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    Building, troubleshooting and testing of these amplifiers should only be
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#26 pre amp

I have a question, still at a deadlock on a design decision. I will be using a stepdown permalloy output transformer (parafeed) its a custom design from ElectraPrint but very sensitive picking up hum from the mains transformers. Really all opt at preamp levels pick up EMI (60 hz hum) from the B+ transformers, so I always use a separate chassis to separate the OPT's from any main transformers (need to especially with sensitive Grado's.)

But I haven't done a DHT build like this and adding the coleman regulators really adds complexity to a 2-case build.

It looks like with the SSHV2, we can have a larger distance between the regulator board and the amp section which helps. But the coleman regs still need to be in the amp chassis.

Is it OK to have the prefiltered supply to the coleman regs in the amp chassis share the same umbilical cord as the prefiltered B+ ?

My goal is one umilical cord. Has anyone found a means to do this with sub -90dB hum ?
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Can Electraprint put your custom transformers in a mu-metal can? That ought to help with noise pickup. The Lundahl LL1676s I'm using for parafeed output are in such cans being that they were originally designed as low level microphone step-up transformers, and they are dead quiet, even though they are next to a strong AC source in my preamp-on-a-board.
Just a thought...
 
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Magz said:
Can Electraprint put your custom transformers in a mu-metal can? That ought to help with noise pickup. The Lundahl LL1676s I'm using for parafeed output are in such cans being that they were originally designed as low level microphone step-up transformers, and they are dead quiet, even though they are next to a strong AC source in my preamp-on-a-board.
Just a thought...
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Its a good idea but we have no luck finding a can to fit these, do you know of a vendor who sells mu-metal OPT cans ?
 
regal said:
Its a good idea but we have no luck finding a can to fit these, do you know of a vendor who sells mu-metal OPT cans ?
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No, but you could make your own cans or shields using TI-Shield, which doesn't lose its shielding properties when it's bent or soldered. It's permalloy clad with copper on both sides. You can get a sheet of it from Michael Percy Audio, which will be more than enough. I have used TI-Shield in various audio components and it works well on both magnetic and electrostatic fields. Be sure to ground the can/shield.

http://www.emsclad.com/examples/emi-rfi-shielding.html


.
 
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So back to my 26 preamp.

My plan is to use the Bent Audio Slagleformers AFTER the output transformers. These are specified to handle up to 10V signal levels. I figure this will give several advantages:

1) increased signal to noise ratio as the preamp noise will be attenuated by the same amount as the signal, which does not happen with the VC at the input

2) increase in the effective input impedance of the following component by the impedance ratio of the autoformer, especially at low volume settings

3) decrease in the output impedance of the preamp by the impedance ratio of the autoformer, especially at low volume settings

My only concern is whether the LL1676 output transformer can handle the inductive load of the autoformer - has anyone here used this sort of arrangement? Any other watchouts from experience?
 
I use the autoformers at the output.
LL1660pp to Intact audio AVCs.
Very happy with the combination, and better than a pot at the input in my opinion.
My AVCs are around 20H, if I remember correctly, but can be stacked to higher or lower inductance.
 
Preferences for Suitable 26 HV Power Transformers

While looking for a suitable HV power transformer for the 26, would the Antek AS-1T175 toroid be a good choice? Antek - AS-1T175 It is 100VA with an electrostatic screen between the primary and secondary, and a magnetic shield around the outside, and has two 175V-0V windings at .28A so separate B+ supplies can be had for each channel. I have some EZ80and EZ81 rectifier tubes and was considering a hybrid rectified supply since it also has two 6.3V 3A windings. Or would a simple single LCLC tube rectified supply using the concentric wound Hammond 369EX (190VCT) Hammond Mfg. - Universal Primary - "Classic" Power Transformers be preferred over a toroid & dual supplies?
 
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 said:
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.
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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.
 
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-1...pt=UK_CCTV&hash=item231c0b341b#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
 
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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
 
I'm getting a fairly quiet shhhh sound from the LM1085 regulator
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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.
 
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