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the aardvark grows a spud!
the aardvark grows a spud!
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Old 28th October 2008, 09:25 AM   #1
aardvarkash10 is offline aardvarkash10  New Zealand
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Default the aardvark grows a spud!

ok, so the junk pile was growing and bits and pieces gleaned from Trademe (kiwi E-bay) kept arriving with a "just in case" frame of mind used in the purchase process. Amongst them, a clutch, well 3 actually, of 6ch6 video output pentodes.

Idylly thumbing through the extensive information available (NOT) for these beauties, I suddenly noted the VERY low drive requirement and the relatively high output (4.5v, 12W at the anode...)

Who'd like their spuds fried I thought???

OK so ideally they would have a little more kick in the g1 area, and lets face it, Class A SE off these puppies is NOT going to make the dance crowd swoon, but they drive my big old 16 ohm OBs quite nicely thanks!

THe power supply could do with serious stiffening (dropped from the PSUD2 designed 250V to a mere 200V under operating load) and a lot of the niceties are, well, lacking... I'm not houseproud at the best of times so don't look to me for under-the-hood perfection. But it made a dreary Tuesday night a bit more fun.

Almost felt like Geoge must of an evening, but without the pyrotechnic display.

Numbers FYI - Anode at 200V, cathode via a 82 ohm resistor giving a -4V bias, g1 driven straight off the CD RCA output via a 25k pot and a .47u cap. Running U/L through a couple of unknown spec SE trannies from an old console. Output best described as stately or inoffensive...
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Old 29th October 2008, 04:35 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Default Re: the aardvark grows a spud!

Quote:
Originally posted by aardvarkash10
ok, so the junk pile was growing and bits and pieces gleaned from Trademe (kiwi E-bay) kept arriving with a "just in case" frame of mind used in the purchase process. Amongst them, a clutch, well 3 actually, of 6ch6 video output pentodes.

Idylly thumbing through the extensive information available (NOT) for these beauties, I suddenly noted the VERY low drive requirement and the relatively high output (4.5v, 12W at the anode...)

Who'd like their spuds fried I thought???

OK so ideally they would have a little more kick in the g1 area, and lets face it, Class A SE off these puppies is NOT going to make the dance crowd swoon, but they drive my big old 16 ohm OBs quite nicely thanks!

THe power supply could do with serious stiffening (dropped from the PSUD2 designed 250V to a mere 200V under operating load) and a lot of the niceties are, well, lacking... I'm not houseproud at the best of times so don't look to me for under-the-hood perfection. But it made a dreary Tuesday night a bit more fun.

Almost felt like Geoge must of an evening, but without the pyrotechnic display.

Numbers FYI - Anode at 200V, cathode via a 82 ohm resistor giving a -4V bias, g1 driven straight off the CD RCA output via a 25k pot and a .47u cap. Running U/L through a couple of unknown spec SE trannies from an old console. Output best described as stately or inoffensive...

The 6CH6/EL821 is a fine tube. I have a quad of Mullard made beauties I hope to build a "Super El Cheapo" with, in my stash. The type will work quite well with O/P "iron" usually associated with 6V6s and EL84s. Tolerance of large value grid leak resistors is low. The EL821 data sheet is here.
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Old 29th October 2008, 07:27 AM   #3
aardvarkash10 is offline aardvarkash10  New Zealand
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Default thanks Eli

yep - the old console trannies were attached to EL84s so no surprises there! Added cathode bypass caps tonight and that seemed ot add a couple of milliwatts of value. Just looking to adjust the HT voltage to get it upward a bit and then I might have a play with pentode / U/L / triode set-ups. Hey, with so few components, its hard to get lost in the maze and easy to identify the outcome of changes made.

Meanwhile Harry Nillson is belting out a few old standards and sounding mighty fine!
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Old 29th October 2008, 08:45 AM   #4
aardvarkash10 is offline aardvarkash10  New Zealand
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Default ahem... errors and omissions...

well color me dumb... OK so the op trannies were wired a little oddly... I had mistakenly transposed the anode and the HT connections. LOL - somewhere I read that transistors have specifications, tubes have guidelines - how fortunate!

Sounds better wired right.

Next - reduced the resistance in the RCRCRC supply from 540 ohms to around 110 ohm. Cap is a reused 40-40-20 350V can. Result - bang on spec HT (248v) but humungeous hum. Bugger.

Now, decent chokes and the like are like the proverbial rocking horse excrement out here in the colonies - but I did have a florecent lighting tube bias choke - so in it went to the supply wired RCCLRC.

