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Thread: Randall RH150 low output

  1. #1
    Junior Member
    Join Date
    Feb 2018

    Randall RH150 low output

    Hi there,

    I've been repairing tube amps for a few years, and have recently started taking in SS amps. Up until now they have been simple repairs, that I've been able to take care of without no transistor knowledge. Now I have this amp that is going to require me to do some learning, and I'm hoping to get some help and guidance from this forum.

    Amp is a Randall RH150. It has one tube, a 12at7, that appears to be a driver tube of some sort. Power section schematic is attached. I should say that I'm still staring at the schematic and figuring out what everything is...

    Also, when the amp came to me it had a 12AX7 in it....

    The amp has really low, farty , kinda crackly output, but I can hear the tone of the amp buried in there, and it sounds pretty good . I'd say its about as loud as a Champ right now, at full bore.

    I've done all the normal cleaning of the fx jacks and ins/outs, and jumpered them to see if that worked, but it didn't. I took the fx send and plugged it into a different, working power amp, and the preamp is fully functional. I took a working preamp I built (MKIIC+), and plugged it into the return, and the amp had the same farty low output.

    So that's as far as I've made it. I've narrowed down the problem to the power amp, which I know nothing about.

    Any thoughts on where I should start, and what I should do, along with possibly a beginner's explanation?

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  2. #2
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Lansing, Michigan, USA
    Don't know that I would call that a driver tube, but if you like, go ahead. I just see it as a pair or triodes in series, like any tube amp. The signal hits the left grid, and the resulting plate signal is fed to the second grid. The second plate signal feeds a source follower to feed the power amp proper at SIGNAL A.

    So it is low and farty. First check power. Looks like it runs on 70v rails, so are they both up close to that and clean? The op amps no doubt run on +/-15v or maybe 12. Are those all up to voltage and clean? The op amp ICs with 8 legs have outputs on pins 1 and 7, is there any DC offset on any of those? Looking for volts here, not a few millivolts. The tube supply is +375vDC, is that close to voltage and clean?

    You know tube amps? OK, so are the voltages around the tube normal? With 375v B+, is the plate voltage on each triode a couple hundred volts more or less? Are the cathodes sitting at a few volts? Did you try a different tube?

    Check out the signal path. It enters on page 2 through IC2A. From there through IC2B. All that stuff left of and below IC2B is basically a graphic EQ circuit that is not adjustable. They set it up for a tine and hard wired it.

    The outputs on the 14 leg IC are the corners.

    So apply a signal to the FX return and scope it through the amp. What does it look like at IC2A? IC2B? At grids and plates of the tube, and at SIGNAL A. That is page 2 and the lead-in to the power amp. If you don't have a clean signal at SIGNAL A, then you won't have one at the speaker.

    Page 1 is the power amp. We'll look at that later.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
    Junior Member
    Join Date
    Feb 2018
    Thank you very much for the response! This gives me a great starting point. I'll report back when I get a chance.

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