Ampeg B15S Buzz

[sjwood3]

Hi All,

Been a while since my last post, learned a lot in the meantime. 

I have finally taken up the project of my B15S that has sat for nearly two years unrepaired.  It had intermittent connections at the power tubes and needed a full recap, all that done.  The amp has new electrolytics, cleaned sockets, cleaned pots, drifted resistors and microphonic disc caps replaced (not that many), ground connections checked, voltages checked, cleaned the green circuit board...in standby or full power, the amp has a slightly audible 60hz buzz.  I've included the schematic below. 

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15s_portaflex.pdf 

Here are the things I've done to see if I can get the noise to go away.

When I pull v1 and v2 the buzz is present, but when I pull v3 the buzz goes away.   With v3 in place and the PI (v4) pulled, noise is also gone.  When I ground the screen at V4 (PI) pin 2 the buzz goes away regardless of standby being on or off.  I lifted R25 and R26, Buzz still there, so I'm relatively certain I've isolated the issue to the V3 area after the channels merge.  I've replaced C18 with a new .01 disc cap.  The bias supply has new electrolytics as well.  D6 diode is fine. R27, R28, R29 are all within value.  C19 tested ok with 0 ESR.  All plate voltages are within a few volts of the schematic, however I did find 110v on the pin 2 grid of V3.   No voltage on pin 7 of v3.   

I performed the signal disturbance test starting with the plates on the power tubes, all ok until I got to V3 and pin 2.  It made a louder sound than any of the other grids, more like an open input.   V2 grids were not as loud as V3 pin 2.  Again, voltage of 110 present on that pin. 

Where the hell is 240v being supplied to the cathode of v3 pin 3?  I'm not an engineer, please explain this to me. 

I'm open to suggestions as to where to look and what to try.  I don't want to shotgun all the parts and find none of it worked.  Preservation is key, functionality is a must.   

I'm knocking my head against the wall on this one.   

I read this post and have looked into the heater issue that was recently discussed, no cigar.
https://el34world.com/Forum/index.php?topic=28288.0

Additional post of interest, I replied to it.
https://www.thegearpage.net/board/index.php?threads/ampeg-b15-hum.2316779/post-34391441

[Latole]

Did you check if hum pot balance and circuit work well ?

[Latole]

J2 contact to ground; contact is good ?

[tubeswell]

When I pull v1 and v2 the buzz is present, but when I pull v3 the buzz goes away… so I'm relatively certain I've isolated the issue to the V3 … however I did find 110v on the pin 2 grid of V3.   No voltage on pin 7 of v3. 

Where the hell is 240v being supplied to the cathode of v3 pin 3?  I'm not an engineer, please explain this to me. 

Double-check V3 pin connections. Make sure that you haven’t got the plate and cathode pin connections bass-ackwards.

[sluckey]

I did find 110v on the pin 2 grid of V3.

That's normal. This is a cathode follower. No problem here. Actually there's about 235V on pin 2 but your meter is loading that voltage down. When you remove the probe the voltage returns to 235V.

Where the hell is 240v being supplied to the cathode of v3 pin 3?

It's a cathode follower. The plate is connected directly to 390Vdc. No plate load resistor, but there is a big 47K and 1K resistor connected to the cathode. The tube is biased such that 5mA flows through the tube. This same current must flow through the 48K connected to the cathode. 5mA through 48K will drop 240V across the resistance. This voltage appears at the cathode. Now this same 5mA current causes a voltage drop of 235V across the 47K. This 235V is applied to pin 2 through a 1M resistor. Since no current flows in the grid, there is no voltage drop across the 1M and the entire 235v appears at the grid.

[sjwood3]

J2 contact to ground; contact is good ?

Yes. I'll recheck all the inputs. 

****input ground connections are all solid.

[sjwood3]

   

When I pull v1 and v2 the buzz is present, but when I pull v3 the buzz goes away… so I'm relatively certain I've isolated the issue to the V3 … however I did find 110v on the pin 2 grid of V3.   No voltage on pin 7 of v3. 

Where the hell is 240v being supplied to the cathode of v3 pin 3?  I'm not an engineer, please explain this to me. 

Double-check V3 pin connections. Make sure that you haven’t got the plate and cathode pin connections bass-ackwards.

The connections are correct for both.

[sjwood3]

Did you check if hum pot balance and circuit work well ?

As far as I can tell it's working fine.  It affects a different level/type of noise when turned, it's clean, measures -64vdc on each of the tabs, its connected to the negative bias. 

[tubeswell]

As V3 has a CF in it, it could be the H-K insulation is breaking down due to the voltage difference between the heater and the cathode exceeding the insulation voltage rating - causing the noise. Have you tried a tube swap?

[sjwood3]

As V3 has a CF in it, it could be the H-K insulation is breaking down due to the voltage difference between the heater and the cathode exceeding the insulation voltage rating - causing the noise. Have you tried a tube swap?

I don't have an extra 12dw7, swapped a 12ax7 in and the noise persisted.

[PRR]

That 12DW7 is rated heater 100V negative of cathode. Here it is almost 300V!!

Changing to a positive reference only reduces the stress to maybe 200V.

I know these things don't blow-up right away. And Ampeg was hoping to sell you a new amp real soon now, 1976. But since you're still using this one, I would think about a dedicated heater transformer for the cathode follower bottle.

Rig two 100k resistors from one cathode to the other of the 12DW7, put 10uFd 300V at the center, and reference your heater winding CT there. That's still over-spec but not by much.

Or recognize that V3 1-2-3 is "just a cathode follower" and replace it with a FET.

[dwinstonwood]

PRR, while you're talking about this, here is a snip from the 12DW7 datasheet.
So, is the 200V number the total for both cathodes?
I'm a bit concerned because the CF in my Hoffman Plexi 6V6 has 131V on its 7025 cathode. The 7025 datasheet shows the same max numbers as the 12DW7.
How safe is that (I'm using 6.3VAC on the heaters)?
Thanks, and sorry for barging in here!

[sjwood3]

PRR, while you're talking about this, here is a snip from the 12DW7 datasheet.
So, is the 200V number the total for both cathodes?
I'm a bit concerned because the CF in my Hoffman Plexi 6V6 has 131V on its 7025 cathode. The 7025 datasheet shows the same max numbers as the 12DW7.
How safe is that (I'm using 6.3VAC on the heaters)?
Thanks, and sorry for barging in here!

It seems relevant. 

[PRR]

...is the 200V number the total for both cathodes?...

"Both"? How would one cathode know how the other one is feeling?

[PRR]

.

[AmberB]

Why would someone put a negative bias on the heater circuit?  Usually if it's not grounded, it has a positive bias, like tied to the cathode of a cathode biased power tube or tubes.