Octal preamp tube dampening

[TubeGeek]

I am working on an amp I just completed building.  It's a Fender 5C3.

Problem I'm having is the preamp tubes.  The 6SC7's aren't exactly plentiful.  I ordered a nice pair of NOS GE's from ebay and ended up they are very microphonic and noisy.  I can hear rattling of the internal components.  From what I've read and been told, this is common of many octal preamp tubes.

I like the sound of this circuit and want to continue playing around with it and comparing it to the 5E3.

When I grip and hold the 6sc7's the noise and microphonics disappear.

I'm considering purchasing a pair of the Duende Criatura Tube Rings to see if it helps out.  Has anyone tried these that could give me an opinion?

http://thetubestore.com/tuberings.html

I'm still weighing my options with this amp.  I'm also considering purchasing some new tung-sol 6sl7's to replace the 6sc7's but not quite yet.

[HotBluePlates]

I laughed when I saw the price for those, then noticed its titanium and teflon. Audiophile snakeoil for sure.

If you think dampeners will solve the problem, you can either add small silicone washers/grommets between the socket and chassis at the socket-mounting screws. Or you could do like Tubenit and make your own dampener rings; I think he cut some out of a silicone pad (maybe meant to keep hot dishes from being directly on your table/counters?).

Please don't spend a bunch of money trying this stuff out.

[TubeGeek]

  Yeah the price is pretty high on those things.

I have already added a flat silicone o-ring to the octal sockets where it meets the chassis.

I'm going to try and find some viton or silicone o-rings to try first.  They are cheaper.

Unfortunately I have to do something in order to use this amp with the octals I have, they rattle and feedback.  I have emailed the seller and hope it's not going to be a problem to return them if I can.  It's also very expensive to purchase 10 different 6sc7's until I find a few that aren't microphonic.  What a can of worms have I opened up with this project. 

[Danskman]

Pre-amp octal or 9-pins tubes are often prone to microphonics, mainly due to their inner structure, many components in a small packages; some are more sensible than others.
I built a two-channels prototype 40W amp based from a JTM circuit (PP-EL34, fixed biased, PPIMV master, LTP), one channel with a 6SJ7 (metal envelope), the other with an EF86; the best result (less microphonics and best sensitivity) I got with these pentodes was obtained playing on the power supply rail. In my amp , the less plate voltage, the best result. There's enough gain in the output stage to make this amp sing 
You could also buy these:

https://taweber.powweb.com/store/tubesktord.htm

(bottom of the page). I bought a set of each size, nice stuff, low cost; no tungsten/moon rock/alchemist secret/snake oil in these, just tough and efficient material.
Just my 2¢...
Bets regards,
Danskman

[tubenit]

Silicon pot holders for cooking are good for 400 degrees  (I think?).  The one in the picture below is rated for 500 degrees, per the ad with it.

You can cut a 3/8" - 1/2" silicon strip and wrap it around the tube.  Then you can use a hose clamp (like for an auto heater hose) to keep the silicon stip on.  


I'd do that waaayyyy before spending the $$$ for those rings. I think the cost on those is silly.

With respect, Tubenit

[labb]

I have built two of the 5B3 amps and encounter the same problem in the second amp. Ended up changing to the new production tung sol 6SL7's for the pre amp tubes. Minor pin out differences. I don't think that the preamp tubes get hot enough to cause problems with most over the counter 0-Rings. If I remember correctly 60 C is as high a temperature that you can touch for very long.  Buna-n is good for 108 C or 225 F. Check with your local auto parts house and try what they have.

Sovtek also is making new production 6SL7's. I did not have much luck with them. The Tung's are a little darker that the 6SC7's that I tried but they do sound good.

[TubeGeek]

Hey there is a good idea! Thanks Tubenit, I'm going to give that a try!

Labb...yeah that's going to be my last resort, changing to the new production 6SL7's.  I have a feeling I will be ordering a pair to try out.

I posted that I am looking for 6sc7's on this buy-trade-sell page and got a couple responses from fellow member's with 6sc7's!  I'll give these a try and hopefully find some good one's that I can use.

[HotBluePlates]

Alternative:
If you think that adding mass to the tube might help keep it from rattling, use one of those hose clamps to clamp something more substantial to the tube envelope.

The idea came from thinking about you mentioning when you hold the tube's envelope, the noises stop. If you attach something that adds mass to the tube, it will mimic that effect and make the tube harder for sound waves to move.

This attacks the problem from the opposite end, rather than cushiony o-rings giving the socket more compliance to move.

[TubeGeek]

I went to the local hardware store and picked up a couple small hose clamps, some teflon tape and o-rings from the auto dept.

I wrapped the teflon tape around the hose clamp and installed around the top of the tube.  I also threw on 3 o-rings on each tube.

This has reduced the feedback considerably.  The tubes are not going into positive feedback on their own now, I can still hear just a little bit of microphonic feedback but it doesn't increase like it used to.

