1975 SVT on the bench, what to do next...

bassmeknik · #1 03-07-2013, 10:44 AM

Hi all, I have owned a 75' SVT since it was new but haven't gigged with it in over ten years. My present large venue amp is a 4 Pro with a BA-115 as the small amp. The SVT came home from a gig many years ago with crackling noises and popping noises. I had it professionally maintained early in its life but haven't had it looked at (by someone else) in quite a while. I am an avid DIYer with tons of electrical experience (including high voltage). I have built several SS amps from scratch and repaired dozens more (tube and SS).

Looking at the SVT it has had a few resisters and capacitors replaced over the years already including a couple of power supply filter caps and plate resisters connected to the driver/PI tubes. R9 (connected to tube 1) in particular is showing signs it was overheated, perhaps just by the resister next to it having failed and replaced years ago OR it overheated itself due to excessive current. It's part of the balance circuit, and I intend on desoldering one end of it to test its resistance. A few more resisters in the driver and PI stage seem to have been replaced and not all are looking healthy (heat blackening on the circuit board). Should I just replace the suspect ones?

I also have all new PS caps but after purchasing found out Nichicon caps are made in China...

so I am now debating whether to recap this amp with China parts when it is all made in USA at this point.

I also have enough Orange drop coupling caps to recap the signal chain coupling caps with the famous "Orange Drops"...

BUT, I am seriously in need of money for bills so...

You on TB with tube amp/SVT repair experience and a feel for the market on old SVT's let me hear your opinion on how I can get the most money out of my beloved SVT. I can recap it with MIC filter caps or save $$$ I don't have to re buy USA made caps. Will the typical used SVT buyer really care about this? please try and put this in terms of dollars as I'm afraid that is the reason for my thread. Should I:

1. sell it as-is (semi-basket case but complete)? this is my last option as it will bring the least amount of cash.

2. recap the PS with my existing Chinese caps and be honest if asked?

3. buy new USA filter caps and recap with American made parts and push the fact that it is all American?

4. should I change the signal coupling caps with the Orange drops?

Any other suggestions on which way to go to extract the most money for this?

I intend to post it in the TB classifieds when I am done with it. If you needed money and had enough skill to do this which way would you go to get the most money out of selling the amp? It unfortunately HAS to go.

bassmeknik · #2 03-07-2013, 10:45 AM

pics coming...

coreyfyfe · #3 03-07-2013, 11:03 AM

You sure it's all made in america? Many of those paper cap covers say made in mexico. Pretty sure the ones in my 74 did.

Anyways, a functional amp is worth more than an original non-functional amp. I'd say fix it and don't worry about where the caps came from.

ngh · #4 03-07-2013, 11:10 AM

Granted I am by no means a pro when it comes to tube amp repair, I have had a bunch of old tube stuff (guitar amps mostly). and find that getting things working with sub par parts is often a better option than having something that is not working. if you are honest about what you have done with it while selling it, the buyer will know that they can always replace the MIC parts with higher quality bits if they feel it necessary with no ill effect. if it was 100% original I would say sell it as is and have the buyer deal with a recap. but as it has already had original parts replaced, just make it run well. the reduction in price for cheaply re-capped or even re-tubbed amps with 70's SVT esque legendary status is negligible if anything at all IME.

the cache is not so much in "all american" as it is in "all original", and frankly what sounds good sounds good! and an amp that runs well sounds better than one that doesn't.

bassmeknik · #5 03-07-2013, 11:19 AM

beans-on-toast · #6 03-07-2013, 11:31 AM

You have a vintage SVT, people know what it is. It is true that the components and tubes in a 75 were not all US made. Since you are fixing it to maximize what you can get out of it when you sell it, I would do the following:

Make the minimal number of changes that you need to make to get it working.

Do not recap the signal path with orange drops. I would hate to see this, they aren't the best option.

Change any out of spec caps and resistors that needs to be changed with a part that is as close to what was originally there. Pay close attention to the plate, screen, and cathode resistors in the power amp. If they are original, I'd just change them.

Recap the power supply, don't forget the cap in the pre-amp. Use good quality capacitors, regardless of where they were made. It is more about how the amp sounds.

Install a three conductor power cord and plug.

Clean the pots, switches, jacks, and tube sockets and pins with Deoxit.

