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Showman Modification
- Thread starter Blue
- Start date
- Feb 12, 2006
- 60,984
- 5
- 220,753
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Adding preamp out and power amp in is not an easy modification because the operating levels are typically quite a bit higher than standard line level and the circuit impedances are also about 10x higher than what an preamp out/power amp in (or effects loop) typically operates at.
There are ways to do it, but it's not easy, it generally requires additional circuitry for impedance matching and level shifting down and then back up.
Now if you are just looking for a preamp out, there is an easier way. IIRC, the Showman, like many Fender amps of that era have a slave out jack which contains a signal padded to line level. This can be used to drive a power amp. This pad can be easily adjusted as needed if it's not quite right.
There are ways to do it, but it's not easy, it generally requires additional circuitry for impedance matching and level shifting down and then back up.
Now if you are just looking for a preamp out, there is an easier way. IIRC, the Showman, like many Fender amps of that era have a slave out jack which contains a signal padded to line level. This can be used to drive a power amp. This pad can be easily adjusted as needed if it's not quite right.
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- Feb 12, 2006
- 60,984
- 5
- 220,753
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Go try an Orange AD200B. Max out the Volume control and set your level with the Gain control. The sound and feel is very close to a blackface Fender and you get about 170W of clean power.
For the record I learned to play on a blackface Twin Reverb and currently own a 68 TFL5000D Showman Reverb for comparison.
The AD200B has a slave out that is driven of the output transformer. I have never used it, so I can't comment on how well it works.
This is my '67 Dual Showman (backdated to look like a '62) with a pair of E-V 15BX speakers. The only enhancements I used or needed were a set of Tung-Sol tubes, a MXR Dyna Comp and a Boss GEB-7 bass EQ that has a boost slider. It played some big rooms and stages, occasionally without FOH support and even with a pair of loud guitars and drummer, this rig was plenty loud and clean. I do believe that much of the magic was in the cabinet which was a huge old Peavey 215 slot port that had been Fenderized. The early 36x24x11.5 Fender cabs were not great for bass, even with D-140F's. I still have the head but it's into a 1x15 now.
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I'll certainly defer to your expertise, but I thought "FX In/Out" - typically after the first tube half (?) was different than "Preamp Out / Power Amp in", which would seem a logical "functional dividing point" in an integrated amp.
P.S.: No Slave Out this era (August '65)
I lucked into a tuned cabinet, so I'm also 1x15 D140. Interestingly, it sounds really different in various locations around my studio - I'm building a little dolly to mount it so I can easily move it around in here.
And for clarity: I have an "old blue" Alembic F-2B (heavily modded), and the driver for this idea was to be able to flex* that preamp into the existing showman power amp (4x6L6GC + Single D140)
The F-2B heavy mods include 5691 tubes in lieu of 12AX7, and one side Fender Tone Stack the other Marshall Tone Stack ... I like the Marshall side better these days, but that might just be novelty of it. Normally that preamp goes into a Million Watt QSC Class "D" then various cabinets. Variety - the Spice of Life!
* "Flex" ... means use it or don't use it, depending on the weather and my daily mood swing.
The F-2B heavy mods include 5691 tubes in lieu of 12AX7, and one side Fender Tone Stack the other Marshall Tone Stack ... I like the Marshall side better these days, but that might just be novelty of it. Normally that preamp goes into a Million Watt QSC Class "D" then various cabinets. Variety - the Spice of Life!
* "Flex" ... means use it or don't use it, depending on the weather and my daily mood swing.
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Sounds like you just need to track down a tube power amp. I would suggest stepping up the power a bit for bass. Consider a Mesa M180, which is the power section out of the D180. The M180 can be setup to run six 6L6GC or six 6550s.
This is one currently available: Mesa M 180 1980s Black | Bass Plucker's | Reverb
Thanks ... Turns out, I personally know Greg at Bass Pluckers ... but I'll probably pass on one more piece of gear. (if you only knew, lol)
Oh come on, give it a spin. At least then your curiosity will be satisfied
.I own 28 amps, so I probably have a good idea about why you are reluctant
.
- Feb 12, 2006
- 60,984
- 5
- 220,753
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Which has the same issue an I mentioned earlier. Interfacing with power amps can be inconsistent depending on the input impedance of the power amp. 10k input impedance loads load it down pretty bad, and open circuit it can swing 20V or more.
It's just one of the drawbacks to that architecture.
I heard ya,
Alembic shares your concern - see last paragraph:
--
The F-2B was inspired by the Fender Dual Showman amplifier. In the late '60's, we routinely added preamp output jacks to Dual Showman amplifiers and substituted an external power amplifier for the built-in quad 6L6's, most usually a MacIntosh 75.
Link RemovedThe F-2B came about to provide the same preamp functionality in a rack-mounting package, as the Dual Showman was considerably wider than the standard 19 inch rack.
Link RemovedWe took the opportunity to make a small improvement in the input jack circuit, while providing identical functionality. Plugging into Jack 1 alone gives full sensitivity, while Jack 2 alone is attenuated 6 dB. If signals are plugged into both jacks simultaneously, mixing resistors combine the signals and give isolation to the two input sources. In the original circuit, the mixing resistors are always in series with the grid of the first stage, and contribute a small amount of Johnson (thermal) noise. Our jacks have additional contacts which completely switch out the mixing resistors when using Jack 1 and the ultimate quietness of the tube is preserved.
