Marshall 2020 transformer reliability

[Mick Bailey]

I've got a Marshall 20-20 on the bench with a history of failed OPTs prior to me getting it; 2 with OC primaries and another with one half of the primary with a shorted turn. I've seen the odd 2020 before with a transformer failure, but this seems unusual. B+ is 400v. The problem is not exclusive to one channel.

Has anyone else had a similar experience? I was thinking of fitting MOVs or flyback diodes for added protection. Given the diminutive size of the transformers and power output they're probably on the limit anyhow and it wouldn't take much to kill them. I've had the main board out to check for bad joints. Couldn't see any but re-soldered the output sockets anyway. The spade terminals are good, new tubes, correct bias.

It all looks good and ready to go. I've got a pair of new transformers but don't want to see it coming back.

Hmmm........

[J M Fahey]

I've ben writing in different Forums about this still unacklowdeged problem, which started with Marshall (Drake transformers) but now has reached even Mesa Boogie.

The "old" wire enamel (old Fenders/Marshalls) was a baked on "oil resin" varnish.
Typically dark brown, is very strong, although somewhat thick which hurts (robs space) in fine wire as in pickups.
When you burn it before scratching/sanding it off it forms "scales".
Easily stands 120/140ºC and even if toasted, keeps insulation.

Then came epoxy enamel, by itself it's transparent, is usually dyed light to mid brown so you notice it's there; you still have to burn it , specially in the finer gauges; when burnt it turns into soot.
Can be bought over the counter with *guaranteed* 180ºC resistance under high vibration (for use in big electric motors) , I guess in a transformer it stands up to 220/240ºC .
I use exclusively this for rewindings and it never comes back.

Both are *excellent* .

But on demand to lower production costs, they invented "self soldering" or "self stripping" wire, which carries a *very* thin layer of something similar to nylon, which *evaporates* at 130ºC and leaves behind a shiny fluxed copper surface, ready to solder.
Perfect for small "2$" wall-warts for phones, games, answering machines, etc. where saving 15 seconds per soldered wire is the key to success, but it has no business inside a >50W transforner and much less in a tube amp. Period.

To avoid using it where it does not belong, it's dyed bright fluo pink or green , I'm sure you now will recognize the wire found inside countless cheap transformers.

Guess what kind of wire I started to find inside dead Marshall transformers?

As in 3 OT replaced inside JCM900, on warranty (new amps) and within 3 months

And then I started getting other brands, including MB.

A few samples I keep as deadweights, to press down on my silkscreen frames when burning new images:

Marshall 100W JCM900:



MB Triple Rectifier (6 x 6L6) with that name it should be a real Macho Man :



For reference, a real Macho Man: 70's Twin Reverb PT ... which by the way works as new, I had to wind a new one because it's 115V only, and we have 220V.
Why not rewind it?

Because it's not only dipped in varnish after finished, but they also used a hole in one cover and filled it with polyester resin, the transformer is an indestructible solid block.



Sp I'm sorry to say thet the new ones you'll get are the same as the older ones, so use MOVs, reverse plate diodes, whatever, to protect them.

If someday you have custom ones wound, insist on "170/180ºC (a.k.a.) ClassF/H) wire, rated for *electric motors*" ,and ask them for a wire sample (a couple inches, can't say no to that) so you burn it with a propane flame and check it.

FWIW my wire supplier thinks me crazy when I buy the wire spools and tells me "everybody else now uses the other one".

[pdf64]

There's the method of wiring a resistor (eg 470 ohms across 16 ohm winding) permanently across the secondary, so that some load is always maintained.
I guess if that's done at the OT terminals, then that's ensuring it probably stays in circuit even in the event of broken tracks etc.
A shorted primary may have been due to over current, rather than voltage spike, most likely due to an output tube shorting; wiring 1 ohm ~0.5W flame retardant resistors in series with the plates and cathodes, to act as circuit specific 'ghetto' fuses, may mitigate for that; slow blow fuses on the PT windings can take too long.
Pete

[Mick Bailey]

I haven't looked into the resistor method enough to come to a solid conclusion. I've recently experimented with this and what suprises me is the fairly high value of resistance across the secondary that will still damp down oscillation and reduce destructive spikes. Even 100k makes a difference, though the smaller the resistance the more pronounced the effect. I even got my skin resistance (a little over 3M) making a difference on the scope. However, if this was the complete answer I would have thought it a cheaper and better method than MOVs/TVS/diodes, but the emphasis seems to be on the latter.

