Hoffman Single Channel AB763

[purpletele]

You might need to reverse your OT primary wires if it's oscillating badly.

Thanks, I had the OT wires scream at me the other day so I switched them and they are much happier.

The oscillation started going away when I turned down the bias setting, but I limited out on the low side with 48 mV.

The oscillation could be affected through bass, middle and treble controls.

The amp felt like it was ready to take flight on its own, so I checked the bias and noted that it was right at double of what it should be, so I shut down. 

I'm pretty certain that I now need to reduce the Bias resistor from 100K 3W to at least 82K 3w, there could be other adjustments as well.

I had originally changed a 470 Ohm Bias resistor to 100 K Ohm due to my transformer not having a specific bias tap for this circuit.  I am using an Edcor Stand Up Transformer.

BV

[sluckey]

Pull V5 tube. Does the oscillation disappear when you pull V5? Recheck bias with V5 pulled.

Bias needs to be set with no signal applied. This also includes any oscillation due to a problem. The oscillation looks just like a signal being applied to the output tubes and causes the tube current to increase (bias mA goes up).

[purpletele]

 author=sluckey link=topic=21976.msg248881#msg248881 date=1521635143]

Pull V5 tube. Does the oscillation disappear when you pull V5? Recheck bias with V5 pulled.

Yes.  The bias is reading 22.5 mV with V5 pulled.

I reset it to 25 mV.  The amp is certainly loud, the chords still have a mild distortion and the end and then the notes fade quickly.  The tone is getting much better.

When I turn the volume up as well as the treble it goes into a high pitched scream.

I also have a decent hum going on constantly.  The reverb gets real buzzy when the turned up.

Definitely getting close but not there yet.

Based upon your recommendations and analysis of the bias, I have reset the bias at 25 mV.  I'll assume that the bias has jumped up because of the interfering oscillation or noise that is emulating a signal.

I'm going to retrace and recheck the components and wiring around that side of the circuit.

Chopsticking around the Reverb circuit I have found V3-6 to be a very loud and sensitive plate.  Not sure where to take it from here.

Bias needs to be set with no signal applied. This also includes any oscillation due to a problem. The oscillation looks just like a signal being applied to the output tubes and causes the tube current to increase (bias mA goes up).

[VMS]

Did you chopstick the yellow wires going to tonestack? KOC recommends twisting wires going to tonestack together.

[purpletele]

Did you chopstick the yellow wires going to tonestack? KOC recommends twisting wires going to tonestack together.

d


I did play around with the tone stack wires and I couln’t Get a reaction.  I think i’ll Post a video, the actual sound might provide indicators that don’t register with my experience yet.

Thanks

BV

[purpletele]

Here is an updated video of the noise I am getting from the Tone Stack.

Also, I am not getting my reverb to work, I am getting very loud sensitive wires at V3-6 & 7.  I am focusing in the reverb circuit to see if I have something wired incorrectly.

I am wondering if I need to create a better ground for the reverb jacks.

Please let me know if anyone has advice on where to look.

Thanks,

BV

[purpletele]

Update to the Video

I grounded the reverb jacks amongst themselves and grounded to the chassis with a bolt.  That made a world of difference.

I am down to a constant hum and my reverb.

I am getting 400 V on V2-1 as well as the red side of the transformer, However I do not get any reading more than 0.0 mV on Pin 2 as well as the green secondary coming off of the reverb transformer.

Shouldn't I be getting a low voltage from the reverb driver rather than 0.0 mV

The tube that I have in there is a fresh 12AX7, I don't have an 12Au7 on hand.  I may have a 12AY7 I can try.

The tone was really nice tonight with room for improvement. 

Thanks for any insights

BV

[sluckey]

I am down to a constant hum and my reverb.

That's progress. So the gremlin sounds are gone? Can you kill the hum by turning any knob to zero? Which knob(s)? Or, is the constant hum associated only with the reverb? IOW does the hum disappear if you turn the reverb pot to zero? Do you get any reverb effect? If you bang on the reverb tank do you hear a crash or thunder sound in the speaker?

I am getting 400 V on V2-1 as well as the red side of the transformer, However I do not get any reading more than 0.0 mV on Pin 2 as well as the green secondary coming off of the reverb transformer.

That's all normal.

Shouldn't I be getting a low voltage from the reverb driver rather than 0.0 mV

no

The tube that I have in there is a fresh 12AX7, I don't have an 12Au7 on hand.  I may have a 12AY7 I can try.

The correct tube is 12AT7. I've had to try several different 12AT7s for this reverb driver to find one that did not hum.

