Paralleled SE, cathode biased questions.

[alvarezh]

Have a friend who loves his single ended cathode biased Carr Mercury. Unfortunately he cannot gig with it due to it's limited power. He has asked me to build him something similar but with enough power for band use.

The Carr he's playing is using a 5881 with (measured) 410 V. on the plates and biased at 50 mA.

I am thinking a pair of paralleled 6550 or KT88s (single ended, cathode biased) running at 500 volts and biased between 65 and 70% dissipation.

Can someone please help me calculate approximate output power?

Also, can a single 12AX7 (12AT7 preferable?) be good enough to drive them? The PDF gives an schematic example (single tube) taken from XA84.com.

Hammond has the transformers, no problem there.

Thank you in advance.

[Phil_S]

Single ended is notoriously inefficient. It is maybe 25% efficient. I'd guess you can get 20W per tube give or take. As I understand it, cathode bias is self limiting (why they also call it self-bias) and the tubes should be biased at idle close to 100%. So, I'm guessing a pair might get you to the 40-50W range depending on what you do with it.

For more volume, look to pair it with an efficient speaker?

I'm not quite sure how to calc this. I don't know if the data sheets give curves for SE use. I'll look later if I have the chance.

[martin manning]

I don't know Horacio, if your guy really likes the sound of the Carr, maybe it would make sense to do a line-out from that and run it into a clean power amp?

[JazzGuitarGimp]

I don't know Horacio, if your guy really likes the sound of the Carr, maybe it would make sense to do a line-out from that and run it into a clean power amp?

Or just get a Badcat Unleash!

SE designs get much closer to 100% idle dissipation than their PP cousins. If you haven't already seen this, here is an excellent, short and concise lesson in SE stage design: http://www.valvewizard.co.uk/se.html It's well worth the read, and Merlin doesn't overdo it in the math department. By comparison, I've _tried_ reading some of the theory in the Radiotron Designer's Handbook, and the math leaves my head spinning in pain. Merlin's approach is a walk in the park by comparison. In his example, he is biasing an EL34 at 95%.

HTH,
Lou

[ToneMerc]

Horacio, I've experimented a little with parallel SE circuits and I agree with all the above. I'm not sure if your will gain that much more in perceived loudness over a single tube, I think you will find that you will need a way huge chunk of output iron and you may lose that vibe that's the whole reason why he enjoys that amp so much.

TM

[TUBEDUDE]

I built a SE kt-88 some years ago using a one electron UBT-1 output xfmr. It made 16 watts with a wide power bandwidth. Single ended I don't think much more can reliably be had.

[alvarezh]

Gentlemen, grateful for your assistance.

Let me begin with Lou:
Thank you for the link, now I understand better on what to do.

@Martin:
Wish he would follow your suggested route, I am afraid he would want the sound of his amp's speaker. He could mike it, but then he would not want to carry the extra gear.

@Phil:
Yes I understand the theory and the reasoning for using PPs pairs on higher power amps (more than 16 watts), but the sound of the second harmonics would be missed, I must use a single ended topology. This guy IS picky! But anything above 20 watts is good to go. I am already thinking on using around 450 volts (after reading the Valve Wizard's document). If the sound is not satisfactory, then I can at least use the PT for a pair of 6L6s (PPs) around 40 to 50 watts (4x6V6s also). I will have to eBay the OT though.

@TUBEDUDE:
This will be two paralleled KT88s, should be a little more powerful.

@ T.M.
Thanks, I know you are an experimenter, the bigger OT together with the KT88s will change the tome somewhat, it will not sound 100% the same (BTW, didn't know the transformer would make a difference). But I have no choice, it has to be single ended, cathode biased and 20+ watts.

I have a used chassis from a previous project, I am going to take the chance. I have all the parts minus the trannies. I am going to go for it. I will report back and we could all learn from this.

