New Year's 6AV5GA SET project

the thermionic watercooler

New Year's 6AV5GA SET project

Postby EWBrown » Fri Jan 07, 2011 9:31 pm

No better way to kick off a new year! :))

Quick before someone pipes up and says Image,
I have real live pictures :))

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The basic circuit of the amplifier is similar to SorenJ's 6AV5GA amp from a couple years ago - I'll append his schematic here:

Image

There are some minor circuit changes, I used 270K for the grid resistors, 1K grid stoppers, 470 ohms, 1W for the 6AV5GA screen grid to plate connection, and 0.22 uF interstage couplers, and there is no input volume control. The OPTs are 3K primary impedance, 10W rating (Transcendars from an e-bay purchase of secveral years past).

The drivers use 12AT7s instead of 5965s, and the 6AV5 GA plate voltage is about 30-40VDC higher, and the RK is 750 ohms, 25W Dale "heatsink" resistors, mounted on the chassis, and using some silicon heatsink "grease". The power supply is a bit simpler than was Soren's, with basic CLCRC filtering.

Rather than compose a whole new write-up, I just copied and editied my posting of a week ago under the "my low watts amps" topic. Why re-invent the wheel, (???) :/

So this contains a commentary of my original 6AV5GA- modified GSG project, and its evolutionary stages, which all served as a test bed for this amp.

I decided to stay with 6AV5GAs, damn the torpedoes, ful speed ahead (666)

The Chassis is the "standard" Hammond steel, black powder-coated, 12 X 10 X 2 inch, which is one of my favorites. There is plenty of room under the hood for neat, clean and uncrowded point to point wiring.

I wired up the Power supply and output stages, using the Transcendar 10W, 3K SE OPTs, and 750 ohm 25W "heatsink" type dale cathode resistors.

Power trannie is a horizontal-mount Hammond 290CX, and a 5H, 200 mA choke of unknown parentage, which I've had in the parts collection for several years. Caps are two 180 uF / 500V, and two 47 uF / 450V, rectifiers are the usual UF4007s, and I used a CL90 inrush current limiter on the AC primary side.

I used the 5VAC filament winding to "buck" the 115VAC rated primary voltage, in order to have more "appropriate" B+ and filament voltages.
This is all in the wiring located just to the left of the fuse holder. The thinner long yellow wire serves as the return for the front switch's internal green neon pilot light.

Yeah, you're not seeing double - there are two AC power switches, one on the rear, and a fancy illuminated one up front, which was a last-minute consideration. I was originally going to mount a 20 mm dia pilot light on the front, and then I found a nice illuminated power switch, so I used that, instead. The two AC power switches are connected in series.

I generally avoid using horizontal mount iron, as the nibbler tool "workout" is a real wrist-buster, but I do like the end results, I may go with this method more often (as I have a quantity of vintage, used and new horiz mount power and output trannies.

With 400V B+, the test 6AV5GAs were dissipating about 25 Watts combined plate and G2. Even though these tubes can take a lickin' and keep on tickin' I decided to throttle things back a bit, added an extra C (47 uF / 450V) and R (333 ohms, 30W, three 1K, 10W WW resistors in parallel), before the choke.. These are "hiding" behind the aluminum angle bracket.

That dropped the B+ to 365VDC, before the choke, and 350VDC to the OPT B+ connections.. Instead of 75 to 80 mA plate current, I now get 66 - 68 mA, and a V P-K of about 280V so I'm down to around 18-19W PD, which is closer to reality. Before I throtled it back, I ran tests on about three dozen 6AV5GAs and culled out the four weakest ones, and one which oscillated uncontrollably.

I even subjected half a dozen 6AV5GTs to this treatment, and they didn't blow up.either. I have some 6AU5s, but I figured they would detonate in short order, they kinda whimpy, in comparison... :'(

I made up another SS 6AX5GT replacement, this time with 50 ohms in series with each uF4007 ,and I added another diode, LED and 680 ohm resistor across 2 and 7 to get a red "filament glow". I tried that out in the "blue plate special" GSG and get the same B+ voltages, though with a slightly lower cathode current, because of its approx 800 ohm vs. 750 ohm RK's.

With this combination, it is noticeably more powerful, better bass, and clear highs, I have run it this way for a few hours, with no ill effects on the tubes, and the PA774 just gets slightly warm. It now has 1.2A less loading on one of the two filament windings, so that saves about 7.5 VA's in the process. It definitely has a much better "Ka-Whump" factor, now (666) O:)

Driver stages are SRPP 12AT7s, running around 3.3 mA, with RK+ RA = 470 ohms. and about 305VDC "top" plate voltage.

