1967 Ampeg Reverberocket 2 Reverb Not Working

[Steelwitch]

Hey ya'll--

I have an Ampeg Reverberocket 2 GS12R on my bench with a strange reverb problem. I'm sort of at my wits end, so I'm calling out for people who are familiar with these amps! I just did a recap job on it, It sounds basically like it should except the reverb has stopped working altogether. Before I opened it up there was an intermittent connection. The filter cap before the reverb was replaced with a sprague 10uf 25v, which should work just fine. All of the original caps test perfectly. Does anyone have any suggestions where I should look for an issue? It just introduces noise, sometimes reverb whistle.

Here's a link to the schematic https://elektrotanya.com/ampeg_gs-12.../download.html

To save some time, I know for a fact that the tank, and the 6U10 is not the problem. Thanks!

[g1]

The filter cap before the reverb was replaced with a sprague 10uf 25v, which should work just fine.

Not sure which cap you mean, where is it shown on the diagram?

[Steelwitch]

Right next to the echo footswitch. Right after I posted this, I found a 22k resistor, which i believe is the ground for the footswitch, is totally open.. maybe I'm wrong about what it does exactly. Does anyone know? For reference, the reverb recovery is ok working. reverb drive section is not.

[nevetslab]

That would be the cathode bypass on pin 3 of the 6U10. What happens if you lift the 20nF cap from ground off the plate of the reverb drive tube? And, I guess we're using the reverb tank's input to discharge the 470nF plate coupling cap. Do you have a scope, where you can look at the input and output of the tank drive circuit? I've NOT worked on these older Ampeg combo amps, just the current generation ones...JET's, Super Jet, Super Rocket SR212TR.

The 22k are you referring to.....part of the grid bias network of the Reverb gain stage? You do need that 22k in the circuit to keep the grid referenced to ground.

[Steelwitch]

22k resistor replaced, but still no reverb. I know there's a lot of info about what each 1/3 of the 6U10 is doing, but do you have more info about how I can check out the input and output on a scope? I do have one, and would be useful in this instance

[Steelwitch]

yes its the 22k off the grid of section 2 (Pin 7)

[Steelwitch]

As I was getting voltages on the 6u10, I was getting pretty much nothing until I put a fair amount of pressure on the tube pins. I'll post voltages for each pin here in a few minutes, but it seems like the connections are very loose and this might be a case of a bad tube socket. There's a ton of RF interference. I love this amp and don't really need an FM radio.. If that is any indication of what might be wrong, I could use some advice as to why! There seems to be a grounding issue. When I was getting voltages the interference was silenced due to the grounding on the chassis through the meter.

Edit: I redact my statement about a bad tube socket. I switch my lead to a clip and it was reading voltages just fine.

[Steelwitch]

P1 - 59v
P2 - 281v
P3 - 9.6v
P4 - 86.7v
P5 - 147v
P6 - 1.6v
P7 - 288v
P8 - unused
P9 - 66.3v
P10 - 284.3v
P11 - 0v
P12 - 57v

[Steelwitch]

Getting no voltage on pin 11 is pretty suspect.. I’m gonna get voltages from V5 and compare to the schematic. If anyone has any additional advice of what’s going on here some help would be welcome! Thanks y’all, as always

[nevetslab]

You have 9.6V on the cathode KT3 (pin 3), so the grid is negative with respect to the cathode. I don't have the Ampeg Reverberocket 2 schematic here for reference, and the one you listed is too grainy to make out. So, putting the 3 stages in this order: Va Pt1 (Pin 10)=284.3V, Gt1 (Pin 9)=66.3V, Kt1 (Pin 4)=86.7V; Vb Pt2 (Pin 5)=147V, Gt2 (Pin 7)=288V, Kt2 (Pin 6)=1.6V; Vc Pt3 (Pin 2)=281V, Gt3 (Pin 11)=0V, Kt3 (Pin 3)=9.6V. So, on Va, Vg-k = -20.4V; Vb Vg-k = -286.4V; Vc, Vg-k = -9.6V. Tube section Vb looks very suspect with 286.4V between cathode and grid. Looking at the data sheet on a 6U10, the maximum ratings for the grid (with respect to cathode) voltage is 50V for any of the 3 sections. Isn't the tank driver section Vb?

