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Old 30th July 2009, 10:29 AM   #11
316a is offline 316a  England
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Quote:
Originally posted by dsavitsk


I've been playing with 3A5's lately, and they seem to be very hum sensitive. This is CCS loaded with a CCS filament supply. I haven't gotten far enough to try shielding, or taking any precautions really, but out of the box there is a lot of noise.

Try backing off the filament voltage slightly . I'm very surprised any of these battery valves would be quiet enough let alone in a three-stage phono . I've used battery types including the 30 , 3A5 , 49 , 3B7 and 1LE3 and found all to have hum problems that could only be cured by wrapping foil around the bottlle or using a screening can . Microphony is another issue I have had , it always seems to be the low power filament types that have poor microphony , type 30 special is the only acceptable battery type I've found to date

316A
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Old 30th July 2009, 11:33 AM   #12
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Quote:
Originally posted by 316a



Microphony is another issue I have had , it always seems to be the low power filament types that have poor microphony , type 30 special is the only acceptable battery type I've found to date

316A
Hi

The low power filament valves will probably have the finest wire in their filaments and as a consequence the filaments are going to be much more susceptible to vibration than the thicker wired higher power types.

I have just made up a makeshift screening can from an old tin can connected to the chassis with a clip lead and this has removed the last vestiges of hum so that it is inaudible even at maximum volume with no input.

Need to find something a bit more elegant now and apply a bit of damping to the chassis while I'm at it.

Common sense tells us that these battery valves should not work in a phono stage, but work they certainly do and properly fettled mechanically they give a superb sound. I reckon Dmitri ought to be congratulated for thinking outside the box as, it were. I have built several phono stages using the usual suspect valves but none has been as good as this one. Whether that says about my building skills than the tubes in question I wouldn't like to guess
Steve
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Old 30th July 2009, 12:04 PM   #13
316a is offline 316a  England
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Quote:
Originally posted by Steve Cresswell


Hi

The low power filament valves will probably have the finest wire in their filaments and as a consequence the filaments are going to be much more susceptible to vibration than the thicker wired higher power types.

I have just made up a makeshift screening can from an old tin can connected to the chassis with a clip lead and this has removed the last vestiges of hum so that it is inaudible even at maximum volume with no input.

Need to find something a bit more elegant now and apply a bit of damping to the chassis while I'm at it.

Common sense tells us that these battery valves should not work in a phono stage, but work they certainly do and properly fettled mechanically they give a superb sound. I reckon Dmitri ought to be congratulated for thinking outside the box as, it were. I have built several phono stages using the usual suspect valves but none has been as good as this one. Whether that says about my building skills than the tubes in question I wouldn't like to guess
Steve
I'm well aware of the pitfalls of DHT's , I've built up quite a considerable stock (close to 300) and have been experimenting almost constantly for the past five years . Currently using thoriated drivers such as 15E , 3C24 and 10Y , battery types are on the back burner as are any oxide filament types apart from output valves .
If you need to damp the chassis , you would be better off getting some damping sheet , cutting into squares and applying to the underside of the chassis . A bit of extra weight may also help , one of the best ways to implement damping is to add large blobs of blu tac to the top of the chassis and once you find the offending areas , use the damping sheet underneath . I find adding trampoline suspension makes matters worse , with the 3A5 , you should find some sprung screening cans . You should also consider backing off your filament voltage , this is what I usually find to be the prime source of hum and microphony

IMO an ECC83 common cathode or ECC88 cascode stage followed by a type 46 etc output stage would be a preferable solution

316A
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Old 30th July 2009, 03:28 PM   #14
Sheldon is offline Sheldon  United States
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Rather than spring the chassis, I've had good success mounting the tubes to a fairly thick sandwich of aluminum and copper, then using pieces of sorbathane (sp?) to set it on the main chassis.

The filament voltage can sometimes be manipulated to good effect. I've done that to reduce distortion in my 801A amps. I can see where it might help with hum. However, in this case it may not be so easy, as the voltage across the filament sets tube bias.

Sheldon
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Old 30th July 2009, 06:06 PM   #15
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Well it turned out that I didn't need any fancy suspensions for the valve sockets. All the problem turned out to be was the top of the metal case ringing and passing its vibrations through to the DCC90s.

Damping the case top on the inside with a sheet of Blu-Tack did the job of suppressing the microphony.

As Sheldon says I can't back off the filament voltages as it is these that bias the tubes in this design. Re the ecc83 or ecc88 cascode stage....been there and didn't like it, far too clinical and "hi-fi" sounding for my liking. The ecc88 cascode was the worst offender, the ecc83 a bit warmer but still as boring.

I do however run 46s in my power amp. They are direct driven by triode strapped, choke loaded C3gs in a DRD/Monkey configuration.

This DHT phono stage does what none of the others I have had could do; it plays music. Couldn't ask for more really.

Steve
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Old 30th July 2009, 08:40 PM   #16
iko is offline iko  Canada
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Thanks for sharing Steve, I've had this phono on my "todo" list for some time and it's very inspirational to read your experience with it.
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Old 31st July 2009, 10:26 AM   #17
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Quote:
Originally posted by ikoflexer
Thanks for sharing Steve, I've had this phono on my "todo" list for some time and it's very inspirational to read your experience with it.
Thanks for the positive comments

All the tweaking is now done and below is a picture of the phono stage sitting between my amp and Lenco GL78 turntable. Tin can screening removed for the photograph
The PSU box can be seen bottom right of the pic.

Click the image to open in full size.

Steve
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Old 31st July 2009, 03:57 PM   #18
Sheldon is offline Sheldon  United States
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Very nice.

For shielding the input tube on my current phono stage (from Steve Bench design), I used some short pieces of copper tubing. They are attached so that they are electrically connected to the chassis.
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Old 31st July 2009, 05:15 PM   #19
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Sheldon

Thanks for the tip about the copper tubing, your phono looks great with them fitted.
I would never have thought of that.

Steve
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Old 31st July 2009, 06:00 PM   #20
Salas is offline Salas  Greece
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You can find ready made where they sell tube sockets. There are plain and golden ones.
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