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1975 AMPEG SVT - Issues (need help Video attached)

J.KEEFE_bass

J.KEEFE_bass

Feb 1, 2008
167
58
Richmond, Va
www.psychobillycadillac.net
#1
Hi everyone-

My SVT is sick and I need some help. Here is a link to a video i made describing my issue.



Have any of ya'll ever ran into this? I have the volume near full blast and have very little output and my amp sounds like it has a distortion pedal in the chain. If anyone can shed some light, I will be most grateful. I'm really hoping all I need is some new 6550 tubes. (fingers crossed)

Thanks for any and all help!


-John
 
#2
Hate to see an SVT down and out....
This could be a number of things that could cause this..
First do the simple basic stuff...process of elimination..
Try changing the pre-amp tube common to both channels..such as the 6C4 ......this is last tube on the far top left hanging upside down in the headbox ...
The SVT has pre-amp out as well as pre-amp input jacks on the back...try to plug into that jack another signal source to see if your power amp section is working properly...
Try changing the 12DW7 on the front of the power chassis right in the front of the removable grill cloth....
If pre-amp tubes don't solve this...then the amp needs to be opened and checked out by someone familiar with working amps like this....
There is High Voltage in the amp and if you monkey with it without knowledge of this type of circuit you could get zapped...
To trouble shoot this circuit is fairly easy and quick to find out the issue.....hope it's not the OT...
Did you do anything recently to the amp??? Change tubes?? Clean it?? take it apart ?? Lend it to someone for a gig ??? Did you disconnect the Molex connector at anytime recently ??
Forget to plug the speakers in when playing ???

Chris
 
#3
I think it sound pretty fabulous, but, yeah, not the kind of tone you can use all the time, and pretty freakin' quiet for an SVT. The distortion is probably because you've got the pre-amp gain turned up so high in order to be audible. And that suggests that your power stage is barely functioning. *Could* just be power tubes. *Could* be that little 12DW7 that buffers the pre-amp signal coming into the power amp (some SVTs were modified to take an easier-to-find tube). Could be other things, like the supply to the power tubes. Do all the power tubes look the same visually as they power up and operate? If you visualize your power tubes as V4-V5-V6-V7-V8-V9 in a row, you can think of 4-6 as "pushing" and 7-9 as "pulling". If you, for example, swap 4 and 7, or 6 and 9, does that make a difference?

And for goodness sakes, be CAREFUL in there, if you do anything other that swap the tubes. There are some lethal charges lurking in those capacitors!
 
#4
In addition, try another bass into the amp.

If you have an active bass, change the battery.

If all that is fine, you can try plugging your bass directly into the power amp in, see if it's distorted.

If you have a volt meter, check the power tube bias.
 
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#6
Salicete said:
Sounds just like mine did before it got its long overdue cap and tube job.
Click to expand...

First thing i would not keep playing it like that just in case it is a problem with the power amp, how long have you played it like that? I would get a pro to look at it if I were you, SVT's are not the easiest amps in the world to trouble shoot. Have you tried the simple things like changing the last tube in the pre amp section, the 6C4? Have you made sure your speaker cables are ok? Have you tried just one cab? Does it sound clean at 9 or 10 o'clock on the volume control like it normally would except for the massively reduced volume? Did this happen all of a sudden or did it get softer over a long period of time? Remember there is 700 volts of plate voltage in these, that's enough to kill you so I wouldn't be taking it apart and fooling with it unless you really know what you're doing. Incidentally the board under the plate that is missing on your main chassis surface has 700 volts on the big power resisters, I would get one and put it on. I would not swap the power tubes, i don't think that would do anything. 1 or 2 bad power tubes would not do that. It would be distorted but still loud. I also feel it is a preamp problem but I still would not keep playing it like that just in case you have a shorted speaker jack (which I doubt) or some other problem in the power amp section. As the previous poster said it's always good to get these things checked up every once in a while anyway. SVT's will play usually for a long time even when they're hurting, bad tubes, incorrect bias etc. but it's not a good idea and this one obviously is beyond that point.
One more thing you should always turn it on with the standby switch in the off position and don't turn the switch on for at least 30 sec, you'll eat power tubes if you leave it on all the time. Are those the original power tubes? Power tubes don't all go at once, when they're completely worn out they usually draw too much plate current get cherry red and blow the plate resister for that tube which takes it out of the circuit almost like a fuse blowing. I don't think this is a power tube problem.
 
