Looking for AB763 transformer advice

[nateflanigan]

Hi everyone,
In a nutshell, I'm gutting a DRRI and rebuilding it with the AB763 board.  I've played every fender amp I can get my hand on in the past few months, and always liked deluxes and vibroluxs.  But I was playing these in guitar stores.  I scored a pretty decent deal on a DRRI and planned on a straight rebuild, while waiting for parts I took the amp to band practice (we're a pretty quiet indie rock band).  I hate it.  Just no where near the headroom or bass response I'm looking for.  So, I'm thinking I should build it more as a vibrolux.  My question is about available real estate, do I have room to replace the stock DRRI transformers with bigger iron?  I'm not worried about drilling new holes, or enlarging cut outs.

Thanks!

[HotBluePlates]

Yes.

Trading up to the Vibrolux you really want is probably the best course of action. Otherwise, you're buying almost everything for a whole new amp.

[Dave]

Take into consideration (no matter what you decide to do) that the PT on the DRRI is wider and longer that an original DR PT.

Dave

[eleventeen]

I have to agree with HBP: You have a very popular and salable amp with the DRRI, new, used, in any condition. To make it a Vibrolux you're going to have to change every transformer, with the possible exception of the choke & reverb. The PT and OT are not really all that much bigger and there's certainly room to do what you'ree talking about.

[I modded a SF DR into a 4x 6L6 effectively a Twin reverb long ago and far away. I recently bought that amp back from the guy I did it for for $350. Used a Genuine Twin Reverb OT and a giant Stancor PT, bigger than a Twin PT. Anyway, this was in an era where power was the ticket more than tone, though that was a mondo amp. Point being, you can physically place a *MUCH* larger OT and PT and you're only talking *slightly* larger than stock.]

But the question is, is this a smart way to go? By the time you buy the new trannys and tubes you're talking at least $200 completely forgetting any work you will have to do, and there could be another $100 worth of electrolytics and misc parts.

I think you should sell your DRRI and just buy the Vibrolux and be done with it. My .02.

[J Rindt]

It's been said, but yeah....sell the DRri. Either buy what you want, or buy a beat up Silver Face something or other that can easily be turned into what/close to what you want.....
Good Luck

[Fresh_Start]

Just to be contrarian, how far could Nate get with a new mondo OT and the right 12" speaker with 6L6 power tubes?  IOW is a DRRI power transformer just not up to the task of pushing 6L6s?  (Nate:  please do NOT follow my advice until someone else chimes in and says you can try this.)

Where I'm heading is, could he buy a new OT, the "right" speaker, and bias the DRRI for 6L6s without gutting the amp then see how close he gets to what he wants tone wise?  

If that's possible, he could at least figure out if a 12" speaker does what he wants or if two 10" speakers are what he needs.  Also, he hasn't lost much if it doesn't work out.  Leave the improved speaker in the DRRI and sell it.  Swap the high quality OT for 6l6s back out for use in whatever he gets next.

My thinking is we don't know exactly what doesn't work right now.  IMLE a speaker upgrade can do wonders for new Fenders.  In my Blues Jr., there's no comparison between the stock speaker and a Weber.  I've tried both a "California" and a "Blue Dog" - both are tons better than the stock POS but they are very different as well.  The California has pretty flat frequency response and is LOUD.  A bigger, better quality OT will get you more bass and the appearance of more clean headroom.  6L6 power tubes definitely will give him more clean headroom.

All this is getting around to my two main questions:  do you like 6V6s or 6L6s tone wise? and, do you like one 12" speaker or two 10" speakers?

Hope this helps,

Chip

[nateflanigan]

Ok, I think a little more information is in needed...

-I've already built the hoffman AB763 board, it cost roughly $150 and took me 8hrs to do two (i'm making one for a friend also).  So In the grand scheme of things not a huge investment.  Plus I really enjoy DIY.

-I got the DRRI for $600 with a weber 12F150 and all JJ tubes.  Thats about $250 worth of parts I would have bought anyway, a cab, chassis and face plate from mojo comes out to about $350.

-I shopped around for silverfaces, everything from princeton's to pro's, the cheapest I found was $900.  I'd still probably want a new speaker, some recapping etc.

So following my thinking, I'm at $750, new transformers will put me around $950.  I don't really think I could do much better. 

Eleventeen, It's really helpful to know that you could fit twin parts in a deluxe cab.  I'll probably put in an over sized PT, start with a VR OT into the weber 12, and experiment from there.

[bigdaddy]

I honestly don't think you need a new PT. I have owned RI Fenders and they sounded pretty good once I had replaced the speaker and biased the amp. As far as I know those PTs will take a pair of 6L6GCs with no issues.