No hum now.

I'm having a blast! So far it has cost $4.50 for a handful of wire 5W resistors, $5 for the four tubes (only used two so far...) and the rest has been recovered from roadside rescue consoles.

Fun shouldn't be this cheap with your clothes on...

Bias is now smack on 4.5v making for a cathode current of 55ma - still a bit of room for drift and the tubes glow a nice, warm orange/red. Sorry - no scope, so no distortion or other fancy measurements.
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Old 2nd December 2010, 10:58 AM   #5
sonata149 is offline sonata149  Malta
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Default 6CH6 for a "Super El Cheapo"?

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Originally Posted by Eli Duttman View Post
The 6CH6/EL821 is a fine tube. I have a quad of Mullard made beauties I hope to build a "Super El Cheapo" with, in my stash. The type will work quite well with O/P "iron" usually associated with 6V6s and EL84s. Tolerance of large value grid leak resistors is low. The EL821 data sheet is here.
Hi Eli,

I built your "El Cheapo" with 6BW6 outputs, all components as per your circuit. Finding out the 6CH6 has the same pin-out as the 'W6 I just tube-rolled between the two valves. There is a lot of drive now, bass sounds full and round, midrange articulate too. After about half-an hour one channel started distorting and noted one of the valves was getting quite red! I replaced it with another one and so far everything has been OK.

The Brimar datasheet for the 'CH6 says anode voltage is 275 V max. (as is the screen voltage) against the 315V (anode) and 285V (screen) for the 6BW6. Now I have about 340V B+ at the OPT primary centre-tap. I wonder if this is too much and so I'm very curious to know whether you or any other member has built the "Super El Cheapo". If so, what changes need to be made to the original circuit?

Regards,
JA
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Old 2nd December 2010, 11:04 AM   #6
SY is offline SY  United States
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the aardvark grows a spud!
Where's the LEDS?
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Old 2nd December 2010, 12:52 PM   #7
sonata149 is offline sonata149  Malta
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Where's the LEDS?
Sent you a PM, SY!
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Old 2nd December 2010, 11:10 PM   #8
piano3 is offline piano3  United Kingdom
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Hi Aardv. I am very interested that you managed to get the desired result passing more than 100 mA DC through a flourescent ballast choke. For what power light was the choke rated?
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Old 2nd December 2010, 11:22 PM   #9
aardvarkash10 is offline aardvarkash10  New Zealand
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it was more than one ballast... I work in a large industrial building with very friendly and compliant electricians!

I actually don't remember how this finished up, but it can't of been brilliant since all the parts are now scattering into several other projects.

Plus ce change...
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Old 3rd December 2010, 12:57 AM   #10
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally Posted by sonata149 View Post
Hi Eli,

I built your "El Cheapo" with 6BW6 outputs, all components as per your circuit. Finding out the 6CH6 has the same pin-out as the 'W6 I just tube-rolled between the two valves. There is a lot of drive now, bass sounds full and round, midrange articulate too. After about half-an hour one channel started distorting and noted one of the valves was getting quite red! I replaced it with another one and so far everything has been OK.

The Brimar datasheet for the 'CH6 says anode voltage is 275 V max. (as is the screen voltage) against the 315V (anode) and 285V (screen) for the 6BW6. Now I have about 340V B+ at the OPT primary centre-tap. I wonder if this is too much and so I'm very curious to know whether you or any other member has built the "Super El Cheapo". If so, what changes need to be made to the original circuit?

Regards,
JA

The big (IMO) thing with many of these tubes is holding screen dissipation down. That's why we chose 1 KOhm to tie g2 to the plate in "El Cheapo". The 'AQ5 called for is "limited" to 250 V. As you can see from successful builder's posts, some overvoltage is safe, provided dissipation limits are not exceeded. TANSTAAFL!

Early in this thread, I mentioned that the 6CH6/EL821 "dislikes" large grid to ground resistors. A "stock" EC uses 470 KOhms. I suspect you red plated because of that. Buffer the LTP by DC coupling ZVN0545A source followers to the plates, increase the interstage coupling caps. to 470 nF., and decrease the grid to ground resistance to 100 KOhms.

I continue to be financially constrained. So, my EL821 amp remains "vapor ware". FWIW, I want to use UL mode "finals", along with separate screen grid tertiary winding O/P "iron". Screen grid B+ will be regulated by a gas discharge tube stack.
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