With all of the heavy iron on the top surface, I decided that it needed some under-the chassis bracing, so I worked up a piece of one-inch aluminum angle bracket to stiffen the chassis top, and to serve as the front mounting points for the power and output trannies. It did take a bit of creative cutting and trimmingh to fit.

The yellow 6.3VAC filament wiring is pre-twisted pair 22 ga teflon insulated (I have a big roll of this stuff) this may seem a light gauge wire for this purpose, but it also serves to trim off a couple of tenths of excess voltage from the 6.5VAC from the power trannie.

FWIW, the 290CX, which was intended as a guitar amp replacement transformer, is just about perfect for an ST-35 build, the voltages and currents are right, and it also has a 5VAC filament winding for those who like to use tube rectification - a 5AR4/GZ34 would be the best choice, and the tube rectifier voltage drop would deliver the correct ST35 B+ voltage..

/ed B in NC
Last edited by EWBrown on Sun Aug 02, 2015 8:25 pm, edited 21 times in total.
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Postby kheper » Fri Jan 07, 2011 9:48 pm

Well laid out. Big choke. Nice, neat wiring.
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Postby azazello » Sat Jan 08, 2011 4:50 am

Congratulation! Nice!
I sow, You use Auricap.....I tried Auricap and Dayton in my SE 300B
and Dayton sounded more better!. Small shunt caps, IMO, aren't suitable
for natural sound /AC HF signal across two different media/. If You will use big shunt caps like Black Gate or ELNA Cerafine, small caps isn't need. Small caps increase HF, but HF isn't natural.
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Postby EWBrown » Sat Jan 08, 2011 10:13 am

The two Auricaps (0.22 uF / 400V) were removed from a prototype linestage project of a couple of years ago. Waste not, want not ;)

I have some Dayton caps, I'l have to try them out in the next amp project, and I've used the larger sized 33 and 47 uF film crossover caps as power supply filters for line stages, they work very well in that application.

I used 2.2 uF / 250V caps across the 470uF / 100V cathode bypass caps for the 6AV5GAs. The operating voltage of approximately 50-52VDC is too close to the 63V rating of the 1000 uF / 63V Nichicon Muse caps I was going to use, and I like to have sufficient voltage safety margin.

The 6AV5GAs actually sound very good, and the amp is improving with continued usage as the transformers and caps are "aging" in.

I have a quantity of DuMont 12AV5GAs - these appear to be Sylvanias, and six of these are very closely matched. I'm considering a push-pull UL amp which wouold also use 12SL7s as floating paraphase driver / inverters.

This is a late 1940s-to-early 1950s vintage circuit, and I'd base this on a design from that era which used 6SL7 and 6B4Gs.

In triode mode, it is good for 10-12 Watts, and in UL mode (not from the 40s and 50s) it should be closer to 18-20 WPC.

I'll try this as a "breadboard" circuit using some octal relay sockets, with screw terminals, so little or no soldering would be necessary.

/ed B
Last edited by EWBrown on Mon Aug 06, 2012 7:43 am, edited 1 time in total.
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Postby TomMcNally » Sat Jan 08, 2011 11:20 am

Looks good Ed! I just got a stash of "tested" RCA 6AV5GA's that
I'll be building something with soon.

I haven't used my trusty Nibbler for years. cutting a hole for a
transformer or IEC socket takes just a few fun spark filled
minutes with a Dremel and a Gyros brand cutting wheel.
Don't use the Dremel brand, they break up and pieces fly
all over.

... tom
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Postby EWBrown » Sat Jan 08, 2011 11:34 am

The Dremel brand abrasive / cutting wheels also generate a really nasty foul stench (sick), usually just before they shatter and turn into malodorous schrapnel =:o

I wonder if a "Zip Tool" with an edge-mill cutter would be suitable for this purpose (???) I figure if they can cut ceramic tile, they should be OK with thin mild steel and aluminum.

Other than nibbling out the power trannie hole, and the two octal sockets, for which I used Greenlee punches, everything else was drilled out with standard drill bits or Unibit step drills - I have 1/2 inch in 1/16 and 1/32 increments, and 7/8 in 1/16 increments. They work very well on the cheapo drill press, much easier and better than using a hand drill.

