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

[wsscott]

Sorry I meant 700 ohm not 1K.

[shooter]

i like moving in slow increments 330, 390, 430, 470


as it sits it's about 2.1W so 5W is fine
ballpark math says a 390 ohm will get you close enough to find n fix everything else


You want a "target" of about 90% tube dissipation, since idling is as many watts as it will draw, as you beat on the strings power will drop slightly in a self biased amp.
then you tweak for tonal happiness   

[wsscott]

Sorry, I don't understand your comment "as it sits it's about 2.1W so 5W is fine".  Thanks.

[wsscott]

Oh, I got you.  You mean I don't need a 10W resistor, but a 5 W resistor would work just fine.  Good.  I've got a selection of those to work with.

Should I change this resistor before trying to resolve the other issue of no output sound?

[shooter]

I would, shoot for anything <14W per tube, use the math I posted to check.  once you change the resistor both cathode and plate volts will change, so re-measure each time

[wsscott]

Thanks.  The smallest resistor I have on hand is a 470 ohm.  So that’s where I’ll start.  Can I attach the cap after I find the correct resistor or should I do it each time I change the resistor?

[shooter]

yes

[wsscott]

Ok, so with a 470 ohm resistor, actual reading is 475 ohms, I get the following:

Pin 8=28.9VDC
Pin 3=405VDC

29/475=0.06105. 61ma/2= 30.5ma
405-29=376 x 30.5= 11.47 W/tube

The only other resistor I had was a 680/667 and it gave me a COOL reading of 9.24 W/tube

So should I settle for the 475 ohm bias resistor, or order a few more in between?

[shooter]

slap the cap on and check the box, you're on to bigger fish

All of the tubes are very sensitive to being even lightly touched,

that's a indication the sockets need re-tensioning

[wsscott]

Ok, here are voltages after the bias resistor was changed.

Filter Cap Nodes 1-6 respectively: 408 371 325 241 213 174.   Increase for each node of about 7%

Filaments on all tubes 6.5VAC

Rectifier Pins 2 & 8 read 408

Pins 2 and 5 for V1 thru V4 respectively:  364/320; 159/186; 154/21; 240/154.  So V3 is suspicious.

Pins 3, 4 and 5 for V5 and V6 are the same respectively:  401 376 28.5

Ok, so I've got V3 acting weird with the filament flash on just one filament at startup; and the weird V3, Pin 5 Voltage
reading.

Still no output from the input jack with guitar plugged in.

So do I try to trace the input signal, or do something else?

Thanks Shooter for your help.

[shooter]

These are all new tubes

you bought new tubes or "they look like new tubes"?


you need the DC volts correct before moving on to AC
so it's tedium time, verify wiring, correct component values, solder connections...etal.


hunt up Dougs "how to troubleshoot" procedure where you trace the circuit, check the parts, highlight....
I typically get a 2X photo-copy of the schematic, pack a bowl of herbs, put on music, KEEP POWER OFF while in this state, but the coloring is fun 

[wsscott]

My buddy whose amp it is, bought all new tubes for the project.

I did a test on that 6SN7 in the V3 position that seems to have an issue with one of the filaments.  I swapped its position with V2.  The problem went with the tube to V2.  So that tube may have been defective.  I don’t think it should be doing that even if it calms down after the initial turn on surge of power.

The swapped tube doesn't have any issue with the V3 position.

[wsscott]

I also checked the Voltages on Pin 5 of V3 and V2, and the low voltage on Pin 5 stayed with the socket for V3.  So it's not the tube causing the low voltage. The voltage issue is with the socket. Pin 5 of the 6SN7 that I moved to the V2 position showed proper voltage.

Just one more thing.

[shooter]

Pins 2 and 5 for V1 thru V4 respectively:  364/320; 159/186; 154/21; 240/154.  So V3 is suspicious.

remeasure V1 - V4 and post like this
V1p 555vdc V1k 22.5vdc
V2........
V3
V4

[AlNewman]

Cathode voltage is just as important as plate voltage when taking measurements.

Seeing as you have 2 triodes drawing from node F, and the current to that node is 1.7 mA, I wouldn't suspect a short at the moment, but perhaps the 47k resistor has opened up, meaning V3 isn't drawing current.  But if that's the case, what caused the resistor to open up could have been a short in either the cathode resistor, the bypass cap, or the coupling cap.  Or, maybe it just got tired.  It's the voltages that tell the story.

