Bias sensing resistors - aren’t?

[Raoul Duke]

Tried searching this peculiar problem to no avail - so here’s my question:

While going through my recently acquired ‘78 Marshall 2204 I installed bias sensing resistors for easier measurement. I left the original jumpers from pin 8 to pin 1, clipped the bus wire to ground, and ran the 2w/1R/1% resistor from pin 1 to ground in its place on both sockets. When firing it up today the resistors weren’t giving me a reading, but I could measure bias voltage at the grid stopper to bias feed connection - which for practical purposes I guess works (?) - just wondering why the 1Rs might not be doing their thing.

Any thoughts are appreciated!
Thanks!

[martin manning]

Either there is another path to ground shorting the 1R's, or the cathode current really is ~zero. What are the plate and pin 5 bias voltages?

[Raoul Duke]

Plates are 400vdc and pin 5 is -40vdc.

I’m using the same ground lugs for my 100R ACT resistors as well. It always worked with my Dumble builds; is this critter that different that it wouldn’t?

[martin manning]

-40V Vg1 might be enough to be near cut-off at 400V Va. You could try bringing the bias voltage up (less negative) and see if you get anything, or use the OT primary resistance to see if there is any plate current as it is.

[Raoul Duke]

That’s measuring from each plate to the OT primary CT (which looks to be attached to the HT fuse), correct?

If so, I’m getting .63k on the inner and .82k on the outer tubes while running which I assume is 630R and 820R?

Bias pot sweeps from -40 to -50vdc if that matters?

These resistors are bugging me - they should be working… that’s why I put them there.

[Raoul Duke]

So, oddly enough after racking my brain and trying to see the problem from different angles - I thought: “Those 1Rs are like little cathode resistors, right? Maybe I should measure the voltage drop across them?”

So I did. I started getting readings in mV that made sense given the resistors I have in the bias supply. So I tweaked until I was at about 65% on one tube and 62% on the other and now it’s ready for a sound test… tomorrow of course - it’s after midnight here now and my wife would kill me, lol.

Hopefully I’ve solved this mystery.

[pdf64]

the resistors weren’t giving me a reading

Where were you putting the meter probes?

[Raoul Duke]

On either side of the resistor - one on the pin side and one on the ground side.

I read an older post about someone having a similar problem on a 2204 build and sluckey (one of my main sources of “passive learning”) advised putting the resistor between the pins and taking the reading across the resistor. So I figured I’d try that - but first I’d try reading across the resistor where it was just see what I got. It seemed to be in the correct range and consistent to both tubes - so I’m hopeful.

I figure if it sounds good, isn’t red-plating, and all voltages are in range then I should be ok. I’m trying to find a good step-by-step guide to grid bias the old-fashioned way just to be safe and prove it definitively but I’m still piecing that together.

Definition:
“Passive learning” - using guidance given by established trusted advisors within the “community” to others trying to solve similar problems discovered by using the “search” function on this and other boards.

Just want to make sure my terminology is understood. I can get answers all day long on the internet - doesn’t mean I’d trust them all, right? I’ve read some pretty wacky ideas out there…

[martin manning]

I’m trying to find a good step-by-step guide to grid bias the old-fashioned way just to be safe and prove it definitively but I’m still piecing that together.

I tried to make a concise guide to biasing and posted it here: https://ampgarage.com/forum/viewtopic.php?f=19&t=37916

[Raoul Duke]

Hey, thanks Martin! I’ll definitely check that out as I cook my ham, lol.

Always learning; great stuff!

[sluckey]

So, oddly enough after racking my brain and trying to see the problem from different angles - I thought: “Those 1Rs are like little cathode resistors, right? Maybe I should measure the voltage drop across them?”

So I did. I started getting readings in mV that made sense given the resistors I have in the bias supply. So I tweaked until I was at about 65% on one tube and 62% on the other and now it’s ready for a sound test… tomorrow of course - it’s after midnight here now and my wife would kill me, lol.

Hopefully I’ve solved this mystery.

Many times the problem is a loose connection between the ears.

[Raoul Duke]

I’ve got that one going on most of the time Steve, lol.

Still bugging me why those 1Rs aren’t reading in the conventional manner. I cleaned and scuffed all the metal below those lugs (including the lugs) and used brand new screws with nylock nuts. I’ve seen some layouts where they have those tubes connected at the cathodes and then run to the B+ filter cap ground. Thought of trying that - but if I can prove what I have here is accurate - I’ll stick with it. Measuring across them is a small variation I can work with if it’s accurate.

[Raoul Duke]

I’m trying to find a good step-by-step guide to grid bias the old-fashioned way just to be safe and prove it definitively but I’m still piecing that together.

I tried to make a concise guide to biasing and posted it here: https://ampgarage.com/forum/viewtopic.php?f=19&t=37916

This is great material as well as the discussion within the thread; thanks all! As I digest this and determine the practical steps to apply it, I have a question about the OT center tap. When measuring - I would assume going directly to the CT wire is the most accurate; however I’ve seen/read folks measuring to the main reservoir cap ground. Is this better?

Also, my OT CT goes to the HT fuse holder where I can access a small bit of bare wire to measure from, but it shares the tab connector with what looks like one of the choke leads - so does this mean this is a bad place to measure from due to interference from that other wire?

Just trying to understand and apply the best way to take these measurements in this particular amp. Any advice is appreciated!

Thanks!

[martin manning]

I have a question about the OT center tap. When measuring - I would assume going directly to the CT wire is the most accurate; however I’ve seen/read folks measuring to the main reservoir cap ground. Is this better?

What are you trying to measure here?

[Raoul Duke]

Martin,
Measuring OT primary resistance and then the voltage drop (each side). My understanding is this is required for determining the plate current for grid biasing?

Please put me on track if I’m wrong here - that connection between the ears might be loose again, lol.