SP14 Valve Pre

[tiguan]

Hi all...
I made a SP14 valve pre almost 3 months ago which I shared the DIY process photos in Gallery part.
schematic of this pre was developed by Roy ( Tubes4hifi) on Aikido design with some changes, using 4 pc. 6SN7 for signal part and 1 pc. 6X5 as rectifier tube.
I do some DIY works but I do not have advanced electronic knowledge and this project is my first entry into Tube World.
This is the Board schematic of SP14

As far as I figured out by reading many webpages and topics, tubes No1 and No2 are driver (voltage regulator) tubes and tubes No3 and No4 are output tubes.

First question Am I right? Did I understood correctly?

Second question, as NOS 6SN7 tubes shall be sometimes very expensive, what would your comments based on effect to audio performance between;
- changing all 4 6SN7 with same best tubes I have in my hand
- changing just 2 driver tubes with the best I have in my hand (say expensive tubes)
- changing just 2 output tubes with the best I have in my hand (say expensive tubes)

Third question is somehow on the same path with second question, should all tubes quad matched or shall be matched by pairs for No1&No2 and No3&No4 will it be enough?

Fourth question,I have 2 rotary swiches for left gain and right gain made each from 11 pc. 10k resistors. SP14 pre output is between 500-600 ohm as I get from Roy. My power is Gamut d200 Mk3 RCA input imp. 20k and balanced 40k. Plus ınternaly there are 3 step voltage adjustment with 2 switches (Sensitivity: 0.77V, 1.55V, 3.1V, 3.9V (full power)) to match the pre with my power do you think I will need still Gain switches shall I directly to connect ladder stepped attenuator to selector switch.

At the end you will say most probably "ears will judge" wchich is correct as a rule of HiFi; but at least technicaly to be on the correct narrow one way path, instead of being at a wide motorway going left right front randomly I need assistance

Besides that I am now making comparisons between tubes which I have already but to extend my stock for 6SN7 I want to move more clever way , if there is any?

[Firebottle]

Hi Yetkin,

A nice project, I believe it is based on the Aikido preamp circuit. There is plenty of info on the web, have a look at http://www.tubecad.com/2004/blog0011.htm

Valves 1+2 are the 'amplifier' stages, nothing to do with voltage regulation, and valves 3+4 are the output valves as you figured.
I would think it better to have the best valves in positions 1+2, but only your ears can tell how it sounds to you.

As far as matching goes, match 1 to 2, and 3 to 4. Your suggestion for leaving out the gain switches seems good as you already have adjustment in the PA.

Cheers, Alan

[tiguan]

Hi Alan, yes you are right it is based on Aikido circuit with octal tubes.
I have
2 matched Sylvania VT231 tubes


4 matched Sylvania 6SN7 GT Chrome Top I guess from 50's


2 matched RCA 6SN7 GT I do not have photo now with me

2 matched Raytheon 6SN7 GTB I do not have photo with me

I am still evaluating. As I do not like to have an opinion fast, generally after any change, I listen my system at least 2 weeks (if it is not so obvious) with different personal moods

I will write in a month my opinions but I am open for recommendations about tubes for Jazz and classical music.

[Mr Kipling]

Hi. What do you have it housed in? There used to be an eBay seller that had an ARC-style pre-amp case, but I can't remember who it was now.

[tiguan]

I made an oak frame, opened front and back. Made the drawings for front,back and top covers in autocad. Gave to CNC workshop for them to cut out from gold stainless steel plate, which is used normaly for elevator internal decoration.

[struth]

Thats pretty Yetkin. nice, quality work.

[walpurgis]

Yes, nice job!

[tiguan]

Thank you

Most recent photo, I changed the knobs which I ordered by mistake different color before. And Ebony Spikes with copper tips.

top cover will be open for some period for tube changing to find the best match from valves I have in my hand.