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Old 22nd January 2012, 02:39 AM   #11
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Default LabRat Update at 25W

(a) Hossanahs, however you spell that. Found my three missing Simple SE boards.

(b) I converted the aforementioned LabRat Simple SE board into a 6AV5 tester; see photos below (sorry, the 1st one is inverted)

Recall that the hard-wired 6AV5 "Clam" had a PST of uncertain parentage, SS rectification, 6SL7GT(W?) driver with CCS on plate & battery bias. It was triode strapped with cathode feedback. Plate voltage was limited to ~370V (I suspect I used diodes to eke-out more voltage from the PST), so I wanted to hear higher B+ and the original 12AT7 driver.

I set-up the LabRat in Ultra-Linear with Cathode Feedback. Haven't experimented yet as I am still mesmerized by the sound, and trying all teh tricky tracks I can think-of. Grateful Dead "American Beauty" on there now.
Never heard the bass fiddle on this track with an SE amp before.

So, here's the formula to get 80mA and ~315Vp out of the stock Simple SE board, which is the "Sweet Spot" for the 6AV5 tube, according to George:
  • Allied 6K7 PST (375-0-375 150 mA)
    [Can still lay hands on the PST; 150mA must be a little conservative.]
  • Std. P/S caps
  • Triad Choke (6H, 150 ohms DCT, 200mA)
  • 5U4GB Rectifier
  • 750 ohm 5W bias resistors (these are properly rated, but get quite hot)

WITH 118 VAC, this combination gives 375-380 volts at the plate, ~61V bias for about 81mA Ipk, and about 25.75W dissipation.

If that's a little rich for your blood, a 5R4GYB rectifier gives ~360V at the plate, ~56V and ~77mA bias, for about 23W dissipation.

Just checked the plates with the lights out (including the computer monitor); at 25-3/4W the plates on both tubes LOOK like they are incandescing VERY VERY slightly. but it's very difficult to see, as it is several "decibels" (or whatever the equivalent log measurement of lumens is) below the glow of the filaments. As it was, I had my nose WAY too close to 400VDC, so that's that. I admit I could be seeing things; in any case, no strong glow from plate or grid from the Sylvanias. YRMV.

I suppose that there are some small sonic differences between the two amps due to the drivers and the battery bias on the 6SL7, but whatever those differences are, it seems like they are very small compared to operating the 6AV5 at ~25W dissipation. So, the preferred 6AV5 recipe DOES appear to be 315Vp, ~80mA, ~25W; just season to taste with pentode w/ or w/o ultra linear, triode strapped, and w/ or w/o cathode feedback.

Note that I generally strongly dislike "global feedback" ala most commercial pentode PP amps; I can never seem to get it to sound "natural". Maybe my mistake was starting with DHT SE amps. In any case, George's cathode feedback is much nicer; like a little cayenne in the shramp etouffe.

I posted the Lab Rat photos to show how I connected the outboard octals for the 6AV5s. So far the long leads haven't caused any trouble, but they're in a shop with lots of 3HP 220 motors, and the amp sits ~3 ft from the breaker box, so I'll keep an eye out.

I used George's TubeLab Euro terminal strip idea with the little white label that fits over the terminals; the Simple SE 6L6 pin number is left and the 6AV5GA pin number is on the right. In addition to the 6AV5s, I want to hear the 6BG6s, which have yet ANOTHER pin-out scheme (and plate top-caps!).

After many hours playing in the shop (to be sure of the 6AV5s) I'll move this PCB to a proper enclosure and hard-wire the outboard sockets and likely make it the dedicated shop amp.

To replace the LabRat I'm gonna build a dedicated test board with all components on the TOP ala George, with screw terminals for all bias components, and with outboard sockets for both driver and power tubes. This way I can tinker with different power tubes AND drivers (e.g. 417A, 6AB4, octals) and biasing schemes.

One of my issues is that I have a (cheap) mono-only LP system in our "music room"; I'm recreating my Phono Dude preamp in mono, and searching for the best MONO power amp for my El Cheapo Baffle Speaker (don't laugh, I said cheap). I'm piecing together a 300B monoblock ala George's original Simple SE (powered by a Heathkit regulated P/S) but all of a sudden a 6AV5 monoblock might be a contender (at least for the R&R stuff) for its BASS and its CHEAPNESS. Besides, it could use the same P/S as a 300B (!).
Attached Images
File Type: jpg LabRat Running 6AV5s.jpg.JPG (633.8 KB, 737 views)
File Type: jpg Off-Board Octal Sockets for 6AV5s.JPG (642.9 KB, 636 views)
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Old 24th January 2012, 03:10 AM   #12
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Default Final Update: Simple SE with 6AV5GAs

I switched PSTs as the 150mA Allied 6K7VG was just getting too hot during "cranking"; using a massive Stancor I found in the stash (records say $10 from eBay about 6 years ago, but shipping was more).

The Stancor yields slightly lower B+ so it's now running about ~23W dissipation.
I think I like it better here; seems like better detail, but still plenty of low end and power (for an SE). Currently listening to Jazz @ Pawnshop, the DHT SE standard bragging record. Not quite as nice as a 45, better than any 2A3 I've heard, likely not as swell as a good 300B amp. But then I bought all 50 of the 6AV5s that I have for less than ONE good 300B.

