New here - VTA SP12 and PH12 project

Thanks Roy, much appreciated.

Well, I connected it up, and something ain't right. I've disconnected the inputs from the SP12 board at the volume control to eliminate any issues there.

After connecting to my power amp and powering up after a few seconds get a lot of hum through the speakers, increasing in volume quite rapidly. The first time I wasn't quick enough and my power amp went into protection mode. No harm done anyway.

Not really sure where to start after that, like I said I disconnected the inputs to eliminate anything from that side. It's only the SP12 connected up.

Tubes being used are:
Brimar 12BH7

Checked all the voltages and everything seems to be OK

12v/heater supply:
Transformer output 13.7VAC
Heater regulated output H+/H- 12.34VDC

B+ supply:
Transformer output 252vac
B+ regulated output 256VDC

Voltage across C4 233VDC, both channels within 1V

Pin 3 of 12x4 tube 13.3VAC - I am guessing I might need to drop this down a bit with a higher value resistor? Standard in the kit is 1 ohm and I reckon I need to go up to around 3.3ohms or so to get 12.6v here. Will have a look and see what I can find in my

My 12v transformer seems quite noisy, I disconnected the output from it and it didn't make any difference. It's one of these:

http://uk.mouser.com/Search/ProductDetail.aspx?R=VPS12-6300virtualkey55310000virtualkey553-VPS12-6300

so you are using 12BH7's in the SP12? Thats an unusual choice for the SP12. The 12BH7's do draw about double the filament current than the 12AU7/6189W @300mA each compared to 150mA each for the 12AU7. As long as your filament supply can handle the extra current, that side should not be an issue.
If you have disconnected the SP12 input, it is 'floating', which would probably cause hum. Re connect the volume control to the SP12 input. Select any input but the phono on the SP12, say AUX. Short the two AUX inputs on the SP12, volume to min on the SP12, connect it to the power amp and see what you get.

Thanks, will try your suggestions.

My filament transformer is rated at 6.3amps so should be more than man enough for the job.

Just a thought on the floating ground - I disconnected the input to the volume control from the selector switch. So the ground is still connected to the volume control. It didn't make any difference to the problem with it connected or disconnected.

OK, I'd still try it with AUX selected, AUX RCA inputs shorted, and see what happens.
Without having a good look inside your preamp, its a process of elimination for now.
Just double check all your grounding connections too. Both pcb's ground should be connected to chassis ground. Mains ground should also be connected to chassis ground.
All RCA jacks should be isolated from the chassis ground and grounded to the preamp pcb and phono preamp pcb respectively.

Thanks again.

Ok, hooked up the volume control and connected it back up and shorted the selected input. Same as it was. I seem to have a lot of microphonics (think that's the right word). If I tap the case in the few seconds before the hum starts to be an issue then I can hear it clearly through the speakers. Perhaps I have a huge amount of gain for some reason?

The grounding is simple:
Bus bar connecting all of the input/output sockets with the exception of the phono input is connected to the main chassis ground at one end only.
A ground wire from the CT connection on the HV input runs to the same chassis ground
The chassis ground is connected directly to the 3rd prong of the AC inlet.
The ground point is bolted firmly to the chassis and the connection is very good, immeasurable resistance on my digital meter.

Either way, here's a few clearer photos of the inside of the case













The power switch contacts have more clearance to the transformer than it looks in this photo!

well certainly cannot fault your workmanship, it is excellent! From what I can see and read, your ground seems to be perfect too!
Sounds to me like, as you've explained, the hum increases after a few seconds, that once B+ comes up and hits the tubes, the problem starts.
Just for giggles, do you by chance have a set of 12AU7's? Would be interesting to see how a different set of tubes would go in there.
Check all your component values again too, although I don't think it would make such a problem unless something drastic was 'swapped'.
Hopefully Roy and other can chime in here as well.

you must have some bad tubes if you've got that much microphonics going on.
Also, the hum MIGHT be related to the power supply, it seems your B+ is pretty low considering your HV input voltage.
With 250vac input you should have around 330vdc before the regulator, and 265v after the regulator.
Check the voltage on each side of D13 (one side is the B+ from the rectifier tube, the other side is after the regulator).
You should have at LEAST a 20vdc differential. If not you've either got a poor rectifier tube, poor B+ capacitor, or poor quality transformer.
But you can correct for that by changing R23 to a lower value. If you're only getting 250vdc after the regulator, change R23 to 300K to get 240vdc regulated,
assuming you have at least 260vdc coming from the rectifier tube. If you still have hum, you have a ground problem (missing ground, or ground loop)

Thanks both. I'll check those things over again later today when I get out of work.

I don't have a set of 12AU7's. I'll have to buy some more valves if I need to try a different set. Although if I do this I could buy new valves rather than NOS vintage ones. To try and prove a point. If it is the valves the set I have will be going back to the supplier as I have no way of testing them myself.

