Ampeg Rocket

[dwhutchens]

A guy I work with was given this amp after his uncle passed away. He had replaced the rotted power cord but was afraid of it because it hummed and smelled hot. I found out about it after he had given it to his brother who was going to strip it for parts or something crazy. I convinced him to get it back, I was going to try to buy it from him but then he wanted me to see if I could fix it for him. I plan to replace the electrolytics and the power resistors. The schematic in the cabinet looks original but is for a Mercury and don't quite match the circuit. The tube chart has M-12 printed on it which has been marked through with a pencil and R-12 penciled in. The input jacks have been rewired and the 47k resistors on the jacks replaced with 1meg. The 25uf cap on v1 pin 3 has been changed to a 50uf, otherwise the circuit matches the R-12 shcematic I found online (if anyone has a more legible copy let me know).

Does anyone have any idea when this was made, I'm guessing late 50s to early 60s. Any ideas on the value. Also does anyone know what the black cylinders on the rectifier socket might be.

Thanks,
David

[Enzo]

Looking at the Mercury drawing, I'd say those are the three 1 ohm resistors in parallel in the recto heater circuit. Does that fit?

This is a simple amp, just how different from the Mercury is it? The R-12 you show has no trem circuit.

When I face one of these with no clear schematic, I get a blank sheet of paper and a pen and draw my own. You can pretty much draw the power amp, is it not more or less like any other power amp? The phase splitter takes one triode or two. And there are only two dual triodes to begin with. DOn't see why a half hour wouldn't give you the drawing you need.

[dwhutchens]

Isn't v1b section the trem ciruit, r3a is speed and r3b is intensity. If I remember correctly the black things go from pin 2 to pin 7 then a wire runs from pin 7 to one side of the 1k 10w power reisistor, maybe some kind of overload protection for the power supply? My only problem with drawing out the schematic is I am partially color blind and I can't make out the color codes on those old, dull color bands on the resistors. I'll see if I can get my brother to help with that. What effect would changing the 47k input resistors to 1 meg other than lowering the sensitivity of the inputs, also the 470k has been removed from j3.
David

[MWJB]

Speaker dates the amp to 48th week of '57. Yes one half of V1 is the trem circuit. These amps don't typically draw a high resale value, maybe 350-400 IF you had a real keen buyer. Even when recapped, these style of Ampegs can hum like crazy - I do not recommend replacing the electrolytics with a can cap, use discrete caps and ground the preamp cap to the input jack, the screen & B+ caps can go to the main chassis ground. You might need to separate the linked up preamp grounds on the board and run individual wires to input jack ground.

Measure plate voltage (pin 3 of 6V6) and cathode voltage (pin 8 6V6), this will tell us whether the amp is running abnormally hot.

With the discrepancies between the 2 schems, better shots of the rectifier socket and input jacks would make things clearer (Enzo has already answered the recto Q, please recheck your hookup description).

Subbing the 47Ks on the R12 schem would greatly increase series resistance at the jacks and hurt fidelity. Removing the 470K would leave the 5.6meg resistor to provide the grid load...maybe previous owner was a harp player?

[dwhutchens]

Ok, I studied the rectifier wiring tonight and the transformer filiment windings go to pins 7 & 8 of the 5y3 socket. The 4 black things jump pin 7 to pin 2 so they are in series with the filiment as shown on the Mercury diagram. A wire goes from pin 8 to the can cap and a wire goes from pin 2 to the 1k 10w resistor. I measured the voltages on the tubes:

v1:
1. 0v
2. 311v
3. 6v
4. 85v
5. 115v
6. 1.4v

v2:
1. 0
2. 182v
3. 2.2v
4. .3v
5. 215v
6. 2v

v3:
3. 346v
4. 340v
6. 123v (I thought pin 5 was the grid but there is no connection there?)
8. 19.5v

v4:
3. 346v
4. 340v
6. 124v
8. 19.5v

B+ a= 250v b= 342v c= 352v

I've read bad things about the red astron caps, how do you check for leakage? Also here is a pic of the jacks.

Thanks for looking,
David

[MWJB]

19.5v at the 6v6 cathodes, accross a 250ohm resistor = 39mA per tube, less screen current...that's fine. Cathode biasedams tendto run higher current than fixed bias amps thus they often get noticably hotter/smellier.

Yes pin 5 is the grid for 6V6, recheck - Ampeg used thin little wires, connections are not always easy to see.

Those 1 Megs on the inputs could well be original, I doubt the missing resistor was 470K, check for a wire going to a grounded large value resistor from pin 4 V1 (where yellow wire from inputs terminates), if you don't find one the missing resistor may have been several meg?

Astrons are good caps, sort the obvious things first before looking for problems you may not have. Change the electrolytics at cathode bypass though.

