Adjusting 300B plate current/operating point

[Deke609]

I've been sitting on a pairs of Lundahl 70H @ 60 mA chokes and big amorphous core parafeed output transformers for months now. They are intended for a rebuild of my SII-45B amp. But my work schedule is pretty tight this year, so I may not have time to even put together a prototype until the end of the year, and possibly not until the new year.

... And I really want to try the new choke/OT combo. So I plan to try them out in my Kaiju. BUT ... I understand that the Kaiju 300B has a plate current around 70+ mA.  My chokes are gapped for a recommended 60 mA.  So I need to drop the plate current.  I see three options: (1) increase the value of the cathode resistor; (2) drop B+; or (3) do both.

What would people recommend?

Follow-up question -- would I need to make any additional changes to the 5670 operating point -- for example, to decrease the pk-pk VAC signal coming out of the 5670 to make up for any decrease in the available voltage swing of the 300B?

This is not a permanent change (although I don't mind doing major surgery to make it happen). This is just to try out the new choke/OT combo. As mentioned, the combo is ultimately destined for my SII-45B rebuild and I will return my Kaiju to its original (albeit slightly modified) state.

The tubes are EML 300Bs and Jac has published a number of recommended operating points @ 60 mA plate current. So I'm hopeful that it will sound ok.

Edit: The recommended 60 mA current for the chokes also assumes that they will see as much as 640 VAC @ 100 Hz (they can be used as either plate chokes or power supply chokes). I understand from Jac (unless I misinterpreted him) that the much lower VAC signal the chokes will see doing plate choke duty means that I have a bit of wiggle room on max current. But this probably very difficult to calculate (and in any event beyond my present knowledge), so I figure the safest course is just to bring plate current down to around 60 mA.

many thanks in advance, Derek

 

[Paul Joppa]

The VAC capability of a plate choke is proportional to frequency, so 640vAC at 100 Hz is 320vAC at 50Hz, 160vAC at 25Hz, etc. Into 3000 ohms, 160v is 8 watts.

With the MonAmour, we have observed that running 60mA at 300v plate-to-cathode sounds clearly better than running 50mA at the same voltage. Of course this was to be expected, but I did not expect it to be as audible as it was. So I would recommend reducing the plate voltage if you want to hear the difference between the iron components rather than the difference in operating point. Be sure to listen to the stock components at the reduced voltage, since there will be slightly more distortion at a given power. I don't expect that to be audible with headphones, but as noted above my expectations are not a reliable guide.

The driver has regulated voltage, so no change is needed

 

[Deke609]

Many thanks again PJ. I will decrease the 300B plate voltage.

Some quick follow-up questions:

If I'm not to make any changes to the 5670 driver circuit, does it follow that I should only decrease the B+ supply to the 300B and leave the existing B+ to the 5670 circuit unchanged? Or reduce B+ for both, and make any needed adjustments to the regulator trimpot to hit 175VDC out? Or will the regulated voltage not be affected by a decrease in B+?

many thanks, Derek

 

[Paul Joppa]

The regulated voltage is 300vDC. As long as the B+ is greater than 310vDC, the regulated voltage will be unaffected.

 

[Deke609]

Perfect. Thanks PJ. I confused the regulator with the bias adjustment. 

cheers, Derek

 

[Deke609]

Finally got the Lundahl 70H/60mA chokes and amorphous core OPTs installed in the Kaiju. That was a bit involved - although there's enough space, the chokes and OPTs have different mounting hole spacings and thus require mounting on standoffs and a particular order of installation that was hard for me to keep straight in my head late at night.

I also decreased B+ and now get about 67mA plate current (previously 75mA). Jac informs me that the Lundahl chokes have about 15% additional headroom, so I'll leave it as is for now. I fired it up and can detect no vibration from the chokes, so they seem reasonably happy. To get closer to 60mA without further decreasing the bias, I may swap in 1.2K cathode resistors in place of the stock 1K.

BUT ... I can't listen to it!!! Two day ago, my right ear started buzzing loudly, and over the course of the day went mostly silent. Since then, my hearing in the right ear has been muted for bass and sensitive to mids and treble (with weird reverb). I hadn't listened to music for a week or more (both my SII-45 and Kaiju were in various states of disassembly), and was not otherwise exposed to anything loud. And today my left ear is a bit wonky -- so most likely a virus.  So critical listening will have to wait until this clears up.  I tried listening only through my left ear last night -- but that was less than satisfactory.

In the meantime, I think I'll install the Kaiju iron in the SII-45 and do my best to tweak the operating point for the 45B.  I'll be able to use the full winding of the Kaiju choke.  And one day I'll get to listen to it.

