]

Swap the left three and right three 6550s to see if the problem switches sides or stays put. If the red light switches, one of the tubes on the red side is iffy. If it stays put, swap the two 12AU7s and see if the problem switches sides or stays put. If the red light switches, the one on the bad side is bad.

If the problem never moves, listen to the other guys here :-)