Metropoulos Forum

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

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

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?

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.

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.

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.

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!

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

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!

Can any one confirm what plate voltage a Classic Tone P.T will yield?

Can you link the Classictone you are interested in? If I remember correctly, some members were seeing 460-470vdc using one of the CT 50 watt pt's in their builds. They do have some models with multiple secondary windings so you can choose your preferred B+.

Don't think my computer skills are up to doing a link but I'm just looking at the equivelant 118 PT JMP era 50w.

I had a flash of inspiration while waiting for iorn to show up. I'm in the middle of a 1987 build and now I'm thinking about going high voltage with 6550 ala Dave Wier for this one and use the iorn that's coming for another build.

Will have to tweak the circuit slightly, 2.7k instead of 820r. Negative FB to 100k on the 4 Ohm tap and the bias splitters. Other than that it's pretty much there.

I contacted Brian awhile back but he said there is no dual/high voltage option on the 50w so this thread caught my eye, I'm still a little confused.

The only other option I could think of was a Classic Tone PT because I had read of the higher voltages but no one has ever confirmed how high.

With out 're reading for the 50w I think Dave said 450v aught to do it so the Classic Tone could be a good option unless of course I've mis under stood and Brian's PT has the option to put 450v on the plates?

AFAIK he only does hi/lo versions now.

Hi Neil, apologies for being a bit if a simpleton but I'm obviously missing something here.

No dual voltage but instead a hi/lo out put?

So on the Marstran PT the low tap will produce somewhere between 420v and 430v on the plates, give or take. What will the high put out?

Having explained what I'm trying to achieve would it be better to to go Classic Tone for the PT and Marstran for OT and Choke? Or would the Marstran PT do the trick?

Cheers

my 118 gives ca 450v with a ss rectifier