Issues with a Fender Princeton Reverb II

[CaptainSucrose]

I have been looking on your website for about a year. Really great job of having detailed information to help someone out of the loop with electronics for a while. Until I got this amp. In the process of researching and relearning what I had done when I was 19, (I'm 36 now)  I realized I should probably do as much of the leg work as I can before I post anything on here. I hope there isn't a post on here already about detailed solutions to what I've been experiencing with this amp already. I have been looking for answers and checking every possibility with a multimeter, I had gone through the amp when I first looked inside with an ohm meter to check components and had a wiring diagram I was writing the values next to each component. I had also checked for continuity with wires leading to the components, pots, tubes, jacks and had done the maintenance work of replacing filter caps, cleaned the pots with spray contact cleaner, and looked over the values of voltage going into the bias section. The bias power resistors are  where my rabbit whole had begun with this amp.

Please understand, I'm not saying I know what i'm really doing with this. I know how to not get shocked and have built a current limiter for accidents but I have a long says to go before i can even say I know what I don't know. I'm learning mostly out of absolute necessity. So why not take it to a repair shop? I have.  After I had taken this amp to a repair shop twice and was told it was electrically sound, it still smelled like burned electronics but I thought it might be a lingering effect of it almost burning. When I put in 2 brand new JJ 6V6 S and 3 ec81 and a 12at7 after I was told it was electrically sound, the tube arced and blew the fuse. After I had got it back the second time and was told it was a bad tube. I had gotten a new set of tubes because of this without charge. I'd like to think its a bad tube except I had looked at the resistors and they're 3 watt metal film flame resistant 390 ohm resistors. They were blue when they went in the amp and now they're turning a nice brown color. Almost like the  T47 light bulb had given them a tan. I had an idea to go to a different repair shop but, all the money i saved to do so was already spent on going to a repair shop twice. I was going to take it by a fender authorized repair shop until my neighbor told me about how they didn't fix his amp either.   

I had gone over to stratopastors website all about this amp in search of any other person who had the same problem and what they had done to fix it. And while I didn't find a solution to the bias resistors overheating, I did find out for certain how to wire it correctly thanks to stratopastor. I had rewired the bias power distribution to factory spec. I'll elaborate on why shortly. So i'll try to be as concise as I can without skipping over details of what I had done with this amp and why from here forward. 

I have acquired a Princeton Reverb II about a year ago. I knew it would need some cleaning and filter caps when I picked it up. So I played it for about 10 minutes when I got home, it sounded like it had potential.

I soon after took the back piece covering the chassis off, looked at the tube chart and noticed it said 6V6GTA on the back. The ones in the power tube section where GT6L6s. So from there, I had decided to start printing the schematics for this amp and go back to the basics of finding out what else might be happening. I noticed a 2 prong outlet adapter on the cord I had taken off that night.

A few days later I had picked up a pair of 6V6s and tested it out. It was short lived as it quickly started smelling like burning electronics. So I unplugged it and looked in the chassis after I was sure there wasn't any smoke coming from it. I noticed 2 large 390 ohm resistors had blackened charing with them and the top emf shielding in the cab had a brown spot from the heat.

[CaptainSucrose]

 
Continuation of original post:

This was not what I had in mind for fixing. I was looking at routine maintenance tasks. Not this. So I had been looking into how the amp worked and how the power was routed. I had found stratopastors web site and shortly after. I was overwhelmed by trying to learn electronics again. I had used a volt meter to check the bias and it had -3 vdc just after the rectifier if I remember correctly. I had used a 1N4007 diode to replace it. I had found 2 100 mfd 100 volt caps for the filter section.  The origional spec was 70 mfd at 100 volts.   I had replaced the two 390 ohm resistors on the solder lugs and the 6.2 zener diode in parallel with a 680 ohm resistor and had to replace the whole solder lug as it was very brittle from so much heat and then I attempted to remove the over heated resistors.

I had installed a bias pot in front of the second filter cap and removed the jumper between the two 220 k ohm pots and the filter caps, and put the 15 k ohm resistor on the right leg of the pot going to ground. After reading about it on stratopastors site and further researching it on Dan Tores's book where he had shown an example with a Deluxe Reverb II with the same bias set up I had decided to do so.

