changing slope resistor on reverb channel on 135w twin

Joe Shelby · From: Walnut Creek, California, USA

I've heard that doing this will modify the tone stack
to give you that 750-800kHz cut.
Has anyone done this? Are there any downsides? What
value components are needed for this mod?
I've been thinking about having it done, but would appreciate any feedback from those who've had it done
or those that simply have their own ideas about it...

Joe

Brad Sarno · From: St. Louis, MO USA

Joe, a bunch of us have messed with this tweak, but a good number of us then return to the stock value of 100k. The deal with the midrange cut is that it's center frequency moves around in relation to the treble setting. If you set your treble around the common 3.5 or so that many steelers use, then you will have a dip centered right around 800Hz. If you tend to have your treble set higher than that, then you could experiment with slope resistor values less than 100k. People were trying 56k, but that's a pretty thick, almost muddy sound unless you really crank the treble pretty high.

Brad

Jon Light · From: Saugerties, NY

I've messed with this too----mainly just because I could. I'm a hack. The problem is, with the time it takes for me to do even a simple job like this, I can't possibly A/B the 'after' with my recollection of the 'before'. I can only say that maybe it was a useful mod. And maybe not. The kind of thing that you try to talk yourself into believing that it was a cool thing to do.
I value ears (and know-how) like Brad's, big time.

However, FWIW, it's a real simple job:

The one to the right is CH1, to the left, CH2.

And be sure that you know and understand all warning re: safety inside these things and the lethal potential of undischarged caps.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

I personally always do the 1 wire mod to add the reverb to BOTH the Normal and Reverb channels first, and leave the old Reverb channel unmodified, and tweak the 'normal' channel, so I have the best of both worlds.

ANYWAYS with that being said, in addition to changing the slope resistor from 100k to 56k, try changing the Bass and Mid caps (.047 and .1 if I remember) to 0.02uF caps for each.

With the traditional all knobs at '7' setup, (7 is where the mids woudl be on Fenders w/o a mid knob-- on twins I like it at about '5'), the stock dip is like at 450hz, with the stock caps and changing to 56k slope, the dip is at about 600hz, and with the changed slope and caps, the dip is at about 800hz, and the tone is thicker and fatter, BUT isn't muddy, you retain the mids where you need them, but cut where you want. -- the difference in tone with the full mod would be like changing the stock 12" speakers out and putting in a 15" speaker ...

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Of course, it's just personal taste, but I think they sound far better with a higher value slope resistor. I've got a 150k in mine, and it gives a nice full tone with the mids all the way off.

Pete Burak · From: Portland, OR USA

I have the 56K in channel 1 (non reverb).
Works great for steel assuming you, like me, want to modify your amp so that you can run the treble at 10 with the bright switch on, and not be too treble-ee.
I jumper Channel 1 to Channel 2 (running through a phase inverter to get 'em aligned). I can get more than enough treble out of channel 2 if needed.
I think of the 2 channels more as bass and treble than non-reverb and reverb.
All knobs/tubes/etc function to shape the steel tone and volume.
Mine is a 76 Vibrosonic.

Joe Shelby · From: Walnut Creek, California, USA

I typically run the treble (no bright switch) 4.5, but recently went to George L's cables and found sometimes I'll take it down to 3.5.
I'm with Donny on the mid setting; 2.0, and feel I could cut it even more to bring out the growl.
Bass usually runs in the 4.5 to 5.5 area and of course all these can vary depending on the room...
Lots of interesting ideas here, thanks to all. Very Happy

Joe

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

I've messed around with the tone stack on a few SF/BF amps. Frankly, I think Leo got it just right for a Fender guitar. But sometimes it's nice to have some options, so we generally put reverb on both channels, modify the normal channel, and leave the vibrato channel stock, as Gino mentions. My Deluxe Reverb normal channel is modified to give more midrange, let's say for a Les Paul. But the vibrato channel is left stock to give the classic Fender clean tone.

For PSG, I think I'm with Donny on increasing the slope resistor for a bit more clarity in the mids. My Dual Showman Reverb has reverb on both channels, a slightly higher slope resistor on the normal channel, but again is left stock on the vibrato channel.

Of course this is purely according to taste. The nice thing about these old Fenders is that they're pretty straightforward to work on - with reasonable care and patience, you don't need black-belt soldering skills. But heed Jon's advice and don't electrocute yourself if you wander in and start mucking about.

Ken Fox · From: Nashville GA USA

I like to do the same, reverb mod on channel 1 and a 56K slope resistor

Donny, the the 150K sounds interesting. It would be fun to try a pot in the stack there to vary the mid slope resistor form 56K to 150K or so.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Ken,
I have done exactly that! a 100k lin. pot in series would give you that sweep... the only drawback, is that that pot will see DC voltage, and can be a bit scratchy no matter what you use.... but it IS a fun experiment..
.. also try using .02uf caps for both bass and mids sometime, with the lower reistor, I think you will be pleasantly surprised-- it makes a vibroluxe with 10's bloom better than a twin with 12's!

Joe Shelby · From: Walnut Creek, California, USA

As I was reading through the posts here, I was wondering about the possibility of a pot. to vary the
mid slope resistor (Ken and Gino). So, even when you
set that pot. at a specific point, you would still have static?
In some past twin reverb threads there was mention of
running the reverb to channel 1, and making you're experiments there, leaving the vibrato channel stock.
That makes a lot of sense.
Even if I learned to properly discharge the filter caps, my soldering skills are somewhere above bungie
cord (Dave M.) level. The good side is that there is a fine tube amp tech (Skip Simmons, for anyone here
in No. Cal) about an hour's drive away.
I'm learning a lot here.
Thanks to everyone. Cool

Joe

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Static> when it's set, its set, and no static, youd' only hear it when turning the knob (think scratchy volume pedal pot). HOWEVER, where that resistor is located in the signal chain is after the first stage (pretty early on), so theres a lot of noise that would be amplified...you COULD minimize it by tweaking at lower volumes, since the volume pot is AFTER the tone stack in a Fender... you have to realize, that we are not talking a volt or 2 here, we are talking about a full 250+ VDC! that is now on that pot that you are turning with your fingers... whereas other pots, such as the volume pot, and tone pots are isolated by blocking capacitors....

... you COULD always add in a large cap paralleled by a smallish cap in FRONT of the tone stack to block the DC and let MOST of the signal through, however, I knwo that will change not only the tone of the amp, it WILL change the behaviour of the tone stack.

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