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Old 27th February 2006, 07:50 AM   #1
tubee is offline tubee  Netherlands
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Default Shunt regulator

I have a trouble with my solid-state regulated diy Pre-amp: its motorboating in the Phono-stage. (Because the current draw from tubes is around the minimum of the regulator TL783, +/- 14 mA)

Have a box full of tubes, and therefore i was thinking of a EL84 combined with a gas-reference (85A1) to swap the solid state for a shunt regulator.

Has anyone experience with this? Thanks.
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Old 27th February 2006, 08:12 AM   #2
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Shunt regulators are great, but have higher impedance than series in my experiments.

How much voltage you need? Here's a SS one I designed specifically for motorboating-tendency preamps (12AT7's are horrible for that).

I have a good tube design too somewhere I can dig out.
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Old 27th February 2006, 08:26 AM   #3
tubee is offline tubee  Netherlands
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Hello Geek, thanks for reply so quick.

Yes the shunt should work great to me too for my pre. Your SS could improve it allready.

In is +/- 270V , want about 210V out.

The line stage is SRPP 6N1P (great tube, but large heater current)
The phono stage is 2 times CCS 12AX7 with passive RIAA inbetween.
I have just got a allmost new Benz Gold MC to play with so now i want to resolve motorboating troubles. The MM had higher output and i could just manage to keep below the motorboating level.

If you can find the schematic it would be great, but i don't have a hurry with it. Have been looking on the net but shunt is not used often as series PS.
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Old 27th February 2006, 10:04 AM   #4
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Hi Tubee,

Funny you mention that tube lineup - one of the projects I'm using the SS reg in now is a preamp with a 6N2P phono and 6N1P in a L-W CF line driver

Here's my tube reg (based on the one by Steve Bench). No shunt ones on "paper" though.
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Old 27th February 2006, 10:45 AM   #5
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Default Re: Shunt regulator

Quote:
Originally posted by tubee
I have a trouble with my solid-state regulated diy Pre-amp: its motorboating in the Phono-stage. (Because the current draw from tubes is around the minimum of the regulator TL783, +/- 14 mA)

Have a box full of tubes, and therefore i was thinking of a EL84 combined with a gas-reference (85A1) to swap the solid state for a shunt regulator.

Has anyone experience with this? Thanks.

two OC3 in series for 210 V
nothing fancier

and-ye,bunch of Ls in LC filter stages,if you can......

edit:
you can always add shunt resistor in parallel to load,if you use series reg as 783
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Old 27th February 2006, 11:08 AM   #6
tubee is offline tubee  Netherlands
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Choky: thought of a shunt R as load on TL 783 also, but sound will improve too with an tubed regulator,a nice goal.
Big or a lot of L's are not an option, no space in cabinet. But will fiddle 1 choke in.

Thanks for the regulator scheme Geek.
Why 2 bx's parallel, to lower the impedance?
I will use one EL84 for the whole Stereo pre-amp, the space is limited, transformer also (2 * 8V AC 1.5A and HT) Don't want to change everything.

My preamp is the upper one of:

http://img169.imageshack.us/img169/8364/setje6ms.jpg

Btw on the left a diy headphone OTL amp, made from scrap tubes, Marcon Lytics, Telefunken ECC85 + two EL95 in triode (the EL95 is the ONLY beam power penthode Philips made) The design is from myself, discovered this morning there are more of such designs: http://www.tubecad.com/january2001/page9.html
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Old 27th February 2006, 10:01 PM   #7
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Hi Tubee,

Quote:
Originally posted by tubee
Thanks for the regulator scheme Geek.
Why 2 bx's parallel, to lower the impedance?
Nope, just extra plate dissipation. You could use a 6AQ5 or 6BQ5 in there tied triode, or tied pentode with extra filtering on g2 for superb ripple reduction.

I just had a box of 12BH7's, but few little power pentodes at the time

If you don't have a zener regulated bias supply, just drop your zener or VR tube in the 6CB6A cathode and ground the divider resistor.

One caveat - use 6CB6A over 6CB6 - warmup time of the "A" is faster and you won't have a few seconds of full unregulated B+ on your circuit like you would with the 6CB6.
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Old 28th February 2006, 06:53 AM   #8
tubee is offline tubee  Netherlands
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Thanks Geek for all tips, will dig this out.

Yesterday looked for a solution for motorboating, with extra cap on PS to phono + extra resistor(from 300 to 1k) its some better but not dissapearing. It starts in the phono stage and with further turning of volume pot the whole preamp starts motorboating. Strange, in the beginning a few years ago it did'nt oscillate.
Maybe some dry lytic caps?

A new PS or regulator doesnt solve the motorboating problem in phono.
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Old 28th February 2006, 11:30 AM   #9
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If the decoupling R is too small, all the C won't help... just make the oscillation frequency lower.

A little rule of thumb I gleaned from experiments is the decoupling R should be 1/10 the plate R of the stage. C can be 10uF or so. More is just waste in my experiments.

Here is an example for my preamp....

Plate load R's for the first phono, 270K. Second phono, 100K. Plate of my L-W CF linedriver are straight to B+ (well, through it's 330R load resistor).

Power from the raw B+ goes through 10K to the second phono and a 10uF 'lytic to ground (10K = 100K second phono load/10). From there, it goes through 27K and another 10uF bypass (27K = 270K first phono load/10).

You don't really want a "star" power point, where raw B+ goes through various decoupling resistors, but a "daisy-chain".

Perhaps illogical, but has been used by designers forever and seems so far foolproof
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Old 1st March 2006, 04:39 PM   #10
tubee is offline tubee  Netherlands
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Geek i agree with small as possible decoupling caps as well. Made a SE EL84 amp with diy output-transformers. Sounded great with about 50uF supply and a choke. Later did it over with same transformers and 6V6gt's, never sounded that good anymore.


I have solved the motorboating problem The TL783 was oscillating, with a shunt R of 33 K ohm to Gnd there is more current-draw from the regulator, and it's quiet now. The phono suffered from it because the high gain.

In meantime i found an interesting supply, its from Frank's Ultimate Tube Preamp.
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