910R ok for 5670 driver bias?

[Deke609]

I want to experiment with using resistor bias on the 5670 driver as another forum user did (can't find his posts).  4.3 mA @ 4V bias makes a 930R resistor. Is there any wiggle room with that value - will 910R do? That would give me 3.9V and change of bias.

Perhaps foolishly, I am thinking of trying silver tantalum resistors that come in 910 or 1K ohms, nothing in between. I'll try "regular" metal film resistors first to see if I like what I hear. 

I suppose the answer is to smoke it and see what it does for me -- but I'd appreciate any insight from PJ and PB, if they have any -- e.g., "We tried tons of small changes to the 5670 bias, and 4V bias @ 175B B+ - not more, not less - was the only one that sounded good" or "Probably won't make much if any difference"

MTIA, Derek

 

[Paul Birkeland]

The bias instructions for the Kaiju/MonAmour/Paramount call for setting the bias voltage for achieving a specific plate voltage.  You give that up with the resistor, so double check for the curves and aim for about 175V DC on the 5670 plate. 

 

[Deke609]

Many thanks, PB. I don't suppose there's any way of tweaking the plate voltage from the C4S plate load side - e.g., by adding some resistance at the input of the B-side C4S? I suspect not since the B-side ties in with the TL431 bias reg board of the shunt reg side of the 5760 (although I have no idea why it does this).

cheers, Derek

 

[Paul Joppa]

There is very little margin for error on that 175 volts - you need to keep the maximum grid/cathode voltage below -1v to avoid grid current in the 5670. I'd use a 2K trimpot. There are bulk foil trimmers available, if you're concerned about quality.

If you don't bypass the cathode resistor, it will add around 30Kohms to the effective plate resistance, which makes it work harder into the 300B Miller capacitance; frequency response would be around -1dB at 20kHz. If you do bypass it, you'll want ~100uF.  (These calculations are crude and approximate, not a full-out design effort!)

The shunt regulator voltage is limited by the C4S transistor; 300v is the maximum for reliable operation  in my opinion.

 

[Paul Birkeland]

There is very little margin for error on that 175 volts

Ah, you don't want to drive the grid to within -1V?  That would indeed make for very little margin of error!

 

[Deke609]

Many thanks PJ. You've saved me quite a bit of trouble.

I'd use a 2K trimpot. There are bulk foil trimmers available, if you're concerned about quality.

Oooh, foil trimmers! I didn't know they existed. But sure enough, Vishay Precision makes some.  Assuming, for the sake of discussion, that resistor composition can affect sound, my assumption is that if two resistors are put in parallel - with one having 10X the resistance of the other -- then, other things being equal, the 10X resistor will have less of a sonic imprint than the 1X. My mostly ignorance-based guess is that magnitude of sonic imprint is proportional to current. If that's plausible, then my plan would be to use a 1K2 resistor with a 10K foil trimmer bypass in parallel.  If I like it, I will shell out for 1K2 silver tantalum resistors - to see if I believe I hear a difference. I'll start with a Dale RN 1K2 for the initial test.

If you don't bypass the cathode resistor, it will add around 30Kohms to the effective plate resistance, which makes it work harder into the 300B Miller capacitance; frequency response would be around -1dB at 20kHz. If you do bypass it, you'll want ~100uF.  (These calculations are crude and approximate, not a full-out design effort!)

That's very good to know! I thought the bypass/no-bypass cap was purely an issue of gain.  I didn't think that it influenced output impedance of the tube (or portion of circuit from 0V to plate). OK, so I don't want to muck with the ability of the driver to drive into the Miller capacitance - so I will add the bypass cap.

I'm not asking for you to do any calculations or provide a lengthy explanation -- but if the following can be answered quickly without effort on your part, I'd really appreciate your insight: For bypass cap sizing, is there some kind of complicated (by my standards) Q/resonance, etc. relationship that needs optimizing, or, instead, can I use a simple dumb rule of thumb: more capacitance = more good b/c more capacitance = less capacitive reactance? I.e., is the 100uF a ballpark minimum figure - or instead a rough estimate of ideal balance of multiple factors?

cheers and many thanks again, Derek

 

[Paul Joppa]

The effect of bypassing cathode resistors is widely covered - for frequency response, it's a shelving filter in the bass, for example. I'm sure you can find a decent description somewhere This is probably good:

http://valvewizard.co.uk/ChoosingBypassCaps.pdf

It's been a ling time since I worked this out and I did it for the minimum capacitance in the bass, for 2A3s and 300Bs. So you'll have to do some algebra and head-scratching. It's an interesting question, so I'll give it some thought, but no quick answers.

 

[Deke609]

Many thanks again PJ. that's very helpful.  I like Blencowe's geometry solution for finding the resultant vector/phase angle.  I'll have to borrow my son's protractor and play around with different values.

cheers and thanks, Derek

 

[Deke609]

Well, I never got around to using the protactor b/c I realized that with a C4S load "Ra" is in megaohms and so totally dominates, so that the equation for minimum cathode bypass cap size simplifies to the much easier formula C = 1/(2*pi*R*f) where R is the cathode resistor in ohms and f is the desired "half boost" bottom frequency.  Assuming that's correct, it looks like a 35uF cap would be good to 5Hz!  So not much needed. 

For voltage rating, I was thinking of going with some small multiple of the voltage difference between cathode and 0V - which in this case is about 4V, so anything above 10V rated would do.  Besides potentially lower ESR, is there any reason for going with a higher voltage rated cap -- e.g., for safety? Off the top of my head I can't see how the the cathode bypass cap would see a big voltage difference. I guess a dead short from cathode to grid or plate could draw a ton of current and drop a lot of voltage across the Kthode resistor. But in that case, I think I'd have bigger concerns than a burnt out cathode bypass cap.

many thanks again, Derek

 

[Paul Joppa]

Your reasoning is good  :^)  The only place where I worry about excess cathode bypass voltage is on output tubes, where if the grid is floating (damaged grid stopper or bad solder joint for instance) the bias will drop to zero and the voltage can go quite high.

 

[Deke609]

Many thanks again PJ. It turns out the voltage rating question is moot since I want polypro film caps and they only come in high voltage ratings. So I have 3 pairs of 500VDC rated block-style dc link caps coming: 25uF, 47uf and 100uf. Not great physical size-wise, but I have a bit of free room in the Kaiju rebuild, so doable. And 500V is a useful rating, so they'll all find a non-Kaiju purpose eventually.

cheers, Derek