Ampeg B15NF bias slowly raising

[boroman]

Guys! I found the issue!

I was wrong in the last post. It was not wired the same. But it's not due to my wiring. It was factory mistake (unotuched solder joint). One of the red wires going from PT to rectifier tube socket was soldered to PIN6 and should be on PIN7 (pin6 shouldn't be connected to anything). That was the difference between those two amps.

Now everything is fine, PV at 450V with 5U4GB and a bit more with 5AR4. Sounds sweet, gosh!!!

 

[beans-on-toast]

Guys! I found the issue!

I was wrong in the last post. It was not wired the same. But it's not due to my wiring. It was factory mistake (unotuched solder joint). One of the red wires going from PT to rectifier tube socket was soldered to PIN6 and should be on PIN7 (pin6 shouldn't be connected to anything). That was the difference between those two amps.

Now everything is fine, PV at 450V with 5U4GB and a bit more with 5AR4. Sounds sweet, gosh!!!

Good going.

It's a lesson that you really have to sweat every little detail in an organized and meticulous way when approaching an amp. You can't assume that connections are correct, you have to check.

 

[agedhorse]

Tracing and finding these kinds of mistakes are why pro techs don't like to follow up on an amp that has been worked on and/or hacked on by somebody else.

I realize that you didn't cause this problem, but imagine if it was caused by another tech (maybe it was?) and you had to charge a customer for the amount of labor involved is sorting it out?

 

[boroman]

Tracing and finding these kinds of mistakes are why pro techs don't like to follow up on an amp that has been worked on and/or hacked on by somebody else. I realize that you didn't cause this problem, but imagine if it was caused by another tech (maybe it was?) and you had to charge a customer for the amount of labor involved is sorting it out?

I mostly restore that amps for myself (I got a lot of ampeg amps), sometimes I sell one to get another. It's fun! I have learned a lot from this group. I'm not a qualified tech but I lov the fact I can completely restore my own amps.

See the photo below - 5AR4 specs. I think it's not the end of the case!... All B15N chematics says PIN6 too, not PIN7!... so why my other B15N has connection to Pin7, not 6? (And I played that for hundreds of hours in the last years, all voltages are right and sounds great_

So now I'm confused! Will look at it tomorrow, again

 

[agedhorse]

Was the pin 7 terminal that's not used in the tube being used as a junction point only?

 

[Wasnex]

I mostly restore that amps for myself (I got a lot of ampeg amps), sometimes I sell one to get another. It's fun! I have learned a lot from this group. I'm not a qualified tech but I lov the fact I can completely restore my own amps.

See the photo below - 5AR4 specs. I think it's not the end of the case!... All B15N chematics says PIN6 too, not PIN7!... so why my other B15N has connection to Pin7, not 6? (And I played that for hundreds of hours in the last years, all voltages are right and sounds great_

So now I'm confused! Will look at it tomorrow, again

View attachment 3786487

Maybe you have been trouble shooting the wrong amp.

 

[aborgman]

Why do you measure voltage in standby? It's entirely possible that you are measuring a voltage that is electrostaticly coupled to a very high impedance node. There are lots of parasitic circuit elements that can cause presumably bad data if you don't understand how this works.

Bingo.

Without the center tap grounded - any voltage you're reading between anywhere and ground is basically not real.

 

[boroman]

Maybe you have been trouble shooting the wrong amp.

Here's a shot of 2nd "identical" amp on my bench that was main amp for years. It's FACTORY soldered to pin7. No signs of solder or any other work on pin6. It even has factory red markings on solder joints there, look (pin7).

Pin 7 is not present on 5AR4/5U4GB. So I wonder why it's wired that way. It works, it worked great for many years. The "new" B15N I'm chasing problems now it's good too. I exchanges three rectifier tubes between those amps, so it's not them.

When powered ON, I have 0VAC on red pin4, and 720VAC on red (unused) pin7. When amp is in playing mode, both pins have equal 360VAC.

 

[Wasnex]

Here's a shot of 2nd "identical" amp on my bench that was main amp for years. It's FACTORY soldered to pin7. No signs of solder or any other work on pin6. It even has factory red markings on solder joints there, look (pin7).

Pin 7 is not present on 5AR4/5U4GB. So I wonder why it's wired that way. It works, it worked great for many years. The "new" B15N I'm chasing problems now it's good too. I exchanges three rectifier tubes between those amps, so it's not them.

When powered ON, I have 0VAC on red pin4, and 720VAC on red (unused) pin7. When amp is in playing mode, both pins have equal 360VAC.



View attachment 3787476

Yep...it looks like you're only getting the benefit of one plate in the tube rectifier.

When the center tap is lifted, you are measure the full voltage across the transformer. Somehow pin 4 is finding a reference to ground and pin 7 is floating. When you set the amp to operate, the center tap is connected to ground so the voltage is split evenly between the windings.

Here is part of a Matchless Thunderchief. This is not my amp but I own one like it. The two rectifiers at the bottom right of the image are run in parallel.

You will note the heaters are connected to pins 2 and 8 and the HV AC supply is connected to 4 and 6. Currently rectifiers in my amp are Svetlana 5C3S which crosses to 5U4G.