Your posted tube voltages are messed up for V2. Would you re-check pins 1, 2, 3, 6, 7, and 8 and post the correct voltages?

[sluckey]

One more thing... Your tremolo is very weak. You should be able to almost get a throbbing tremolo effect with 6V6s.

[davidwpack]

All my reverb problems went away when I unplugged the footswitch. I was using a switch that wasn't intended for reverb switching. (Needs to be shielded). It introduced awful noise. I couldn't tell if you had one plugged in. If so, try that in addition to what Sluckey said.

[purpletele]

Thanks guys!

[purpletele]

That's progress. So the gremlin sounds are gone?

Not entirely, when the volume is maxed it sounds like an x ray machine on idle.  If I max out the volume and turn the tone stack uniformly up to 12:00 it gets real noisy.

Can you kill the hum by turning any knob to zero?

I can kill the noise but I am left with a constant hum, which certainly could be a pre-amp tube, but it's pretty loud.

Which knob(s)? Or, is the constant hum associated only with the reverb? IOW does the hum disappear if you turn the reverb pot to zero?

When all knobs off and I turn the reverb pot up full it seems to be the same frequency and cycle noise, only louder.  I put an Ay7 in V-2 and that is noisy, I am trying to get some AT7's today.

Do you get any reverb effect? If you bang on the reverb tank do you hear a crash or thunder sound in the speaker?

Yes, when I turn the reverb up and drop the tank it sounds like a crash cymbal.

Your posted tube voltages are messed up for V2. Would you re-check pins 1, 2, 3, 6, 7, and 8 and post the correct voltages?

I updated the voltages on V-2 please see attached.

I'll put together a quick video later,  Thanks for the ideas.

BV

[VMS]

Have you tried the amp in its cabinet, circuits can sometimes pickup all kinds of noises from lights and other things on your workbench.

[purpletele]

Have you tried the amp in its cabinet, circuits can sometimes pickup all kinds of noises from lights and other things on your workbench.

I have not, good point.

[purpletele]

Update on Deluxe Progress

1. Had full power yesterday with really nice blooming tone but I had noise in the background, and the reverb was not working.

2. I was probing without any findings when I had a pop, not sure if I caused it with a wire but I couldn't repeat it or make it change.  It took the amp into a real flubby tone like a rectifier going bad.  The rectifier is red hot .

3. I didn't have a replacement so I decided to clean up the grounding.  I changed my scheme to the way I have been building my last three amps, which is to have the CT's and one ground connection from the Capacitor Reservoir.  The control wires ground go back the a negative buss on the circuit it came from.

4. I went ahead and grounded the ground tabs on the reverb and tremolo jacks. I took that ground back the a negative buss in the reverb circuit.

5. When I started it up the noise was gone but the volume and tone was flubby as it was before.

6. I picked up a new rectifier, 5AR4 and loaded it in.  Prior to energizing I pulled V-5 and reset the bias at 25 mA, it was at 24 mA.  I replaced the tube and then started it up.

7. The bias was right at 24.6 mA with the tube in, which I hope means that it is not picking up noise or oscillation in the circuit.

8. I went through and updated a voltage chart including a new column for todays voltages so you can see the delta.

Question

What would cause the rectifier to get really hot?  Too much voltage?  How do I regulate or affect the rectifier voltage?

[ Invalid YouTube link ]

[sluckey]

Check the AC voltage going into the rectifier. Check the dc voltages at all filter caps.

Tell me about that reverb tank. Who made it? What is the number on it? Where did it come from?

[purpletele]

Check the AC voltage going into the rectifier.

5. 2 VAC on the yellow

678 VAC on the Red Secondary's

Check the dc voltages at all filter caps.

Node A = 434 V  two caps wired in series
Node B = 431 V
Node C = 426 V
Node D = 410 V

Tell me about that reverb tank. Who made it? What is the number on it? Where did it come from?

The tank is from Antique Electronics
MOD P-RMOD8AB2A1B
10 Ohm
2575 Ohms Output Impedance

[sluckey]

Voltages look fine. No clues here. Reverb tank is suitable also.

[purpletele]

Voltages look fine. No clues here. Reverb tank is suitable also.

It is curious, and I don't have a full grasp on how DC voltages are flowing to and from the rectifier.

The only thing physical thing that could be affecting the voltage of the rectifier output could be the bias tap, correct?

I don't understand what would cause the rectifier to be so hot other than it is getting energy backed up.  I am going to change the bias tap wire to an 18 GA wire just to see if that has any effect. 

