ShoBud Telestar amp

Dennis Bailey · From: Fort Worth Texas

Has anyone seen one of these amps?
Its a ShoBud Telestar custom. It has three 6l6 power tubes...don't know yet what the other tubes are. It has a 15 inch Standel speaker. And a fuzz knob that says
"Sho-fuzz"
serial number written on the back is 3042. The previous owner thought that it may be one that Jim Evans built.

Ken Fox · From: Nashville GA USA

Yes Jim built it. The 3rd 6l6 is for a voltage regulated supply for the screen grid voltage

The following is from my personal files from Jim Evans, one of many letters he wrote to me:

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.

Ken Fox · From: Nashville GA USA

Jim and I were very good friends. Years ago in Dallas he asked me to take over repairs for the older Evans amps. He shared a wealth of info via emails to me. I saved them all, they were so full of knowledge.

The following is a letter he sent to me that will tell you more history on the amp:

Hi Ken,
Excuse me for a moment, while I try to shake this "nonstalgia-headache". I suppose this proves that everyone, at least once in a lifetime, should plunder through their attic basement, outoor-barn or whatever, and look for paper-trail-evidence, that the things they think they remember really happened, and that it was in this life---even in this universe.
In this instance, the encouraging part was finding out that most of the things I've written to you the last couple of days were basically like I remembered them. The attached schematics, not unlike museum exhibits, all relate to the bulk of what I have told you by e-mail over the last couple of days.
The main schematic is a scanned copy of the exact original for Albert Talley's amp. You may enjoy grinning at the date shown (June 1962). Xerox copiers and scanners had not yet been invented, but pencils and paper had, so that was what generated this schematic. It shows that Albert was correct when he reminded us that this amp (prior to Sho-Bud days) was called a "Telstar", as the forerunner of the Compactra-100. The part I was correct in trying to remember was, that its basically the same schematic without an internal-sensitivity-control between V8-B and V4-A, to set the pre-amp gain. You might want to have this schematic in hand when you flips his amp over and look at the wiring.
The next interesting schematic in this attachment is the marvel of engineering, where Bob Crooks was ahead of anyone else I know, in using a triode phase-splitter (instead of the typical signal tapped phase inverter stage), and drove it with a high-gain pentode in the same envelope. This permitted a complete three-tube power amplifier, all within a single fedback loop. He was also running a close race for first, in regulating the screen-grid voltage of the output tubes rather than the plate voltage. This saved about five pounds of power-transformer iron and still gave a tighter performance. Last and not least, he used a fast-response pulse-tube of excellent linearity for the power outputs, while some skeptical designers ridiculed in ignorance (I was not one). Bob told me in person that he got this concept from his other R&D work in Industrial Electronics.
The other thing you will see here is the application of the 6U8, which was not originally made for audio any more than the 6DQ6's were. But look who jumped onto the technology: It was RCA, with their 7199 which now seems to have bit the dust. RCA also did a good job of incorporating a similar beam-power structure, for the linearity their 7027's. But RCA did not follow through, to use tight screen-voltage regulation, as Crooks had done. We did the same thing in a different manner with the 6L6 cathode-follower. For this reason, along with adjustable AC-signal-balance and DC-idling-current-balance, and additional negative feedback, the Compactra-100 actually outperformed what the RCA specs promised for this circuit.
The third interesting attached schematic shows the thing I mentioned, where Standel later (in the transistorized hybrid ) diverted to the dual triode (7025 or 12AX7) to replace the 6U8 Pentode/Triode. I've already "used up" my commentary-space on this.
Finally, for those who support the "big-bang" theory, attached also is the RCA schematic that was our "big-bang" for the Telstar/Compactra-100. The things we tried to do for upgrade are all obvious when you compare the two schematics.
So, back to my nostalgia-headache, I'm pausing for a coffee break. Check you later. JIm Evans

Ken Fox · From: Nashville GA USA

Dennis Bailey · From: Fort Worth Texas

Wow! Thanks Ken.
Thats a lot of information.
The normal channel works, but the tremolo channel just hums when a guitar is plugged into it.
The previous owner purchased this amp in 1977 and it was used when he got it. He did say that new tubes were installed a few years ago, but the caps are probably the original ones.
I'm going to put it in the shop and have it gone through.
Thanks for the schematic.

Tony Prior · From: Charlotte NC

wow very interesting amp !

Thx Ken for sharing details !
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Jim Goins · From: Michigan, USA

Looks almost identical to my old Evans compactra 100, I bought it used about 1974. Jim Goins.

Godfrey Arthur · From: 3rd Rock

Be nice to hear this amp.

Need a U2b upload for posterity.
_________________
ShoBud The Pro 1
YES it's my REAL NAME!
Ezekiel 33:7

Dennis Bailey · From: Fort Worth Texas

It really is interesting Tony! Especially the sho-fuzz feature.
Jim Evans was a really forward thinking guy to put this feature on
an amp of that era.
ok thanks Jim, It will be interesting to find out if this amp was the predecessor
to the Compactra or vice/versa.
Right now the amp sounds very thin and tinny...even with the treble turned down
and the bass turned way up, and the mid knob set to about 12 o'clock.
Perhaps new caps will improve the tone?
Its a very heavy amp..have not weighed it yet but its heavier than a Nashville 400.
That magnet on the speaker looks like it could pick up a small car.
OK Godfrey...will put something up on youtube when it comes back from the shop.

Joel McCoy · From: Georgia, USA

Hi Dennis. As an owner of two Compactra's I can tell you that these should NOT sound tinny with no bass. That amp of yours is an early one if i'm not mistaken. I would LOVE to hear that fuzz-tone. I wonder if it's like a Boss-tone? I run my Compactra into a 4x10 Marshall cab rated for 150 watts (model 1965A) and never want for tone...
Good-luck with an awesome and rare amp!
_________________
'64 Sho-Bud Fingertip D-10 9+1, Goodrich VP, Sho-Bud/Evans Compactra 100 Custom, Sho-Bud/Evans Compactra 100 Head unit, '75 Tele, '77 Guild D25, '71 YBA-1, Marshall 4x10 and a Les Paul.

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