Sho-Bud Compactra

Dale Goode · From: Virginia, USA

That was a quick add. Thanks. Right up front, I am not a pedal steel player; but I like old tube amps. I recently traded for the Sho-Bud in the photos. I want to restore it but also want to know exactly what I have. Little info on these to be found but good comments in general. This one is SN 4083. May have been a prototype based on some of the construction techniques and since it does not always match the schematic (like 30K hum balance pot vice 200 on schematic). I powered it up on a current limiter and it works and sounds pretty good despite the musty smell indicating that it sat in a damp basement for some length of time. Anyway I want to bring it back to reliable condition and maybe install some upgrades. Any info on this amp will be much appreciated. Thanks. Cheers.

Ken Fox · From: Nashville GA USA

The first amp was called a Telestar. It may be closer to your amp

Cartwright Thompson · Posted 5 Aug 2022 2:45 am

Ken,
Could you tell me what the 6L6 does?

Ken Fox · From: Nashville GA USA

Voltage regulator circuit

Ken Fox · From: Nashville GA USA

The following is part of an email from Jim Evans many years ago. He was my mentor and engineering hero for sure. I have many emails from Jim that I have kept in the Cloud. He was way ahead of his time with the Compactra amp

I hope you guys enjoy this excerpt from Jim. We were discussing my possibility of building a super Compactra with four output tubes. I finished the design years ago but realized the cost and weight of such an amp would limit sales

I would also still recommend the easy-balance of triode as the phase splitter. For best results it would be driven by a pentode in the same envelope. Here we are back to the tube 7199, which was very forgiving to variable-circuit-parameters, that result from component drifts or voltage drops. Thje 7199 will also take a great deal of negative feedback in the power loop, without bordering on instability. If you notice from the schematic I used a single 56pf mica capacitor across the feedback resistor, and never even came close to parasitic oscillation.
Other pentode/triode tubes have been used for this, but I have never tried any that come close to the 7199 from my evaluation standpoint. I noticed on the internet that the 7199 was again being made.
My friend Robert Crooks (who created Standel) tried some of the other Pentode/Triode tubes, which worked fairly well, but then dropped back to using the same dual triode as his preamp tubes,(12AX7 / 7025) for both the driver and the splitter. This will work, but the gain of the driver does not permit the degree of negative feedback that I would prefer, to still maitain the desired gain of the power amplifier. Still I would consider this probably the next best approach, if the 7199 is not available or affordable.
Now in the phase spiltter, many engineers went to great extremes to use precision resistors, to get the AC signal-balance for the power tube grids. Even then, two other factors are not automatically corrected, namely unmatched output tubes and slight impedance-mismatches in the output transformer's center tapped primary.
What I did here was to use the old McIntosh technology of an AC-Balance trimpot in the cathode side of the splitter. In this way, the signal can be set for optimum performance by simply using an intermodulation-distortion meter to set the AC balance pot. This not only takes care of all the variables with one simple tweak, but also permits a "100,000-mile-tuneup" every year or so, to restore linearity as the components age or drift.
The next item that would merit my concern is regulating the screen voltage of the output tubes. This is far more important that regulating the plate voltage by some "super-designed" high-voltage main-supply. If you hold the screen-grid voltage constant in the output tubes, you can get away with all kind of "sags" in the main DC supply to the plates. In fact RCA engineers learned that it would take more than ten times the regulation of the main supply (lots of weight in the power transformer core, heavier windings, and much larger filter capacitance) to match simply regulating the screen-grid voltage. The technique I used for this came from a friend and master-engineer (Al Hart) in Chicago, who designed all the "Grommes" and "Precision: amps in the early Hi-Fi revolution. This was simply an "emitter follower regulator" which for convenience and affordable performance was a standard (inexpensive at the time) glass 6L6.
Interestingly Robert Crooks likewise regulated the screen voltage for the (TV horizontal sweep) tubes he used for audio outputs. But since these required only a regulated +150 VDC he used the same gas tube regulator as was used in tube TV circuitry. He and I not only exchanged some ideas, but in 1960 when I worked in Dallas in Industrial Electronics, I bacame his area service-rep, and repaired Standel amps for his Dallas dealer.
One more approach I used in the Compactra-100 ,for output balance, was the "DC-Balance" I picked up this technology from the upper-end Hi-Fi amps, built by Fisher.
The same argument can be presented here as for the AC-Balance: Instead of expensive precision resistors, simply use an inexpensive trimpot to balance the plate current of the two output tubes.
One final operating parameter that was important in this power amp was the reduced bias (around -35 VDC) that made the tubes pull slightly more idling current, and eliminated even the slightest trace of Class-B (crossover) distortion. This runs the output tubes at adling current of about 135 mA, instead of the typical 70 or 80 mA recommended by most manufacturers. If the output tubes had been standard Beam-power-pentodes like the 6L6 this would have reduced their life expectancy by 10 to 15 percent. But I trusted the assurance of RCA engineers that the 7027 was still just "coasting" at this idling current.
The negative bias supply was obtained by dropping the power transformer's secondary HV-AC with the reactance of a capacitor rather than dealing with the I/R (wattage-heat) drop of a power resistor. It was then rectified by a simple diode and filtered before being sent to the bias pot. In later amps like the Hybrid-300 I had the power transformers custom wound and employed a special lower-voltage winding for th bias. But the Compactra-100 was built from off-the-shelf parts, and quite often from the surplus market that had over runs, where I could schedule purchases in increments 50 or sometimes even 10 at a time. Bear in mind this was the early days, and I think dinosaurs may have still roamed the earth, at least figuratively speaking.

