Bassman 10 Project - Fender Power Transformers

[cowpuncher]

Hello gentlemen,


I posted an intro in the appropriate forum earlier, and now I have my first question for the board. 


I have an old Fender Bassman 10 that was given to me for free long, long, long ago... back when I was a teenager in fact! (I'm 44 now, wow time flies).  No speakers in the cabinet, but the amp was fully functional and I would use it now and then instead of my Marshall JMP50 for a different sound.  It's not one of the ultra-linear bassman 10's and with a stomp box to kick it into gear, and the amp cranked, actually didn't sound half bad.  It's sat in the shed for many years now, and the cabinet has languished in an old closet in the folks house. 

http://el34world.com/charts/Schematics/files/fender/Fender_BASSMAN_10.pdf


I've always figured I would eventually refurb it or rebuild it into something else.  Last night I had some time on my hands, and looked at one of the schematics for it in the tube amp schematic library on this site.  I noticed something:


Power Transformer:
TR1 - 125P5DX
TR1 - 125P5D


The schematic shows the secondary voltage as being 365-0-365V.


Then I look at a few other amps.  That same power transformer is listed for a few other amps. 


The AB763 Super Reverb and AB763 Pro Reverb show TR1 as the same part number, with a secondary at 360-0-360V.  Okay, I can believe that.  Likewise, the AB568 circuit shows that same power transformer part number, with 365-0-365V.


The AB668 Pro Reverb uses the same transformer, and the AA1009 and AA270 Pro Reverb shows it and has 375-0-375V listed as the secondary for the same part number power transformer.  Higher voltage ratings for the secondary, with the same part # power transformer.  Interesting. 


Then I look at the AA165 Pro Reverb, which also used the same Part #125P5DX or #125P5D power transformer, and now it's listed as 340-0-340V.  The plot thickens...


Finally, I see that the AA763 Vibroverb is also listed as using this same power transformer as the Bassman 10, with 355-0-355V given as the secondary voltage. 


So my questions:


1 - Was their really THAT much variation at the Fender factory for the same part # power transformer?  I have listed examples from schematics showing it as having a 340-0-340V secondary and as high as 375-0-375V.  Quite different voltages, and all of these amps should be at similar power levels with a pair of 6L6GC output tubes... so I have a hard time believing that bias and current draw would be radically different. 


2 - Since most of these amps, particularly the Super reverbs, have a lot more tubes drawing heater current than my Bassman 10 does with it's 3 x 12AX7's and 2 x 6L6's, and the transformer is the same part number as these other amps... that leads me to believe I've got at least another 1 amp of heater current to play with if I want to modify this amp and still be within safe margins. 


3 - Is it possible that Fender showing these transformers with such differing secondary voltages is because they would test-select them for different models?  Those that were in the 360-365V range going to the Super Reverbs... those at the 340-355V range going into Pro's and Vibroverbs, etc at the factory?  I'm not familiar with what was actually going on in the factory at the time. 




If I've safely got a surplus of heater current to throw away.... I'm probably going to end up gutting this Bassman 10, and building something like a 5F6-A circuit with EL34 output tubes, and maybe a bias switch to allow me to also swap in 5881's when I'm in the mood.  It's a Silverface Bassman 10, probably won't ever be worth much as a vintage collector piece... and hell - I got it for free ages ago anyway. 


Thoughts?  Anybody care to elaborate about all of those different amps having such different ratings for the power transformer's secondary, with the exact same part number power transformer, according to Fender's own schematics? 

[sluckey]

Try to not overthink it. 

[cowpuncher]

gee thanks sluckey :p 

Don't get me wrong:  At the end of the day, I'm going with "hold my beer and watch me stick EL34's in this sucker".... that decision is made. 


I'm just wondering about so much variation on the high voltage secondaries given for the same part number power transformer.... as well as surprised by the fact that apparently the power transformer in my boring old bassman 10 is the same as was slapped into Super Reverbs, Vibroverbs, and other much more exciting and interesting amps. lol 

Surprised because elsewhere on the net if you mention putting substituting EL34s into this amp or a similar one you read endles "OMG OMG OMG OMG HEATER CURRENT OMG OMG OMG" replies. 

Meanwhile... apparently it's got got gobs of extra heater current to spare, if the same part # was used in Super Reverbs, Vibroverbs, etc. 


Like I said - In the end, no matter what, I'm doing what I'm doing.  It was just interesting to me to discover that same part # used in so many other (better) amps... as well as the variation in high voltage secondary specs given on each of the schematics. 

