Mesa 400+ power transformer

[GainFreak]

Hi,

A friend of mine bought an abused Mesa 400+ which needs a power transformer replacement.
I'll be ordering one at the local transformer shop but I'm not very sure about the HV winding specs.
The only replacement with specs I found is from TAD but the HT secondary is rated at 750mA which for 12 6L6s doesn't look enough to me unless the amp is biased in class B or something.
In the same time the original PT doesn't look very huge so I would appreciate any info and/or advice on that.

[nosaj]

Hi,

A friend of mine bought an abused Mesa 400+ which needs a power transformer replacement.
I'll be ordering one at the local transformer shop but I'm not very sure about the HV winding specs.
The only replacement with specs I found is from TAD but the HT secondary is rated at 750mA which for 12 6L6s doesn't look enough to me unless the amp is biased in class B or something.
In the same time the original PT doesn't look very huge so I would appreciate any info and/or advice on that.

Well here is a suggestion. Disconnected the PT and find out exactly what is wrong with it. You may have working secondaries that you can derive some specs from it with Ohms law.

nosaj

But damn are his intentions to rebuild the house everytime he plays it?

[GainFreak]

The thing is he bought it for dirt cheap while he was recently in US and to save from shipping costs the PT was removed so that unfortunately is not possible. The PT was due for replacement because of the US voltages anyway.

[g1]

I would expect it to be biased fairly cold. Also, (aside from idling) only 6 tubes will be 'on' at a time, so the number doesn't seem so far out?
Maybe this can help:
Calculate Tube Amp Power Transformer Current

[Leo_Gnardo]

In the same time the original PT doesn't look very huge so I would appreciate any info and/or advice on that.

I take it you're avoiding overseas shipping so a purchase from Mesa or Mercury isn't going to happen.

Since you do have a local transformer shop, you have the option of beefing up the hi voltage winding, thicker wire for instance so you can deliver more current. Might result in a larger transformer but I'd say do it, as long as it fits the mounting footprint & doesn't crash into other things in the amp. You might wind up with a better than factory spec amp, wouldn't that be nice.

[GainFreak]

Maybe this can help:

Thanks for the link, I found that today while searching for specs.

I take it you're avoiding overseas shipping so a purchase from Mesa or Mercury isn't going to happen.

ANYTHING from MM is out of the question, heavier stuff from US either. After I add the shipping cost, VAT and taxes prices become unacceptable at least for items like this. I'll have that done locally.

I'm not very good in theory so is there any very simple rule of thumb or formula how to determine the current capability of the HT winding when a given number of tubes are used provided a single HT secondary and bridge rectifier?

[J M Fahey]

to save from shipping costs the PT was removed

Excuse the French but stupid decision..

Even more so on such a weird amplifier, it isnīt your typical 4 bottle amp for which there are many examples both sides of the pond, a PT for a Twin Reverb or a Marshall Plexi are broadly interchangeable but a Mesa 400 wonīt work even with n Ampeg SVT transformer.

The most practical solution would have been to ship the original PT by ship or cheapest option, including consolidated cargo, and worst case paying the luggage excess fee which would not have been as expensive as buying or ordering a PT either side of the ocean (I guess you are in UK) and then use it with a 240:12V autoformer which by definition and functioning principle weighs *half* what its power rating indicates.
Youīll have to trust me on his, donīt have the time or inclination to explain why.

To boot,even if custom wound, an autotransformer is a stupid easy one to wind, since it only has a single winding, all same wire end to end, with a center tap, so they are cheap by definition.
Even insulation is cheap, since in any case "primary" and "secondary" are connected, being a single continuous strand of wire.

Compare that to a proper PT: primary , HT secondary, filaments, probably LT windings for preamp or relays, all different wires, all different advance setting in the winding machine (I know, I wind transformers commercially) ... did I say *expensive*?
Plus it must be very well insulated to stand HIPOT primary to secondary insulation tests.

Is it too late toask for theoriginalone and use it with a locally sourced autotransformer?
If not, consider that option.

Worst case, let me calculate what you need (post the full schematic here) but consider that a 2 bottle amp uses a 500mA +V fuse, a 4 bottle one a 1A one ... so a 12 bottle one wouldneeda 3A one.
Not saying the winding must be rated 3A but wouldnīt be surprised at 2A or not far from that.

EDIT thatīs not a garage band amplifier for sure, but a World Touring Band class one (Bruce Springsteenīs Bass player used one, coupled with 2 x 4 x 10" aluminum cone Hartke cabinets) so I wouldnīt be surprised at it having 2 x 120VAC primary windings, able tobe wired series or parallel.
Of course, it might also have just a single 120V one.

[GainFreak]

Hey Juan, thanks for chiming in.
First of all I'm not in UK but in that part of EU where I can get one piece of this transformer custom done for less than 100 Euro so I'm definitely going for it. Local transformer shop which is a small business would do single orders without any problems. I've used their services in the past so it can be considered a done deal.
They guy is touring extensively so it's not going to be a bedroom amp
The only schematics I found are of very poor quality and low resolution but I'll post them here for reference together with a pic of the amp insides that I found in the net where you can see that the PT is not very much bigger that the OT. In fact there isn't much difference between 400 and 400+ except for the number of tubes and the lower plate voltage in 400+.

Mesa400+ schematics.zip



This one from TAD is listed as 400+ replacement:

Mains transf. for Mesa Boogie Bass 400+ - Mains Transf. for Mesa

[GainFreak]

Since I'm ordering a brand new PT and because it's a bridge rectified HT supply I was thinking of omitting the center tap which from what I can see is used only to decrease the overall voltage (via 3 Ohm/5W resistor). A 360V secondary should get me into the 460-480V range at idle after rectification.
Anyone see any problem with that?

An EI120x70mm stack has ~450W capability so there is room to beef the secondary up to 1A.

Also for the sake of experiment I was thinking of trying a not very deep NFB. Which place is best to do that? Can I add a 50-100 Ohm tail resistor to the 470 Ohm one in the last stage or else?