Roy, the B+ transformer is 553-VPS230-110 from Mouser with both primary and secondary wired in series. The AC inlet voltage is approx 230-235V here in the UK, though it varies by +/-5v depending on the time of day you check it.

I'll check those other voltages later and report back...

I've spoken to the supplier that I ordered the 12BH7's from and they said they test all valves before dispatch. So I have no reason to believe that there's a problem with them.

I've also ordered some new 12AU7's anyway to try out.

one thing, and this applies to all newbies to tube gear, is this so called 'tested' tubes subject.
First thing I say is 'caveat emptor'....buyer beware.
Most tests performed on tubes are rudimentary, at best. Stick them in a tube tester, and see if they pass, if yes, they are 'tested'. Now, there is nothing wrong with this basic of test, but ALL that shows is that the tube tested passes the minimum requirements, with a lot of tube testers out there, thats a basic 'good/bad'.
Very few commercial tube suppliers, even private sellers on eBay and such, have anything more than a basic tube tester, it tells them the tube is good or bad, nothing more.
To do a comprehensive test on a tube, takes a very sophisticated, and expensive, tube tester and would/should involve a number of different testing procedure, or a custom made tube tester that will pretty much check ALL parameters of a tube.
I won't go into details here, as there is plenty of reading material out there on the web on this subject, but for example even a good tube tester does not have the ability to supply the high B+ voltages for a given tube, as this would make the tube tester too expensive and complicated, so most testers are a compromise, enough voltage levels to do basic tests un tubes.
Your supplier is probably telling you the truth though, they may well have tested the tubes they sell on an every day tube tester, and as far as they are concerned, the tubes passed.
For interest sake, ask you supplier what tube tester they use, it would be interesting to see what reply they come back with.
I have a number of tubes in my stock that test well, and my tube tester, although not top of the line, is better than most basic tube testers out there. But when I plug them into an preamp, I get some hum or other noises.
Now, apart from using expensive or custom made tube testers, the single best test for ANY tube is?.....plug it in to your amp and listen...that is THE best test you can perform on any tube!
So, be VERY careful, and take it with a grain of salt, if a supplier tells you that a tube has been tested.
Most of this applies to "NOS" stock, newly made tubes are generally good, some do need 'running in' time, and will get better with time.

yes, as Montana has said, tube "testers" really just apply a voltage and read the current flow thru the tube.
They do not check things like noise, microphonics, or other characteristics. The best tube tester is your amplifier (or preamp).
Most tube suppliers offer a 90 day return policy. I've only had a few "bad" tubes, but I've had a lot of tubes that were noisy.
Some tube suppliers do offer additional "noise" testing for a premium price.
For USA buyers, I've had very good luck with Jim McShane, TubeDepot, and Yen Audio for new tubes. For vintage tubes, TCTubes.com

Thanks both, very insightful. I've had valve amps before, but never really played with them as such. So never really considered the testing process. I buy a lot of my valves from HotRox who supply a lot of guitar amp related kit. Although I struggled to find the 12x4 valve in the UK for some reason, so ordered that and the 2 JAN 6922 tubes in the PH12 from Langrex.

Here's the result of the voltage testing:

AC input voltage at mains 232VAC
TX output 252VAC

B+ @ J4 jumper 256VDC

Voltage at D13 from rectifier tube 272VDC
Voltage at D13 after regulator 256VDC

flak monkey wrote:Thanks both, very insightful. I've had valve amps before, but never really played with them as such. So never really considered the testing process. I buy a lot of my valves from HotRox who supply a lot of guitar amp related kit. Although I struggled to find the 12x4 valve in the UK for some reason, so ordered that and the 2 JAN 6922 tubes in the PH12 from Langrex.

Here's the result of the voltage testing:

AC input voltage at mains 232VAC
TX output 252VAC

B+ @ J4 jumper 256VDC

Voltage at D13 from rectifier tube 272VDC
Voltage at D13 after regulator 256VDC

as tube4hifi stated, your B+ is low, your 272VDC should be around the 330VDC mark and your 256VDC should be around the 265VDC. OK, thats 'only' a 9VDC difference, but that could/would make a difference in the overall circuit performance.
Change those resistor values at the power supply regulator as per tubes4hifi suggestion and see if the resulting increase in your B+ makes a difference in the hum. If not, then it is either a single or more bad/noisy 12BH7's or still a ground related issue.

Ok, thanks. I'll swap out R23 and see if that makes any difference.

My other new tubes should be here tomorrow too, those are old stock tubes, they are new production 12AU7EH Gold's. They were low cost to put in there and try it again.

flak monkey wrote:Ok, thanks. I'll swap out R23 and see if that makes any difference.

My other new tubes should be here tomorrow too, those are old stock tubes, they are new production 12AU7EH Gold's. They were low cost to put in there and try it again.

