Marshall PA 20 1917

Nice amp from 1970 (I’m sure it is now out of waranty :-) )

Marshall PA 20 1917 frontfaceMarshall PA 20 1917 backfaceSexy, isn’t it ?

Marshall PA 20 1917 bulgin 1Marshall PA 20 1917 bulgin 2

Bulgin power cord …

Let’s have a look under the hood …

Marshall PA 20 1917 from 1970

1970

Marshall PA 20 1917 output transformer

Output transformer

Marshall PA 20 1917 silicium diodes

Diode rectification …

Marshall PA 20 1917 badly modded

WTF ??? OK, this amp *needs* a service !

PT has previously been replaced on this amp, but voltages are too low (300V instead of 390V at the diodes)

Marshall PA 20 1917 Schematic

Marshall 20W Lead 2022 Schematic

Marshall 20W 2061X RI Schematic

Marshall PA 20 1917 Layout ... work in progressLayout … work in progress – Based on excellent work from http://www.amparchives.com

Cleaning process …

Marshall PA 20 1917 cleaning

Marshall PA 20 1917 cleaning processMarshall PA 20 1917 cleaning process - Oxidation

Oxidized contacts … to be cleaned !

Marshall PA 20 - Shinrock IGPW SOT-M18R Output transformer

F&T reservoir cap, new Shinrock / IGPW  (Marshall 18W) power transformer

Marshall PA 20 Bulgin powerswitch

Marshall PA 20 Bulgin powerswitch 2Bulgin powerswitch …

Marshall PA 20 - 2022 Lead mod

“2022 Lead” mod (2 capacitors added : 4.7nF & 500pF, for a brighter sound on one channel, easily removable …) – Wiring cleaned.

Marshall PA 20 - 8Ω + 16Ω outputs

Marshall PA 20 - 8Ω + 16Ω outputs (2)

8Ω output added (blue wire) – This amp has now 8Ω + 16Ω outputs

After PT replacement, voltages are as on the schematics.

IGPW / Shinrock Technical Data
Power: 103 VA
Primary 0/230/240 Volt
Sec. 1: 290/0/290 Volt(AC) @ 0.12A (DC)
Sec. 2: 0/5/6,3 Volt @ 2/1A ( AC) (if Marshall 18W EZ81 rectifier used – unused in case of Marshall PA 20)
Sec. 3: 3,15/0/3,15 Volt 3A (AC)

Bias :

I used a 150R resistor for EL84 bias. Vk # 12V, Ik # 36mA (measured with a bias system). I could not be able to measure Ig2 : 2mA ??? (no resistor …)
Va = 370V
Vg2 = 360V
Pa # 12.6W (if Ig2 is 2mA) quote hot !

Considering relative high anode and screen grid voltage, I chose 6P14P-EV tubes (EL84M equivalent, designed for VA = 400V) …

Marshall PA20 6P14P-EV EL84M

Marshall 1917 Cabinet 2045

With Marshall 2045 cabinet, how sexy !


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7 Responses to Marshall PA 20 1917

  1. 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

  2. enginerd says:

    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

  3. t-geek says:

    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.

  4. Nic says:

    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.

  5. guillaume says:

    Hi,

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

    Guillaume

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