[PRR]

> Was their really THAT much variation

It's only 10%. All tube-work is 20% slop. I doubt "selection". Once the turns-count is set, the only variation is the local wall-voltage, which did vary. And may not have been the nominal number on the plan. And meters. 10% is not a big jump on a needle-meter. Factory meters get banged-up and beat, so maybe the calibration drifts.

I would not fret about any sane number of small tubes added. EL34 from 6L6 is a big jump right there. But as you say, this is a super-common transformer. If you smoke it, you can replace it.

[DummyLoad]

that is a kick-ass amp just as it is. a no choke bassman. i just sold one that had been stored a lawnmower shed for over 20 years. it was stinking so much of mold i left it in my garage for quite some time before i brought it in to go over it. the closed back cabinet with 4 x10 adds a nice punch and really sounds good with a guitar with single coils. just freshen up the filters before you gut it and give a shot, you may be pleasantly surprised as i was and leave it in it's original vintage state.


--pete

[cowpuncher]

that is a kick-ass amp just as it is. a no choke bassman. i just sold one that had been stored a lawnmower shed for over 20 years. it was stinking so much of mold i left it in my garage for quite some time before i brought it in to go over it. the closed back cabinet with 4 x10 adds a nice punch and really sounds good with a guitar with single coils. just freshen up the filters before you gut it and give a shot, you may be pleasantly surprised as i was and leave it in it's original vintage state.


--pete

Good point, and I am going to do that and see what I think.  When we were all 18 or 19, my cousin (great drummer), our bass player buddy, and I wanted to jam all night at max volume, everything cranked, and enjoy some good beers one night.  My dad owns a large machine shop/diesel mechanic business... and this particular weekend there was no big equipment or trucks taking up the main part of the shop.  I cleaned up a 40' x 40' area, we managed to load all 3 of us, plus a full drum kit, an old Kustom bass stack, guitars, mic's, and an old Kustom PA with a couple speakers in my 77 Monte Carlo.  We still to this day can't remember how we managed to do it, but we did that and made it to my dad's shop in 1 trip. lol  It took some engineering and contortion.  I played through that Bassman, pulled out of it's cab, and running into an early 60's Bandmaster 2x12 cab that I also got for free (dad's buddies were always giving me hand-me-downs and it was great).  The drummer cousin swears to this day that Bassman, with everything dimed and my Les Paul Jr plugged into it, was the best guitar sound I ever got.  So your point about just trying it stock after a re-cap has a lot of merit rather than getting carried away. 


I've already got all the filter caps for the job, F&T's.  I've got a pretty enviable stash of parts in general... comes with years of buying stuff and then putting off projects. :)  I've also picked up a bunch of Sovtek 5881WXT's in matched pairs, and I've got some JJ 6L6's around as well.  I'll clean her up nice, give her all new electrolytic caps, check everything over... and try it with the 5881WXT's.  I might change the screen grid resistors to 1K ohms since I've heard the 5881WXT's sound better that way. 


Likewise I could make the few changes to also slap EL34's in it and try them as well, rather than re-inventing the wheel and gutting the thing. 


For the 4x10's I picked up some Jensen C10 clones from amppartsdirect's Ebay store last year for the project as well.  "Real Vintage Custom Series 10" Speaker"  They were $25 each plus shipping, I figured it was worth taking a chance on them.  I ended up putting one in a 5F1 Champ that I built for my niece and they aren't too bad.  So 3 of them, and probably an actual Jensen C10 as well, will go in the 4x10 sealed cabinet. 

Not guaranteeing I won't gut it and build in a 5F6-A circuit, but your point has merit and appeals to the lazy and time-challenged part of my nature. 

[sluckey]

In addition to what PRR said, there's another factor that can cause the same transformer to put out different voltages. Loading. There are differences in the circuits you mention and biasing becomes a factor as well. Hotter bias puts a bigger current drain on the PT causing the voltage to sag more than a cooler biased amp. Anything that changes the load on the PT will affect the voltage it produces. A reverb driver is a significant load. You could probably take one PT and connect it to each of the amps you mentioned and get a different voltage measurement in each case. Or you could pull all the tubes of those different amps and measure the unloaded secondary voltage and probably get less variance in voltage readings.

If you need to replace an "XYZ" PT you would probably like to use an "XYZ" PT. But how will you know if you will get one of the high output "XYZ" PT or one of the lower output "XYZ" PTs?   

Now I'm confused. I think I'll try not to overthink this.

[frankenxtein]

Specs for replacement part? .... They wouldn't be exactly the same specs , not too much more and nothing less. Seen it in a Silvertone Sam's pdf