New build debug...

[Paul-in-KC]

All,

I finally finished initial assembly on the 3rd-4th Gen Hybrid 50 W ODS Low-Plate-Classic build.

But I think I have something wrong at the power tube sockets. Not sure though. The reason that I suspect that, is because I normally use a "Bias King" tool to set power tube bias - and for some reason, it's not seeing any idle current on the power tubes in my new build. I have never seen this before - which is why I suspect I have something wrong at the power tube sockets.

I know I'll probably need to take some pics - but here are the numbers...

[PT = Mag Components - Classic Tone #40-18005]

B+1 = 465 VDC
B+2 = 464 VDC
B+3 = 442 VDC
B+4 = 308 VDC
B+5 = 299 VDC

I think that only the first two are relevant to the power tubes - but that's all of them.

On the sockets:
P1 = nc
P2 = 6.6 VAC (between 2 and 7)
P3 = 463 VDC
P4 = 462 VDC
P5 = -48 VDC (adjusted for maximum negative bias voltage)
P6 = nc
P7 = 6.6 VAC (between 2 and 7)
P8 = ground

Any glaring problems with these numbers?

I would expect the Bias King to display the idle current - but it's not.
I wonder if it expects pin 1 to be tied to pin 8 (which it's not in my case - at least not yet )

If these numbers look as they should then I'll have to investigate why the bias tool isn't displaying idle current draw.

As always - many thanks.
-Paul

[martin manning]

Your voltages look reasonable. (Edit: suppressor grid) and pin 8 are connected internally in 6L6, it’s only for EL34 that you need to connect the socket pins. You have pin 8 tied directly to ground, with no current sense resistor? IMO it’s a good idea to have that option for checking cathode current. I guess you have tried raising the bias voltage from -48 to see if you could get a current reading, and didn’t see anything happening? What about the screen resistors? Are they connected from pin 4’s to terminal strips? What is the voltage across them?

[Paul-in-KC]

Your voltages look reasonable. Pin 1 and pin 8 are connected internally in 6L6, it’s only for EL34 that you need to connect the socket pins. You have pin 8 tied directly to ground, with no current sense resistor? IMO it’s a good idea to have that option for checking cathode current. I guess you have tried raising the bias voltage from -48 to see if you could get a current reading, and didn’t see anything happening? What about the screen resistors? Are they connected from pin 4’s to terminal strips? What is the voltage across them?

Martin,

I am using 6L6GC's which don't appear to have pins 1 and 8 connected - as I have no continuity between the pins. But, to test the theory that the Bias King tool might expect them to be connected, I put a jumper on each socket and no difference.

Correct, I have pin 8 directly to ground - and you are undoubtedly right - I should (and will) put a current sense resistor in there. But I shouldn't need to have it for things to work.

Yes, I tried reducing the negative bias (all the way to the most positive) to see if I could get a reading, and nothing.

Voltage across the screen resistors on pin 4 (470 Ohm 5W) is 0. Which doesn't make sense to me (but may indicate the problem). I get B+2 voltage from either side to ground - which is consistent with 0 across, but makes it seem like I'm supplying that voltage from both sides. I have measured the resistance across both resistors and they are both very close to their nominal value.

BTW - there is no output to speaker - which seems consistent with no cathode current.

-Paul

[martin manning]

Sorry, I miss-typed... the suppressor grid is internally connected to the cathode (pin 8) in 6L6, where in EL34 it’s connected to pin 1, and you have to connect it to the cathode on the outside. Pin 1 on 6L6 is not used. All types of 6L6 with glass envelopes are the same in this regard, metal envelope 6L6 had pin 1 connected to the envelope.

No voltage drop across the screen resistors says there is no screen current, which is consistent with no cathode current. It’s as if the cathodes aren’t grounded!?

[sluckey]

Gotta be something simple. Are the filaments glowing?

[Paul-in-KC]

Gotta be something simple. Are the filaments glowing?

Yes the filaments glow on all tubes.

BUT - here's something...

