Vibroclone Project Considerations

Vibroclone Design Considerations

The Chassis
A Bandmaster Reverb (BMR), Super Reverb or a Pro Reverb chassis can be used for a Vibroclone. Although most would agree that a BMR chassis is preferred because a Super Reverb or a Pro Reverb have their own unique tone and characteristics. Thus most of us purist would prefer to leave these amps as is and play them for their unique characteristics. A BMR, like the Bandmaster and Bassman heads, are more commonly accepted for modification.

Fender manufactured BMR amps between the years of 1968 through 1980 with several circuit varieties. The most desirable years are the earliest years from 1968 to 1972, with 1968 and 1969 being the most desirable years. The earlier the year, the more similar the circuit, topology and components are to blackface era amps and thus the more easily these chassis can be converted to a blackface circuit and achieve the construction and tonal qualities of blackface amps. In 1976, a master volume (MV) was added to the BMR along with increased B+ voltages making these amps more challenging to blackface with diminishing blackface tonal results.
BMR Chassis Circuit History Summary
1968-1969 BMR amps used the AA768 circuit with a GZ34 rectifier tube. The AA768 circuit had an output tube balance control, was cathode biased and had grid to ground suppressor caps. However, wiring topology was very similar to blackface era amps. This circuit is very easy to blackface.

1969-1970 BMR amps used the AA1069 circuit with a 3-prong AC ground, 3-prong AC accessory outlet and a three way ground switch. These amps used the 5U4GB rectifier tube. Due to poor customer response to the AA768 circuit changes, CBS designers removed the cathode bias on this circuit but maintained the output tube balance control and ground suppressor caps. Wiring topology was still very similar to blackface era amps. This circuit is also very easy to blackface.

1970 - 1975 BMR amps used the AA270 circuit. Circuit is very similar to the AA1069 but as the years when on, CBS used less quality components, converted to all plastic insulated wires and deviated from the blackface topology. Circuit wiring became very messy with longer then necessary wires and poor wiring topology. These amps are a little more difficult to blackface.

1976-1980 BMR continued to use even sloppier wiring topology. CBS added a master volume in around 1976 and starting waxing the fiberboard to help reduce the probability of parasitic oscillations due to the lack of wiring topology detail. By far the hardest to blackface.
Output Transformer
The output transformer used on the BMR is a rather wimpy (low iron content) 4-ohm transformer. When building a Vibroclone to power a single 15" 8-ohm speaker, if the existing stock 4-ohm output transformer is used, there will be an output impedance mismatch causing a loss of power and earlier then desired breakup. A Twin Reverb / Dual Showman output transformer when used with only 2 output tubes and an 8-ohm load provides excellent tonal quality. Finding a used black or silverface Twin Reverb / Dual Showman output transformer is becoming more difficult. I've used after market Twin Reverb / Dual Showman output transformers from Mojo and New Sensor with very good results.

Faceplates
Most of us thus far are using the existing BMR faceplate. If you use the existing BMR silverface faceplate, use a black tolex cabinet with the silverface grill cloth of Blue White and Silver. At least this way, the over all cosmetic appearance would be consistent of the silverface amps. For alternative faceplates, Larry Rodgers is the only one reproducing Fender faceplates at this time..
Cabinet Considerations
When considering a Vibroclone cabinet, I would suggest, unless you are an experienced cabinet builder, that you consider cabinet builders Gregg Hopkins or Larry Rogers. Both Gregg and Larry build yellow pine finger joint glued cabinets just like the originals. Any other material like plywood and stables just don't hold up over time. A finished clone with a Twin Reverb output tranny and a JBL D130 or equivalent speaker is one heavy amp. So you need a really well built cabinet. Plus it's really hard to beat their prices. Gregg and Larry both offer vintage tolex and "aged" grill cloth for that "smoked on" aged look. The original Vibroverb came with tilt back legs which can also be purchased through Gregg or Larry for an additional cost at the time the order for a cab is placed.
Speaker Consideration
Original Vibroverbs used a variety of speakers from the easy to break up Jensen C15N to the clear as a bell JBL D130. When considering a speaker, you really need to consider what sound you're after. If you're after a soft tone with easy breakup then a C15N or equivalent speaker will do the job. If your looking for maximum headroom as in the early Stevie Ray Vaughan sound, then a JBL D130 or equivalent full range high power speaker is what you need. JBL D130 speakers can still be found but expect to pay $100 to $150 for an original cone. With an original cone, we are talking about a speaker that is on the order of 30 years old. So if you plan on cranking your clone and I suggest you do, the cone is likely not to last very long. So you better plan on having it immediately reconed or reconed sometime in the near future. With the cost of a used D130 plus the cost of good recone adding up to maybe $250, you might want to consider the new high power series speakers from Ted Weber. Ted modeled the California CA15 after the JBL D130 including the aluminum dust cover. Ted offers three flavors of his high power 15" series. Check them out at Weber VST.