After i had set the bias to -33 volts that the test point on the schematic which is just not suitable for this amp. I had gone back through using the output transformer method of measuring the resistance from the centertap to pin 3 on each of the output tubes it was 211 on both sides. The voltage dissipation from the center tap to pin 3 was 5.8v on the outside and -6 v on the inside tube. This was at 396 volts. So the plate dissipation was around 10.92 watts for the outside and 11.31 for the inside tube. From what Ive read thats way too much. So I had lowered the bias to -39 volts at the test point when I had brought it down to .18ma for the 396 plate voltage. So the tubes should be taken care of now and the amp should be ready to play except, the resistors that burned up before, they were getting very hot now. very quickly.  I had taken a infrared thermometer to check it. Within 5 seconds, they're at 103F. the wire leading from the 680 ohm resistor goes the red pedal jack. the connection from this resistor was also at about 100F. All other parts except the resistors were at 78 F. Had started looking into a short. So I took the known ground connection at the solder lugs and lifted it off the chassis. The chassis ground to the known ground point read something in 3 meg ohms.

[CaptainSucrose]

It had blown the 12AT7 shortly after I had gotten the bias pot installed and started to play it for a short time. It has the reverb input and the second half is the lead channel driver. The red pedal connection with the wire leading to the resistor that gets hot, goes to this lead channel through a vactrol for the -6.2 to activate the optocoupler inside it. I had run the diode check on this vactrol and the one for the reverb pedal and they both only have a signal from my meter from one way.

I'm not sure if its the cause or the result of the 12AT7 going but, the reverb pan had a short in the input transducer too and the reverb was working before the tube went.

When I was looking for possible reasons the reverb would have this all happen at once, the rca jack for the input for the reverb on the amp had a loose wire for the center conductor with an intermittent open. So I've recently replaced this jack. The ground for the speaker wire had come loose and I felt it break free in the wire when I was plugging in the speaker so I had re soldered this as well. 

I had gotten some interesting info about this amp from stratopastor he said this one was exported to Canada, and was possibly one of the last ones made going by the transformer, speaker numbers.

So most recently, this is what Ive done:

*Since it had those EC81s, it developed a hum that changes volume with the presence pot. I also had just installed a capacitor kit with the cap can which has 80 mfd 40 and 2X 20 mfd caps all at 450 volts , the 70mfd 100 volt filter caps , the 4.7 mfd 450 volt cap, and 3 25 mfd @ 25 volt coupling caps.   

Before making any post here I did go through with a highlighter and check all the wires and components I had done any work with to make sure they were connected

I had double checked all the wiring in the power section and picked up a 140 watt soldering iron to make a good solder joint with the cap can to the chassis. Cleaned the wires and soldered them as best I can. I don't think I'm great at it anymore but, it has continuity, holds in place, isn't a cold solder joint.

I had gone through the eyelet board and lifted it up to see if there was a blob of loose solder under the first layer of it. Even made a shop vac attachment with a straw to see if there was anything like that I could get out. I didn't see any wires under this first board. I keep wondering if there should be any though. Especially with the 15 K ohm resistor in parallel with the 70 mfd cap in the bias circuit. It's not on the wiring diagram but, its shown on the schematic and confirmed by stratopastor about it being there. I'm not sure if it could have a seperate ground wire but the schematic makes me wonder yet all the stock photos of the chassis show it in place parallel with the 70 mfd cap going to the same ground point.   

I pulled the output transformer out to see if there were any shorts in the windings to the iron of the transformer itself. I'm greatful it's not that. When I checked the transformer with an ohm meter in the chassis, it says 3.5K from the speaker side of the transformer. I can't say I know if thats right, or if this is my next clue to follow with this amp.

I replaced the tube sockets as the old ones where very loose. I had tagged all wired and which pins they went to.

The main reason besides the socket being loose was because when I would move the amp from room to room, it would not sound the same at all. Like something shifted out of place inside the amp. So I wanted to rule this out of a possibility for this cause 

I had also checked the connection with the speaker jack again and I don't get the same reading on my meter twice. I havent gotten the same reading from the ohms meter when checking different points like the speaker. The reading goes up and down now.

