SP10 problem =(

Hi all! I just finished building Roy's SP10 preamp and there is practically no gain. The line stage tubes (I tried both 12AU7s and 12BH7s) don't light at all and I can barely here the music, even with the potentiometer cranked to 11.

Has anyone else built this preamp? Any ideas on what I should check? I'm just a newbie and a hobbyist, but I've built one of Roy's phono preamps without any problems and I'm pretty sure I've got this one wired correctly as well.

Any ideas / help will be much appreciated!

Thanks!

I should have mentioned too - the power supply side of the PCB seems to be fine. The 12X4 glows properly, so I know it's getting AC. The B+ voltage seems fine too, since I shocked the hell out of myself touching the B+ solder connection on the line stage side of the PCB. I checked to make sure the F+ / F- jumper was wired correctly and it seems to be too... *scratching head*

Hi Marty,
email me directly, as no one else has this exact version, although the circuit is identical to my SP6 and Zmod circuits.
You should have 24v across F+ to F-, about 290vdc on B+, and 145vdc on pin 3 of V4 (junction of R25 and R28) as the schematic shows.
If that's all correct, it should work perfect with lots of gain!
Roy info@tubes4hifi.com

martyingels wrote:
The line stage tubes (I tried both 12AU7s and 12BH7s) don't light at all...

Now, THERE'S a pretty good hint!

Hi Roy! I will send you a personal email, but I thought I should post my measurements here as well.

With the red test lead on F+, I measured 51 V.
With the red test lead on F-, I measured 50 V.
With the red test lead on B+, I measured 270 V.

I should also add that the above measurements were the same on both sides of the jumpers.

BUT, with the red test lead on pin 3 of V4 (assuming I was testing in the right location), I only measured around 370 mV.

Also, both sets of tubes that I tried were brand-new, EH tubes.

it appears you don't have any filament voltage, check for 24vdc between F+ and F-

Honestly, I'm not sure I know how to check the voltage between F+ and F-... but here is what I did and the resulting reading.

I placed the multimeter black test lead on F- and then the red test lead on F+. I did this on both sides of the jumper and both times I got a reading of 1.38V.

Did I measure that correctly? I apologize for being such a simpleton. All help is very much appreciated.

I appreciate you chiming in too, Bob!

1.38V is too low. At that voltage the tube filaments won't light up and with very low emission from the filaments, the tubes will barely function, if at all.

Sorry for the late reply - I was away from the computer for the weekend.

My multimeter is auto-ranging, so it's supposed to automatically detect whether you are measuring AC or DC voltage (or, at least, that's the assumption I am laboring under).

Roy sent me an email with a few more things to measure, so when I take those I will update this thread with the results.

Thanks to all!

Ok - Roy asked me to remove all the tubes from the PCB and take the following measurements: voltage across F+ to F-, voltage at input to the filament circuit, and the voltage at the input to the filament regulator.

I removed all tubes (including the 12X4) and got these readings:

1.78V across F+ to F-
8.22V at AC input to filament circuit
16.5V at input to filament regulator

Also, out of curiosity, I checked the voltage at the output from the filament regulator and it read 1.78V - the same as the voltage across F+ to F-.

So, it seems that the power transformer isn't delivering enough current, but I'm also guessing that the filament regulator might be bad? What do you think? By the way, the power transformer is a PA211, purchased from Dynakitparts.

I think we figured this out, Marty may report back in a day or two. I think the two resistors for the filament regulator were reversed,
my fault, not his. But just reading over the postings reminds me of how much I dislike "auto-ranging" meters.
Using a meter that has a mind of it's own, tends to let you not use your own mind! You need to know the difference between AC and DC !!
I'll take a $20 meter that I can set on the correct scale over a $200 meter anyday!! Fact is, I have about 6 of them!
That way I can measure every single AC and DC voltage in an amp all at the same time!

Hi all!

Indeed, once I reversed those two resistors, the voltage jumped to 12.62v across F+ - F- (that's without the 12X4 in place). Problem solved!

And lesson learned regarding auto-ranging meters. I think I'll buy a second meter that isn't auto-ranging and just leave the auto-ranging one in the tool box as a back-up.

Thanks so much to all for the help!