Here are two different techniques for mounting a JBL with a Twin Reverb output transformer. The first is to off-set the speaker cut-out. The second is to center mount the speaker, but unique speaker screw placement is necessary. Both require the Twin Reverb output transformer to be off-set mounted towards the choke.
Example: BMR chassis date code 1969, Circuit AA768.
As you can see from the pic, this chassis is very similar to older blackface amps with respect to wiring topology and still used mostly cloth wire. Where plastic insulated wire is used, the topology is still neat and follows the blackface layout convention. A complete electrolytic cap job was performed and the tone critical "brown chocolate" coupling caps were replaced with Orange Drop caps. A blackface mod was performed and a new production Twin Reverb output transformer was installed. Amp was then retubed with NOS Philips 6L6GC's, biased and then thoroughly tested. One minor cap value was changed in the vibrato channel tone stack to tighten up the bass when driving a JBL D130 and walla, killer tone with the breakup characteristics of a blackface amp.
Example: Super Reverb chassis date code 1975.
Here's a late '70s chassis that was shipped to me from a player, John, out of Chicago, to be use to build a Vibroclone. This chassis was about the last year before a master volume was added. Note the use of thick plastic insulated white wires and waxed fiberboard. Again, the later amps makes maintenance and restoration work a little more challenging. Someone else had already installed a Twin Reverb output tranny, though it needed to be re-mounted to fit the use of a JBL D130. A complete cap job, blackface conversion and all other necessary restoring changes were completed including replacing a low B+ Hammond power transformer with a correct new production tranny from New Sensor. We used Philips NOS 7581A's and a mixture of NOS and new production preamp tubes. John elected to use a Larry Rodgers BF Super Reverb faceplate and a custom Vibroclone cabinet from Gregg Hopkins. So my job on this one was pretty easy. Just restore the chassis, change out the faceplate, re-set the output tranny and assemble the pieces. I organized the buying of the cab and faceplate. Clone turned out real nice. John had also sent me the JBL he acquired locally. It had been reconed, but broke up a little earlier then most JBL's, so the amp has a really sweet bluesy tone.
Example: Bandmaster Reverb chassis date code 1968.
Here is another example of a Vibroclone. This is a 1968 Bandmaster Reverb chassis purchased by Dennis out of Texas. Dennis bought the amp from a shop in Seattle and had the amp dropped shipped to me for restoration, Vibroclone conversion and some mods. Unfortunately, someone had install an old unmarked bogus power transformer with out a 5v rectifier tap, had performed a cap job using under voltage rated caps and performed a half baked Blackface conversion. So the first order of business was to get the amp to a healthy state. A new power tranny from New Sensor was installed and the under voltage rated caps were replace with the appropriate voltage value Sprague caps. Dennis wanted a dual channel reverb mod, a switch-able tremolo mod and sole control added for maximum tonal variability. We use one of Schumacher's new Twin Reverb output transformers. Dennis had Larry Rodgers drop ship a Brownface faceplate with cream barrel knobs for me to install. Dennis also had Larry make a brown tolex Vibroclone cab and is going to use one of Ted Weber's California series 15" speakers. When Dennis completes the amp assembly, he promised to send some pics of the final product which I will then post. In the mean time, here are some pics of the chassis work.