Rob Robinette's bias calculator

[dude]

I put a 6L6GB in my Vibro Champ, used Rob's calculator to bias the amp. He says the 6L6GB is rated for 19 watts. Putting in the voltages and cathode R valve, I get the bias reading from this calculator at idle: Plate cathode current 63.4mA for the one tube (minus approximately 5.5% for screen current). The dissipation on his calculator says that's 95.8%.  I assume at idle the Champ class A is fine at that dissipation. 


I notice in his specs on his calculator Max Cathode bias for that tube is at 66.2% mA's. I assume it means at 66.2mA's that is 100% dissipation for that 6L6GB (tube data says 19 watts, too)..?   


Plate to cathode: 287.2 vdc
voltage across C R: 16.58
cathode R value: 247 ohms
B+ to ground = 302vdc
Screen voltage from Ground =266vdc
Amp sounds great, no red plating. I assume I'm good to go?

[shooter]

double check with your own calculator;


Vk/Rk = Tube current
(Plate V - cathode voltage) * Itube = tube dissipation
subtract a couple mA for G2 if you're a perfectionist   

[pdf64]

Are you happy that the PT isn’t getting overly stressed?

[sluckey]

I put a 6L6GB in my Vibro Champ, used Rob's calculator to bias the amp. He says the 6L6GB is rated for 19 watts. Putting in the voltages and cathode R valve, I get the bias reading from this calculator at idle: Plate cathode current 63.4mA for the one tube (minus approximately 5.5% for screen current). The dissipation on his calculator says that's 95.8%.  I assume at idle the Champ class A is fine at that dissipation.

yes

I notice in his specs on his calculator Max Cathode bias for that tube is at 66.2% mA's. I assume it means at 66.2mA's that is 100% dissipation for that 6L6GB (tube data says 19 watts, too)..?

This is based on the tube type and the plate to cathode voltage you provided.

     19W/287.2V =  66.2mA
 

Plate to cathode: 287.2 vdc
voltage across C R: 16.58
cathode R value: 247 ohms
B+ to ground = 302vdc
Screen voltage from Ground =266vdc
Amp sounds great, no red plating. I assume I'm good to go?

Yes, unless your PT is being stressed.

file:///C:/Users/dude/Desktop/Rob%20RobTube%20Bias%20Calc.html

This link points to a file on YOUR computer. We cannot see it. Better to post a link to the calculator on Rob's site. Looks like this...

     https://robrobinette.com/Tube_Bias_Calculator.htm
 

[dude]

Shooter, l did the math got the same as Rob’s calculator, Sluckey did the math too, same. Thanks for the reply.


Pdf64, l was thinking about the PT too, but after half hour with all knobs at 10, not hot, barely warm if that. Thanks for asking.


Sluckey, l need to figure out Windows again been using a Mac lately, was wondering why the link wasn’t blue.
Thanks for rechecking the math too. l think Rob’s bias calculator is right on the money. As l mentioned above the PT seems good, that why l decided to go with a 6L6GB, not 25 or 30 watts, just 19 watts max dissipation.
I figured probably increased amp wattage to around 10 watts? Amp is loud, clean up to 6 just starts to break up at 7, nice tone at the edge of break up, 10 is Neil Young 5E3 territory or close. I’m glad another member posted a video about raising the the dropping R’s in the Vibro Champ, l used 15k instead of 1k to screen and no screen grid R. That gave me about 40v’s less than the plate. No blocking distortion at 10. With a bigger old paper OT, guessing about 10 watts and a 10” alnico speaker, tone is sweet. I don’t think the impedance is exactly correct, the OT measure 34:1 and l’m using an 8 ohm spk. But l think close enough.
Thanks again for your help.

[pdf64]

Output power is set by the HT voltage, OT impedance, and valve saturation voltage.
I don’t see a reason why a 6L6 type valve here would push out more power than a 6V6.
By saturation voltage, I mean the anode voltage when Vg-k=0, given the Vg2 etc operating point.

[dude]

Output power is set by the HT voltage, OT impedance, and valve saturation voltage.
I don’t see a reason why a 6L6 type valve here would push out more power than a 6V6.

