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Old 24th May 2012, 05:57 PM   #1
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Default 808 SE Build

Greetings all, my name is Steve. I've been a member here for a little while and primarily read things here, so this is a rarity for me.

I've built a few things in the past using HV (PP 807's was my most recent project) so I understand the dangers and costs involved with building big amps.

I want to build a SET Amp using the 808. Right now, here's how far I am:

I have a big old Kenyon transformer that can do EITHER 2000VCT, 2500VCT, or 3000VCT at about 250ma. Based on the datasheet I have for the 808, it looks like at max plate voltage (1500) I'll be using the 2500 taps.

I have two globe 866's that test new and are pretty sweet, as well as a filament transformer with CT and a 2kv+ rating. I know people advise against MV rectifiers, but there's no way I'm not using these. They'll be beautiful.

Have a choke, 12H at 250ma. Here's where it gets fuzzy... I don't know the HV test rating of it. I cannot find data anywhere. All I know is it's pretty big. That being said, what are the disadvantages of putting the choke on the CT of the high voltage? Will this mess with my MV tubes?

The next area of concern is using the 808 class A. I know it's a radio transmitting tube and designed for B or C but I know shishido did it class A if I remember correctly. If it is indeed possible, what would be a good tube to drive it with? a pair of 300b was my first thought? Prefer some kind of tube that will look as pretty as the 866's and the 808.

Curious to hear thoughts.

Steve
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Old 24th May 2012, 08:16 PM   #2
djn is online now djn  United States
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Cool looking tube. Post many pix as you go.

Vh Ih VaMax PaMax Notes
7.5 4 1500 50 Ia max=150

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Last edited by djn; 24th May 2012 at 08:19 PM.
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Old 24th May 2012, 08:18 PM   #3
agent.5 is offline agent.5  United States
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maybe 3c24 as driver?

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Last edited by agent.5; 24th May 2012 at 08:43 PM.
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Old 24th May 2012, 08:26 PM   #4
djn is online now djn  United States
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That would sure look cool.....like daddy and son!
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Old 24th May 2012, 10:04 PM   #5
7N7 is offline 7N7  United Kingdom
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I love to see people starting projects like this.

I do think that 808 is a bit of a problem for audio with its very high anode resistance and mu.

The only way I should think would be in class A2; there is a Japanese bloke who has obtained good results, if I can find a link I'll post it here,

To drive the large amount of grid current, you would need something more suitable than 300B I think. I suppose a transformer would be the answer but I prefer glass, so I would make a super-beefy cathode follower using a power valve with plenty of poke. If it had to be a bit different, a cathode follower built using a QQVO6/40 (sorry forgotten the American number - it's 5xxx} Running the two sections in parallel as beam tetrode cathode follower would perhaps provide the necessary grunt! There are still beefier versions of these double beam tetrodes - YL1071 also has a US number - 8xxx - sorry!

Lastly I did always quite fancy the 806 which looks like the 808 but has a base like an 845; it looks as though it might be a bit less demanding.

Edit regarding the choke, perhaps you know someone who has an insulation tester? Of course it would have to be a high-voltage one and if we are talking of 2kV then we're looking at proper porcelain insulators etc., which might be outside the realm of your choke!
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Last edited by 7N7; 24th May 2012 at 10:08 PM.
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Old 24th May 2012, 11:43 PM   #6
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The only real disadvantage of using the choke in the return side is that stray capacitances to ground tend to bypass the choke rather than adding to the "real" caps. No big deal usually.
Also, because the MV rectifiers will want to see a choke input, the first choke will see a *lot* of AC voltage across itself. Always a good idea to minimize high voltage peaks as possible. Some folks will add a snubber across the input choke - can't hurt.

All good fortune,
Chris
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Old 25th May 2012, 12:12 AM   #7
multi is online now multi  Australia
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Shishido used 350 volts for 15 watts output with 120Ma current.
Only my opinion but I don't think this tube would sound very good at 1500 volts.
You would have to run it at 5-10ma or burn it out. the 808 only has 50 watts plate dissipation!!!
These tubes work best at lower voltages and high current.
the 805 sounds terrible at 1200 volts Single end, 20 Ma, but very good as Shishido uses it at 800 volts and 120-140Ma
I can scan Shishido's circuit if you wish.
Phil
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Old 25th May 2012, 12:13 AM   #8
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Since you haven't said that you actually have some 808s in your possession, please be very cautious if you need to buy them. I can tell you from experience that these are old enough that 50% of what's out there will be either very gassy or have gone to air. Most sellers don't have a clue what the real condition is for any they may have for sale. Buy only with a full guarantee and return privilege. Regular normal tube testers can't test them. Also, the filaments are very fragile and demand very careful packaging for shipment.
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Old 25th May 2012, 12:48 AM   #9
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Shihido's 808 amp
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File Type: jpg 808 SE.jpg (184.8 KB, 315 views)
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Old 25th May 2012, 01:04 AM   #10
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Thanks for the replies guys. I have two 808's at this point, both were properly tested before I purchased them and I have checked that the filaments are still good in each, and have lit the filaments as well recently.

Low voltage may be a better option then, I didn't really think of that.

Interesting point on the choke too, I will probably look into that more and add a snubber.

I can't wait to get started on this thing... I also have two 811's laying around, but I don't know if those would be any easier and I'd rather stick with the 808.

Also will look into more options for driving it. I too am a fan of glass, and would prefer to not have an interstage transformer in it at all.

Steve
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