Clean the cabinet and chassis.

Change any tubes that are necessary but you don't need to provide a new set.

Set the bias and balance.

bassmeknik · #7 03-07-2013, 11:31 AM

Steve Dallman · #8 03-07-2013, 11:31 AM

Last I looked, Mexico is part of America, as is Canada. I have had good luck with Xicon caps...actually any caps ending in "con." I wouldn't replace any coupling caps that didn't leak. It's the electrolytics that fail with age. As electrolytics age they tend to drift up in value (in a very sloppy way.) As they drift up in value, they drift down in power handling, and eventually fail by either opening up or shorting. Modern Xicons in a tube amp would not be a concern to me.

beans-on-toast · #9 03-07-2013, 11:38 AM

When you change those burnt resistors, it helps to mount the new ones a bit off the board. It helps with heat dissipation and stops the board from charing. Use flameproof resistors if necessary as per the original schematic.

bassmeknik · #10 03-07-2013, 11:45 AM

B-string · #11 03-07-2013, 12:01 PM

Excellent tips on the resistors, using over-rated as an example 2 watt instead of 1 watt never hurts either. I have had very good experience with Nichicon caps and would not hesitate to use them. I don't think you will find MIA electrolytic caps for anything other than NOS. The environmental impacts moved commercial production out of the US a long time ago.

coreyfyfe · #12 03-07-2013, 01:41 PM

Quote:

Originally Posted by B-string

I don't think you will find MIA electrolytic caps for anything other than NOS. The environmental impacts moved commercial production out of the US a long time ago.

Sprague Atoms are still MIA aren't they?

B-string · #13 03-07-2013, 02:11 PM

Quote:

Originally Posted by coreyfyfe

Sprague Atoms are still MIA aren't they?

I can't say as they were bought by Vishay. COO is hard to find in the corporation literature.

beans-on-toast · #14 03-07-2013, 02:33 PM

Vishay has its corporate headquarters in PA but manufacturing facilities are all over the world. As far as I know, Sprague Atom used to be manufactured in the US but hasn't been in some time.

Steve Dallman · #15 03-07-2013, 02:38 PM

When I do a cap job on a vintage amp (I'm working on a Gibson GA-20T right now) I gut the filter and other electrolytic caps, and replace the innerds with modern caps. Modern caps are smaller and will fit in the cans, cardboard tubes, or whatever were used. I seal them up and it all looks stock.

will33 · #16 03-07-2013, 03:25 PM

+1 to just using decent quality stuff regardless where it came from. The real techs here can advise you to what is quality nowdays as some already have. That's what a good tech at a shop would do. You're unlikely to find much if any parts like that that are truly all USA made nowdays and if you do, it'd be throwing money down the toilet for anything other than maybe a museum piece.

It's a '75 SVT. Just make it reliable and make it sound like a '75 SVT should. Let's not forget the new ones are wholly China made and ain't cheap. And the Heritage line, though maybe put together here, are still full of Chinese parts, and not all top-o-the-heap either, and they still sound great, though expensive as hell.

Good luck with the re-furb, though if you do a good job, you might be inclined to keep it...I know I would.

jastacey · #17 03-07-2013, 06:54 PM

I'd keep the signal chain stock, I've had good luck with the F&T caps when used in the power supply, as to the discolored burn resistors, test/replace where needed, use flameproof .. most people will understand that an amp that old will have stuff changed out over the life of the amp

dmusic148 · #18 03-07-2013, 07:11 PM

Quote:

Originally Posted by B-string

I can't say as they were bought by Vishay. COO is hard to find in the corporation literature.

They're finally tearing down the old, long-empty Vishay plant just down the road from me in Monroe, CT. Kind of depressing. It seems we don't manufacture hardly anything anymore.
On topic: agree with using whatever caps are available. Using all US stuff won't help resale.

walterw · #19 03-07-2013, 08:14 PM

1. snip all the wires and pull the board out of there entirely

2. install a genz benz shuttle 6.0 under there, and splice the pots and jacks to the external SVT pots and jacks

3. don't tell anybody

fdeck · #20 03-07-2013, 08:36 PM

In my view, find the root cause of the popping and crackling, and address that issue, rather than prophylactically replacing parts and hoping for the best. When it is up to gig worthy condition, decide whether or not to sell it.