6550/KT88 SET amp aka "Uber Beam Machine"

Len_Dreyer · « **Reply #280 on:** 7 Jul 2008, 03:23 am »

Jim,

I didn't have any luck with google either. I've been following this thread and the group buy discussion piqued my interest. Right after that, I stumbled upon the Tube Vendor thread and started plowing through it. On the second time though, I found the rectifier. So I figured I let you guys know and I emailed Eddie the info. I also ordered one.

DaveC113 · « **Reply #281 on:** 7 Jul 2008, 04:55 pm »

Been listening to the amp for a little while now... I'm still very happy. I have mainly been using it in UL mode, but have switched it over to triode a few times and am really impressed by how it sounds this way. For some kinds of music I think its an improvement over UL. Its not quite as fast, sharp, and authoritative as UL, its more relaxed, but doesn't give up detail and still has a fairly sharp transient attack if the music calls for it. I think it sounds more natural with a lot of acoustic music, jazz, vocals, etc.. With rock, most electronica, etc... UL is probably the way to go... but music like Bela Fleck and Medeski Martin and Wood splits the difference and sounds great either way, and I mostly prefer triode mode for this kind of music.

I haven't tried cfb yet.... but judging from how good it sounds without it I doubt it will be an improvement.

nodiak · « **Reply #282 on:** 8 Jul 2008, 01:59 am »

That's good news Dave. I've found I am more a triode guy from amps I've had, but listen to alot of electronica so am glad I have UL option on the transcenders.
Don

JoshK · « **Reply #283 on:** 8 Jul 2008, 01:26 pm »

That is a quite often said feature of UL and pentode, that it is punchier. Purests say its a pentode affect, a coloration or distortion of sorts and prefer the purity of a triode. I have no opinion as I haven't heard the two compared in a controlled way. I however am more of a rock lover, but have a penchant for other styles as well, san jazz. Having options is a good thing.

Last night I putzed around with my dovetail jig some more. I think it is going up on ebay and I am going to order a porter cable jig instead.

jrebman · « **Reply #284 on:** 8 Jul 2008, 02:11 pm »

Josh,

The Porter Cable tool is decent, but also take a look at the Incra stuff that you use with your existing router table. It's probably a bit more flexible.

-- Jim

DaveC113 · « **Reply #285 on:** 8 Jul 2008, 03:26 pm »

Josh, I have a Keller jig. I've only used it for one project so far, but it was very easy to use and produced perfect results. I think its about 1/2 the price or less compared to PC style jigs.

I've seen Incra jig stuff, looks really good too.

http://www.amazon.com/Keller-Dovetail-System-135-1500-Journeyman/dp/B000022420/ref=tag_dpp_lp_edpp_ttl_ex

jrebman · « **Reply #286 on:** 8 Jul 2008, 03:44 pm »

Hot tube tip: I sold my PP KT-88/6550 amp to another guy recently, and he soon installed the cryoed TS 6550s in the amp and said they were a big improvement over the chinese KT-88s that came with the amp. This past weekend he dropped in a cryoed set of the Gold Lion reissues (from Cryoset) and said that took it up another couple of notches -- right out of the box and with no break-ink, that is.

So, if you've got a pair of these, don't be afraid to give them a try -- maybe better than the TS 6550s.

-- Jim

DaveC113 · « **Reply #287 on:** 8 Jul 2008, 03:59 pm »

Here's a nice 5ar4 for anyone looking

http://cgi.ebay.com/One-MULLARD-GZ34-5AR4-Metal-Base-Audio-Rectifier_W0QQitemZ120278982991QQihZ002QQcategoryZ73377QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

JoshK · « **Reply #288 on:** 8 Jul 2008, 04:40 pm »

Jim,
I have a pair of TS 6550's and I got that pair of Genalex KT88's you sold me, so I have plans to compare myself. I like the idea of having options.

I have to decide on what jig system I want. I guess I got to think it through some more. I just really don't like the craftsman. Almost all the templates use the 0.4" guide bushing. The bushing that comes with the jig doesn't fit any of my routers without me going out and buying a special base and no one seems to sell the standard brass guide bushings (which do fit) in 0.4". I went to sears and they sell only one set of guide bushings that includes a 0.4" one but it is too long for the 9/16" box template. Ultimately the system is far too frustrating to be worthwhile so up on ebay it goes.

