6SN7 Tubes in SP14

Hi,

I finished my SP14 2 days ago, really enjoy its sound, but have a question and an issue.

Question first. Which type of 6SN7 tubes can I use in the SP14. There are 6SN7GT, GTA, GTB and several others. Which ones can be used or are they all compatible?

My issue. I currently use NOS Sovtek Russian 6SN7GT (6H8C). When the amp is cold, there is no microphonic. After about 45-60 minutes, once the amp is really warm, they become microphonic. When one uses the selector switch, for example, you hear a "clonk" through the speakers. The same happens when I knock on the chassis. What is your experience? Is this related to my tubes? What are you guys using? Or is there hope that it goes away, ones they are broken in?

Thanks for any feedback.

Michael

I've read several posts on different forums about Russian and Chinese tubes that are microphonic. Microphonics seem more apparent on tubes (octal & miniature) where the micas do not touch the inside of the glass envelope.

Got pictures to share?

Here are two pictures:

Zimmer64 wrote:Hi,

I finished my SP14 2 days ago, really enjoy its sound, but have a question and an issue.

Question first. Which type of 6SN7 tubes can I use in the SP14. There are 6SN7GT, GTA, GTB and several others. Which ones can be used or are they all compatible?

My issue. I currently use NOS Sovtek Russian 6SN7GT (6H8C). When the amp is cold, there is no microphonic. After about 45-60 minutes, once the amp is really warm, they become microphonic. When one uses the selector switch, for example, you hear a "clonk" through the speakers. The same happens when I knock on the chassis. What is your experience? Is this related to my tubes? What are you guys using? Or is there hope that it goes away, ones they are broken in?

Thanks for any feedback.

Michael

You can use all those variants and more.
If you want good low microphonic, low noise 6SN7's, Kevin Deal from Upscale Audio is a well respected dealer. They burn all their tubes in for 72 hours and then grade them.  After passing tests for noise and microphony, each tube is tested individually for triode balance. Once this process is complete, tubes are then matched and sorted into three different grades and you pay only about $5 more for the top grade tubes than other dealers where it's up to luck what you get. There probably are other companies that do this but you will find plenty of people recommending him on the various audio forums.  

http://www.upscaleaudio.com/6SN7/

you can use any variety of 6SN7s, and yes, some tubes can be microphonic

Michael, It's mainly the gain stage of the SP14 that is likely to be affected by microphonic 6SN7's and in this amp the gain stage is the pair of tubes closest to the inputs (left side of the PCB). I suggest you move the tubes around as not all of them will necessarily be microphonic. Maybe you could number the tubes so you can keep track them. The right pair of tubes is the buffer stage and has no gain and shouldn't be affected by microphonic 6SN7's.

Another option is to try tube dampers, they might help.

Further to my previous post, I should make it clear that the gain stage is the first tube in each channel on the left closest to the inputs and the buffer stage is the tube on the right in each channel. The idea is that you put the microphonic tubes in the buffer stage and the good ones in the gain stage.

Thank you all for the advice. I contacted my supplier. They told me that the Rusian 6H8C they sold me are not optimal for use in a preamp and they offered to take them back and replace them with new production Tung Sol's (could have told me earlier...). Anyway, I now ordered Sylvania's (NOS from the 60's) from Andy at Vintage Tube Services. Will take usually 2 weeks to arrive here in Switzerland, but I will post my experience once I have them.

Best

Michael

I tried some of the RCA 5692 Red Base tubes and found them no better than plain Sylvania 6SN7GTBs. Even had a microphonic 5692 in my SP14 at one point, so it isn't just the new Russian tubes. Guy who sold it to me said he had no problems in his amp and to his credit he took them back.

The Sylvania tubes that you have coming will treat you really fine.

Update: Received 1954 made Sylvania's from Vintage Tube Service 2 days ago. What a difference. It is a completely different amp. Worth every penny of the $260 plus shipping that I paid! Microphonics gone, much better dynamics, much clearer sound. Incredible. I wont waste money on cheap tubes ever again. Life is too short  

Michael

FWIW:

Please go on an auction site and look at the prices of 12SN7s as compared to 6SN7s.  Sylvania, and I agree they are some of the better tubes of this type, made both 6SN7s and 12SN7s, as did RCA, Tung Sol and others.  

A 6SN7 chrome dome Sylvania is the same construction as a 12SN7 chrome dome Sylvania, just the filaments are different.   To look at them they are identical.  However the 12SN7s are available at pennies on the dollar as compared to the 6SN7s.  Same with Tung Sol 6SN7s and 12SN7s.  Same construction, with the exception of the filaments, yet look identical.  For the Tung Sol 6SN7 you'll pay $50.00 per tube, and for the 12SN7 maybe $5 to $10 dollars per tube.

If one is intending to shell out a couple of hundred dollars for a set of four 6SN7s, I am suggesting one consider changing the 9 volt filament tranny to a 15 volt tranny ($30.00?) and R21 from its 6 volt value of 470 ohms to either 910 or 1K instead.  This will set the filaments at 12+ volts and then you can use 12SN7s.  I actually went with 100 ohms on R20 (normally 120R) and 1K on R21.  

As an example I recently picked up a quad of brand new in the box, never used, RCA coke bottle
12SN7s for $25.00.   It would have been at least four times that for a set of 6SN7 coke bottle RCAs.

Something else to consider:  the voltage drop from 15 to 12 volts is a lot less than the drop going from 9 to 6, so it ends up being more efficient with less waste heat.  

I can detect no difference in sound between a same type 6SN7 and its corresponding 12SN7.  If anything the 12SN7s sound better as you can find them new and unused, unlike most good vintage 6SN7s.

Just my two cents.  Not intended to offend anyone in anyway, but offered out of my experience as an alternative to consider.

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http://electronics.mcmelectronics.com/search?cataf=&view=list&w=torroid&x=-917&y=-88

Don't let the misspelled "torroid" stop you.