When I play the amp with its half power tone I still get decent response and pinch harmonics.  It feels like I'm a half step from the correct note, real close.

Thanks for reviewing and assisting

BV

[sluckey]

The only thing physical thing that could be affecting the voltage of the rectifier output could be the bias tap, correct?

No.

[bnwitt]

Every time I've had a pop and then a change in signal or output but some partial output is still there, it has been a bad connection or solder joint at a component lead or wire. I'd aggressively chopstick every board component and every tube pin wire.  Do you have under board wires?  If so did you bring the leads up inside the turrets and fold them over ala Sluckey.  In cases like this it helps to have an audio signal generator, a dummy load and a signal tracer (listening amp) to trace the signal through the different stages of the amp.  If you don't have those below are ebay links to the ones I use.


https://www.ebay.com/itm/Heathkit-IT-12-Signal-Tracer-Working/273133747401?epid=1300239703&hash=item3f980a3cc9:g:AOgAAOSw93ZavUVC


https://www.ebay.com/itm/Heathkit-audio-generator-model-IG-5218/132549445825?hash=item1edc9008c1:g:zFsAAOSw5i5atX0E

[purpletele]

Barry,

Thanks for the thoughts and insight;

Every time I've had a pop and then a change in signal or output but some partial output is still there, it has been a bad connection or solder joint at a component lead or wire. I'd aggressively chopstick every board component and every tube pin wire.

I have fondled every connection and wire a few times.  I know there is a bad connection but I can't find it yet.

Do you have under board wires?  If so did you bring the leads up inside the turrets and fold them over ala Sluckey. 

I learned that lesson a while back.  I have solid wire jumpers with a hook lead over the turrets.  Doesn't mean I didn't suspect them and then proceed to tear it apart to verify.

In cases like this it helps to have an audio signal generator, a dummy load and a signal tracer (listening amp) to trace the signal through the different stages of the amp.  If you don't have those below are ebay links to the ones I use.

I have a tone generator on my Ipad and I made a test probe that hooks into another amp.  I have ggod signal coming into V1-2 and I loose it coming out of V1-1.

I have an ESR coming today to check Capacitors, but I suspect it is a bad ground joint.

I invested in a new Hakko iron about half of the way through the construction of this amp.  My Weller was giving up the ghost.

I'm going to keep searching, it was there and I lost it.

[sluckey]

I have a tone generator on my Ipad and I made a test probe that hooks into another amp.  I have ggod signal coming into V1-2 and I loose it coming out of V1-1.

Does that mean there is no signal at V1-1? If so, that should be really easy to find. Start by measuring the voltage on V1 pins 1, 2, and 3. What have you?

[purpletele]

Does that mean there is no signal at V1-1? If so, that should be really easy to find. Start by measuring the voltage on V1 pins 1, 2, and 3. What are your voltages?

V1-1 = 271 V
V1-2 = 3 mV
V1-3 = 1.9 V
V1-4 =
V1-5 =
V1-6 = 273 V
V1-7 = 0 mV
V1-8 = 1.88 V
V1-9 =

[sluckey]

With those voltages you should hear a signal on pin 2 and you should hear a louder signal on pin 1.

Your home made probe has a dc blocking cap in it, right?

[purpletele]

With those voltages you should hear a signal on pin 2 and you should hear a louder signal on pin 1.  Your home made probe has a dc blocking cap in it, right?

I rebuilt a probe the other day with the same results.  The probe has a .1 uF 400V Cap with a resistor in series and another resistor going to ground.  It works well at V-2 as I mentioned.  I'm going to make a video of that testing as well as the muddy tone.

I pulled all of the Orange caps and tested them, I also tested the by pass caps in circuit with an ESR, that was cool.

I took photos of the wiring and cross checked it.

I replaced the caps but utilized some Jupiter caps to utilize the space more efficiently so I can see the connections.

Plugged it in and not a thing changed and I couldn't get any probing reactions.  Muddy tone with pinch harmonics, weird.

I re-measure the transformer voltages again, 678 V.  The Edcor is rated for 640 V, it sure feels like an overheated or dying rectifier.

Here is a video of the probing etc.

http://www.youtube.com/watch?v=lkUIHE1xvYQ

[sluckey]

That's very odd!. I wonder if your probe is a factor. All you need inside that probe is a series .1µF cap. I suggest removing those resistors and try probing V1-1 again.