[CaptainSucrose]

So I dismantled the bias pot and went back to the stock configuartion now instead of 390 volts at pin 3 to ground it droped to 370 volts, now I'm getting 323 volts instead. Plate dissipation is suprisingly consistant from the first time I checked it at -33 volts with the bias pot. The first time I used the output transformer method I had 5.8 volts from center tap to pin 3 and -6 volts to the inside tube. Now they're at 5.67 and 5.57 volts for plate  disipation and ohms lawing it to get 26.7 ma and 26.27 ma for current but with the lower voltage at 323 volts, it's within reason for the wattage at 8.6 watts. A bit too much but not at 10.92 watts and 11.31 watts for it's initial voltage reading of 396 volts. It begs the question: Where did 50 volts go? and it also makes me wonder if the output transformer method would hold true if there's an underlying problem it doesn't identify. I don't think it's bypassing the laws of physics , but maybe it's not really disipating 5.57 volts how or where it should.

When I made my notes on a wiring diagram, the values of a path for resistors leading to the speaker jack line out were changed to a 22k ohm instead of a 2.2 k ohm and a lot of resistors leading to the presence and master volume had been going out of spec. Some where intentionally changed from the color code. on the bottom of the chassis I saw a note from a repair man where it said "low output" in pencil.

I'm not sure what someone was really doing with this amp or trying to achieve besides overcoming the low output.

If anyone has a suggestion of what do look for or check, I'm capable a of safley doing most tests. no variarch is all. I'm stumped on this for now.

I'll be pacing the hallway and see if I don't come up with anything else until I check here again.

Also Ill log in with my phone for photos of the actual amp and put them on here next.

[AlNewman]

So it sounds like you fixed it?
8.6 watts is good for fixed bias, about 61%.
Voltages may have changed when you rebuilt the power supply or the tube sockets.  Perhaps something had been set up differently if they were running 6l6 instead of 6v6.
If you're worried about it you can put 1 ohm resistors from cathode to ground and measure your current there.
You may want to build a lightbulb limiter if you don't have one and want to keep working on amps, might have saved you some resistors and headaches. Helps protect you as well.
Does it make noise?

[sluckey]

I had replaced the two 390 ohm resistors on the solder lugs and the 6.2 zener diode in parallel with a 680 ohm resistor and had to replace the whole solder lug as it was very brittle from so much heat and then I attempted to remove the over heated resistors.

These components are not part of the bias circuit. They simply tie into the -55V bias supply on order to drop the voltage to -6.2V. This -6.2V is used to operate the two Vactrols (and probably some LEDs on the footswitch). If you measure 6.2V across the zener this circuit is working properly. Those two 390Ω resistors should get hot. They have about 48.8V across them so together they are dissipating 3 watts! I highly recommend replacing with 5 watt resistors and give them lots of free air space to dissipate the 3 watts of heat.

Can you provide a larger pic of the layout/wiring diagram? Never mind. Already have a good layout/schematic.

[CaptainSucrose]

So it sounds like you fixed it?
8.6 watts is good for fixed bias, about 61%.
Voltages may have changed when you rebuilt the power supply or the tube sockets.  Perhaps something had been set up differently if they were running 6l6 instead of 6v6.
If you're worried about it you can put 1 ohm resistors from cathode to ground and measure your current there.
You may want to build a lightbulb limiter if you don't have one and want to keep working on amps, might have saved you some resistors and headaches. Helps protect you as well.
Does it make noise?

It had started to make huming tied in with the presence knob after I put in an ec81 for phase inverter

I wouldn't be as concerned with lower voltage at 323 but, I think it means more current is going through it or its getting the voltage from b+ now? Since 330v is what b+ runs at from the filter cap can. It should be at 390 volts.

And I'm on the same page about the current limiting light bulb. Already built one. Is a 200 watt incandescent heat lamp bulb a good one for this?

I had read many modifications around a 6L6 change over. They seem to also have a speaker with 4 ohms for getting the right impedance and not ruining the output transformer. The speaker wasn't changed in this one. It shows 6 ohms on multimeter.

[CaptainSucrose]

I like the 5 watt 390 ohm idea, I think I'm going to try this next and see if they don't get to 160 degrees.

Is there a way to find out where I'm losing 67 volts across pin 3 to ground from the output tubes?

And the original half wave rectifier was built for a 1 amp 800 volt diode. The 1N4007 runs at 1 amp 1000 volts. Could this be the problem in the  overheated 390 ohm resistors?

[CaptainSucrose]

Here's a picture of whats in here for now. I have a feeling its going to get better than this.

[CaptainSucrose]

Should I make a small eyelet board for here to have the space for bigger resistors and more air flow to them so the heat from the first 390 ohm resistor isn't going to the second one?

