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Old 25th January 2004, 10:15 AM   #1
Jax is offline Jax  Sweden
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Default Progress of my 813 SE

I'm pondering about the input stage of my 813 SE project. I need some 50V RMS with low impedance on the 813 grid. Others seem to go the single stage with a high current pentode but I would like to have triodes only. So it has to have 2 stages.

Below is what I came up with after a couple of glasses white wine. I'm probably crazy but simulation shows good result, it can drive a load of 30k in parallel with 1000pF way beyond 20 kHz and 100Vpp. Power supply is +-200V.

Am I completely nuts, or?

Click the image to open in full size.
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Old 25th January 2004, 11:33 AM   #2
Fuling is offline Fuling  Sweden
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Ah, fler i detta avlånga land som håller på med 813 SE!!

My idea for an 813SE amp is as follows:

PC86 - DC-couping - 6C19 - Lundahl IT - 813 - Hammond 1628SE

Just a thought: Since your´re using a +- supply, maybe you can get rid of the coupling cap?
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Old 25th January 2004, 11:55 AM   #3
Jax is offline Jax  Sweden
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It's possible to get rid of the coupling cap to the 813 if I use -300-350V but that would complicate the power supply.

Getting rid of the other coupling cap is also possible but I would have to set the grid to +85V on V2B and this baby is asked to swing 100Vpp. So I would need more + voltage, say +300V.

That would set V2B to +300V plate and -80V cathode, a bit much for the poor tube

Lowering V1A plate to -88V would only move the coupling cap to the input.
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Old 25th January 2004, 11:59 AM   #4
Fuling is offline Fuling  Sweden
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As I said it was just a thought, I didn´t look very close at the schematic.

Whats parts are you using around the 813? (Power transformer, OPT, rectifiers etc)
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Old 25th January 2004, 12:16 PM   #5
Jax is offline Jax  Sweden
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Power:

From my junk box, two 685VAC transformers for 900VDC, a lot of different chokes. Somewhere I have a couple of junk 350-0-350VAC transformers for the +-200V supply. 6SN7 needs a 6.3V biased to around -100V so may need an extra 6.3VAC.

Need: 10VAC 5A for each 813. Perhaps rewind a couple of 80VA toroids.

I'm thinking of letting Sowter wind me a couple of boatanchors for output. 8.5k primary, 8 ohm secondary, their P-sized core, it will handle 20W at 25Hz with 90mA DC. Same core can go to 20W at 20Hz if I reduce primary to 6.5k.

this project is definitely not for the newbies!
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Old 25th January 2004, 01:02 PM   #6
Fuling is offline Fuling  Sweden
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Interresting! Let me know if you´re going to buy anything from Sowther, I might be interrested in a similar pair of OPT´s. Maybe we could save some shipping costs.

Newbies?? Definitely not! These power supplys could probably kill an elephant! It´s sad that 813´s has plate caps.
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Old 25th January 2004, 01:55 PM   #7
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Hi all,

Quote:
Originally posted by Fuling
It´s sad that 813´s has plate caps.
I like plate caps

I use a 10y to drive the 813, works for me


cheers, Jan
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Old 25th January 2004, 02:21 PM   #8
Fuling is offline Fuling  Sweden
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Plae caps look really cool, but at ~900V they might be a bit hazardous. Not much of a problem for me though (knock on wood), since I live alone without kids or pets or anything.
If i had kids I´d think twice about using these tubes
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Old 27th January 2004, 06:47 PM   #9
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Hey-Hey!!!,
I have a small boy, and I am gathering parts for a U-L connected PP 813. Look to another industry specializing in protection. Toddlers and Firearms comes to mind. I am going for ~700 B+ and those top caps make me nervous too. I am thinking some sort of lock and key for the AC power supply and then still excercise caution. Let's see, maybe one of those cool key type switches the moovies always portray the Nuclear weapons with...
regards,
Douglas

As to voltage being lethal, when backed by a few uF of high quality capacitor, it is ridiculously unsafe at voltages usually associated with SE EL84's.
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Old 27th January 2004, 06:59 PM   #10
Fuling is offline Fuling  Sweden
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Rememer to put some bleeder resistor in the PSU to discharge the caps.
OR, why not put some kind of cage over the tubes?
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