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Old 7th July 2004, 09:47 PM   #1
Fuling is offline Fuling  Sweden
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Default A safe (?) DC coupled amp

Good evening!

I have a 6B4G SE amp that for various reasons needs a little makeover, so why not have a little fun meanwhile?

Maybe I should begin with telling a bit of its currrent condition:

6B4G SE with Hammond 1627SE OPTs. 5AR4 + CLC PSU, CLC filters for the filaments. Input stage: Choke loaded (Hammond 156C) 6SN7. Thin bass and plenty of hum

My idea is to rebuild it completely, using a new mains transformer that gives +460V after proper filtering. Way too high for a regular 6B4G circuit but ideal for a direct coupled one.

The choice stands between a "Monkey" or "Free Lunch" design that requires expensive plate chokes for the driver or somekind of active load referenced to the cathode of the output tube.

Since the gain was too low with the 6SN7s Im going to replace them with PC86s. I have good experience with mosfets as plate loads in -followers so this might be one way to to.

Have a look and post your opinions!

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Old 8th July 2004, 10:44 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Yay for 6B4s! I use them in my SE amp, a totally nice tube. Really need the DC filaments for them though, they just have too much hum otherwise.

I know its heretical, but have you tried driving them with a cathode follower? I have a very simple 6SL7 cathode follower driving mine and just get the most fantastic sound- great bass, very neutral and no hum at all.

Otherwise, have you considered a choke input power supply to drop the B+?
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Old 8th July 2004, 12:48 PM   #3
Fuling is offline Fuling  Sweden
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Yup, its a great tube!

I used 6SL7s with cathode followers before I changed to 6SN7s.
Sounded good but I know I can get better

The point is that the new power transformer allows DC coupling, which is a good thing. If I would want lower B+ Id use the original (Hammond) transformer.

Ive been thinking a bit about the posted schematic and I feel that a pair of Lundahl plate chokes might be worth the money.
Getting Mosfets involved is a bit heretic...
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Old 8th July 2004, 01:40 PM   #4
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FETs are fine for duties other than voltage amplification. Think heaterless pentode. Frequently, a combination of tubes and FETs does a better job than either device type alone does.

Be pragmatic; it's the music that counts. IF "sand" acting as a "spear carrier" for tubes improves performance, so be it.
Eli D.
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Old 8th July 2004, 06:51 PM   #5
Fuling is offline Fuling  Sweden
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Im facing a hard decision here...
Im guite fond of inductive loading but price is definitely an issue.
Two 270H 10mA Lundahl chokes would probably cost me about 160-170 Euros. The mosfets can be bought for like 1/100 of that.

I guess the mosfet circuit would result in a very good frequency response, something that cannot be taken for granted when inductors are involved.

IF "sand" acting as a "spear carrier" for tubes improves performance, so be it.
Well said!!
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