Presto K8 amplifier extremely loud (center-tap volume pot?)

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Angus McCarthy
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Joined: Mon Aug 23, 2010 6:22 pm
Location: Bloomsburg, PA, USA

Presto K8 amplifier extremely loud (center-tap volume pot?)

Post: # 48068Unread post Angus McCarthy
Sun Oct 08, 2017 7:52 pm

Well, after sitting on this equipment for nearly 7 years I've finally gotten my head screwed on straight, gotten off my ass and re-capped the amplifier in my Presto K8. After replacing the failing output tubes it now plays and cuts reliably, but at an extremely high volume level. I know that we're talking about around a 20 Watt amp, but this gets loud enough that I'm afraid of blowing the speaker if I pushed it all the way, to say nothing of popping the cutterhead.

What's more, the phono and microphone sides of the volume control seem to be bleeding into each other. I can have the pot nearly all the way to the left (phono side) and still hear some sound coming from the microphone input. Is this normal for these amps? It was my understanding that grounding the center tap of the volume pot should short out whatever half the wiper is not contacting. I measured the resistance across the center tap and wiper lugs, and it never drops below 13Kohms (from a max of 50Kohms) before suddenly showing an open connection right at the point where the center tap should be.

Can anyone familiar with Presto's on-board amps offer advice? I'd hate to think that the volume pot has gone bad, because I have looked and looked and there seems to be no place to find new 100Kohm center tapped pots suitable for tube electronics. They apparently just aren't made any more.

On a somewhat related note, is anyone aware of a source/workaround for the (perennially missing) screw-on protective cap for the mic input? There's a constant low-level buzz that sets into the system when it's not connected to a mic, and I'm wondering if grounding the input through that missing cap would solve the problem.

Thanks in advance,
Ben W.

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