1967 Dual Showman Resurrection

[RB]

I acquired this amp about a year ago. It was a total basket case. The cabinet was bare wood with the finger joint glue broken making the cabinet shall we say, very flexible. There was no hardware or front and back panels. The amp appears to have been manufactured in the first half of 1967. I was slogging around Viet Nam on my first tour when this baby was born.

The chassis had no tubes. It appeared that the amp had been dropped on it’s top. The chassis was bowed in from the weight of the transformers. You could just get the 6L6s in but they were leaning so much from the bent chassis that they touched the power transformer. It must of fallen from a considerable height.

The first challenge was to figure out what I was going to do with the amp. I wasn't sure if I would restore it to original condition or keep it as a testbed for experimenting with UltraPhonix mods. I had moded a Twin to Hotel Hog specs found on the Dumble section of this forum and was really impressed with the outcome. I wasn't exactly sure of all the details of the UltraPhonix mods so I was a little hesitant but I figured they could be reversed if it didn't work out.

I regret not having taken before pictures. I never intended to post anything about the amp but everyone that has played the amp has liked it so much I thought I would describe the process.

I started by puting the chassis in a hydraulic press and getting the floor of the chassis as straight as possible. It required various arrangements of metal and wood blocks and locating ideal points of pressure to get it back to something looking like normal. Having accomplished that I was on to the circuitry.

The filter board was rebuilt using F&T caps and metal oxide dropping resistors. The bias board was also rebuilt.

The phase inverter was rebuilt and rewired to Tweed Bassman/Marshall Blues Breaker values and fitted with a JJ803 long plate triode.

The preamp stages are variations of the Dumble Bluesmaster circuit. The coupling caps are all Orange Drop SP6 polyester. Resistors are a mix of metal film and carbon films used in the usual Dumble locations. I didn't use any local negative feedback in this amp. All cathode bypass are TVA 25vdc.

The Normal channel tone stack was configured with 250pf treb, .02 mfd Mid, .02 mfd Bass and 33k slope. The bass pot was changed to 500KA. I did not add a mid pot to this channel.

The Vibrato tone stack was configured with 250pf treb, .02 mfd Mid, .02 mfd Bass and 56k slope. The midpot was changed to 25K.

The Vibrato intensity pot was changed to a pot with a switch so the Vibrato could be disconnected from the circuitry to get some gain back caused by the loading of that connection. The remaining 12AX7s are all Ruby (Chinese). The 6L6s are a matched and burnt-in quad of JJs.

The Normal channel is the most open and chimey. The Vibrato channel is also great but more focused and mid forward (I think because of the 25K mid pot). Both have their strengths. I didn't think they would be so noticeably different.

I think most Fender owners would agree that the volume on 4 and a little above is where the magic and singing begins with these amps. This is true with this amp also but it will hold together a few more notches up the dial and sustains like crazy. It takes pedals really well also. I think I got close, actually very close to what I hear in the UltraPhonix YouTube demos on YouTube. I am not sure I have the correct or the complete formula for UltraPhonix mods but it's close enough for me. The functionality/tone of the amp turned out way above my expectations and all who have plugged into always give you that look after they played for a few minutes. You know, that “what the hell is this” look.

[shupe13]

Nice.

Sent from my REVVLPLUS C3701A using Tapatalk

[dorrisant]

Sweet! I'm hoping others will. Comment on the circuit relative to the UP. Let us know if you have any clips.

I reworked one of the 135w Twins into a Hotel Hog a while back. That amp was really sweet as well.

[martin manning]

Great work. Nice to hear of an amp brought back from the edge. I too would like to see a more organized collection of the D mods. Maybe somone will take that on.

[Stevem]

Nice body work on that chassis there!
I bet your happy you put the needed effort into its resurrection!

A few years ago I picked up a 67 Bassman that had been left out in the rain over the Summer due to a bad PT, left out in the rain OUT of its cabinet mind you!!

It got very rusty on the inside and lawn mowing dust and grass clippings covered the inside, but I cleaned it all out , used automotive rust converter on the chassis , replaced the PT with a low voltage Marshall for a JTM 45 that I had on hand and the darn thing worked, no socket arcing or board arcing , I was amazed!

[M Fowler]

I have saved bent up old Bassman's and Pro's bending the chassis back into shape using a ratchet clamp that pushes out instead of pulls in.
Not sure how these amps get such bent up chassis.

Mark

[Paul-in-KC]

Great that you saved that bit of musical history. And made her better than new.

That is fantastic. Wish I could hear what she sounds like.

-Paul

[pompeiisneaks]

I have saved bent up old Bassman's and Pro's bending the chassis back into shape using a ratchet clamp that pushes out instead of pulls in.
Not sure how these amps get such bent up chassis.

Mark

I think just due to the sheer weight of the transformers, any kind of significant impact can cause major torsion on the chassis.

~Phil

[M Fowler]

Yes the transformers seem to be bent on some but I've seen new amps with leaning transformers

[Stevem]

I should have taken pictures of the new Ampeg SVT that I had to look at last week at work!
It was dropped in shipment and due to the way they mount the transformers now with just nutcerts the OT fully broke loose , ripped the leads out of its primary side, took out all 6 6550 output tubes and the 3 preamp tubes on the lower chassis , quite a mess to say the least, but that's what takes place when a 28 lb Chunk of iron rolled around , lol!!

[Stevem]

I should have taken pictures of the new Ampeg 320 watt SVT that I had to look at last week at work!
It was dropped in shipment and due to the way they mount the transformers now with just nutcerts the OT fully broke loose , ripped the leads out of its primary side, took out all 6 6550 output tubes and the 3 preamp tubes on the lower chassis , quite a mess to say the least, but that's what takes place when a 28 lb Chunk of iron rolled around , lol!!