Link RemovedAs in its predecessor, the instrument signal is amplified by the first stage before any volume control. Without the wide dynamic range provided by the 300 volt supply, the first stage could be easily overloaded by large transients which are characteristic of electric instruments.
Link RemovedThe tone control section follows, with a circuit attributed to Tom Walker, wherein the Bass and Treble controls contribute mostly boost and the Mid control provides cut only. While there is no absolutely flat position, the response is reasonably flat with the controls set at 2 - 10 - 2, with the bright switch off. The circuit can be described as interacting, such that the frequencies affected by each control is changed somewhat by the settings of the other controls. Additionally, when all 3 tone controls are turned to 0 then no signal is passed at all. These effects, while not suitable for a music reproduction system, have nonetheless proved highly pleasing for electrical instruments and have been adopted by a wide group of manufacturers.
Link RemovedThe Volume control immediately follows the tone control section. The Bright Switch connects a capacitor around the volume control, the value of the capacitor being selected to bypass only the high frequencies. The effect of the Bright Switch is also interactive with the Volume Control, and there is no effect if the Volume control is turned up to 10, and the magnitude of the effect changes, depending on the setting of the control.
Link RemovedThe signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used. While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.
Alembic shares your concern - see last paragraph:
--
The F-2B was inspired by the Fender Dual Showman amplifier. In the late '60's, we routinely added preamp output jacks to Dual Showman amplifiers and substituted an external power amplifier for the built-in quad 6L6's, most usually a MacIntosh 75.
Link RemovedThe F-2B came about to provide the same preamp functionality in a rack-mounting package, as the Dual Showman was considerably wider than the standard 19 inch rack.
Link RemovedWe took the opportunity to make a small improvement in the input jack circuit, while providing identical functionality. Plugging into Jack 1 alone gives full sensitivity, while Jack 2 alone is attenuated 6 dB. If signals are plugged into both jacks simultaneously, mixing resistors combine the signals and give isolation to the two input sources. In the original circuit, the mixing resistors are always in series with the grid of the first stage, and contribute a small amount of Johnson (thermal) noise. Our jacks have additional contacts which completely switch out the mixing resistors when using Jack 1 and the ultimate quietness of the tube is preserved.
Link RemovedAs in its predecessor, the instrument signal is amplified by the first stage before any volume control. Without the wide dynamic range provided by the 300 volt supply, the first stage could be easily overloaded by large transients which are characteristic of electric instruments.
Link RemovedThe tone control section follows, with a circuit attributed to Tom Walker, wherein the Bass and Treble controls contribute mostly boost and the Mid control provides cut only. While there is no absolutely flat position, the response is reasonably flat with the controls set at 2 - 10 - 2, with the bright switch off. The circuit can be described as interacting, such that the frequencies affected by each control is changed somewhat by the settings of the other controls. Additionally, when all 3 tone controls are turned to 0 then no signal is passed at all. These effects, while not suitable for a music reproduction system, have nonetheless proved highly pleasing for electrical instruments and have been adopted by a wide group of manufacturers.
Link RemovedThe Volume control immediately follows the tone control section. The Bright Switch connects a capacitor around the volume control, the value of the capacitor being selected to bypass only the high frequencies. The effect of the Bright Switch is also interactive with the Volume Control, and there is no effect if the Volume control is turned up to 10, and the magnitude of the effect changes, depending on the setting of the control.
Link RemovedThe signal loss of the passive tone controls and the volume control is made up by the second stage, also a common-cathode vacuum tube stage. The second stage plate circuit feeds the output jack. Since the output impedance of the tube circuit is fairly high, the power amplifier(s) should be located adjacent to the F-2B so that short connecting cords can be used. While the output circuitry of the F-2B can satisfactorily interface with the vast majority of power amplifiers, the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms or if the power amplifier has very low sensitivity.
Last edited:
- Feb 12, 2006
- 60,984
- 5
- 220,753
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Yes, I am very familiar with the principle guys behind Alembic, they are relatively local and were a part of the fabric of the industry that I grew up in.
We all have deep connections to the early pro audio industry, it was the Wild, Wild West back then. In many cases we made it up as we went along as there were no standards that applied to our applications.
We all have deep connections to the early pro audio industry, it was the Wild, Wild West back then. In many cases we made it up as we went along as there were no standards that applied to our applications.
I have a Bandmaster Reverb that Ive had "alembic mods" preformed on amongst other mods (Alembic mods are changing out a hand full of components making bass and middle control more responsive as well as a Preamp out). Though to be honest I use it more for guitar applications than I do for bass. Usually using preamp section of fender into an old SS bogen PWR amp into some JBL speakers a la Jerry Garcia.
It's been done, here's the proof.
Late 70's they were selling those 1x15 cabinets - asking $400 each - and while it'da been a fun talking point, I passed the opportunity.
I would think there's more than a few of them still around, somewhere.
Might be ... been literal decades, and they didn't have an infinite supply - iirc, there were two of those rigs - leap-frogged across venues until even that was too much "work" (or something) .. I saw it once and it was pretty cool. Say what ya want about the music, but their tone was always to die for, state of the art - especially the drums.
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