Ampeg SVT-3 Pro Tube Gain Question

I just picked up a used SVT-3 Pro. Early 90's model. Everything works as expected except for one thing. On all others I have tried, when you adjust the Tube Gain from max to min, the sound gradually mutes and then comes back (as the little decal on the face plate suggests).

On this one, instead of gradually muting and coming back, I get a rather odd buzz before it comes back. It could be that I never had it loud enough before or I wasn't paying close enough attention, but can other SVT-3 Pro users clarify this? Otherwise, the Tube Gain control itself works as I remember with regards to tone.

On another note, the fan sounds like a lawn mower with leaves caught in it. Upon further inspection it appears to have some serious grime buildup. I just ordered an exact replacement from DigiKey, part number 032127.

Hey everyone, thanks for the replies! Guess I'm OK. Gonna test the tubes on my trusty old Mercury tester and replace any if necessary. Anything else I should be aware of with this amp? Read about the MOSFET bias adjustment, but I think I'm OK in that regard. What I'm wondering is however, how come people have that distortion problem on new units? What can cause it to pop up over time?

Anyway, regarding the fan, I did blow it out and it still sounds like it's "rubbing". However, if it's normally obnoxious, has anyone ever found a suitable replacement? It's a DC fan, so I'm wondering if an ultra-quiet PC fan with similar specs can be substituted. There are fans out there that can move just as much air but are much more quiet. It is definitely an issue in a small home studio.

OK. Tubes all tested good except for the 12AU7. Came up "Weak" on my tester. Could it be due to the "Made In China" sticker on it?

Been doing some reading on tube replacement (option anxiety overload), seems an RCA Clear Top 12AU7 would be a good start.

On that topic, I searched around but didn't see much regarding the SVT-3 Pro in reference to tube replacement recommendations. The stock tubes (in my amp) are: 1x Groove Tube 12AX7, 3x Sovtek 12AX7's and the one Chinese no-name 12AU7.

So... If one wanted to re-tube the entire head, what tubes would be suitable if not better?

Thanks for the info! Was actually thinking of going with www.eurotubes.com. However, I've read some conflicting posts on using eurotubes (JJ) vs. NOS tubes. Any thoughts there or is this opening up a whole 'nother can o' worms?

So I am finally done futzing with my SVT-3 Pro...

I have to say, NOW I am impressed and very happy!

I did 3 things:

1) Complete retube. I went with eurotubes.com and ordered 4 standard JJ ECC83S tubes to replace the 12AX7's, and 1 JJ ECC82 for the 12AU7. There are options within these choices, but I went with the "standard" tubes. I dealt with Jay who was very helpful and thorough with explaining all the options. The improvement is nothing short of AMAZING! The amp literally came to life. I have more gain, but I didn't lose ability to get a nice clean tone (which was a concern). I also have much more overall volume (which I believe is due to the ECC82?). I no longer need to engage the EQ and bump the EQ level slider to get more volume. The tone is rich, warm, fat and punchy. If I want growl, I can get growl (although at the moment I still like my SansAmp Programmable Bass DI for the dirtier tones). The fan seemed to run a bit slower after the retube (albeit still noisy). It still ramped all the way up when the amp was first turned on, but it gradually slowed down to the point where I didn't hear the motor anymore, just the grinding sound I described before.

2) Replaced the fan. There was definitely something wrong with the original one. The (exact) replacement is by no means quiet, but it is a significant improvement to the degree where I'm no longer distracted by it when playing at low volume (the grinding noise is gone).

3) MOSFET bias. I read here in the forums that the 12AU7 is what feeds the MOSFETS, so I assume that replacing the 12AU7 (if not all of the tubes) had an effect on the amount of signal feeding the power amp section. I did notice some very low "solid state" sounding distortion/fuzz after the retube, which sounded exactly like what others have described on the forums. I carefully used a plastic potentiometer adjustment tool and nudged the MOSFET bias pot ever so slightly clockwise until the fuzz was gone. It literally took a hair's thickness of a turn. Since I did it by ear, I would like to try and find out the procedure for doing this with a meter (i.e., the right way) so I know it's correct for my new setup. Can anyone help in that regard?

So there you have it. Thanks to all the people here on TalkBass for your replies and helpful suggestions!

I have read elsewhere here on the forums and heard from an electrical engineering buddy of mine that the retube should not have affected the MOSFET bias. Rather, it can "drift" over time.

It's possible that I simply didn't notice it before and was paying more close attention after the retube. I also played the living out of it that night.

To jeromebass, as for swapping out the tubes, just power down and unplug the amp, unscrew and remove the top cover, gently pull out each tube (might have to wiggle them a bit) and replace them with the new tubes. Just be sure that the ECC82 goes in the 2nd socket from the back to replace the 12AU7. The rest are all the same. The pins on the tubes are arranged so that it would be almost impossible to install them incorrectly, unless you used a hammer...

I hope you are as happy with the results as I am! Keep us posted.