6550/KT88 SET amp aka "Uber Beam Machine"

slbender · « **Reply #360 on:** 18 Oct 2008, 10:46 am »

Hi Josh,

I think I posted previously in this thread most of the details, still, another level of complexity has been incorporated since then.

This 2A3 equivalent SET Amp now has a startup sequence. Upon power up, the tube filaments are DC powered by a switching power supply through three series connected current inrush limiters and power thermisters. So the 12 Volt filaments are started up slowly and get about 8 Volts in about ten seconds time, while the separate negative bias supply is fully powered on. A timer holds off the full filament voltage and the high voltage power supply. After 15 seconds nominal the relays kick in and the three thermisters are shorted, letting the fuil filament voltage flow, and the high voltage is sent out to the pair of capacitor banks. After another 2 seconds the cap banks ( one for each channel ) are charged and the high voltage is present on the plates.

At power on, the set slowly pulls power from 0 Watts to about 50 Watts during the first 12-15 seconds, then it hits around 175 Watts for a few seconds and slowly settles down to a nominal 105 Watts. The power supply has an IEC inlet with its own A.C. power line filtering unit. The IEC feeds both the switching power supply and the high voltage power supply, which incorporates a true AC line 1:1 isolation transformer, then fast high voltage diodes (like Hexfreds) and a pair of 1000 uF. caps in a classic voltage doubler.

Each channel then has 1,200 uF. @ 330 Volts in its cap bank. Each audio channel consists of a 5687 used as two voltage gain stages, and zero loop feedback in typical SET fashion. The voltage gain stages are coupled by a 2% polypropylene cap, which feeds another 2% polypropylene cap driving (3) 5687's in parallel (6 triodes per channel) for the output stage. There is a separate cathode resistor (small value) for each output triode, and bias voltage pots for each gain stage and a bias pot for each pair of triodes within each tube in the output stage, also some variable loading resistors. Thus it requires the triodes within each tube to be matched within 10%, or better, to prevent current hogging. Will see how well this works out over the long haul. I actually used all "used" 5687 tubes for the output stage so they would be highly "burned in" already.

Ten test points for measuring each of the bias voltages are provided near the front of the amp for ease of setting up. I suspect the value of the input cap is probably too small in value, resulting in the 24-27 Hz. -3dB point, making it somewhat larger should solve that. More tests to be run soon, along with a full distortion panel. That input cap consists of a 2% polypropylene cap bypassed by a Russian teflon cap, bypassed by a smaller polystyrene.

Operation is ~ 310 Volts @ ~ 8.5 to 9 mA on each of the 6 output triodes for a nominal 50mA quiescent current and its power into 8 ohms before clipping is right around 5.0 Watts. The Z Output calculates to: ~ 0.52 ohm; giving a Damping Factor of: 15.3, rather unlikely values for a no-global feedback amp!

I'd be interested in what the various other builders of KT-88 / 6550 / EL34 / 45 / 2A3 / 300B users calculate on other SET Amps for the equations: Z out and DF.
Post 'em!

Z out = Rp / (Ra / Rsec)

Df = Rsec / Z out

Rp = Tube Rp total
Ra = Z Primary winding
Rsec = Z Secondary winding

-Steven

Quote from: JoshK on 18 Oct 2008, 03:21 am

Cool Steve,

Are you going to give us any details? Like what kind of amp is it, other than tube?

I started the shellacing of my chassis. Here's hoping it turns out.

JoshK · « **Reply #361 on:** 19 Oct 2008, 12:38 am »

Well so far so good. I am pleasantly surprised by how nice the results look after the first couple coats of shellac. I put on another couple today, now I am going to wait a few days as stated and rub it out. I plan to use the renaissance wax that I used on the copper top-plate.

The internet rocks! It can make a woodworker out of even me.

Steve,

I'd calculate the Zout and/or DF of the e-linear, but it isn't going to be as straight forward. The reason is that the amp is UL output and uses plate to plate feedback, which lowers Zout.

Niteshade · « **Reply #362 on:** 19 Oct 2008, 08:59 am »

Does anybody have a single ended KT-88 amp finished? I'm really curious about their sound relative to the highly touted 300B.

DaveC113 · « **Reply #363 on:** 19 Oct 2008, 03:30 pm »

Quote from: Niteshade on 19 Oct 2008, 08:59 am

Does anybody have a single ended KT-88 amp finished? I'm really curious about their sound relative to the highly touted 300B.