The size of the transformers in these amps must be a contributor to their early demise. Here'a a photograph of my hand (which is a girly size - Someone once said to me "it's about the width of a man's thumb. Or two of yours.") holding one;



With a B+ of 400v and Marshalls bias of 35ma per tube the amp will push well over 20W. Through that transformer.

But, some run in studios all day long, year after year.

Now, it could be that the guy has dodgy speaker leads or something. Marshall were down to their last 5 transformers and I've now bought 2. I like the resistor method. What's the downside?

[pdf64]

Wow, it's looks more appropriate for a micro amp; or an old Gibson.
I'm haven't found any drawback with the resistor method; with an amp cranked through a (Fluxtone) speaker, flyback diodes or MOVs on the primary seem to affect the waveform, whereas the (secondary) resistor doesn't appear to.
The Vox AC50 used it, as did some Traynors; the global negative feedback (820 + 100 ohm resistors) on BF and later Fenders could be seen as a 'kind of' implementation of it.
I think it's worth adding to any amp, as however careful we are, one way or another they all get driven into an open circuit once in a while.
Pete

[J M Fahey]

[ATTACH=CONFIG]23803[/ATTACH]

Just curious, can you please post the iron measurements?
I need the size of the visible EI rectangle and the stack thickness.
Don't remember if you posted the primary DC resistance, please do.

I might wind one on a rainy weekend and test it .
With proper wire, of course

By the way, the ones in the immensely popular Tiny Terror look roughly that size.

A Musician came a few days ago to buy one of my most efficient cabinets (he really needs all the help he can get), and I was amazed at the Tiny T ................ (ransformers).

Yet it worked well, within its power.

[Mick Bailey]

The visible stack size is 57.5 wide, 48 high and 20 deep (mm). Primary DC resistance is 405 Ohms (193 + 212). The mounting hole pitch is 48x38mm.

The previous transformer still failed even after new tubes had been fitted, plus I still have the original tube set which the owner dropped by. It doesn't look like tube failure. I'm also going to get hold of the speaker cabs and leads to make sure there are no external problems.

[J M Fahey]

The visible stack size is 57.5 wide, 48 high and 20 deep (mm). Primary DC resistance is 405 Ohms (193 + 212). The mounting hole pitch is 48x38mm.

Ouch !!!
That transformer is cr*p , in capital letters.
It's *much* smaller than I thought.

Let's see: commercial cores do not come in random sizes, there's a "standard" list and you choose from there.
So it's easy to pick which one is that.
Core width is 1/3 the 57.5mm you measured, or around 19mm (it's an exact number in inches), and commercial plastic cores are always *slightly* larger for ease of insertion.
In this case the plastic core is 20x20mm , what you measured.
This works out as a transformer which will saturate beyond all of .... 1.9^4=13W
They must be kidding.

But magnetic saturation is nothing, it certainly won't kill the transformer ... but resistive losses will, definitely.

There are not set rules for this, but a 100W OT often has between 25 and 45 DC ohms per side, a 50W one between 45 and 60 ohms ... here I'd expect around of 100 ohms .... tops.

You measured around 200 ohms per side .....

Way too fine wire (maybe they couldn't fit a thicker one in the small window), small core, that's a very lossy transformer, and losses turn into heat.

I know Drake makes *good* transformers, but if pressed to cut costs to the bone, and on the other side are offered huge orders ..... you end up with stuff like this.

Oh well.

PS: just curious: do they exchange dead transformers on warranty or your customer is paying through the nose for each of them?

The previous transformer still failed even after new tubes had been fitted, plus I still have the original tube set which the owner dropped by. It doesn't look like tube failure. I'm also going to get hold of the speaker cabs and leads to make sure there are no external problems.

Well, now you know why

[Mick Bailey]

No exchange for dead ones and the amp is well out of warranty. The real problem is I have to guarantee the repair and if they pop again I lose money. I ran the amp up for a while and the transformers really cook. lowering the tube current to anything below 65ma per pair introduces a lot of crossover distortion (same on both channels). I've looked at installing larger units, but it's a 1U rack and there's no space. The only other option is to mount larger drop-through transformers in the lid. There's no room to do anything inside the amp; The main PCB almost touches the casing and a sheet of plastic is stuck under the board to prevent shorts.

I'm now thinking that lowering the B+ and screen voltages may give the transformers a slightly easier life. Even that isn't easy to do - there's just no space.

The new pair are labelled up Dagnall Transformers TXOP 00009. I'll drop them a mail to see what they think. Marshall must have agreed the spec originally.

[J M Fahey]

No exchange for dead ones and the amp is well out of warranty. The real problem is I have to guarantee the repair and if they pop again I lose money.

