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Old 7th March 2007, 07:02 PM   #1
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Default *very* basic plate resistance Q

All,
When hunting around for a tube to use, what sort of ratio do you look for between the plate resistance of a preamp tube, to the input impedance of your power amp ?
Im looking at 47k for my input impedance, and have been browsing though some tubes that I would like to try; but I just want to be sure that their plate resistance isn't too high.

Thanks !
Steve

PS - I did search for the answer, but no matter what string I tried, I couldn't quite find what I was looking for...
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Old 7th March 2007, 07:30 PM   #2
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Hi. The output impedance of a common cathode stage, (the most typical for preamps), is the tube's plate resistance [rp], in parallel with the plate load resistance [Rp]. When an unbypassed cathode resistor is used [Rk], the tube's resistance is rp + mu*Rk. In the latter case the output impedance is therefore (rp+mu*Rk)parallel Rp.
In other words, the output impedance is always much lower than the tube's rp on the datasheets. This gives you a lot more options in choice of tubes.
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Old 7th March 2007, 07:36 PM   #3
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Thanks ! So I guess the question is; what range should I be looking to score ?

Thanks again,
Steve
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Old 7th March 2007, 07:46 PM   #4
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Quote:
Originally posted by Steven-H
Thanks ! So I guess the question is; what range should I be looking to score ?

Thanks again,
Steve
Equilibrium.

For better power transfer it should match. For better linearity it should be lower.
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Old 7th March 2007, 11:58 PM   #5
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Yes, but power transfer isn't what we're looking for here... the preamp is a voltage amp, and loading it with the "matched" impedance will reduce the voltage gain and possibly push it into a less linear region.

You'd like to have an output (source) impedance much lower (1/10th?) the impedance of the load. This is also good for driving long cables... the capacitive load is less significant with a low source impedance.

So... low plate resistance tube, cathode follower, step-down transformer, voltage feedback are things to consider. Also - 100 pF (5 ft of cable, 20 pF/ft) is 80K reactance at 20 KHz. That's a few dB roll-off if you have a 50K source impedance.
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Old 8th March 2007, 01:04 AM   #6
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Ok, understood. So I suppose my question is this:
When selecting various triodes to try in a preamp (irregardless of topology), how much of a factor is (should ?) plate resistance be ? Judging by how the final output value can be somewhat manipulated, is it really such an issue ?

Thanks !
Steve
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Old 8th March 2007, 03:22 AM   #7
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Quote:
Originally posted by Steven-H
Ok, understood. So I suppose my question is this:
When selecting various triodes to try in a preamp (irregardless of topology), how much of a factor is (should ?) plate resistance be ? Judging by how the final output value can be somewhat manipulated, is it really such an issue ?

Thanks !
Steve
I don't worry about it. Linearity is more important so that good open loop performance can be attained. That leaves fewer messes to clean up. The only other consideration is g(m) and whether or not the current demand for slew rate performance can be met at the higher frequencies. It's seldom much of a problem when driving another voltage amplification stage. Finals can always be driven with cathode or source followers.

The plate resistance will vary all over the place with Q-Point anyway, so you really don't know what you'll get anyway.
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Old 8th March 2007, 03:15 PM   #8
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Actually, your question has a slight flaw built in. it is nearly impossible to judge which tride to use solely on Rp, REGARDLESS of topology. A good example would be a cathode follower - here gm would take front row with Rp being of much lower importance.

I have recently been in your shoes, designing a simple line stage, and my reasoning on this is to design with a 'safety factor'. In my case, although the input impedance of the power amp is the standard 47k, I did my calculations with a low 10k - this should cover the vast number of output stages out there. Also, accounting for about 2m of interconnect plus stray capacitances, say 100-200pF capacitance total) is not a bad idea. With worst case conditions (highest Rout and highest capacitance) we want to have a -3dB high roll-off at least one, and preferably two or more octaves above 20kHz. So, for 40kHz and 200pF the output impedance of the line stage should be less or equal to 20k, preferably much less than that. How Rp willfigure into this depends heavily on the topology.

From this point on i can only give you the example of my own design. The tube chosen has a Rp of about 9k, and was chosen primairly because it has a sufficiently low mu and is very linear at chosen Q point
There is no cathode resistor (fixed bias is used), and there is a 22k anode resistor. Even though mu is about 21, the resulting gain of ~16 is too high for a line stage, since I was aiming at a gain of about 4. This could have been reduced by using an unbypassed cathode resistor, but that in turn increases the output impedance signifficantly, so anode feedback was used instead. This introduces a feedback resistor which is a load and in parallel with the output impedance, but because it is a large value, we can disregard it in the calculation. Hence, the output impedance without feedback applied is 9k || 22k == 6.38k. At this point it should be noted that using a lower value load (input resistance of power amp) actually helps with the capacitive part of the load, as the input resistance of the power amp goes in parallel with the output resistance of the line stage - but this comes at an expense, which is reduced linearity. This is why MilesPrower mentioned that you should concntrate on finding a linear tube, as it will have more consistent sound with various loads.
In most preamp aplications, anode resistors will be at least equal, and mostly several times the tube Rp. Given the above calculations, it follows that in this configuration, an Rp of ~30k or less (preferably much less) is a good idea.

At this point i will return for a second to the use of feedback. Because voltage feedback is used, the output impedance of the line stage is reduced by feedback action, by the same factor as it's amplification. I am aware that NFB gets a bad rap, but in this case it was considered a fair trade off because it alowes me to use an easily obtainable, and more linear tube. Reasonably priced and easily obtainable low mu (in the range of 6-8) triodes tend to be trioded power pentodes or triodes for voltage regulation, which are not as linear as i would like. The feedback is loacal and comes out as apporximately 12dB. It also lowers the effective output impedance to about 1.6k, which is a nice bonus. i am mentioning this as a serious consideration, precisely because it may alowe you to use a higher Rp tube 'in a pinch', assuming, of course, you take care to deal with all other repercussions fo using feedback in your design.

Finally, a footnote on using fixed bias in a preamp - a clean DC supply was available, so it was easy to generate a fixed negative bias, providing the opportunity for eliminating a cathode resistor bypass capacitor 9since indeed there is no cathode resistor to bypass), which would have to be an electrolytic type. Instead, however, an input coupling cap is needed, but fortunately, this can then be of a much smaller value and foil type.
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