Ampeg B15NF bias slowly raising

[boroman]

I have two '69 B15Ns on the bench. One is my beloved one (from profile pic!) and one is a new one. I have recapped it the same way I did the old one. Nothing messed up with ground positions etc. Changed 100uf bias cap, put 20k pot in series with 33k resistor to have variable bias.

The case is, when I hit standby (after amp is warmed up) the bias is slowly rising up. From 0 to 30mA in about 30 seconds.. so - a LOT! I swapped tubes from the other one (which stabilizes bias in like 3 seconds!), I have changed the diode, changed another 100uf cap too. I looked on my another B15N from early sixties and late 70s, and they all do the same stuff. and in this amp, when I hit standby it behaves like it "just warming up" (the same way like I would be hitting power and standby on cold start together). I have really no clue what's going on. I know that I could put lower value bias cap so it would rise just a bit quicker, but on the other one I have 100uf and its perfect.

Any thoughts? Amp sounds great in general. Just this issue that would not allow me to play right after I hit standby which is kind of odd.

 

[agedhorse]

What is happening to the negative bias voltage?

Is this a new problem or did it start after the amp was "fixed"?

Did you "fix" one "problem" at a time and then test step by step, or did you replace a bunch of parts and then discovered this?

 

[boroman]

What is happening to the negative bias voltage?

Is this a new problem or did it start after the amp was "fixed"?

Did you "fix" one "problem" at a time and then test step by step, or did you replace a bunch of parts and then discovered this?

Well I bought the amp as non-working - completely covered in dust and dirt, with original tubes, untouched electronics, with exploded/leaked caps (bias cap was corroded too), 2 prong power cord etc and blown fuse. I cleaned it, recapped, retensioned sockets and turned on - it sounds great, dead-quiet, no hum/hiss at all. Tubes tested good, only rectifier tube was shot (one of its halves). I could not even test it before.

Bias behaviour on warmed up amp: Dropbox - video.mov - Simplify your life

 

[agedhorse]

Next time, my suggestion is to start with the power supply. Replace the power cord with a grounded cord/plug, and disconnect (remove) any of the caps from line and neutral to ground or chassis, disconnect the power supply from the amp (often you can simply remove the tubes) put the designated load onto the supply to draw say 30mA from the plate supply, 5mA from the screen supply, 10mA from the preamp supply and a couple mA from the bias supply, and then measure the voltages compared with the schematic to be sure these are good before you do anything else. The point is to rule out what you can confirm is operating correctly which is the essential concept of "differential diagnostics" (one component of structured troubleshooting).

What are your voltages? Measure before and after any dropping resistors.

 

[boroman]

I know that it could be done. But I think I don't have proper equipment to do that. I first powered it up without tubes and put lower value fuse to check power transformer.

Voltages are OK. On standby I have around 200VDC plate and like 5-6V less screens. When in playing mode I have around 490V plate and a bit less screens too. Schematics calls for 450V but I've seen/had (recapped) B15Ns from 420v to 510v so nothing wrong with that. Wall voltage 121v. I have tried 5U4GB today to bring it down to around 480)

There is a multicap: 40/40/40uf + separate 30uf. Since I could not source that I always recap with three separate 47uf (with common ground) and one 33uf. Plus change 100uf/100v bias cap. Then I do the first listenning test, so nothing could be wrong here as all the + and ground points are the same as original and the same as my other (identical) amp.

 

[agedhorse]

The voltage will fall under load, which is the reason for placing a load on the supply to test.

What is the negative bias voltage, and is this stable at turn-on?

With the tubes installed, what happens to the G1 voltage on each power tube as the bias increase? does this become less negative?

 

[beans-on-toast]

When you added the adjustable bias did you remove R34 and replace it with the adjustable circuit. R36 often goes out of spec, check that resistor when it is hot. Is the replacement diode a 1N4007 or similar.

Using two parallel resistors can be helpful to get the right value.

 

[boroman]

What is the negative bias voltage, and is this stable at turn-on?
With the tubes installed, what happens to the G1 voltage on each power tube as the bias increase? does this become less negative?

Sorry for my lack of knowledge (still learning), but what is G1?
And yes, when standby is engaged, it becomes less negative

@beans-on-toast I'll try to replace 100k today. When amp is off, it's in specs. And about the bias resistor - I did it similar thing with the other '69 - two pararell 68k's + 20k pot. When one of the resistor fails, there's always another one so better chance to keep at least a bit negative voltage

 

[Wasnex]

? I am not following your problem. 30mA is only 50% dissipation for a 6L6GC at 490V plate. Are you concerned because it's taking too long (30 sec) to reach 30mA? You want this to occur in 3 sec?

 

[Wasnex]

Sorry for my lack of knowledge (still learning), but what is G1?
And yes, when standby is engaged, it becomes less negative

@beans-on-toast I'll try to replace 100k today. When amp is off, it's in specs. And about the bias resistor - I did it similar thing with the other '69 - two pararell 68k's + 20k pot. When one of the resistor fails, there's always another one so better chance to keep at least a bit negative voltage

G1 is the control grid (pin 5).

 

[agedhorse]

Does the bias voltage at the top of C15 and then C10 remain stable or does it become less negative?

