Premier 90 Reverb Unit

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CJ

CJ

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Joined
Jun 3, 2004
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nice antique on the bench today, kind of a noisy knockoff of the Fender 6G15 standalone reverb box,

weird, but i never seen this guy before, and right after i fixed one, another one came in, wtf?

kind of cool that the second one had a 6AK6 instead of a 6AQ5 which is seen on most schematics,

these things sound cool but feature 60/120 Hz cycle hum and reception from the local radio transmitter.
pr1.jpg
 
strange chassis bend which might be part of the reason it picks up radio real well.

the radio carrier manifests itself as distortion of the guitar signal,

pr3.jpg
 
carbon comp kludge with a cap mod showing, replacing the filter caps does not help much, so we had to do a complete rebuild, using a 12AX7a instead of the 6EU7 thereby utilizing the lo hum center tapped heater. that riveted terminal strip was done right over the paint which probably did not help the hum problem much.

notice the 680 ohm cathode resistor which differs from the 6AQ5 1K cathode resistor. this is how we knew that a tube swap was not the reason for the 6AK6, and also by the jumper from pin 7 to pin 2 which is not done on the 6AQ5 version.
pr4.jpg
 
this version had the virtual center tap heater circuit (the two 100 ohm resistors), the previous version we had in featured a transformer center tap for the heater circuit.

input jacks and pots mounted over paint, so we took some 220 wet or dry and made some good grounds even if that was not the grounding scheme attempted by the engineer.
the results tend to favor the improved grounds.
pr5.jpg
 
fastest way to rebuild these is to just rip all the parts out, this saves wear and tear on the tube sockets as trying to wrangle resistors out of the eyelets is a tiresome and destructive process, this is because the people who wired these things probably did mil-spec wiring during WW 2. This means tying a knot in the lead so that the circuit will still work without solder. this was done just in case somebody missed a solder joint so as to not bring the airplane down over enemy territory.

pr6.jpg
 
the 6AQ5 version had a red reverb knob, it is white on the 6AK6 version.

notice MIKE instead of MIC, maybe Mike was the engineer and wanted props.
pr8.jpg
 

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this is how we wired the first one, soldered resistors right to the chassis. worked like a charm, unit is completely quiet, which is 140 db different from the horrific and unusable box we started with.
i need to grow a longer arm so i can pat myself on the back.

notice this AQ5 ver. has no paint.
pr9.jpg
 
and this is how we wired the second 6AK6 version, terminal strips instead for more service ease for the next guy. first one we did had film caps, this one ceramic caps which we re-used, all CC resistors swapped out for film, as they were pretty crusty, 180K measured 220 K etc.
bridge rect top right.
12AX7a heater wired in,
pr10.jpg
 
here is somebody else's re-draw.

another mod we did was to make the screen of the 6AQ5 and 6AK6 not be at a higher potential than the plate. this was done by running the screen off the preamp B+ node. this also seemed to calm down the overkill reverb that was a little too much.premier_90_reverbunit_sch.pdf_1.png
 
here is a marked up schemo showing the hard to read mods we did along with some voltages.

the .001 off the first plate killed the radio station problem. and these things are real bright anyway, so it centered the tone also.

B+ went from about 130 to 159 with the addition of the bridge rect instead of 1/2 wave single diode circuit. and this helps with 120 Hz ripple also.
note that if you do this mod you will need to get rid of the cardboard multi-cap as it is only rated at 150 VDC.

this guy used a different approach with the diode ground and all that stuff.

we kept the 2 lead power cord as to not diminish resale value as some people are picky about such stuff. this guy dates back to the 50's i believe.
don't ask me about the fuse.

note lack of fuse.

link for the schemo below>
https://noisemikers.com/premier-90-reverberation-mods-schematics-and-impulse-response-irs/
these things go for about 600 on Reverb.

Premier-90-Reverb mods.jpg
 
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I’ve serviced a bunch of these, and also a bunch of thier weirder amps (like the suitcase one with organ toggle tone controls). Never had a single one that wasn’t noisy as hell. I have no idea why they fetch the prices they do, but if people wanna keep paying my hours to make them useable… who am I to
Say no? 😆
 
CJ said:
Exactly!

did you get rid of the 6EU7?

install a fuse?
Click to expand...
Always fuse and properly grounded cord. Nothing leaves the shop without getting as close to modern safety standards as it can.

i usually leave the tube arrangement as close to the original as possible- mostly cus that tends to satisfy the client who wants it “original” 😆

but i do swap in a full wave rectifier, and play with the grounding arrangement similarly to get the noise down.
 
I also tend to install the gnd loop suppressor shown above(diodes with resistor in parallel, I also add a parallel cap) on older equipment designed to intf with other line powered stuff. I've not had problems, but am curious if you guys ever had any? I'm not the amp police:), so don't take this the wrong way, but is it strictly legal in the U.S?
 
musipol said:
I also tend to install the gnd loop suppressor shown above(diodes with resistor in parallel, I also add a parallel cap) on older equipment designed to intf with other line powered stuff. I've not had problems, but am curious if you guys ever had any? I'm not the amp police:), so don't take this the wrong way, but is it strictly legal in the U.S?
Click to expand...
It's not technically breaking the safety/earth ground reference, which I've seen some people do, so I would imagine it's likley legal in the US. I don't know if it would pass a CSA sticker inspection, but hard to say. Those grounds would be on the "other side" of the transformer's galvanic isolation...

From a personal/business liability standpoint- not to mention morality and ethics- I actually have a standing policy that anything that comes in leaves meeting code as close as I can make it. That includes grounded cord with the ground lead to it's own dedicated threadlocked bolt, proper 'fuse first' in the hot line, removing the death cap, unpolarized 'accessory' plugs and dodgy old pushbutton breakers etc.. etc... I tend _not_ to install anything between my power/audio/chassis grounds- personally just tie all the ground terminations at the single chassis point. But I think you can implement something like the above and still meet codes.
 

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