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Old 4th January 2021, 04:40 PM   #1
hareynolds is offline hareynolds  United States
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Default I am BAAACK...with my box full of 6AV5 tubes!

Brief history: haven't done thermionic stuff for about 8 years.

When last we spoke. I had several Rat-Lab test set-ups, and was seriously trying to pin-down the best circuit and operating points for a truly simple 6AV5 SE amp using George's SSE board...

Then Real Life intruded. Like George, I worked in a highly cyclical, boom-bust industry (in my case, the "upstream" oil & gas business). It has always been a roller coaster ride, but in 40 years I had survived every single purge & layoff by making myself as big a bargain as possible, which usually meant 50-60 hour weeks, and jumping into stuff which made everybody uncomfortable. The last 8 years have been like that, on steroids, but nothing that I haven't seen since I started in the early 1980s.

Until this year. On April 15th, I was laid-off. On April 25th, I turned 67. On April 26th, my (former) company declared Chapter 11.

(A) I re-read my posts here from ~2012. Honestly I barely recognize that guy, but perhaps I can live-up to his ambitions...

(B) I discovered that I had made a Simple SE flexi-amp, using the SSE board, a healthy PST & choke, and dual sockets for both the 6L6/EL34 family AND the 6AV5..see photos below.

(C) It APPEARS that folks (at least in the US) are still interested in the 6AV5 in SE application, but there doesn't seem to be any longer term data on running the thing CRANKED, ie at roughly double the rated dissipation.
I think I can work on that; otherwise, my heirs will find themselves staring into a box of 6AV5s making unkind comments about me, God Rest His Soul.

Here's some photos of where I am now.

1. very old 6AV5s test mules of uncertain provenance (was gonna say parentage, but it was Sylvania what birthed 'em; ALL my 6AV5s are Sylvania...) GLOWING at 25 watts dissipation: 380V plate, 61V cathode across 780 ohms, 78 mA x 319V = 24.95 W

After a few hours at this op point, these old horses just started spitting & DIED.

(2) the open-back shop speakers. Rat shack 8" so-called "full range" with little Foster horns in ancient surplus kitchen cabinets dyed red. Can't remember the cross-over, but it's low-order and pretty high frequency IIRC. Ain't climbing up there today to find out. This set-up ACCIDENTALLY works good, as the Homasote ceiling reflects the bass nearly perfectly. Very Very open, natural lively. I am seriously tempted to get some 12" Eminence Delta 12LTAs for these boxes. There it is again, WHO AM I KIDDING? Old downside is that at higher volumes, the tools on the galvanized pegboard can start to rattle (see: ZZ Top "LaGrange") which always makes me think the 80 year old Thordarson PST is coming apart

(3 & 4) the Simple SE Flexi-Amp with "auxiliary" sockets, Gen 1.
Should have added an OCTAL input tube socket with off-board biasing (e.g. NiMH batteries, which I really like).
I might actually DO a Gen II...

Current status:
Replumbed the P/S to give ~
  • 410V plate (increased C1 from 2.2uF to 18uF), which of course makes the nice THordarson Swinging Choke less relevant.
  • Raised Cathode Bias resistors to ~1000 ohms (2 x 10 W in series)
  • Running a "new" set of 6AV5s; at 120VAC in off the Variac, I get 409VDC plate, 67VDC cathode for 67mA x 342V =22.9W dissipation. No glow on these puppies with the lights out.
  • Contemplating wiring-up a 120V hour meter I found in the stash, to actually KEEP TRACK. Novel idea, huh?

Near this operating point seems to be the sweet-spot for a Simple SE implementation of a 6AV5 SET amp. IMHO, this MAY be the cheapest best SET I have built.

I had an old engineering professor (Fred Hooven, you could look him up) who said that "Engineering is doing for 50 cents what any JACKASS can do for $5". I do believe that's what George has accomplished with the Simple SE board...

More later, muchachos..
Attached Images
File Type: jpg 2021-1-3 6AV5 at 25W dissipation.jpg (111.9 KB, 164 views)
File Type: jpg 2021-1-3 Open-back Shop Speakers.jpg (140.7 KB, 162 views)
File Type: jpg 2021-1-3 Simple SE Flexi-Amp Guts.jpg (162.0 KB, 168 views)
File Type: jpg 2021-1-3 Simple SE Flexi-Amp.jpg (136.5 KB, 164 views)
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Old 4th January 2021, 05:09 PM   #2
vinylkid58 is online now vinylkid58  Canada
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Quote:
Originally Posted by hareynolds View Post
[*]Running a "new" set of 6AV5s; at 120VAC in off the Variac, I get 409VDC plate, 67VDC cathode for 67mA x 342V =22.9W dissipation. No glow on these puppies with the lights out.
That's impressive. You running UL taps on the screens?

jeff
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Old 4th January 2021, 06:03 PM   #3
hareynolds is offline hareynolds  United States
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Yes, they are wired Ultralinear.