The tube rectifiers are used when the amp is in half power mode and produce 500V plate. In full power mode, the diodes connected to the left tube sockets are switched into the circuit and produce 540V plate.

I have never traced out how the standby is actuated, but from looking at this image it appears that it is done by grounding the center tap of the PT's secondary, just like your amp. The center tap is the inner switch in the top right corner. The small wires on the left side of the switch turn on panel lights that run off the 6.5V heater supply. I believe the heavy wire on the right side of the switch is the center tap.

I had previous assumed the standy switched the plate supply on and off. Note the small green wire running to the standby, and also the small green wire going to from the Half power switch to the HV fuse.

I would have to open the amp and take some readings to know for sure, but I know think the standby grounds the center tap.

This amp develops higher bias voltage when it is in standby and takes at least 30 seconds for the current to ramp up when the amp is set to operate. I have always assumed it was designed to operate this way.

 

[boroman]

Wow, that looks great. Your knowledge is top @Wasnex!

That leads to another question. Why earlier (cathode biased) and later (fixed bias, different PCB but the same schematic) have all fast ramp up of bias voltage, and the PT HV is connected to pins 4+6? That's confusing. I'm just curious WHY. Need to understand that issue

 

[Al Kraft]

Tracing and finding these kinds of mistakes are why pro techs don't like to follow up on an amp that has been worked on and/or hacked on by somebody else.

I realize that you didn't cause this problem, but imagine if it was caused by another tech (maybe it was?) and you had to charge a customer for the amount of labor involved is sorting it out?

Just went through this with scenario with my preferred tech (who is among other things a former Hughes radar technician) on a Warwick bass that a lesser qualified person had gooned up. I needed a quick repair and the other individual while generally qualified and competent got in over his head on some micro-elecrtronics in the bass. Long story short, it cost me an hour and half of troubleshooting just to discover that the best course of action was a complete replacement of "repair induced damaged components".

I was happy to pay for his time, but my friend felt badly about charging me to basicially spend time arriving at the conclusion that a previous repair I had paid for caused more damage than it partially fixed.

 

[agedhorse]

Just went through this with scenario with my preferred tech (who is among other things a former Hughes radar technician) on a Warwick bass that a lesser qualified person had gooned up. I needed a quick repair and the other individual while generally qualified and competent got in over his head on some micro-elecrtronics in the bass. Long story short, it cost me an hour and half of troubleshooting just to discover that the best course of action was a complete replacement of "repair induced damaged components".

I was happy to pay for his time, but my friend felt badly about charging me to basicially spend time arriving at the conclusion that a previous repair I had paid for caused more damage than it partially fixed.

This is something I generally shy away from because most folks don't understand how difficult and frustrating it is, and how expensive it can be. They often become quite upset and sometimes get very angry in the process... when they abandon their gear rather than pay for the time, everyone loses because it's not economically feasible to recover the cost of repair based on the amp's value when working.

And, it happens all the time. It seems that everybody is a tech these days.. until they aren't!

 

[Wasnex]

Wow, that looks great. Your knowledge is top @Wasnex!

That leads to another question. Why earlier (cathode biased) and later (fixed bias, different PCB but the same schematic) have all fast ramp up of bias voltage, and the PT HV is connected to pins 4+6? That's confusing. I'm just curious WHY. Need to understand that issue

IMHO, a cathode biased amp has nothing to do with this since it does not have a bias supply.

My Thunderchief has very high value resistors in the bias supply. My assumption is the high resistance is behind the time it takes for the bias to stabilize. It's been a long time since I had it open, but I believe the bias supply is behaving just like yours and charging up in standby. That did not previously surprise me since I assumed the standby on my amp switched the plate supply instead of the center tap.

As I mentioned previously, the reason the bias supply charges up in standby is a bit of a mystery to me as well, but it seems clear that it is somehow charging up across ground and the capacitors through the bias diode and one side of the tube rectifier.

There's probably a bit more going on but this is the basic idea. I have just erased one of the diodes in the half wave rectifier so you can see which diode sections are most likely interacting to create the voltage

This is the same circuit drawn a different way.

I have just erased one of the diodes in the half wave rectifier so you can see which diode are most likely interacting to create the voltage. Here are the originals.

If you read the link @beans-on-toast referenced on capacitor coupled bias supplies, it gives you more of a hint. But our bias supplies are missing some components that are supposed to be required to function, so it's still a bit of a mystery.

Here's the link again. Scroll down near the bottom of the page to the section titled "Capacitor Cupled Bias Supply" The Valve Wizard

Here are the images from the discussion

Obviously our amps don't have capacitor coupled bias supplies since they are missing C1 and R1, the bridge rectifier, and R3, but something similar is basically what is going on. Keep in mind, the values needed for some of these missing components can be inferred by other components in the amp.

Another thing to consider is most of my amps have a second low voltage tap for the bias supply.

Perhaps I am wrong, but I suspect the lower voltage taps would be less prone to starting the process that leads the bias supply to charge up without a grounded center tap. Pretty much all of my amps with this setup have solid state rectifiers and the standby is on the plate supply, so I don't know if the bias supply would charge up or not.

