Dual Showman Pre-Amp Tap mod:

Dual Showman Pre-Amp Tap mod:

Postby Pete B. » Fri Oct 01, 2010 12:54 pm

Hey Guys,
So like almost a year ago now I picked up a '70 Dual Showman Reverb Head.
It had a Pre-Amp Tap mod already installed (although I did not have it documented, it did work).
This mod includes one added 1/4" output for the pre-amp tap output, and a switch on the back that basically shunts the power amp section to ground (uhh, I think).
The pre-amp tap is always active, the switch turns on/off the power-amp section (or so it appears).

So check this out...
I gigged with it last night, and I ran a 2x12 Jerry style cab with two 4-ohm JBL-E120's.
I ran one E120 from the DSR Speaker-Out (100W Tube power, like normal, non pre-amp-tap'd use, a match for the 4-ohm DSR/JBL), and ran the other E120 from the pre-amp tap to Channel A of a MosValve 962 (160W bridged @ 4-ohms).

So I crank up the DSR thru the bottom speaker and dial in a tone with the volume in the 5-6 range (I use a volume pedal, or else that would be really loud).
Then, I throw the switch so the DSR speaker turns off, and bring up the MosValve power to the top speaker, and dial in a level (I ran the volume mostly in the 12-2 o'clock range, it also has a presence knob whitch I ran pretty low as the rig is pretty bright as it is).

Then I throw the DSR speaker back on, and get a good match between the DSR speaker and the MosValve speaker.

I experimented with Pre-amp volume and mix of the two speakers a bit during the first set.
At the beginning of 2nd set, I played like the first 4 songs with just the DSR speaker, then added the 2nd speaker as the set progressed.

That was a cool way to run the rig!
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Re: Dual Showman Pre-Amp Tap mod:

Postby Pete B. » Fri Oct 01, 2010 2:34 pm

One other thought...
One thing that was cool is that I was able to demonstrate to myself and a coupla gear head Buds in the house, exactly what the difference between a Pre-Amp'd Twin to Solid State power amp, and a Pre-amp Twin to Tube power amp, sounds like.
All other things were equal (same pre-amp signal, same speakers in the same cab).
I did a few blatent dial in, dial out, mix together, A/B tests to demonstrate during setup, and a little at break.
I thought the sound from the Tube power was a little richer sounding... The SS amp sounded very close and obviously has more headroom depending on the amp.
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Re: Dual Showman Pre-Amp Tap mod:

Postby tigerstrat » Fri Oct 01, 2010 7:13 pm

That's interesting Pete, doubly sorry I missed last night .

Semi-related, after years of powering my own DSR preamp and E120(8) w/ a SS power amp, and then just recently running them w/ the power section of my Boogie Mk III, I've now gone back to plugging in power tubes in the DSR (just the outer pair, for about 40-45w), and running it as a full head to drive the 120. I used to be able to tell the difference- maybe I just thought I could. :D
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia
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Re: Dual Showman Pre-Amp Tap mod:

Postby JonnyBoy » Sat Oct 02, 2010 12:28 am

This wasn't the first time you played through a preamped Fender twin/DSR ? I have to say I can't live without mine. I use to use a Mosvalve SS amp and moved to the Mac and noticed a great degree of warmth returned using that amp. Pretty cool experiment, and at a gig no less. :cool:
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Re: Dual Showman Pre-Amp Tap mod:

Postby Pete B. » Sat Oct 02, 2010 9:32 am

JonnyBoy wrote:This wasn't the first time you played through a preamped Fender twin/DSR ? I have to say I can't live without mine. I use to use a Mosvalve SS amp and moved to the Mac and noticed a great degree of warmth returned using that amp. Pretty cool experiment, and at a gig no less. :cool:


I want to get a Mac at some point. I haven't seen one blatently pass before my field of vision yet, nor purposely hunted one down (yet).
Because I have my other Head (Vibrosonic Reverb) modded to Waldo's JG TR spec, I have been using it (I too often run it with just the outer pair of tubes, this makes it want to see 16ohms though... A DSR with only two tubes would want to see an 8ohm load).
This was actually the first time gigging with the DSR, although I had it up and running at home since I got it (I had the power cord replaced to 3-prong like the day after I got it, besides that, no mods other than the pre-existing pre-amp tap).

The other side of the story is...
I strained my freakin' right Bisept muscle lifting the DSR!
It fuckin' still hurts! :mad:
Now I want that light two space rack with a SMS JG Pre and a Velocity 300! :hail:

This was an un-mic'd gig in a place where it's possible to be too loud with a Twin Reverb style combo amp (The White Eagle here in Portland).
I was thinking with the SS amp around 2 o'clock was pretty similar to the DSR power output, and was trying the mix & match thing with the speaker outputs.
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Re: Dual Showman Pre-Amp Tap mod:

Postby JonnyBoy » Sat Oct 02, 2010 10:17 am

+1 on the velocity and SMS. Jon S. really hit a home run in my book with that simple and compact set up. His tone is great too. You would love an MC50. I have a 250 also, but it needs a little love plus it is twice the size and weight. The 250 is great because it has 2 amps, or one to use and a backup, but the MC50 is so light so I prefer it and keep the mosvalve on backup.

My MC50 blew a fuse last night 30 min before show time, I have no idea what made me check the fuse first during the "What the hell is wrong" inspection phase. luckily I had enough time to get to a near by radio shack to find a 1.5 amp 250 volt fuse before we played. To get my Mosvalve to work I would have had to change the speaker connections from screw down posts on the mac to 1/4" jack for the Mosvalve. Not being prepared to change amps, I was wondering how I was going to do that without a soldering iron or another speaker wire.
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