#26 pre amp

[mach1]

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?

 

[10y]

Forgot to share filament supply: two Rod Coleman's board that work great!!

Ciao,

Anto

 

[andyjevans]

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

 

[mogliaa]

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

 

[mogliaa]

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

 

[10y]

Andy,

what is the less microphonic in your experience? It depends from brands, structure or between same type is random?

I too have bought from usa vendors on ebay.

Regarding hum I must separate the two woods around 1,5 metres for a inudible noise.

Ciao,

Anto

 

[10y]

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

 

[mogliaa]

I've been running preamps for nearly two years now and haven't had any valve issues

 

[10y]

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

 

[mogliaa]

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

 

[10y]

Ale,
will do like you suggest

Thank you !

Ciao,

Anto

 

[DimZ]

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

 

[mogliaa]

Anto,
The rubber dampers are not cheap, but worth adding them. If you don't want to put the money on them, you can cut some rubber squares from a spare bicycle tube and add them between the screws and the base of the amp.
Time to experiment my friend!
Ale

 

[Magz]

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.

 

[Wavebourn]

I used mouse pads to mount sockets of "microphonic" tubes. Punched holes in the chassis bigger than tube sockets, mounted sockets on circles made from mouse pads, then mounted this construction on the chassis.

 

[mogliaa]

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...

 

[Magz]

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...

 

[Magz]

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.

 

[mogliaa]

Why? VR will keep the CCS node at same voltage. When anode voltage goes down, then more current is drawn, but difference is small to take VR out of regulation.
Can you explain your point?

 

[Magz]

From what I've read when I was researching VR tubes and their application, you're basically increasing the output impedance of the VR tube as seen by the 26, so you get poorer regulation. Will it be enough to make an audible difference in that circuit? I don't know.