Gibson 60's optocoupler replacement with VTL53C

[OahuClay]

I'm going through a gibson GA-45RVT circuit, actually labeled a EA12-RVT in an epiphone cab but trust me its the 45RVT, de-tone shrilling it by removing those crappy "T"filters and general clean up.If I had worked on that assembly line I surely wouldn't admit to it.Anyhoo its got a bad sylvania optocoupler PL8224C, I'm sure it was standard equipment in 60's gibsonS and I want to apply a VTL-53C to it, anyone ever do this? If so where did you start with dropping resistors for the LED?

 

[Bob Womack]

I haven't done it but I recently picked up a GA-45RVT with a working trem.

I'd love to see the the T filters. Are they what is choking off the mids on the amp? The GA-55RVT (whi8ch I had in the '70s) had a mid control that had to cranked wide open to get a decent tone. I've compared the two circuits and see that there appears to be a fixed mid control on the GA-45RVT. Is that it?

Bob

 

[OahuClay]

It's a design error Bob, nicely outlined in a term paper by an engineering student, YOU CAN DOWNLOAD IT HERE https://courses.physics.illinois.ed...rrmann/Mark_Herrmann_P398EMI_Final_Report.pdf

The scan of the schematic is blurry in the report but you get the idea, he painstakingly graphs the freqency cuts of the filters before and after the mod, a simplified view can be seen on uncle Dougs you-tube channel where he de-filters a GA-35 and has a before and after video of operation.I havent checked but I would assume Gibson engineers wasted a lot of money replicating this mistake in many models, basically we need to take out 5 parts and install one cap and one resistor in about 3 places. I have noticed people finding components in the wrong place in some of them so keep an eye out. I feel the 60's GA amps have great potential, they just need a good hard tweak!

 

[tubelectron]

@OahuClay :

I did a similar change in my SF Fender Deluxe-Reverb, where I replaced the photocell by a VACTROL VTL5C1 with success :

Here is a quick sample giving the sonic result :



I do not know if this mod is applicable to the Gibson GA45-RVT though...

-tbln

 

[Bob Womack]

The scan of the schematic is blurry in the report but you get the idea, he painstakingly graphs the freqency cuts of the filters before and after the mod, a simplified view can be seen on uncle Dougs you-tube channel where he de-filters a GA-35 and has a before and after video of operation.

Fascinating. When I compared and traced the schematics of the GA-55RVT and the GA-45RVT I saw this little circuit and thought it had to be the culprit and the research paper and Doug's excellent video confirmed it for me. Now I've got a little job to do, eh?

Bob

 

[OahuClay]

Great Bob, and a note for anyone in the future reading this post, my amp is a 66 Epiphone EA-12RVT Futura but the Schematic for the 12-RVT is not the amp as labeled, mine is a 1966 production with the 45-RVT circuit, with 6L6GC outputs.For the reccord the earlier models with the 7591 outputs also had "T" Filters.

 

[OahuClay]

@OahuClay :

I did a similar change in my SF Fender Deluxe-Reverb, where I replaced the photocell by a VACTROL VTL5C1 with success :

Here is a quick sample giving the sonic result :



I do not know if this mod is applicable to the Gibson GA45-RVT though...

-tbln

It is but the Fender was designed with a higher voltage operating a NE-2 type bulb so i will need a larger dropping resistor, It's been done beffore, i was just hoping to find someone who already figured out the appropriate resistance for the drop I need for the 45rvt . Thanks for the circuit though, it will get used!

 

[OahuClay]

For others who may be trying to solv the LDR trem roach problem, someone retro engineered the gibson roach, which has an incadescent bulb and they are selling them on ebay for $20.