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SVT-VR problem.

brothernewt

brothernewt

Some people call me the stormtrooper of love...
Apr 13, 2004
1,634
28
Gladstone, OR
#1
I know “take it to a tech”… and I will. I just wanted to show up with some info so I don’t get ripped off.

It’s a US made SVT-VR. I got it new a few years ago and have mostly babied it. The Bias and Balance lights on the back look normal and everything visually looks correct (to my untrained eye). I play in a cover band… 4 to 8 gigs a month, 3 to 4 hours each. I run it at relatively low volume, correct impedance, allow warm up/cool down time, and it’s usually moved/stored in a road case. It works and sounds great for a couple hours… then usually late 3rd set, it starts making a horrible intermittent static noise, almost like a bad cable crackle. The sound doesn’t stop or change, the crackle is just on top of it. I’ve replaced all the cables, used a different cab, etc. I cannot recreate the problem at home.

I’ve had poor experiences with the two major local shops (Inner Sound and All Service). I’m pretty sure they’ll re-tube the entire thing (both channels of the pre and all) at full retail price on the tubes, you can work out what that’d cost. The one highly recommended independent guy doesn’t want to work on it (thinks recent SVT’s are a pain). He suggested it may be a bias issue or a poor tube connection that only acts up when hot, and recommended taking it to Eurotubes for re-tubing (less expensive than the 1st two) but this still seems like it’s not the issue. Why would it work for so long without problems if the tubes were bad? Granted, I’m not a techie kind of guy but this seems illogical to me.

Any advice, suggestions, input, or recommendations on another place to take it would be appreciated.
 
#2
If it only happens towards the 3rd set, it could very well be a heat issue.
The only other variable I can think of is volume; towards the 3rd set have you lost a bit of hearing sensitivity, is the crowd noiser, and therefor the volume of the band increases? Could be something as simple as a dirty pot at a certain gain level.
 
#4
I don't turn up... pretty much a set it and forget it guy. Last time it happened I hadn't touched the amp at all between sets. It was a weird area and my amp was on the other side of the drummer pointed sideways. I might try more ventillation... maybe adding a 2nd fan (I saw pics of the V8 innards last night and it seems to have 2).
 
#8
MikeyB said:
+1 - Johnk posted a fix for the SVT-VRs that were going into shutdown - it involved bypassing the relay. I did that mod to my own VR and it's good to go.

Johnk is a nice guy and knows his SVTs!

-M
Click to expand...

Yah, I did the same as well, after 3 gigs, where the voltage was below 110v, ended up using my sansamp those gigs...... No problems since.
 
#9
that's a very difficult problem to diagnose over the internet, but i do think that it is either heat related (i.e , a cold solder connection/bad connector) or a preamp or driver tube that is eventually acting up.

once it does it, have you checked to see if it does it on both channels? if it does, and it is a tube (?), it is either the last tube in the preamp (on the top chassis), the phase inverter, or one of the two driver tubes (on the lower chassis).

this is the type of problem that i would probably perform the 'chopstick' test on, where you get it to demonstrate the problem and them tap on various connectors/components to see if it makes any difference (i.e. goes away). intermittent and/or heat problems can be difficult to find as they must be repeatable to locate them. sometimes i have used blow dryers or a heat gun (at a distance) to speed up the process of heating up the amp/components, instead of waitng or playing thru it for 2-3 hours for it to get it running hot.
 
#10
I lean towards something other than a tube, only because the tubes are the FIRST things to warm up, and in 20 minutes they are pretty much as hot as they will get.

Since it seems to take much longer than that, heat is presumably getting to something else in the unit.

What I would do on the principle of trying easy and possible things first, is to try the "chopstick test", and/or have a substitute tube to swap in. You'd need one 12AX7 and one 12AU7, and in a practical sense you would only get to swap the ones in teh power amp...... but that might be enough. Slap it in standby, grab a rag, and do a lightning tube swap. Almost lightning, you will probably pull a power tube to reach, but you get the idea.

if no go there, it might be preamp, or power tube, or chassis component.

As I said, I think it will come down to a chassis component just because the time is so long.

The description kinda reminds me of "Fenderitis"... Bad plate resistors used to do a similar thing in Fenders, back in the stone age.
 
#12
Thank you all for the input... it's much appreciated. I'm pretty sure I tried the other channel and it continued, but I'll double check that next time it happens (or if I can manage to repeat the problem at home). I'm pretty sure also that I have a 12ax7 and a 12au7 laying around... I'll try swapping those out as well. I also planned on having the relay bypass mod done when I took it in, even though that's never been an issue for me.

J
 
#13
Interceptor said:
This is also where a can of freeze spray is fun.

My hunch is a cracked solder joint or a flaky resistor. I wouldn't be looking anywhere but the PA or driver sections.
Click to expand...

Wuh-woah................. watch the freeze mist around tubes. They make a nasty little 'clink" noise and the vacuum all runs out........

yea, it's harder to screw up with the chassis open, but............. and the OP doesn't sound like he ought to open it.... I know good techs who are not fond of the 660V B+......... and don't care for working on SVTs. Non-techs should stay out.
 
#19
Jerrold, I see your point regarding freeze spray.

I had figured that anyone who is sharp enough to be digging around in a hot SVT is also sharp enough to not hit a tube with the stuff.

I spent many years working on tube based radio and TV transmitters. I am exceedingly careful and have taught many a tech how to stay safe. I got bit by a 2200 Volt supply once. Only once. I now religiously powder test my gloves.

I assumed (yeah I know) that anyone rooting around in a high power tube amp was well schooled in personal safety AND in keeping the gear safe. I made bad assumptions.

For the record, if someone hasn't been well trained by a competent tech, and haven't had their work methods observed by a solid tech, they have NO BUSINESS inside a hot amp. And even then, they shouldn't be doing it alone.
 

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