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Post subject: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 3:53 am
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Hey!
I posted a similar thread on TGP, but since I just read the DRRI bias thread, I thought, it might be of interest here too...

I started biasing my amps a few months ago. I'd love to hear from experts like shimmilou or Arjay, if I am getting things right.

I have two 6v6 - Fender amps (PRRI and a '66 BFDR) that both luckily have access to the biaspot from the outside, so I do not have to open the chassis everytime I measure bias.

I use a bias probe (with a 1 Ohm resistor) to measure the plate voltage at both powertubes in mV. (If you have tightly matched tubes, the readings should me almost the same.)

According to 'Ohms law' the plate voltage divided by the resistance = current
So, since the resitor in the probe is 1 Ohm, the mV (voltage) reading can directly interpreted as the mA (current). Correct?

Now, as for the "right" value to aim for:
The plate voltages in my amp are around 420 V and consulting the Weber Bias calculator (http://www.webervst.com/tubes1/calcbias.htm) a value of 20mA would be a plate dissipation of 70%, the maximum recommended value for my lovely 6V6 tubes.

But as shimmilou pointed out in another thread, and correct me if I am wrong, what we are really getting by measuring the plate voltage is actually the plate current and not the needed value for the cathode current, which is actually about 3ma lower.

So if I am reading 23mV the cathode current is actually about 20ma, the max. recommended value if I want my precious tubes to last a while. (And, BTW, the Fender schematic of the PRRI recommends 23mV at the testpoints inside the amp.)
I often read in forums that people set their Deluxes up to 28ma, I wonder if they interpret the 28 mV they get as the cathode current....and actually their bias is about 25ma or even lower.

Anyway: I usually did set the voltage reading to 22mV to get about 19ma. (To be little below 70%.)

Strange thing was, that the amps still sounded a little cool to me, and as soon as I turned the bias higher it started to sound a lot better. So I was torn between longer tube life and having better tone. :(
But, yesterday I finally had the great idea of measuring the value of the resistor in the bias-probe! I measured 1.2 Ohm on my multimeter, which is 20% more.
So according to Ohms law, I have to divide my readings with the figure 1.2 instead of 1 to get the proper current, right? Which means, when the multimeter showed me 22mV, the plate current was 18,3333mA instead of 22mA.
And that would explain, why the amps sounded a little on the cool side. So I set the bias according to the new calculations, and violá: I am happy with the sound! 8)
So my advice to all: Check the value of your resistor (in the probe or at the testpoints)!

Please let me know, if I am misinterpreting something here, and I am on the way to giving my tubes a sweet but short life-span. :wink:

The reverend


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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 7:25 am
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You have an excellent grasp of it! (E is volts, I is current, R is resistance, P is power in watts). I'll just point out that the Plate Voltage measured at pin 3 on the tube will be in hundreds of volts not mV, and the Plate current is lower than the Cathode current, (the Cathode current is that current through the "one ohm" resistor). Cathode current minus grid current(s) equals Plate current. So, I think that you mean that the mV reading at the "one ohm" resistor, divided by it's resistance, equals the current through the resistor (which is Cathode current), and you are correct (I = E/R).

To take it one step further, the calculated current through the resistor (Cathode current), times the measured Plate voltage (in hundreds of volts DC), roughly equals the wattage dissipated by that tube at idle (P = I x E). Deducing the actual Plate current using the Cathode current minus a few milliamps, and measuring the actual value of your "one ohm" resistor, just makes your bias setting a little more accurate.

The PRRI for example, if your Plate voltage is about 420VDC, at your setting of 20mA Plate current (23 mA Cathode current), that would be 8.4 watts (8.4 = .02 x 420). For a 6V6 considered to be a 14 watt tube, 8.4 watts idle would be roughly 60%. Measure your Plate voltage, multiply by your Plate current, to get the actual wattage of the tube dissipated at idle. :)

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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 8:19 am
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Thanks for the clarification, shimmilou!
Sorry, I mixed up plate and cathode current. :oops:

Just to make this more confusing for me :) :
You are using 14w as the maximum plate dissipation for the 6V6s. But the Tung Sol RI I have in the Princeton are rated 12W, according to a spec sheet I found. (http://www.newsensor.com/TubeSpecification.aspx)
So that would make my 8.4 W more on the side of 70%, right?

