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    SP10 problem =(

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    martyingels

    Posts : 8
    Join date : 2012-06-20

    SP10 problem =(

    Post by martyingels on Wed Jun 20, 2012 2:57 pm

    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!

    martyingels

    Posts : 8
    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Wed Jun 20, 2012 3:17 pm

    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*

    tubes4hifi
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    Re: SP10 problem =(

    Post by tubes4hifi on Wed Jun 20, 2012 5:41 pm

    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

    GP49

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    Re: SP10 problem =(

    Post by GP49 on Thu Jun 21, 2012 12:23 am

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

    Now, THERE'S a pretty good hint!

    martyingels

    Posts : 8
    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Thu Jun 21, 2012 3:53 am

    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.

    tubes4hifi
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    Re: SP10 problem =(

    Post by tubes4hifi on Thu Jun 21, 2012 12:54 pm

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

    Bob Latino
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    Re: SP10 problem =(

    Post by Bob Latino on Thu Jun 21, 2012 12:59 pm

    EH small signal tubes have sort of a "hidden filament" that is hard to see in a well lit room. The thing to do with EH 12AT7, 12AU7, 12BH7 tubes is to make sure the room is dark to try to see if the is filament lit OR feel the tubes to see if they are warm after the amp has been on for a minute or two. It goes without saying to never touch any tube after it has been on for longer than a minute or two ... you could get a serious burn .... The inside of many tubes are in the 350+ degree range after they have been on for a while ...

    Bob

    martyingels

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    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Thu Jun 21, 2012 2:34 pm

    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.

    martyingels

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    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Thu Jun 21, 2012 2:40 pm

    I appreciate you chiming in too, Bob!

    GP49

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    Re: SP10 problem =(

    Post by GP49 on Thu Jun 21, 2012 4:46 pm

    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.

    Bob Latino
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    Re: SP10 problem =(

    Post by Bob Latino on Thu Jun 21, 2012 4:59 pm

    Just double check that you are on the DC scale on your meter when you measure F+ to F- ... I have myself at times (not thinking) had my meter on the AC voltage scale when trying to measure DC voltage. If you are truly getting the 1.38 VDC between F+ and F- .... As GP49 has said, that is way low. Check all your solder connections on the preamp's filament wiring. Resolder any that look suspicious ...

    Bob

    martyingels

    Posts : 8
    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Mon Jun 25, 2012 1:55 pm

    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!

    martyingels

    Posts : 8
    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Tue Jun 26, 2012 12:56 am

    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.

    tubes4hifi
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    Re: SP10 problem =(

    Post by tubes4hifi on Thu Jun 28, 2012 4:26 pm

    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! Very Happy

    Bob Latino
    Admin

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    Re: SP10 problem =(

    Post by Bob Latino on Thu Jun 28, 2012 4:40 pm

    I agree with Roy ... I also have a dislike for "auto-ranging" meters. Sometimes they do not "auto-range" properly and as a consequence, they give erroneous readings. I had a VTA amp sent to me one time that "would not bias". The amp biased here in my shop just fine. The problem was traced back to a customer's bad auto-ranging meter whose auto-ranging sensor did not work properly.

    Another thing to consider is to have at least TWO meters. If one starts acting funny and gives weird readings, try the other meter. If BOTH meters read the same, then you can feel pretty safe that the reading you are getting is accurate. I have a B&K meter that I use everyday which is backed up by a Fluke meter that spends most of its time in a drawer ready to "pinch-hit" for the B&K.

    Bob


    Last edited by Bob Latino on Tue Jul 03, 2012 2:14 pm; edited 1 time in total

    martyingels

    Posts : 8
    Join date : 2012-06-20

    Re: SP10 problem =(

    Post by martyingels on Tue Jul 03, 2012 1:40 pm

    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!

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