What I would do is simple; I would use a SS rectifier, get a new OT from David Allen TO35MT or TO40MT, put the board in and call it a day. Doug also carries good transformers. The amp will sound amazing. You might have to work some things out with the bias supply and a few other issues like the coupling cap before the PI. You maybe like a different value, I liked 500PF caps there to cut down on the lows, if I wanted more I just turned up the bass and mid control.

I would add a mid control by using the second input jack and using a small old style Fender knobs that everybody sells, David Allen has black ones. You could also mess with those values, anything from 10-50KL. The 12" speaker will be louder and have more punch and lows then 2x10" speakers unless you get some real heavy duty ones like the Weber 10F150. That's why I say to change the coupling caps since the amp was designed for 10's or a in BFDR a cheap low sensitivity 12.

If you at any time burn up the PT which I doubt, then you can get a new one, they are not that expensive unless you get a Mercury made with pixie dust.....great trannys but a little over priced IMHO.

[eleventeen]

Nate, the Stancor PT I put into the Deluxe was a monster, bought new-surplus, significantly bigger footprint wise and heightwise than a twin reverb tranny, maybe add 1/2" to both dimensions. It is so danged tight inside the cabinet that I had to make a special tool to get the 8-32 chassis nuts onto the threaded ends of the screws for the chassis straps! No, a regular nutdriver won't do it because the nutdriver handle gets in the way and it has to approach the end of the screws (at that side of the amp) at an odd angle! Oh yeah, to mount the speaker I had to make a new baffle board and slightly rotate the spider of the speaker (had to be a shallow JBL D-120, too) because the (Twin Rev) output tranny pokes into the space between two arms of the speaker spider!

As I said, don't ask me HTH I got all that crap into the Deluxe cab!

Two 6L6's occupy the former 6V6 sockets, one is in the rectifier socket, went to SS rectifier, and one 6L6 socket was newly punched, located inboard, between the PT and the choke. At the time, it didn't occur to me that the 6L6 circuit would likely want a beefier choke. Now, 25 years later, I will want to use a smaller tranny, which creates its own metalworking issues. I'm sure I can fab a plate to reduce the footprint. Could kick myself for modding an AB 763 SF Deluxe Reverb. But hey, life was simpler then and there sure didn't seem any practical limits on the number of Deluxe Reverbs one could buy if one wanted.

[nateflanigan]

Thanks for the input Bigdaddy, it's hard not to get caught up in the hype around mercury mags, and boutiqie type transformers in general.  I think I'll take your advice on the PT, if nothing else it'll be more interesting to change it out later and hear a before and after. 

Eleventeen, that sounds rough. 

[Fresh_Start]

Nate - there is some basis for claims of better performance from "boutique" transformer makers.  IMHO Fender is putting the cheapest stuff they can get away with in their "vintage" reproductions.  It's just that the output transformer is a whole lot more important - and directly involved - than the PT in most amps.  (Tweed Deluxe is an exception.)  You'd be surprised by how much change swapping the OT for one of Dave Allen's made on a simple Tweed Princeton.  I was.  In that case, I didn't actually want the greatly improved bass response but it made a HUGE difference in the amp's sound.

Mercury Magnetics transformers are great, but only if you pay the +25 unit volume price.  If you have to pay full retail for MM iron, there are a lot better alternatives starting right here with Hoffman's transformers.  I built two Princeton Reverbs.  For the first, the customer insisted on MM iron.  I put Hoffman's transformers in the second build for me.  If anything, I like mine better.  The cabs & speakers are identical and the circuits are very close.

2CW

Chip

[tommytornado]

I have done the exact thing you're talking about..  DRRI, original PT with the 30-35W output transformer that one would find in a 5E7, 5F4, tweed and BF Pro..  And to my ears the owner ended up with and amp that is as loud as a Vibrolux.  I would try the conversion using the stock PT and try that first.  I would suggest getting a http://musicalpowersupplies.com/3.html OT40PP OT.  Check the specs and you'll see that it's setup with 3.3K/4K primaries with 4,8,16ohm secondaries/output... so you've got all the options power tube and speaker ohm's wise.  Plus it's an exposed/no bell covers design, so it will give you a little more room to clear the speaker.  Biased at around 25mA, I'm getting 410ish V on the plates of JJ 6V6.  I tried Tung Sol 5881 RI's in it as well.  They provided more headroom, but I felt it pushed the sweet spot a little too loud.. so I changed back to the JJ 6V6's.  The PT got hotter a little faster with the 5881's as well. 

Also about the PT..  It's a different physical design than the stock DR PT.  MM is the only place I know of who makes a direct drop in replacement, but it's pretty expensive. 

Here are some suggestions and lessons learned during the project.