The OPTs are the EDCOR GXSE-15-5K (15 watt 5K ohm, currently about $42 each) which are the One Step Up from the super cheap EDCOR XSE-15-5K open frames (currently $22).

I haven't done an A/B test between thee two OPTs. I'm not sure the difference is that great, but IIRC I bought the G model for the Wife Acceptance Factor.

I have quite a few of the XSE cheapies, which I use when bread-boarding SETs...

I have noticed that the RatLab prototype is currently wired as NON-uL, with a very simple GLOBAL feedback instead of George's more-local (ie to the power tube) feedback.

All implementations of the RatLab 6AV5 used a 6SL7 driver with George's Constant Current source and NiMH battery on the cathode, so THAT may have been the big difference. Generally, I dislike Global feedback as it sounds more "constricted" on my open-backed/baffle speakers, so I wonder what happened exactly.

More later as I continue to dig..

Last edited by hareynolds; 4th January 2021 at 06:08 PM.
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Old 4th January 2021, 11:17 PM   #4
egellings is offline egellings  United States
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That red plate doesn't look good. Need to back off on the plate current a bit.
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Old 5th January 2021, 06:33 AM   #5
hareynolds is offline hareynolds  United States
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Default Red Plate at Night, TubeLab's Delight

The Red Plate Tube was one one of a pair of ancient test mules running (at that time) at ~25 Watts, AFTER seeing abuse in the high 20-something-watt range over the years.

Indeed, these tubes ground to a halt about an hour after the Red Glow, and were replaced with a fresh pair. This is the first set of 6AV7s that I've actually destroyed, so I figure they donated their little Bottle Bodies to science.

Based on George's earlier observations, it appears that the tell-tale warning sign is the concentration of the glow, rather than an even distribution across the plate. Don't know the mechanism of failure, but from the sound of it, the plates warped into/too close to the grid or screen. Perhaps annealing and consequential loss of strength? or maybe just plain ol' MELTING.

I didn't toss them; considering a TubeLab-style autopsy, if I can find a hammer bigenough and a place to stand...
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Old 5th January 2021, 10:06 AM   #6
Parafeed813 is offline Parafeed813  Netherlands
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No need to use a hammer: do it in style and smash it with a transformer
Wrap a towel around it and gently break the glass.
Safer and easier to clean up the splinters.
I'm really curious about the cause of death in this case.
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Old 5th January 2021, 03:35 PM   #7
Tubelab_com is offline Tubelab_com  United States
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Quote:
Originally Posted by hareynolds View Post
Don't know the mechanism of failure, but from the sound of it, the plates warped into/too close to the grid or screen. Perhaps annealing and consequential loss of strength? or maybe just plain ol' MELTING.

I didn't toss them; considering a TubeLab-style autopsy
I usually put the tube inside a thick paper bag with the end rolled shut, place that on a hard surface then tap the tube gently with the round end of a small ball peen hammer, or give the glass end a little squeeze in a vise. Those tubes have a big empty space at the top, hit or squeeze that area to avoid damaging the guts. I then cut the tops off the plate tabs and grid rods where necessary to slide the top mica off. Then you can cut through the plate tabs at the bottom where the wires are attached and slide the plate off. If the tube just suddenly stopped working, the usual failure is a warped screen grid touching a beam forming plate or the control grid, or a tube arc taking out the little strap that connects the cathode to the base pin. Continued operation in the red zone usually leads to vacuum contamination causing grid current leading to a runaway condition.

It's pretty hard to melt a plate, even if that's what you are trying to do. Usually the glass will melt, crack or shatter first.

When someone gives you over 100,000 used 50 year+ old tubes that were all stored in an abandoned and vandalized warehouse in the Florida heat, rain and humidity, you have a lot of "test subjects." Nearly 1/3 of the tubes were broken. It's a sad site to find the remains of maybe a hundred transmitting triodes like 211's and 845's that had been thrown against the concrete to break them. It looked like multiple paintball style battles had been staged with tubes as the projectiles. That stuff all went into the dumpster on site. As did most of the corroded and mangled tubes. The rest were transported 200 miles to my warehouse where I began to spend nearly 5 years sorting them all.

First you find and toss all the really nasty and useless tubes that are still mixed in to boxes and drums full of tubes. Then you sort the big boxes full of randomly mixed tubes by size. Then you sort them by type number. In the process you find some gems like the WE417A's and 5842's. But you also find some strange stuff. What are the odds of finding a tube with a spare heater stuffed inside the glass? What are the odds of finding TWO of them with the same type number?

I also found a damper tube with a hole burned through the plate. Even so the tube tester said it was good, so I decided to perform further testing to see if it really worked......it did, and met the specs for current handling and voltage drop. Now, I didn't want to ever use this tube in something, or let it get mixed in with the stuff I kept, so I decided to "test" it a little more.

I already had it connected up to a small power supply to run the heater, and a bigger power supply to feed the plate. The first picture is a bit out of focus, but clearly shows the hot cathode through the hole in the plate with no plate current. The tube is a 6AX4, rated for 165 mA of continuous plate current.

