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Ampeg Reverberocket II GS-12R project

 
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Mon Jun 27, 2011 2:59 pm    Post subject: Ampeg Reverberocket II GS-12R project Reply with quote

I picked up this amp from a craigslist seller a couple of weeks ago. It's suffering from low volume, heavy unpleasant overdrive above 5 on the volume, and poor tone. Here are a couple of before pics:

I replaced the big red cap in the gut shot with a 40uf one and also replaced a couple of the resistors in this pic. I replaced the 70/40/40 cap can as well.

Now, the amp sounds a little better, but voltages are off. The OT was buggered up by whoever installed it. A very poor job of connecting new wires to the OT is hidden by the electrical tape, and the OT was hanging by one bolt. I mounted the new OT in the original stock location.

I purchased a replacement OT (http://www.triodeelectronics.com/tf65wscta48o.html) and this is the wiring diagram (
Thumbnail, click to enlarge.
).

I've replaced the OT, and I wanted to double-check here to make certain that my reasoning is sound regarding the wiring. A schematic is attached. As a side note, I also installed a 3-prong plug and will pull the death cap.

1. The brown and blue plate leads go to Pin 3 on V1 and V2 respectively 7591 tubes, http://www.duncanamps.com/tdsl/show.php?des=7591).
2. Black goes to ground.
3. Green goes to the 10k resistor adjacent to the large 140 resistor, you can see it in the gut shot coming out of the small hole between the two power tubes. It appears at junction A at the bottom of the schematic. Or, does it go to the speaker jack, or both?
4. Red goes to the 70uf cap.

Now, I'm left with the two leads marked Screen on the OT diagram. The old OT didn't have these leads, so I'm trying to locate their connections. Or, they may not be used at all, which is what I'm thinking.

I also used one of the existing vent holes in the back of the chassis to mount a speaker jack (non-invasive mod). I'm a bit confused on the green 8ohm speaker lead. In the schematic and other pictures I've seen, the line looks like it would go to the 10k resistor I mentioned above. If I wire it that way, do I need to run a wire from the 10k connection to the jack, or where should the jack be connected? Do the screen leads need to be connected, and if so, where? Alternatively, should the green and black wires run to the new speaker jack as in other amps I've seen?

Thanks for helping!

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Firestorm



Joined: 25 Jan 2008
Posts: 2568
Location: Connecticut

PostPosted: Mon Jun 27, 2011 4:26 pm    Post subject: Reply with quote

What you have is an ultralinear OT (with screen taps) and both 4 and 8 ohm secondaries. To rebuild to stock, ignore the screen taps, cap them and stow them. Similarly, ignore the 4 ohm secondary cap and stow that. The green lead can go to the 10K feedback resistor and you can connect the hot lead for the speaker jack there. I can't tell how the speaker ground was connected; you may want to isolate the jack and make the ground connection directly to that ground point (unless the original grounded to the chassis near where you'll put your jack.)
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Mon Jun 27, 2011 4:28 pm    Post subject: Reply with quote

Firestorm wrote:
What you have is an ultralinear OT (with screen taps) and both 4 and 8 ohm secondaries. To rebuild to stock, ignore the screen taps, cap them and stow them. Similarly, ignore the 4 ohm secondary cap and stow that. The green lead can go to the 10K feedback resistor and you can connect the hot lead for the speaker jack there. I can't tell how the speaker ground was connected; you may want to isolate the jack and make the ground connection directly to that ground point (unless the original grounded to the chassis near where you'll put your jack.)


Thanks! The original ground was grounded at the cap can. I can do that and run the jack's ground to another point closer to the jack.

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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Tue Jun 28, 2011 1:11 pm    Post subject: Reply with quote

I hooked up the OT as described, and had the same distortion when I cranked it up. The distortion lasted a few seconds, then a loud squeal came from the speaker and I switched the amp off immediately. I've seen people post about amp squealing, so I'll do a bit of research. Can't imagine what's wrong, it seems simple to hook up. I set it aside until I could do some checking.
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xtian



Joined: 19 Apr 2010
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Location: NorCal

PostPosted: Tue Jun 28, 2011 1:36 pm    Post subject: Reply with quote

Did you replace the filter caps? If they're as old as that red one...
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Tue Jun 28, 2011 1:39 pm    Post subject: Reply with quote

xtian wrote:
Did you replace the filter caps? If they're as old as that red one...


Yes, I've replaced the filter caps. I have a Variac and brought the power up slowly after they were replaced. As yet, I have not replaced coupling caps and others on the board.

Things to try:
1. Swap tubes. Unfortunately, I don't have a set of 7591s on hand, but I do have spares for the others.
2. Check the wiring of the new OT.
3. Bypass the new jack and test.
4. Remove the new jack ground and run the OT ground directly to the jack.
5. Unplug the guitar and test. Then try another guitar and cable.
6. Swap the OT leads to the power tubes. I think I'll try this first.