Or is it just bad soldering? I'm not the best at this anymore. About 17 years out of practice.

[sluckey]

Is there a way to find out where I'm losing 67 volts across pin 3 to ground from the output tubes?

Remove the output tubes and measure the voltage at pin 3 of both sockets. Also measure the AC voltage between the red secondary wires of the PT. And measure the voltage across that zener diode. What have you?

And the original half wave rectifier was built for a 1 amp 800 volt diode. The 1N4007 runs at 1 amp 1000 volts. Could this be the problem in the  overheated 390 ohm resistors?

no

Should I make a small eyelet board for here to have the space for bigger resistors and more air flow to them so the heat from the first 390 ohm resistor isn't going to the second one?

No need. Just leave the 5W resistor leads long enough to provide adequate separation. You should be able to neatly install on that terminal strip so there will be about 1/2" air clearance around each resistor. Your rusty soldering skills are showing!

[AlNewman]

It had started to make huming tied in with the presence knob after I put in an ec81 for phase inverter

I wouldn't be as concerned with lower voltage at 323 but, I think it means more current is going through it or its getting the voltage from b+ now? Since 330v is what b+ runs at from the filter cap can. It should be at 390 volts.

And I'm on the same page about the current limiting light bulb. Already built one. Is a 200 watt incandescent heat lamp bulb a good one for this?

I had read many modifications around a 6L6 change over. They seem to also have a speaker with 4 ohms for getting the right impedance and not ruining the output transformer. The speaker wasn't changed in this one. It shows 6 ohms on multimeter.

Try measuring voltages without the tubes.

The hum in the negative feedback?  You said you weren't getting proper readings across your output transformer, maybe there's a bad connection.  Theoretically hum should increase as negative feedback goes down. 

[CaptainSucrose]

Hi there, I've been repairing a signal generator I made and getting an oscilloscope running. Along with understanding the basics of how to use one again.

So I think your both on the right track with measuring voltage without the tubes and checking voltage from pin 3 on both sides.

I do have a hypothesis about this bias section as part of it is well understood how it works, with the 2 390 ohm resistors but, the other half of this connected to the 680 ohm resistor paralleled with a 6.2 volt zener. Yes, it provides voltage to the vactrols. The electrical way it does so, anytime it noticed there was less than 6.2 volts it provides more current until it sees 6.2 volts again. Thus heating the resistors and heating up the zener next to it as well. Sometimes around 160 degrees.

This past weekend it had started at 390 volts on pin 3. The zener diode was a 7.2 threshold instead of 6.2 volts. It had a stable voltage base for the bias except it would overheat those 2 390 ohm resistors because of there inadequate wattage rating. I was increasing the bias voltage to the tubes at pin 5 to -36 to -39 volts seemed to be around the right current at .18 ma with 390 volts at pin 3. The trouble with doing it the way had done is it would trigger the zener diode to bring in more current until it saw the -6.2 volts.  I think this is where I've been overlooking this whole circuit thinking it's unrelated to the issue of overheating. Its a voltage divider that I've got to have as close to 0 ohms of resistance for the path to ground and move the bias filter cap ground to the other sheet metal screw away from the transformer ground on the other common ground lug at the other sheet metal screw. Should I get a balance so the zener diode won't start sending a lot of current to the red pedal jack while sending more voltage to the bias circuit? There should be about 15 volts to spare. The bias circuit has -55 volts, so including the zener in this taking -6.2 volts at the very end of the bias circuit, leaves about - 48.8 volts to use for biasing. I think it'll come down to finding a different zener diode voltage to work with and a different place to add a resistor to the bias circuit to leave enough voltage to the zener to not trigger it to send so much current to the red pedal. When its triggered to send more current, it seems to be continuously doing so to the point it overheats and changes the break down voltage it starts to send more current. When I checked the zener when the pin 3 to ground voltage dropped to 370 volts, it was at 5.6 volts on my meters diode checker. And now, when it was running at 323 volts to pin 3 to ground the diode check said 4.3 volts. So could it be possible to change the bias voltage by only changing the zener diode and putting a 5 watt potentiometer 100- 600 ohms instead of 2 390 ohm 5 watt resistors?

I think this is really whats happening? And I think maybe this could be a solution along with improving my soldering skills before touching anything on this amp again.