[wsscott]

As I've said, the wiring in this amp doesn't match up with any schematic.

The V3 Pin 5 plate is fed from Node F at 174VDC.  It goes through a 100K (actual 103K) resistor, NOT the 47K as shown on the schematic.  So maybe this is what's cutting down the voltage so much.

I don't know enough to know how to calculate the voltage drop, but maybe you do.  If so, what would the resultant voltage be at Pin 5?

Thanks.

[shooter]

don't worry "what" just measure V1-V4 plate AND cathode VDC, also take the time to MARK-up a schematic so we in the cheap seats can have useful knowledge.

[wsscott]

I haven't had a chance to MARK up the schematic, but here are the plate and cathode voltages for V1a/b through V4a/b

V1ap 364vdc  V1bp 320vdc
V1ak 14.6vdc  V1bk  7.67vdc

V2ap  159vdc  V2bp  186vdc
V2ak  7.92vdc  V2bk  8.05vdc

V3ap  154vdc  V3bp  21vdc
V3ak  6.93vdc  V3bk  5.41vdc

V4ap  240vdc  V4bp  154vdc
V4ak  2.58vdc  V4bk  2.18vdc

Let me say this about V3 and the schematic, the wiring is totally different. I would comment also that the V3bp is fed by Node F at 177vdc.  It connects to a 100k resistor and reads 177vdc going into the resistor, and 20.05vdc coming out of it, which then connects to pin 5.

So that resistor which is marked as 100K and reads 103K is where the voltage is lost.  Connected to the plate side of that resistor is a .005uf cap which leads up to the "Dimension" pot for the Echo.  The schematic doesn't show components with these values.

I hope this helps you understand what I'm dealing with.  HaHa!

[shooter]

is the cap I have circled in the circuit?
IF yes, lift the "left side" where it connects to the coupling cap and pot, THEN re-measure plate vdc

[wsscott]

No, there's no other cap, no 680pf or 47K resistor.

Here's a photo with my red line showing the path.

Node F comes in at the top of the board on the blue wire, connecting to a 100K resistor, then down to the eyelet, where a green wire runs to the Plate (Pin 5) of V3.  At that eyelet there's a pink 0.005uf cap connected that goes up to an eyelet at the top of the board, and then a green wire runs from there directly to the outside, left tab of the "15K" (actually is a 25K) "Dimension" pot that controls the Echo.

[shooter]

 
which leads right to the 680pF cap strapped on the pot

[wsscott]

Sorry, but no cigar.  That cap is not connected to the lead going to the pot.  The green lead is connected to the pot on the far right in the photo, and that cap is on the one to the left of it.  The tone pot.

[shooter]

the TS doesn't indicate a 680pF so there's another schematic "update" then

[wsscott]

Easier said than done.  I think I've learned from watching a lot of Youtube videos on Ampegs, and this model in particular, that you never know what you've got and the schematic is a "starting point", but quality control seems like it might have been lacking.  I get a headache when I'm trying to trace the wiring and compare it to the schematic.  It probably would have been easier for me to just to start from an empty eyelet board and build it from scratch.  But then it wouldn't be "authentic"!!  And of course, what schematic could I follow to build it??????

[shooter]

I get a headache

see previous post on tracing schematics, herbs, music, think of it as where's waldo 


The math on V3's volts seem good
156vdc / 100K = 1.5mA
5.41vdc /3.3K = 1.6mA
so any smart kids wanna help guess if this is a herring or problem

[AlNewman]

It's been a long time since I was a kid, OR smart, but without some sort of a schematic, there isn't much anyone on this side of the internet can do.

By the voltages, all tubes are drawing current, by the schematic, the nodes don't draw proper current compared to voltages at the tubes.  Probably the dropping resistors aren't what is drawn, maybe cathode resistors aren't what's drawn, definitely plate resistors aren't what's drawn.

We do know there isn't a signal, so unless somebody is going to trace the circuit, perhaps the best option would be to go through with a listening amp and find where you lose signal.

[wsscott]

I think I can trace a signal.  I've got a simple small digital OSC.  Is that the best way to do it, and input a 1K signal with a dummy load on the output jack?  Thanks.

[AlNewman]

Yes, just be careful around high voltages with your scope, IE, measure at the 0V side of the coupling caps.

I'd also, before hooking your scope up, just check resistance to ground at your cathodes, and compare to known, by reading the bands, values of your cathode resistors.  Maybe you have a bypass cap that's shorted.  While you do this, print out the schematic, and mark the values as you go.  Like Shooter said, sometimes it's fun to colour.