I recorded a few clips for documentation, please excuse my sloppy guitar playing!

http://soundcloud.com/glacier/5c3-bounce-no-clamp-on-tube

http://soundcloud.com/glacier/5c3-bounce-playing-around-with

[echuta13]

www.mcmaster.com carries silicon o-rings for a wee bit less than those other ones...

[HotBluePlates]

... they are very microphonic and noisy.  I can hear rattling of the internal components.  From what I've read and been told, this is common of many octal preamp tubes.

I wanted to point out something else:

Not all tubes have low microphonics, some by their very design. It's not that they are built to have microphonics, but there were some tubes intended for early computer or other on/off (non-audio) service.

I used some 9-pin dual-triodes in a guitar amp that I thought would be good, cheap NOS tubes, but they howled horrendously when the amp volume was above 4-5, even with no input. Not the tubes' fault, they were made for computer service (they would either be full-on or full-off in a digital circuit). Since 50's-60's computers didn't have speakers, and the tube would be in cut-off or saturation, audio frequency microphonics were not a problem.

Note also that some NOS dealers get all misty-eyed about "long-plate" tubes, and charge a lot more for them. There may be some sonic differences between long-plate and short-plate preamp tubes, but manufacturers went to shorter plates and/or extra mica spacers to stiffen the internal structure and reduce microphonics.

Octal tubes may have added risk of microphonics because their larger internal structure. Of course, some types and some variants of those types show a greater tendancy towards microphonics than others.

Just extra food for thought.

[gldtp99]

This is why i've turned to building with 12ax7 preamp tubes---- i'd rather alter the circuit in old Tweed (and other) designs instead of dealing with types of preamp tubes that are often rare/unstable/unusable--- i'm not a fan of small pentode preamp tubes----- I don't believe that there are any magic preamp tube types, but it is the circuit that matters most.
Using 12ax7's also makes it very simple for the customer to swap in another 12ax7 in case of a tube failure---- most players i know don't have a stock of octal preamp tubes on hand, but they can easily access another 12ax7 to get them through-----even a plain jane new prod 12ax7 sounds better than a squealing/inop amplifier.
I do keep a small supply of metal octal preamp tubes near my bench--- I use them as "socket stuffers" when assembling octal output sections--- they keep the tube pins nice and stable when assembling/soldering----- so i guess they do have a good use.
I think that amp function is more important than staying true to an old design that uses parts that are more trouble than they're worth.
I'll bet you can guess what i think about "rubber tube rings"-- -- ------ and they get How Much for them ?.................................gldtp99

[TubeGeek]

I am learning the difficulties with octal preamp tubes.  I figure I'll try a few more sets of octals and if I cannot find any that are not microphonic or noisy, I'll convert to 12ax7's.  As of now I am expecting a set of new production 6sl7's to arrive tomorrow, I'll install them and see what happens, after rewiring of course.

This project is solely for my own collection so I don't care what it takes to get it right.  I began this build so I could analyze the differences between circuits and hopefully learn in the process. It's all about the journey for me.

This project has opened up a can of worms for me though...when I compared this 5c3 to my 5e3 I was shocked how crappy my 5e3 actually sounded!  Now the 5e3 is on the bench being re-wired as I built it 8 years ago and my wiring skills have gotten much better since then. Good thing I'm not under any deadline to get these finished.

[jojokeo]

the best result (less microphonics and best sensitivity) I got with these pentodes was obtained playing on the power supply rail. In my amp , the less plate voltage, the best result. There's enough gain in the output stage to make this amp sing 

Yes in "playing with the supply rail" but - not so for me in what works. Actually increasing voltage thereby lowering the gain of the tube is the best way to control microphonics. You accomplish this in several ways; take a point from the supply rail earlier on, or lessening the dropping resistor, and/or lowering the plate load resistor are simple ways. Take note of geezer & tubenit's designs with a 5879 pentode where they like to use a 56k load resistor. Similarly w/ an EF86, you can lower the standard 220K the same way but just remember to adjust the screen resistor too if using a pentode to keep the same break-up characteristics.

[TubeGeek]

Just a quick update...

I received the tung-sol 6SL7's yesterday and installed them this morning.  They sound much better and controlled.  I describe it as more closed in and dry sounding.  I still have the sag/compression I expected.  The gain seems relatively equal to the 6sc7's I had before.  Sounds very nice with a Strat and Tele and quite gritty and creamy with a Les Paul and SG. There are no troublesome microphonic's with these new tubes but I do hear a bit of a rattle.  The more I play the amp the more I'll determine if this is a problem for me.

I'm going to play the amp with these tubes for a few days and then try some other 6sc7's that are on the way to me.

[smackoj]

I built a Valco 'Supreme' which is similar to the Fendy with a 6SQ7 pre and 6SC7 PI. It took some work to get it quiet and producing full power but in this amp it was the SQ7 that wanted to squak. I found the tone of the amp really pretty brown and tried 5 or 6 diff. 12" spkrs with not much improvement. I finally switched to a 12--7 socket and preamp tube which gives it a more guitar friendly tone IMO.

Most of the rave I have heard about the octal amps comes from the harmonica players and they get some wicked tone for that purpose for sure.

Jack D