So my poor soldering has added 6 ohms to the return path to ground from the secondary and the filtercaps for the bias. Also, I created a ground loop at the same time. I think this is where the hum at the presence pot is coming in from.



On the schematic and working models of this amp, they are somehow able to work on the 2 watt equivalents. So after reviewing the ohms law of this as Suckerly had pointed out where 5 watt resistors would be good, I remembered coming to this same conclusion but thought I had done the math wrong since all other models had no problem with 2 watt equivalents. With the 780 ohms, at -55 volts I'm getting .0705 ma. So it would have about 3.8775 watts its really working with? Wow I'm glad I thought I over rated the resistors. Hind sights 20/20.

Could this be the longterm fix this thing has needed for the correct bias?

[sluckey]

You have one wheel in the ditch captainbooger. Tell us about your adjustable bias mod.

[pdf64]

When making circuit voltage measurements, I suggest to always also measure the mains voltage being supplied into the amp. eg at the amp’s mains power switch.
As power supply voltages will tend to track any variation in mains voltage.

[EL34]

Captain,Your emails are bouncing back
It says this

Reason: Remote SMTP Server Returned: 452-4.2.2 The email account that you tried to reach is over quota. Please direct

Please fix your email so I don't have to take any actions. Thanks
I will subscribe to this thread and wait for your response

[CaptainSucrose]

Oh, ok I'll see what its saying that about my email for. The joys of spam.

[CaptainSucrose]

Thats a really good idea for measuring mains voltage first. I'm going to start doing this before hand.

First Im going to check the voltage with a fresh zener diode. If I find its not getting 6.2 volts at the red pedal, address this first before anything gets experimenting with the bias. I'm not sure it is or isn't. It seems like resistance checking the vactrol suddenly has different results. When i used a signal generator it had changed signal levels  abruptly the same way. Without using lead level switch. The 6.2 volt led the zener is for would be for a footswitch I don't have. I'm not planning to get it either. Would it work without the zener diode in place? Just the 680 ohm resistor to ground instead?

If all is well in the lead channel select vactrol the red pedal leads too, for the bias mod I'll find the voltage drops at the resistors in bias circuit first, experiment on paper with resistance values to find best place to alter bias while minimizing lower voltage going to the zener.

Shouldn't it be captain sinus infection? Theres way more i did wrong than right at first.

[AlNewman]

Shouldn't it be captain sinus infection? Theres way more i did wrong than right at first.

I think maybe Steve was referencing toungue in cheek the fact you referred to him as Suckerly.  I'm sure it was by mistake, just like Captain Booger...  I couldn't even type that without laughing.

[sluckey]

Shouldn't it be captain sinus infection? Theres way more i did wrong than right at first.

If you won't respect me enough to get my name right, then you should expect the same from me!

[CaptainSucrose]

Ahhh, I see what I did now. Ill be inserting foot in mouth indeed. My apologies for that. Really it was not my intention,  Sluckey.

 I thought it was a reference to my lack of attitude for this type of work.  not a lack of reading.

[CaptainSucrose]

*So in previous post aptitude was the word used. Not attitude.  Auto correct got me on that one.

[CaptainSucrose]

I had some time this past evening to run some test measurements we had discussed previously.

The voltage at pin 3 to pin 3 was 0 volts

Ground to pin 3:  413 vdc at both sockets

The voltage at the zener diode - 6.4 volts and same for 680 ohm resistor in parallel with it.

The voltage across the 2 390 ohm resistors- 29 vdc

The voltage at bias seconadary output 53.3 vac

Voltage at rectifier - 53.3 vdc and 2.3 vac

At bias test point -32.3 vdc no vac

B+ was at 322 vdc all through the board.

Voltage at outlet: 122.2 vac

I had put a new zener diode in. It measured  .72 on my diode check function before installing. After it was installed, it measured .46. The old one measured .46 on diode check too while installed. The old one measured at .622 out of circuit.

Sluckey, I went through with a infrared thermometer after this on the solder lugs the 390 ohm resistors measured about 99 degrees, after I had installed the new diode. The temperature of the diode, was at 194 degrees. So I think you have the right idea of getting two 5 watt 390 ohm resistors and I'm also going to get a 5 watt equivalent diode at 6.2 volts as well. To see if there's any improvement in stability of both voltage and temperature.

Hopefully a lower temperature on the diode too as the derating I've seen on spec sheets shows a direct relationship to higher temperatures.

