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Old 26th November 2011, 11:23 AM   #3751
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Hoyan: Fabulous work. Excelent build and 125W is a lot of SE power.
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Old 26th November 2011, 02:53 PM   #3752
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Default Electric Chair Amplifiers...a reminder of safety folks!

Hoyan,

Lovely amplifiers and the video sounds very good for just a cell phone camera.

I need to make a comment though of concern about building an 'electric chair' amplifier that has very lethal 1000 volts exposed and no warning sticker. The original builder and owner might be alone or very careful and have no pets, BUT, what happens after they are gone and the amps fall into the hands of a new owner who may not know the DANGER? I would never build an amp like this without a shield in place to keep inquisitive fingers away that would shut down the high voltage if the shield was removed.
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Old 26th November 2011, 06:29 PM   #3753
noyan is offline noyan  Turkey
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rcavictim,
Yes, you are right. I will organise a plexiglass safety cover.
Thanks
Noyan
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Old 26th November 2011, 07:17 PM   #3754
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Quote:
Originally Posted by noyan View Post
rcavictim,
Yes, you are right. I will organise a plexiglass safety cover.
Thanks
Noyan
Noyan,

Sorry I misspelt your name. I copied it off the previous reply.

I bought one of the very early MingDa MC3008AB, 805 SET amp. S/N 10. Uses 300B as a cathode follower to drive an 805. PD on the 805 is 120 watts, full tube rating at 1050 VDC. Plate shows just barely perceptible color in the dark. The plate volts climbs to 1600 VDC before the drive appears if I do not manually delay switch the hi B+ switch. This is due to a choke input Hi B+ filter in a totally new power supply I made for this beast. I could have tamed the choke during no load with a big wirewound 200 watt power resistor bleeder but this amp makes enough heat as it is thank you very much! I installed solid state relays in the primary sides of the various power xfmers and a 5 VDC power supply which is always live to run power through the chassis banana jacks that receive the banana jack pin fasteners used to secure the removable tube cage. My miniature toggle main power switch only has silent LV DC running though it too so no hum radiation to the audio circuits as the power switches are in the same location as the volume control. Remove the cage and the amp will not run. A jack shorting cable can be inserted in the banana jack receptacles on top of the chassis to restore operation by a technician if desired. A dress chrome rack handle could be made to drop into the cover jacks as well if I wanted to run the amp cover off and have it still look nice. They would be at chassis earth potential. Frankly I prefer the look of my amp with the tube shield cover on. My cats appreciate this too.

I'll post a picture of my creation if I can find the file. BTW it makes 45 watts RMS per channel and sounds very good to me. With near 100 dB sensitive speakers it doesn't work very hard. In a class-A the first watts are the cleanest.

Plexiglass may heat and warp. A metal perforated or wire mesh cage would not impede cooling air. In my highly modified amp there is a plexi top cover over the 805's present in the original factory design and it has deformed slightly during the first few hours use which stopped after I rebuilt it and installed a cooling blower.

RCA (not me, the other guys) says that at full ratings cooling air needs to be directed to the spot between the grid and plate terminals from a nozzle and blower to prevent failure of the seals. Other than this the tube is radiation cooled. Whatever you do for a tube box/cage you don't want to impede the cooling of these big hot tubes. Judging by the plate glow on your amp I'd say you are near full continuous PD ratings.

Regards,

rca
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Last edited by rcavictim; 26th November 2011 at 07:20 PM.
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Old 3rd December 2011, 07:27 PM   #3755
alex mm is offline alex mm  Europe
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my Gromes 260A modified is growing slowly just missing case ....
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Old 4th December 2011, 01:54 AM   #3756
tomchr is offline tomchr  United States
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Quote:
Originally Posted by rcavictim View Post
I need to make a comment though of concern about building an 'electric chair' amplifier that has very lethal 1000 volts exposed and no warning sticker.
A warning sticker does nothing to protect the user. A cage would be a better solution.

Nice reading light ... eh, I mean .... "amp"... Which driver tube is used? 2A3?

~Tom
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Old 4th December 2011, 02:19 AM   #3757
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Quote:
Originally Posted by alex mm View Post
my Gromes 260A modified is growing slowly just missing case ....
Looks like you are loaded for bear now. Remote volume control too. Sweet! Why does that power xfmer look too small?
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Old 4th December 2011, 02:56 AM   #3758
AJT is offline AJT  Philippines
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my new build:

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underneath:

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Old 4th December 2011, 04:18 AM   #3759
GloBug is offline GloBug  Canada
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UFO!

RH84 stuffed into a light fixture.
Changes - 6V6, 5Y3
Telefunken 5k OPt's

Sounds out of this world, no pun intended.
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Old 4th December 2011, 04:27 AM   #3760
pageboy is offline pageboy  United States
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I want to see the underside of this one when you get it wired up.
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