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mr mojo 5th January 2006 02:39 PM

Bias and max current for my 7591 AB amp...
 
Howdy folks!

I've got a question about the current levels in my 7591 PP amp.

IIRC, max current rating for a 7591 is 85ma. This may seem silly, but is that for one tube or is that for a pair in PP?

The amp I built is adjustible fixed bias with AC/DC balance pots and test points. Each 7591 has it's own 27ohm resistor to ground at each of the four test points.

If I'm doing this right, then: 27/.9=30ma. Schematic lists 33ma at this bias point so I'm wondering if my math or the schematic is incorrect.

Next question: I know biasing the tube further into class A territory will give better sound at the expense of tube life.

Where exactly is the golden mean here?

I can go: 27/.8=33.75ma

Or: 27/.7=38.6ma

Or: 27/.6=45ma

My HV secondary is rated for 750ma so the extra current is no problem. All HV filter caps are rated for 900v, so voltage-as long as it doesn't exceed max plate and screen ratings-isn't an issue either. If my math above is correct, which of these bias points would give the best balance between sound while sacrificing just a bit more tube life than the most conservative settings?

Thanks for any and all help!

Best,
mr mojo

mr mojo 5th January 2006 03:37 PM

Forgot to mention I'm using a pair of 5AR4s in parallel, so current draw won't be an issue there as well...

JojoD818 5th January 2006 03:50 PM

hello there mr.mojo

if R = 27 ohms, and you read 0.9V at one of the 7591s cathode, then

V=IR, I=V/R

then it should read I = 0.9/27 = 33.33mA per 7591 tube

again, if you read a 0.6V at one of the cathodes, then your current would be:

I = 0.6/27 = 22.22mA

I should also remind you that this amount of current is actually plate + screen current.

cheers,
JojoD

mr mojo 5th January 2006 03:59 PM

Hey Jojo!

Boy do I feel dumb!! I appreciate the clarification. That's what I get by going from memory rather than double checking Ohm's law to be sure!

So, with the math corrected:

1/27=37ma

1.25/27=46ma

Any idea if the 85ma max rating of the 7591 is for a single tube or a pair?

If the 85ma max rating is for a single tube, which of the above bias points would give the best compromise between best sound and moderate tube wear?

Thanks again for your help-I sure do appreciate it!

Best,
mr mojo

JojoD818 5th January 2006 04:17 PM

Hi,

No problem, I'm here to help as well as to be helped. :D


Found in a datasheet that 85mA is per 7591 (for one tube), proper bias point can be found in the datasheet of the tube. I think this depends on your plate voltage too.

JojoD

mr mojo 5th January 2006 04:42 PM

Jojo,

Thanks again for the help! Duncans tube spec page isn't working and I couldn't find definative info on whether that spec was for one or two tubes.

I'm running plate voltage at 465v and 33ma. the screens are fed by a pair of 0A3 VR tubes to give a somewhat constant 75v drop.

Max plate for a 7591 is 550v. Max screen is 440v.

If I bias them for 40ma or even a bit more my plate voltage should actually decrease a bit-right?

What I'm looking to do is bias them into relatively high class AB1-not into class A; trying to get the best possible sound, even if it shortens tube life by a year or two. But I don't want to cross the line and shorten tube life by 4 or 5 years or risk outright damage.

The output transformers are rated for 70watts and with AC and DC balance I should in theory be able to bias the 7591s hotter without running the risk of unbalancing the output transformer-is that right?

Or, is this whole situation more complicated than I think it is?

Thanks a lot for the help!

Best,
mr mojo

Giaime 5th January 2006 04:46 PM

Quote:

Originally posted by mr mojo
Any idea if the 85ma max rating of the 7591 is for a single tube or a pair?

If the 85ma max rating is for a single tube, which of the above bias points would give the best compromise between best sound and moderate tube wear?

Hey Mr.Mojo, I think you haven't understood a very important thing in tubes and power stages.

Max current doesn't say a thing when you must bias an amp: 85mA (yes, it's for each tube) means that if you try to pass more than 85mA the tube will have a very short life.

But when biasing, it's not the current limit that counts, it's the POWER limit. A tube's plate has a finite capacity to dissipate heat, the capacity is indicated in the datasheet as Maximum Plate Dissipation. For those tubes, it's 19W. You must bias the amp mantaining the quiescent plate dissipation under that limit.

Plate dissipation is calculated by multiplying cathode current (I'm semplifying the things a bit) for the plate-to-cathode voltage. So You have to measure plate voltage, write it down on a piece of paper and figure out the max current you can have without exceeding that limit.

So I = P / V, you must divide the Max Plate Dissipation (19W) for the plate voltage to get the maximum current. For 450V, it's 47mA for each tube, MUCH LESS THAN THE 85mA you're taking as "limit".

