#26 pre amp

[Magz]

I'm going with a Hammond 374AXP. Potted, 360-0-360V @138mA, electrostatic shield, 5V 3A winding for rectifier, 6.3V 3.5A winding for the remote control boards. Choke input supply with Bendix 6106, LCLC, followed by VR tubes to drop to 225V. Simulates great in PSUD.

Total overkill? Perhaps.

Hammond Mfg. - Universal Primary - "Classic" Potted Power Transformers

[jamesdb]

Thanks all, I had missed that the Hammond 369EX has an electrostatic shield because it is not shown on the hookup diagram like the 374AXP is. After rereading the notes for the 369EX I saw the shield wire is internally connected to the bell, thus not shown on the schematic.

[Bas Horneman]

Verpas - Groothandel in doppen en kunststofproducten

Artikelen zijn per stuk leverbaar en in ieder gewenst aantal te bestellen. Volgens de website.
Translation: Articles can be ordered per 1 and in any quantity. According to the website.

[merlin el mago]

Originally Posted by Bas Horneman

Artikelen zijn per stuk leverbaar en in ieder gewenst aantal te bestellen. Volgens de website.

[Bas Horneman]

I've added a translation.

[triode_al]

Originally Posted by Bas Horneman

Artikelen zijn per stuk leverbaar en in ieder gewenst aantal te bestellen. Volgens de website.
Translation: Articles can be ordered per 1 and in any quantity. According to the website.

merci (thanx)
Of course i shot from the hip once again.

I'll try to revamp my old projects (as I have the tubes DC90, 3A5, 10, 10Y, 67) using good mechanical isolation this time.

[merlin el mago]

Originally Posted by Bas Horneman

I've added a translation.

Thanks for letting me know Bass

[andyjevans]

I did some playing around this week with a new preamp. My goal was to see what tubes could be used in filament bias with some standard commercial 12v DC power supplies I have lying around - rated 1 watt. I tried VT67. Very clear but treble a bit hard. 31 was equally clear and good overload - needed over 12v supply though. Treble was again a bit hard and the sound was faintly boxy. 01A was quite good but treble again a bit hard - I'm very sensitive to classical violins. ended up back with the 26, which was clearly the best of the bunch, and the least microphonic. 01A were most microphonic.

So I tried to find an operating point for a 12v regulated DC supply feeding a current reg. I tried the original Rod Coleman supply posted a while ago here. I couldn't get a margin of 6v which seemed to be needed to get it properly in regulation. I chose an operating point of approximately 105v anode, 3mA, starved filament of 800mA, 7.2 ohm cathode resistor (10+25 ohm), 6.3v bias on top of cathode resistor. With just under 12v DC supply I had about 4.5v headroom for a regulator. the LM1085 worked fine here, and the setup is playing as I speak and sounding fine, with the two series Hammond 156C anode chokes and teflon FT-2 caps. So this is a handy formula to know for a faster kind of build - it works. Here's the kind of thing -

http://www.ebay.co.uk/itm/4-X-12V-12...ht_1751wt_1099

I have a question I'd like an answer to - I'm getting a fairly quiet shhhh sound from the LM1085 regulator, audible at the speaker. Rod's regulator was silent. what's this all about? It's within regulation so where is the shhhh coming from - the DC supply or the LM1085 or what? The regulator is well within regulation - it starts to work OK at 10.5v supply. Grateful for ideas here. I think the PSU is linear rather than switched - it weighs 650g. Haven't looked inside - it's sealed with special screws. If I could take the case off I could put four in a box and get 24v twice. That would make it possible to use Rod's regulator and a nice operating point like 135v, 5mA, 10 ohm cathode resistor like I usually use. Otherwise four separate PSUs are a bit of a dog's dinner. I'm assuming I can run two together to get 24v.

andy

[dhtrob]

Hi Andy,

Interesting report! Only valve I liked more than the 26 is the 10...

I am using the 26 at a Va of 95Volt, biased through a 3.9ohm resistor, running it at ~6mA.. So using Rod's regulators I can do this with a 10 Volt supply on a not starved filament.

I had the same hissing sound when using other (vccs) implementations, only Rod's regs were quiet in my set-up..

Rob

[triode_al]

I'm getting a fairly quiet shhhh sound from the LM1085 regulator

Could this power supply be a buck regulator (sans the traditional power line transformer, that is) - those could give very high band noise. I have never tried such power supplies as I have read a lot of warnings against them.

another thought. A regulator uses zeners internally, while zener noise is very hard to get out of a system, is my finding.