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Author Topic: Old 70's Randall Amp  (Read 1620 times)

g1

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Re: Old 70's Randall Amp
« Reply #15 on: October 11, 2018, 09:00:51 PM »

Have a look at this thread, I think the schematic should be very similar:

https://www.ssguitar.com/index.php?topic=4236.0
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galaxiex

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Re: Old 70's Randall Amp
« Reply #16 on: October 11, 2018, 09:34:55 PM »

Well if it's not a CMI product,
they at least sourced their chassis and PC boards from the same place.

Lots of other design similarities, front panel and knob type to mention 2.

Thanks for the pics. :)
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Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

that0neguy

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Re: Old 70's Randall Amp
« Reply #17 on: November 10, 2018, 11:25:43 PM »

Also, I bet those are Centralab pots.

Pull one and I bet it says Centralab on the bushing right under the threads.

Finally took out the chassis again tonight, pulled several pots and found that they are not, in fact, Centralab pots but CTS.  So good news / bad news...
- the good:  they are (hopefully) still made, or some equivalent
- the bad:  I still don't know what their specs are

I thought about just switching the pots from the left channel over to the right channel, but now I'm having second thoughts about that.  It's late, I'm tired, and I'll more than likely screw it up or do something else equally stupid.
« Last Edit: November 10, 2018, 11:33:01 PM by that0neguy »
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that0neguy

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Re: Old 70's Randall Amp
« Reply #18 on: November 11, 2018, 12:01:45 AM »

Have a look at this thread, I think the schematic should be very similar:

https://www.ssguitar.com/index.php?topic=4236.0

I took a better look at the schematic and found one part of the question, as in what type of pot(s) they are.  Unfortunately, no indication for the brand.

Also, the other dude that also had a Randall Comander I, where'd he go?  I wish he'd stuck around longer.
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g1

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Re: Old 70's Randall Amp
« Reply #19 on: November 11, 2018, 01:46:41 PM »

Also, the other dude that also had a Randall Comander I, where'd he go?  I wish he'd stuck around longer.
Click on his username and you will see an option to send him an email.  Worth a try, he was last here in 2017 so it's not that long ago.
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phatt

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Re: Old 70's Randall Amp
« Reply #20 on: November 12, 2018, 03:45:59 AM »

Don't over think it all, Many pots will fit and do the same job. 8)
As the amp has basic controls there are only really two possible pot types,
Linear and Log.
Take them out one at a time and measure them.
Linear pots will read half total resistance at half rotation.
i.e. A 100k pot will read 50k at half rotation while a 100k Log pot will read about 10k~20k at half rotation.
(Meter on the bottom and centre lugs)

So jot down which pots are lin and log then order some 24mm CTS or similar pots. By the pics looks like they are the old 1/4" solid shaft.
Phil.
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teemuk

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Re: Old 70's Randall Amp
« Reply #21 on: November 12, 2018, 11:05:04 AM »

Here's the deal with similarities...

Standel was sold to Randall Instruments, Inc. in 1972 but for a short while the company also continued its venture with the Standel amps as well. Robert Crooks held his position as president, but was also appointed vice president of engineering at Randall. His son was also an executive in both companies.

The cabinets, chassis, knobs, hardware, etc. of early Randall amps... yes, all those parts were pretty much sourced from Standel's inventory. Others, such as transformers, Don Randall had taken with him when he left Fender. They even kept "Standel-ish" cosmetic styling for a while including the "faux" wood panels. Standel largely designed and produced some of the earliest Randall amps (Alpha and Delta series), which were hybrid designs with 8417 tube -based output stages and solid-state preamps.

I would take a wild guess that these designs largely served as a basis for future Randall amp models that followed.

In 1973 Standel was eventually sold to CMI, or Chicago Musical Instruments (also known as Norlin Industries, which actually was the company that owned CMI). Most prominent company under CMI umbrella was Gibson, and Gibson happily adopted Standel's (solid-state) designs as part of their new "G-series" line of solid-state amplifiers and once again they were largely built using the same Standel parts inventory.

Another side venture of that were the "SG Systems" amps, which were mostly hybrids with guess what... 8417 tube -based output stages and solid-state preamps. Yep, another "Standel design". In difference to Gibson G-series amps these largely relied on FET preamp circuitry, which is actually very, very similar to circuitry you'll encounter in an early Randall amp: Same FETs used in very, very similar circuits and overall circuit architecture. Also, once again they largely relied on existing parts inventory, so hardware is similar in Gibson, Standel and (early) Randall amps.

Eventually, Standel operations were seized completely, Gibson stopped building amps for a while under Gibson trademark (I think because they moved their production plant to another state) and then had a brief side venture with "Lab Series" amps. The SG Systems amps on the other hand simply weren't a very long lasting product.

So, Bob Crooks eventually went to work for Barcus-Berry (where he pioneered the concept of "Exciter" products relying on dynamic phase shifting) and Randall simply hired an interesting character called Gary Sunda to take care of amplifier product design. Gary was a former jet propulsion engine designer from Lewis Labs and did all sorts of freelance electronics design (e.g. he had built the power supplies for world's largest "slot car" race ways) and operated an amplifier and speaker repair business. Since mid 1960's he has produced a series of transistorized "Sunda" guitar amplifiers (manufactured by Lewis Labs), which Randall had found quite impressive. After working a shortwhile as a freelance designer for Randall, Sunda was eventually hired on their payroll and became chief engineer. He is largely credited for tweaking the distortion circuitry of Randall amps, and in that he basically continued from where Standel had left. It is possible that Sunda also co-operated in design of those SG systems amps as well.
« Last Edit: November 12, 2018, 11:08:31 AM by teemuk »
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that0neguy

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Re: Old 70's Randall Amp
« Reply #22 on: November 25, 2018, 10:10:54 AM »

Teemuk, thank you.  That is a plethora of information I didn't know I needed, but definitely useful and wanted.  I've had this amp for years and, until several years ago, never saw another one remotely like it.  And to find info about it on the 'net??  It might as well not have even existed.  Maybe you should write a book on the early years of Randall... : :cheesy:

And phatt, thank you.  You've re-centered my quest to (hopefully) make this thing work. 

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