Thinking about Martin's last post, I decided to take the Bias King out of the equation - thinking that it was keeping the cathodes from ground.

Sure enough. Now we have cathode current!

I see about 20 VDC across the screen resistors (~42.5 mA). Unfortunately, I now also have a disturbing noise coming from the general vicinity of the filter caps!
It's not very loud. Sound "comes up" as the filter caps charge. It's not immediate when switching to "Operate", but pretty quick. So, I'd say must be coming from the filter cap(s). Sounds a little like a DC motor.

Needless to say, I don't let it do this very long. Only long enough to take a voltage measurement across the screen resistors.

Anyone ever hear anything like that?

-Paul

[Paul-in-KC]

Gotta be something simple. Are the filaments glowing?

Yes the filaments glow on all tubes.

BUT - here's something...

Thinking about Martin's last post, I decided to take the Bias King out of the equation - thinking that it was keeping the cathodes from ground.

Sure enough. Now we have cathode current!

I see about 20 VDC across the screen resistors (~42.5 mA). Unfortunately, I now also have a disturbing noise coming from the general vicinity of the filter caps!
It's not very loud. Sound "comes up" as the filter caps charge. It's not immediate when switching to "Operate", but pretty quick. So, I'd say must be coming from the filter cap(s). Sounds a little like a DC motor.

Needless to say, I don't let it do this very long. Only long enough to take a voltage measurement across the screen resistors.

Anyone ever hear anything like that?

-Paul

CORRECTION - the "buzz" is coming from the OT.
(that's with no input signal)

[sluckey]

42.5mA seems excessive for a 6L6. I'm thinking the plate current is excessive too. That will buzz the OT until it smokes. Do you still have approx. -50VDC directly on pin 5 of each tube?

[martin manning]

The screen current is way too high. I think the bias circuit needs to be adjusted to get more negative voltage, say around -60.

[Paul-in-KC]

42.5mA seems excessive for a 6L6. I'm thinking the plate current is excessive too. That will buzz the OT until it smokes. Do you still have approx. -50VDC directly on pin 5 of each tube?

Yes - still have a little less than -50 on each (like -48, -49).

[Paul-in-KC]

The screen current is way too high. I think the bias circuit needs to be adjusted to get more negative voltage, say around -60.

I don't have that on my bias tap.?.?

Would dropping the plate voltage to something around 435 VDC (or lower) also work?

I wasn't aiming for that high a Voltage. I was just following the schematic for the Gen 3rd Gen Low-plate 50W ODS (which shows a nominal B+1 = 440 and B+2 =437).

-P.

[sluckey]

I agree with Martin. If -48 is the max bias voltage you have then you need to modify the bias circuit to give you a maximum of about -60v. You can pull the 6L6s until you have more bias voltage.

How much AC voltage do you measure on your bias tap? Show us your schematic.

[sluckey]

I found the schematic. You can increase the negative bias voltage by increasing the 27K resistor on the bias pot to 33K or 39K. Or, you can decrease the 3.3K bias range resistor. I wouldn't go below about 470Ω. Either of these changes should increase the bias voltage enough for 6L6s. It's OK to change both resistors but I doubt that will be necessary.

[Paul-in-KC]

I found the schematic. You can increase the negative bias voltage by increasing the 27K resistor on the bias pot to 33K or 39K. Or, you can decrease the 3.3K bias range resistor. I wouldn't go below about 470Ω. Either of these changes should increase the bias voltage enough for 6L6s. It's OK to change both resistors but I doubt that will be necessary.

Thank you sir!

I'll see what I can get.

In the mean time - if I wanted to share a PDF (such as a schematic) - can that be inserted in a post, or would I need to store it on the internet somewhere and insert a link (the way pictures are inserted)?

I see PDFs shared here, so I know there is a way.

[sluckey]

Sure. Just add the file as an attachment or post a link to it. Either works fine. Here's a link to the message that contains your schematic...

https://ampgarage.com/forum/viewtopic.p ... 71#p163003

And I also added the pdf as an attachment...