Ampeg R-12-R Reverberocket Amp-What a mess inside

[SEL49]

PS... The footswitch was also hardwired (no jack)

[wsscott]

Yes, I have the footswitch but it was cut off of the terminal board!

[SEL49]

Reverb will work without the FS, but Tremolo requires a FS to be wired to the board in order to function.

[wsscott]

SEL49-So when the tremolo circuit's 1.2K cathode resistor goes to Chassis Ground when the footswitch is closed, the Tremolo function is then running?

If so, I could just ground the wire connecting to that resistor on the circuit board, and I wouldn't need a footswitch for it. It would be "on" all the time. I could just turn it "on/off" by controlling the Speed and Intensity pots?

And if the footswitch is only needed to turn the Reverb OFF, I wouldn't need it at all.  I wouldn't have to re-wire anything for it.  I could just control the amount of the Reverb by adjusting the "Dimension" pot?

I ask these questions because my buddy who owns the amp would like to do away with the footswitch and the cabling, just to "neaten" things up.

[SEL49]

All correct

[wsscott]

Still working on the hiss problem.

I've replaced the carbon resistors on the plates and the guitar input jack with no effect.

When I remove V1 the hiss is gone, regardless of whether a guitar is plugged in.  Of course there's no signal to process to be sent on to the Output tubes.

Same thing if I put V1 back in and remove V2.

There's no DC on either the Volume or Tone pots.  I have put in new caps connecting these pots to their plates.

I measured the 1M Volume pot and it reads about 1.35M.  The Tone pot reads slightly below its value of 500K.

Could the Volume pot be causing the hiss?  The volume of the hiss and the guitar increase or decrease in turning the Volume pot.  Tone becomes more treble or bass, but volume of hiss/guitar signal doesn't change other than the pitch.

[AlNewman]

Try grounding the grid of V1, (labeled pin 4 on your schematic), see if that helps with the hiss.

[wsscott]

No, still have hiss.  I clipped Pin 4, grid, at the tube and clipped the other end to the chassis, and also tried it clipped at the Input jack, but still hiss.

[shooter]

a scope is your friend

[wsscott]

I have a small digital scope, but I"m not very experienced in using it to know what to look for.  I guess now is the time to get some more experience.

[Willabe]

Or use a listening amp.

Scroll down to find it. A lot of guys use them here.

 https://el34world.com/Hoffman/tools.htm

[AlNewman]

You could try switching V1 with the PI, see if that helps.

[wsscott]

When the scope is in AC mode, 1khz signal, 135mVrms, I get a nice sine wave through V1 Pin 4 grid, and through the wipers of the Volume and Tone pots.  The signal is 135mVrms at the input jack, 79mVrms after the input jack resistors, and 2.25Vrms--So a lot of gain.

There is hiss in the background that's also loud.

When I disconnect the scope's probe, leave it's negative lead attached to the chassis, and switch it to DC coupling, with no signal input at the input jack, I pick up a signal on the scope of about 15khz and 25mVrms.  That signal is jumpy and fuzzy looking.  Not a nice sine wave.  So I assume that's the Hiss.

If I move the scope's probe away from the chassis, with its ground lead still connected to the chassis, that signal disappears or is smaller on the scope.

[shooter]

of about 15khz and 25mVrms.

That should be visible in AC coupled mode with the scope set up to "view" that frequency.  Start to the wiper to the VOL pot after V1a.
Don't inject a signal yet, just see if you can get "eyes on".  25mV will be close to the scope "baseline noise" so you might have to spend quality time looking in the weeds for a "coherent signal"

EDIT: 15khz might be ambient noise in your environment, LED lights, wall warts, so don't go chasin CCP technology 

[wsscott]

SEL49 can you help me with the wiring of this OT?

After I added the 10K resistor, and turned it on to test, my 8 ohm dummy load, which is a 3W 8ohm resistor wired onto a 1/4" jack, that I plug into the Output Jack where the Black and Brown secondary wires are connected, got really hot.  So I turned it off.  It hadn't done that previously.  So I don't know if I have improperly wired in that 10K resistor.