Also, I'd like you to temporarily install a 1/4" phone jack for testing purposes. See attached drawing. This will allow you to connect your guitar directly into the PI of your amp. Sound will not be as loud as playing through a properly working amp. But it should be bedroom loud and it should be clean. Make a recording and post for us to hear and evaluate.

[purpletele]

That's very odd!. I wonder if your probe is a factor. All you need inside that probe is a series .1µF cap. I suggest removing those resistors and try probing V1-1 again.

My probe was originally a simple BillM nail to a .1/400V Cap, it worked well and picked up the signal, but when I lost it immediately I looked on line and tried another version.  I'm curious and will rule out every thing.

Also, I'd like you to temporarily install a 1/4" phone jack for testing purposes. See attached drawing. This will allow you to connect your guitar directly into the PI of your amp. Sound will not be as loud as playing through a properly working amp. But it should be bedroom loud and it should be clean. Make a recording and post for us to hear and evaluate.

I'll put that together soon.

. 

Thanks for the ideas.

[purpletele]

That's very odd!. I wonder if your probe is a factor. All you need inside that probe is a series .1µF cap. I suggest removing those resistors and try probing V1-1 again.

My probe was originally a simple BillM nail to a .1/400V Cap, it worked well and picked up the signal, but when I lost it immediately I looked on line and tried another version.  I'm curious and will rule out every thing.

Also, I'd like you to temporarily install a 1/4" phone jack for testing purposes. See attached drawing. This will allow you to connect your guitar directly into the PI of your amp. Sound will not be as loud as playing through a properly working amp. But it should be bedroom loud and it should be clean. Make a recording and post for us to hear and evaluate.

I connected the jumper jack and hooked up the wire as directed, but when I connect my guitar to that jack the amp goes silent, no mild hum, no signal, nothing.  When I plug into the front end then I get muddy tone.

No need for a video on that one.  I'm not sure what the field result means, I hope that gives you an idea.

. 

[sluckey]

Really! Show me a close up pic. I want to clearly see the jack and the wires attached to it. I want to clearly see the wire connected to the cap on the board. And I want to clearly see V5 pin 2. Take more than one pic if necessary to show me all three of those things.
 

[purpletele]

Really! Show me a close up pic. I want to clearly see the jack and the wires attached to it. I want to clearly see the wire connected to the cap on the board. And I want to clearly see V5 pin 2. Take more than one pic if necessary to show me all three of those things.

[sluckey]

I'm beginning to think you are cursed. Take that same guitar and cable and plug into a known working amp. Does it work properly?

[purpletele]

I'm beginning to think you are cursed. Take that same guitar and cable and plug into a known working amp. Does it work properly?

I have a very nice amp right behind me that I play, I just finished 45 minutes of staring at a circuit and playing guitar.  I have the same muddy tone on the amp as it has been for 3-4 days now.

I could not find information on the maximum secondary voltage hitting the GZ34, but there is a tremedous amount of AC voltage going to that rectifier.  Am I hunting in the wrong forest with the rectifier and the 678 V?

[shooter]

the 678 V?

warning, I did not read the other pages;

rail voltage is easy to check and monitor, even during play;
is it close to "spec" DC?
what does it fluctuate when being hammered?
Is there gobs of unfiltered ripple?

[purpletele]

t

he 678 V?  warning, I did not read the other pages;

rail voltage is easy to check and monitor, even during play;
is it close to "spec" DC?

I posted the As Built Voltages below.  Looking for a posted Voltage Chart for that amp?

what does it fluctuate when being hammered?
Is there gobs of unfiltered ripple?

The voltage is at 394 Vdc on the B+ rail.

When I hammer a chord it drops to 350 Vdc.

[sluckey]

Ha. I wrote you were cursed but I meant the amp is cursed. Sorry about that.

Am I hunting in the wrong forest with the rectifier and the 678 V?

Absolutely. We covered this earlier. That 678VAC is measured from one plate to the other plate. Perfectly normal. If you measure from chassis(center tap) to either plate you will see 339VAC. You could say your PT puts out 339-0-339. Or you could say your PT puts out 678VAC center tapped. Both descriptions mean exactly the same thing. Your GZ34 is FINE. My Sunn Sceptre puts 430-0-430 (or 860) on a GZ34. Data sheets list 550-0-550 (1100) as max.

You have gremlins in that amp or you have gremlins in your test equipment, or you have gremlins in your procedures. Maybe all three. There is no logical explanation to not be able to plug your guitar into the PI and hear some music. Given the normal voltages you gave for V1, there is no logical explanation for having a test signal on V1-2 but having nothing on V1-1.

