Fender 75-Entertain Me

[Cygnus X1]

Fender 75 is a mangled mess of an intertwined circuit with a good clean and unusable distortion channel.
Fender seems to be great at that...

So entertain me.
Mod it?
Strip it out?

Tweed Deluxe one side and JCM style on the other?

(Someone said D'Overdrive it...but my limit is 300 dollars).

[teemuk]

Uh, I know it's a kind a boring option but why not just sell it...?

[Cygnus X1]

I would...but it isn't mine.

Someone wants me to do something
cool with it.

I have some ideas.
Wanted to see what this group thought, or had experience with the 75 specifically.

[Ian444]

Well its got 2 trannies I presume, a few power supply caps, pots etc all the hardware is there, what you can make of it depends on the power and OP trannies and your imagination I guess. What does the owner want? If you measure up the OPT that will determine what output tube choices you have, and then check that the power tranny can support the choices.

"Tweed Deluxe one side and JCM style on the other?"
That sounds pretty good

[FUCHSAUDIO]

Makes a great D clone with reverb and a loop. Nice iron, healthy B+, many output taps too.

[Cygnus X1]

Makes a great D clone with reverb and a loop. Nice iron, healthy B+, many output taps too.

Not sure if I could manage it give the pricing limitation.
Could you suggest any mods to bring the drive channel to D-Style territory?
Cheap?

Also...is this OT ultra linear?
(Forgive me for not looking it up).

[Ian444]

Whats the budget, $20?

[pdf64]

Whats the budget, $20?

Should be plenty, though may to to stretch to $30 if including 'black goop' of a real 'clone'.
But yes, I'd love an insight into Andy's approach to this.
It's a wide open platform with many options. Most recently I put it close to a AB763 reverb channel, with a 'clean' boost at the end, then master vol and fx loop before the power amp.
The key thing has been to remove the 'plate to cathode' 500pF snubber caps to get the sparkle back.
Yes it's UL. Put another B+ node in for the screens.
I've found that the combination of a 500V B+, stiff supply, UL and current 6L6 can result in very short tube life if overdriven.
Pete.

[FUCHSAUDIO]

Well, we do a complete rebuild with our mod board, but it's certainly doable if you have the smarts and/or support. Plenty of room, plenty of voltage and current in there.

We do a mod for about $ 1-K and it includes a loop, keeping the reverb, there is room for triode/pentode in there, remove UL (if it exists) add a choke (if it doesn't), put in new filters and a separate transformer for the relays. Not impossible by any means.

[Cygnus X1]

So here we are, somewhere between $20 and 1K.
The more I look, the more I want to just toss the UL OT, work with the PT and strip out the boards to build a completely different circuit into it.

It's a mess of interdependent circuits between the two channels!

I am used to looking at...say, an AB763.
Familiar territory.

I knew what I was getting into when I took this on and the owner has given me complete freedom (with a budget).

Great suggestions, I'm just watching...keep them coming.
Imagine it's yours to do whatever you want with.

[Ian444]

You don't have to toss the OT, just disconnect the UL windings and run pentode mode.

If the existing boards have more complexity than you are happy to deal with, then rip the board/s out and start afresh.

[Jackie Treehorn]

Why would you disconnect the Ultralinear taps? My D style is built on a Fender 75. I actually just adapted the existing boards. But my advice is to use the Ultralinear taps but not the global feedback, that way you get the advantages of negative feedback in the power tubes without having to wrap global negative feedback around the phase inverter. I would put 100k plate loads on the PI and a post pi master because it will clip very nicely. It's kind of a best of both worlds situation, having advantages of both negative feedback amps and non negative feedback amps. The other side of the PI also makes a great reverb return, so you can also get a really nice reverb.

The output transformer is 8k, so KT66's would be a good choice, but I've used E34L's with success.

[Cygnus X1]

Sounds good Jackie.
Do you have a schem, or point me to the right direction to study what I need to do?

I guess you left the normal channel alone and tweaked the drive side?

[pdf64]

The OT is excellent. The nominal primary is 4k to match a pair of 6L6GC with 2, 4 and 8 ohms secondary taps, though if an 8ohm load was connected to the 4 ohm tap, the reflected primary impedance would be ~8k.
Unless you can afford to fit sylvania STR387 it would be best to re-configure away from UL operation; because in UL mode the screens dissipate over 20 watts when overdriven and no current 6L6GC can take that for long. A friend killed off a pair of Winged C in less than 20 hours and they're supposedly equivilant to STR387, with a 500V screen rating.

I guess you left the normal channel alone and tweaked the drive side?

It's single channel. The 'lead' mode uses vactrols to switch in the 'drive control (which bypasses a big potential divider) and a second master vol. That's why it's not too hard to re-configure it as an AB763 reverb channel with a boost at the end. Pete.

[FUCHSAUDIO]

Remember: YOu can combine UL and screen resistors if you like. It will allow you to run UL with modern tubes, although it does hit the screen harder.