I have finished reverse engineering the Carr minus the power scaling (complicated circuit), for there is no need for it, he already has the Mercury which he can use for lower volumes. No reverb either, if it works, then the final amp will carry it.

On the Carr, V1B (the input side) is 150k/1.5K/22mf, V1A is 100K/2.7K/22mF. Coupling cap out of V1A is .003mF (waoo! talking about restricting bass.) V2 and V3 are the reverb duo. 6L6 presently the octal.

The tone stack is pretty much a copy of a Champ with a 6.8K resistor for mids and a very awkward 150K feeding the volume pot (high side) from the treble wiper. Additionally, there is a 47pF cap from the treble wiper to the volume wiper, weird stuff. I am going to change the tone-stack cap values and install a mid pot in leu of the 6.8K resistor, he's complaining he doesn't have enough and no control over the mids. I will use the tone-stack calculator and try to "skyline it" without going full on Dumble's take. A mid boost circuit could be useful too. He would love it!

In the end (hopefully), I think that a stronger mid presence and a little more power should better serve his needs.

Ah, one thing I found out about the Carr, it's using a Celestion Red, White and Blues. No wonder that little bugger is so loud, the speaker has a 101 dB sensitivity, nice trick! He will get that same speaker for sure!

One last thing, you guys think I should have another (cathode driven, maybe) stage to drive the KT88s. Obviously, none is needed with a single tube. Do I need more driving power with a pair?

Again, thank you all very much.

[Phil_S]

Have you thought about using a single KT-120? The data sheet say 20W in Class A, but I'll bet you can make 30W from it without making it break a sweat. In PP, this is a 60W tube. There are some good reviews on this tube. I have no direct experience with it.

Even at 20W it should be considerably louder than what he's got in the Carr.

[M Fowler]

I have a pair of KT120 tubes and they are great sounding and I've tried them in a large variety of PP amps but not in any of my SE amps.

A guy I know in Minneapolis-St. Paul area put a set in his Little Walter 50 to get more volume and really likes it. I'm sure he also went with SS recto to get the plate voltage up.

[Firestorm]

It seems like the theoretical max power out for a KT120 in SE should be 30W. A pair of KT88s, about 40; two 6550s just under that. But in all cases the challenge will be to find an OT that is rated for the standing current for those configurations at idle. The Hammond SE OTs that can do it are "HiFi," heavy, expensive and probably sound, well, too HiFi. You might be able to parallel a couple of OT's from the 125SE series. To realize maximum output, you have to get the plate voltage, primary impedance and drive voltage just right.

If you want to boost mids, you have to scrap the Fender tone stack and go with something like the James. Because its insertion loss is so high (especially when set for mid boost), you may need to add gain in the preamp.

[JazzGuitarGimp]

Horacio, I am looking very forward to following your progress! This one is interesting, and I've got a pair of KT88's on the shelf that I took in trade for a repair job.....been wondering what to do with'em....

Lou

[tubeswell]

Calculate the reflected load for centre bias Class A operation for one tube using the idle voltage and the maximum plate dissipation,

Zout = Va/(Pa/Va)
Where:
Va = Anode voltage.
Pa = Maximum anode dissipation.

and then halve that for the parallel tubes.

Then use cathode biasing on the KT88s to get them idling at 100%

The OT (and any whole-of-supply HT choke) will need to be DC-rated to handle about 1.2x to 1.3x the total idle current of both tubes. 1.5x to be 'safe'. (While one might think the DC draw gets up to 2 x the idle current for SE Class A, they don't get anywhere near it usually)

[alvarezh]

Great fresh ideas gentlemen, most thankful to all.

I have to investigate on the K120 (Thanks Phil, I missed that!). Last night I was looking at a single 6146B and the 813s. The 6146B its only a little bit more powerful than a KT88, and the 813 is way too much.

