Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Keyboy on October 16, 2023, 08:59:59 pm

Title: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 16, 2023, 08:59:59 pm: Hello,

I am looking for help proofing a recreated layout for an old amp.

I am planning a Marshall 1917 20 Watt PA amp build and couldn’t find an original layout. The original schematic is available and generally readable, and the 20 Watt Lead amp schematic is almost identical and is an even more readable reference.

There may be an original layout out there, but I figured I would at least learn something by recreating a layout based on the original schematic and photos of the actual amp chassis.

I will attach the new layout file in this post and the original schematic in the next. I understand this is a significant favor to ask, and understand if no one has time – just thought it was worth a try. Also, I am the opposite of an expert.

(I did notice that in almost every chassis photo I found the tech seems to not use the schematic’s 680K resistors and 1M resistor connected to V2.)
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 16, 2023, 09:17:19 pm: As mentioned, pls see the attached, original schematic. thank you!
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: sluckey on October 16, 2023, 11:57:00 pm: I see two issues... Pin 9 of both EL84s should be connected to the violet wire. Pin 1 of both tone controls should NOT be connected to the ground buss.

Everything else looks good.
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 17, 2023, 08:00:14 am: Thanks Sluckey!
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: JB on October 17, 2023, 12:38:04 pm: Unlike Fender, Marshall didn’t do layout drawings, just schematics. You could search for chassis pics of the original amp and if you find one work from that.
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 17, 2023, 03:04:19 pm: Thank you JB, this is my first Marshall build and I did not know about them not creating layouts. I did look through dozens of 20 Watt Marshall 1917 photos earlier and they were a huge help. Thanks again!
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 20, 2023, 10:30:03 am: Hello,

I'm just about ready to start the Marshall 20 watt 1917 build and have two questions.
1) The old schematic does not identify the voltage ranges, and I think I'm good, but there is a 50uf cap I wanted to check which is attached to the EL84 cathode on both tubes. I'm guessing a 50uf/50V would work fine. When looking at real examples of the amp's circuit board - this cap is pretty small.
2) The old schematic labels the two diodes as "1008" - I assume a 1n4007 (or maybe 1n4008?) would be fine?

I have attached a snapshot of the schematic and where the two components in question are. THANK YOU!!
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: sluckey on October 20, 2023, 12:03:29 pm: 50µF @ 50V and 1N4007 (or 1N4008) are fine.
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on October 20, 2023, 12:25:06 pm: Awesome, thanks Sluckey!
Title: Re: 20 Watt Marshall P.A. Amplifier 1917 - layout check
Post by: Keyboy on November 05, 2023, 05:43:50 pm: I'm sure there are endless questions about Cliff Jacks, and hate to ask another, but...as I researched wiring these for my Marshall PA 20 1917 build I see dozens of variations of Cliff input jack wiring schemes online.

I finally just drew up a quick layout based strictly on the original PA schematic. I have attached the schematic and layout and would really appreciate your feedback.

Thanks in advance