Anto -
Get a selection of 26 tubes. They vary a lot in how microphonic they are and how much they can hum. You probably need to choose some good ones. There should be a lot on ebay.com. There is usually plenty. I get mine from US dealers - I cleaned out my usual guy!
Andy
Get a selection of 26 tubes. They vary a lot in how microphonic they are and how much they can hum. You probably need to choose some good ones. There should be a lot on ebay.com. There is usually plenty. I get mine from US dealers - I cleaned out my usual guy!
Andy
Problem to solve in the next:
1) great microphonic effect.... all noises generated near pre will sound on speaker
Ciao,
Anto
Ciao Anto,
Look at the rubber dampers here. DHTRob posted a link before if you are interested in contacting the supplier:
http://www.bartola.co.uk/valves/wp-content/uploads/2012/10/IMG_8680.jpg
Try to starve the filaments. A slight starvation (e.g. 900mA) may help reducing the microphony. Also try using 26 instead of 226, as the latter tend to be more microphonic
Cheers,
Ale
I'm wondering why there is an 330 ohm RC stage after the shunt reg (which would have op impedance of approx 90 ohms according to S. Bench's site).
Surely lower op impedance is better?
I just wanted to tweak the anode voltage and provide additional decoupling. Also wanted to try Morgan Jones' caps!
Ale
Ciao Anto,
Look at the rubber dampers here. DHTRob posted a link before if you are interested in contacting the supplier:
http://www.bartola.co.uk/valves/wp-content/uploads/2012/10/IMG_8680.jpg
Try to starve the filaments. A slight starvation (e.g. 900mA) may help reducing the microphony. Also try using 26 instead of 226, as the latter tend to be more microphonic
Cheers,
Ale
Hi Ale,
I was thinking about similar rubber support, thanks for your confirmation.
Starving the filament could shorts tube life?
Ciao,
Anto
I've been running preamps for nearly two years now and haven't had any valve issues
OK, will try tomorrow to reduce filaments voltage to 1,2-1,3 V.
Where do I find those nice rubber's base ?
Anto
Anto,
Reducing filament current will increase Rp and impact the operating point. Don't know what circuit you are using, but try and listen. You may take the bias point far too out and the distortion may increase. I'd suggest to try adjusting the Coleman regulators and testing microphony and sound of the preamp. Then you can measure new bias point.
Re the rubber suspenders I got them kindly from DHTRob. There is a seller in Netherlands, they are not cheap though. I think Thomas Mayer may supply a different set of dampers if you fancy that alternative. Otherwise search this thread as the link of the supplier was posted time ago.
Ale
Reducing filament current will increase Rp and impact the operating point. Don't know what circuit you are using, but try and listen. You may take the bias point far too out and the distortion may increase. I'd suggest to try adjusting the Coleman regulators and testing microphony and sound of the preamp. Then you can measure new bias point.
Re the rubber suspenders I got them kindly from DHTRob. There is a seller in Netherlands, they are not cheap though. I think Thomas Mayer may supply a different set of dampers if you fancy that alternative. Otherwise search this thread as the link of the supplier was posted time ago.
Ale
for the rubber suspenders take a look here
PB Fastener Gewindepuffer 110004 Außen-/Außengewinde Schwarz im Conrad Online Shop | 546346
and here
4x Anti Vibration Damper Isolator for FPV Aerial Photography GIMBAL ECILOP XA650 | eBay
PB Fastener Gewindepuffer 110004 Außen-/Außengewinde Schwarz im Conrad Online Shop | 546346
and here
4x Anti Vibration Damper Isolator for FPV Aerial Photography GIMBAL ECILOP XA650 | eBay
I wasn't aware of this - it could be useful since you can adjust the final voltage more accurately. What's the minimum size resistor to give adequate separation?
Also it could help with strike voltage if the HT is restricted.
Putting a resistor of that size in series with the VR tube will kill the regulation. If that doesn't matter to you, then go for it.
Putting a resistor of that size in series with the VR tube will kill the regulation. If that doesn't matter to you, then go for it.
Resistor is not in series. VR is fed by CCS directly and RC is fed out of it...
Re: microphonics, if you look back a while in this thread, you can see what I did. I mounted teflon sockets on a heavy support I made from two 4mm thick aluminum plates sandwiching a layer of soundcoat damping material, then mounted the support to softwood brackets using Deflex bushings and washers.
I used wood to mount them because I wanted dissimilar acoustic impedances, I.e. aluminum-wood-aluminum-soundcoat-teflon.
In any case it worked great. I can tap on the shelf or even the preamp itself and not hear anything through the speakers. A lot of work, though...
I used wood to mount them because I wanted dissimilar acoustic impedances, I.e. aluminum-wood-aluminum-soundcoat-teflon.
In any case it worked great. I can tap on the shelf or even the preamp itself and not hear anything through the speakers. A lot of work, though...
Resistor is not in series. VR is fed by CCS directly and RC is fed out of it...
It looks like it's in series between the VR tubes and the 26 to me. As far as I know, that hinders the ability of the VR tubes to regulate the B+ going to the plate. If that doesn't matter to you, then no problem.
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