I kept cranking up the current until things got interesting. Of course the plate got red when I was somewhere over half an amp of plate current. A neat blue ray started to appear at nearly 1 amp. I cranked it up to 1 amp where the blue got pretty intense and grew into a plasma that enveloped the top half of the tube. I let it run at this level for over a minute expecting the plate or the glass to melt, but the tube started arcing, and the glass cracked letting air inside and killing the show. The autopsy revealed several new holes in the plate, and flying bits of metal likely caused the crack which can be seen across the top of the tube in the last picture.
Attached Images
File Type: jpg UglyPins.jpg (166.5 KB, 88 views)
File Type: jpg Box_o_tubes.jpg (195.8 KB, 34 views)
File Type: jpg Sorting_rack_A.jpg (61.8 KB, 32 views)
File Type: jpg WE5842.jpg (185.1 KB, 27 views)
File Type: jpg two_of_them.jpg (164.6 KB, 26 views)
File Type: jpg HoleInPlate.jpg (55.1 KB, 24 views)
File Type: jpg BluRay.jpg (103.5 KB, 20 views)
File Type: jpg Plasma.jpg (101.2 KB, 22 views)
File Type: jpg Arcing.jpg (106.6 KB, 27 views)
File Type: jpg Autopsy.jpg (162.8 KB, 30 views)
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Old 5th January 2021, 04:28 PM   #8
hareynolds is offline hareynolds  United States
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[I just KNEW that the phase "Tube Autopsy" would yield another excellent Epistle from George]

Excellent stuff, George.
When you do these "experiments", what's your Personal Protective Equipment?
Didi you scrounge a bullet-proof shield from a 7-11 dumpster?
Which is to say, what's the worst that can happen, exactly?
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Old 5th January 2021, 05:24 PM   #9
Tubelab_com is offline Tubelab_com  United States
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Lets just say that I'm not exactly the model example in the PPE category. For everything from table saw, to soldering, to experiments involving hot tubes and electricity, I am usually dressed in a pair of shorts. I'll trade them for jeans in the winter. This comes from growing up in Miami Florida in the days when only rich people had air conditioning. I left Florida at age 62, and old habits are hard to break. I have been seen shoveling snow in just jeans and flip flops, no shirt, nearly every winter.

Seriously I do ware a pair of polycarbonate glasses when soldering or playing with power tools or electricity. I have a big power supply capable of 650 volts at 1.7 amps continuous. It has 1000 uF inside wired directly across it's output. It can and has made stuff explode violently, including two tubes and several electrolytic caps.

I have a 1/4 inch thick sheet of Lexan that I put between me and the experiment when there is a danger of exploding parts. It was also used when testing stuff whet deadly voltage levels. All meters were connected, the sheet installed, then power is applied.

I also learned long ago to remove the power cord and place it out of reach when wiring stuff up. Too many things have a black cord, so simply pulling the plug is not enough. I yanked the power cord on an amp I was working on only to get a shocking surprise. I had unplugged my soldering iron instead. Removing the cord makes two steps necessary to power up the experiment.

I insulate my feet with flip flops whenever any possibility of current passing from hand to foot exists. I learned that lesson early in life when I had to work on whatever junk I dragged home as a kid outdoors, usually in a utility shed with a bare dirt (mud) floor. Here in my basement lab there is a layer of plastic on the concrete floor and carpet over that around the benches that are used for electronics experiments, even those that see 12 volts max.

A small tube like your 6AV5GA's can usually be smacked directly with a hammer and all the glass will just fall to the ground below, there is not enough glass mass for it to fly very far. A paper or thick plastic bag will usually contain all the glass unless it happens to get cut during the strike. That rarely happens. I simply place the tubes to be broken inside one of those double Trader Joe's bags, reach in to the open top and whack it with a pair of pliers, then take out the carcass and break off the remaining glass letting it fall into the bag. When the autopsy is done, all goes into the bag which gets rolled up and thrown into the trash.

A CRT is a different story, and people have been killed by the glass flying from dropping one. I disposed of me excess CRT's as a kid by floating them in the lake and shooting them with a rifle.

Electrolytic caps will usually vent if overheated. The scored crisscrossed lines on the top will split open and a stinky steam will vent preventing an exploding cap.....Unless the overload is too rapid for this to occur. A cap the size of C2 in an SSE can explode with a bang not unlike a cherry bomb. The shrapnel will be hot and can be sharp. This will not happen in something like the SSE since the resistances in the power transformer, rectifier tube and choke, will limit the current to a low enough level to prevent explosions. I have had SSE experiments go wrong enough to cause an electrolytic to vent. It stinks, but the goo can be cleaned off the board and the cap replaced.

I have had two vacuum tubes explode, one was rather violent, but the glass does not have enough mass to penetrate bare skin. Glasses were definitely an eye saver, as I was pretty close to that tube. Again the monster power supply was involved along with a really stupid arithmetic error. I was expecting to apply 35 watts to a 24 watt tube......it really did NOT like 350 watts.....not once, but twice. The second tube just cracked though.
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