1973 Marshall Lead 50W needs a replacement 1202-164 Power Transformer

[AndyD]

Thanks Neikeel, that is similar to what I have heard on the variation of voltages across years.

I have a ‘74 50w lead and bass with B+ of 365v, a ’78 2204 with B+ 370, and in the past a ‘77 2204 with 365v. The trouble is, yours could very well have had a B+ of 420v. My lead and bass is basically a bass amp. With its lower B+ it breaks up early, but not in an aggressive way like 5000pf bright cap. The early break up is smooth and predictable. It’s perfect for a crunchy rhythmn tone which sings with a pedal to push it. I have a JTM 50 clone (which I also love) with 420v B+ which has much more headroom, but behave similar to the L&B. If you have a choice, I might be tempted to go for the higher B+. Hope that helps!

 

[LPman]

My '69 JMP 50 has a '74 PT. I'm in Europe, we have 230 wall voltage officially. When the amp's voltage selector is on the 240/250 V position, the B+ is about 360 VDC. On the correct 220/230 V position it puts out 400 VDC (biased on the colder side at 60%), sometimes as high as 410 VDC and sometimes only 395 VDC depending on wall voltage fluctuation.

So does this '74 PT has a slightly above average B+ for the period? The power tubes' heater voltage was measured at around 6.7V if I remember correctly, it was certainly higher than 6.3V but still in the factory acceptable range.

 

[AndyD]

My '69 JMP 50 has a '74 PT. I'm in Europe, we have 230 wall voltage officially. When the amp's voltage selector is on the 240/250 V position, the B+ is about 360 VDC. On the correct 220/230 V position it puts out 400 VDC (biased on the colder side at 60%), sometimes as high as 410 VDC and sometimes only 395 VDC depending on wall voltage fluctuation.

So does this '74 PT has a slightly above average B+ for the period? The power tubes' heater voltage was measured at around 6.7V if I remember correctly, it was certainly higher than 6.3V but still in the factory acceptable range.

Hi, it has always been my understanding that in Europe, we should be setting the voltage selector on the 240/250v position. Is there anywhere in the world that has higher wall voltage than us? I would say that 360V B+ was spot on with a '74 transformer. My '74 50w Lead and Bass gives a B+ of 365v. If you want to be sure, check your heater voltage again on the 240/250 setting. The closer to 6.3v (but not less than 6.3v) the better. Hope that helps.

 

[Pete Farrington]

The closer to 6.3v (but not less than 6.3v) the better.

Provided it’s not lower than 6.0V at the end of the heater string, I think it all should be ok.

 

[Pave Dog]

Thanks for the advice, I got the Heyboer 8023 and finished wiring it tonight. Of course it created other issues but will ask on another page

 

[LPman]

Hi, it has always been my understanding that in Europe, we should be setting the voltage selector on the 240/250v position. Is there anywhere in the world that has higher wall voltage than us? I would say that 360V B+ was spot on with a '74 transformer. My '74 50w Lead and Bass gives a B+ of 365v. If you want to be sure, check your heater voltage again on the 240/250 setting. The closer to 6.3v (but not less than 6.3v) the better. Hope that helps.

You have to because in the UK you have higher wall voltage than in the EU (240V compared to 230V). We have to set it to 220/230 V. My tech says anything between 6V - 6.8V is good for heater voltage.

 

[AndyD]

You have to because in the UK you have higher wall voltage than in the EU (240V compared to 230V). We have to set it to 220/230 V. My tech says anything between 6V - 6.8V is good for heater voltage.

Our wall voltage was standardised with the EU years ago! Our wall voltage is 230v -6% +10% which gives a range of 216v - 253v.

 

[playloud]

Our wall voltage was standardised with the EU years ago! Our wall voltage is 230v -6% +10% which gives a range of 216v - 253v.

Yes, but my understanding is that this wide range is a bureaucratic "fudge" in order to achieve harmonization. Nothing has actually changed since the old days, when continental Europe mains was ~220V and UK mains was ~240V. It just happens that the new specification encompasses both regimes. Perhaps some more informed members can confirm?

It's a cheeky move, because I would usually assume without context that a range of x-y implies a truncated normal (or at worst log-normal) distribution around the mean.

Of course, best practice is still to monitor the actual mains voltage you are plugging into or - in the case of tube amps - the heater voltage from the PT.

 

[Pete Farrington]

Yes, the 230V -6+10% euro norm is something of a cynical fudge.
It means that amps tend to run around 10% hotter than intended.
Thankfully most modern Fenders have a 240V tap that can be accessed inside that chassis,

 

[AndyD]

Of course, best practice is still to monitor the actual mains voltage you are plugging into or - in the case of tube amps - the heater voltage from the PT.

Concisely put! And I still think the heater wants to be closer to 6.3v which in this case means putting the mains selector on 240/250 Giving a B+ in the mid 300s.

 

[LPman]

Concisely put! And I still think the heater wants to be closer to 6.3v which in this case means putting the mains selector on 240/250 Giving a B+ in the mid 300s.

We've traditionally always had 220 V wall voltage where I'm located in the EU, which was standardized to 230V a couple of years ago, so using the 240/250V on the selector instead of the intended 220/230V makes no sense to me. Wall voltage here is fairly consistently 230V.

 

[friendofotto]

https://www.marstran.com/Wiring Diagrams.html
Marstran sells a 1202-64 @ $230
The point re voltages, early ones (circa 1969 and 1970 when first introduced were same as the -118 (around 420v post rectification) as the series went on towards 1980 the voltages went down. The voltages went back up again with the 1202-324, in fact higher to 470v or so.

I recently got a 1971 JMP 50 it has a replaced 1202 164 with date code for 1978. I'm interested in finding a correct voltage 1202 164 replacement.

In checking the Marstran site, they have their 1202 164 listed as 370V in circuit.

I'm under the impression that 370v is too low to be correct for a 1971 50 watt.

Please advise, and thanks

 

[Pave Dog]

360V - 420V is correct for a 1202-164 in a 50W early 70's JMP