'65 reverberocket

[firemedic]

My drummer scored a '65 GS12R Reverberocket from a guy outside a Guitar Center, they had offered the dude $170 for it so said drummer paid him 200. It still worked, but he asked me to look at it since the volume drops after @ 10 min of playing. Otherwise he thinks it sounds good. It has this very ugly cheap tolex covering it but is otherwise stock, w/ original everything. Even the speaker appears in good shape. I'm pretty sure I can fix it.

So I notice it's still got the original filters, 'lytics, 2-prong plug etc. I break the news that this may cost a little, even if I do the work for free. (BTW any good sources for an 80/40/40 can cap?) Now he is considering reselling it!

My question is- is this the very desirable amp I think it is? Am I missing something?

 

[HotBluePlates]

Antique Electronic Supply has a multisection cap rated at 80/40/30/20 @ 525v. You could parallel the last 2 section for 50uF; a bit over stock, but no big deal. You'll want to eyeball their measurement and compare to your original to see if it'll fit.

[tubenit]

I'm pretty sure I can fix it.

I'm confidant that you can!  I think that's a pretty nice amp. I had a chance to play one yrs ago and liked it.

Tubenit

[Willabe]

Some guys like the verb in the Ampegs better than in the Fenders.


              Brad       

[firemedic]

I especially like the Compactron 6U10 in it. I offered to buy it for 200, rehab it properly (more or less) & keep it at his place for the foreseeable future, primarily cuz his practice space has A/C. HBP I saw that same cap can, that was exactly what I had in mind.
Still waiting on his reply......

[spacelabstudio]

I have one of these that I fixed up and love it.  Well worth it, imo.

Chris

[firemedic]

jojokeo's post about a conjunctive filter on the PT got me thinking: I plan to replace the 2-prong cord with a 3-prong, & ground the chassis. Do I need to eliminate the .05 uF 1000v cap that goes from the AC mains hot feed to ground? I had planned on taking it out but I really don't know what it does.

[firemedic]

I looked around the forum & see that it is the infamous death cap. Out it goes.

[ThermionicEngine]

I've re-capped about five of these amps.  I rate it very close to the Deluxe Reverb.  You can get some nice 7591 power tubes from JJ and the CE 80/40/30/20 @525 worked very well.  They are dimensionally close to the original Mallory, allegedly made with Mallory's equipment.  Using them for a B-15 on my bench as I write.  Cheers

[firemedic]

I've recapped the thing, including all 4 cathode bypass caps. That CE multi-cap can fit perfectly! Also replaced the screen wire which looked ratty, and power tube cathode resistor/ cap.
So I fire it up & man does this amp have a sweet tone. Per expectation, it really comes alive when turned up.
But the loudness isn't really there; that must be what my drummer meant when he said it sounded like it had lost half its power after playing @ 10 minutes.

The voltages are nominal per the GS-12R schem.
Except the driver stage grid, 63v. Nominal voltages for the stage are plate=250v, grid=30v, cathode=84v.
Now I have an embryonic understanding of this stuff. Would this grid voltage cause the apparent loss of power without the tone suffering? It still overdrives with the volume turned up, but is not loud.

Update: I replaced the coupling cap to the grid, also checked the 5.6M resistor connecting B+ to the grid & it is within normal limits. I'd really like to get that grid voltage down but don't know how & I'm out of ideas.....

The 7591s are original- could old worn out power tubes just fade out the volume like that?

[firemedic]

I have researched a little & see that the stage in question is a fixed-bias cathodyne PI. So the bias voltage for this stage is twice nominal, roughly. It's a wonder it works at all. I suppose i could daisy-chain a few 1M resistors to rule out a bad 5.6M. But it looks fine on the DMM!?!?? 

[Willabe]

Try checking the 1.5M grid R. It forms a voltage divider with the 5.6M. If it drifted up in value....      


              Brad    

[sluckey]

Except the driver stage grid, 63v. Nominal voltages for the stage are plate=250v, grid=30v, cathode=84v.

I think the 30v number on the schematic is wrong. You have a B+ node that's 330V. Then a simple voltage divider of 5.6M and 1.5M. If you do the math you'll get 69.7V across the 1.5M resistor.

