'76 SVT - Issue w/ preamp? power amp? can't tell?

[JMacBass65]

Short version, I have a 1976 SVT head that has not been serviced in at least 20 years. Clearly no cap job or anything. New power tubes and biasing was last done long ago. Didn't get much use starting 15 years ago, but for the past five, it gets out maybe 5 times a year.

It has had intermittent noise issues for several years. Not very loud, and sometimes not at all. Weird erratic sounds, kinda like a combination of static, buzzing, and morse code, a hovercraft....if that helps. I joke to my bandmates that the aliens are trying to contact us through my amp. If I smack it on the side or top, they sometimes get louder, quieter, or stop. Sometimes the amp runs as if there is no issue at all. This morning it was quite until I played a couple of notes, then the noise came. I continue to use it because even at their loudest, you really cannot hear it beyond the stage, and once we are playing, forget about it.

All of the amps functions - volume, tone controls, both channels, all (seem) to work as expected. And - it still sounds flipping AMAZING, which is why I have put up with the noise.

The noise, by the way, is at a constant level regardless of volume/tone settings. Happens with or without an instrument plugged in. If I tap or knock anywhere on the amp you can hear it come through the speakers, and it sometimes changes the character of the noise. Noise, when happening, only goes away when on standby.

This morning, while experimenting with using an outboard preamp into the power amp of my SVT (for reasons having nothing to do with the noise), I discovered that as soon as I plugged a cable into the "power amp in" jack, ALL NOISE went away - dead silent. Connected the outboard pre and turned up and all was good - no aliens at all.

SO - Question is - Have I managed to diagnose and isolate the issue to something in the preamp?? And if so, is it likely a tube, based on description?? And, I am no expert on the signal flow with this thing, but is the issue likely after the volume and tone portion of the pre, since they have no effect on the noise?

 

[JimmyM]

Yes, it's the preamp, and while I'm not the most educated on tube amp repair, sounds like a tube to me. Tap them lightly with a wooden stick while it's turned on (don't use anything metal for obvious reasons) and see if you can get one to misbehave. Or you can swap each tube one at a time with a known good tube. If that doesn't work, it's something else (obviously).

 

[JMacBass65]

Yes, it's the preamp, and while I'm not the most educated on tube amp repair, sounds like a tube to me. Tap them lightly with a wooden stick while it's turned on (don't use anything metal for obvious reasons) and see if you can get one to misbehave. Or you can swap each tube one at a time with a known good tube. If that doesn't work, it's something else (obviously).

I knew you would chime in.

Excellent, thanks. Backing what I was thinking so far. No spare tube lying around so I'll need to go get one, NP. Question - the three tubes that sit on the chassis, are those also part of the preamp circuit? Something in my grey mater seems to recall that they are.

 

[Mark Reccord]

Definitely in the preamp as Jimmy said. I'd put money on a noisy 6C4, but it could be something else. The 3 smaller tubes on the chassis are the phase splitter and driver tubes in the power amp section.

 

[JMacBass65]

Definitely in the preamp as Jimmy said. I'd put money on a noisy 6C4, but it could be something else. The 3 smaller tubes on the chassis are the phase splitter and driver tubes in the power amp section.

OK - cool, excellent info. Funny you mention the 6C4.......

Digging around some more I found an RCA tube box for a6C4. It has a tiny tube in it, Magnovox with the numbers:
37
11
28

Written that way. I suppose I swapped this out at some point. Who knows.

Why do you say 6C4 I wonder?

 

[JMacBass65]

Found this in another thread.

OK - FWIW - I have a 12AX7 in the power amp V1 position - I seem to recall that being a common mode, or acceptable substitution.

I see there is only one 6C4 tube in the amp. Might yank it and try this Magnoavox one, just to see what happens.

 

[Mark Reccord]

OK - cool, excellent info. Funny you mention the C64.......

Digging around some more I found an RCA tube box for a C64. It has a tiny tube in it, Magnovox with the numbers:
37
11
28

Written that way. I suppose I swapped this out at some point. Who knows.