Yes... and if you send me a 300B amp, I'd be happy to let you know

Dave

jeffdavison · « **Reply #364 on:** 20 Oct 2008, 03:07 pm »

ditto.

All I can say is this has been the best sounding amp I've experienced...

in about 2 months I'll be, hopefully, finishing up a 2A3 amp....keep tuned

JD

Quote from: DaveC113 on 19 Oct 2008, 03:30 pm

Quote from: Niteshade on 19 Oct 2008, 08:59 am
Does anybody have a single ended KT-88 amp finished? I'm really curious about their sound relative to the highly touted 300B.

Yes... and if you send me a 300B amp, I'd be happy to let you know

Dave

DaveC113 · « **Reply #365 on:** 23 Oct 2008, 03:27 am »

I'm trying some re-issue Tung Sol EL34Bs in the amp. Very nice.... compared to the re-issue TS 6550s, there isn't as much bass, I had to raise the x-over freq. on my sub and turn up the level a bit. The mids are nice, and seem to be a bit more detailed. Highs aren't as pronounced, but aren't rolled off either. All in all they bring out the midrange more. I'll have to wait for them to burn in a bit longer, but so far I don't see them replacing the 6550s. They will be nice to have around for a change and to have backup tubes around.

I'm also thinking of converting my cfb switch to a el34 / 6550 or kt88 switch, right now I'm running a 560r cathode resistor... I could optimize the operating point for each tube type by switching a resistor in or out.

jeffdavison · « **Reply #366 on:** 17 Nov 2008, 05:59 am »

bump!!!

What's happening crew????

I've built my 2nd and now I'm doing up a 2 stage 6C45p/300B (actually a WE437a/320B-XLS) DRD using a stacked power supply.

Haven't seen any activity here so I thought to bumpererup to see waz shakin

JD

JoshK · « **Reply #367 on:** 17 Nov 2008, 02:34 pm »

Alls quiet on the western front here... Actually that isn't true, its been too busy to get much done. Case is done and was putting everything together when life had other plans for my time. Roof sprung a leak, college friend came to visit, big wedding to go to, etc, etc. Hopefully it stay quiet for a bit, before I am off to Mexico on vacation, so I can finish up.

Your plan sound really cool. You actually sourced WE437a? 320BXL is EML? What iron?

JoshK · « **Reply #368 on:** 19 Nov 2008, 03:24 pm »

I was curious as to who all was actually building amps. I know Jeff and Dave are done with at least one each. Steve is working on his creation (almost done?). I am making slow progress. Anyone else I've missed?

Did we lose Dr. Dan a long ways back?

JoshK · « **Reply #369 on:** 23 Nov 2008, 08:31 pm »

Made some progress on the e-linear amp.

Not finished wiring up the underside, but got 40-50% done yesterday. The cap clamps still need to get painted (black). And I got a couple fastener holes to true up. But I couldn't help taken a peak at what she'll look like.

JoshK · « **Reply #370 on:** 26 Nov 2008, 03:54 pm »

Steven,

Have you considered 6S4A's instead of 5687's? Similar specs, single triodes instead of double, which might be advantageous if trying to match, and pretty cheap to boot. More sockets though.

JoshK · « **Reply #371 on:** 26 Nov 2008, 03:58 pm »

P.S. my wife convinced me to make the cap clamps in cherry finished like the case instead of painting the maple ones black. She also was wondering if I was going to paint the champagne OPT and chokes black.

That's what I get for showing her pictures of Josh Stippich's stuff.

Actually, I had her looking at Josh's stuff for half hour last night. She is into metal arts and was trying to see if she could learn about his processes. She had mentioned a few times working with me to do artsy tube amp designs.

slbender · « **Reply #372 on:** 27 Nov 2008, 03:03 am »

Hi Josh -

Your amp is looking SUPERB! Bitch'in Great... Mine looks almost cheesy in comparison. NO Wood, sigh. Maybe I'll have my friend make up faux wood sides for it at some point. I'm rather space limited due to the small chassis's I've been using, so no extra room for many more sockets, unless I can pull a Dr. Who in space, in 3 dimensional space that is

.

My All 5687 Amp has traveled to two other systems recently and on one it was the highlight of the evening, on the other it was so-so due to a rather large room, and an inefficient speaker - that was also a difficult load. The owner uses a 250wpc amp on it, so 5-6 Watts was sucking on complex material, but it did sound extra nice on less stressful stuff and even a drop bass heavy, as I increased the input cap from .096 uF to .566 uF just before packing it up for the trip.

