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Old 15th June 2005, 12:20 PM   #1
morfeas is offline morfeas  Greece
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Default Preamp hiss noise

In my latest pre 6DJ8 SRPP I have excesive HISS noise. It's clean hiss, not buzz, not hum.

Every few seconds the hiss is going louder.
On the scope the only thing I can see is some hi frequency stuff riding on top of <1mV of 100Hz noise. Scope is upto 15MHz.

Heater PSU is AC with one node thru 100nF to AC earth. Heaters are not on DC potential.
HV PSU is 220uF-25H-100uF-1K-100uF.


Circuit runs on 205V ~7mA/channel

Any ideas welcome.

PS: replacing the 6DJ8 with 6N1P reduces the hiss by 50%.
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Old 15th June 2005, 01:15 PM   #2
SY is offline SY  United States
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You've probably got an oscillation. A 15MHz scope is not adequate to see it.

Local bypassing and grid stoppers will work wonders.
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Old 15th June 2005, 02:11 PM   #3
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SY probably nailed it, but I once had a noise I couldn't figure out and then when I connected the heater to a positive voltage it went dead silent. So it turned out to be leakage from the heater. The old RCA books recommend offsetting the heater to positive 20-40Vdc, whether using AC or DC heater current.
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Old 15th June 2005, 02:35 PM   #4
SY is offline SY  United States
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SF, excellent point, and especially so if the top tube in the SRPP doesn't have its heater properly biased- running near or over the max heater-to-cathode voltage rating is poor practice.
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Old 15th June 2005, 02:55 PM   #5
morfeas is offline morfeas  Greece
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Thank you for the replies.

Have been using SRPPs without elevating the heaters and I got away with it. Maybe not this time, even though I'm still in the tube's max specs.

Will update you on this...
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Old 16th June 2005, 02:13 AM   #6
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It could also be a dead tube, I've got at least a few 6GH8's that make a mess when swapped into Frankenhouse. But that you tried at least one other tube, that's unlikely, especially if new tubes (the 6GH8 I speak of probably saw decades of a television, ugly service for anything ).

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Old 16th June 2005, 07:10 AM   #7
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Quote:
Originally posted by SY
SF, excellent point, and especially so if the top tube in the SRPP doesn't have its heater properly biased- running near or over the max heater-to-cathode voltage rating is poor practice.
Yes.

I solve the problem by using a seperate bottle for the "topside" and floating the heaters, bypassed to ground with a low value (~2.2uF) 350V 'lytic. That way if you do get a heater-cathode short, not a lot will happen (and your cap won't go through the roof!). Bottom bottle can be DC grounded, attached to the rest of the system string.

It's a method I've used for years and has proved quite foolproof
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Old 16th June 2005, 09:21 AM   #8
morfeas is offline morfeas  Greece
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Quote:
Originally posted by Geek


Yes.

I solve the problem by using a seperate bottle for the "topside" ...
Can you please explain?

Tried the following.

Metalic clip-on shield top of the tube, no imporvement.
Elevated heaters on ~50V, big improvement.
Local bypassing, no improvement.

Hiss is still there listenable on my 100db horns.

Will try ferite rings on grids, replace the volume pot.

Can it be a bad socket? I use some teflon, silver plated, British made salvaged from military scrap.
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Old 16th June 2005, 09:32 AM   #9
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'k, no problem

The noise is induced from the H-K, probably caused by near-breakdown levels of voltage on the top tube.

6DJ8 has two elements. Two tubes are requred for SRPP stereo operation (4 elements total). Rather than using one tube to handle each channel, use one as the L+R bottom of the totem pole and one as the L+R top. The "bottom" tube's heaters can be tied into the existing heater string. Add a small transformer whose sole purpose is to feed the heaters of the "top" tube. Leave the leads isolated from DC, but not AC with the capacitor.

Here is an example from my Akai tuner I "tubeified":
http://geek.scorpiorising.ca/contrib/Geek/6N1P_srpp.png

The 6N1P on the bottom has a DC grounded heater, the 6DJ8 ontop, has an isolated one.
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Old 16th June 2005, 09:42 PM   #10
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ecc88 SRPP topology, [about 25-30db gain with bypass capacitor] and an amplifier very sensitive, with 100db horn speakers ....if the tubes has no problem,, i think it is normal some hiss,,,,,
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