Why do you say C64 I wonder?

Tiny tube sounds about right. The 6C4 is well known for being microphonic and otherwise noisy. It's always the first tube I look at in a noisy vintage SVT for that reason. Luckily, they're cheap. When the original one in my '76 SVT got too noisy to handle I bought a pile of NOS 6C4s and went through them and picked the quietest one.

 

[JimmyM]

Get the 12ax7 out and put in a 12dw7. Only half of a 12dw7 is a 12ax7.

 

[JMacBass65]

Tiny tube sounds about right. The 6C4 is well known for being microphonic and otherwise noisy. It's always the first tube I look at in a noisy vintage SVT for that reason. Luckily, they're cheap. When the original one in my '76 SVT got too noisy to handle I bought a pile of NOS 6C4s and went through them and picked the quietest one.

Sweet - good stuff. BTW - the thread where I found the tube layout also has some other good info. Mentions that the 6C4 is the summing tube for both pre channels - so that also makes sense, and another reason why that is your go to tube.

 

[JMacBass65]

Get the 12ax7 out and put in a 12dw7. Only half of a 12dw7 is a 12ax7.

I seem to recall hearing that too.

I am going to link to this other thread - funny as it mentions DW7 OR AX7 in some positions.

 

[JMacBass65]

Good stuff here:
SVT Tube Preamp Layout | TalkBass.com

 

[JMacBass65]

Holy crap guys, that was it!!!!!!!!




I pulled the v5 out and replaced it w/ the one in the box and ALL IS RIGHT WITH THE WORLD!!!. No aliens, no nothing. tapping and knocking on the outside produces no sound, induces no noise, ITS ALL GONE.

@JimmyM and @Mark Reccord , can I buy you guys drink or two??? This is awesome - I have been putting up with this noise for five years and I had the solution here on TB and in a parts box right under my nose. I feared taking it to a shop and hearing it was a cap job I needed (which I bet I do need, or will sometime soon, anyway).

But this is amazing.

What's funny is that the tube I put in was a Magnavox - likely original for a '76. What was in there was an RCA, that clearly came out of the box I had here. So why did I change it?? I must have bought a NOS RCA and dropped it in there at some point just because........somebody said it would be a good idea, I don't know. Clearly at some point it started going south on me.

This has inspired me to finally get a supporting membership - way overdue, this place is awesome!!

 

[coreyfyfe]

Tiny tube sounds about right. The 6C4 is well known for being microphonic and otherwise noisy. It's always the first tube I look at in a noisy vintage SVT for that reason. Luckily, they're cheap. When the original one in my '76 SVT got too noisy to handle I bought a pile of NOS 6C4s and went through them and picked the quietest one.

I did the same. They're cheap enough it's worth getting a bunch and swapping as needed. I got some NOS/JAN stuff from Invalid Link Removed.

 

[beans-on-toast]

You did a great job at figuring it out. If any tube in the preamp is going to give you static, it's the 6C4.

BTW, here is the service manual for your amp. It's good to have a copy for when your amp needs to be serviced.

 

[JMacBass65]

Thanks @beans-on-toast that's very cool!

I am going to need to investigate the DW7/AX7 thing. The thread I linked to (you are in there) indicated an Ampeg service bulletin going out at some point, to mod the amp due to a shortage of DW7 tubes. But it kinda goes around in circles about what was stock and when, etc.

The one time I had it serviced, at Chuck Levins Washington Music Center back around 1989, the dude said my problem was some changed resistors, blah blah.

Long time ago. But I wonder if another owner had already done the mod, and the Chucks guy reversed it? Or was the mod not done, and this guy did it?

Receipt is long gone so all I remember is it had something to do with resistors.

 

[Mark Reccord]

Holy crap guys, that was it!!!!!!!!




I pulled the v5 out and replaced it w/ the one in the box and ALL IS RIGHT WITH THE WORLD!!!. No aliens, no nothing. tapping and knocking on the outside produces no sound, induces no noise, ITS ALL GONE.