Thinking ahead to my eventual rebuild of the SII-45B amp: What are people's opinions on the pros and cons of fixed bias instead of cathode bias for the output tubes?  All this playing around with B+ and plate current got me thinking that fixed bias might give some additional control over operating points -- or put differently, less dependence on how off-the-shelf iron interacts with the tubes.  I was already planning to have regulated B+ (Linear Audio makes a regulator kit that can handle 600V+ at 200mA+). Maybe I could add a regulated negative bias supply. Pete Millet makes one, and maybe there are others out there.  Among other things, I figure this would allow me to use the full B+ as plate-to-cathode and thus to use lower B+ than would be required with cathode auto-biasing (which drops a lot of voltage across the cathode resistor).  I haven't looked into this in any detail yet, but if the regulated bias supply used trim pots to dial in the -ve voltage, I could install some voltage meters to monitor bias and some sort of set-screw mechanism to make adjustments on the fly.

Anyone played around with fixed bias and have some thoughts?

cheers and many thanks, Derek

 

[Paul Birkeland]

What are people's opinions on the pros and cons of fixed bias instead of cathode bias for the output tubes?

There's already a little too much plate voltage available in a Stereomour to run a 45 with cathode bias, hence the additional dropping resistors.  Going to fixed bias makes this problem even worse.

 

[Deke609]

Thanks PB.

But this would be tailor-built for the EML 45B, many of the recommended operating points of which call for 400V+ plate to cathode, 48mA plate current, and nearly -90V bias.

So I'm wondering whether fixed bias might make sense in such an amp.

cheers and thanks, Derek

 

[Paul Birkeland]

Why not take your Lundahl iron set and pair it with a power transformer that will make you about 500V DC so you can properly use these?

 

[Doc B]

Could be your ear problem is allergy related. My right ear gets clogged and my hearing gets messed up a little every spring. Flonase helps.

 

[Deke609]

Why not take your Lundahl iron set and pair it with a power transformer that will make you about 500V DC so you can properly use these?

Well, I honestly dunno. That was the original plan. Now I'm wondering whether there might be some advantage to employing fixed bias -- but never having used it, let alone designed a circuit using it, I'm hoping for some input from people a lot more experienced than me about the pros and cons of fixed versus auto bias. 

cheers and thanks, Derek

 

[Deke609]

Could be your ear problem is allergy related. My right ear gets clogged and my hearing gets messed up a little every spring. Flonase helps.

That's an idea. Thanks Doc. Didn't consider that possibility. I don't usually get bad allergies, but I have been sneezing without any other symptoms for about a month -- maybe all this indoor time without much outdoor exercise (I usually cycle to work) is exacerbating things. I'll take a Benadryl.  Don't have anything else on hand.

 

[Paul Birkeland]

Fixed bias advantages:
1.  You need less B+ voltage.
2.  You have some added flexibility in terms of using different tubes in approximately the same circuit, assuming there's lots of adjustment range in your bias supply.
3.  At turn on, the grid will almost instantly be at the desired negative bias voltage, whereas a cathode biased tube will start at zero bias (or worse for a directly coupled amp).
4.  No heat generated in the chassis from cathode bias resistor(s).
5.  For directly heated tubes, you can heat many tubes on one filament supply but bias them individually by measuring plate current and adjusting the bias voltage to the grids individually.

Fixed bias disadvantages:
1.  Cost.  You need some AC winding to feed the bias supply, at least a diode, a couple of caps, a resistor or two, and a sturdy pot.  You also need to add in some small resistance so you can measure current to be able to set the bias, or really you may be better off with a meter so you can keep an eye on things.
2.  Bias doesn't self adjust for aging tubes.
3.  There are additional ways that the amp can self destruct.
4.  Grid leak resistance can't be as high as it could be in cathode bias.

Cathode bias advantages:
1.  A cathode bias resistor and capacitor don't cost much.
2.  The circuit will self adjust as the tube ages.
3.  The circuit will limit current draw in the event of a catastrophic short.
4.  Grid leak resistance can be higher.

Cathode bias disadvantages:
1.  Bias resistors get hot.
2.  Extra B+ voltage is necessary for biasing.
3.  Poor performance in class AB/B circuits (not really applicable here).

 

[Deke609]

Wow! Many thanks PB for the very thorough explanation.

I think I understand everything except: additional ways to self-destruct (but my take away is that the build is riskier, particularly for a novice like me who sometimes has things go poof) and the benefits of higher grid leak resistance. But I've previously read about the latter and can look that up.

thanks again, Derek

 

[EricS]

I appreciate the clear and concise list of pros and cons of these approaches, Paul!