But... wait a minute!!!! You can't bias the tube at the maximum dissipation in a class AB amp. You cannot bias them at more than 70% of the maximum plate dissipation, so I = 0.7*P / V = (still taking 450V as a guess, you must substitute your plate voltage here) = 33mA for each tube, 66mA for the pair.

Obviously, biasing them to the 70% max Pd will shorten their life but you get good sound, less than 70% you'll increase tube life.

Those considerations are valid in a class AB amp, in a class A amp you can run the tubes to 100% max Pd.

You still can surpass those limits, but the reliability of the amp will be greatly compromised. Some tubes can't stand their maximum Pd (especially cheap chinese ones), imagine what they would do if you surpass that value.

Hope this helps


;)

Giaime 5th January 2006 04:49 PM

And for a complete treating of the argument, see here:

http://www.aikenamps.com/Why70percent.html
http://www.aikenamps.com/Biasing.html

mr mojo 5th January 2006 07:58 PM

Giaime,

WOW! Thank you for taking the time to post such a thoughtful and detailed response-I REALLY appreciate it!

Your response and the links have made things much more clear.

I now know that the max PD safely allowed per tube is (19).7=13.3 watts.

The article from which I built the amp lists 475v on the plates and 33ma cathode current at idle, giving us:

(475).033=15.675 watts; well above the 70% PD!

Since I've got 465 on the plates; staying within the 70% PD rating gives us:

13.3/465=.028ma

And with the 27ohm cathode resistors at the DC balance test points, that gives us:

.028(27)=.756v

So, for my amp the bias needs to be adjusted for .756v at the cathode test points to give me 13.3 watts PD.

What is unclear though, is with a max PD of 13.3 watts, that gives 26.6 wpc as the max safe output from a class AB 7591 amp.

The article states 35 wpc, and at the plate voltage and idle current listed on the schematic gives 31.35 wpc. IIRC, the Scott 299C is said to be rated for 35wpc as well. Are these instantaneous peaks rather than average output?

Once again, thanks so much for your help. I keep this up and maybe in 10 or 15 more years I'll actually start getting a handle on this whole "DIY tube amp hobby!"

Best,
mr mojo

Giaime 5th January 2006 08:20 PM

Hello MrMojo, glad to have been helpful.

Quote:

Originally posted by mr mojo
What is unclear though, is with a max PD of 13.3 watts, that gives 26.6 wpc as the max safe output from a class AB 7591 amp.

The article states 35 wpc, and at the plate voltage and idle current listed on the schematic gives 31.35 wpc. IIRC, the Scott 299C is said to be rated for 35wpc as well. Are these instantaneous peaks rather than average output?

Don't confuse plate dissipation with power output, they're two very different things. This leads to another problem...

Quote:

Originally posted by mr mojo

I now know that the max PD safely allowed per tube is (19).7=13.3 watts.

The article from which I built the amp lists 475v on the plates and 33ma cathode current at idle, giving us:

(475).033=15.675 watts; well above the 70% PD!

Since I've got 465 on the plates; staying within the 70% PD rating gives us:

13.3/465=.028ma

And with the 27ohm cathode resistors at the DC balance test points, that gives us:

.028(27)=.756v

So, for my amp the bias needs to be adjusted for .756v at the cathode test points to give me 13.3 watts PD.

You see that the amp designer said 33mA, why now we are biasing at 28mA? Don't you sense that something is wrong? :smash:

Yes, something's wrong here. That's because we've done some semplifications:

in a pentode, cathode current is equal to plate current PLUS screen current. So what you measure at the cathode is the sum of these two currents. We cannot bias the tube according to the cathode current, because that's not a clear indication on how much current the plate does pass. So, we have to determine REAL plate current. To determine this, simply determine quiescent screen current and subtract this from the cathode current to get plate current.

How to calculate screen current? Surely your amp has some kind of screen resistor connected to the screens of the power tubes (pin 4 and 8), usually one for each tube or one for each pair of tubes. Measure the voltage drop across this resistor: put your meter on the highest voltage scale, and place (carefully, there's high voltage juice here!) the probes across the resistor. You'll get a voltage, now to calculate current you have to do I = V/R, where R is the screen resistor. You should get about 10mA per tube.

Now that we know screen current, we can subtract this to our bias calculations. So now, you have to determine plate current.

We said you must stay under the 70% of the Max Pd, so 0.7 * 19 / 465 = 28mA. But cathode current will be 28mA + the screen current, so to calculate the cathode current you need you must add 28mA to the screen current that you measured. I expect around 38mA here, so you can safely bias your tubes up to 37/38mA. See now, your 33mA value wasn't excessive, in fact that's a little bit conservative, and I must add that you should follow this 33mA rule, because that's not good to use the tube right at its limits.

About the power output, my datasheets for the 7591 says that under class AB1, 450V on the plates, 400V on the screens, 33mA of plate current, 9.4mA of screen current and a 6600 ohm load, you'll get about 45W!


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