I had a 6V6 (5Y3) in at first after l upped the screen dropping R from 1k to 15k 350 on plate to cathode, screen was 303, 470 ohm cathode R. Tone was ok, but not what l was hoping for. So, tried the 6L6GB (19watt tube). Changed the components for the 6L6, wanted as close to 100% dissipation l could get, 250 ohm R gave me 95% and the amp sounded fantastic and (to me) much louder. Maybe it’s still pushing 5 watts power but it sure sounds much louder then the 6V6.
The slightly bigger OT, 10 watts vs 5, doesn’t saturate like that tiny original.
I could gig with this in a small bar (very light drummer) or mic it for a big venue. I mean at 7 it just starts to break up, that’s where l like my tone. Run it on 10 and roll a strat’s volume down a little for a nice rhythm, up for a lead.
Anyway, thanks for all the info you have posted on my questions, l’m no master tech that’s for sure.

[tubeswell]

I figured probably increased amp wattage to around 10 watts?

If you were running a 30W tube and had it center-biased with an optimum reflected load and fully driven (but not clipping) with good SE iron, you could make maybe 12W tops.


If you replace '30W' with '19W' in ^that sentence^, then not quite 8W tops (or less if you weren't able to drive the tube to max clean, and didn't have it centre-biased at Pmax with the optimum load etc.)

[pdf64]

I figured probably increased amp wattage to around 10 watts?

If you were running a 30W tube …

Maybe a 6L6GC?
Just to note that design max rated valve types (eg 6L6GC) put the onus on the equipment designer to apply a suitable derating, it shouldn’t be used ‘as is’, in the same way that a design centre rating (as per the 6L6GB) can be.
My guess is that a design centre rating for a 6L6GC would probably be about 25-26W.
The full design max rating should only be dissipated under the worst case foreseeable limit conditions, eg mains voltage at max allowable limit and load impedance at lowest tolerance limit. 
From https://www.one-electron.com/Archives/RCA/RCA-AppNotes/RCA%201958%20AN-174%20Design-Maximum%20System%20for%20Rating%20Electron%20Tubes.pdf


A grievous flaw in these online bias calculators is that they ignore the above and make no distinction between anode limits derived under different rating systems.  They treat all ratings as being comparable, on an equal footing, intended to be used identically, which is not the case.

To be fair, none of the current valve manufacturers provide info on the rating system they use, and also the online articles justifying the ‘bias to idle at x% of anode limit’ don’t make any distinction between the rating systems either. I’ve tried to nudge Randell Aiken to clarify, but he’s not playing 

[tubeswell]

My point wasn’t about design center, it was about ‘optimum’ load and center bias based on running the tube at Pmax (whether the tube manufacturers intended to state it was ‘actual’ Pmax or not). If the tube is run at some lower power configuration, then the result will be lower power output.

[shooter]

once i quit gnashing teeth on SE builds, i just started using KT88's somewhere near Pmax, tweaked for sound, not tube life.  Then simply used the volume pots to control loudness, life became worth living again 

[pdf64]

My point wasn’t about design center, it was about ‘optimum’ load and center bias based on running the tube at Pmax (whether the tube manufacturers intended to state it was ‘actual’ Pmax or not). If the tube is run at some lower power configuration, then the result will be lower power output.

Apologies, I didn’t intend to contradict / argue, it’s just that the bee in my bonnet started buzzing, and I was driven to explain the issue (in case anyone isn’t aware of it)

It’s also worth noting that the 9k reflected load on the primary that’s currently used here isn’t likely to be near optimum for max SE power output.
So idling near the anode’s limit is unlikely to result in centre bias, whichever rating system the anode is rated under. Rather it’s likely to be somewhat hotter than centre.
So the hot bias is more about its effect on tone / gain, rather than power per se.

[PRR]

> the 9k reflected load on the primary that’s currently used here

Where was that given?

[dude]

> the 9k reflected load on the primary that’s currently used here

Where was that given?

Maybe cause I mentioned l used a SE 10 watt OT with a  34:1 ratio, 8 ohm spk with a 6L6GB
EDIT: 34x34=1156/8=9248, no cigar, 4 ohm spk would be on the money

[PRR]

...Maybe cause I mentioned...

Thanks.