New here - VTA SP12 and PH12 project

Firstly, hello everyone. Joined up here as it seems to be the place to be for people with VTA kit

A bit of background: I've been into hi-fi kit for about 20 years - bought my first budget kit when I was in my early teens. Since then I have been through a huge amount of kit - sometimes making steps forward and sometimes backward. I've done quite a few DIY electronics projects before, but this is the first time I have undertaken a valve pre-amp project. Maybe not a sensible place to start, but we will see. I'm an engineer by trade - graduated about 10 years ago, and have my own well equipped workshop - but it's mostly full of custom motorbike and car projects. I've built my own ECU's for these before.

I've always been interested in electronics and I've done a lot of reading on the subject of valve electronics recently - but obviously this is no substitute for experience. However, we all start somewhere

The reason for wanting a valve pre is that I really miss my old Audio Note M2 Signature and wanted to get something to replace it. No way I can afford another one of those though. So after a bit of reading around and a few fruitless web searches (mostly turning up Chinese junk) for suitable kits to build something I came across the VTA equipment and was impressed with the write ups.

After some more deliberation I've settled on the SP12 and PH12 kits. I have found Roy very helpful indeed in the short email exchanges we have had to date.

I've opted for the PCB kits only, and they are on their way to the UK. So I'll be looking for a suitable case, transformer, switches and pots etc. Shouldn't be a major problem though.

First subjective question - does anyone have any sensibly priced valve recommendations on the following? I have in the past used JAN/GE valves but these are getting increasingly difficult to find at sensible prices.

4 off 12AU7/ECC82
1 off 12x4
2 off 6922/6N11/E88CC

Also is anyone able to tell me approximately what sized enclosure I need to be looking for to fit the SP12 and PH12 in sensibly?

I intend to keep this up to date as I get on with the build. Probably with a few questions as I go.

Thanks
SJ

Welcome to the booby hatch, Flak Monkey.

If you're looking for NOS tubes and decent new tubes there a couple of very good sources here in the Americas.
Jim McShane at mcshanedesign.net
Andy Bowman at http://vintagetubeservices.com/
(Don't email Andy, just phone him up. He doesn't do email.)

12x4 rectifiers are plentiful, you could likely find them locally for under U$D 10;
As to 6922s, the best ones were made in Holland, on your side of the pond, so look around locally as well.
The 12AU7s will likely be most critical to your sound quality. For those, I would recommend Andy or Jim. You might talk to Roy about substituting 12BH7As, which have worked well for me.

Best of luck with your builds.

Cheerio,
David

Hello David, and thank you

We have a few tube suppliers on this side of the pond too, I have previously used:
Watford Valves
HotRoxUK

I'll take a look at the sites you linked to as well

I'd be interested in substituting 12BH7 as I understand they are a 'better' sounding valve but need a bit more filament current. As I don't have a mains transformer yet, this might be a worthwhile upgrade.

SJ

Well, we're getting a bit out of my area of expertise here.
What I can tell you is that I've used 12AU7s and 12BH7as in the driver holes in my VTA power amps, and much prefer the latter.
You could also go bug-nutty and stuff some octal 6SN7s into those slots since you're scratch-building and don't have sockets yet. I've not tried same but others on this board have. Ask around.
Bob Latino has the best handle on xformers, custom built, but they are US voltage. I suppose a 120/240 converter would work, but it probably adds a bit of noise. I am sure sKiZo and Bob will weigh in here at some point; listen to them.
Cheerio
-d-

Thank you

My previous pre amp used a pair of 6SN7's in the line stage and sounded sublime. Infact, I still have some of those valves on my shelf. But I have a feeling they are a very different beast indeed and might need more than a few minor changes. 12SN7 might also be possible without heater circuit changes?

I don't have any problems getting a transformer in the UK, if I've got the spec

Open to all suggestions to be honest

Been doing some research and have decided I'll simply upgrade the transformer and fit the 12BH7 tubes. Changing to 6/12SN7 looks like it's considerable modification, to met at least, so will go the simpler route!

Been speccing out the other parts from Mouser and have it more or less sorted.

Anyone able to suggest suitable impedance for gain control? My guess is linear curve, centre detent RK27's at 10k?

Thanks,
SJ

That's a question best asked of Roy (tubes4hifi) on this board. The ones he fitted me up with are superb.
Also neglected to mention a tube source who sells on canuckaudiomart, goes by the handle of Dan0, I think, he's in Quebec. PM me and I can zap you his email. Never did me a bad turn.

Thanks David, I'll look him up

This didn't take too long to sort out. Got the jumpers to do yet. Will get the PH12 assembled tomorrow.

Flak,
Lookin' sweet. Glad you found Europe-voltage xformers.
Can't wait to hear how they sound to you. Book report due on Friday or thereabouts assuming your soldering iron doesn't flame out.
The 12BH7A is a fantastic and long-living tube, provided they are not highly microphonic, which is not a brand issue, just a matter of experimentation or your confidence in the supplier.
Cheerio,
-d-

I'd recommend a 100K audio taper volume control, you don't want a center detent (that's something that is normally a balance control)
Using 12BH7 you'll need double the current (1.5A total for the tubes, so spec a 3A transformer) and bigger heat sinks on the filament regulator.

