Marshall PA 20 1917

7 Responses to Marshall PA 20 1917

1. David Bradshaw says:

   January 28, 2012 at 11:22 PM

   Hi, you left a comment on my enginerd blog about your PA amp.

   When I was doing some research on the 20W, I created some graphical amp layouts that were placed on the web. (http://www.amparchives.com/album/Marshall/Schematics%20&%20Layouts/20W%20Layouts/slides/20W%20PA%20Layout.html) I believe the only other differences between the amps are bright caps on the volume pots.

   Your voltages are quite low compared to the vintage values. Have you checked the voltages on your heaters? If they are much different from 6.3 Volts AC, then perhaps you have a bad transformer. You mentioned that the transformer is not original so its more likely that it was replaced with the wrong type. I’ll have to look back at my research to see if an 18W transformer will fit the 20 Watt chassis. If so, then you have a lot of options for replacing the transformer. If not, than I am only aware of Mercury Magnetics that sells (pricey) replacements.

   Modern EL84 will stand up to the heat. Especially JJ’s and the russian types. I had these tubes installed in a DSL401 with a VERY hot bias, with 425 VDC on the plates and they did fine.

   Feel free to contact me again via email. I am at my other computer right now, so I’ll need to dig through my research later. I think I might have voltages of my 20W amp from using an 18W transformer but I’ll have to check.

   Best of luck with your restoration!

   enginerd

   Reply
2. enginerd says:

   January 29, 2012 at 2:40 AM

   I just checked my research on the 20W. I never found out if the 18W power transformer fits a 20W chassis. You could probably find out by measuring the spacing between the mounting screws on your chassis and comparing to these dimensions:

   http://www.mojotone.com/amp-parts/transformers-marshall/Marshall-Style-18-Watt-Power-Transformer

   As for voltages, here are some examples using an 18 Watt style power transformer:

   Measurement 1: Post rectifier= 360VDC, Output Transformer Center Tap= 356VDC, EL84 screens=350V, Phase inverter node=325V, Preamp Node=303V

   Measurement 1: Post rectifier= 350VDC, Output Transformer Center Tap= 344VDC, EL84 screens=334V, Phase inverter node=300V, Preamp Node=301V

   Voltages posted by Soultone Amps using Marshall’s 20W transformers:

   Measurement 1: Post rectifier= not documented, Output Transformer Center Tap= 345VDC, EL84 screens=339V, Phase inverter node=310V, Preamp Node=290V

   Reply
3. t-geek says:

   January 29, 2012 at 11:58 AM

   Thanks a lot David !

   Yes, the PT has been changed and it delivers way too much “low” voltage and need to be replaced … I think I’m gonna buy the Magnetic Mercury P20JM ToneClone (expensive, for sure :-/

   I will give a try to russian tubes 6P14P-EV (EL84M) that supports quiescent Ua up to 400V and a little more power dissipation at anode, I think (this last point to be checked …)

   I “discovered” your layout a few days ago (downloaded, printed and full of pencil marks now ) : very useful !
   I noticed my PI has same values resistor as 2061X (e.g. 2x220k going to the grid of PI instead of 2x680k, and 2x220k instead of 1Meg between both grids g1 of PI). This does not look as a mod, seems to be Marshall genuine, and hence it differs from Marshall schematic (and from your layout) … Just a piece of “historical information”.

   Thanks again for all this information.

   Reply
4. Nic says:

   October 26, 2012 at 5:06 AM

   hey! I have assembly 291! Mine is great shape (fantastic actually) PA 50w. so… cheers . It was a canadian export and sounds amazing when cranked.

   Reply
   • t-geek says:

     October 26, 2012 at 3:16 PM

     That’s nice. Any pix / links ?

     Reply
5. guillaume says:

   July 22, 2015 at 2:55 AM

   Hi,

   What is the size of the headcab? W,H,D and the cut out please? thanks, it’s importante for me

   Guillaume

   Reply
   • t-geek says:

     July 23, 2015 at 2:02 PM

     Hi,
     510 x 220 x 210 mm
     Thickness = 12mm
     Hope this helps.

     Reply