Marshall 1974X combo

[M Fowler]

My friend texted and want to come over and visit to talk about amps.

He brought his friends Marshall 1974X 1-12" combo amp, Celestion G12M green back.

That amp sounds fantastic with biting lower tone at any level, apparently the original transformer set in this amp is working quite well. Internet chadder isn't so kind to the original set.

Mark

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[M Fowler]

More

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[M Fowler]

The components are same as most 18w Marshall varients I've looked at but the PT is 550vdc 275-0-275 150mA; 6.3v 3A; 5v 2A

Bypass caps are 22uf/100v
slope resistor 56k
tremolo caps are film type .01
cathode bias 470uf/100v and 100R/10w
2MA speed pot
1kL intensity pot

I'm going to take voltage readings soon because my friend wants to clone his friends 1974x

Trouble most Marshall 18w PT replacements are 290-0-290 so I would have to reduce that down for the 550v I want.

[sluckey]

The Hammond 270FX has 275-0-275 @ 173mA. It also has a 0-115-125 primary in case you have high line voltage. It's a stand-up style though. I used it on my Vox AC-15.

[M Fowler]

Steve, I looked at the 270FX and the 270Ex 550vct 144mA but that was also x mount standup.

I might just call Heyboer for another custom job.

Mark

[M Fowler]

I talked to Heyboer about a replacement PT for this combo and everyone is using the higher voltage so I ordered my PT from Mojotone.

[M Fowler]

Been working some on building my friend an 18w combo.

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[petermcateer13]

For what it's worth, you might also try Trinity Amps in Ontario Canada. They make a (reportedly very good) 18W Plexi kit, and I believe they sell the PTs and OTs separately.

Sent from my P01Z using Tapatalk

[M Fowler]

OK 18w Marshall experts I need your help please!

Been working on two 1974x 18w tremolo combos and neither one tremolos.

The voltage is off on the oscillator tube. V3 12AX7 see chart


GDS chassis, PT and OT

2ML Speed pot

1KL Intensity pot

All known good tubes. Been resoldering and trying to get these amps going but know. First channel sounds great and loud.

Tremolo channel is lower volume and no go on tremolo.


Mark

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Marks 18_watt_voltage_chart.doc

[sonicmojo]

I noticed a Mica cap on the trem switch jack that I haven't see on schematics. Long shot but what if you bypass that?

[M Fowler]

I just added that to try get tremolo to work.

So it makes no difference. It also makes no difference if that jack is disconnected.

[sonicmojo]

Is the resistor on Pin1-Pin6 of your V3 a 220K? I am only seeing a few volts drop there.

[sluckey]

Has the trem ever worked on these amps? Here's the schematic with voltages on mine with a working trem…

http://sluckeyamps.com/18w/18w.pdf

[sonicmojo]

Looks to me like your cathode resistor on V3 is 820K which should be 820Ohm.

[sluckey]

I think you got it!