Bias range question.

[martin manning]

-100V is definitely more than -10V. Which will push more current through a resistor?

That depends on what the other end of the resistor is connected to.

Which would cause more pain if accidentally touched?

If it's 100F outside I know what clothes to put on and what precautions I need to take. If it's -100F outside, I'm not going ;^)

[sluckey]

If it's -100F outside, I'm not going ;^)

That's my point. -100° is definitely more cold than -10°.

[Raoul Duke]

Raoul,

To be more specific, what is the bias voltage going to the tubes? It can be measured at the wiper of the bias pot.

Also to make sure we are all on the same page, when you say you turned the bias voltage DOWN, do you mean closer to zero (smaller number) or more negative ( bigger number)?

I’ll check the bias voltage again tomorrow and report back.

Yes, turned down to a smaller number. I followed Martin’s guide for initial power up and static bias setting - so I turned the bias to the lowest setting anticipating I’d be working my up. The bias swing with the JJs was something like -32 (full CCW) up to -55 (full CW) IIRC.

With the JJs set up correctly (I think):

- Bias voltage going into the pot is -48.4vdc (CCW lug fed from diode and 3k3 on bias supply board)
- Voltage at pins #5 averages -42.5vdc
- Voltage on the lead of the 1R resistor that is soldered to pins #8 averages .042vdc
- Voltage on those same leads with DMM in DC mV scale averages 43mvdc (this is usually the scale I use for setting bias)

I don’t think I’m doing this wrong but please correct me if I am.

[martin manning]

Sounds good to me. What is the plate voltage now?

[Raoul Duke]

Plates hover between 448 and 455 depending on what’s coming out of the wall and how warm it is. It’s closer to 455 on start up but settles right around 450 after 10-15 minutes of warm up.

Now that it’s out of the cab again - I’m thinking of putting the1k8 in and trying the Tung Sols…

Any suggestions or anecdotal advice regarding this? I don’t mean to sound indecisive; just something about the TSs being in the 2nd gen amp “seems” right-er, lol.

[martin manning]

Ok, I wasn't sure which set you had in it. I'd try the Tung-Sols. It's easiest to parallel another resistor to find the right value before removing and replacing. You might find something that has the range to cover both the JJ's and the Tung-Sol's, maybe -40 to -60V Vb would do it. Seems like you have the +/- 3s sets of tubes there ;^)

[Raoul Duke]

Tung Sols are in with a 1.5w, 1% 1k8. Here’s the latest readings:

451 on the plates
Bias voltage at the pot is -58.5
Pins #5 avg. -54
At the 1R: avg. .044vdc, 44mVdc

I don’t understand the disparity between pins 5 and pins 8? These readings were pretty much aligned with the JJs (which still bias with the 1k8).

Got to be something going on in the tubes, right?

[martin manning]

They are just very different in plate current at g1 voltage (pin 5), and the Tung-Sol's are strong in cathode emission, requiring more negative g1 voltage to throttle them back.. The 44 mV across 1 ohm is telling you the cathode current (plate + screen) is 44 mA.

[Raoul Duke]

So still bias my 65% at the 1R and I should be good then? Or should I be somewhere in between?

[martin manning]

451 x 0.044 = 19.8W, 19.8/30 = 0.66, or 66%, including the screen current, which might be ~5%. Actual plate dissipation is ~61%. I think it's fine there,

[Raoul Duke]

So still take the reading at the 1R as the definitive measurement for bias; and continue to look for the reading there as the 60-65% yard stick- check.

Thanks Martin! I was just reading a post you were part of from 2018 that details how to get “there”. At first pass, I struggle to understand - but I’ll keep at it until it penetrates the cranium, lol.

I appreciate the advice and lesson (once it sinks in).

I hope these things sound decent…

UPDATE: Finished reading the 2018 post and ran through the above example a few times and I have a much better understanding now. Makes a lot more sense now. With the previous back and forth I was starting to really doubt what I was doing.

[GAStan]

I'm happy to see you got this sorted out. I apologize for any confusion I caused.

Also to make sure we are all on the same page, when you say you turned the bias voltage DOWN, do you mean closer to zero (smaller number) or more negative ( bigger number)?

When I wrote the above post I was thinking in terms of absolute numbers, so should have written, "smaller absolute number" and "bigger absolute number".

I'll edit my original post for future readers.

[Raoul Duke]

Nothing to apologize for Glenn, I appreciate your willingness to jump in and help a greenhorn (like me) out.

My confusion was mainly due to my over-thinking and second-guessing; which I attribute to my own inexperience.

It’s all good