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    B+ supply to board good, but different V at pin 1 of 12AT7

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    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    B+ supply to board good, but different V at pin 1 of 12AT7

    Post by Pooch on Thu Feb 28, 2013 1:08 am

    This is on a mk4 VTA board. I checked R13 and R11(150K,68K) and they checked A-OK, but I swaped them out anyways just in case they went off value when under load. No cigar, I'm still getting 147v on one board and 163.8v on the other one. This is taken at pin 1 of the power 12AT7 (is that what it's called?)the non splitter one. Could it be C13, C15 or R15,R19? I'll pull and check those tommorrow, but I thought I'd ask wiser minds, first. The reason I started tracing the Volts is because I can't get the bias up past .36 on the amp with the low V on pin 1. Tubes all test good on a Precision 10-12 tube tester. B+ is 423 VDC at the board terminal.
    Gordy

    tubes4hifi
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    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by tubes4hifi on Thu Feb 28, 2013 2:51 pm

    both of those voltages are within nominal limits considering that tubes and tube circuits easily can have a 10% difference in operation.
    Seems your real problem is setting the bias, so don't worry about the voltage on V1 that's within spec.
    If you can't get enough bias, then you'll need to change R39 to a lower value. You've probably got a 10K there.
    Try changing that to a 4.7K or even a 2.2K and then try to set the bias again, and I'm sure all will be fine.

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by Pooch on Thu Feb 28, 2013 5:02 pm

    It seems I already have a 4.7k at R39. The schematic called for a 10K, but your parts list has a 4.7K listed. Which is what I put in. I checked The Volts on both amps and here's what I got....55 VAC on the board from transformer (good)
    -74 VDC after the diode, -74 VDC after 1st cap and -68.5 after the 4.7K resistor. These figures apply to both amps, with cables and speakers plugged in. Should I still drop the resistor to 2.2K on both amps, or check elsewhere?
    Thanks,Gordy

    tubes4hifi
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    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by tubes4hifi on Thu Feb 28, 2013 5:13 pm

    if you can't get your bias up to 0.40vdc, then you need to change the range of the bias pots, by changing the value of R39 to a lower value,
    or you can change BOTH R37-38 to a lower value. Are you using some unusual output tubes? Usually the stock range is good for most tubes,
    although it seems using KT77s the R39 value needs to be very low. Not below 1K or you'll start getting some noise in the bias.
    Your -74v after the diode is pretty high, so you may want to change those R37-38 resistors from 27K to 22K or 18K.


    Last edited by tubes4hifi on Thu Mar 07, 2013 9:48 pm; edited 1 time in total

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by Pooch on Thu Feb 28, 2013 9:14 pm

    Ge EL34s in the amp in question. I'll lower R39 and see what happens. I have some Chinese EL34s I can put in ,to see what happens.

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by Pooch on Thu Feb 28, 2013 11:22 pm

    I lowered R39 to 2.7K (didn't have a 2.2K), and with the GE tubes I couldn't get the bias past .324VDC with both pots turned fully counterclockwise. So I changed the GE EL34 for the no-name Chinese EL34s and I was able to bias to .4VDC with both pots fully counterclockwise. I'm guessing it's a tube related problem. I'll change R39 down to 2K and see if I can get some leeway on the bias pots.

    I'm not sure what R27-R28 you're talking about, Roy? My mk4 parts list and schematic only shows an R27 with a value of 1K.

    tubes4hifi
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    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by tubes4hifi on Fri Mar 01, 2013 6:15 pm

    oops, sorry, not R27
    R37 is the other end of the bias voltage divider. It's usually 27K but may be a 33K. Making it smaller will also give you more bias.
    The bias voltage divider consists of R39, the bias pots, and R37. On an VTA70 board, it's R37 and R38 in addition to R39.


    Last edited by tubes4hifi on Thu Mar 07, 2013 9:49 pm; edited 1 time in total

    Pooch

    Posts : 53
    Join date : 2009-12-01
    Age : 66
    Location : Wa

    Re: B+ supply to board good, but different V at pin 1 of 12AT7

    Post by Pooch on Thu Mar 07, 2013 9:12 pm

    thanks, Roy. Yup R37 was the problem. I had 33k in there, but failed. It was what looks like a wirewound and measured ok when lifted, but must be drifting in circuit. All I know is I replaced with a 1 watt metal film 33k and all is well.
    Gordy

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