[AlNewman]

You do have SOME noise at your speaker, correct?

[wsscott]

Thanks. Yes the noise I have is the crackling of the tubes in the sockets.

[AlNewman]

Right, I think I recall somebody suggesting to retension, or clean something along the way.  Anyways, you'll be fine.  Just remember, Shooter is the best with scopes on this forum.  If there's a guy who can get a result from an oscilloscope, it's Shooter.

[wsscott]

I found this Voltage chart for the amp last week and meant to post it, but somehow got distracted.  It was prepared by someone with the amp and he shows his voltages vs. voltages he managed to find on an Ampeg schematic(s).

As you can see, the V3 Pin 5 voltage is 109vdc and Pin 6 is 4.4vdc.
Mine is 21vdc and 5.41vdc.

Its also like these readings for my tubes are "flipped" with his readings for V3 and V4.

Regardless, V3 Pin 5 is certainly not close to my reading of 21vdc.  So probably not a "red herring", but still "fishy"!

[bmccowan]

V1, V2, V3 etc are just identifiers. Typically manufacturers keep tube locations and functions consistent. Key word; typically. To troubleshoot using measured voltages, you need to know the function and connections of each one of those tubes. You have identified several inconsistencies. PIA, I know, but you need to track those connections, even if it does make your brain hurt. That's how to resolve your comment on V3&4 being flipped, because they might be.

[shooter]

my reading of 21vdc

because you are having so much fun....
Re-measure the plate BUT use a couple different "ground points"


1st use the "bottom" of V3 cathode R (3.3k) on Joe's schematic
2nd Chassis
3rd the NEG side of the Cap that feeds that Tap (F if I recall)

[wsscott]

Thanks guys.  Will re-check with different ground points.  I thought I might just go ahead and replace that 100K resistor.  Maybe something is going on with it just when voltage is applied that causes it to act differently when I check its value without voltage.

[wsscott]

Scooter-I checked the Plate voltage again using the 3 different ground points, and they all read right around 20vdc.

So I guess I'll trace the signal before I do anything else with the circuit, ie. like changing the 100K resistor, and see what results tracing yields.

AINewman:  I checked the cathode resistors, and their values from cathode to ground read the same as their resistor bands.  So no short.  However, although the cathode resistor for the plate in question reads 3.3K which is consistent with the schematic, the cathode resistor for the first half of the tube reads 5.6K and the schematic shows 330 ohms.  No where on the schematic I'm using is there a 5.6K cathode resistor.  But the plate voltage is ok.  So who knows???

[wsscott]

Ok, I connected the scope and checked out all of the Grids on the 4 tubes.  Some of the images were jumpy, and I think that's probably a lot to do with needed the sockets tensioned.  But I did get wave signals, at least at first, on all of the grids, a couple with beautiful sine waves, and others distorted and really jumpy.

I did save pictures of some of these images on the scope.

The grid on V1, P4 was clean smooth, and pretty but that's where the signal comes in from the input jack.
It's P1 grid was rough and jumpy but still there.
I got signal on the V2 grids but not pretty and steady.
Then on V3, I originally got sine waves on both of the grids, but when I checked again later I couldn't get any signal.
Finally on V4, I got signal on both grids, and it was pretty clean but got a little jumpy.

Finally I checked the grids on the Output tubes.  And unbelievably I was getting a Sine Wave with its amplitude changing as I turned the volume and tone pot!  This was using the 1K test signal.

So I figured if there's a sine wave on the grids of the output tubes, I must be getting something at the speaker.  So I unplugged the dummy load, plugged in the speaker, and I could hear the 1K signal through the speaker and its volume and tone varied with the volume and tone pots.

So I said well let's see if it will play a guitar sound.  I plugged it in an VOILA, I'm now getting sound from the guitar through the test speaker.  Its volume is still pretty low even at the volume pot's highest setting, but signal is coming through.

So some great progress!!  Now to figure out how to get some proper volume.  I'll let my buddy work on the sockets over Christmas and maybe that will improve things.

Any idea what could have happened??  Thanks.

[shooter]

Any idea what

 
Mine is not to wonder why, mine is to fix shit and go play

[wsscott]

Shooter--You just have no curiosity about you!!

Problem its not loud enough to be heard 20 feet away!

So I've still got to resolve that V3 plate issue, and also get some input from you guys on the forum on what to pursue on the volume.

