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  1. This amp is 58 years old, and with amps this age,...

    This amp is 58 years old, and with amps this age, you really need to take what I call a "global" approach, which means that you can't take anything for granted. To be honest, I wouldn't even turn on...
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    My take from personal experience having played...

    My take from personal experience having played around with Dynaco amps for years and having rebuilt two pairs of Dynaco MkIIIs with improved driver boards is that the Dynaco driver circuit shows its...
  3. That's how it's done in transistor combo organs,...

    That's how it's done in transistor combo organs, but Hammond's mechanical tonewheel generator is locked to the AC mains frequency with a synchronous motor, so that approach is not possible.
    ...
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    True, but as you said in your post, leakage is...

    True, but as you said in your post, leakage is less likely to be the culprit here.
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    Electrolytic caps can have a number of different...

    Electrolytic caps can have a number of different things wrong with them. DC leakage at working voltage is only one of them.

    It's possible for a capacitor to have high ESR or even to look like an...
  6. Is it accurate to describe vintage Magnatone vibrato as pitch-shifting?

    I just finished a long, frustrating restoration of a non-working Magnatone M15A with a huge list of problems, so I became pretty familiar with the circuit. Its vibrato circuit was marketed as a true...
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    Many thanks for posting all the photos! It's a...

    Many thanks for posting all the photos! It's a big help.

    I also mounted an IEC socket. This one had had mice in it and required a lot of corrosion-related contact cleaning.

    I lowered...
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    Magnatone M15A Reverb Tank Mounting?

    I'm ready to finish a long Magnatone M15A restoration, and I need to mount the new spring tank in the cabinet. This is a cabinet with an angled baffle between the amp and the speakers.

    What I...
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    I think the problem with the LED idea is that the...

    I think the problem with the LED idea is that the neon lamp is not just the light source for the LDR as it is in the Fender circuit. It's also the LFO. There's no separate oscillator driving it.
    ...
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    Magnatone M15A Echo click

    I've been slowly working on a bit of a basket-case Magnatone M15A, and I've now got most of it sorted out. The most obvious issue remaining is an annoying click when Echo is used.

    This is one of...
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    The other rule of capacitors is that you hear...

    The other rule of capacitors is that you hear their effects far more if their value places them in the middle of the audio frequencies you're listening to. If you had, say, a 0.1uF ceramic cap...
  12. I like this explanation, including taking UL...

    I like this explanation, including taking UL operation into consideration. A huge proportion of the circuitry of this amp goes into Magnatone's complex stereo vibrato circuit, so it would make sense...
  13. What's the design purpose of hybrid fixed/cathode bias?

    I've been slowly working on a bit of a basket-case Magnatone M15A, so I've become very familiar with the circuit.

    It uses a 2 x 7189A push-pull output stage with a -19V fixed bias supply, but...
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    Yes, NP0/C0G types are the best ceramic types in...

    Yes, NP0/C0G types are the best ceramic types in terms of being closest to being ideal capacitors. They are extremely stable with respect to temperature and age. They are similar in many ways to mica...
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    Conclusion: Flipping the PT around to hang...

    Conclusion: Flipping the PT around to hang underneath the steel chassis drops the hum from 11mV to 5mV, which is acceptable for an amp like this.

    New mounting holes have to be drilled for the...
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    I'm pretty sure it's being picked up in the tone...

    I'm pretty sure it's being picked up in the tone control section. I tried pivoting the transformer on all its axes. No significant difference. What looks like it might work is to hang the PT...
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    Sometimes you need to post about something for...

    Sometimes you need to post about something for the solution to come to you.

    I realized I hadn't tried moving the power transformer, so I unbolted and moved it. Hum disappears. I should have known...
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    Gibson G30 solid-state amp hum

    I've been working on a 1972 Gibson G30 SS amp, and the hum (60Hz) at the speaker is 11mV, not affected by any of the controls, including the volume control. It's just a constant background hum.
    ...
  19. Quite true, but square waves are a good...

    Quite true, but square waves are a good diagnostic tool for determining whether or not an audio circuit has flat frequency response or not or if it's exhibiting any other problems like ringing,...
  20. Fender AB763 Vibrato Channel Square Wave Frequency Testing

    A friend of mine with a BF Super Reverb was asking me about the Vibrato Channel of his amp because he doesn't like it as much as the Normal Channel. As far as I can tell, there's nothing wrong with...
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    Since it's a preamp, probably with low current...

    Since it's a preamp, probably with low current demands, you could just use a slow warm-up tube rectifier for the power supply without too many sacrifices instead of silicon diodes.

    You can also...
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    I suppose it depends on the cathode follower...

    I suppose it depends on the cathode follower circuit. In a bootstrapped CF, the control grid voltage is still controlled by the cathode voltage, not the plate voltage. If the CF is biased by a...
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    What looks like a Sprague black tubular capacitor...

    What looks like a Sprague black tubular capacitor would not be original either if the amp is from the early 1950s.

    At this point in history, it's not unusual to be doing a 2nd refurbishment of...
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    Though it's not the question being asked, all...

    Though it's not the question being asked, all aluminum electrolytic capacitors leak a tiny amount of DC current. That's why old capacitor checkers that test caps for leakage at full rated voltage...
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    The output stage has bias feed resistors (220k)...

    The output stage has bias feed resistors (220k) on the high side for KT88s. Many HiFi amps with KT88s in fixed bias don't go over 100k.
  26. On the topic of diodes, it helps to learn to see...

    On the topic of diodes, it helps to learn to see the whole picture. In a typical tube amp, the biggest current draw is going to be the output stage. There will be an initial surge at turn-on to...
  27. I've made it a habit to do some additional...

    I've made it a habit to do some additional forming on CE Distribution can capacitors before installing them, which I do with an ancient Heathkit Condenser Checker that I've modified slightly to give...
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    In the Hammond world, we tend to acknowledge the...

    In the Hammond world, we tend to acknowledge the necklace reverb as the best-sounding spring reverb, but its drawback is that it's so sensitive to any motion around it like someone walking past the...
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