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#26 pre amp

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andyjevans

andyjevans

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Joined 2003
artsandmedia.com
prinzinio said:
I have tested several rectifiers like: Marconi 83, Fivre 5Z3, Fivre 5V4G, RCA 5Z3, RCA 5U4G ribbed plates, Philips 5u4G, Cunningham 80, Telefunken AZ1 globe mesh and Valvo AZ1 mesh.
5Z3 and 5U4G played a bigger sound, more lushed, IMO - oversized.
If I would not tried AZ1 - I would settled on Cunningham 80, however I found VALVO AZ1 globe mesh to be the most 3D and airy while keeping apropriate smoothness and musicality. Telefunken is IMO too analitical.
Click to expand...
I've been doing some rectifier rolling in my amp, which is 26 into EL12n. The 26 has high voltage Schottly diodes so the rectifier only affects the EL12n. However, the 26 is in there as well sound-wise. I tried GZ34 Sovtek, GZ32 generic, 5U4G Russian and a few 5V4 including Brimar 5V4G and Sylvania/GE 5V4GA. The 5V4GA was the best, audibly ahead of the rest, followed by the Brimar and the 5U4. B+ voltages varied between 308V and 326V but this didn't seem to be the deciding factor since both GZ34 and 5V4 were 326V. The 5V4GA has plates with 3 vertical slits in them similar but not the same as the RCA 5V4G/5V4GA. It was brighter than the rest, more alive with more detailed highs. I think this complements the warmth of the 26. Acoustic tone is excellent and I've been playing one track after another, enjoying the increase in clarity. I'd like to try the RCA version next. These are very good value. No silly prices since they go under the radar. 175mA max, but that's more than enough for me.
 
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Rod Coleman

Rod Coleman

Member
Joined 2004
The 26 runs very low current, so if you like rolling, you could try UF4007 Diodes. These have lower capacitance than SiC diodes, so the undamped energy storage is reduced.

Adding series resistance of 330 to 680 ohm to each diode will shape the forward current path as a vacuum rectifier does.

Or you can put a cheap TV PY8xx vacuum diode in series with the positive output of the rectifier.
 
andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
The 26 preamp I built for a friend still has hum issues which I need to eradicate. Unfortunately his speakers go down very low so any mains hum is more audible on his system. Plus he has a small room and sits close to the speakers. So the hum must achieve very low levels and be virtually inaudible. According to him the aluminium tube shields cut down 80% of the hum, so that's progress. Presumably that leaves the B+ and filament supply. I'm using an older version of Rod's regs which should be quiet and is quiet in my own system, but I really must convert to V9 since this is even quieter and better all round. The B+ could do with a Salas reg maybe. Never used one or any equivalent, though I have some glow tubes. The voltage drop means stepping up the B+ which I'd prefer not to do. The alternative is to add a second choke and more capacitance.

Looking over my notes I see that Rod has provided the following..

"The surest way to eliminate this kind of (common-mode) noise: a transformer with an electrostatic shield. The Antek AS-series is an Audio Grade transformer, and the 50VA or bigger versions have a shield. Other Audio-grade trafos made by Canterbury Windings and Air-Link in the UK, and Toroidy in Poland have shields and are designed for reduced flux in the core, which is also very helpful. R-core trafos are also likely to be good. But, Beware: If you still have a steel chassis, that may be confusing matters. And watch the cathode resistor - Inductive resistors act as antenna, producing more or less noise/hum."

The cathode resistors I use in filament bias are wirewound types situated above the top plate because of the heat they generate. That could maybe be an issue looking for an alternative solution. The cathode resistors get hot so a problem inside the case. All my cases are aluminium.

Ah, the trials and tribulations of getting a 26 preamp totally quiet. Can we hear from those who have achieved this?
 
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E

euro21

Member
Joined 2004
Paid Member
andyjevans said:
The cathode resistors I use in filament bias are wirewound types situated above the top plate because of the heat they generate. That could maybe be an issue looking for an alternative solution.
Click to expand...
I used Dale NH (non inductive) wirewound resistors with old (now very old) passive Pentium heatsink (from HP server)... it was a decade ago.

Grounding.jpg #26 preamp.jpg

The self noise is quite good.
#26 preamp self noise right.jpg
 
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euro21

Member
Joined 2004
Paid Member
For DH preamps I exclusively use filament bias ... if it possible.
Some DH tube biasing are problematic -for example 300B-, and some of them sensitive to vibration too, so not all are suitable for preamp.

This is my old #26 preamp schematic:
My #26 preamp.jpg

I used Stevens & Billington TVC as output volume control.
 
andyjevans

andyjevans

Member
Joined 2003
artsandmedia.com
Thanks Bela. I don't like solid state active loads, so I'll stay with a resistor load. And I'll try ordinary cathode bias with DC Link bypass caps and see what that sounds like. The resistor would be small and inside the top plate which should help with hum. The aluminium tube shields are a big step forward so it must now be possible to make the thing quiet for guys with sensitive speakers that go down low.

I looked for toroids with electrostatic shields but I don't think Airlink do them and Canterbury Windings is no more - the guy has retired. Toroidy would be expensive to import..... maybe there's some other UK supplier I can dig up.
 
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ErikdeBest

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Joined 2003
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