Newly aquired Tube preamplifier - need opinions

[mohd216]

Hello dears
I have recently aquired tube preamplifier from one of my friends, he claims that he got it from Singapore, i saw same design on ali excpress though. Anyway, pre is very nice, in very good shape, sounds amazing, however, i noticed small shhhhh sound when i put my ears next to speakers, this shhhh were audible on my solid state preamp before also, but was twice lower,

This preamp is pushing allot of power to my power amp were i need to raise volume only on 1 ( 8 o'clock ) , i will try attaching some videos and photos for the pre and willing to get your kind openions.

Tube pre (unnamed)
Power amp (krell kst 100)
Speakers ls50
https://photos.app.goo.gl/RxKK8sPHEoCdSdrY9

Thanks

 

[andyjevans]

That looks like 6SN7s. I believe Stephe reviewed it on YT. Gain would be around 20. Much too much.

If there are two octal sockets it could be rewired for 6G6G for instance - half the gain.

 

[mohd216]

Dear andy

Tubes are JAN CHS 6SJ7GT

And yes as you mentioned they seems to be loud
Clear and musical but bit loud

 

[Bigun]

don't put your ears next to the speakers

(adding some / increasing negative feedback will reduce distortion, gain and noise, improve frequency response and lower output impedance)

 

[rayma]

The excessive gain makes the excessive noise.
A change to reduce the gain will also reduce the noise.
It would be unusual to need more than a gain of 5 times (14dB) in a line stage today.

 

[mohd216]

Thanks for your response
I have added link with demo in the thread
I couldn't upload the video due to file size
Appreciate giving it a look and tell me what do u think😊

 

[rayma]

A schematic diagram would be necessary to properly redesign the circuit to reduce the gain.

 

[mohd216]

Dear Rayma,

Unfortunately i dont have the schematic

 

[vinylkid58]

Tubes are JAN CHS 6SJ7GT

Are they triode strapped? A resistor connecting pin #6 to pin #8 (G2 to plate)? Can't tell from your pics.

jeff

 

[mohd216]

Hi jeff

To be honest im not an expert, but i can send you some pics tomorrow for the preamp

Thanks for your response

 

[andyjevans]

https://frank.pocnet.net/sheets/049/6/6SJ7.pdf

Gain in triode is indeed around 20. The pinout is odd, unfortunately, so less obvious to find a lower gain alternative.

I would consider rewiring it for a 6G6G. Only 125mA at 6.3V so no problems there. I've used this in triode, gain 9.5, and I liked the sound. It's very clear. But it would have to be rewired by somebody who knew what they were doing.

If it were mine I'd fit a UX5 socket and use a 37 tube. Gain around 9. Very good sound. I use 37 or the 76 myself. Not easy to find these tubes though. The 6G6G is still fairly common and cheap.

 

[vinylkid58]

To be honest im not an expert, but i can send you some pics tomorrow for the preamp

No worries, I was just curious.

I would consider rewiring it for a 6G6G.

Or triode strapped 6V6.

jeff

 

[andyjevans]

Triode strapped 6V6 would be a little more heater current - 900mA instead of 600mA. Probably OK but who knows.

Same pinout as the 6G6G anyway, and with that pinout a 6P5GT could also be used, gain 14 and a true triode. Not so common any more though.

I've been playing around with all these tubes and the 37/76 are the stars of the show. Higher output impedance than 6V6 and 6G6G though so would expect to see 100K input impedance on the amp.

 

[chrisng]

i saw same design on ali excpress

Vacuum Tube Preamp 6j8p 6sj7

Are they triode strapped? A resistor connecting pin #6 to pin #8 (G2 to plate)? Can't tell from your pics.

 

[andyjevans]

Are those 2 output transformers I see on each side of the chassis? Or are those chokes? No OPTs marked on the schematic. No photos of the inside I can see. Looks a good candidate for a rebuild with a better tube.