Maybe its these weird component values that deviate from the schematic that's causing all the headache!

[shooter]

 
I watched it kill to many cats


when you troubleshoot for a living, after the "honeymoon phase" it's just become the most efficient, competent, you can, then go live the life you want, mine was mostly stuck to a cliff-face 200 ft up, thinking Huston, we might be in trouble.


once you have the socket issue solved, the amp is stable, not crackling, popping etal then you can scope out each section, measuring input V output of that section to see where gain is made and gain is sucked out.


if it's still set up for scoping;
set all TS to 5, gain n volume to 8, measure/adjust your input signal at the input jack typical would be ~~ 100-140mVac, then measure the speaker +  Typical ~~~ 8-10Vac

[wsscott]

Will give it a try.  Is the input signal value you mention Vrms or Vpp?

[Willabe]

So I've still got to resolve that V3 plate issue,

This is the tube that the plate problem goes with when you put it in a different socket? It follows the tube?

If so, just buy another tube.

[shooter]

rms or pp, dealers choice but DO NOT mix n match AND state which.  I came from the RF world, everything was done rms.  The audio world likes PP because it sounds "bigger" and they can charge for the "extra power" 


aside;
wife n I were chill'n on the couch after about 20 years married she says "I never once worried you'd leave me for another woman, there isn't a woman alive that you could love more than them damn machines you fix"

[bmccowan]

Lucky for her there are not a lot of women with a bunch of broken tube amps.

[wsscott]

Willabe-no, the brightly glowing filament at startup follows the tube, but the low plate voltage stays on V3 Pin 5 plate.  I'm sending the tube back to Antique Electronics Supply tomorrow for replacement since I just got the JJ from them a week or so ago.

[shooter]

tube amps are just a hobby, she was referring to the $7 million worth of MRI's I was responsible for, the CT scanners before that.  She is one of the amazing Techs I dealt with, I could leave a meter on-site and she would understand my instructions, measure what I needed to know, at the other end if I got a call from a sketchy Tech; "Just power down and go to lunch or something, DON'T touch anything!"


now I specialize in landscaping n gardening, the closest I get to electronics is making SURE the electric fence is powered down before entering the garden 

[Willabe]

Willabe-no, the brightly glowing filament at startup follows the tube, .....

That's common enough, not a problem. At turn on in rush of current hits the filament causing that brighter glow for a second or 2. Because the filament is cold, it has low resistance, so draws a lot more current, and this causes the brighter glow. A few seconds, as filament gets hot it's resistance increases, and the filaments glow lessens. This is how/why a light bulb limiter works.

Probably why your seeing it on 1 of the filaments in this 1 tube, cathode sleeve is a little low, so filament is sticking out the end of the K sleeve some. Probably nothing to stress about. We've had guys see this before, post about it and the tube was fine.

..... but the low plate voltage stays on V3 Pin 5 plate.

Maybe you have done this already and I missed it but, with that socket, measure the the dcv on the sockets plate pin, K pin and grid pin.

If the plate dcv is much higher, looking normal, then it's got to be that tube, if not, if it's still looking low, then you trace backwards from plate pin to B+ supply node with your meter. Measure other side of plate R, then measure at the filter cap + for that B+ node. You measure both because it could be a solder joint, could be the wire. It can also be the plate pin in the socket.

This shouldn't be that hard to find what is causing this low plate dcv.

[wsscott]

Willabe-my post #117 comments about the subject voltages.  The subject plate, ie. 20vdc, connects to Node F carrying 177vdc through a 100K resistor.  There's 177vdc going into the resistor, but only 20vdc coming out and making it to the subject plate.  So the resistor reads good, but it's losing a lot of voltage at that point.  Why is that?  Could the resistor read properly but something else in the resistor be causing the drop?  I don't know enough about this stuff to even guess.  Or is the Node F voltage of 177vdc too low?  Based on another guys Voltage Chart that I posted earlier, my Node F voltage seems fine.

So what's pulling the voltage down?

[shooter]

So what's pulling the voltage down?

the easy answer is high current, but the math says there's only ~~1.5mA
another possibility is "the method of measuring"  that seems to be eliminated
the tube not drawing "proper" current seems to be eliminated by multiple tubes causing the same result.


once you eliminate all other possibilities, the problem will be come obvious 
 

[wsscott]

Shooter, and the answer is???  Nothing is wrong?  If so, I'm skeptical, but what do I know.  Its my gut.