[firemedic]

Thanx Sluckey- I've since found some other nominal voltages from other forums & they get the same voltage.
I only fixated on it since it was the only abnormal signal path voltage off the schem, but we've cleared it up now.
I also found another guy who had the same low volume problem, finally he replaced the OT & speaker to fix it.
Just for giggles I hooked up my 2-12 WGS cabinet and it was louder but not quite enough.
So, any good sources for a OT-151A? or equivalent upgrade?

[ThermionicEngine]

As I recall volume was good until the amp was on 10 minutes.  If so, I would start searching for a component that drifts way off value when it heats up.  I use a Fluke 62 IR thermometer to find hot spots.  Also, you probably already did this, check the tubes after the volume goes down in case it is a malfunctioning tube, even new ones can fail.  Bad solder joints sometimes fail only when the amp warms up.  Chopstick should find a bad joint and possibly a bad component.  The most prevalent cause of this symptom I have encountered has been a resistor failure after heating up.  A big issue is working in a live chassis to find the problem.  If you have not done it, work with someone who has.  If you are proficient at it, use caution anyway, complacency is a killer.  All the best, Larry

[spacelabstudio]

Since we're talking about this amp today anyway, a friend of mine and I were looking at the tremolo circuit and trying to figure out what the switch in it does.  I can describe the effect--when engaged (not sure if open or closed, probably closed, it's engaged by cranking the depth knob all the way to the right, then it hits the switch) the tremolo sounds like it's following more of a square wave than a sinusoid.  Looking at the schematic it looks like it switches in a filter (2 resistors and a capacitor) on the LFO output:

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_GS12R.pdf

Does anyone have more than a vague notion of what's going on there?  It sounds pretty cool, albeit not particularly useful most of the time.

Chris

[firemedic]

As I recall volume was good until the amp was on 10 minutes.  If so, I would start searching for a component that drifts way off value when it heats up.  

Well I'm not sure exactly about the original problem, I only know what I experienced myself after replacing caps, to wit: Works fine, good tone, volume isn't all there. Then after playing a while I fancied there was an ever so slight drop in volume. It was so slight it could have been my imagination.
 
None of which negates your point, I understand. I did look at the tubes throughout the whole process & didn't see anything out of whack, redplating or otherwise.
Tomorrow I will open it up again & have a look with the thermal imager. Will a bad resistor get real hot then? 

[ThermionicEngine]

Well I'm not sure exactly about the original problem, I only know what I experienced myself after replacing caps, to wit: Works fine, good tone, volume isn't all there. Then after playing a while I fancied there was an ever so slight drop in volume. It was so slight it could have been my imagination.
 
None of which negates your point, I understand. I did look at the tubes throughout the whole process & didn't see anything out of whack, redplating or otherwise.
Tomorrow I will open it up again & have a look with the thermal imager. Will a bad resistor get real hot then? 
[/quote]

Some resistors drop a lot of voltage and it is normal for them to be hot- but usually resistors fail by going open or becoming a dead short.  Sometimes they split the difference.  The shorting ones get hot.  The open ones do not. Except for big volt dropping resistors or cathode resistors in cathode biased power amps, most resistors should not be hot.  Look for an adjoining component that might be drawing a lot of current.  I'm afraid looking at tubes will not divulge much- voltage at the pins would be very helpful.  Tapping with a chopstick will often reveal a loose or poorly soldered connection. 

[firemedic]

Okay, I went over all the voltages, once when they settled down after initial startup, then let everything cook for 45 minutes and rechecked. Nothing drifted more than a volt or so.
So I'm ready to wrap it up, satisfied that the guts are working the way they're supposed to. Probably replace the speaker with a WGS Reaper when I get to it.
I realized also that I had mistakenly put in a 200R cathode resistor where I meant to use a 150R. Pretty typical of me. So I paralleled a 270R 10W with a 330R 10W, it should be pretty stable & run cooler.
I'm done, this is a tone machine, even if it's not so loud for now. Thanks guys!

[firemedic]

Same amp, new question.
Has anyone got any experience with this amp? as near as I can tell, per weber bias calculator, this RR2 is pushing 34w. 360 plate volts (plate-to-cathode), 15v cathode, 150R resistor.
 
It sure doesn't sound like 34w.