I connected one end of the 10K to V4 Pin 3 cathode where one end of the 220 ohm resistor is also connected, and I connected the other end of the 10K resistor to an empty eyelet.  Then I ran a new lead from that eyelet to the Output Jack tip lug where the Brown OT secondary is also connected directly from the OT.

Is that correct?  Or does the 10K need to be wired "in-line" with the Brown secondary before it is connected to the lug on the Output Jack tip.

[SEL49]

The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?

[Willabe]

After I added the 10K resistor, and turned it on to test, my 8 ohm dummy load, which is a 3W 8ohm resistor wired onto a 1/4" jack, that I plug into the Output Jack where the Black and Brown secondary wires are connected, got really hot.  So I turned it off.  It hadn't done that previously. 

That's at least a 15w amp and you are using a 3w R for the dummy load?

[wsscott]

True, but I haven't gotten past 1 or 2 on the Volume pot, so I assumed it would be okay for low volume, short test periods.

[Willabe]

The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?

Is the OT in this amp original with the 2 black secondaries and 2 brown secondaries?

Or is this a replacement OT with just single +/- fly leads?

[Willabe]

And why are you using a dummy load to test if you have the NFB hooked up correctly?

You need to hear what it sounds like.

The dummy load wont protect the OT from a short, use your LBL if that's what your worried about.

[Willabe]

True, but I haven't gotten past 1 or 2 on the Volume pot, so I assumed it would be okay for low volume, short test periods.

What tests are you doing with the volume on 1 or 2? 

If your using it to look at the signal with a scope, turning the volume pot only to 1 or 2 is not going to give you enough signal to see much of anything if something is wrong. 

[wsscott]

It is not the original OT.  The OT had been removed from the amp by someone before my buddy was given the amp, so a new one was purchased with just the 2 secondary leads.

I had the dummy load in when I was injecting a signal to trace it through the circuit and see where the signal went crazy, hoping it would be an indicator of the source of the hiss.  I got as far as the tone pot and the dummy load resistor heated up so I removed the dummy load and connected it to the speaker. 

[Willabe]

Ok, then try what SEL49 suggested.

The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?

[wsscott]

So yesterday I spent some time replacing carbon comp resistors, and replacing them with metal film of equivalent or near equivalent values.  I had most of the values I needed but there were a few that were a little lower or higher.  The 5.6M at the input jack was replaced with a 4.7M since I didn't have a larger value.

I've noticed that the hiss seems less, but also the gain is much lower now.  Maybe that's because the gain is less.  I can turn it all the way up and the volume is about where it was at a 2 before these changes.  Although the hiss is less on Input Jack 1, it's almost negligible on Input Jacks 2 and 3.

Someone had a 25/50 cap installed across the 22K that connects with the 3.3K and 2.2K resistors on the V2, Pins 3 and 6 cathodes. And instead of the 22K connecting to a 2.2K on one end, a 5.6K had been installed.  Neither of these are on any schematic.  So I removed them, and replaced with values shown on the schematic.

I'll check the values on the resistors I replaced to see if I misread something.

I used the guitar at the input for a signal level comparison and although the hiss is lower so is the volume.

I didn't change any wiring yet on the output jack to the 8 ohm speaker.  I did check the AC voltage on that jack and it's reading 14.5 VAC.  Does that sound correct?

[shooter]

I'll check the values on the resistors I replaced to see if I misread something.

if you did this step 1st, you'd already know 
always ohm ANY and ALL resistors 1st, the paint colors n schemes of the modern components are NOT from our 64 Crayola box!

[wsscott]

I checked, and I had swapped the positions of the 3.3K and the 2.2K.  Their bands and readings are correct, just in the wrong spot.

Shooter-any thoughts on the Output Speaker Jack AC voltage of 14.5 VAC?  Is it too high/low/or just right?  Like the 3 bears.

[SEL49]

Shooter-any thoughts on the Output Speaker Jack AC voltage of 14.5 VAC?  Is it too high/low/or just right?  Like the 3 bears.

With no input signal to the amp there should be zero volts at the speaker jack. The speaker jack must be mounted to the chassis to provide a ground to one side of the OT secondary. Alternately, if the speaker jack is just dangling in the air, you must connect a wire between the jack ground lug and chassis ground. Grounding one side of the OT secondary will likely make that 14.5V meter reading disappear.

