Marshall Heads too big!

ICBM

Ché said:

I'd mentioned this to my tech when he serviced my Mesa MK I head and my Vintage Modern. I asked why the Marshall head was so big and he replied that he found them easier to work on as the transformers and valves weren't as closely spaced.

Which is quite a good thing given how often you're likely to have to work on a Vintage Modern compared to a Mk1 .

Ché

ICBM said:

Ché said:

I'd mentioned this to my tech when he serviced my Mesa MK I head and my Vintage Modern. I asked why the Marshall head was so big and he replied that he found them easier to work on as the transformers and valves weren't as closely spaced.

Which is quite a good thing given how often you're likely to have to work on a Vintage Modern compared to a Mk1 .

Hah!... you're probably not wrong :P

I'd rather something be big and therefore servicable rather than tiny and cost less to sent it to a land fill and buy a new one than repair it. That's why I like "vintage" gear.. they actually had instructions on how to mod and fix it in the manual! Covered by warranty! Anyway, I digress.)

Get a JCM800 4101 (or orther 1x12) combo and chop it down to a head.

martinw

John_A said:

^ A work of art Martin and I would love one  

ThePrettyDamned said:

Excuse me while I just wipe away my saliva.

Yummy.

thorpy6 said:

@martinw That looks fantastic. Whats with the doughnut power transformer?

albanov said:

MJW electra looks great by the way.

Thanks!

@thorpy6 What they said about toroids!

jpfamps said:

You are also not supposed to use centre tap full-wave rectification with toroids which would preclude the use of a valve rectifier in the conventional topology, however you can employ a valve rectifier in a hybrid bridge configuration which also reduces the voltage stress on the rectifier: a win-win situation.

I've read that before, but not been able to find a reliable reason for it. Can you shed any light?

The ones I use come with 2 separate secondaries which can be connected CT style which I assumes makes the difference.

ICBM

martinw said:

I've read that before, but not been able to find a reliable reason for it. Can you shed any light?

The ones I use come with 2 separate secondaries which can be connected CT style which I assumes makes the difference.

I don't think so - it's that you're not supposed to run DC through the winding, which is what happens with a centre-tap full-wave valve rectifier - the HT current flows "upwards" from ground through the winding to the rectifier. (Hence why some old amps have the HT fuse in the centre tap to ground connection.)

A modern centre-tap setup to provide positive and negative rails for a solid-state amp isn't the same, because there is no net DC on the winding, and that's how they're normally used.

jpfamps

ICBM said:

martinw said:

I've read that before, but not been able to find a reliable reason for it. Can you shed any light?

The ones I use come with 2 separate secondaries which can be connected CT style which I assumes makes the difference.

I don't think so - it's that you're not supposed to run DC through the winding, which is what happens with a centre-tap full-wave valve rectifier - the HT current flows "upwards" from ground through the winding to the rectifier. (Hence why some old amps have the HT fuse in the centre tap to ground connection.)

A modern centre-tap setup to provide positive and negative rails for a solid-state amp isn't the same, because there is no net DC on the winding, and that's how they're normally used.

I think that's the reason. You only need a very small amount of net DC on the secondary to cause the transformer to go into saturation (probably one reason why a toroidal OT isn't a great idea).

Possiblely there also may be more of a problem with ringing when one side of the winding shuts down each half cycle. This is speculation on my behalf.

Having said that, I've seen several toroids wound for full wave valve rectifiers, so I expect it will work OK.

Regardless, employing a hybrid SS / valve bridge rectifier is a much better solution: the transformer is used more efficiently and the PIV on the valve is halved.

jeztone2

Id actually like a DSL401 2 x 12 combo, I'd also like a PRS Archon 2 x 12 combo. So Im at the opposite end of the scale. but I thought the Marshall 1987x was quite a compact head?

ICBM

jpfamps said:

I think that's the reason. You only need a very small amount of net DC on the secondary to cause the transformer to go into saturation (probably one reason why a toroidal OT isn't a great idea).

I recently had an amp with a toroidal OT as well as PT. The original one did blow, however… no idea if that might have been a contributing factor.

jpfamps said:

Regardless, employing a hybrid SS / valve bridge rectifier is a much better solution: the transformer is used more efficiently and the PIV on the valve is halved.

The only problem is that someone will look at the schematic, see the diodes and say "that's not a proper valve rectifier!"…

jeztone2 said:

but I thought the Marshall 1987x was quite a compact head?

No…

http://medias.audiofanzine.com/images/normal/marshall-1987x-263729.jpg

jeztone2

But it could be

ICBM

Yes, if it was put in a smaller box. From memory the chassis will fit in the same size box as a 20-watter, although not in an actual Marshall one due to the shape of the front panel.

Gassage

ICBM said:

It's particularly ludicrous when you see a 70s 50W model with the back off...

http://www.vintageandrare.com/uploads/products/34370/883537/original.jpg

Even the new ones have a lot of spare space inside the chassis really.

I wasn't very impressed to find that the Blackstar ID series heads contain at least 50% empty space, either.

Yeah but the space is surely to give the amp more headroom?