So I'm replacing the speaker. I need to know the actual wattage so I'm not getting an under rated speaker. Everyone who has one of these amps has a different take on the actual wattage: some say 18w max, some say 25w, some say 35w. Some say 30w. I don't want to get a 50W speaker if I don't need to, for purposes of sensitivity. OTOH, the Weber calculator values have so far been right on with my other amps. Will the 50w WGS HP Reaper be sensitive enough for this application?

[EL34]

No advice here, only want to say that some of the best tone I have ever heard came from ReverbRockets in my shop.

They are really cool amps

[HotBluePlates]

Same amp, new question.
Has anyone got any experience with this amp? as near as I can tell, per weber bias calculator, this RR2 is pushing 34w. 360 plate volts (plate-to-cathode), 15v cathode, 150R resistor.
 
It sure doesn't sound like 34w.

Everyone who has one of these amps has a different take on the actual wattage: some say 18w max, some say 25w, some say 35w.

34w idle doesn't equal 34w to the speaker. Actual speaker power depends on true operating class.

a true 7591 is a 19w tube. A pair of them idling at 34w is do-able. If this was purely class A, you'd never get more than 17w output. This amp probably strays into rich class AB, call it a 20w amp.

If you think you might ever slap a square-wave fuzz in front of it, plan on 45-50w of speaker; the square waveshape results in higher average power (as much as double zero-distortion sine-wave power). If you're just planning on using it as typical, I'd feel safest with a 30-35w speaker, maybe as low as 25w if I wasn't gigging with it.

Since we're talking about this amp today anyway, a friend of mine and I were looking at the tremolo circuit and trying to figure out what the switch in it does.  ... when engaged ... the tremolo sounds like it's following more of a square wave than a sinusoid. 

Your ears told you the story.

A typical free-running oscillator like we use in a guitar amp will probably clip. The feedback ensures the tube turns on and slams into one rail, then turns off and slams into the opposite rail (ground). You'll almost always find some kind of cap to ground in the circuit (or some other way to lose treble) to roll off the distortion present in the output. Further, since even an output of 15Hz is almost unusably fast trem, there is no need of anything that could be remotely mistaken for treble from the oscillator.

The "Intensity switch" shorts in a low-pass filter. Low-pass to the LDR, that is. Highs are shunted off in a shelving manner, which makes the oscillator output more closely approximate a sine. Switched out, you get more square-wave, and you should hear a choppier trem sound.

[firemedic]

Thanks HBP for the wattage info.
This is a shared amp for the time being. If it was just me playing on it, volume on 7 & just barely distorting, I'd use a 25W. To me it's a Fender, to put it simplistically.
My drummer/amp co-owner keeps it turned up all the way though, all the time, & his pickups are hotter. To him, it's a Marshall.
I suppose my next question is, would I be giving up too much tone with a 50w speaker? They're not much more expensive than 30 watters. Huge difference or not?

[Shrapnel]

7591 Per RCA RC-23:

FIXED Bias:
Plate: 350VDC, G₂: 350VDC, G₁: -15.5V Z_a-a6600 Power Out: 30W
Plate: 450VDC, G₂: 400VDC, G₁: -21V Z_a-a6600 Power Out: 45W

CATHODE Bias:
Plate: 450VDC, G₂: 400VDC, R_k: 200R Z_a-a9000 Power Out: 28W



Ampeg: (ReverbRocket 3)
Node A (Plates via transformer): 360V, Node B (screens): 350V: R_k: 140R
Node C: 290V, Node D: 275VDC

Ampeg: (Joe Piazza schem: GS12R)
Node A, Node B: unspecified, Node C: 340V, Node D: 200V, R_k: 140R

Both of these Ampeg schems I have do not specify Z_a-a but Identify the OT as OT-151A

I would guesstimate your power output at about 30W IF Z_a-a is 6600R

[firemedic]

Thanks shrapnel. I'll get the 30W Reaper & cross my fingers.

[spacelabstudio]

Thanks shrapnel. I'll get the 30W Reaper & cross my fingers.

I have a 25W Weber in mine, although I tend not to run it at full tilt.  No problems so far and it's been in there a few years.

Chris

[firemedic]

Yeah, this one is going to be dimed a lot. I imagine the 30w will hold up though. Thanks for your info.