I have been making incremental steps on my projects. Nothing to write about, but a little here and a little there on most of them in tandem. I'll drill out the chassis's of a few projects together, then I clean and primer them together, then paint together, etc. This seems to be the most efficient use of my here and there time.

jrebman · « **Reply #289 on:** 8 Jul 2008, 05:12 pm »

Dave,

Nice rectifier -- yet he doesn't even specify year or getter type, but at currently at over $500, well, not even something I can remotely consider. This one should last a couple lifetimes though

Josh, sounds like how I'm going about things -- a little here and a little there as I can. I was actually going to take the chassis over to the little shop here and drill the small holes for transformers, circuit board standoffs, RCAs, volume pot, etc, and then discovered that my nice titanium drill set has disappeared, so I'll have to replace that first. Then it will be easy to take the chassis over to the machine shop for the tube holes, holes for the motor run caps, and the IEC inlet or power entry module -- whichever I end up using. After that there will be a light at the end of the tunnel. Still no real predictions on when it's all going to come together.

-- Jim

JoshK · « **Reply #290 on:** 10 Jul 2008, 01:46 am »

I think i may have the patina for the oil-rubbed bronze look i am going for. you have to use 'liver of sulphur' and not just potassium sulfide. there must be something else in it that works a bit different. the copper has to be super super clean too. as in you must polish the surface with steel wool, sandpaper or other fine abrasive surface and then wash it with detergent, rinse well then apply the liver of sulphur immediately after. this gave me a much darker and non-flakey surface. i will wait till it dry fully and then hopefully i can brush it out for the look i am after, but i am hopeful based on the reaction i got with los.

p.s. sorry for the no caps, this keyboard's shift key is broke.

JoshK · « **Reply #291 on:** 10 Jul 2008, 01:37 pm »

For all of you looking to spend big bucks on NOS rectifiers, consider this. I know not many people here will listen to my opinion, but consider what Lynn Olson has to say about rectifiers. This is taken from his Amity/Karna/Aurora pages.

From page 1.

Quote

Moving on to the main B+ supply, I first tried a 5R4-GY rectifier (a traditional choice for 300B circuits), but was dismayed with the arc-overs and poor reliability in several examples. Maybe they were old and weak, but this sort of failure should never happen in the first place. By contrast, TV damper diodes, including the New-Old-Stock 6C*3 family and the new Svetlana 6D22S, have more-than-ample peak curves, and derating for continuous use gives more headroom in current and voltage than the traditional tube rectifiers seen in 2A3 or 300B amps. The low voltage drop (15V), huge peak currents (2A), and slow warm-ups (30 seconds) are just additional bonuses.

Matt Kamna also demonstrated a technique for zooming in on the waveform on the power-transformer secondary (about 10V/div on the scope screen). The rough appearance around the zero-crossing was very obvious with solid-state diodes. HEXFRED's gave a small improvement, but conventional tube rectifiers looked much smoother, and the TV damper diodes were by far the smoothest of all. So even in low-current preamp applications, TV damper diodes give the least noise. I know from experience in the Tektronix Spectrum Analyzer division that it's much easier to eliminate noise at the source than filter it afterward. If there was an even quieter device, I'd use that, but as far as I know, TV damper diodes are the quietest from the viewpoint of switching noise. Considering that the main B+ supply is switching five hundred volts, this is not a small consideration, since switch-noise is radiated in all directions, into the B+ supply, the interior of the chassis, and back into the power cord.

On the amp design philosophy point #5 he says this

Quote

In terms of ragged waveforms, solid-state diodes are the worst, followed by Schottky diodes and HEXFRED's, followed by conventional tube rectifiers, followed by TV damping diodes, which are the smoothest of all in terms of the AC waveform on the power trans secondary. This, along with 2 amp peak current, is why I use them. The 30 second warmup is just a bonus.

Links to the whole article...
Amity
Amp Design Philosophy

JoshK · « **Reply #292 on:** 10 Jul 2008, 01:43 pm »

Also note that damper diodes at present are typically only a few bucks each and readily available given that they were used in many TV sets.

1) Cheaper
2) Typically far more robust, much higher PIV (peak inverse voltage), higher voltage rating, higher current rating (typically) and were meant to not be replaced for the life of the TV set (long life).
3) lower switching noise
4) Slow turn on (soft start)
5) lower voltage drop

Most damper diodes are single diodes, rather than two diodes in a typical tube rectifier (remember, rectifier is just a term used to denote a tube with two diodes convenient put together for this use). This means you have to use two of them for most circuits. If you use leads off the pcb, it wouldn't be hard to retrofit George's pcb for damper diode use.

