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Old 30th December 2011, 04:41 PM   #1
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Join Date: Oct 2009
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Default HEllo and a few suggestions please

Hello everybody,
I am very new to this forum. Well actually I joined about two years ago, but got distracted by AK. I have just refound you folks.

I am a tube amp builder wanabe. I have built about 20 amps, but all from predesigned schematics and with help from friends at ak. I just figured out how cathode bias works this morning - my point is I am a bit slow and teaching myself as I go.

I am currently working on a project made from leftovers and scraps. I have a pair of Pilot el84 pp OPT,s and a PS from a Zenith 7D31. I hae found several schematics that might work, but I was wondering if anyone hada suggestion of a favorite for this configuration.

The PS puts out about 730v(supposed to be 660 @ 117VAC). I have a pile of drivers(12ax7's, 6u8a, 7199.s 6eu7's, etc)

So, anyone have a suggestions. Oh, the OPTs, even though they are pilots, area little small. PRobably about the same size as the oes on the magnavox 93** chassis'.

TIA!

MArk

AKA cricket
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Old 30th December 2011, 06:11 PM   #2
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Location: Monroe Township, NJ
The power trafo will droop under load.

You will do fine using the Pilot O/P trafos, IF you triode wire the "finals". The reduced power O/P of triode wiring leaves magnetic headroom for a GNFB induced low freq. error correction signal to work, without causing core saturation.

There are many good options available. Of course, I'll "push" a design I'm associated with. The "El Cheapo" schematic I've uploaded will work, without parts value changes, using any 6V6 family O/P tubes. EL84 family tubes will work, with a cathode bias resistor value change.
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Old 30th December 2011, 07:55 PM   #3
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My power supply circuit will obviously be different. What can you say about those CC circuits? I have never seen or heard one. Interesting ide for sure.

Here area few pics of my build so far
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Old 30th December 2011, 08:44 PM   #4
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The baby Huey is another popular choice. PP EL84, 12AX7 LTP, UL wired.

EL84 Amp - Baby Huey

The thread only has about 1200 posts.........better brew some coffee.

...another is Poindexter's 6GK5/6V6 music machine. Triode wired 6V6 PP for about 6W output.

Do a search here for music machine.....

and finally, SY's red light district EL84 PP Pentode wired.

The Red Light District - another PP EL84 amp
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Old 30th December 2011, 08:45 PM   #5
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Quote:
What can you say about those CC circuits?
A constant current sink (CCS) in the tail of a differential gain block is "killer". Symmetry is forced upon both O/P signal phases. In "El Cheapo" the CCS "glues" the driving signal and the NFB signal together.

Quote:
My power supply circuit will obviously be different.
Looks like you're setting up for a 5 V. tube rectifier. Obvious 250 mA. capable types (in descending rail voltage order) are the 5AR4, the 5U4GB, and the 5R4. Make the 1st cap. of a CLC filter 20 μF. and you will be safe to experiment with all 3. Settle on the type that brings the B+ rail voltage in "perfect". The 5U4GB draws 3 A. of filament current, unlike the 2 A. of the other 2 types. Make sure the power trafo can take the strain. BTW, use the "potato masher" 5R4 variant, as other 5R4 varieties might arc over.
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