New here - VTA SP12 and PH12 project

I'm going to alter the B+ regulation as soon as I can grab some more resistors

I haven't hooked the PH12 back up yet, got side tracked with other things. Will keep you updated.

Thanks again for the help guys, really appreciate it.

I've been speaking with a company in the UK that builds custom toroid transformers for valve audio equipment and asked them to supply me with a suitable one.

This should get my voltages correct on the supply side and mean that the regulation will have more 'room' to work properly. It will also get rid of my noisy heater transformer.

I sat and had another quick listen to the amp last night, and even at this stage can tell its a cut above the best! I'd certainly compare it directly with my old Audio Note M2 Phono Signature. I'm using the 12AU7's at the mo - and have the 12BH7's to try at some point.

I'll get around to a proper review at some point.

have you hooked up the PH12 again? As this seems to be your problem area.

I've just got the PH12 hooked back up. And everything is now working properly. The SP12 is dead quiet at all bearable listening levels, no background noise at all. The PH12 has a bit of hum, not too bad though. I expect this might be caused by either my noisy heater transformer or having not hooked up the ground cable from my turntable as I haven't fitted a ground binding post for it yet.

If anything the volume control is too sensitive - 100k alps blue - it's getting to the limit of being too loud after only 1/3 of the travel.

I changed the filament resistor on the 12x4 to a 3r3 and the voltage there is now 12.9v. Could probably go a bit higher on the resistor if need be to drop the voltage a touch more.

Also changed R23 for 310k. Forgot to write down my voltages this time, but it seemed to be better.

well glad to hear you got it all sorted out!

Thanks. Just for fun I swapped the 12BH7's back in and have the same problem as before. I reckon there's a duff valve in the set. Will be sending those back to the supplier.

Will be replacing the 2 power transformers with a suitable toroid in a few weeks when I have a bit of cash...

if you've got hum with 12BH7s in there, then your filament transformer is probably marginal on current. Those tubes use twice as much current as 12AU7s

Thanks Roy. My filament transformer is a 6.6 amp one, I thought that would be enough as the tubes are only about 300mA each?

flak monkey wrote:Thanks Roy. My filament transformer is a 6.6 amp one, I thought that would be enough as the tubes are only about 300mA each?

that is more than adequate.
I have had some issues with 12BH7's as well. I found that 12BH7's can be all over the place. I like their 'sound', but for some reasons they seem to be more prone to noise than say 12AU7's, 5963's etc., so there is a good possibility that your 12BH7's are 'noisy'.
Remember the rather long winded reply I wrote regarding 'tested' tubes from vendors. THE best test for any tube is actually running it in your amp/preamp. Thats why it is a good practice, and fun to collect, a number of different tubes of the same model for tube rolling.

yeah, 6.6 amps of filament current is more then plenty, so not sure why you are having trouble only when you plug in the 12BH7 tubes!!
yeah, as Montana says, possibly bad tubes!!

I sent those 12BH7's back to the supplier and they refunded me - I haven't yet sourced any others to replace them, but I am quite happy with the 12AU7's for the time being - they aren't anything special Electro Harmonix Gold's but they sound good to me. I would like to try some more 12BH7 in the future though.

I spent the refund on a custom wound toroid for the power supply and replaced the pair of old fashioned frame transformers with one toroid. It's got rid of the transformer mechanical noise and there is considerably less background noise too. The voltages are more like what they should be too, though I neglected to write them down!

Sounds great - will get around to tiding up the cables in the next couple of days and then finally get around to just having a damn good listen to it.

Forgive the indulgent photo's - nothing like the glow of tubes to soothe the mind

you asked about the volume control, but never got a reply. One thing, the blue ALPS RK27 isn't the best volume control by anyone's imagination,
they are good but not great. The $30 21 step attenuator we now use looks cheap but is very high quality internally, and uses high quality metal film
chip resistors, much better quality. Not likely you want to spend $170 on a Goldpoint or $180 on a Khozmo, but those are the best.
You can reduce the sensitivity of the volume control by adding 50-100K of resistance in series with the input (in other words, between the selector out and the volume in,
add 50-100K of resistance on each channel) and that will cut your gain in half.