*  I didn't do this, but will if I ever do another DRRI conversion.  Start fresh and replace ALL the tube sockets.  The one's in the amp are cheap and it ended up being more work pulling all the wires and cleaning up the solder on the pins. 
*  Mount your speaker when the chassis is emptly (except for the PT) and make sure that you positon the OT in a place that it doesn't butt up against the speaker. 
*  Wire the normal channel to go through the reverb and trem circuit's..  and voice the normal channel differently.  I went with the bassman/marshall tone stack 25pF/.022uF/.022uF w/56k slope resistor...  This channel will play better with hot humbuckers than the fender tone stack will.  If you want a little more dirt, the brown era (6Gxx) tone stack works nicely.
* I installed a SPST switch between the speaker jack tip and the NFB resistor.  You'll get a nice volume and dirt boost when NFB is disengaged.
* Speaker suggestions:  Weber Signature Series 12B, Jensen MOD 12-70, Eminence Legend 125, Weber 12F150 (see Weber's suggestions), Emi Red White and Blues.

Feel free to email me if you have any questions.  tomcat.arnold at gmail d0t com.     

[nateflanigan]

Awesome advice guys!  That's a pretty unbeatable deal for an OT.  A big part of wanting to do this project was to have a classic fender channel, and the "normal" channel to tweak around with.  I haven't put in any leg work searching, so I feel bad asking, but what's involved in routing normal to reverb?

[Fresh_Start]

Awesome advice guys!  That's a pretty unbeatable deal for an OT.  A big part of wanting to do this project was to have a classic fender channel, and the "normal" channel to tweak around with.  I haven't put in any leg work searching, so I feel bad asking, but what's involved in routing normal to reverb?

The Normal to reverb topic comes up often.  Check out this thread:
http://www.el34world.com/Forum/index.php?topic=9397.0

I like the idea of a Marshall tone stack in the Normal channel too.  You could add a MOSFET cathode follower fairly easily too (google "MOSFET Follies") to boost the gain.  Or you could go toward the Brown Vibroverb shown in the schematic I linked to in that thread above.

Rock on!

Chip

[tommytornado]

ooops..  I forgot a few things: 

I found the bright channel was too bright with the 47pF cap on the volume control.  But if you want that option, you can get a 1m pot with a spdt push/pull switch (if you can find one that will fit, the big a$$ weber's I had didn't) and make the bright pull/push switchable.

I installed mid controls for both channels on the back pannel using 25KA pots.  They allow you to dial in a little more grit/fatness vs. the stock fender 10K mid pot.

I used a 100KA (audio) reverb pot vs. the stock 100KL (linear).  Much more control tweakability with the audio taper pot. 

[HotBluePlates]

Well, we can find transformer charts that tell us what model transformer was used in the old amps. The downside is we don't usually get actual transformers specs with those charts, so we don't know for sure what the capabilities are/were.

If we note that the supply voltages are very similar in the Deluxe Reverb and Vibrolux, then to get more output power we have to have other changes in the output circuit. Power being voltage times current, to get more power we need a bigger voltage swing, a bigger current swing, or both. If voltage is held constant, we could halve the load impedance and double the current swing to get double power. That appears to be how Fender did it.

Radio Daze lists some specs for Hammond made Fender replacement transformers, which is more than you find with most replacement transformers. Hammond's actual posted specs are a little higher if you look at their website, but the trend is interesting. The model that is a Deluxe replacement is given as a 120mA secondary where the Vibrolux replacement is a 180mA secondary.

Of course, switching from 6V6 to 6L6 means you need ~1A more heater current, which the Vibrolux PT has over the DR PT.

Without going through the equations, when I ran the numbers the Deluxe PT looked a little more capable than needed to make its 20w or so, while being underpowered to provide a full 40w. Swap in 6L6's, and you should get more power than the Deluxe Reverb, providing the heater winding holds up to the extra load. The Vibrolux PT guarantees 40w of output and a heater winding that will certainly support 6L6's.

[nateflanigan]

Again, thank you everyone!

I've thought about the pull pot bright channel mod, I've never really cared for the bright channel on real fenders though.  I've got the audio taper pot for the reverb.  Playing around with the mids seems fun, especially for the abnormal channel.  I've seen some dumblesque mods also, which seems to amount to a mid-heavy fender channel.

Thank you for the PT math HBP, it seems like the amp will function with the orignal iron but could be better with more.  I forgot about the hammond trannies, I've mostly been looking at mojo just cuz they have some specs available.  The site mentioned earlier has some very affordable PT's as well.

[mresistor]

Why not use an Allen TP40D PT which will handle the 6L6s and the extra heater current no sweat. I'd bet it's made by Heyboer. It is 3/4 inch taller than a stock pt but that shouldn't be a problem. I think I have one in my DR. Give a look at the TO26 OT while there.

[Bp-Plickner]

2nd on the Allen power tranny. I have used them several times including in my 66 DR.
Allen also sells some pretty nice out put transformers. I have used his bassman OT in a couple of 5e7 builds and like them as well as his vibrolux replacement that might fit a DRRI chassis. 
They sound good with 6v6, 6l6, EL34 and KT66 output tubes.
I  belong to the big iron is good club