SVT problem

countrybassist · #1 09-29-2010, 06:45 AM

I bought a '73 SVT about a month ago, and until the other day it was working fine. After about 45 min the sound had changed quite significantly. It became "hi Fi" sounding and lost most, if not all, of it's bottom end. After turning it off and playing it again the next day, it worked fine for awhile and then the same problem started again. The bro I bought it from had told me that he had new power tubes and caps put it in, but some of the pre-amp tubes are original. Does this sound like it may be a tube? Any thought are appreciated.

Rickett Customs · #2 09-29-2010, 06:48 AM

If a few of the pre tubes are still original and you're saying that your lowend is disappearing, then yes I'd say there's a good chance that it may be the tubes.

nysbob · #3 09-29-2010, 06:58 AM

Just for fun, work the rocker switches back an forth a few times. They can get a piece of dirt in them or be partly switched and change the sound dramatically.

I leave all the boosts off on mine, if that ultra hi switch accidentally gets turned on, it sounds awful.

hover · #4 09-29-2010, 07:05 AM

Quote:

Originally Posted by nysbob

Just for fun, work the rocker switches back an forth a few times. They can get a piece of dirt in them or be partly switched and change the sound dramatically. .

Between this and maybe a preamp tube going bad....

fu22ba55 · #5 09-29-2010, 07:17 AM

Lots of things could have gotten jiggled around in shipping (if the head was shipped to you)... I'm sure it was carried by hand everywhere by its previous owner, but who knows who handled it in shipping. (Unless you bought it local).

When I moved, my SVT went in a flight case and I saw them put it on and take it off the truck, but it still needed pretty major service after being banged around by the movers. (some power tubes, bias circuit issues...)

If you don't have one already, find a well-trusted, competent, tube amp tech in your area and have him/her bless your SVT. You're going to be meeting them eventually...

countrybassist · #6 09-29-2010, 07:34 AM

Although it was shipped in a road case, I'm sure that it got bumped around alot. It did work fine for about a month. The rocker switches should be ok. When I first fire it up, it works perfectly for about 45 minutes, so I'm assuming that something is happening as it gets hot.

JimmyM · #7 09-29-2010, 09:17 AM

that makes me think it's a tube issue. however, won't be the first time i was wrong if it's not

Alex1984 · #8 09-29-2010, 10:35 AM

If it had an effects loop, ie. preamp out/power amp in, I'd check those jacks and short them if necessary.

timv · #9 09-29-2010, 12:59 PM

All the above suggestions are good. Also, could be dirty preamp tube sockets. Sounds like it needs a good simple going over and clean up.....

When it happens, try flipping the rocker switches and you could try gently wiggling the preamp tubes, with the amp on, you may get a couple snaps and pops but make sure to turn the volume down a little bit first, this might point out a certain tube or socket.

Also, with it turned off, you could remove each preamp tube and just reseat it and see if the problem comes back....
Good luck....

Thumpking · #10 09-29-2010, 02:15 PM

Quote:

Originally Posted by hover

Between this and maybe a preamp tube going bad....

+1. Keep you fingers crossed it is not the PCB.

sedan_dad · #11 09-29-2010, 03:57 PM

Check that stupid 6C4.
Is it weak in both channels?

zachoff · #12 09-29-2010, 03:59 PM

Something similar to this happened on my guitarist's V4 and it ended up being a bad bass pot.

ghostsontheroad · #13 09-29-2010, 04:05 PM

I had a similar issue with a V4 and we took all the power tubes out and one was going bad...what you are describing sounds like a tube issue, especially the loss of low end and the change in tone. I would meticulously go through all the tubes and see if they are all ok. 1st go ahead and swap all the preamp tubes, before you mess with any others. It could be a 5 minute fix.

countrybassist · #14 09-29-2010, 05:06 PM

Thanks for the replies. Now one more question, I'm not much of a tech, but I do know how to remove and replace tubes, but in the SVT what is it that I'm not suppossed to touch if I don't want to get fried! LOL!!

nysbob · #15 09-29-2010, 09:05 PM

Stay away from anything that looks like a can. You should be able to get the tubes in and out without touching anything else.

In the long run, though, you're going to need to establish a relationship with a decent tube amp tech, so there's no time like the present. He should be able to track it down with an hour of bench time...plus maybe you can get him to check the caps.

Jerrold Tiers · #16 09-29-2010, 09:11 PM

Sounds heat-related, and very possibly NOT a tube, but rather some other part that "goes open" when it heats up.

You might see if any controls affect it next time, and if one channel only or both is affected. Also if a good thump on top affects it....

Tubes are as hot as they are gonna get in a few minutes, generally, although they CAN do that also.

BassmanPaul · #17 09-30-2010, 11:45 AM

Quote:

Originally Posted by Jerrold Tiers

Sounds heat-related, and very possibly NOT a tube, but rather some other part that "goes open" when it heats up.

This was my first thought too. May I suggest that you do NOT open the amp. Thermal problems can be a bear to find. It's why freeze sprays were invented. If Jerrold's advice doesn't get you anywhere please take the amp to a tech. His bill will be way lower than what you life is worth.

Groover · #18 09-30-2010, 12:01 PM

Have you wiggled and giggled all switches and such make sure oxidation isn't the culprite?

BassmanPaul · #19 09-30-2010, 12:06 PM

Quote:

Originally Posted by Groover

the culprite?

A culpable sprite perhaps?

WalWarrior · #20 09-30-2010, 12:10 PM

These amps scare even the most talented techs. Don't screw around if you don't know what your doing. These amps can crank 700 volts (this is approaching electric chair voltage) across the plates. Take it to a qualified tech.