[wsscott]

Boy do I feel dumb!  So I could do it as you suggest, or just wire the speaker terminals directly to the 2 secondary wires?  I now assume that’s how it was originally done—as shown in the schematic.

[shooter]

you must connect a wire between the jack ground lug and chassis ground.

even if you hardwire the speaker


Math:  14.5vac ^2 / 8ohms = 26W  .....  When it's actual music and sounds like it "should" 

[wsscott]

If I decide to mount the jack to the chassis, there’s an existing hole next to V1.  Is that location ok or is it to close to the preamp section so as to cause noise?

[SEL49]

Don't put the speaker jack near V1!

[shooter]

^^^^^^^^^^
you want large signal wires and sensitive signal wires as FAR apart as possible,  Look up "Inductive coupling" "signal cross coupling" for your evening reading as you sip a fine single malt n unwind the day 

[wsscott]

Since the amp I'm dealing with didn't have an OT when my buddy was given the amp, and no output jack on the chassis, does anyone know how this amp was physically wired?  Was the speaker wired directly to the OT, and a wire added to the OT COM side that then connected to the chassis somehow?

[SEL49]

Have you forgotten reply #198 already?

[wsscott]

SEL49-I remember the post, but since my OT only has 1 black and 1 brown wire, I'm trying to figure out the best place to connect the ground wire to the chassis.  I just wanted to "see" how the Ampeg came wired from the factory.

[Willabe]

2 ideas come to mind, both without a 1/4" output jack.

1. Run the OT secondary pair of wires directly to the speaker and solder them in place on the speaker tabs along with a 2nd pair of wires that go back into the chassis. The 2nd pair of wires you run into the hole in the chassis that Ampeg used. You might need to put a rubber gromit in that hole or enlarge that hole a little so you can get a gromit in it. Attach those 2 wires that you brought back in to the solder eyelets/turrets on the board. Those 2 wires are for the -FB loop.

2. Run the OT secondary wires into the chassis through the hole that Ampeg ran them through, might need a gromit. Attach them to the board where they go for the -FB loop. And run a 2nd pair of wires from those 2 solder eyelets/turrets back out through that same chassis hole to the speaker and solder them in place.

Either way, you will need 2 pairs of wires to get the OT secondary hooked up to the speaker and the -FB loop.

And you have to run them through the chassis hole that Ampeg used. That chassis has no room for a chassis 1/4" jack.

[Willabe]

It shows in this layout drawing where to connect the OT 8 ohm secondary -FB loop wire to the eyelet/turret board and where the 10K -FB loop R gets placed/hooked up. 

Just need to solder the  chassis OT secondary ground wire (2nd OT ground wire to eyelet board, 1st OT ground wire to the speaker) to ground inside the chassis.

https://el34world.com/Forum/index.php?action=dlattach;topic=18071.0;attach=48337

[wsscott]

Willabe-Thanks.  Yes I used that Layout in wiring the OT. First, I connected the 2 secondary wires from the OT to the lugs on a 1/4" jack to be connected to the speaker. Then I ran the 10K Resistor to that eyelet from V4, Pin 3 cathode.  Then I ran a new wire from there to the 8 ohm lead on the Speaker Jack.  Finally, just today, I ran a new ground lead from the Ground lug on the Speaker Jack to a ground on a terminal strip inside the chassis.

So I think I did it correctly.  Please tell me if something is wrong with it.  It now shows no VAC on the Speaker Jack when there is no input signal, and it shows VAC when there is an input signal.  The voltage increases as I turn up the Volume.  I stopped when it read about 6VAC on the Speaker Jack because it was really loud.

But I've still got the hiss.

The schematic shows the Input Jacks as 1-Guitar, 2-Accordion, and 3-Mic.  The amp is wired with the 470K resistor on the Guitar Input jack, and not the Middle Accordion Input jack.  The 5.6M is also wired to the Guitar Input jack and of course Pin 4, Grid.

The hiss is worse on the Guitar Jack, and much less on the other 2 jacks.  The hiss is there regardless of whether or not the guitar is plugged in.

