1974x PT replacement

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foreverstrung
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Re: 1974x PT replacement

Post by foreverstrung »

hammond1.jpg
hammond 2.jpg
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Re: 1974x PT replacement

Post by geoff 1965 »

Marshall-1974X-18W-Schematic.pdf
you're quite a bit off! here's the 2004 version of the 1974X schematic for you to study and see where your mistakes are i.e. you can't have both the 120V & 240V pirimary wires going to the switch,the red/yellow HT centre tap goes to ground via the HT fuse,the 2 red HT wires go to pins 1&7 of the ez81,the yellow/black wire is rectifier filament 5V and is not used etc,etc
note someone has been in there and changed the 91R bias resistor to 200R.
see if you can work out the wiring also do the mounting bolts of the 290 paz match your existing holes?
good luck
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Re: 1974x PT replacement

Post by JMPGuitars »

The unused wires get clipped and covered with heat shrink tubing.

Why are you using the Hammond if Marshall is sending you a replacement? I would use the Hammond for another build.
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Re: 1974x PT replacement

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Mounting bolts line up perfectly.
No telling when the Marshall replacement might be here. Maybe a couple weeks, IDK, but I've got this one here now
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Re: 1974x PT replacement

Post by foreverstrung »

Does anyone know where I might find a Dagnall 4909 PT drawing? I can't find one anywhere and Dagnall has not responded to emails.
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Re: 1974x PT replacement

Post by foreverstrung »

I might wait JPM, but I'd like to be able to install this one If I can. With the Marshall, I'll likely have the same color/location issues with the primary and secondary that I'm having now. A Dagnal 4909 drawing would take most all the mystery away, but I can't find one anywhere and Dagnall hasn't responded
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Re: 1974x PT replacement

Post by geoff 1965 »

hammond 2.jpg
add a pic of the wiring from the mains in socket,remember to draw a layout of existing wiring colours for when the marshall transformer arrives,there looks to be 2 types "the pic i posted shows tags and yours has leads"
you won't get any response from dagnall,i've contacted them twice asking about their 18W transformers and had no reply!
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Re: 1974x PT replacement

Post by foreverstrung »

Thx for posting that geoff 1965. I. I've got this down now. 100%, but now that I do have my head wrapped around it, I'm thinking of waiting at least a few more days to see if the Marshall replacement shows up. I'm more of a purest than pushing this through. At first there, I couldn't leave it alone, because I didn't understand. Now that I do, I can wait a bit......Either way, the extra tranny I have I'll use to build a 18w clone....Love this stuff. Thanks everyone for the help and support........I'll post pics and updates as I move fwd
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Re: 1974x PT replacement

Post by geoff 1965 »

Those hand wired marshall’s are a work of art! and I would wait and fit the Marshall PT for originality.check the bias resistor and capacitor and if you don’t have a tube tester I would monitor the bias voltage when you first power on.which brand of ez81 do you have?
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Re: 1974x PT replacement

Post by foreverstrung »

I've got a Tesla EZ81. Old original Tesla, not JJ. I did just order a NOS RCA EZ81. It'll arrive soon.
I don't have a tube tester, but 2 NOS Mullard EL84's.....They were NOS when I put them in after I purchased this amp. Maybe 25 hours of playtime on them.
When you say test the bias voltage, you mean, get the plate voltage and them mA of the EL84's and then do the math, right?
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Re: 1974x PT replacement

Post by 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.
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Re: 1974x PT replacement

Post by 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
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Re: 1974x PT replacement

Post by 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.
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Re: 1974x PT replacement

Post by 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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Re: 1974x PT replacement

Post by 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.
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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

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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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