@JimmyM and @Mark Reccord , can I buy you guys drink or two??? This is awesome - I have been putting up with this noise for five years and I had the solution here on TB and in a parts box right under my nose. I feared taking it to a shop and hearing it was a cap job I needed (which I bet I do need, or will sometime soon, anyway).

But this is amazing.

What's funny is that the tube I put in was a Magnavox - likely original for a '76. What was in there was an RCA, that clearly came out of the box I had here. So why did I change it?? I must have bought a NOS RCA and dropped it in there at some point just because........somebody said it would be a good idea, I don't know. Clearly at some point it started going south on me.

This has inspired me to finally get a supporting membership - way overdue, this place is awesome!!

I'm glad I was able to help you get it sorted out quickly! Lots of great resources on TB for sure, though sometimes you have to wade through a bit of noise to get to the good info. Not this time, though.

I did the same. They're cheap enough it's worth getting a bunch and swapping as needed. I got some NOS/JAN stuff from Invalid Link Removed.

I got a bunch of them from a buddy of mine in Nova Scotia who in turn got them from the Canadian Navy when they cleared out all of their tube stocks in the early 2000s. I think he charged me like $25 for 20 of them. Of course, half of them were basically too noisy to be useful...

You did a great job at figuring it out. If any tube in the preamp is going to give you static, it's the 6C4.

BTW, here is the service manual for your amp. It's good to have a copy for when your amp needs to be serviced.

You know, I didn't have a copy of that service manual. Until now, that is! Thanks for posting it.

 

[Mark Reccord]

I think a 12AX7 should be OK in the PI position but I'm not 100% sure. It's getting towards the limits of a 12AX7 triode's current capabilities though. I'd be inclined to get a 12DW7 in there if I could.

 

[beans-on-toast]

I think a 12AX7 should be OK in the PI position but I'm not 100% sure. It's getting towards the limits of a 12AX7 triode's current capabilities though. I'd be inclined to get a 12DW7 in there if I could.

When 12DW7's were hard to find MTI posted a procedure to convert to 12AX7's for the SVT-HD. Here is the SVT-HD service manual. The conversion procedure is on the last page. They recommended a straight swap in the power amp. In the pre, there were some resistor changes.

In the early amps they had a 12DW7 and two 12BH7's in the power amp. In modern amps Ampeg went with a 12AX7 and two 12AU7's.

​

 

[beans-on-toast]

Thanks @beans-on-tbeans that's very cool!

I am going to need to investigate the DW7/AX7 thing. The thread I linked to (you are in there) indicated an Ampeg service bulletin going out at some point, to mod the amp due to a shortage of DW7 tubes. But it kinda goes around in circles about what was stock and when, etc.

The one time I had it serviced, at Chuck Levins Washington Music Center back around 1989, the dude said my problem was some changed resistors, blah blah.

Long time ago. But I wonder if another owner had already done the mod, and the Chucks guy reversed it? Or was the mod not done, and this guy did it?

Receipt is long gone so all I remember is it had something to do with resistors.

Very possible that the mod was done. Details are below in the SVT-HD service manual (not the same as the one that I posted earlier), last page.

Then again, some techs tend to put in whatever they have in stock when they are doing repairs. So you never know what might have been changed at some point.

 

[JMacBass65]

When 12DW7's were hard to find MTI posted a procedure to convert to 12AX7's for the SVT-HD. Here is the SVT-HD service manual. The conversion procedure is on the last page. They recommended a straight swap in the power amp. In the pre, there were some resistor changes.

In the early amps they had a 12DW7 and two 12BH7's in the power amp. In modern amps Ampeg went with a 12AX7 and two 12AU7's.

Right, I have the 2 BH7s, and then a AX7 in place of the DW7. So, if I understood you correctly, for the power amp section the AX7 is OK as per Ampeg. That would be cool because then I could simply stop thinking about it.

I'll need to check the pre amp tubes to see what's in there.

THANKS!!!!!