I'll stick with this configuration and see what happens over the longer term.

Specs, front to back:

375-0-375 ~200mA PST (that's conservative; it's stone cold)
12AT7 in std Simple SE trim, no Battery Bias or similar tricks
0.22uF Russian PIO coupling caps, ~$1 each in quantity a few years back
Sylvania 6AV5GAs biased ~73mA with 800 ohm resistors
Cheapo 15-8-5K Edcor OPTs
Rat Shack "full range" 8" speakers with tiny Foster horns in junkyard open-back cabs (like your Dad's Motorola Console, circa 1961, only ugly)

I'll start a new post if anything develops over the long term (e.g. melted tubes).

Long story short: the 6AV5GAs are magic in combination with the cheapo Edcors, just like George said. They likely aren't BETTER than a real single plate DHT properly done, but I think they'll hold their own against ANY 6L6/6V6/EL34 I've tried in the Simple SE, and I personally like the detail and "punch" much more than any of the half-dozen 2A3 SEs I've built.

The closest non-DHT SE amp I've built was a Direct-Coupled Double (parallel) Darling with 1626s, but the 6AV5 blows the Darlings away for power, staging and snappy transients.

Possibly the biggest bang for the buck since my 1957 Ford F100 ($75 in 1972). Unlike the Ford, I ain't selling this one.
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File Type: jpg photo.JPG (661.3 KB, 627 views)
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Old 24th January 2012, 02:00 PM   #13
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I think they'll hold their own against ANY 6L6/6V6/EL34 I've tried in the Simple SE
Another dark horse is the 6EZ5. I like it better than a 6V6, but it isn't up to a good 6AV5. Lower power and needs a lower B+ voltage. Again tested with the little Edcors.

The 6EZ5 was on everybody's dollar list for over a year, then zap they were all gone. Somebody somewhere likes them......and has them all.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 5th February 2012, 12:46 AM   #14
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Default REALLY the Last Update. Well, Until I Get New Edcors

The original diodes on teh RatLab board finally gave out (shorted) and flashed the JJ 5U4G. I cut the power and lopped the diodes off the board, and restarted with the JJ. There are pyrotechnics inside the restifier on start-up, but so far it's working. I have a handful of those GE C-90 soft-start thermistors; if I remember, maybe I'll install 1,2, or 3.

In any case, been using the 6AV5 LabRat as the shop amp for a couple of weeks. Still spectacular. Hendrix, ANY Hendrix, gets cranked to 11. Best R&R SE amp I've build yet.

In order to test the role of the cheapie Edcors, I ordered
(a) a pair of GXSE 15-8-5K OPTs ($38 each) and
(b) a pair of CXSE 25-8-5K OPTs ($84 each)

I'll try BOTH pairs on the 6AV5 LabRat and see what difference there is.
I'll likely NOT change (can't imagine that any iimprovement would be worth losing the bragging rights about how cheap this amp is now) and use the GXSEs on a Simple SE we use for the TV, and the CSXEs for SE amps.

MEANWHILE, I rebuild a decrepit Heathkit PS-3 power supply using 6AV5s as pass tubes. So far works great; as expected the 6AV5s don't seem to mind 80 ma straight across continuously. I'll make a separate post with photos.
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Old 23rd December 2012, 02:40 AM   #15
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These army type 6B4G look a lot like 6AV5, looks like these are not urban legend...


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Old 23rd December 2012, 03:08 AM   #16
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Kind of amusing when you can get NOS 6AV5's for $5 or less.

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Old 25th December 2012, 01:11 PM   #17
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looks like these are not urban legend...
I had heard about the legend when I decided to test the 6AV5. Several years ago I found one of the tubes like the one in the Ebay auction. It had a milky white getter so I busted it open to study the guts. Looked just like a Sylvania 6AV5GA to me.

These tubes used to show up from time to time and can be priced anywhere from $5 to $25 each.

The best "6AV5" type is the 6FW5. It was only made by GE and had all their best stuff inside. There were too many different things in the wild labeled "6AV5" and some would just melt in a larger TV, so the 6FW5 was created. You can find 6AV5's with 12, 15 and 17 watt plates. All 6FW5's have 17 watt plates. Unfortunately they aren't too common.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 30th December 2012, 06:20 AM   #18
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Agree, 6AV5s are a bit variable, using 6FW5 in SE (variation on the TSE design with damper rectification and triode-strapped 6AG5 as driver, UBT-3 o/p transformer), the 6FW5 is fairly predictable with ~320 volts across tube at 55-60 mA idle current. Sounds and measures good too, -3dB points at 1 watt are 12Hz and 54kHz respectively, plays well on mk1 Klipsch Heresys.
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Old 30th December 2012, 07:49 AM   #19
cotdt is offline cotdt  United States
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Try going all out and build the TSE with 6AV5's and with high end iron. I'm very curious about that.
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Old 30th December 2012, 07:54 AM   #20
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Sounds interesting, what iron will you be using?
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