Looking for a custom made 100W power transformer . . . .

[Ghostman]

Why are you so obsessed with 510 volts? How is this the magic number that is going to make the world come crashing down upon you? The magic number for face melting tone? The resonant frequency of wet farts passed through a cinder block wall?


I say play the damn thing! Goddamn!

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[RickyLee]

How about a Mesa dual rectifier take out? Should be cheap enough. You get that rectifier tap and what did they say? 470 plate volt?

I might have one here at the house. I am hoping it will run no higher than 470V. My DC-10 ran about there if I am remembering correctly.

What stumps me is why do many of the Mesa owners swap out their stock transformers? I can see the hype of wanting a Mercury Magnetics for the output transformer. But the power transformers Mesa uses seem very robust and well built. I just can't find any specs on them though. As in the HT mA or current ratings and such. And every Mesa PT I checked does not have multiple HT secondary taps. They only did that extra tap on the primary (bold/spongy) that drops the primary down to about 105VAC. That lowers EVERY secondary tap and you end up with a cool lower plate and screen, BUT a very low heater tap around 5VAC or less.

Mesa used to use Schumacher power and Fender output transformers. But now I can't find where these new ones are coiming from or where they are made. That link I posted for new Mesa trannies is Tube Amp Doctor and they are in Germany. I thought I read Mesa only uses USA parts?



Maybe that was the first older Mesas?

 

[Woods]

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Come over and let's talk about it. I'm just off Nickerson st.

 

[Ghostman]

Come over and let's talk about it. I'm just off Nickerson st.

 

[Woods]

Same to you good sir.

 

[mickeydg5]

BUT, this PT is doing the exact same thing as that last Mitchell.

ADD: Another question I just thought of. Should the bias supply circuit have more than one capacitor? This one just has the one reservoir cap. I added in a few parts/trim pot to make it adjustable. Wondering if I need another cap at the tail end of the supply?

I put a new reservoir cap of 47uF 100V value.

The other Mitchell had low filament and high HT voltages just like this one?


The Mark-I has a simple half wave rectifier for the bias supply. I presume the Mitchell is similar.

 

[RickyLee]

The other Mitchell had low filament and high HT voltages just like this one?


The Mark-I has a simple half wave rectifier for the bias supply. I presume the Mitchell is similar.

Yes and Yes.

My buddy found a similar power xfmr but does not know exactly if it is a Twin or the Bandmaster type less powered. Bummer is it only has the 606-XXX numbers stamped on it and no other code numbers. Other odd thing is it has two yellow wires which tells me it has a 5V tap. I do not recall Twin Reverb having two unused/non terminated wires inside. They do not have a tube rectifier.

Anyone recall seeing an old Twin or Dual Showman Reverb with unused 5V tap inside? There is no way for me to know the mA rating on the HT tap on that one he has. Bummer as I could get it for cheap too.

I was just playing the amp at some good volume and it is a monster only running two 6L6 and wall mains set to under 115VAC. Still have some work to do as it had some crackling when I cranked it. And the crackle would happen if i tapped on the head box. Hopefully something simple as tube socket tangs or bad Ground/solder joint. Amp has an awesome big bold clean tone. But a noisy reverb circuit.

I am also trying to verify the output transformer impedance. It has a 4/8 label where a toggle was, but hole plugged with cap. The output transformer is just a single secondary tap. It is a Twin Reverb so figuring 4 ohm or 8 ohm since I am running two 6L6. I think my other Mitchell actuall gas the 4/8 switch. Hoping I can dig the other Mitchell out here shortly.

It is just like this one pictured but does not have the "FENDER" stamped. Codes are: 022897 606-3-50. So not sure if it is 73 or 83. Could date it to 1983. But there is a sticker inside from a repair done in 1982. Caps had date codes from 77 to 79. So I am wondering what era Fender put that "FENDER" stamp on these OT? Or could that be just for ones purchased by Fender? Atwin code uses 022889, but I found this chart which shows the 022897 as other Fender models. BUT I am figuring they used the big Twin output transformer in 40W amps/two 6L6 and ran it 8 ohms (4 ohms in Twin). I am quite sure it is the same OT. It is big LOL. Guess I will have to find Twin Reverb OT dimensions to verify LOL.

http://www.bustedgear.com/res_Fender_output_transformers.htm

 

[mickeydg5]

Well if it (022897/125A30A) is good for a Showman it can handle 100 watt design.

606 is Woodward-Schumacher manufacture.

That transformer was used in the Fender Showman AB763 design. So I would say 1963 week 50. Most manufacturers started using double digit year designation by the end of the 1960's.

 

[RickyLee]

Well if it (022897/125A30A) is good for a Showman it can handle 100 watt design.

606 is Woodward-Schumacher manufacture.

That transformer was used in the Fender Showman AB763 design. So I would say 1963 week 50. Most manufacturers started using double digit year designation by the end of the 1960's.

Wow, you think that output transformer could be s 63 huh? I would take that in a heartbeat. I think it is a 73 though.

I was playing the thing for awhile with chassis pulled out and of course it would not do the crackling problem LOL. The thing is a loud monster though. Oddly, Mr. Mitchell had a 56K resistor to Ground after the input 1 on V1A grid. I stuck a small 1MEG trim pot in series with that 56K and was playing around with the amp. Oddly, you do not lose that much signal going from 1MEG to 56K. But the 1MEG does give it much more thickness and grind. Still getting used to that odd MXR 6 band EQ built in right on the Input 1. I guess it can do some cool things with tone shaping.