Thanks, yes I have a 100k audio taper pot for the volume control. I was specifically talking about the left/right gain controls when I mentioned the centre detent pot.

I've been working on this on and off, got the PH12 built last week I and have started work on the chassis. Just need to finish drilling all of the holes in the front panel for the volume and power controls then I can start sorting the internal layout. I haven't put the Mouser order in yet for all of the other bits yet.

I have been working on this on and off, in between the other projects. I finished drilling the case for all of the other bits, fitted all the RCA sockets etc.

And this week I am finally getting around to actually fitting everything into the case and wiring it up. Made lots of twisted pair cables in different colour combos. A lot of it is still only connected up one end so far as I am waiting for a Mouser delivery of my transformers and a few other bits. I need to tie all the cables up to tidy them up once I have tested everything.

A couple of quick question ref grounding on this set up, I'm wary of setting up any ground loops and want to do it right first time - I have linked all of the ground lugs on the RCA jacks (but not the phono input) with a piece of copper stripped from some household cable. All of the cables to the selector switch are grounded at the input end only. Should I run a ground cable from the copper connecting the ground lugs back to the main case ground which also runs to the earth pin of the power connector?

There is a ground connection on each of the PCB's which currently has only a piece of cable attached. Should I also simply run these back to the same case ground?

it looks good! just leave the twisted pairs ungrounded on the selector end. The ground buss of the RCA jacks should connect to AC earth ground on the AC input connector.
I usually keep the phono input ground completely separate and ground it only to the PH12 input ground.
The twisted pair outputs of the SP12 board will complete the ground to the RCA buss ground.
You may want to re-orient the SP12 board 90 degrees so that the AC input voltages aren't right next to the RCA jacks, and as far away as possible from the PH12 phono.
Not sure you got the answer for the left/right gains, those can be either linear or audio taper, doesn't matter, and I usually use 100K.
They aren't really balance control with a center detent, they are used as left/right channel gain, which of course can be adjusted to balance left/right gain in that sense.
It's a much more flexible arrangement than a simple balance control, because it's system dependent on your amplifier gain and speaker sensitivity and your room setup.
Set it up once and forget it, then just use the volume control.

Thanks Roy. I am going to re-route the L/R outputs from the SP12 away from the AC inputs, if I have problems then I'll look at turning the board around.

Following your explanation above, the ground point on each board - near Q1 on the SP12 and near R3 on the PH12 - can these just be left disconnected?

One other quicky - best place to pick up B+ and GND supply to the PH12? I've taken it from the jumpers, J3 and J2, but would it have been better to take them from somewhere else?

connect B+ & GND on the PH12 to A and GND on the SP12 (one on either side of Q1)
and the SP12 GND between the two diodes at the far right end of the board should go to AC earth ground

Perfect thanks. Will move those over later, nice and simple

Finished wiring it all up tonight, waiting for my tubes to arrive now, which should be tomorrow.

Connected power and checked voltages:

Getting just over 13VAC at the input of the heater circuit and 12v dc out. All good there.

Getting ~260 VAC at the input of the HV circuit. Can't check the output yet as I don't have the rectifier tube. Getting around 130VAC between ground and neutral and ground and live - at the unused CT connection at the HV input. Sound right?

Reckon I'll fit some anti-vibration pads under the transformers later this week.

Getting a bit of hum from the heater transformer - the larger of the 2 in the pic. Not sure if it will cause any issues yet as I still don't have a full compliment of tubes to test this thing with.

Although 4 NOS Brimar UK 12BH7's arrived today from HotRox

you may need to add some copper or mu-metal shielding around the transformer(s)

Thanks, I was planning on making a shield to fit between the transformers and the boards I will have a look and see what I can find.

tubes4hifi wrote:you may need to add some copper or mu-metal shielding around the transformer(s)

ya took the words right out of my mouth!
I would go for mu-metal before copper, it does seem to work that much better!
Or, if that does not do it, try a toroid to power the HV and one set of filaments and a smaller toroid for one set of filaments only, if you have the room.

Thanks for the input guys. I'll have a look and see if I can find a source of Mu-metal in the UK. Found lots in the USA. Might be that we call it something slightly different here. I know there are issues with bending/drilling it so I'll do a bit of research in that. I was initially planning to make a simple copper/steel shield at some point anyway.

I'm interested to note that a few people have fitted additional filtering to the PH12 power supply. Can anyone shed any light on what this entails and the benefits of doing so?

I am hoping the rest of my valves arrive today so I can give it a test tonight.

on the older versions of the PH12, there was no provision for additional B+ filtering, so a few customers added a choke between the SP12 and PH12 B+,
but for the past year or so all PH12s have a B+ regulator on them, which works a lot better than a simple choke.