I don't know how serious you are wishing to get back into tube sound, but is always adviseable, and can easily become an obsession! ,to start collecting tubes of different manufacturers of the same model, as well as equivalent models, for example 12AU7, I really like the 5963's, or their European equivalent ECC82, they do tend to have a really nice sound, particularly RCA brands, this way, you can tube roll and should one or more tubes sound a bit noisy, swap them over until you're happy with the sound!
As for the rectifier you are using, not much can be substituted for the 12X4, its pretty much on its own.
And as you perhaps build up your collection of tube power amps, again, start collecting different brand and model tubes to suit that amp.

Montana had it mostly right, but changing R23 isn't going to increase your B+, what it's going to do is take your reduced B+ that's available,
and use a set value for the regulator that will let it do it's job. You've got a 16vdc difference input to output, which is marginal, but it should work
as the regulator has about a 15v dropout, but I like to run it at 20-25v difference. Using a 300K resistor for R23 will make the regulator put out 241vdc,
which will give you a 31v difference. That's a little low on B+, but it should work fine.
If you put a 300K in SERIES with a 10K resistor the regulator will put out 249vdc for a 23v difference, right in the right ballpark.
When the regulator is actually working as designed (using the correct set resistor to give > 20vdc difference) then your background noise
will be reduced by 60db, yes, you read that correctly, the SP12 & SP14 are DEAD QUIET.
Here's a partial quote from an email I got today from a new user . . .
Moment by moment, the sound of this preamp is revealing its magic with absolutely zero detractions.
Among its attributes, I am stunned at the black background on silent passages in music.
It was no bother with the HK Citation 1, but this starts and stops at a black wall of silence. A SS preamp could not be more quiet.

Great, thanks again guys.

I've just tried a different set of tubes and I still have the same problem. So it's time to start having more of a dig into other things I think.

The noise (hum) isn't there immediately, it takes maybe 5 seconds to start to build and then increases quickly until my power amp goes into protection mode.

I am thinking it's a grounding issue somewhere.

flak monkey wrote:Great, thanks again guys.

I've just tried a different set of tubes and I still have the same problem. So it's time to start having more of a dig into other things I think.

The noise (hum) isn't there immediately, it takes maybe 5 seconds to start to build and then increases quickly until my power amp goes into protection mode.

I am thinking it's a grounding issue somewhere.

that delay in the onset of the hum is the B+ ramping up and 'awakening' the tubes, for one of the better explanations.
Yes, I agree, sounds like a ground issue, and you've pretty much isolated it to the line preamp. For interest sake, short the phono input RCA jacks, select phono on the preamp. I'm pretty sure you'll get the same result, but it is worth trying anyway...nothing to loose.
Check all your wiring, solder joints/connections, component values, just go over the whole assembly again. Thats the best I can suggest for now.

Thanks again. some success to report!

I sort of got it working now. Checked all of the components and values, everything ok. The R23 value is 316k anyway, but will change it to 310 if it will help things along.

Removed the HV to the ph12 and it now powers up with no noise at all. I also redid the jumpers on the SP12.

Get a few pops which trip the power amp if you turn that on first, which I know you shouldn't. Bad habits.

I haven't really had chance to listen to it properly yet, but I'm impressed so far!

flak monkey wrote:Thanks again. some success to report!

I sort of got it working now. Checked all of the components and values, everything ok. The R23 value is 316k anyway, but will change it to 310 if it will help things along.

Removed the HV to the ph12 and it now powers up with no noise at all. I also redid the jumpers on the SP12.

Get a few pops which trip the power amp if you turn that on first, which I know you shouldn't. Bad habits.

I haven't really had chance to listen to it properly yet, but I'm impressed so far!

so you're saying B+ to the PH12 for now is isolated? What about F+ & F- to the PH12?

I left the filament supply connected. I removed the B+ and ground wires between the PH12 and the SP12 and its all ok now. I haven't tried reconnect it again yet. The valves aren't fitted to the ph12.

I'll be checking the ph12 components tomorrow to see if I screwed something up.

flak monkey wrote:I left the filament supply connected. I removed the B+ and ground wires between the PH12 and the SP12 and its all ok now. I haven't tried reconnect it again yet. The valves aren't fitted to the ph12.

I'll be checking the ph12 components tomorrow to see if I screwed something up.

were the tubes ever fitted to the PH12 when you had the problem?
As for the PH12 ground wire, I would go directly to your star/chassis ground from the PH12 pcb, thats how I connect my builds.

The valves were fitted yes. I am about to fit the B+ between the 2 again, will run a seperate ground back to the main chassis/star and see if it's any better.

I'm pleased that I hadn't screwed something up completely!

flak monkey wrote:The valves were fitted yes. I am about to fit the B+ between the 2 again, will run a seperate ground back to the main chassis/star and see if it's any better.

I'm pleased that I hadn't screwed something up completely!

OK, keep us posted

connecting the PH12 will drop your B+ slightly, so I'm still pretty sure your B+ is right on the ragged edge of regulation,
and lowering the 316K value of R23 to 300K or maybe 310K will solve the problem.