I dug out some of my parts stash. Turns out I have a full set of older era Dual Rectifier transformers, power output & choke. That power transformer would be a good mate to this Mitchell amp. That Dual Rec output transformer is the same size as the Twin Reverb OT but with black end bells. And it has an extra secondary tap. I am guessing it will be 8 & 4 ohm taps.

Did not dig the other Mitchell out. Will get that out one day after work. The even more rare Mitchell Deluxe 1X12 is near that other as well. Wil get that one out too. I think we had a discussion on that one quite a few years back.

That little Mitchell Deluxe deserves a picture as it has a wicker cane grill with a light brown stained cab shell. That one was a bit odd too as it is 50W, two 6L6 but ran about 560V plate and screen LOL. I just can't recall if I swapped the power transformer in that one, Or maybe I stuck KT77's in it. It has much smaller transformers. I am curious to see what the circuit is now that I have all this new knowledge of amplifier innards.



ADD: Just was thinking, if I can get the fourth Mitchell I have out (Mark 125 head) I will try to get some pics up. Another oddball as it had two GE 6550's with about 575V plate and screen.

 

[RickyLee]

Check this out:

http://www.ebay.com/itm/Super-Rare-...935239?hash=item33aeea7d87:g:vU4AAOSwSdZWfe2n

http://www.ebay.com/itm/Super-Rare-...789148?hash=item4887129c5c:g:DT8AAOSwcOFWbLO1

 

[Steeveejay]

I like your idea of adding in the front end dropper resistor. How come know one does this instead of the circuit with the zeners dropping the voltage?

Also, this PT in this amp does not have a 5V tap either. So no tube rectifier option there.

But I need to replace this power transformer regardless. It has a weak heater supply for some reason, and drops to under 5VAC if I run all four 6L6's. I can't figure out what make and model this power transformer is as it has no numbers or ID on it at all.

I was hoping to find a new Boogie replacement PT, but can not find where you would buy one.

I do have an old Mesa transformer in my stash I am going to dig out and see if it will work out. I think it might be from a very early Dual Rec. But it might have a somewhat high B+ voltage as well. Was just holding out hope I could find a new one that had multiple secondary tap voltage options similar to the awesome Classic Tone one I put in my Plexi build. That one gives you three different B+ options.

At this time, I am just running it in half power mode with two 6L6's. And then my idea of the KT77's is good for the high screen voltage issue. But then even worse for this weak heater supply issue as KT77's draw quite a bit more current than 6L6.

Hey Ricky, there's another way to go here if you have room in the chassis.
I would be looking for a separate filament transformer , BUT, first i would be measuring the current of the heater winding in circuit . KT 77 is a heavy duty EL 34 , You're going to need 6 amps on the heaters alone . so even if you run the preamp and splitter heaters of a separate traffo , .

 

[bluesaxe]

Ah, didn't see the part about the low heater voltage initially.

I agree with Mickey; if the HT is too high and the heaters are too low, probably easiest just to find a different OT.

 

[RickyLee]

The other Mitchell had low filament and high HT voltages just like this one?


The Mark-I has a simple half wave rectifier for the bias supply. I presume the Mitchell is similar.

I dug out the first Mitchell Pro 100 I have had. Looks as if I swapped in an old Mesa power transformer into that one to remedy the high HT and low heaters. Took some readings on both Mitchells just a bit ago. That first Mitchell is a Plain Jane version as it is minus the reverb circuit and the graphic EQ (built in MXR 6 band EQ LOL). The preamp PCB is smaller in #1 and the third 12AX7 for the reverb is not there. So he copied Mesa in that way as he offered his Mark copy in a stripped down version as well.

Other difference is that first Mitchell Pro 100 I will now call #1, has a generic output transformer. This OT is almost exact same size (maybe 1/8th inch less thick but almost spot on width height as Mitchell #2 which has the Twin Reverb OT, but this #1's generic OT has 4/8 ohm secondary taps. No numbers on the dark brown end bells. Looks like a possible punch stamping of "C1 603" on the laminates but hard to make out.

Mitchell #1 Mesa 561140 power transformer:

Wall/Mains: 120VAC (VariAC)
HT: 694VAC
Heaters: 6.43VAC
6L6 plates: 469VDC

Looks as if I also hooked up that lower primary voltage tap and here's the readings with it switched to that. This would be the same Bold/Spongy switch on a Dual Rectifier or switching to Tweed mode on some other Mesa amps. I am running both amps with only two of the four 6L6's installed.

Wall/Mains: 120VAC (VariAC)
HT: 585VAC
Heaters: 5.39VAC
6L6 plates: 396VDC

Now here is Mitchell #2 with the stock power transformer.

Wall/Mains: 120VAC (VariAC)
HT: 729VAC
Heaters: 5.90VAC
6L6 plates: 496VDC

Don't know why I did not think of it, but Weber has a Twin Reverb power transformer and it has two secondary HT options of 640V/600V. And they are not that expensive either. I was not able to get much volume up on these two amps this evening but I might be able to play them both tomorrow with some good volume.