#7
cerrem said:
Hate to see an SVT down and out....
This could be a number of things that could cause this..
First do the simple basic stuff...process of elimination..
Try changing the pre-amp tube common to both channels..such as the 6C4 ......this is last tube on the far top left hanging upside down in the headbox ...
The SVT has pre-amp out as well as pre-amp input jacks on the back...try to plug into that jack another signal source to see if your power amp section is working properly...
Try changing the 12DW7 on the front of the power chassis right in the front of the removable grill cloth....
If pre-amp tubes don't solve this...then the amp needs to be opened and checked out by someone familiar with working amps like this....
There is High Voltage in the amp and if you monkey with it without knowledge of this type of circuit you could get zapped...
To trouble shoot this circuit is fairly easy and quick to find out the issue.....hope it's not the OT...
Did you do anything recently to the amp??? Change tubes?? Clean it?? take it apart ?? Lend it to someone for a gig ??? Did you disconnect the Molex connector at anytime recently ??
Forget to plug the speakers in when playing ???

Chris
Click to expand...

Chris Thanks. I don't have any spare tubes yet so i jumped straight to testing the preamp out. As you advised i took signal from the preamp out into another source and a got a clean sound. So is the power amp section is working?
 
#8
J.KEEFE_bass said:
Chris Thanks. I don't have any spare tubes yet so i jumped straight to testing the preamp out. As you advised i took signal from the preamp out into another source and a got a clean sound. So is the power amp section is working?
Click to expand...
OK...That is really good news.... because dealing with the pre-amp is much easier and simpler...and a lot less expensive, since most stuff in the pre-amp cost a few dollars to replace...
I suggest looking to substitute the 6C4 first just for process of elimination... My next suspicion would be the single filter cap in the pre-amp stage, my money is betting that it is that filter cap in the pre-amp that went south .......
 
Last edited:
#9
J.KEEFE_bass said:
Chris Thanks. I don't have any spare tubes yet so i jumped straight to testing the preamp out. As you advised i took signal from the preamp out into another source and a got a clean sound. So is the power amp section is working?
Click to expand...
Hang on, what do you mean here "took the signal from the preamp out into another source"? Are you saying that you ran the pre-amp out of another amp into the SVT power stage, and it came out clean, or you took the preamp out of the SVT and ran it to another amp? If the former, then, yeah, your power stage might be fine, and, as cerrem suggests, that 6C4 is an obvious suspect, along with the filter cap. If the latter, I'd look at that 12DW7 and its supply.
 
#10
Sid Fang said:
Hang on, what do you mean here "took the signal from the preamp out into another source"? Are you saying that you ran the pre-amp out of another amp into the SVT power stage, and it came out clean, or you took the preamp out of the SVT and ran it to another amp? If the former, then, yeah, your power stage might be fine, and, as cerrem suggests, that 6C4 is an obvious suspect, along with the filter cap. If the latter, I'd look at that 12DW7 and its supply.
Click to expand...

I took the SVT preamp out into My SWR 350 receive. My SWR 350 was plugged into a speak cab. I plugged my instrument cable into the SVT and what came out of the speaker was super clean and sounded great. I'm on the 6C4 & the filter cap. Thanks so much guys!!!!!!!!!!!!!!!!
 
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#12
This is confusing. Let's start again.

You connected the SVT preamp out into the SWR power amp in. You plugged into the SVT preamp in and it sounded clean. This means that the SVT power amp has the issue, the preamp is working properly.
 
#13
There are three small signal tubes in the power amp, a 12DW7 and two 12BH7's. You said that you don't have a spare 12DW7 to replace the one in the power amp. If you have a spare 12AX7 you can use that for testing.
 
#14
beans-on-toast said:
This is confusing. Let's start again.

You connected the SVT preamp out into the SWR power amp in. You plugged into the SVT preamp in and it sounded clean. This means that the SVT power amp has the issue, the preamp is working properly.
Click to expand...

here is the signal path that produced the clean sound.
instrument cable into the SVT channel one input> SVT preamp out into SWR 350 Receive> SWR 350 speaker out to Speaker Cab.

so are we still thinking 6C4 & the filter cap or have i confused everyone. LOL.
 
#17
Just to be clear, make sure that the SVT speaker cab is connected to the SVT whenever the amp is powered up.

When you were testing the pre out on the SVT into the SWR, you should have had the speaker cab connected to the SVT.
 
#19
As an aside, vintage SVT owners should thank their lucky stars for the world wide web and the collapse of the Soviet Union. Back when I was repairing my own 70's SVT, in the late 1980s, it was nearly impossible to find oddball pre-amp tubes like 6C4s, and not easy to find 12DW7s, even in major urban areas.
 
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