Morgan AC20 layout or schematic

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coldengray
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Joined: Wed Sep 28, 2016 8:38 pm

Morgan AC20 layout or schematic

Post by coldengray »

Anyone ever put together a layout or schematic for a Morgan AC20? I owned one briefly and wish I had created them myself but it was before I got into building amps.

If anyone is willing to share in private please PM me.
lovetone
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Re: Morgan AC20 layout or schematic

Post by lovetone »

Private is not fair we would all like to see.
frankdrebin
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Re: Morgan AC20 layout or schematic

Post by frankdrebin »

it is so simple you can do it by yourself watching pics on google,nothing special anyway.
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jelle
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Re: Morgan AC20 layout or schematic

Post by jelle »

In the spirit of this forum it would not be appropriate to post schematic and layouts of amplifiers that are still in production for the purpose of cloning them.

On the other side of the equation of your cheap amp, there is a person trying to make a living by making amplifiers.

Why not work off the schematic of a vintage Vox instead, it may be the same thing. No idea, never seen a Morgan. But you would learn more in the process of tweaking it to your liking. And that is what this place is about. :D

or build the Rocket, maybe a scaled back version? :idea:
Badside
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Location: Montreal, Canada

Re: Morgan AC20 layout or schematic

Post by Badside »

If you MUST copy that amp, there are some very clear gut shots if you Google for it, you can clearly make out the circuit and read most cap and resistor values. Looks like a pretty basic AC15-type circuit, with a switch to select an EF86 or 12AX7 preamp tube, and a neat little VVR control on its own little PCB.

Both the EF86 and 12AX7 seem like "textbook implementations", the 12AX7 uses only one of the two triodes. Only one volume knob (the only tone control is a typical Vox-type Cut control), one of the switches appears to add in a bright cap (can't see the value, could be anywhere from 100 to 500pF), and the other one seems to select between two coupling cap sizes (can't read the value, but looks like a 22nF and possibly a 2.2nF, based on size), The EF86 seems to have only a 1uF cap (I'm not familiar with the amp, but this would mean this switch would only work on the 12AX7 mode).
Phase inverter is classic Long Tail topology, with the 2nd input grounded. Power amp is classic cathode bias pair of EL84s.
Compared to an AC15, the PSU is a bit stiffer than you'd expect: I see 47uF on the plate and another 47uF on the screens, then 3 22uF caps, the two preamp tubes are clearly fed off a single one, I THINK two might be paralleled for the PI node, that's the part I'm unsure (or it might have to do with the VVR implementation, not an expert on those)
Interestingly, the EF86 appears to be triode connected (but I'm not EF86 expert, never built anything with one).

And of course, as with all Morgan amps (that I know of), Mercury Magnetic iron all around, including a choke.

As usual with a boutiquey amp like that, the secret of the sauce is probably not just in the general list of ingredients but also in the execution and careful selection of each component (although, to be fair, I see Cliff jacks, M150 signal caps, 1W carbon film resistors, Belton sockets, Alpha pots and generic electro caps, nothing fancy except for the hollow cathode bias resistor). The point being, you could try cloning it part for part and not necessarily get what you're expecting. There's always a part of "black magic" in the construction of an amp.

Also, the Mercury Magnetic iron is bespoke, you'd have to convince MM to sell them to you, which they might not be authorized to (Morgan's own specs, he might have an exclusivity deal).

Point is, if what you want is an AC20, you'll have to fess up and buy one. He spent a lot of time and money developing these amps.
If you're just interesting in building a Vox type amp for fun, using some of Morgan's twists, look at the gut shots and have at it.

As far as I'm concerned, I can see why he made some of the choices he made, but if it was me I'd send the 12AX7 channel on the 2nd input of the PI, so I could run both at the same time (or A/B switch between them), probably using the 12AX7's 2nd triode to revert the signal so it's in phase with the EF86 channel (the 2nd input on an LTPI is phase inverted). But I always say: if you're gonna build it yourself, make it different, make it yours.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
coldengray
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Re: Morgan AC20 layout or schematic

Post by coldengray »

All great points. Since my original post I've come across the Trinity TC-15 and it seems to have most of what I wanted in a Vox build, namely a Top Boost channel and EF86 channel. I'd rather just buy a kit with purpose-built chassis, board and iron. Much easier though also not quite as fun.
coldengray
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Re: Morgan AC20 layout or schematic

Post by coldengray »

After further research I might build the Brownote Foxy 18.
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jelle
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Re: Morgan AC20 layout or schematic

Post by jelle »

That is a fun amp! Maybe check with Moss to see if he has all in stock.
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randalp3000
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Re: Morgan AC20 layout or schematic

Post by randalp3000 »

Here you go, posting because their is nothing unique about this circuit.


UPDATED, FIXED SCHEMATIC
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Last edited by randalp3000 on Tue Oct 04, 2016 3:53 pm, edited 3 times in total.
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randalp3000
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Re: Morgan AC20 layout or schematic

Post by randalp3000 »

more for reference
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Badside
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Re: Morgan AC20 layout or schematic

Post by Badside »

Interesting. So my analysis wasn't too far off. The EF86 is not triode-strapped (that didn't make much sense too, but I'm no EF86 specialist), and the switch also switches B+ (turning off the unused tube, and sending the signal through the same coupling caps, which explains why the coupling cap selector works on both modes).

This schematic shows the EF86's screen connected straight to the cathode though, from what I know of pentodes that would effectively mute the tube. I'm guessing they just put the 0.1uF cap on the wrong side of that connection (it should be between the screen and cathode or ground, IIRC)

Still feels like a wasteful design to me, it's kind of an amp for the undecided who can't commit to the 12AX7 or the EF86, plus that unused 12AX7 triode…
I would give the 12AX7 its own channel and feed it to the inverting input of the PI (use both triodes so that the end result would be in phase with the EF86 channel, allowing running both at the same time, perhaps make it a pseudo Top Boost channel by having the 2nd triode feed a MOSFET Follower driving a tone stack)

EDIT: This schematic also does not include the VVR control. But those are available as kits (which the Morgan's seem to be, it's on it's own small PCB)
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
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randalp3000
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Re: Morgan AC20 layout or schematic

Post by randalp3000 »

I think I screwed up on the morgan schematic, I believe the vox is correct, going to double check and fix.

thanks for catching this.
arsimoes
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Re: Morgan AC20 layout or schematic

Post by arsimoes »

Hi randalp3000!

Were you able to identify what is wrong with the AC40 circuit?
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