Author: steve R

my twins

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(reposted from rtopiaradio with edits and amplifications)

(post in progress)

chapter 1 – the origin story

1991 Norco CA – I bought a 1977 Fender Twin (ser A768591) from some guy named Viv, who lived in a geodesic dome.

Date and time unknown – my friend Jim “Flames” Brennan bought a 1977 Fender Twin (ser A769024) from somebody in the wilds of San Bernardino County CA.

Both amps were initially serviced by Smokey “The Amplifier Man” Stover.  Both amps had the Pre CBS mods done. They were also completely recapped (including some caps replaced with polystyrene). Flames also opted for Smokey’s “Grunch” mod (which removed the master volume and tremolo and replaced the “intensity/speed” controls with “grind/crunch” controls) .

I’ve forgotten exactly how the “grunch” mod works on a technical level. I just remember that I wanted to keep that part of my Twin stock.

Flames used his Twin (the Flames Twin) until approx 1995 before selling it to a friend.

After nearly 30 years, a couple circuit mods and a few speaker and power tube configurations (KT88’s and currently 6550’s) it’s been my primary amp.

In 2013 I was able to buy the Flames Twin from the person he sold it to. The cabinet was already in bad shape when Flames originally bought it. The repairs that held it together for nearly 20 years didn’t survive the cross country journey (it appears most of the damage happened on on a crater infested section of I40 between Garth Brooks Blvd and OKC – that section of interstate has been improved).

chapter 2 – the reconstruction

The cabs were made from scratch by the same vendor that built my eurorack cases. Theyre solid pine with some joint enhancements (better than CBS did) to make them stronger.

step 1 raw
roundovers 1 (note: circuit housing stored on shelves)
roundovers and final sanding
rear view (note: half closed vs open)
waiting for cabs to be finished
“norco”
“flames”

Rocktron Vertigo Vibe Mod

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crossposted from rtopia.net (my gear building alter ego)

Got this used on Reverb ($35 – couldn’t pass it up)

Most of the reviews on the Vertigo were pretty favorable. The biggest complaints were “range of speeds via the rate control” and “noise”. Once I got it in my signal chain and worked with it for a couple of hours, I thought it’s attempt at the “univibe sound” was adequate and the ramping (gradual change between settings) RATE/FAST switch was a cool feature (The RATE/SWITCH will either go max rate or whatever the RATE control knob is set to. )

MODS PERFORMED:

  1. added variable control to FAST option on RATE/FAST switch
  2. extended range of RATE control (wanted a really slow sweep)
  3. true bypass
  4. replaced LEDS (red/green = zzzzzz)

Items 1 and 2 are documented below (presuming every modder knows how to do TB and part swaps)

This is the PCB and controls out of the enclosure

The switches are hooked in with connection headers (nice touch). I jumpered the ACTIVE switch so the circuit would always be engaged (rate always evident) on either side of the TRUE BYPASS DPDT switch:

While inspecting the board for circuit clues (couldn’t find a schematic) I noticed this diode installed on an LED pad. I was hoping this might be an indicator for the FAST rate. I installed an LED and it didn’t illuminate. I left the LED in (acting as a diode). It didn’t affect the tone/operation of the circuit. It’s interesting that Q7 is facing the LED pad. I wonder if there’s any chance there may have been an attempt at doing a light sensor configuration?

This is the back of the PCB opposite the RATE/FAST switch header (note: pads for LED to the right of the switch pads are tied into LFO section). I removed R57 (10K) and replaced it with a 25K pot (wired as a variable resistor)

To extend the range of the RATE control I replaced R24 (2K) with a 500R resistor.

(note: excess solder flux on the pads in the comb filter section were already there, perhaps these were post run adjustments made at the factory)

RESULTS: worked! (with some conditions)

The FAST control (small knob between foot switches) and the RATE control knob interact with other. Adjusting one will effect the setting of the other. Starting with FAST control knob and setting an initial speed, then going back and forth between the two until I got a FAST vs SLOW balance that I could use didn’t take very long to dial in. Another benefit was being able to dial in more DEPTH for the FAST setting.

With TRUE BYPASS installed, the LEVEL control will always be set to MAX (I also tweaked the feedback resistor on the output buffer to give it a bit more gain)

I attempted to do some tweaks to the comb filter section (changed C22 from 10uf to 1uf) but it started to sound a little phasery (changed it back). I’m gonna run “stock” for a while, analyze a bunch of different univibe schemos and maybe go back in for some tweaks.

In the meantime – I got a day of “Machine Gun” and “Bridge of Sighs” to get to : )