Schematic opinion

Marshall Amp Discussion

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studiodunn
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Schematic opinion

Post by studiodunn »

I have a donor amp and was thinking of going the EL34 high gain 2 channel route.
I'm not sure where I got this Schematic from, but it looks interesting.

Any glaring issues with it, and or words of caution?
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Stevem
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Post by Stevem »

Well it's all there but for the power supply, and it does have the better Cathode follower driven effects send also, not that there good for much ( effects loop circuit wise ) in a high gain amp but for popping a noise gate in there!
What are you shooting for in regards to output wattage?
Some 450 volts will get you some 45 to 50 clean watts and 500 volts will do 60 watts dependant on the chosen PTs current output!
A replacement Highwatt PT for there 50 watt model will get ya 60 watts easy with strong output tubes, ot the Mojotone low power Marshall JTM power trans will get you 40 watts and you have the option of running a tube recto.

The main word of caution in doing a high gain build is the layout!
The grid wires need to be as short as possible and the tone stack and or frequency tweet responce components need to as close to there respective tubes and control pots as possible, this all in regards to keeping grid wires short!
This is very very the case for the output wires on the PI feeding the OT as there is big signal level there than can impress itself back into a lower level preamp section that is in phase with it!

Big rule of Thumb here is to use shielded wire for the grids of the first two gain stages and if there is a long distance that needs to be covered between two gain stages then it's best to make the plate lead long before hitting the coupling cap for the next stage as this keeps with the first goal of keeping our grid leads short!
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Post by Stevem »

Here's a easy way to run that first preamp tube heaters on DC for low hum.

Note that if you build with a Hum balance pot then that will replace the 2 100 ohm resistors.
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JazzGuitarGimp
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Re: Schematic opinion

Post by JazzGuitarGimp »

It all looks good except for the EFX Return jack, which is wired incorrectly. The tip and tip switch should not be tied together: connct the tip only to th PI input, and connect the tip switch to the tip of the EFX Send jack, as well as the output of the preceding CF.
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Stevem
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Post by Stevem »

Nice catch!, yup the sleeve should go to ground.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
studiodunn
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Re: Schematic opinion

Post by studiodunn »

Thank you guys.

That is great info on the DC heaters. I will work that into my layout for sure.

The chassis is Huge, so I will have to take some time laying it out. There is allot of space between the pots and the tubes.

It seemed to me the standard Wreck filtering(called out on the notes) would be a bit low considering the type of circuit. I only know enough to notice 22u filters as opposed to 50u used in Marshalls.
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Post by Stevem »

If you want good tight punch as you get the amp cranked out ( above 3/4) then run 100 uf feeding the OT and 100 uf feeding the output tubes screens.
Preamp wise just make sure that you run no more than 2 gain stages off of any one power supply filter node, if you stick to this then a 22uf filter is fine !

Chassis / layout wise you want very short leads out of the PI feeding the grids of the output tubes.
The general layout of a Marshall is the way to go with a big chassis, all AC and PT on the right side, OT in the middle , speaker output jacks on the AC side and input jacks on the front left.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Jered
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Re: Schematic opinion

Post by Jered »

Those back to back diodes on master #2. What effect does that have on the sound? Thanks
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martin manning
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Re: Schematic opinion

Post by martin manning »

Stevem wrote: Wed Feb 08, 2017 1:00 pm Here's a easy way to run that first preamp tube heaters on DC for low hum.
Note that if you build with a Hum balance pot then that will replace the 2 100 ohm resistors.
Why does this circuit have two ground references?
Jered wrote: Fri Jun 02, 2017 3:12 am Those back to back diodes on master #2. What effect does that have on the sound?
It's a diode clipping circuit. As stated in the notes it's a version of the "Jose mod" often seen in Marshalls. The diodes should be Zeners, though. As drawn they will have no effect.
Jered
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Re: Schematic opinion

Post by Jered »

What value zeners would you recommend? Thanks
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martin manning
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Re: Schematic opinion

Post by martin manning »

Google Jose Mod Zener. I believe 20V is used, but you will find a number of variations.
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Colossal
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Re: Schematic opinion

Post by Colossal »

martin manning wrote: Tue Jun 06, 2017 12:19 pm Google Jose Mod Zener. I believe 20V is used, but you will find a number of variations.
Yep. 5 and 16V are also common values. Lower values will dump more volume, which isn't always a bad thing.
studiodunn
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Re: Schematic opinion

Post by studiodunn »

Doesn't the Jose Mod typically go pre-EQ?
Cameron
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Re: Schematic opinion

Post by Cameron »

studiodunn wrote: Tue Jun 06, 2017 9:36 pm Doesn't the Jose Mod typically go pre-EQ?
Yes ....but I remember when he made the amp ...telling him the zeners should be before the tone stack......he said it sounded pretty cool like it was.....so he left it ....I think there are clips somewhere of it ......so the schematic should work and did sound good. The other thing I didn't like about it was....the loop before the tone stack and clipping circuit....but as I said ...it did end up being a very cool amp.....
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