Ampeg Reverberocket R-12R

[dkirk]

I recently purchased a cosmetically mint Reverberocket, S/N 001478, from the original owner. There is no audible output from the speaker when amp is powered up - no hum - no reverb crash when amp is rocked. Pilot light works and all tubes glow normally so heater circuit is working. Tube compliment is all original factory equipment.

I plan to replace the power supply filter capacitors as standard practice, but would anyone be able to pinpoint the problem area in the R-12R circuit just from this problem description?

[Hangman]

It might help to have a look inside the amp. Another forum member had a similar problem and found that the screen resistor was measuring way too high.
Did you measure the D.C. Resistance of the speaker? If the speaker is blown that would certainly cause your problem.

[dkirk]

Dear Moderator,

Thanks very much for the tips. I'll check both items you mention and will let the forum know what I find.

[Hangman]

Dear Moderator,

Thanks very much for the tips. I'll check both items you mention and will let the forum know what I find.

I meant that you should post a photo of the inside of the amp.

[dkirk]

Was out of town for a few days but now back home and proceeding with my Reverberocket repair.

Measured the speaker resistance with my VOM and got a consistent 1.0 ohm resistance. Also did the 9V battery test which generated an audible "pop" and cone deflection, thus speaker tests good.

Took pictures of the amp internals and these are shown below:

   

   

   

   

I'm certainly no amp tech but I have replaced filter caps on my Fender amps and have read several books on tube amp theory and repair. With a little coaching from those with more experience, I'm hoping to get this nice amp operational again.

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The tube chart is interesting, to say the least! Note the alterations made via pen to the printed chart. The actual tube compliment corresponds to the pen-written description. Pre-amp amp tubes are 12AX7, 12AU7, etc., and are the original GE (AN) tubes, along with the 7591 power tubes which are Ampeg branded. As stated before, I know the original owner and he never had this amp serviced. Thus, I'm assuming the tube chart was altered by factory techs at the Ampeg factory.

Another interesting observation - the model appears to be an R-12-R-M...another pen revision. I'm assuming the -M designates the use of 12AX7 tubes versus what was originally printed on the tube chart?

The rectifier tube is metal-jacketed and hard-wired and firmly attached to the chassis - have never seen this before.

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Here is a picture of the entire backside of the amp with cover removed. Speaker date code indicates a manufacturing date of December of 1964. Former owner purchased the amp sometime in 1965 so this checks out.

   

[dkirk]

Still plodding ahead on my Reverberocket restoration. Chassis is out of case and all tube sockets cleaned and capacitors checked. First step is to replace the two large 20 mfd caps located on each side of the chassis - checked values showed some drift. These caps are on order from Antique Electronic Supply. The large can (combined) cap, which I first thought was a rectifier tube, actually checked ok regarding capacitance values, with very little drift. A few other caps are questionable but I'm progressing in a step-wise manner due to my amateur status as an electronics tech. There is no evidence of any resistors overheating, all components visually look ok, and all solder joints look good.

According to the chassis date code of 001478, this amp was manufactured in January of 1965. Everything is cleaning up nicely with the chrome plating on the chassis being free of any defects...truly a beautifully made amplifier.

   

   

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A couple of additions - I measured the speaker resistance with leads disconnected and got a consistent 8 ohms dc.

This amp has a solid-state rectifier. Circuit is designated R-12-R-M and is very close to the R-12-R-T schematic.

[dkirk]

Replaced both the 20 mfd filter caps and powered amp up on the bench. Still no output signal to speaker with volume control at 100%. There is a faint pop at the speaker when I turn the power switch off.

Here is the schematic that I found that most closely matches what I have:


  Ampeg-Reverb-Rocket-12RT-Amp-Schematic.pdf (Size: 20.62 KB / Downloads: 10)

Moderator previously commented on another forum member with a failed screen resistor. Could someone please point out the screen resistor(s) on this schematic so I know what to check next?

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Checked AC voltage into rectifier - 288V. Checked DC voltage out of rectifier - 380V. Do these values sound correct? Does anyone know of a published diagram with checking voltages at specific test points?

[dkirk]

Here is the schematic for the R-12-R-T, which is the closest I can find to my R-12-R-M, converted to jpeg for easy viewing:

   

Note this shows 7 tubes??? What gives here?

Starting at the power tubes, and referencing voltages from an RCA tube manual, I'm finding the following plate voltage:

Pin 4 - 378 VDC
Pin 3 - 375 VDC

This is the same for both power tubes.

Moving to phase inverter (12AX7), with amp idling (no input signal), I'm only reading .3 VDC at pin 1, either with tube plugged in or removed. According to manual, this should be several hundred volts. Does this sound normal? Comments please?

[Hangman]

dkirk,
I apologize for my radio silence, had a family emergency and wasn't checking the forum.

one thing about old ampegs, they rarely match the schematic. The owner Everret Hull was infamous for doing custom builds for well known jazz musicians, and if the circuits were constantly in flux. Your amp does seem to be a very unique example of this.

The Joe Piazza schematics are awful. Don't use them. The schematic you have shown only uses 6 tubes, but two of the tubes are triple triodes. which means you have two unaccounted for triodes. This couldn't be very similar to yours. I've added a new schematic under the docs section "R12R Reverberocket"
The first one should be pretty similar

I would focus on the fact that you have no plate voltage on your phase inverter. can you measure any voltage on the other side of the plate resistor?
what about power supply point C?

[dkirk]

Hangman,

Not a problem - wishing you the best with the emergency and hope the outcome was good.

I got the impression that circuit design was constantly changing at Ampeg, as I can find nothing in the literature on the -M version of the Reverberocket. I do like the fact that the -M set up for the easy-to-get and inexpensive 12A-type preamp tubes.

I will cease using the Piazza schematic as, even though neatly drawn, it was confusing. Thank you for updating the R12R schematic in the docs; I will refer to this from now on.

Lack of inverter tube plate voltage was a bit disconcerting. I'll check for voltage on other side of plate resistor and then look at point C in the power supply and report back. Thank you for the valuable guidance.