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Old 15th October 2012, 05:32 PM   #2171
euro21 is offline euro21  Hungary
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Unhappy Vibration

Try E.A.R. grommets: E-A-R -- noise control, vibration damping, sound insulation, shock cushioning

McMaster-Carr sell this type: McMaster-Carr

.... but not outside of States.

"Due to the complexity of U.S. export regulations, McMaster-Carr accepts international orders only from our established customers. This decision also applies to orders shipping within the United States, because it is based on the final destination of the items. We cannot accept this order or future orders." :-(
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Old 15th October 2012, 05:38 PM   #2172
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by ikoflexer View Post
Ale, I just stated the fact. Whether a higher output impedance is of any consequence in your preamp that's another matter. Most likely not noticeable.

Magz, in my older implementation with the 156C plate choke I could clearly hear the VR noise as a hiss when no music was playing and the volume was way up. Didn't bother me much; and the warm glow was a winner in my book too.
Did you feed glow tube with a CCS? That is the only way you will get no noise (85dB rejection over the raw supply) from the glow stage.

Regarding the output impedance. Looking from the transformer side, the 10u cap is in parallel with 3k3 in series with the glow tube. The cap impedance is about 800 ohms at 20Hz. If neglecting the VR impedance then the equivalent is somewhere below 800 ohms which I can't see that negative for a preamp with quiescent current of less than 6mA...
Ale
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Old 15th October 2012, 05:55 PM   #2173
DimZ is offline DimZ  Greece
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Quote:
Originally Posted by euro21 View Post
Try E.A.R. grommets: E-A-R -- noise control, vibration damping, sound insulation, shock cushioning

McMaster-Carr sell this type: McMaster-Carr

.... but not outside of States.

"Due to the complexity of U.S. export regulations, McMaster-Carr accepts international orders only from our established customers. This decision also applies to orders shipping within the United States, because it is based on the final destination of the items. We cannot accept this order or future orders." :-(
similar Anti-vibration bushings are available in Europe too (Germany) ...

https://www.buerklin.com/default.asp...=ArtNr_12H3788

there you can also find threaded metal blocks, similiar to those Thomas Mayers is using for his excellent idea/design for suspending sub chassis ...

https://www.buerklin.com/default.asp...p=ArtNr_17H912
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Old 15th October 2012, 06:39 PM   #2174
iko is offline iko  Canada
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Quote:
Originally Posted by mogliaa View Post
Did you feed glow tube with a CCS? That is the only way you will get no noise (85dB rejection over the raw supply) from the glow stage.
I'm talking about the intrinsic noise of the VR tube, which is not dependent on what the VR tube is fed by.

Quote:
Regarding the output impedance. Looking from the transformer side, the 10u cap is in parallel with 3k3 in series with the glow tube. The cap impedance is about 800 ohms at 20Hz. If neglecting the VR impedance then the equivalent is somewhere below 800 ohms which I can't see that negative for a preamp with quiescent current of less than 6mA...
Ale
As long as you're happy with that, I'm happy.
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Old 15th October 2012, 08:23 PM   #2175
mogliaa is offline mogliaa  United Kingdom
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Only reason why I added the RC is to provide the necessary voltage drop to achieve the operating point I wanted. I will measure noise level when I get the chance to do so and will also test removing the RC. Bringing up the operating point for 30-40V won't make a big difference though.

I think if I don't experiment here I won't learn or potentially discover other ways of improving things. So far I have been doing everything that everyone else recommended here (and in other posts): filament bias, gyrators, OT, DC filament regulation, etc.

Cheers, Ale
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Old 15th October 2012, 11:47 PM   #2176
iko is offline iko  Canada
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Ale, in my opinion the entire community has benefited from your experiments. Great exploration!

Why not use a VR tube and a zener below it? 1N4757A (51V) or 1N4758A (56V) so you can get 126V or 131V out respectively? The zener can be bypassed with a larger cap if you want to muffle its noise.
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Old 16th October 2012, 02:54 AM   #2177
disco is offline disco  Netherlands
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Quote:
Originally Posted by mogliaa View Post
I think if I don't experiment here I won't learn or potentially discover other ways of improving things.
I'm with the disco-verer: To notice or learn, especially by making an effort.
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Old 20th October 2012, 08:47 AM   #2178
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What cable arrangement are people using for outboard DC filament supplies? I don't know if this topic has been covered - it's such a long thread. For connectors I use 4 pin XLRs and currently just twisted unscreened pairs of 1 metre length. I have 10,000uF capacitors inside the main signal chassis

So what kind of cable is best for this job? The DC supply in filament bias is actually in the signal path at the tube socket, though the supply is floating. So low noise and hum should be a priority. So what do I consider here:
- twisted pairs
- twisted pairs with screen
- 2 twisted pairs
- 2 twisted pairs in one screen

And so on....

Also, should the screen be connected at one end (which?) or both?

All recommendations appreciated!

Andy
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Old 20th October 2012, 09:35 AM   #2179
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Hi Andy, the actual implementation will depend on the components you use, and their location. My thoughts:

A 2-chassis solution is easier to make quiet. If you have 2 chassis, put all of the mains transformers on the power chassis (none at all on the valve chassis). All mains transformers will leak a little current into their frames, and any stray field from them will induce small eddy-current in the chassis. The effect may not be big, but if you want the best, it may add up to something!

The power supply chassis should also have the rectifier, and big reservoir caps. If these are mounted on the signal chassis, the big recharge current-pulses will transmit EM field all around the room!

The Regulator (I assume you always use them!) is best mounted on the signal chassis. This way, the filament exposure to common-mode pickup is minimized.

It only remains to connect the raw dc supply to the regulators. Twisting the cable will reduce differential-mode pickup, but with my regulators, differential mode noise will be highly rejected anyway.

Shielding the cables helps reject common-mode pickup, and so is desirable. common-mode rejection can also be improved by mounting a common-mode choke on the signal chassis (away from any choke or OPT), and shunting the noise to the chassis (using e.g 47nF 630V MKP stacked-type, or for +, one for -). In this case, connect the cable shield at both ends, and maybe even use a clamp-on EMC ferrite on the cable. Or, you could connect the 2 chassis together using some wide braided copper.

The safety earth should be attached to the power chassis by means of a bolt/nut that is not used to mount any other component or bracket etc.
A separate conductor should be similarly mounted on the signal chassis and connected to the safety earth. An EMC ferrite on this last cable may also be worth trying.

The HT B+ supply must be in a separate cable, preferably shielded, using cable and connectors rated for 440V ac rms.

Use fuses for both supplies, mounted in the PSU chassis.
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Old 20th October 2012, 10:01 AM   #2180
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Hi Rod!

There's a lot to digest there. Right now I just want to wire up a few cables for my filament supplies. So looking at cable types. My filament supplies may go up to about 2 amps but usually not over 1.5A so first decision is what thickness wire. I'm thinking 20AWG is sufficient - not sure about 22AWG.

Then if I ground the shield one end this is looking more like a 5 pin XLR. Easier for me to put this at the PSU end since all my amp chassis are wired up with 4 pin XLRs. Just ground the screen one end.

How necessary is the screen? And if it is necessary, then is grounding it at the PSU adequate?

Just trying to get the simple decisions right first. Just filaments here, not HT.

Andy
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