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Old 15th April 2008, 10:18 AM   #1
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Default phono stage hiss

Hi all,

I fixed up a tube phono stage for friend. It was in bits, based on PCBs but all the parts were intact. So anyway I got it up and running and I ran a low level signal through it last night from an ipod just to see if everything was connected properly. I got back a distorted signal - which is what I expected since I didn't use an inverse RIAA signal.

So anyway, this thing uses 2 x EF86 and 2 x 12AX7 and I would bet it based on a fairly standard circuit.

My question:

I've sorted out the hum part, but there is a fair level of hiss/rush from this phonostage. Is there much that can be done with that?

I have some 5751 that I can sub in instead of the mullard 12ax7 thats in there now...

Fran
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Old 15th April 2008, 05:11 PM   #2
kevinkr is offline kevinkr  United States
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Before you do anything try it with the appropriate phono cartridge or a resistor having the same dcr as the cartridge used. You could also short the input to ground to do this although this returns the most optimistic result.

Replacing the 12AX7A with 5751 may or may not make any difference. I rather suspect the EF86 is the first tube in the signal chain and this will establish the overall noise floor of the phono stage.

Can you post a schematic?

Slightly lowering the heater voltage may reduce thermal noise from the cathode, worth a shot. Carried to an extreme it may result in dull sound, and shortened tube life if the cathode emission is compromised at the operating point used. Voltages as low as 11V/5.5V have been used in vintage phono stages for this reason.
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Old 15th April 2008, 05:23 PM   #3
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Hi Fran,
Changing tubes is the first thing to try and something that you've probably already done. After that, suspect the plate load resistor(s), especially those in the first stage. Try replacing them (or it) with a new one of the correct value. Also check the old one to see if it has increased in value, which is not uncommon, and would increase the gain of that stage causing more noise.

Victor
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Old 15th April 2008, 06:16 PM   #4
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OK,

thanks for the suggestions. I don't have schematic to hand but will try to dig one out. I think the heaters might be a bit low already - 12V I think, but I need to leave it on for longer and test again.

Part of the problem is that I don't have a MM cart set up - and I don't intend setting one up either!!

Might see if theres something handy lying around I could use...


Fran
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Old 15th April 2008, 07:55 PM   #5
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Quote:
Originally posted by woodturner-fran

thanks for the suggestions. I don't have schematic to hand but will try to dig one out. I think the heaters might be a bit low already - 12V I think, but I need to leave it on for longer and test again.


Fran
fyi - EF86 uses 6,3V filaments. I'm assuming you're using 6.3 for both the EF86 and 12AX7.
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Old 15th April 2008, 08:06 PM   #6
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dodgy cap somewhere?
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Old 15th April 2008, 08:59 PM   #7
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Theres 2 outputs from the PS, a 12V one for the heaters and a 285V one for the rest.

I think what they did was to wire the EF86 in series and then the 12AX7 paralleled. Although I need to sit down with this thing a work through it a bit better.


Fran
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