We have already found multiple errors. I'm sure even more are hiding. I think you need another set of eyes and hands on your amp.

[purpletele]

I'm going to keep looking but at a less intense pace.

I agree it could be a number of things, I sure don't understand the Audio Signal testing results.  I ended up with the same result's with two different probes, which was a loud signal at V1-1 and then gone. 

I'll find the issue but it's a bit humbling, especially after I have esentially rebuilt it and corrected it along the way.

There not that many places to hide.

I have to keep working forward on the solution, but I also have some other projects lined up that I have started.

[bnwitt]

sometimes it helps to walk away from an amp and come back later with fresh eyes

[purpletele]

sometimes it helps to walk away from an amp and come back later with fresh eyes

I am setting it aside for know, I am hoping I'll come across something on another amp that will inspire a resolution.  've probably rebuilt that amp three or four times already, but it will not rule me.  Although it has knocked down some confidence.

It's will be resolved!

[bnwitt]

When you do figure it out it will be a lesson that lasts.

[purpletele]

The Luckey Deluxe Lives!



I'm sure I punished the OT with an early Flame Job  .  The OT was just barley passing current.

I was able to remove and replace without destroying anything.


I have a little issue with the reverb, but that can be cured I'm sure.


Felt like a monkey on my back

[purpletele]

This Deluxe Reverb is up and running strong with exception to the reverb part.

I went through and cleaned up the wiring, I cannot find any blatant errors.

I am getting the following voltages:

Node B   = 382 V
V2 1 & 6 = 378 V

0.0 mV @ Reverb Secondary

V3-1 = 170 V
V3-2 = 0.0  mV
V3-3 = 2.64 V


V3-6 = 168 V

V3-7 = 0.0 mV

I'm not getting anything at all from the reverb. 

I am not getting any signal from dropping the tank.

I tested the .0033 cap and it is fine.


Question: Is it possible to have the secondary not function properly but not indicate a short? 



Thank you all for any advice.

BV

[sluckey]

Question: Is it possible to have the secondary not function properly but not indicate a short?

The secondary of the RT (green/black wires) should read about zero ohms between them.

[PRR]

> I am not getting any signal from dropping the tank.

Then tank pickup or recovery stage.

Unplug the recovery cable and put your finger on it. Does it BUZZZ! ? If not, carefully put a finger on the reverb return control pot. (Don't have any other body part touching amp or anything grounded!) Does it Buzz! there?

[purpletele]

Question: Is it possible to have the secondary not function properly but not indicate a short?

The secondary of the RT (green/black wires) should read about zero ohms between them.

I am getting a reading of 2.5 ohms.  Not sure if that indicates a failing transformer.

Replacement transformer comes tomorrow.

[purpletele]

> I am not getting any signal from dropping the tank.

Then tank pickup or recovery stage.

Unplug the recovery cable and put your finger on it. Does it BUZZZ! ? If not, carefully put a finger on the reverb return control pot. (Don't have any other body part touching amp or anything grounded!) Does it Buzz! there?

PRR,

I am not getting any buzz from the return cable when I touch it.

However, I get a good buzz when I touch the Pot for the reverb.

Does that give you an indication of something?

[sluckey]

I am not getting any buzz from the return cable when I touch it.

However, I get a good buzz when I touch the Pot for the reverb.

You won't get any buzz from the return cable if you unplugged it from the amp. PRR wants you to unplug the cable from the tank and put your finger on the center pin. You should get a louder buzz than you got by fingering the reverb pot. If not, there's a problem with the reverb recovery triode.

[purpletele]

I am not getting any buzz from the return cable when I touch it.

However, I get a good buzz when I touch the Pot for the reverb.

You won't get any buzz from the return cable if you unplugged it from the amp. PRR wants you to unplug the cable from the tank and put your finger on the center pin. You should get a louder buzz than you got by fingering the reverb pot. If not, there's a problem with the reverb recovery triode.

Got it, thanks for the clarification

[purpletele]

> I am not getting any signal from dropping the tank.

Then tank pickup or recovery stage.

Unplug the recovery cable and put your finger on it. Does it BUZZZ! ? If not, carefully put a finger on the reverb return control pot. (Don't have any other body part touching amp or anything grounded!) Does it Buzz! there?

(Update) PRR,

I have the return cable unplugged from the tank and I am not getting any buzz from the return cable when I touch it.

However, I get a good buzz when I touch the Pot for the reverb.

Does that give you an indication of something?

[PRR]

If I read your tangled quote correctly: the problem is in the reverb recovery stage.