Also keep in mind the tubes and diodes have small values of capacitance between their elements that might become significant as the applied voltage goes up.

Sorry I can't give you a clear answer as much of the world is a mystery to me. Some things you just observe and take on faith.

I would suggest correcting the wiring on the amp with the taps currently connected to pins 4 and 7 to see how it behaves. It might start behaving like the amp you have been concerned about.

 

[boroman]

I did a PDF of your last post. These are all great informations... I need to learn.

I would suggest correcting the wiring on the amp with the taps currently connected to pins 4 and 7 to see how it behaves. It might start behaving like the amp you have been concerned about.

I actually did it now. No fireworks included. Amp plays fine. Not sure if I have "less sag". Sounds about the same.

Downsizes:
- Bias ramp up slowly but not so slow like on the other one that we're talking here (takes 10 seconds to reach steady value, acceptable)
- After powered on I have voltages around 200V all over power supply caps, 6L6GC anodes and some other voltages in bias supply like I haven't got before.

Is it OK to be like that?

 

[beans-on-toast]

I actually did it now. No fireworks included. Amp plays fine. Not sure if I have "less sag". Sounds about the same.

If only one plate of the rectifier was working, the amp would have more hum. If you changed the wire from pin-7 and nothing changed in terms of hum, something odd is going on. I would look for traces on both sides of the circuit board at the tube rectifier socket just to be sure. Unused rectifier socket terminals are ofter used as tie points, below is an example. In this case of a Fender amp, diodes were added to help the tube avoid flashover. Here pin-3 and pin-5 are used as tie points. Pin-7 can also be used as a tie point. It goes without saying that if these sort of mods are performed, other rectifier tubes should not be substituted in the amp without checking the wiring and confirming requirements.

In an unmodified amp, the high voltage windings (red wires) are connected to pin-4 and pin-6, the 5VAC heater (yellow wires) connects to pin-2 and pin-8.

This is the schematic for the wiring above.

 

[boroman]

Thanks @beans-on-toast

Both amps did not hum with one of the HV red wires tied to PIN7. They are exactly the same in terms of hum when soldered to pin6 or pin7. Almost dead-quiet on idle, which is typical to B15N. I don't hear more/less sag, but I may not be pushing the amp to its limits (not in the strudio right now). I would leave them as is, but those 200V voltages on the caps & plates (before playing mode on) makes me confused.

Does anyone of you have B15NF to check it for me? Maybe it's just the way it should be in this amp?

 

[boroman]

@beans-on-toast @Wasnex @agedhorse

Let's drop the PIN7 vs PIN6 debate (let's say PIN6 is correct), and the slower bias ramp-up.

I have opened today '63, '68', 69' and '73 today.

The last three after powered on have about 170-200VDC on 6L6 plates and power supply caps. Some minus voltages in bias section (arund -80V). The '63 have close to zero voltages after powering it on in these sections.

The ON/OFF and standby is wired the same on four of them (green->ground, hot->fuse->on/off switch->PT, neutral->PT). The difference is only standby light which on the later ones is inserted in the strip between power switch and transformer, and on the early one the plexi logo gets its voltage from the heater circuit. Besides the PCB "look" changes, the circuit (and cap layout: 40/40/40/40 vs 40/40/40/30) looks similar on all of them. All four sound excellent, no hiss, no 50/60/120hz hum. I have grounde

Maybe slight circuit design changes makes '63 different from later ones (e.g. the '63 has grounded transformer. but later ones I dont know - like @aborgman said - maybe here's the case)? That's why I'm looking for somebody who could measure their fixed bias, post '67 B15N for me.

 

[bogson]

Wow. Entertaining thread! Mine too has 190VDC on the plates in standby. And bias voltage of about -78vdc.

These go to 465VDC plate, and -49VDC bias within a few seconds of taking off standby.

I’m assuming this is normal.

For those of you with plate voltages in the 460-490 range, what kind of plate current are you seeing? Mine are only about 25ma. So by my calculations that’s only 40% of max plate dissipation for 6L6gc. Wouldn’t I need to decrease my negative bias voltage quite a bit to get it up the 65-70% target? Something doesn’t seem right. Maybe the power tubes themselves?

 

[byacey]

I haven't really figured out the return path for the electrons in the bias circuit when the amp is in standy, but I believe they have to go through one side of the tube rectifier. See my thought train in post #32. If I understand your notes, one amp is developing 200V on the plates in standby and one is not. So it's possible the bias circuit is exactly the same on both amps, but something in or related to the plate supply is different in one of the amps. I would pull the tube rectifier and see if the bias voltage still develops. My guess is it won't.

If the bias voltage does still develop, I would wonder if the PT is getting a partial short to ground somewhere. Bad standby switch, bad wiring, bad PT? Just spitballing. There has to be a return path for the electrons and you have to find it.

Disconnect the standby switch and re-test. There may be carbon track leakage inside the switch from arcing.

 

[bogson]

Disconnect the standby switch and re-test. There may be carbon track leakage inside the switch from arcing.

I can just pull the center tap off the switch to test, yeah? And do you get 0VDC on your plates when in standby?