'70 twin mods

[Howard Palmer]

Hi All,
I've had a '70 Fender Twin for a few years now and recently got a '68 Dual Showman head that I like better so I've been using it instead. This led me to thinking about doing some tone mods to the twin. I've started doing some research on old Forum posts which are very informative, it's incredible how much information is available here, but it seems like what the best mods to do to the twin evolved over time. I read a lot of stuff about the Dan Healy mod and saw that a lot of folks tried it out but reviews were mixed.
I want to get reverb in both channels, tame some of the grainy high end issues, and get better control of the mids. The range of useful adjustment on the twin mid control is really narrow. I seem to always end up with treble around 3, the mids aroung 5, and the bass around 4 but am still unhappy.
Is there any consensus on how to modify the tone stack on these beasts for Pedal Steel? Is it a good idea to put the tantalum caps in per Healy?
Any help would be greatly appreciated.

[Tim Marcus]

Reverb on both channels is only a good idea if you move the two channel summing resistors to before the reverb input. Otherwise the two channels will interfere with each other, that will add more gain to the normal channel though and get you tremolo on both channels as well.

If you want to tame high end issues, ditching the small value ceramic caps in the preamp is a good place to start. Replace them with mica - vintage mica if you can find it. Also, you can replace the coupling caps because they might be killing some of your low end. There is a 10p on the reverb mix resistor and a 250p in the tone stack of each channel.

[Ken Metcalf]

A good thing to try for an old twin is new speakers and fresh power tubes.
Speakers can be tailored to suit your preferences these days.
I like a clear clean tone and have 25 watt greenback Celestions in my 67 Twin and put a 50 watt Weber 12F150 in my Deluxe for less break-up in the Deluxe.
I am very happy with the results using JJ power tubes.

[Stephen Cowell]

I tried mica (definitely vintage) and went back to the original ceramic for the 10pF... that one cap makes a huge difference to the amp, most of the signal goes through it, regard it as a 1/4" high-pressure hose. I tried a bunch of different 10pF caps, the only one that sounded good was the original Fender one. You can play with the 3.3Meg value some... this RC network really sets the voicing for the amp, at least the reverb side.

Another thing... you need a good tube in your first stage (gain hole). Trying to voice your amp with a crappy tube in there is worse than chasing your tail. Get several NOS tubes and swap them out so you're familiar with the differences between them... RCA, Mullard, Telefunken, Tung Sol are best... Phillips/Sylvania not as good IMO, still maybe better than current production. The difference a $100 Mullard makes is startling, if you can get one that's not microphonic. Tap on the tubes so you can tell if they're noisy/microphonic... listen for hiss and hum differences.

'Grainy' high end may be self-oscillation... the later amps had plastic wire, this wire is not as good as the cloth-covered stuff... the main pre feedback is set by a twisted-pair of wires leading from the input jack and volume pot... these can't be twisted as tight when plastic is used, that's why a lot of these amps have a 10pF cap bypassing the plate resistor. There's a reason the later Twins are not worth as much... I had a friend rebuild his using cloth wire and copying the layout of a BF Twin... the difference was magical.

Good luck seeking The Grail...

[Brent Torgrimson]

I can't speak to changing the tone stack, but there's a book called "Great Tube Amp and Guitar Mods" by Mike Thompson. It has all sorts of Fender amp mods... master volume, adding tone coloring stacks, pull the Bass knob for more bass, etc. etc. With that book + some stuff I found on the internet, I rewired my '70 Twin to (supposedly) be like a Dumble Steel String Singer (with some added extra stuff), and it works very well. That said, Dumble amps are overrated IMHO, and certainly way overpriced. Remember to ground the power filter caps - it's dangerous to open an amp & start dinking around because the power filter caps store a good jolt...