If that's the only option, send it back as is, because you can't guarantee what's physically impossible.
The problem is a way underdesigned part, on which you have no control, and there's no space to fit a better one, even if available.
1U tube amp? Ridiculous !!!!

Obviously designed by the Marketing Dept. who saw the Mesa Boogie 2020 success (same thing) and wanted a piece of the action.
The Mesa, at least, has *way* more free space but its transformers are as tiny as Marshall's ... you can't beat Physics.:

I'm *sure* Marshal engineers hated the task, but the beancounters run the Company everywhere.
Definitely Dagnall hated this even more.
Now I understand, they were ordered "a 20W OT which fits inside 1U" and had to comply.

I've looked at installing larger units, but it's a 1U rack and there's no space. The only other option is to mount larger drop-through transformers in the lid. There's no room to do anything inside the amp; The main PCB almost touches the casing and a sheet of plastic is stuck under the board to prevent shorts.

That's the point. No space inside.

I think the realistic options are 2:
1) as said, send it back.
2) "mount larger drop-through transformers in the lid"
That's a *good* one.
At first I thought it impossible from an aesthetics point of view, but now I found a way to please all:
a) mount 2 *good* transformers above the cover .... "as if it were a regular Marshall amp chassis".
2) build or get (or have the owner take care of that) a "Marshall type" empty cabinet, where the bottom sitting chassis is the 1U rack amp.
Of course it will have at least 2U "free space" "upstairs", ample for any transformer.
Everything else stays the same.
It will be a COOL looking compact head.
Think about it

I'm now thinking that lowering the B+ and screen voltages may give the transformers a slightly easier life. Even that isn't easy to do - there's just no space.

That's the point.

The new pair are labelled up Dagnall Transformers TXOP 00009. I'll drop them a mail to see what they think. Marshall must have agreed the spec originally.

I found them:

Even with the best goodwill in the World, they can't offer you a different option.

FWIW, googling "marshall 2020 schematic" with NO other qualifications (not that I'm specially looking for bad apples), brings as first page search results :

one of the channels does not work

Hi all, A friend just brought in a Marshall 20/20, the unit doesn't work,

Marshall 20/20 down again - Marshall Amp Forum

My Marshall el84 20/20 just died!!!! SOS

EL84 20/20 poweramp... help!

Does not look good .
Plus .... same first page shows a lot of different shops offering replacement OTs .... something about which I didn't ask.
They must be selling like hot cakes

[Mick Bailey]

This is certainly an amp to avoid repairing in the future. No wonder Marshall are nearly out of stock on the transformers. They could have designed it better with toroidals and shifted things around a bit. There's wasted board space around the mains socket that could have been better thought out. Even so, it would be a compromised design.

I wonder how many of these amps failed under warranty?

I'm going to think through what to do next. I think lowering the current through those transformers to a 'safe' level would mean that it wouldn't have much output at all.

[J M Fahey]

That's the point.
They are already, say, 15 (real) watts per channel.
What will you need to lower them to, to have a *significant* advantage ? ...... 7W?
Don't think the owner will like that

Curiously Mesas don't seem to show the same amount of trouble ... but the usual complain is that "the fan is very noisy because of the *huge* airflow"
The Boogie Board ? View topic - mesa boogie 20/20 -> annoying fan noise

now, ONE thing really got me annoyed though - on low volumes the fan is just superloud - it exceeds the fan noise of my 50/50 by far since its one of those small size fans with high rpm. my question is: are there any replacement fans for a less noisy unit .................... help appreciated, i didnt expect such a loud unit ................ first thing my girlfriend said when she entered today to check my new unit: wow, whats that noise?!

i was so fed up with the fan noise that i simply disconnected the fan and played 1hour with open case, since i couldnt enjoy my rack at normal bedroom level at all thanks to the stupid fan.

I also find the fan pretty noisy Part of it is mechanical noise form an old fan but a lot of it is the airflow itself.

*Maybe* you can try something like that .... but how can you actually be sure?
After all, the Mesa OT might be better than Marshall's.
Not a gamble I'd like to take.
Oh well.

[Mick Bailey]

Looks like the Mesa transformers go the same way.

Someone was asking on another forum where they can get them in Europe. Take a look at the price....Gulp!

Output transf. for Mesa Boogie 20/20 Power Amp - Output Transf. for Mesa Boogie

[Tom Phillips]

246 Euros for that little transformer? Ouch!

[Gtr_tech]

If I were in yer shoes, I'd decline the repair and explain to the owner why it ain't worth throwin' any money at. Who needs an amp that is quite proven to be unreliable and is a shitbag design to begin with? Scrap pile fodder....