Interesting point on the James tone-stack Firestorm, will be looking into that tonight also. On the subject of the OT, well Fire, life is a compromise that we are all forced to live with. Not even Warren Buffet can have a different colored private B747 for each flight he takes. Thank you for the tip though. Once the prototype is done I will try to voice as close as possible to the Carr. Nevertheless it will not end up a being a Carr, but it will be single ended, cathode biased, and will have double the power and more notable mids. The owner (a friend) called Carr offering to buy a bigger version, the answer was "I don't make it". Hell, I would of done it! charge the guy what ever is reasonable for a one off beast ($10,000.00 ? it just needs a bigger power section) but don't let the opportunity slip away, you never know how many more customers that may bring. An extra page in the website, and all the R&D paid by the first customer is a deal, no matter if he puts customers on a six month waiting list and charge $5,000.00 per specimen.

Mark? Fowler, thanks for the tip. My friend has sharp ears, but he's no Neil Young, at least not yet! I am heavily betting that the tone should come out close with either a pair KT88s or a KT120.

Lou, in case I go the KT120 route, let me advance this info: I have an original Sunn 200S amp (2XKT88s PPs) that I rebuilt a couple years ago. That thing sounds good, with nice round, warm and full notes. Compared it side by side with a Traynor 100 watt transistor bass amp. The Sunn had more presence, not to mention the difference in tone. And most important, the Traynor was struggling in a rock band, the Sunn was coming through nicely. Anyhow, this project should take time, I feel confident I have enough knowledge (particularly with the kind help from you guys) to come through with it. But all the amp building electronics I know is self taught, lots of reading, some experimentation, but not much (design) experience. I'm 62, clearly not as sharp as when I was 20, wiser maybe, but with the will to learn equal to a passionate young musician! Will post every significant advance, but this will take one to two month to build, the biggest drawback is falling on how fast I can obtain the transformers, I should have the rest of the parts here with me already.

Tubeswell, thank you kindly for the lesson, I am sure it will come handy, already copy/pasted to my computer (you can tell I don't rely on the internet, much less the new "cloud" computing).

Thanks again to all.

[martin manning]

The OT (and any whole-of-supply HT choke) will need to be DC-rated to handle about 1.2x to 1.3x the total idle current of both tubes. 1.5x to be 'safe'. (While one might think the DC draw gets up to 2 x the idle current for SE Class A, they don't get anywhere near it usually)

Hi Tubeswell, do you have a reference for this rule?

[tubeswell]

I recall figuring it out once upon a time - lets see if I can repeat the feat.

Say your plate is idling at 400V 70mA with a 2k5 load. The plate isn't going to go all the way to 0 on the downward swing because of the cathode voltage, which is roughly (say) 40V through 560R (to get your 70mA) at idle. So suppose theoretically for a moment that your maximum plate swing is not 800pp but instead 720pp, therefore even if you could get the plate swing down to 40V (which you won't because the tube is cathode-biased so the cathode voltage will rise as the tube current rises) the maximum hypothetical tube current is then 0.9 of 140mA, which is 126mA, of which (say) ~10mA? is screen current (assuming we've got good screen grid resistance to dissipate the excess), so you're looking at 116mA plate current at this supposed minimum plate voltage. But as I said, in reality you still never even get as low as 40V on the plate, because when the tube current goes up, the cathode voltage also rises, and if there was 116mA through 560R that would be 65V at the cathode, which means the plate is now not even swinging 720pp, but 670pp, so the plate current is 83% of the 140mA that the load line says it should be on the y axis. But then you factor in some inefficiencies in the pre-amp driver stage and in the OT and speaker, so you don't get 2k5 load resistance at every frequency and you can maybe figure that the plate swing is at best more like 600pp at full tilt, which is 3/4 of what it could be if the universe was perfectly linear, and its probably even less efficient than that, so I sad 1.2 x to 1.3 x. I probably did my maths all wrong and dropped some carry-overs when I shouldn't, but I'm keen to hear other people's views on it.