Marstran 1202-118 PT

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Turd Ferguson
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Marstran 1202-118 PT

Post by Turd Ferguson » Fri Feb 14, 2014 11:15 am

Need advice with Marstran 1202-118 PT I am building Metro 50 and need help with wiring Trans. it has terminals for secondary's they are listed like this below need to know where each wire connects to such as I believe that high HT go to bridge rectifier

A=5VAC
B=5VAC
C=HT HIGH
D=HT LOW
E=0
F=HT LOW
G=HT HIGH
H=COMMON
I=3.15 VAC
J=0
K=3.15 VAC

Thanks
Turd

gldtp99
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Re: Marstran 1202-118 PT

Post by gldtp99 » Fri Feb 14, 2014 12:00 pm

Here's the Marstran Wiring Diagran page------ your PT's info is listed there:
http://www.marstran.com/Wiring%20Diagrams.html

dan_tramble
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Re: Marstran 1202-118 PT

Post by dan_tramble » Wed Feb 28, 2018 2:34 pm

My question is whats the voltage difference HT HIGH to HT low?

I am going to do my bridge with my 68 spec amp 4 diodes like an original I want to create as close to a real replica as I can

Does anyone know the B= of HT HIGH vs HT LOW?
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neikeel
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Re: Marstran 1202-118 PT

Post by neikeel » Wed Feb 28, 2018 4:03 pm

If you are wanting to be like an original 68 you will wire the diodes straight off the HT terminals on the rectifier. They are NOT a bridge.
Two pairs in series and hooked to the HT fuse.
After that they used the block rectifier (again NOT a bridge as the ground lug floats).
Later still (with the first pcbs) you have to pairs of 1N4007 or similar on the board, again not bridge.
The B+ of an original would be 420v.
I have a Marstran -118 in my 50w 2 in 1 clone but it is an earlier single voltage version.
Neil

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Freebird
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Re: Marstran 1202-118 PT

Post by Freebird » Wed Feb 28, 2018 4:10 pm

I once measured AC voltage at HT High and Low without burden at 230 VAC socket. They are

- HT High: 665 VAC
- HT Low: 590 VAC

I then just measured the anode voltage of EL34 on PIN 3 with burden (all tubes are in) only for HT Low. It was at 230 VAC socket about 406 VDC. It depends on the bias current value.

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neikeel
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Re: Marstran 1202-118 PT

Post by neikeel » Wed Feb 28, 2018 4:22 pm

Freebird wrote:
Wed Feb 28, 2018 4:10 pm
I once measured AC voltage at HT High and Low without burden at 230 VAC socket. They are

- HT High: 665 VAC
- HT Low: 590 VAC

I then just measured the anode voltage of EL34 on PIN 3 with burden (all tubes are in) only for HT Low. It was at 230 VAC socket about 406 VDC. It depends on the bias current value.
The ac and rectified voltages fit, although I would expect 345-0-345 pre-rectifier.
Neil

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Re: Marstran 1202-118 PT

Post by dan_tramble » Wed Feb 28, 2018 4:49 pm

Very good guys! So HT LOW is where I want to be. I want to shoot for 430 plate volts. Around where a 68 would have been...

And yes Neil, it’s two diodes in series off each tap joined and wired to the HT fuse... I’ll pick up bias current off one lug of the HT. I have NOS mustards and PEC pots. Just ordered a 5 way voltage selector for this amp. I’ll use my 31 mm knockout punch and make a hole in between the power tubes.... I want this one as close to a primo amp as I can get it.

I have wing C power tubes and 3 Mullard 12ax7 for it....
Probably no master volume PPIMV. I’ll use the attenuator

Thanks for the insights. Muchly appreciated.

All I have for resistors are 1 watt CF. I’d love to find some pheir or iskra. Never built an amp “this close” before

I want this one hopefully to be the pinnacle of my amp building. I’m in no rush!

Thanks again guys!
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

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Re: Marstran 1202-118 PT

Post by danman » Wed Feb 28, 2018 7:03 pm

This seller had quite a few Pihers last time I checked...https://www.ebay.com/itm/5x-Piher-820-O ... 1438.l2649

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neikeel
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Re: Marstran 1202-118 PT

Post by neikeel » Thu Mar 01, 2018 2:48 am

just to confirm, a 50 watt plexi never had plate voltages as high as 480v, that is 100w territory (490 is the correct voltage for most)

I am pretty sure that depending on tube current draw 410 to 430 is the area you will get with a stock Marshall PT or clone.

If you buy a Classic Tone PT they go higher, but not vintage correct.

The lo HT tap on the Marsstran will be 380-400v area that you find on the early 70s amps - so a browner sound.

Edit - sign of ageing, I read your 430 as 480 without my glasses on!
Neil

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