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Old 28th January 2008, 06:51 PM   #11
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Gents,
Thanks for the help ! I made a mistake on my schematic; there will be 300v B+ - not 400 as shown.
As far as the gain goes; I am thinking of building a passive preamp; so I don't think it will be much of an issue ?

Thanks again,
Steve
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Old 28th January 2008, 07:44 PM   #12
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Steven-H
Gents,
Thanks for the help ! I made a mistake on my schematic; there will be 300v B+ - not 400 as shown.
As far as the gain goes; I am thinking of building a passive preamp; so I don't think it will be much of an issue ?

Thanks again,
Steve
Hi Steve,
Passive pre should work fine in this application. I still recommend a slightly lower plate voltage (250V) for the 45 unless you are using cathode bias in which case 300V is just right. In my application they sounded a bit strained at 275V, and yes they measured better, but the 3rd harmonic came up relative to the 2nd which also decreased slightly relative to the 250V operation and overall it actually sounded considerably worse. (I'll admit this was into a 4K load however. IIRC this was the case both for constant dissipation and constant current comparisons. ) Fine tuning is a good idea.
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Old 29th January 2008, 06:44 PM   #13
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Kevin,
Thanks for the help ! I don't think there are any other tubes that I can sub in for the 5842 that would be lower gain...
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Old 29th January 2008, 08:45 PM   #14
JoshK is offline JoshK  Canada
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5687 is a higher gm tube with mu about 1/2 that of the 5842.
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Old 29th January 2008, 09:12 PM   #15
kevinkr is offline kevinkr  United States
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Yeah Josh is right the 5687 would be a pretty good choice here if you need less gain.
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Old 29th January 2008, 09:22 PM   #16
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You guys are awesome. I'll check out the 5687 but I guess it is a twin triode - so I would have to rewire the socket to work on the board (which is fine, as I was assuming to mount the sockets to the chassis with short leads to the board.....). I really would like to get more use out of my soon to be preamp

Thanks again,
Steve
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Old 29th January 2008, 10:09 PM   #17
Gluca is offline Gluca  Italy
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Quote:
Originally posted by Steven-H
Gents,
I ordered up a set of 5K 1:1 IT's from Electraprint...So am I going about it the proper way here ? I am also thinking about maybe going fixed bias on the 813 - but that would require yet another transformer in the PSU; so I'll have to think about that some more....If I do go that route - I have seen where the negative voltage gets fed directly into the secondary of the interstage - sound about right ?

Thanks guys !
Steve
Nahhhh .... You don't need any transformer and forget caps and diodes. Grab a bunch of 12V batteries (size MN21) and you have got a clean cheap and easy negative rail. I made it a number of times and it works. Never mind, the batteries will last long (you are not biasing that 813 in A2, aren't you?). OH! Those big transmitting triodes really like to have the cathode at ground.

Ciao
Gianluca
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Old 30th January 2008, 11:46 AM   #18
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Gluca,
That sounds really interesting...Do you have a schematic so I can get a feel for what it would actually look like ?? I'd need like 10 of 'em though...

Thanks,
Steve
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Old 30th January 2008, 12:30 PM   #19
Gluca is offline Gluca  Italy
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Click the image to open in full size.

Schematic is pretty simple. Series connect the batteries to match the desired bias (err... only discrete multiples of 12V) and connect the + terminal to the cathode (center tap or negative lug of the DC filament) and the - terminal to the secondary of the interstage tranny.

Ciao
Gianluca
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Old 30th January 2008, 02:08 PM   #20
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Interesting Stuff ! How do you keep tabs on your battery voltage though ? I would imagine that as the batteries age - that your bias would change ?!

For the filament supplies, I bought the TentLabs 5A supplies. So I suppose that I would connect them to the cathode, then ground one side out right at the tube pin ?

Also - while I have your eyes - I thought that the IT needed to be loaded somehow ?

Lets see if this schematic looks correct....
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