[HotBluePlates]

7591 Per RCA RC-23:

FIXED Bias:
Plate: 350VDC, G₂: 350VDC, G₁: -15.5V Z_a-a6600 Power Out: 30W
Plate: 450VDC, G₂: 400VDC, G₁: -21V Z_a-a6600 Power Out: 45W

CATHODE Bias:
Plate: 450VDC, G₂: 400VDC, R_k: 200R Z_a-a9000 Power Out: 28W

Yeah, we don't have the OT impedance info, so we're all guessing.

Note the 350v condition is 66% of the 450v condition, based on the fixed bias numbers. 66% of the 28w for 450v cathode bias is 18.5w, but that's a low guess in my opinion. I'm sure Ampeg used a lower OT impedance to keep this amp comparable to Fender's Deluxe Reverb (which would be the sensible competition, given size and what I recall of the acoustic output of these).

Anyway, 20-30w, pick your poison. No specific wattage is "best" but the higher wattage speakers with bigger voice coils tend to have a clearer, more authoritative sound. Smaller speaker with smaller voice coils tend to break up faster. But folks like WeberVST have so many options available, you can't really call that a solid rule of thumb.

[spacelabstudio]

Since we're talking about this amp today anyway, a friend of mine and I were looking at the tremolo circuit and trying to figure out what the switch in it does.  ... when engaged ... the tremolo sounds like it's following more of a square wave than a sinusoid. 

Your ears told you the story.

A typical free-running oscillator like we use in a guitar amp will probably clip. The feedback ensures the tube turns on and slams into one rail, then turns off and slams into the opposite rail (ground). You'll almost always find some kind of cap to ground in the circuit (or some other way to lose treble) to roll off the distortion present in the output. Further, since even an output of 15Hz is almost unusably fast trem, there is no need of anything that could be remotely mistaken for treble from the oscillator.

The "Intensity switch" shorts in a low-pass filter. Low-pass to the LDR, that is. Highs are shunted off in a shelving manner, which makes the oscillator output more closely approximate a sine. Switched out, you get more square-wave, and you should hear a choppier trem sound.

Makes sense. Thanks!

Chris

[firemedic]

I went ahead & replaced the speaker w/ a Jensen C12Q 35W, didn't make a heck of a lot of difference. Very nice tone, but not real loud.
Time passed.....
I paid off my drummer, now the amp is mine. BWAHAHAHAHA! Now I can get busy & really work on this thing. It now has a handle!
I thought long & hard about upgrading the OT, & which one to use. As usual the solution was right under my nose, while I surfed the web for "Ampeg replacement OT".
I have a 014318 EXT in the TOS I've been meaning to upgrade; let's use that. The impedance should be about right for cathode biased 7591s.

So THAT was the problem. I guess the old OT was "tired" whatever that may mean. Read: "too small". Nice, loud amplifier, now. Kind of tweedy. The tremolo is just wild if I turn it all the way up.
Hopefully this post will help the next guy who is wondering why his Reverberocket 2 is so quiet.

 

[spacelabstudio]

I went ahead & replaced the speaker w/ a Jensen C12Q 35W, didn't make a heck of a lot of difference. Very nice tone, but not real loud.
Time passed.....
I paid off my drummer, now the amp is mine. BWAHAHAHAHA! Now I can get busy & really work on this thing. It now has a handle!
I thought long & hard about upgrading the OT, & which one to use. As usual the solution was right under my nose, while I surfed the web for "Ampeg replacement OT".
I have a 014318 EXT in the TOS I've been meaning to upgrade; let's use that. The impedance should be about right for cathode biased 7591s.

So THAT was the problem. I guess the old OT was "tired" whatever that may mean. Read: "too small". Nice, loud amplifier, now. Kind of tweedy. The tremolo is just wild if I turn it all the way up.
Hopefully this post will help the next guy who is wondering why his Reverberocket 2 is so quiet.

 

Hmm, now I'm wondering how loud this amp is supposed to be.  How should the when working as designed output compare to say a Deluxe Reverb?

Chris

[firemedic]

I just figured it should be at least as loud as a 6V6 p-p, and it wasn't even close. I googled quite a bit & slowly gathered that the stock RR2 OT commonly was causing low volume issues.

Update: I got some new 7591A-EHs (also a RCA 6U10), and it's even LOUDER. Between the new tubes & new speaker I've got some breaking in to do. Looks like it's about wrapped up....