1974x PT replacement

[geoff 1965]

No when you first power on connect to pin3 of one of the el84’s and chassis “ground” with your multimeter and monitor the voltage,it should gradually go up then level off and stabilise.
Check the resistance of the 200R bias resistor and capacitance of the 470uf cap.

[foreverstrung]

Glad I waited. New PT from Marshall came today. I installed and trimmed it out. powered it up with all tubes in place and it didn't blow up, so theres that. I'll put it back 100% tomorrow and plug in a guitar and see how she holds up. Attached is the pic after I trimmed it out a bit, but before tubes.
Geoff, how exactly do I check the bias resistor and the 470 cap? Pretend I'm an idiot. With much of this stuff I am. Some call it "touched" . Special

new PT.jpg

[geoff 1965]

Looking good! Does your multimeter read capacitance?
Can you add a better focused pic of the transformer and wiring to the switches etc.

[foreverstrung]

Yes, it can read capacitance. But I've never checked while the cap is in place. Please describe......Also, that 200R bias resistor you mentioned should be a 91R, I think, right? What's that going to do? How can I check to see is this is good or damaging?

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[geoff 1965]

The 200R will “cool”bring the plate dissipation of the el84’s down,wait till you have it up and running then if you can safely check some voltages we can help you fine tune the amp.be careful!there does’nt look to be a bleed resistor so make sure capacitors are drained of voltage before you work on it!
Just try reading the capacitance and resistance of the bias resistor and cap in place but you might need to unsolder one lead of the capacitor for a true reading.
With my 18W,same 275-0-275 HT as yours and with a 160R bias resistor i’m at 90% dissipation and close to 12 volts on the cathode.

[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

[foreverstrung]

PS....I'd prefer not burning up tubes. LOL. Just to clarify, but I'd prefer that than running too cold

[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

[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

[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.

[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.

[foreverstrung]

this voltage stayed steady, in both tubes, for well over 10 min...323.5 in one el84 and 321.5 in the other

[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?

[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

[foreverstrung]

stuff.jpg

[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.

[Bieworm]

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!

[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

[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.

[foreverstrung]

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