/ed B
Last edited by EWBrown on Mon Feb 14, 2011 8:40 pm, edited 1 time in total.
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Postby azazello » Sat Jan 08, 2011 3:19 pm

50-52 v is very suitable for Your Nichikon 63 v. The cap works better on about 80% of it max. U.
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Postby Ty_Bower » Sat Jan 08, 2011 4:06 pm

kheper wrote:Big choke.


Indeed. Hasn't it been said the choke ought to be at least the same size as the power transformer, if not larger?

Nice looking amp, Ed. How do those Transcedars compare to the Edcor GXSE? I've sometimes wondered if Gery's 3K:8 would be a good upgrade for my GSG.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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Postby EWBrown » Sun Jan 09, 2011 8:53 pm

The big choke was just an "unknown heritage" one from the parts stash, it may be one of the (infamous) Handwound Transformers products, from e-bay, from a few years ago, it has the same silver-gray "hammertone" paint on the end bells. I had measured it at 5H, and as I recall, it was rated at 200 mA. It's big enough for that. I thought of repainting it black just to match everything else, but later decided to let it be, it does add a little "contrast" to the "basic black" theme.

My original plan was to use a 5AR4 tube rectifier, and mount the choke closer to the front of the chassis, but I decided that SS rect and heftier electrolytics would be more suitable. I did have to "waste" some B+ voltage by using the 5V filament winding to buck the primary AC voltage, and 333 ohms worth of resistors before the choke. I probably would have been better off with the 5AR4, in the long run. There's always a "next time"...

I'd say that the Transcendars are at least as good as the GXSE OPTs, and most likely to be a step higher in quality, they do sound very good, with solid bass, crystal clear highs, transparent mids (OK, I really don't know all the "fancy" descriptive terminology of the audiophiliacs) - Like the Edcors, these trannies have 8 ohms secondary only, with no 4 or 16 ohm taps, unlike the Transcendar 15 watt 3K:8 which had 16, 8 and 4 ohms available.

I haven't yet tried Edcor's new series of blue-end-bell GXSEs, just the older "open frame" versions from a couple years ago. .

I have one 10K:16 to try out sometime, as a 5K:8 ohm, for a single channel UL-SE prototype.

I bought the two 10 watt 3K Transcendars several years ago on e-bay they were cheap enough to convince me to get a set of them, for a yet to be determined future project, which just happened to be this 6AV5GA project. I've used the 15 Watt 3K OPTs in a 300B project, and they worked and sounded very good in that application.

Save the Edcors for UL-SE projects, like 6AV5GA, 6550, etc, and the untapped primary OPTs for triode power stages...

/ed B
Last edited by EWBrown on Tue Dec 30, 2014 6:41 pm, edited 2 times in total.
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Postby kheper » Mon Jan 10, 2011 9:44 am

Ty_Bower:
Hasn't it been said the choke ought to be at least the same size as the power transformer, if not larger?


There is another theory. Smaller, multiple chokes (in series with filter caps) are supposed to suppress more ripple than one big, honkin' choke.

EWBrown:
I'd say that the Transcendars are at least as good as the GXSE OPTs, and most likely to be a step higher in quality, they do sound very good, with solid bass, crystal clear highs, transparent mids


I bought and used 2 pair of Transcendar transformers: 1 PP UL (30W) and 1 SE (~10W) for $90 a pair. Recently, they have gone up in price. Transcendars are high quality and remain a good bang for the buck.
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Postby EWBrown » Mon Jan 10, 2011 1:04 pm

There are a couple other pin-compatible (6CK base diagram) BPTs which might be useable in this or similar circuits, the 6FW5 (18W PD) and 6BD5GT (10W PD, with 325V rated VP and VG2). I'd suspect, like with the 6AV5GA, that these tubes can handle a higher PD with relative impunity (666)

/ed B
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Postby sorenj07 » Mon Jan 10, 2011 3:55 pm

Don't listen to what they say, it's all about the big manly choke :)

Nice build!
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Postby TomMcNally » Mon Jan 10, 2011 8:47 pm

I'm still waiting for a chance to use this 50 pounder ...

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Postby EWBrown » Tue Jan 11, 2011 10:34 am

That monster might be a good candidate as the inductor for a DIY carbon arc lamp project (666) ;) (lol)

Image

What are the inductance and current ratings ? (???)

/ed B
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Postby TomMcNally » Tue Jan 11, 2011 12:58 pm

It's probably about 2 Henrys at 3 amps. It was replaced with
a bigger one when the transmitter was upgraded to a higher
power level. There is a high voltage transformer in the building
probably good for 6000 volts DC at 3 amps too.
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