(in excruciating detail)
I had a '62 Fender Bandmaster head that I'd been keeping around as a backup, but I've become hooked on the warmer brownface sound, and decided to make better use of this amp by converting it into a combo amp. The Fender Amp Field Guide describes a Bandmaster 3x10 combo amp with the brownface front panel, released for only one year in 1960, but they don't have a photo of this amp. Greg Gagliano's had written an article for TCG magazine that described the conversion to a 3x10 combo and it sounded like a good idea. I also received positive feedback from the folks on the Fender Discussion Page Forum. ( see the discussion thread here ), so I decided to give it a go.
I'd originally envisioned this as a poor-man's tweed Bassman 4x10, but eventually settled on a re-creation of the brown 3x10 Bandmaster combo released in 1960. So, I decided to post some photos of my combo conversion for anyone who might be contemplating a similar project.
|This is the Fender Bandmaster head used in the project. This amp was manufactured in 1962 with the 6G7-A circuit. It has a generic replacement 4-ohm output transformer (OT). One constraint I had given myself was to make only reversible modifications to the amp.|
|The combo cabinet was built by JD Newell to be a faithful reproduction of the 1960 brown tolex 3x10 combo. He was recommended to me by one of the helpful folks on the Fender Discussion Page Forum. The cabinet was very reasonably priced and he was willing to work with me to give me what I wanted.
Note: Someone has pointed out to me that the original combos had the three speakers arranged with two on the bottom and one on top. I recall now, that I remember reading that the original top speaker came with the bell cover removed to clear the chassis. This is how I specified the baffle to be made, based on a photo from a similar combo conversion. I may sometime look into switching the baffle, but it really doesn't seem top heavy to me like this.
|In order to get the cabinet quicker, I ordered it without glides, logo and handle. I installed these myself.|
|Here is the empty cabinet ready for loading with speakers and the amp.|
|Three new Jensen P10R speakers were used for this project. I tried the 'break-in' procedure using a 6.3V transformer as suggested on the Weber VST site, for about 12 hours. Did it help? I dunno. But it gave me something to do while waiting for the parts to come in :-)|
|Now, on to the amp itself. Three 8-ohm speakers in parallel gives a nominal impedance of around 2.6 ohms. This is not a significant mismatch for the 4 ohm OT, and it would probably be fine with this, but I wanted to try a 2 ohm Bassman style OT, to see if there was any impact on tone. This photo was taken when I had the Bassman OT patched into the circuit for testing with the 3x10 cabinet.
With this 3x10 open back cabinet, (to my ears) there was a noticeable improvement in bass response and clarity with the larger Bassman OT, so I decided to change out the OT.
|In order to avoid drilling holes in the chassis, I fabricated an adapter plate, as others have done. It is made of a scrap of thick aluminum plate with two 8-32 threaded holes that match the two mounting holes of the original transformer. The four 10-32 threaded outer holes match the mounting tabs on the larger Bassman OT.|
|The other modification I made was to replace the diode stack in the power supply with a Copper Cap rectifier from Weber VST. I wanted to tame the output a bit for these speakers and hopefully get easier breakup with the slightly lower output voltage. I got the version that supposedly emulates a 5U4. Installation was easy, since the chassis already had a cutout for an octal socket. All I had to do was slap the socket in there and move the AC lines from the power transformer from the old diode stack to the socket and install a wire from the output of the copper cap to the output of the old rectifier circuit. I left the old diodes in place, so that it would be easy to restore the circuit to its original configuration. The bias supply remained untouched. It would have been easy to install a real 5U4 (I've got several lying around) but I didn't want to mess with mounting a 5V filament transformer. Note that I also installed tube locks-- I felt that this was important, particularly for the heavy copper cap unit.
While I had the amp up on the "rack" I decided to take some voltage drop measurements. I took a 4-ohm THD Hotplate attenuator, throttled the sound down and turned the amp volume to 10. Then, I watched the B+ voltage level at the output of the copper cap, or the diode stack with a digital voltmeter. I played some chords through the amp and recorded the minimum voltage indicated. I know the impedance is a little off, but I did it the same way with the copper cap and with the original diode stack, to compare the two.
The B+ supply with the original diodes had a voltage drop of 26V, or roughly 6%, while the WU4GB copper cap had a drop of 70V, or 18%. According to the Weber VST web page, the WU4GB is supposed to have a voltage drop max of only 50V at a max current of 275 ma. I didn't measure current draw so I cannot attach any particular significance to the observed drop of 70V with my simple test, but the important thing I suppose is that the tube-like sag is certainly there.
From the Weber product literature, I expected a slow turn-on of the B+ supply, but in my case, it came on instantly, just like it did with the original diodes. Maybe the current is limited, I don't know.
Anyway, I like the way the amp sounds and that's all that really matters.
|OK, amp's done; it's time for final assembly. The two figures on the right show the completed combo amp. I was a little concerned, since the amp mounting holes in the cabinet were pre-drilled, but the fit was perfect. Speakers were wired up with cloth covered 18 ga. wire and a new F-plug installed. Original white knobs were also replaced with new Fender brown knobs.
As you can see from the closeup, the faceplate is scratched and bent in several places. If anyone has a lead on a replacement faceplate in excellent condition, drop me a line.
|I got a repro tube chart from Peter at firstname.lastname@example.org, who was very helpful in my decision as to which tube chart was appropriate. He also supplied me with the chart for my other Bandmaster restoration project.|
|Here is a nicer photo of the completed Bandmaster combo amp. This combo cabinet is a bit taller than the original but the Victoria Luggage Co. cover from vintage-amp.com for a Fender Concert amp fits it perfectly.
So, what did it cost? Total cost was about $650 (excluding the amp itself), which included (with shipping) the custom cabinet, speakers, handle, logo, glides, cloth covered speaker wire, F-plug, knobs, tube chart, amp mounting straps, tube locks, output transformer, and the Weber copper cap. The cover was an additional $80.
Bandmaster Combo Conversion
(Gratuitous gear porno)
Note: I no longer have this amp. It was replaced by a tweed 3x10 Bandmaster reproduction that I built from a Weber 5E7 kit.
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