[MWJB]

Second thoughts on the jacks - still think that it might be original & that you are not missing a 470K, but are you absolutely sure that something has been clipped off J3 (Guitar) input (picture slightly blurry)? Is that cap in parallel with the 1meg that sits between J3 Accordian input & J2 Guitar input?

The jack wiring is different enough from either schemo to be totally unrelated to either.

[dwhutchens]

I looked at the jacks again and something has definitly been unsoldered from the switch lug on the guitar jack. The 470k shown on the diagram would have connected the jack to ground when no guitar was plugged in. The middle jack has had its switch contact cut off and that lug is being used as a common point, the guitar jack is connected there by a 1m resistor and the cap in question. The accordion jack is connected by a 1m resistor and the mike jack is connected by a wire. Another wire then runs under the board to pin 4 of v1. Also there is a 1m resistor going from that common point to the ground lug of the accordion jack but it was never soldered As for the power tube grids I completely missed the fact that the 1k grid resistors were soldered between pin 5 and 6 they were hiding down under the tube sockets next to the chassis.

[dwhutchens]

I am putting a 3 wire grounded cord on this amp. I have the black wire going to the fuse then through the fuse to the switch and then to the transformer. The white wire to the other side of the transformer and green to ground. Can the orientation of the transformer primary wire have an effect on hum? In other words does it matter which wire goes to line and wich goes to neutral?
Thanks,
David

[olddawg]

I have a Ampeg Rocket amp made in New Jersey in 1960 that I recently restored. I found that the R-12A schematic drawn by Joe Piazza 4/18/97 was the closest. My amp was unmodified and all original. A brand new 4X20mf filter can cap is available from Fliptops for $28 which will resolve most of your hum issues. I also replaced all of the Astrons with mylars. After recapping this amp and replacing the power cord with a 3 wire earthed chassis ground, I have no issues with hum, even when I slam it with a Bad Monkey and a Rat. Sometimes I run it through a Marshall 1965A 4x10 cab to thicken it up. It is a great old amp for small venues. It absolutely screams.

[michael peck]

i just found one im gonna use in the studio cause my fender hot rod deville is humming too much for recording, this little ampeg rocks, it has 3 inputs guitar,accordian,mic, hum is much less than fender, how many amps is this thing? what year?

[olddawg]

Originally Posted by michael peck

i just found one im gonna use in the studio cause my fender hot rod deville is humming too much for recording, this little ampeg rocks, it has 3 inputs guitar,accordian,mic, hum is much less than fender, how many amps is this thing? what year?

There is probably a date on the tube chart inside the cab. If not, use the pot codes or serial number to date it.

[michael peck]

ill check pots, schematic is 1/2 torn off, ill have to get that one offline and print it out..i used amp last nite in rec studio and it worked great, love the sound better than my pc of junk mexican fender, know at least 2 people who had mexican hot rod devilles have wires rattle loose like me, 1st sign of trouble is when you have to hit top of amp to stop static and crackling sound, fender has about 5 yrs of giggin on it.....had it fixed twice now....not impressed

[michael peck]

i found serial# its 212150, i dont think its a 50 watt, too light only wieghs 25lbs, but has fat chunky sound at low volumes even, do you know age from this or wattage?

[capnjuan]

Hi Michael; gonna need more information to date that amp. The pots have date codes stamped on them; what do those numbers indicate? Tube lineup; what do you have in there? 6V6s out? Rectifier; 5Y3? How about a pic or two? CJ

[michael peck]

tubes are 6sL7 6sL7 6v6 6v6 5y3. volume pot# 1376218 and ba811-1532, tone pot 1376241 and ba811-2308, have to borrow a digital camera to get ya pic but input jacks are angled and say guitar / accordian/ mic, also has tremelo pots , speed and intensity. amp is very loud, and has beautiful sound, better than my fender 60 watt tube, this amp im using exclusively on our next cd for guitar, but i plug my mandolin in direct to mixer

[guitician]

I have an Ampeg M-12 that was all original when I got it. It never had any "drive" to the circuit so I checked all the signal paths and found that the input jacks were mis-wired from the factory. I wired them per the schematic on the back panel and now it ROCKS. It's a dual channel version of yours with a third 6SL7. I also wired a 3 conductor power cord and replaced a couple of the "frako" bypass caps that failed.

[Prairie Dawg]

Originally Posted by michael peck

i found serial# its 212150, i dont think its a 50 watt, too light only wieghs 25lbs, but has fat chunky sound at low volumes even, do you know age from this or wattage?

Look at the speaker-indicates it is a 1957 vintage speaker. The amp's around the same vintage if it is an original speaker. Check some EIA codes on the iron, that should tell the tale.

XXX first three numbers are the manufacturer
X last digit of year
xx week of the year.

[olddawg]

Originally Posted by michael peck

i found serial# its 212150, i dont think its a 50 watt, too light only wieghs 25lbs, but has fat chunky sound at low volumes even, do you know age from this or wattage?

It's around 17 watts RMS.