I'm not really sure which diode would be a good one for this job. Some have specified their purpose of stopping transient voltage spikes, not continuous operation. I'm considering trying out a 1N5341B 6.2V 5W. I'm not sure if there's a specific rating besides wattage I should upgrade too. There is a 50 watt 6.2 volt chassis mount but, mounting it to the chassis seems like I'd make another ground loop.

I found an interesting app for designing and testing circuit designs unfortunately they don't have tube design in mind. Still its helpful to see where putting a potentiometer in circuit would have minimal impact on the zener.

So hopefully I've got a good game plan to work with for a solution with less overheating and im also working out some details to see if its possible to install a potentiometer with the least amount of change in voltage to the zener diode for plan b if 5 watt power rating helps and voltage and temperature is lowered in components.

Thank you everyone for your advice and help on what to do for this amp. Especially since I had no idea after all the things I had tried at first. I had the tunnel vision going on haha.

[sluckey]

I had put a new zener diode in. It measured  .72 on my diode check function before installing. After it was installed, it measured .46. The old one measured .46 on diode check too while installed. The old one measured at .622 out of circuit.

You cannot check a zener diode with the diode check function on a DMM.

[EL34]

Captain Sucrose:
Your emails are still bouncing back. I will have to freeze your postings if you do not fix this.


Message delivery has been delayed to the following recipients:
Recipient: [SMTP:sullivankk8944@gmail.com]
Reason: Remote SMTP Server Returned: 452-4.2.2 The email account that you tried to reach is over quota.
Please direct 452-4.2.2 the recipient to 452 4.2.2  https://support.google.com/mail/?p=OverQuotaTemp h7-20020a5e8407000000b0070a9ccf198dsi2464069ioj.2 - gsmtp

[sluckey]

Doug, if he won't clean out his email in-basket on google, maybe you could point him to how to turn off notifications in his forum profile. Or just do it for him.   

[CaptainSucrose]

Hi there, I had started a new email since theres a lot of spam mixed with a few important emails I really shouldn't delete too.  Theres a new email attached to this profile. It should be free of this problem. Sorry about that.

[EL34]

Hi there, I had started a new email since theres a lot of spam mixed with a few important emails I really shouldn't delete too.  Theres a new email attached to this profile. It should be free of this problem. Sorry about that.

Ok, thanks

[CaptainSucrose]

[/quote]You cannot check a zener diode with the diode check function on a dmm  [/quote] 

Well, atleast I'll stop doing that then. A 9v battery, resistor and voltmeter?

[sluckey]

If you measure 6.2V across the zener this circuit is working properly.

[CaptainSucrose]

So, after taking the voltage readings and noting the 3.2 vac on the heater circuit, and only able to get 6.3 volts on half of heater wires, the hum balance 100 ohm pot would only go to 25 ohms max. Not 50 ohms in the case of a single parallel resistance.

So I had unwired the heater circuit made sure pin 7 from 6v6s went in parallel with pins 4 and 5 on preamp tubes. And pin 2 from 6v6s to pin 9. Still the same problem happened.

Next I alligator clipped the transformer wires to the light bulb sockets one at a time. It had 100 ohms. I checked the other winding and it went to 100 ohms. Next trying the parallel connection, it went to 25 again. When I would unplug the light bulb It would go to 100 ohms. The contact for the light bulb base has a short to ground. There was a small spot half way down when I would press the light bulb in the holder where it still had 50 ohms but, atleast I have a lead on where to look next.

So this makes me wonder, is the zener diode trying to soak some of the current from this short since its in the voltage range of this zener at 6.2 volts?

[sluckey]

I'm sure that in your bizarro world this makes perfect sense. I am entertained.   

[tubenit]

CaptainSucrose,


Could you tell us IF the amp is working properly, please?    And is there some concern that you are trying to resolve and what that might be? 

I can't discern what you're currently doing at this point? 

Respectfully, Tubenit

[CaptainSucrose]

CaptainSucrose,


Could you tell us IF the amp is working properly, please?    And is there some concern that you are trying to resolve and what that might be? 

I can't discern what you're currently doing at this point? 

Respectfully, Tubenit

What am I doing? Replacing the pilot light assembly.

It seems to have a short to ground through the light bulb socket.


