Ampeg SVT 6C4WA replacement?

RRJackson · #1 04-23-2011, 03:06 PM

I have a really microphonic 6C4WA in my SVT-HD. In reading some of the posts about this tube's issues I was wondering if there's a replacement other than just another 6C4WA, since everyone seems to have issues with them?

BTW, since the amp is opened up right now, a big thank-you to its father, Dave Ryan. I've had the amp since '89 and it's always been great. Right up until I was sweeping a filter trying to find a tone with a bass a couple of days ago and it suddenly became insanely microphonic.

Jack Ryan's brother? | Flickr - Photo Sharing!

-Rob Jackson

JimmyM · #2 04-23-2011, 03:56 PM

dave's on here occasionally. hasn't been on for a while, though. nice guy.

microphnic 6c4's are common. i have 4 of them here and all are microphonic. i chose the least microphonic of the 4 and put it in.

Jerrold Tiers · #3 04-23-2011, 04:01 PM

We had a ton of trouble with those when BUILDING the SVT-HD run also.

That kinda explains why the later versions don't have them, despite that being a "difference from original"........ The 6C4 wasn't ever really intended for audio, and these days, finding a good one can be a challenge.

I don't know offhand of a pin-for-pin replacement, but there may be one. However, it is likely to be even more obscure than the 6C4, not sure that would be an improvement.

RRJackson · #4 04-23-2011, 04:04 PM

Quote:

Originally Posted by JimmyM

dave's on here occasionally. hasn't been on for a while, though. nice guy.

microphnic 6c4's are common. i have 4 of them here and all are microphonic. i chose the least microphonic of the 4 and put it in.

Yikes. Well, the practical realities of the world are sometimes not the kind of stuff that makes good ad copy. Heh...

It was so strange, though. It's never really been a problem and then as I was sweeping a filter with a high Q the wah-ish sound suddenly started this strange oscillation and the amp basically became unplayable. I'm fairly sure the 6C4 is the problem, but it happened so suddenly it kinda thew me.

-Rob

RRJackson · #5 04-23-2011, 04:06 PM

Quote:

Originally Posted by Jerrold Tiers

We had a ton of trouble with those when BUILDING the SVT-HD run also.

That kinda explains why the later versions don't have them, despite that being a "difference from original"........ The 6C4 wasn't ever really intended for audio, and these days, finding a good one can be a challenge.

I don't know offhand of a pin-for-pin replacement, but there may be one. However, it is likely to be even more obscure than the 6C4, not sure that would be an improvement.

I appreciate the info. I guess just getting a few of them and seeing which one works best actually does sound like the prudent move.

BassmanPaul · #6 04-23-2011, 04:17 PM

The 6C4 is very close to half a 12AU7. You might think about pulling the seven socket, opening the hole to 3/4" and fit a nine pin socket. Just wire to one triode and leave the second unused.

beans-on-toast · #7 04-23-2011, 06:36 PM

6C4WA equivalents are: EC90, CV4058, M8080, 6100, 6135

Sometimes stabilizing the tube with a vibration damper can help suppress microphonic noise. I use a Cinch 5020B tube cover on the 6C4WA in my SVT.

RRJackson · #8 04-23-2011, 06:45 PM

You know, I actually don't think it's a microphonic tube now. I just had it apart and was checking to make sure nothing was improperly grounded or anything. BTW, this is the tube that was in it that I was suspicious about:

Anyway, I put everything back together and wrapped a piece of foam around that tube. I fired up the amp and the oscillation noise was worse than ever, but it's also got a periodic sound like something is grounding out. I'm officially in over my head. I need to take the amp to a local guy and see what's going on.

I appreciate the comments, though. I'll certainly post what the resolution turns out to be. I've had over 20 years of faultless service from this amp. I guess I couldn't expect it to keep on going forever without any repairs. It's been hauled all over the United States. One time it was right next to a bass of mine when a fork lift drove over the bass. I thought the amp had been murdered as well, but it was six inches outside the kill zone.

Even if a tranny has fried it's a small price for two decades of stellar service, IMO.

-Rob

RRJackson · #9 04-23-2011, 06:48 PM

That's an excellent idea!!! Thanks!

Quote:

Originally Posted by beans-on-toast

6C4WA equivalents are: EC90, CV4058, M8080, 6100, 6135

Sometimes stabilizing the tube with a vibration damper can help suppress microphonic noise. I use a Cinch 5020B tube cover on the 6C4WA in my SVT.

RRJackson · #10 04-23-2011, 07:07 PM

BTW, just in case anyone likes amp porn...heh...here's the amp while I was checking it out to make sure there wasn't a screw or something grounding out inside the chassis.

Bottoms Up! | Flickr - Photo Sharing!

RRJackson · #11 04-24-2011, 04:32 PM

BTW, if it does turn out to be the output transformer, does anyone know of a source for original replacement trannies for these amps? I'm sure it's going to be pricey, if that's what it turns out to be.

-Rob

JimmyM · #12 04-24-2011, 05:46 PM

Quote:

Originally Posted by RRJackson

BTW, if it does turn out to be the output transformer, does anyone know of a source for original replacement trannies for these amps? I'm sure it's going to be pricey, if that's what it turns out to be.

-Rob

i seriously doubt it's a transformer. but just in case, Welcome to Fliptops -- Parts for Ampeg and other Vintage Amp Lovers carries them, and mercury magnetics and hammond electronics both carry them, albeit for a lot more than what the transformers at fliptops cost. are they better? don't know since i've never had to replace one in my svt.

RRJackson · #13 04-24-2011, 06:06 PM

Quote:

Originally Posted by JimmyM

i seriously doubt it's a transformer.

I really hope you're right. My hope is that I've got a bad capacitor somewhere or something minor like that, but I always kinda prepare for the worst so I'm not too dizzy when I get the news.

Thanks for the leads, though.

-Rob

JimmyM · #14 04-24-2011, 06:30 PM

that's a far more likely scenario, bro.

RRJackson · #15 04-14-2012, 08:28 PM

I just got a tube tester and went through all the tubes. The problem was a bad 6550. Nothing major at all. I know it's been a while, but I wanted to put a period on this.

-Rob

RRJackson · #16 04-14-2012, 08:31 PM

BTW, thanks a lot for that suggestion. A vibration damper exactly like that went on that 6C4WA before I tried anything else. I was hoping that would solve my problem and I could lay a, "You saved the day!" on you. It was still a good call and a mod I've added just in case.

Quote:

Originally Posted by beans-on-toast

6C4WA equivalents are: EC90, CV4058, M8080, 6100, 6135

Sometimes stabilizing the tube with a vibration damper can help suppress microphonic noise. I use a Cinch 5020B tube cover on the 6C4WA in my SVT.