6U10-GE.pdf

Also, with regards to the loose connections on the tube socket on this tube, can you re-tension them? On tube sockets that give you enough access, I'll use my smallest Wiha bladed screwdriver to pry them to close a bit, which usually helps. Some sockets you can't get into for doing that.

[Justin Thomas]

nevetslab,

It may or may not matter, bit there are 2 versions of this amp: one using a 7199 PI & 1 using a 12AX7. Also, my old 67 had a hand-drawn addition of elevated heaters on the 7591, drawn in dark pencil. As it was about 95% original I presume it came from the factory like that. Mine was the 12AX7 verdion (and came with a pair of VERY fine Amerex Bugle Boys!)

I actually wondered if it might have been worked on by Ken Fischer himself as part of thr backlog clearing...

Anyway, just an FYI to check both vafiations for differences.

Justin

[Jazz P Bass]

Here is the linked schematic: Ampeg_GS12R.pdf

[Steelwitch]

The one in question has a 12ax7 PI

[nevetslab]

nevetslab,

It may or may not matter, bit there are 2 versions of this amp: one using a 7199 PI & 1 using a 12AX7. Also, my old 67 had a hand-drawn addition of elevated heaters on the 7591, drawn in dark pencil. As it was about 95% original I presume it came from the factory like that. Mine was the 12AX7 verdion (and came with a pair of VERY fine Amerex Bugle Boys!)

I actually wondered if it might have been worked on by Ken Fischer himself as part of thr backlog clearing...

Anyway, just an FYI to check both vafiations for differences.

Justin

OK, back at my shop, and pulled up both schematics of the Reverberocket. I see the one having the 1AX7 as the PI shows plate, grid and cathode voltages for the 3 6U10 tube sections that seem sensible....though on pin 11 the grid voltage MUST be a typo, it showing 250V, with the cathode at 8.6V. There's no circuit path to Pin 11 that could produce 250V. That would be 0V.

But, looking at the tube pin readings posted, this looks like the other schematic, using the 6U10-C as the PI.

The voltage reading for pin 7 must be wrong....288V on the grid, with 1.6V on the cathode and 147V on the plate. The schematic for this 6U10-B section shows 135V plate, 1.4V cathode, and no doubt 0V on the grid (not stated, but no source to obtain 288V, unless an internal or external connection near pin 10, where there's 284.3V.

So, you'll need to verify Pin 7 voltage reading. If it is high like stated, there's something in the circuit area that doesn't belong.

As for where to connect your scope probe to look at the signal going to and coming from the reverb tank (once you get the circuit voltages corrected for Section B, you can look at input to grid of 6U10 pin 11, junction of a 2.2M, 220k & 0.1uF plate coupling stage preceding this stage, then look at the connection following the 0.47uF cap off the plate of 6U10 pin 2 (6U10-A). This would be the reverb drive signal. The Return signal can be found at the wiper of the Reverb pot (when the pot is at max CW), this being from the plate of 6U10 pin 5 via 0.005uF. The Reverb signal mixes with the dry signal at the grid of V3 12AX7-pin 7.

A quick check on the Reverb tank for DC resistance on the tank input and tank output will usually confirm the tank being ok, and not indicating an open tank xfmr coil.

[nevetslab]

The one in question has a 12ax7 PI

Same schematic I'm referring to....12AX7 is feeding the 6U10 driver (PI), that feeds the two 7591 output tubes. Anyway, read thru my post of 10:01AM minutes ago, as it refers to a problem on the 'B' section of the 6U10.