• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Driver Tubes (SVT)

#21
Hey B-string -- could you tell a little more about how you made your 6c4 to 12au7 adapters? I've just gotten into a '75 SVT and after taking it out to a half-dozen gigs the 6c4 has gone microphonic. I ordered a sleeve of 5 JAN Phillips NOS, and there seems to be only one decent one in there, the rest are like baby rattles...

Looks like it's going to be a recurring issue with this head, so I thought I might build some adapters. I was thinking of getting a nine-pin socket, soldering some leads on, bending these around to match the 6c4 pinout, then wrapping some cardboard around and pouring an epoxy "plug" to insulate and hold everything. Is this similar to what you did, or is there an easier way?
 
#22
You could also try some dampers. I bought a few 6C4s for mine, found the least noisy, and put a metal damper on there. I'm trying to find the exact link for the one I bought but TB member beans-on-toast recommended them and if you send him a PM he may be able to help you out. There are others out there - rings and such - but I haven't tried those to see if one style is better than another.
 
#24
FritzM137 said:
Hey B-string -- could you tell a little more about how you made your 6c4 to 12au7 adapters? I've just gotten into a '75 SVT and after taking it out to a half-dozen gigs the 6c4 has gone microphonic. I ordered a sleeve of 5 JAN Phillips NOS, and there seems to be only one decent one in there, the rest are like baby rattles...

Looks like it's going to be a recurring issue with this head, so I thought I might build some adapters. I was thinking of getting a nine-pin socket, soldering some leads on, bending these around to match the 6c4 pinout, then wrapping some cardboard around and pouring an epoxy "plug" to insulate and hold everything. Is this similar to what you did, or is there an easier way?
Click to expand...

Details eh? It was fun once ;)
Used a glass cutter to "donate" a 7 pin base from a very noisy 6C4. Attached 22awg buss bar to the required pins. Covered buss wire and pins with teflon sleeve. Filled with high temp epoxy to cover pins 1/16" below edge of remaining glass.
Used a PC mount 9 pin socket so I could cover the pins easily with shrink tube after connecting. Connections were made as short as possible while allowing for a slight strain relief. Adjusted buss bar for maximum spacing between leads and level between bottom of 7 pin base and top of 9 pin socket.
Now the hard part.
Used a old 9 pin split tube shield and reformed it to hold a waxed fish paper tube form around the assembly. A juice drink straw was installed in the center hole of the 9 pin socket to fill the space between the base and socket using a large syringe. The high temp epoxy was low viscosity, high dielectric from a firm I was working for that built infrared sources under mil spec. A dead old radio chassis lent itself as a holder for the 7 pin base during fill and cure (also allowed me to tap the chassis during fill to minimize air pockets). Non stick cooking spray was applied to the radio's 7 pin socket and surrounding chassis.
After curing for a full 12 hours the form was removed and rough trimmed, after another 24 hours final trim and clean up was done.
Reformed a spring wire to attach to the preamp chassis to prevent the 12AU7 with added weight of the adapter from vibrating loose during gigging or transport.
Wasted two prototypes before I had a good one. Only installed three of them, but at last report they were going strong.
Think I got it all down? Was over 15 years ago now :)

Did forget something. After all connections were made to the 7 pin it was fully cleaned. Then buffed with jewelers paper around the outside to provide some tooth for the epoxy. Cleaned again before finial potting.
 
#25
Thanks guys.

I actually ordered one of the cinch dampers in my last order from fair radio, having seen Beans' recommend previously on that other thread. But despite this, the 6C4 I had in the amp continued to worsen, to the point where it would make a loud pop if the preamp chassis was jostled slightly, and even intermittently cut out from cabinet vibrations on low notes when the rig was being played at stage volumes.

I was amazed at how "jangly" most 6C4s seem to be -- out of a sleeve of 5 that I ordered, four seemed like they would be problematic just based on the vibrations I could hear in the elements and heaters by holding them up close to my ear and tapping lightly on the envelope! One seems mechanically sound and worth a try. But I wonder how long that will hold up, and I want this to be a working amp, so that's why I'm interested in trying the adapter route as well.

Incidentally, Beans, do you do anything for insulation safeguard at the bottom of the cinch around the tube socket pins?
 
#27
The microphonic 6C4 in my 69 SVT has lasted about 3 years now. I don't use the SVT all the time but I do use it semi-regularly. I bought 4 hoping to find one not microphonic but they all are. But the one I picked seems to hold up, and it still sounds awesome, so I'm not sweating the problems with them anymore. I'll use it till it burns out, then drop another of my microphonic 6C4's in it ;)
 
#29
FritzM137 said:
Incidentally, Beans, do you do anything for insulation safeguard at the bottom of the cinch around the tube socket pins?
Click to expand...

I haven't found it to be an issue. The shield can be pushed over the tube as far as you want it to go. If you are concerned, just leave it set back from the tube socket a bit. You can always trim the length of the shield a bit. I've done that with another type of amp where I had a shorter tube.


David
 
#30
As Sean mentioned, a commercial version of the 6C4 works well. They are more rugged and less prone to microphonics.

I've had good luck with the 6C4WA, also known as the CK6100. Here is the data sheet. It is recommended when mechanical shock or vibration is encountered so it keeps microphonics down.
 
#31
On another pre amp tack, I've just literally had a horrible pre amp microphonic time with my 67 B15N, I've tracked it down to one of the 6SL7WGT pre's in there, it's been popping, halving volume and generally wandering from mic' to ok. I thought dry solder etc in the amp, but first I test the valve in my tester and it's jumping there too, ahh well at least the amps ok, now I look at the pins and see if they could be cleaned more and hey presto, two pins have lost ( or never had any ) their solder, soldered up and we're cooking on gas, what a sweet sounding Ampeg this is:D.
 
#34
beans-on-toast said:
Interesting.

I've seen the glue on a base of octal tubes come undone from the glass. It goes to show that it pays to check everything over carefully. You saved on buying a new tube.
Click to expand...

Days like that are great when they come, instant fix on a rainy Sunday and straight back into playing through something that sounds better than it did before. Major result on the valve, now I got the chance to hear it working properly, it would be a bitter pill to go new modern.
 
#36
In theory all you need is one but I would still buy multiples. Tubesandmore.com sells Philips ones for $4.50 each and other makes for $3.75. At that price, it pays to buy a handful at a time just in case.

I've got quite a few and whenever I need one it is good but in all honesty I probably pre-screened them when I bought them years ago and simply forgot.
 

Latest posts

Top Bottom
Back