Dale Goode · From: Virginia, USA

Thanks Ken Fox for the schematic. Your schematics are more legible than any others I have found for Sho-Bud amps. I am slowly but surely discovering what I've got here. It seems to be a transition amp between the Telestar and production Compactras, but who knows? I am verifying all components and wiring and working on a layout drawing. It doesn't look like anyone modded it after the initial build, but it has extra circuitry connected to two unlabeled switches on the control panel. I've seen these switches on one other Compactra, but they appear to be an option one could ask for. It's turning out to be a bigger project than I expected, but what the heck. It'll be fun getting this amp cleaned up and fully working again and knowing how it works.

Dale Goode · From: Virginia, USA

...and thanks for all the other info. All these pieces are putting the puzzle together.

Cartwright Thompson · Posted 5 Aug 2022 9:27 am

Thanks Ken, fascinating stuff.
Too bad you can’t still get a 7027 that’s as rugged as an RCA.

Dale Goode · From: Virginia, USA

Well, not much interest in this post, but I thought I'd share what I found out about this amp and ask a few more questions. Based on the schematic, I'm still not sure if this amp is a prototype and changes were made to later units or if changes were made to this unit during production. Maybe a customer just asked for certain options / changes. Anyway, the first photo is a schematic of my Compactra along with some circuit additions (red dotted outline) I want to make. I numbered most discrete components to match with the second photo to come close to an illustrated parts breakdown (IPB). This may save some time for other Compactra owners / repair techs. The third photo is a kind of Fender style layout drawing I made to illustrate the purpose of the front panel switches. It is not a complete circuit layout. The fourth photo shows partial circuit diagrams in more detail than the schematic. I hope all these prove beneficial to someone.
I always like to give the designer / builder the benefit of the doubt, but I have some questions about the amp. I'd appreciate any responses.

1. Mr Evans did not use any grid stopper resistors on the input of either channel. I plan to install some as there appears to be a benefit and no downside. Thoughts?
2. All of the tube heaters are connected to the same PT winding, including the 6L6 regulator (cathode follower). I can see why this is potentially bad. Thoughts?
3. This amp has a 220VAC primary winding on the PT. Are other Compactras like this?
4. This thing is really heavy. Any reason why I should only preserve it in its original state? A head and speaker cab would be much more manageable.
Thanks and cheers,
Dale

J Fletcher · From: London,Ont,Canada

Well I think it's a pretty interesting thread. Seems like the amp is kind of over engineered for steel guitar use , lots of those ideas in the design are perhaps more useful in a hifi application. The notes from Mr Evans are most enlightening. He was a clever man who knew his stuff.

Dale Goode · From: Virginia, USA

Thanks, J Fletcher. I like getting my hands on innovative equipment other than the big name stuff. I think I hit the mark and am close to ordering refurbishment parts. Just a few more details to work out.
Cheers

Bob Lawrence · From: Beaver Bank, Nova Scotia, Canada

_________________
Franklin D10, Telonics (E9)True Tone(C6) pickups, Fender Steel King , Evans (FET 500LV), Nashville 400, Quilter MicroBlock 45 amp's, Telonics FP100 volume pedal, PodXT(effects only), Boss RV3(delay & reverb),Steel Guitar Black Box,Bill Lawrence cables, Walker Seat,Peterson Flip Tuner, IVL Steel Rider, IK Multimedia IRig Pro DUO for recording.

Bob Lawrence · From: Beaver Bank, Nova Scotia, Canada

If anyone wants a PDF version I can email it to you.
_________________
Franklin D10, Telonics (E9)True Tone(C6) pickups, Fender Steel King , Evans (FET 500LV), Nashville 400, Quilter MicroBlock 45 amp's, Telonics FP100 volume pedal, PodXT(effects only), Boss RV3(delay & reverb),Steel Guitar Black Box,Bill Lawrence cables, Walker Seat,Peterson Flip Tuner, IVL Steel Rider, IK Multimedia IRig Pro DUO for recording.

Dale Goode · From: Virginia, USA

Bob and all,
The complete schematic (with the missing right side part of the drawing) by Jim Webb and provided by Ken Fox can be found here, https://bb.steelguitarforum.com/viewtopic.php?t=378747&sid=ba99a4549ce325d11e43a5f2c3241a01.
Cheers

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