V5 on SVT300 preamp

[pksvt]

Valve 5 on my svt300 preamp, with the 6C4 tube, does not have one of those metal sleeves. Is this normal? Mine is an older model, from about '72-'75.
I am having trouble with crackling noises in my amp, and those noises seem to be associated with Valve5. I have tried new 6C4 tubes; still crackling. Shall I replace that V5 tube socket? If so, where may I get one? Someone said I may need to retension the socket. How do I do this?

 

[Raw N Low]

1. Don't attempt the work on it yourself.
2. Take it to a professional tech and share this information.
3. Internet forums are breeding grounds for armchair QB's = conjecture.

Note: Make sure to let the tech know it's an early Ampeg SVT. I wouldn't use the 'SVT-300' nomenclature since there was an all tube power amp called the Ampeg SVT-300. That's a different animal.

 

[pksvt]

I appreciate that, Raw N Low. I wish I was in the Denver area. I'd bring it to you. I don't know any professionals in my area. I had a good guy who changed the caps, but he left town and the state, in fact.
So, you mean I should just call my amp an early Ampeg SVT?

 

[coreyfyfe]

The 6C4 did not have a shield. It is a common noise source in these amps, they’re sensitive to vibration and not great tubes. They were replaced in later versions of the amp with a common dual triode instead of the single triode. This tube is used to sum channels 1 and 2 into the power amp. I have a number of 6C4WA/6100 tubes in my bin so I could find one with the least noise and also run a cinch tube shield on mine to help with mechanical vibration.

The 6C4WA tubes are cheap, $5 here if you want to grab a couple and find one with less mechanical noise/microphonic issues.
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If a new tube doesn’t help have a tech diagnose the amp further.

 

[Jim C]

I'm not aware of an svt300 preamp and I've owned many Ampeg bass amps. A photo would solve this mystery.
If it weighs about 85 lbs. it would probably be an SVT.
To have it serviced professionally, look up the Ampeg approved service centers in your area.

 

[Wasnex]

Every indication suggest you are talking about an SVT.

No shield on the 6C4 and they tend to be microphonic. In the 87 SVT, none of the preamp tubes have shield. I have owned three vintage SVTs and my 87 is the quietest.

I have seen it suggested that the Mullard M8080 is better than most. Alsoe recommended, 6100 and 6C4WA. I have a stash of different 6C4 type tubes made by different companies. The 6C4 in my 87 rattles noticeably, but it's usually not a problem at the volume I typically play. Maybe someday I try some of my stash.

It's possible to find a shield that fits:
https://www.talkbass.com/threads/nad-new-nightstand-day-my-very-1st-svt.1347426/page-4#post-21402748

Although it's true that current SVTs use half a 12AU7 in place of 6C4...you can't run a 12AU7 in a vintage amp...it uses a different tube socket.

Replacing a tube socket requires tools and skill. I would try rolling a few fresh 6C4 into the amp. If you actually need to replace the tube socket, it's probably best to pay a pro. This is to protect you and the amp.

 

[JimmyM]

Despite most 6c4’s being microphonic, I never felt it acted negatively on my 69 SVT.

I’m not saying this is the cause of crackling in your amp, but for me it’s almost always been V1 as the culprit, except for one time when it was a power tube. Pull each tube out and reseat it and see if that helps.