And what is the max. rating for the NOS RCA in my Deluxe Reverb? (GT, not GTA) Also 12W?

Thanks,
Reverend


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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 8:53 am
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The data sheet for the Tung Sol 6v6 provides two sets of ratings, one for a single triode configuration (12W), and the other for a push-pull configuration (14W). The PRRI is a two-tube push-pull. Also, you can rest assured that the NOS RCA will have the same plate dissipation rating as the Tung Sol.


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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 10:13 am
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Whew!

Pretty weighty Sunday-morning read for a geezer still on his first cup of coffee.

But +1 to all posts and to the OP. If the T-S re-issues are overbuilt to any degree they should be fine so long as the idle bias remains at 23mA or below. I have NOS gray-plate GE's in my DRRI and they do quite well there. As to the T-S data sheet, that info should be correlated to the typical operating characteristics found in the RCA Receiving Tube handbook, and changes noted accordingly. And mef1207 is correct -- the dynamic parameters will differ significantly between class A (single-ended) and class AB (push-pull) applications due to the 100% duty cycle of the former.

(time for that second cup now......)

:lol:

Arjay

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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 10:43 am
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thereverend wrote:
Thanks for the clarification, shimmilou!
Sorry, I mixed up plate and cathode current. :oops:

Just to make this more confusing for me :) :
You are using 14w as the maximum plate dissipation for the 6V6s. But the Tung Sol RI I have in the Princeton are rated 12W, according to a spec sheet I found. (http://www.newsensor.com/TubeSpecification.aspx)
So that would make my 8.4 W more on the side of 70%, right?

And what is the max. rating for the NOS RCA in my Deluxe Reverb? (GT, not GTA) Also 12W?

Thanks,
Reverend


This will help you in your bias calculations:

http://www.webervst.com/tubes1/calcbias.htm

I use it all the time.

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Post subject: Re: Biasing - Am I getting this right?
Posted: Sun May 01, 2011 11:54 pm
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bluesky636 wrote:

This will help you in your bias calculations:

http://www.webervst.com/tubes1/calcbias.htm

I use it all the time.


Yes, thanks, I know the link and also use it, but for a correct value you need to take the proper Watts value, and up until now it thought that only 6V6GTAs and the new JJ's were 14W.

And I also noticed the two different values in the TungSol sheet for single- and push-pull configuration, but on page 1 it says this: Max Plate dissipation: 12W
Isn't that the proper value?
And to add to the confusion :wink: I just saw in the EH6V6 tubesheet, that their max. plate dissipation is 14W. :?:
So what am I missing here? Should I treat all 6V6s as 6v6GTAs in the calculator? Because at 420V this would make a difference of 3.3 ma (70% would be 20ma for 12W, and 23.3ma for 14W)

Arjay, I hope you enjoyed your second cup! I am on my first right now. :)
So are you shooting for 23mA in your DDRI? And are you treating the NOS like a 12 or 14W tube? Or, to put this more clearly: For how many % of dissipation are you going? Is the sonic difference between 60% and 70% really big in your experience?

Thanks again for all your comments so far, great to have people with much experience chime in!!

Reverend


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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 12:19 am
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Have a bit of bad news. I bought a "matched" pair of reissued TungSol 6V6GT for my 1966 Pinceton Reverb. Sounded great. Biased at about 8 watts per tube, at idle. Had about 40 hours on the tubes. Actually sounded good, except for a odd overtone that started at about "6" on the volume.

When I checked the tubes... the plates were bright, glowing red. I pulled them and put Hytron made 6V6GTY (1950's) into the sockets and measured the bias. Still steady 8 watts per tube. No more red plates. No more odd harmonics at volume.

Going to pull the chassis and double check the bias power supply. Esp the resistors. Anyone else have this issue?

Thanks!


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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 12:23 am
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thereverend wrote:
Arjay, I hope you enjoyed your second cup! I am on my first right now. :)
So are you shooting for 23mA in your DDRI? And are you treating the NOS like a 12 or 14W tube? Or, to put this more clearly: For how many % of dissipation are you going? Is the sonic difference between 60% and 70% really big in your experience?


