1974x PT replacement

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foreverstrung
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Re: 1974x PT replacement

Post by foreverstrung »

I've got a Craftsman MM. 82024. This one does not get capacitance. Resistor does measure 200 ohms. I'm not sure what to do just now. I don't want it running cold. Rather hot. I don't care if it burns up tubes......should I order a 150R bias resistor? 160R? Change them out, see what sounds best? what's the best step by step to dial this thing in?
It does not have a bleed resistor, I do it manually. I triple check before sticking my fingers in there. Everytime, just FYI
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Re: 1974x PT replacement

Post by foreverstrung »

PS....I'd prefer not burning up tubes. LOL. Just to clarify, but I'd prefer that than running too cold
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Re: 1974x PT replacement

Post by JMPGuitars »

The ONLY correct thing to do is to measure your tube dissipation. You want the power tubes running between 75 to 85%. That's the sweet spot for tone and reliability.

You can use a calculator: https://robrobinette.com/Tube_Bias_Calculator.htm

Thanks,
Josh
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Re: 1974x PT replacement

Post by foreverstrung »

Thx... Picked up a Fluke MM. I needed to replace this craftsman for a while now. I've used this link before. I'll go through everything this eve.....Plugged in a bit ago and it does not sound as good as I'm certain it can.. Like I said, I'll go thru it tonight....Thx for the link JMP
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Re: 1974x PT replacement

Post by geoff 1965 »

Monitor the bias voltage of the el84’s for a good 10 mins it should be stable after the tubes have warmed up.you need to be certain there was’nt an issue that caused the PT failure.Tubes can leak internally.
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Re: 1974x PT replacement

Post by foreverstrung »

I've got 323v Plate to Cathode voltage, using the link and directions JMP sent, but I'm not sure where to go from here. I realize that this is all new to me, it's not like the bigger amps. Not that I knew a lot about them either, but I'd biased more than a few. Always with a trim pot and a MM, but with these el84's and a cathode bias, it's back to square 1. I appreciate the guidance.........where do I go from here to determine if I'm running cold and then what to do about it.
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Re: 1974x PT replacement

Post by foreverstrung »

this voltage stayed steady, in both tubes, for well over 10 min...323.5 in one el84 and 321.5 in the other
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Re: 1974x PT replacement

Post by geoff 1965 »

That is your plate voltage,bias cathode voltage is measured from pin3 of the el84’s socket to ground “chassis” what does that read and is it stable after the tubes have warmed up?
Last edited by geoff 1965 on Wed 10/20/21 8:26 am, edited 1 time in total.
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Re: 1974x PT replacement

Post by Bieworm »

To use the robrob calculator Josh linked you...

1. Select tube
2. Fill in A to K voltage ( voltage pin 7 minus voltage pin 3)
3. Press calculate
4. Type 2 where it says number of tubes that share cathode resistor
5. Enter resistor value
6. Enter cathode voltage ( voltage of pin 3)
7. Press calculate

Now you have the result. Make sure you look at the cathode biased percentage
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Re: 1974x PT replacement

Post by foreverstrung »

stuff.jpg
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Re: 1974x PT replacement

Post by foreverstrung »

cathode voltage started at 12.5. After 10 min it was 12.8....then @ 15 min it was 12.68........IDK if that significant flux of not.
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Re: 1974x PT replacement

Post by Bieworm »

foreverstrung wrote:
Wed 10/20/21 11:45 am
stuff.jpg
Wrong tube choice. It should be EL84 or 6bq5 . What tube did you select?
According to your voltages you've got 82.5% .. so that's near perfect!
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Re: 1974x PT replacement

Post by foreverstrung »

I deleted that pic because I was going to run the numbers again. I had the plate to cathode number wrong. should have been 323.5, not 330.....and if I need to use the el84 12w, then it'll be diff again. Give me a few and I'll do it again
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Re: 1974x PT replacement

Post by geoff 1965 »

cathode voltage looks okay,that was my concern especially with the PT failure but if it dropped to 12.6 from 12.8 after 15 mins that's pretty stable.
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Re: 1974x PT replacement

Post by foreverstrung »

This is it. Checked and rechecked. Let it heat up for 15 min fully powered. No significant change from 1st to last readings
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