New SP9 PH12 owner with ST-70

Hi All,

I just recently picked up a used VTA SP9 (TCLAv4) with PH12 phono board. I've been using a ST-70 with my Sansui 555A feeding it (great combo) and the SP9 is a nice upgrade as the pre.

Couple questions as a newbie.
- with the V1/v2/v3/v4 12AX7 tube slots, can you use different matched pairs there? if so which slots need to be matched? Which pair is the most important. i.e. spend the better money on v1/v2 or v3/v4?

- what are the size/value of the caps that are often upgrade? I believe they are the C2 & C6 positions.

- is it possible and have any of you added a switch to turn on/off the phono board? I don't always listen to vinyl and would like to prolong the life of the phono tubes.

Also not sure what caps are on my board, brand wise.

Not quite sure, but is that a Zu SDFR in your avatar?

Hi Brap. It’s the Zu 260FRD. Omen Dirty Weekends.

Kray wrote:Hi All,

- is it possible and have any of you added a switch to turn on/off the phono board? I don't always listen to vinyl and would like to prolong the life of the phono tubes.

Interesting idea. I suppose just turning off the filament supply to the PH12 would work. I'll be interested to hear what replies you get.

.

The most important tubes are the pair on the PH12 board and the pair on the left of the SP9 board which should be selected for low noise. Nothing needs to be matched, only push pull power amps like the ST-70 need to be matched. If a switch was used it would need to break both the filament and HT supplies. Preamp tubes should last for many years and personally I wouldn't bother with a switch. Also, the point that Peter has raised may apply as well.

Email Roy, the designer of this amp and he will answer all your questions, including what caps were used here. Attach the photo. He usually responds quickly: info@tubes4hifi.com

Here’s a reply I got from Roy regarding the switch for phono board.

“You could use either a DPDT switch or a DPDT relay and a switch to disconnect filament and B+ power to the PH12.”

.

mijohn wrote:The most important tubes are the pair on the PH12 board and the pair on the left of the SP9 board which should be selected for low noise. Nothing needs to be matched, only push pull power amps like the ST-70 need to be matched. If a switch was used it would need to break both the filament and HT supplies. Preamp tubes should last for many years and personally I wouldn't bother with a switch. Also, the point that Peter has raised may apply as well.

So the left tubes are V1/V2 and are input tubes correct, then V3/V4 are output?

So you're saying the input tubes are more important than the output?

Yes, V1/V2 is the input gain stage and should have the best and quietest tubes you can find. V3/V4 is the buffer output stage and doesn't contribute as much to the sound quality. That's not to say that quality tubes would go unnoticed there, but in answer to your original question "Which pair is more important to spend the better money on?' I would say V!/V2.

Thanks @mijohn for the details.

I’m guessing it’s probably next to impossible to remove all hum from this Preamp with my high efficiency 97db speakers?

It’s not bad at all but it’s ever so slightly there.

Kray, enjoy the hum just like I do with my Klipsch horns; means you have nice speakers!! Dale

Dale Stevens wrote:Kray, enjoy the hum just like I do with my Klipsch horns; means you have nice speakers!!  Dale

will do Dale!

Kray wrote:Thanks @mijohn for the details.

I’m guessing it’s probably next to impossible to remove all hum from this Preamp with my high efficiency 97db speakers?

It’s not bad at all but it’s ever so slightly there.

Hum it is not likely caused by tube selection. It is normally due to power supply artifacts, either 60 or 120 Hz, getting into the signal path. The main causes are improper grounding plan, and wire routing.

Assuming those are correct, and additional source can be insufficient pre-regulated B+ voltage. I had this problem with my SP13/PH12 build. The regulated B+ output was fine until I added the SP12. The additional current draw caused there to be an insufficient voltage drop across the regulators resulting in ripple in the regulated B+. Roy helped me fix this by changing a couple resistors to drop the B+ target voltage. Now B+ is ripple free.

It looks like your SP9 has a B+ regulator, so could have a similar situation. Just a thought.

Speaker efficiency alone has nothing to do with hum or hiss. Only the total gain matters (amp+speakers).

rjpjnk wrote:

Kray wrote:Thanks @mijohn for the details.

I’m guessing it’s probably next to impossible to remove all hum from this Preamp with my high efficiency 97db speakers?

It’s not bad at all but it’s ever so slightly there.

Hum it is not likely caused by tube selection. It is normally due to power supply artifacts, either 60 or 120 Hz, getting into the signal path. The main causes are improper grounding plan, and wire routing.

Assuming those are correct, and additional source can be insufficient pre-regulated B+ voltage. I had this problem with my SP13/PH12 build. The regulated B+ output was fine until I added the SP12. The additional current draw caused there to be an insufficient voltage drop across the regulators resulting in ripple in the regulated B+. Roy helped me fix this by changing a couple resistors to drop the B+ target voltage. Now B+ is ripple free.

It looks like your SP9 has a B+ regulator, so could have a similar situation. Just a thought.

Speaker efficiency alone has nothing to do with hum or hiss. Only the total gain matters (amp+speakers).

Thanks for this. I'll try to lookup more on the B+ and how to measure it, etc. I'm not super familiar with building stuff myself but can easily follow instructions and have a multimeter.