The next amp (currently under construction) will be a curious horse of a different color, as in Russki Red. I hope this next project will provide 12.5 Watts per channel (if it doesn't blow up that is). It could even be 18 wpc. if I can score a higher power transformer than the one I already have mounted (but not yet soldered in place). Again assuming it doesn't simply blow up when actually driven... no I didn't prototype this, it just came to me a few weeks back, and I'm gonna go-with-the-flow of ideas as I build it. I'll give you a hint, it does contain a pair of 6550c's. So its a kind of Uber Beam Svet Machina. If it doesn't work I can always start chopping and throw a quad of old 1625's in there, just got some big white porcelain 7 pin sockets and plate caps, so will see; and if it does work, then the next-next amp will have the 1625's and plate caps.

Fireworks? Stay Tuned.

-Steven

Quote from: JoshK on 26 Nov 2008, 03:54 pm

Steven,

Have you considered 6S4A's instead of 5687's? Similar specs, single triodes instead of double, which might be advantageous if trying to match, and pretty cheap to boot. More sockets though.

JoshK · « **Reply #373 on:** 28 Nov 2008, 07:55 pm »

I had written a detailed response and then lost it somehow. That hasn't happened in a while.

1625's look cool. I have a lot of projects slated and have a lot of the iron already. Couple years worth likely. Speakers are the next on the list though.

jeffdavison · « **Reply #374 on:** 16 Dec 2008, 05:26 pm »

THIS THREAD SHOULD NEVER GET TO THE SECOND PAGE!!!!

BUMP!!!!

JoshK · « **Reply #375 on:** 16 Dec 2008, 07:02 pm »

Well, I could sticky it, but it seems a bit self serving since I have a lot of interest in this topic. Instead let's just discuss more.

I was on vacation for the last 10 days, in Mexico, so basically I have made no substantial progress. I did finish Morgan Jone's Valve Amplifiers on the plane.

Last night, I polished up my cap clamp. I decided against using the black clamps and made a really attractive single "clamp" out of a piece of the cherry, so it matches the chassis. I'll try to take some detailed pictures to show how I did it since it may inspire someone else to think a bit differently about how to mount the motor run caps.

Basically, the clamps are a block of wood with 2" holes bored out with a forstner bit. Then there is a 2 1/8" countersunk hole 1/8" deep bored with a forstner bit too. (Actually the order of boring is reversed to make it easy.) This allows the motor runs to slide tightly into the hole and the lip to also be covered. The wood then can be bolted to the chassis holding the cap to the top plate if the holes in the plate are only large enough for the tabs to fit through. This result is much more attractive then cap clamps.

jeffdavison · « **Reply #376 on:** 24 Dec 2008, 05:27 am »

well, not really 6550 / KT88 anymore.... but here's an update of the latest work in progress that I posted about earlier. These are far from being done. All the wiring and wood finishing left, But I got real excited yesterday when the top plates came in from F.P.E. that I had to mock it up. DRD circuit. 320B-XLS driven by a WE437a using stacked power supplies. Hopefully, these will make my 6550 Uber ap sound like a cheap 30 year old Japanese transidtor radio...

JD

JoshK · « **Reply #377 on:** 24 Dec 2008, 03:27 pm »

Your fast! I saw some of your questions on tube diy asylum, so I knew you were building this. What are the other two tubes? One is a rectifier, but the other? Dual rectifiers for the stacked supplies?

Looks really sweet!

jeffdavison · « **Reply #378 on:** 24 Dec 2008, 06:07 pm »

Yep, once I get a design concept finalized, I start accumulating the parts. I usually decide on a footprint early on and start builing the structure. As the parts arrive one by one I take measurements and start the top plate design. Tho I take measurments of spec sheets to get me started, actual parts are used to adjust things and to make sure everything is where they need to be.

Yep, 1 output tube, 1 driver, and 2 rectifiers (one each for each of the stacked supplies). A stacked supply is a full power supply where the groun of one is the B+ of the other. Works very well in a DRD.

Here's the lates schematic (subject to more tweaking)

JD

JoshK · « **Reply #379 on:** 24 Dec 2008, 06:56 pm »

Love the use of stacked supplies in DRD. I sketched this same idea the other day. I like how it allows fixed bias on the output tube (no cathode resistors).