JoshK · « **Reply #293 on:** 10 Jul 2008, 01:51 pm »

Lastly before I get a million pm's....some examples of damper diodes

Svetlana 6D22S
6C*3 (6CJ3, 6CG3, etc)
6A*4 (6AU4, 6AX4, etc)

data...
http://www.nj7p.org/Tube4.php?tube=6AU4gt
http://www.nj7p.org/Tube4.php?tube=6Ax4gt
http://www.nj7p.org/Tube4.php?tube=6cj3
http://www.nj7p.org/Tube4.php?tube=6CH3

A quick scan of AES shows they sell for $3-5/ea, compare that with $150 for Mullard GZ34.

DaveC113 · « **Reply #294 on:** 10 Jul 2008, 04:39 pm »

Here's a post by kevinkr over at diyaudio on the subject:

http://www.diyaudio.com/forums/showthread.php?postid=1552097#post1552097

Since everything seems to be working fine, I think I'll leave things as-is and remember to let the amp cool for a while before I power it back up. I've been giving it about 10 min to cool....

The metal base 5ar4 I posted was just for fun... I find it hard to believe it sold for that price. While some of the most desireable tubes are really pricey, it looks like $75-$100 is the going price for a NOS re-branded Mullard thats still made in GB. Mullard branded ones in perfect shape are $150. Used ones that test high (better than any new tube) are selling frequently for about $50. Under $10 for diodes is still far cheaper though...

DaveC113 · « **Reply #295 on:** 11 Jul 2008, 04:55 am »

Josh, could you point me to some reading about distortion characteristics for UL vs Triode operation? UL does seem like some sort of positive feedback mechanism, so I am interested to know if distortion increases as a result... Also, the UL tap appears in wiring diagrams to be taken out of the primary windings about 1/2 way between the B+ and the power tube's output... what does this mean in terms of the signal being fed back to the tube's grid?

I did a volume check, the amp has plenty of power for the Omega XRS. I hit 103 dB peaks (dolby c fast weight, radio shack meter) at the listening position playing White Zombie. This is without a sub and the amp happened to be in triode mode. There was still power to spare, I just didn't want to destroy the drivers.

jrebman · « **Reply #296 on:** 11 Jul 2008, 03:30 pm »

Dave,

For lowest distortion you want to have a pretty high ratio of transformer primary impedance to plate impedance of the output tube -- i.e. higher load impedance than source impedance, and generally 3x seems to be a minium magic number.

When you triode strap a pentode you reduce the effective plate impedance by a factor of about 2.5 times over pentode mode, thus increasing the impedance ratio and lowering distortion and at the same time increasing damping factor. UL is a compromise that has lower distortion than than pentode but greater than triode strapped and better damping factor as well, so sounds snappier as it has these characteristics and almost as much power as pure pentode mode. Pure triode mode, while having lower distortion and better damping has considerably less power than pure pentode or UL modes.

Generally the magic number for a UL tap is somewhere between 40% and sometimes as much as 50%, with 43% being the most common sweet spot.

HTH,

Jim

jeffdavison · « **Reply #297 on:** 13 Jul 2008, 05:06 am »

Been reading on Georges web site (TubeLab) and his links to CFB..... I'm thinking of instead of just switching in and out of feedback from the 8 ohm tap, I'd put a 3 way switch to have the option of feddback from the 8 ohm, the 4 ohm, and no feedback.... For reason he says are valid, that I couldn't find reference to anywhere else, is that George reverses the polarity of the outputs of the OPT's,, that is he wires common as 8 , and 8 as common. He says different transformers may or may not be wound in opposite directions. He does reference other OPT's ( http://www.tubelab.com/Output_Transformers.htm ) but doesn't elaborate further about the reverse output polarity... or did I miss something?
My OPTS (Silk S-325's http://www.sacthailand.com/Transformer_SilkS-325.html ) have 4 and 8 outs.

JD

Quote from: DaveC113 on 7 Jul 2008, 04:55 pm

I haven't tried cfb yet.... but judging from how good it sounds without it I doubt it will be an improvement.

JoshK · « **Reply #298 on:** 22 Jul 2008, 06:35 pm »

My top plate so far...

nodiak · « **Reply #299 on:** 22 Jul 2008, 08:53 pm »

Hey Josh, I like that kind of thing. Leads to some need to choosing wood that doesn't clash imo, gets more interesting this way. Are you thinking clear coat, satin or gloss or ? Wll be interesting to see how that color could accentuate tube glow. Inspired here. I have some silver caste figures had thought of setting on transformers and a turtle figure may work on a volume knob.
Don