Kray,

Best thing to do is ask Roy how to measure the regulator voltage for that particular board. On my SP13 it was as simple as measuring the voltage across a single resistor and making sure it was at least 20V. Likely the same on yours, but need a schematic to say for sure. I searched and could not find a schematic online.

Incidentally, I noticed in the picture your PH12 looks different than mine. I don't see a B+ voltage regulator on it. Maybe it is an older version? or perhaps something other than a PH12? I would send the pic to Roy and ask him if he has schematics for those exact boards.

Here is a nice build thread of a SP9/PH12 combo. My SP12 looks like this one.
http://www.tubes4hifi.com/SP9-2016.htm

EDIT: On other thing I noticed. That red ground wire from the SP9 to the RCA ground buss looks suspicious. Are the L and R output twisted pair grounds (white wires) also connected at the SP9? If so that would be a ground loop. I notice in the build thread above the SP9 seems to be grounded directly to the AC line jack. Any ground loop can definitely cause hum. Worth looking into.

EDIT 2: Also looks like the phono input grounds are connected to the other input grounds. I'm pretty sure the phono inputs (both positive and negative) should go directly to the PH12 and nowhere else. This would be another ground loop since PH12 is grounded already by connection to the SP9 (The B+ ground -- Black wire).

Curiously, the build thread above also shows the phono grounds connected to the RCA ground buss, whereas all the other builds I've seen have the phono isolated. Maybe the SP9 has some special requirements that I am not aware of, but to me these connections look suspicious and are at least worth clarifying if you are getting hum.

rjpjnk wrote:Kray,

Best thing to do is ask Roy how to measure the regulator voltage for that particular board. On my SP13 it was as simple as measuring the voltage across a single resistor and making sure it was at least 20V. Likely the same on yours, but need a schematic to say for sure. I searched and could not find a schematic online.

Incidentally, I noticed in the picture your PH12 looks different than mine. I don't see a B+ voltage regulator on it. Maybe it is an older version? or perhaps something other than a PH12? I would send the pic to Roy and ask him if he has schematics for those exact boards.

Here is a nice build thread of a SP9/PH12 combo. My SP12 looks like this one.

EDIT: On other thing I noticed. That red ground wire from the SP9 to the RCA ground buss looks suspicious. Are the L and R output twisted pair grounds (white wires) also connected at the SP9? If so that would be a ground loop. I notice in the build thread above the SP9 seems to be grounded directly to the AC line jack. Any ground loop can definitely cause hum. Worth looking into.

EDIT 2: Also looks like the phono input grounds are connected to the other input grounds. I'm pretty sure the phono inputs (both positive and negative) should go directly to the PH12 and nowhere else. This would be another ground loop since PH12 is grounded already by connection to the SP9 (The B+ ground -- Black wire).

Curiously, the build thread above also shows the phono grounds connected to the RCA ground buss, whereas all the other builds I've seen have the phono isolated. Maybe the SP9 has some special requirements that I am not aware of, but to me these connections look suspicious and are at least worth clarifying if you are getting hum.

Thanks rjpjnk. I've sent an email to Roy with some questions.

- My PH12 board is v12a, it does have a B+(red) and Ground (black) in middle of board. might be hard to see on pic.
- I thought the red ground wire looked suspicious too. The thread shows the ground wire going to AC line jack, then the black wire from AC line to underside of board (not sure where it connects from there but looks like the same ground point where my PH12 black ground is connected to SP9 board). My AC line black wire connects back up to the power tree.
- yeah the other SP9 builds I've seen show the PH grounds connected to RCA ground bus. but one thing I noticed is they have each side red/white with a ground connection to the board. mine only has a white ground connection vs the regular build -> attached since I can't post links yet.


I'm wondering if I swap that red ground wire and the black ground to match the other builds if that would help.

I got an email from Kray about a week ago and answered the questions he had at that time.
This is one of the very early SP9s & PH12s, and this PH12 does not have it's own B+ regulator like newer ones did
(note I discontinued the PH12 a few months ago, and the SP9 about a year ago, but due to very slow sales).
rjpjnk was correct about the possibilities of a few ground loops in the photo.
The inputs jacks to the PH12 should not be connected to the RCA ground buss, only to the PH12 board.
And that red wire from the SP9 board to the ground buss shouldn't need to be there if the output connections are already grounded.
You could do a live listening test and snip that wire to see if it was helping or hurting.
Kray, any other questions email me direct as I check emails almost every hour, and only check into this forum once a week.

My SP14 and ST120 were dead quiet with my PSB Golds (88db efficiency).  I now play them through my Zu DW Mark II's and with the higher efficiency, I can hear a little noise when there is no music playing, very little.  Once the music starts, everything is wonderful.

The PSB's were 4 ohm speakers, running on the 4 ohm and sometimes 8 ohm tap.  As you know, the Zu's are 12 ohm speakers (97db efficiency).   I added a 16 ohm tap to my amp and am using that with the Zu's.

It is a shame that the sales of the SP9 was so poor. I found it a spectacular preamp with the compensation found necessary for a variety of vinyl sources. I found many older LPs in need of tonal compensation.
Paired with a PH16 I have a really nice preamp.