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Masco



Joined: 29 Nov 2007
Posts: 192
Location: Maryland

PostPosted: Tue Jun 28, 2011 2:54 pm    Post subject: Reply with quote

#6
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Wed Jun 29, 2011 2:08 am    Post subject: Reply with quote

Thanks for all the help. The lead swap fixed the problem and the amp sings sweetly now at low volume except for a bit of hum. A bit trebley, but I still have components to test and replace, including the caps on the board. I still have breakup above 5.
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Last edited by Travst on Sat Jul 09, 2011 10:14 pm; edited 1 time in total
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Sat Jul 09, 2011 10:13 pm    Post subject: Reply with quote

I've now replaced the caps on the board and installed the new OT. The tone is much better than before, but I'm still getting heavy breakup (fuzz) above 5. While this early breakup is often reported with these amps, there's still a problem(s) somewhere. I've tested all the resistors and replaced 2 that were out of spec. I've also tested all the tubes and swapped with spares, except for the power tubes. I did swap them into each socket to see if voltages changed with the swap.

Voltage from the PT seems to be off in some cases, particularly for the heaters and grids on the 7591s. I've attached a voltage chart and would appreciate any thoughts you may have. Some of the voltages I've marked as zero show just a few millivolts. A schematic is also attached.

The good news is... all the hum is gone and the amp is dead quiet.

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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Sat Jul 30, 2011 11:00 pm    Post subject: Reply with quote

I've been working on this amp with the help of Structo, Tres Mellows, Snorklemonkey, and Andy Fuchs as well as the guys from AG that posted above. There appear to be 3 schematics related to the amp, none of which is exactly correct for my amp. These links will allow people to download full-size copies. The third schematic is a scanned copy from my back panel.

http://i40.photobucket.com/albums/e227/Travst/AmpegGS-12R3.jpg

http://i40.photobucket.com/albums/e227/Travst/AmpegGS-12R.jpg

http://i40.photobucket.com/albums/e227/Travst/AmpegGS-12R-Travst.jpg

The pics below were taken after I was done. The last pic shows the dropping resistors discussed below.

To quote from Tres' comments:

Quote:
Now that's interesting... That schematic matches the earlier of the two RR2 schematics I sent you. But,looking at your photo in the thread, your amp is not wired quite like that. According to that schematic, the 2W 1K resistor should be dropping from the A rail to the F rail, which then drops again to C with the 22k. The dropping resistor between A & B should be 470 ohms, which would get you the 355V, a 5V drop from the 360V at A.

Instead, your power section is wired like the later schematic, which doesn't use the 470 ohm resistor between A&B (I don't see it in your photo) and uses the 1K between A & B. Funny thing is, looking back at the photos of mine, it's done the same way. Mine, however, still splits the PI over 1/3 of the 6u10 and 1/2 of a 12ax7, but you said yours uses one 12ax7 as the PI (like the later schematic). You'll have to compare your board to the schematic to see which is more accurate for your amp...gotta love Ampeg.


So, I don't have the 470 ohm resistor marked on the schematic on the back of my amp. Whether it was removed or not, I can't tell. There was a 22k resistor in place in the position marked downstream from the 1k resistor. However, this resulted in 258 volts at the screens rather than the 345 on the schematic. After looking at all 3 schematics, I realized that the resistor was supposed be a 2.2k resistor. Replacing the 22k with a 2k put my screen voltage spot on. The cathode voltage is a bit low, but I didn't have a higher 10w resistor to sub in.

At any rate, the amp sounds great and I'm posting this in hopes that it'll help someone else down the line. I also located an original footswitch and swapped out the original CTS for a V30, which added volume and crunch. Feel free to comment and discuss.

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Liquids



Joined: 23 Feb 2011
Posts: 14
Location: CT, USA

PostPosted: Tue Aug 30, 2011 3:37 pm    Post subject: Reply with quote

Nicely detailed posts!

I just scored a Reverberocket 2. Mine seems pretty nice. I'm debating about a transformer replacement, though mine works fine (and the amp score - though cheap - left me broke for the time being)! Curious about the tone, and a 4 ohm tap would be nice.

I really like the sound of the amp! I'm gonna screw with biasing it differently though. I know some think that 'hotter bias is better' but in other Ampegs (unlike Fenders) I've really like the fidelity and preferred the tone when the amp was biased 'cold.' Speaker-wise, the CTS is pretty classic, but there's plenty of (and plenty more) girth available with my 2x12 avatar cab hooked up.

I keep wondering about the chosen value of the 7591 cathode bypass cap. I'm considering beefing it up and seeing if I prefer the tone. Is there a calculator out there for cathode bypass caps on power tubes, for frequency response?
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Travst



Joined: 20 Oct 2008
Posts: 159
Location: Birmingham, Alabama USA

PostPosted: Thu Sep 01, 2011 1:01 pm    Post subject: Reply with quote

I experimented with the bias on my amp a bit and found that the stock setup sounded as good as anything I tried, so I left it stock. As for speakers, I tried stock, a V30, old Eminence/Randall, Eminence Texas Heat, and a late 50s Jensen. The V30 won out because it wasn't too dark, didn't break up so early, and was very clear.

Try this for calculating cathode bypass caps:

http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/

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