1974x PT replacement

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foreverstrung
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1974x PT replacement

Post by foreverstrung »

Hey. I'm new to this site. this is my 1st post.
I've got a 2004, 1974x I bought a few months ago online. I've probably got a total of 20-25 hours of play time on it since I purchased it. A few days ago I had it in standby and then got distracted. As I was chipping away at my distraction, I noticed a strong electronic smell. When I realized it was the amp and the mains fuse was blown. I replaced it and it and the new fuse blew immediately. Pulled rectifier tube and both EL84's and it did it again. Upon researching I found these amps had PT issues and there was a recall thing awhile back....anyways, I purchased a new PT from Tube Amp Doctor out of Germany. I purchased the PT made as an upgrade for this issue specifically by Marshall, tho I did consider the MM tranny, I thought the Marshall tranny would be more to the point. I got the MJTM18WP, I believe the MJTM15WP is what's in there and burned up........Appreciate any thoughts.
To qualify, I've been in more than a few amps. Bias many Marshalls. Some Fender Mods. Modded pedals and have built numerous guitars. I do realize that an amplifier can be quite dangerous if the caps aren't bled or if I'm an idiot.......TIA
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Re: 1974x PT replacement

Post by Bieworm »

What makes you sure it's the power transformer that's defect?
There are other things that make fuses blow. Did you detach all wires from the PT and checked for continuity at the secondary wirings and their center taps?
Each pair + center tap has to be measured individually, so no continuity checks between filaments and HT wiring.. 😉😉
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Re: 1974x PT replacement

Post by foreverstrung »

Thx for that. I'll follow up. But it's likely going to get a new tranny since I've already bought it. I'll check continuity before it arrives. Suppose to be here Wednesday afternoon.
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Re: 1974x PT replacement

Post by geoff 1965 »

What are the specs of the PT from tube amp? you have to be careful here because the 1974X reissue is a different beast to the original 1974’s.yes they did have problems with the early PT’s and the new ones are 275-0-275 HT @ 150mA. The reissues are biased hot and that might be why they beefed up the current rating of the HT.
FullSizeRender-4.jpg
Note; it’s a pity your original transformer fried,we’ve had a couple of postings with the reissues but as yet no reference voltages of the 90R biased el84’s.
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Re: 1974x PT replacement

Post by foreverstrung »

Thx for that. I'll post the specs when it arrives. It'll have all of that with. I'm pretty certain I've gotten the correct tranny for this "upgrade". I've ordered the MJTM18WP that was/is used to replace the "recalled" MJTM15WP in these reissues
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Re: 1974x PT replacement

Post by foreverstrung »

Just pulled this from the Tube Amp Doctor website of the PT I've got coming. Appreciate the feedback.

Product information "Mains transf. for Marshall 18W Plexi / TAD Amp-Kit"
Can be used with Rectifier tubes EZ81 (6.3V) and GZ34/5AR4 (5V).
Vintage correct slotted lamination. Made in Europe to Dagnall specs.
For 2x EL84 and up to 4x ECC83, lay-down type: Lamination 3,75"
Perfect replacement/Upgrade for the Marshall 1974X or 2061X, replaces D4969 and D5323

Mounting: 79x64mm

Primary:
240V Red
120V White
0V Black

Secondary:
290V@120mA Red
0V Green/Yellow
290V Red

3,15V@3A Green
0V Green/Yellow
3,15V Green

6,3V@2A White
5V@2A Orange
0V Yellow
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Re: 1974x PT replacement

Post by foreverstrung »

As far as current specs, IDK. Whatever stock 2004 Tranny is
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Re: 1974x PT replacement

Post by geoff 1965 »

Product description is questionable “made to dagnall specs” and “perfect replacement/upgrade” when the HT 120mA is 30mA less than the dagnall! It might be okay but I would monitor the amp for a good few hours the 90R bias puts the dissipation of the tubes well over 100% at idle.
Have you tested the bias resistor and capacitor and do you know if the tubes are good?
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Re: 1974x PT replacement

Post by foreverstrung »

Tubes are good, NOS Mullards, but even if they weren't, when I removed them, still the mains fuse blew. I havn't checked the bias resistor or cap, but still, with the tubes pulled, this would not be an issue, right? With the rectifier tube pulled, there should be no current running up that way, right? I'm not an amp pro, just a guy that likes to fiddle with his stuff. Like I said earlier, I've set the bias on more than a few amps, have built dozens of guitars, modded pedals. I understand the finesse needed here, but I'm pulling all the 411 I can get online and from you guys. Appreciate all the feedback, so to clarify, I know little, but from what I understand, when the tubes are pulled and I power up and it still blows the mains fuse, chances are extremely good that it's the PT, right? Especially with the history. This is a 2004 with the stock tranny. good bet this is issue..........however, are you saying the PT I bought does NOT look like it will fill the bill? Or should I proceed? I can return it and get a MM. $100 more, but I'm not as concerned with that as I am a reliable PT and a relatively easy install
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Re: 1974x PT replacement

Post by geoff 1965 »

Just pointing out that Marshall upped the current rating on the HT for a reason,it says yours is for the 18W plexi and kits which are not the same as your 1974X.
Have you tried getting a dagnall? They are under £100 here in uk including post!
Also something like a bias fault might have caused your PT failure so it pays to check over the circuit you can’t take for granted it was the PT.
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Re: 1974x PT replacement

Post by foreverstrung »

Understood. That makes sense to check what may have caused the PT failure.
Do you think I'm wasting my time with this tranny I've purchased? It's arriving today and I'm looking fwd to installing. I really thought that this was the replacement tranny used by Marshall to upgrade after the recall. I guess that's not true. That being said, if it's not the right tranny and there's any chance that it's going to fail, I'd just assume get a MM Tranny. It's pricey, but that's not that big a deal. The big deal is getting the best one, quick as possible with easy installation.........I had thought when I purchased this one, that if it was the designed replacement specifically made for this amp, that it would be "color coded" so to speak, relating to wires. Not a lot of thinking involved.
So, return this tranny? Get a MM? Or shoot the moon and put this one in and see how it holds up?....Appreciate the help
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Re: 1974x PT replacement

Post by geoff 1965 »

Notice the Marshall “dagnall” transformer has tags on and most replacement type transformers have wire leads except the GDS transformers so you would have to improvise when installing.
If it was my amplifier and a genuine Marshall not a clone I would try to keep to originality,I don’t want to advise you on using a transformer with a lower HT current,what is the spec on the MM HT current?
Try contacting GDS amplifiers and getting the specs of their 18W power transformers,they are heyboer with a good reputation and solder tags on them.
Where are you based Europe or across the pond?
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Re: 1974x PT replacement

Post by foreverstrung »

Heres the Mercury Magnetics spec sheet. Can you tell me, does this fit the bill?

Radiospares P18RS Power Transformer
PRI SEC
CT 0V B+
316V
0V
6.3V
0V COM
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Re: 1974x PT replacement

Post by geoff 1965 »

The 316V open circuit voltage is okay but what’s the current rating?
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Re: 1974x PT replacement

Post by foreverstrung »

I was unable to copy and paste the entire spread sheet. I believe the MM is 120mA as well. MM said this was the PT to use for this amp as a replacement for the 1974X, even with 120mA. Lots of talk about how nice these transformers are from MM. Never heard a bad thing yet......
So Marshalls replacement tranny for this recall is a tranny with 150mA? Just to clarify/
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