 

[boroman]

? I am not following your problem. 30mA is only 50% dissipation for a 6L6GC at 490V plate. Are you concerned because it's taking too long (30 sec) to reach 30mA? You want this to occur in 3 sec?

Yes. On all of my amps bias stabilizes after 1-3 second, here it's starting from scratch, like on the cold amp or like amp that has power & standby switched at one time. And yes, I always set them on the cold side 50-60% dissipation.

Does the bias voltage at the top of C15 and then C10 remain stable or does it become less negative?

If C15 is a bias supply cap: -78V at the standby then, in playing mode, slowly raising to -50V
The same case when measuring from G1 to ground.

Plate (Pin3 to ground) 200V at standby off, 490V when sin playing mode (after 1-2 seconds). Looks OK here.

Where should I look for C10? Is it one of 0.1 caps?

 

[Wasnex]

Yes. On all of my amps bias stabilizes after 1-3 second, here it's starting from scratch, like on the cold amp or like amp that has power & standby switched at one time. And yes, I always set them on the cold side 50-60% dissipation.



If C15 is a bias supply cap: -78V at the standby then, in playing mode, slowly raising to -50V
The same case when measuring from G1 to ground.

Plate (Pin3 to ground) 200V at standby off, 490V when sin playing mode (after 1-2 seconds). Looks OK here.

Where should I look for C10? Is it one of 0.1 caps?

I have an amp with a similar power supply setup that behaves the same way, regarding it taking about 30 seconds for the bias current to ramp up and stabilize.

Honestly I am having trouble understanding why you are getting any plate voltage when the amp is in standby. When the center tap is lifted, the tube rectifier plates should not be referenced to ground through the PT, and thus the cathode should not be producing any voltage.

Also since the bias supply is taken off one of the same taps, I don't see how the bias supply is generating -78V in standby, since none of the taps are reference to ground in standby. I was under the impression the voltage for the bias supply was generated between the center tap and the outer winding of the PT, not across the entire secondary. So with the center tap lifted, the bias supply should go dead. Perhaps I am missing something.

 

[beans-on-toast]

In the image of the modification that you posted above....it would have been better practice to mount the pot and two resistors on a small perfboard, then run solid covered wires to the PCB. The board mounted with nylon standoffs and they can be gooped down to the PCB.

As it is, the delicate lead of the bias pot is sitting in a blob of solder. Not a good way to connect things. Solder is not a good conductor. The solder joint may be bad.

 

[beans-on-toast]

By any chance is the wiper of the 6.3 heater hum pot connected to bias circuit? This is done to provide a DC offset to the AC heater voltage and can help with noise. If so, remove it and ground the wiper while debugging your issue. Doing this takes one factor that doesn’t have to be in the bias circuit out of play.

 

[boroman]

I have replaced the 100K as suggested - no difference

I have checked the center wire of the hum balance pot, desoldered and grounded, no difference, the same behaviour. By the way, center tap goes to 220k not 10k/diode (circled in the photo). The same on another B15N. Or I am reading the schematic wrong?

Measured voltage at the caps (working amp)
33uf - 490v
47uf - 490v
47uf - 418v
47uf - 418v

I really have over 200V at pin3 when not in playing mode, I read the same voltage on the 100K resistor. When I hit standby, it disapeears from 100K and I have full 490V on pin3/PV in second.

 

[agedhorse]

Yes. On all of my amps bias stabilizes after 1-3 second, here it's starting from scratch, like on the cold amp or like amp that has power & standby switched at one time. And yes, I always set them on the cold side 50-60% dissipation.



If C15 is a bias supply cap: -78V at the standby then, in playing mode, slowly raising to -50V
The same case when measuring from G1 to ground.

Plate (Pin3 to ground) 200V at standby off, 490V when sin playing mode (after 1-2 seconds). Looks OK here.

Where should I look for C10? Is it one of 0.1 caps?

Sorry, it's hard to read on the schematic on a phone, it's C19 which is the second 10uF cap in the CRC filter section.

If this voltage remains relatively stable than I would look at the D.C. voltages at G1 (pin 5) of the output tubes V4 and V5 to see if they remain stable or drift upwards (less negative).

Do not change parts, the point is understanding what is happening so that logical decisions can be made. This is what structured troubleshooting is all about.

 

[boroman]

Sorry, it's hard to read on the schematic on a phone, it's C19 which is the second 10uF cap in the CRC filter section.

If this voltage remains relatively stable than I would look at the D.C. voltages at G1 (pin 5) of the output tubes V4 and V5 to see if they remain stable or drift upwards (less negative).

Do not change parts, the point is understanding what is happening so that logical decisions can be made. This is what structured troubleshooting is all about.

This version does not have dual 10uf caps, it has one 100uf cap (my 1st post) - you can also see on the photo attached.
Schematic: https://drtube.com/schematics/ampeg/b12nf-jp.gif
Voltages at G1 does not drift... pretty stable!

 

[beans-on-toast]

I didn’t realize that you had the 100uF revision. They revised the earlier one 100uF cap version in favor or the two cap bias circuit, it is quieter and has better performance.

 

[boroman]

I didn’t realize that you had the 100uF revision. They revised the earlier one 100uF cap version in favor or the two cap bias circuit, it is quieter and has better performance.

I think they changed it along with the new board (fully PCB with PCB mounted power tubes) somewhere in early 1970s.