If I remove V2 then there’s no sound at all because the signal stops and can’t get to the Phase Inverter for the final processing.

Half of V1 is for the Pre-amp, and the other half is for the Tremolo circuit.  The hiss isn’t affected by my turning either the Tremolo Speed or Intensity pots.  And of course there’s no Tremolo.  So my guess is that the Tremolo circuit is not causing the hiss.

I have replaced all the resistors and caps from the Input jacks through to V3.  And I’ve replaced 90% of them through the end of the circuit board.

Because the hiss is less at Inputs 2 and 3 (accordion and mic) its my guess its something either with the jack wiring to the circuit board, or the Volume pot.  The jacks have been cleaned as well as the chassis where they attach.  The resistors on the input jacks have been replaced.  As stated before I replaced the 5.6M with a a 4.7M since I didn’t have the larger resistor on hand.

[Willabe]

Yes I used that Layout in wiring the OT. First, I connected the 2 secondary wires from the OT to the lugs on a 1/4" jack to be connected to the speaker. Then I ran the 10K Resistor to that eyelet from V4, Pin 3 cathode.  Then I ran a new wire from there to the 8 ohm lead on the Speaker Jack.  Finally, just today, I ran a new ground lead from the Ground lug on the Speaker Jack to a ground on a terminal strip inside the chassis.

Only 1 thing, how are you going to keep the 1/4" jack and 1/4" plug from swinging/flopping around? Did you fasten them somewhere?

So yesterday I spent some time replacing carbon comp resistors, .....

I've noticed that the hiss seems less, but also the gain is much lower now.

I stopped when it read about 6VAC on the Speaker Jack because it was really loud.

So is the amp back to full volume again?

[wsscott]

Yes, I will stabilize the speaker jack.

Yes, the amp is back to full volume.

Is that 5.6M resistor or that 470K resistor at the input jack a potential cause for the hiss?  Why are they both so large?

[Willabe]

Yes, I will stabilize the speaker jack.

You really didn't need that 1/4" speaker jack, extra work without a need.

[Willabe]

Is that 5.6M resistor or that 470K resistor at the input jack a potential cause for the hiss?  Why are they both so large?

Just change them to MF(metal film), then you'll know if they were a problem.

They used those values to set the bias where Ampeg wanted it that 6SL7 tube. 

[wsscott]

I already changed them to metal film.  Still the hiss. 

[wsscott]

I just noticed that the ground lead in the shielded cable connecting V1 P4 grid to the Input jack is not grounded on either end.  It's just the shielded lead that is connected to the tube socket and then to the input jack.  Could this cause the hiss?

[shooter]

NO INPUT.....GROUND pin 4 of V1 with a gator clip, did the hiss go away?

[wsscott]

YESSSSSSSS!  NO HISS!!

Okay, so I'll connect the ground shield of that wire to the chassis ground up at the input jack and leave the other end at the tube socket cut off and not connected.

[wsscott]

Ok, so connecting the ground shield doesn't work.  So that's not the solution. 

So Shooter what is the solution?

[shooter]

replace the input jack with a proper one that GROUNDS the input with nothing plugged in, maybe the shielded wire also if it's been solered to death over the years

[wsscott]

The jack is a grounding Switchcraft type jack with the grounding tongue on the tip lug.

With nothing plugged in the jack, if I check continuity between a chassis ground point and the tip lug on the jack there is continuity.  If I put in a plug there is no continuity to ground.  Seems proper?

If I plug in a guitar with the alligator clip connected from Pin 4 to Chassis Ground, I get no sound at all.  No guitar, no hiss, nothing.

But if I remove the jumper, I get sound and hiss.

Is that jack wired wrong?  I disconnected its connection to the other 2 jacks just to eliminate them from the circuit.  I thought let me get 1 input jack working right, and then I can deal with the others.

[wsscott]

So Shooter, is your analysis that the jack is leaking since the circuit is quiet when Pin 4 is jumped to ground, but its not quiet when I rely just on the jack?  So even though there's continuity with chassis ground, there's still a bad connection in the jack itself that's causing this problem?