So the heater circuit for the 12ax7s and 6v6s are supposed to be getting 6.3 volts on all heater pins when wired in parallel only the current should be added or subtracted,

When I was asked what the voltage was at different points in the amp, I found the heater circuit was getting 3.32 volts ac on both sides of the heaters for each tube. So I checked into the heater circuit more.

When I adjusted the hum balance, only half of the parallel connection was able to get 6.3 volts to it. So either pins 4 and 5  could get 6.3 volts ac or pin 9 could get 6.3 vac.

When the amp was powered off I found there was only 25 ohms of resistance with the hum balance pot, it should go up to 50 ohms if its the only parallel connections to ground from the heater circuit. Indicating another connection to ground somewhere besides the pot or the pot was defective.  It was not the potentiometer, went up to 97 ohms out of circuit. In circuit, connecting only one wire was connected to the pilot light would show 50 ohms too. Showing another parallel connection between the transformer, pilot light, and hum balance still.

When I had taken the light bulb out, I was able to see the potentiometer  go to 50 ohms on the multi meter with both wires connected. Indicating where the short to ground was, in the light bulb.

I did the same test with just the transformer and potentiometer it only showed 50 ohms for both sides of the hum balance pot when wired in parallel.

My only question then:

Could this ground short connection impact the zener diode in how it functions? If its zener voltage is at 6.2 volts, and the partial short to ground is from a 6.3 volt supply would it be a possible cause for this diode to over heat too? Only working with the negative half of the sign wave from 6.2 volts until it startes to degrade from reaching 195 degrees and allows more and more voltage and current to come through?

Does the amp work?

I'm waiting on a light assembly. As soon as it arrives you'll be the first to know it if does work or not.

In the meantime I'm cleaning tube pins and soldering the heaters back to the pins so there's only 2 transformer wires and 2 heater wires to solder and find out if this amp does or doesn't work.

Sorry about the confusion. Trying to work on this amp and only do so when my son is asleep.

I don't want to leave a to be continued but, I tried to find a light assembly in a local music shop to save the wait time for you. No luck finding one there.

Does this help?
 

[CaptainSucrose]

If there was a drawing of only the heaters, the light bulbs were the closest thing with no tube symbols available. The first bulb is the pilot light.

The ground point at the first bulb is the short.

The potentiometer is wired like the heaters are exactly with the wiper to ground.

I'd connect the lines but it shows yellow dots all over and is distracting for the point of this.

[PRR]

Why are you measuring to ground? The heaters are not in the B+ circuit. Measure what the heater "sees": from p9 to p45.

[CaptainSucrose]

Oh, I didn't know that. Thank you

[CaptainSucrose]

Why are you measuring to ground? The heaters are not in the B+ circuit. Measure what the heater "sees": from p9 to p45.

Thank you, I

On V4 from pin 9 to 4 and 5, there's 6.4 volts ac into 2 50 ohm resistors where the hum balance used to be.

The hum balance also started showing inconsistent ohm readings like a short too so, I removed it as suggested somewhere on this site.

No sudden voltage dropout after removing the pilot light and the hum balance, and completely redoing the heater wiring. A lot easier to make sure there's consistency in wiring and different colored wires now.

Other changed components:

I had gone through and cleaned out  the solder cups in the bias circuit, put new solder in and replaced the components coming from the rectifier like the 10 k ohm resistor, the 15 K ohm resistor,  replaced the 220 k ohm resistors leading to pin 5 of both output tubes as one showed 216k and the other was 228 k ohm.  The 82 k ohm was replaced when it showed 89 k ohms,  and 100 k ohm resistor was at 91 k ohms and a .22 mfd cap showed no signs of life from checking its capacitance with a fluke 117. Replaced both 70 mfd /100 volt bias filter caps with 100 mfd/100 volt,

When I checked the voltage at the heaters at p 9 to 4,5 it was at 6.45 vac

The bias after replacing the components with schematic based values other than bias filter caps, was a bit low, at -30.32 vdc.

The volt check I had after the half wave rectifier before replacing these components was -53 volts dc and 2.34 vac. After removing the pilot light and hum balance, replacing the bias resistors and one bad cap, voltage after half wave was -53.55 and only .345 vac. And

Other notable improvements, there's no humming sound coming from one of the transformers anymore. And the high voltage center tap was reading 412 vdc to ground consistently, not 322 vdc anymore.

My current limiting light bulb wasn't half bright while testing the circuit this time after these changes where made