I'm using vintage NOS tubes so I take my data from the RCA tube handbook -- thus, I'm treating them as 14W bottles. With the calculated idle bias at 23mA that's an RCH under 70% vmax plate dissipation for my particular DRRI.

Arjay

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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 5:45 am
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BMW2002Ti wrote:
Have a bit of bad news. I bought a "matched" pair of reissued TungSol 6V6GT for my 1966 Pinceton Reverb. Sounded great. Biased at about 8 watts per tube, at idle. Had about 40 hours on the tubes. Actually sounded good, except for a odd overtone that started at about "6" on the volume.

When I checked the tubes... the plates were bright, glowing red. I pulled them and put Hytron made 6V6GTY (1950's) into the sockets and measured the bias. Still steady 8 watts per tube. No more red plates. No more odd harmonics at volume.

Going to pull the chassis and double check the bias power supply. Esp the resistors. Anyone else have this issue?

Thanks!

I have a pair in a '65 DR right now,but keeping a close eye on them,time will tell.
I've always used NOS tubes so the new products are a whole 'nother ballgame for me.
I ordered some NOS Tung-Sol 5881s from Greg Levy at Hi-Test Guitars,Sat.,and we talked for awhile about issues like you mentioned,he knows so much more about tubes than I do so I won't try to quote him,but quality of metal in the new tubes were the main topic.


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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 8:38 am
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thereverend wrote:
bluesky636 wrote:

This will help you in your bias calculations:

http://www.webervst.com/tubes1/calcbias.htm

I use it all the time.


Yes, thanks, I know the link and also use it, but for a correct value you need to take the proper Watts value, and up until now it thought that only 6V6GTAs and the new JJ's were 14W.


Not true. Suggest you re-read it. The Jim Jone's Bias Table clearly says "12 Watts" for 6V6GT, not 14 watts. Been that way for as long as I have been using it, about a year now. In the very first calculator you have the option of selecting a 6V6 or 6V6GTA.

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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 9:15 am
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bluesky636 wrote:
thereverend wrote:
bluesky636 wrote:

This will help you in your bias calculations:

http://www.webervst.com/tubes1/calcbias.htm

I use it all the time.


Yes, thanks, I know the link and also use it, but for a correct value you need to take the proper Watts value, and up until now it thought that only 6V6GTAs and the new JJ's were 14W.


Not true. Suggest you re-read it. The Jim Jone's Bias Table clearly says "12 Watts" for 6V6GT, not 14 watts. Been that way for as long as I have been using it, about a year now. In the very first calculator you have the option of selecting a 6V6 or 6V6GTA.


Yes, bluesky, that is exactly what I meant, maybe you misunderstood my post...

I was suprised, that people treat their 6V6GT (Tung Sol RI i.e.) also as a 14W tube, not only the GTA's - Because 23ma at 420V would be more than the suggested 70% max. dissipation for 12W, but just under 70% with 14W tubes.


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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 9:36 am
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That site appears to max watts PER tube:

http://www.webervst.com/tubes1/calcbias.htm

Not push-pull pairs. I had my Princeton setup for 8 watts per tube. Which is much more conservative than the rated 12 watts the site states for 6V6GT tubes. My Champion 800 puts out about 6 watts and it's an SE-amp, Class "A" amp.

The tubes were actually almost flaming red glow from top to bottom of the anode plates.

The Hytrons 6V6GTY, at the same bias, didn't glow. Even little spots on the plates.

I'm thinking either I got a crummy pair of TungSols. Or they need to be biased around 5-6 watts per tube.


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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 9:54 am
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Something is wonky with that T-S glass. Swapping in the geriatric Hytrons is the proof. I'd ask for an exchange or a refund.

JMO

Arjay

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Post subject: Re: Biasing - Am I getting this right?
Posted: Mon May 02, 2011 10:37 am
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thereverend wrote:
Yes, bluesky, that is exactly what I meant, maybe you misunderstood my post...

I was suprised, that people treat their 6V6GT (Tung Sol RI i.e.) also as a 14W tube, not only the GTA's - Because 23ma at 420V would be more than the suggested 70% max. dissipation for 12W, but just under 70% with 14W tubes.


Guess I'm still confused as to your point. Oh, well. Happens all the time. :lol: I'm a 6L6GC man myself. :D

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