#26 pre amp

[Bas Horneman]

anufacturer Zetex (Diodes inc) ZTX868 is probably nearest equivalent.

Well if the 1051 does not need heatsinking..I guess it won't be a problem soldering it...since it is quite a big part I suppose.

[Original Burnedfingers]

Its too bad we don't have a group project going on this 26 preamp. A ready made list of the proper procedures and parts to buy to make this. I personally cannot made this preamp and end up with something that is hum free.

[andyjevans]

Originally Posted by Rod Coleman

Andy, I think the umbilical idea is perfect.

Have you tried locating the caps right up close to the transformer? I try to keep the wiring between the rectifier and caps (and the trafo) as short as possible, because the CURRENT running along those wires is a pulse train of 2 to 5A peak, with fast rise time and high-band recovery pulses (even with schottkys!). With long wires there is a risk of electromagnetic coupling into the amp.

I've started to put the reservoir cap for the filaments in the PSU box. Well actually I split it - 10,000uF in the PSU box and 10,000uf in the signal chassis. How about that?

andy

[Rod Coleman]

Originally Posted by andyjevans

I've started to put the reservoir cap for the filaments in the PSU box. Well actually I split it - 10,000uF in the PSU box and 10,000uf in the signal chassis. How about that?

andy

It'll make a difference for sure. Especially with a little bit of choke (ferrite will be fine, 1mH or more, 5A rated at least, to cover the peak current) on the output of the trafo box.

Then, to ride first class, use a feedthrough capacitor on the wall on the box to run the +ve through. Wire the box to the rectifier/cap neg using thick braid.

[Richard]

Hi All,

Rod, you’re a wealth of knowledge, thank you once again for taking the time to design a heater supply for us. I’ve already ordered the parts for this psu. Yay!

I've started to put the reservoir cap for the filaments in the PSU box. Well actually I split it - 10,000uF in the PSU box and 10,000uf in the signal chassis. How about that?

Andy, I’m doing something similar but I though about adding a pre-regulator, i.e. transformer – diodes 160000uf Lm7809, all in the filament psu box. But I’ll go with Rod’s suggestion.

Rod, is this circuit attached below correct? Does it need to be a metal box or is wood ok?

Also regarding your final all fet heater regulator, how close does it have to be to the valve, is 17cm too far?

Cheers,

Rich

[Rod Coleman]

Rich, the feedthrough cap looks like this:

Rapid Electronics - Electronic Components

I think Farnell have these too.

The two leads are B+ in and B+ OUT. The body and nut form the other terminal of the cap.

Feedthough caps only make sense if you have a metal enclosure that you are trying to keep electromagnetic pollution contained within. The enclosure can be partly meshed, to allow ventilation of your trafo. Use braid to wire the cap -ve to the enclosure. you don't need the braid all up to the preamp itself.

If you use a CCS and a gyrator (I think I'd use the FET gyrator now) and have enough headroom, the 7809 will not help much (unless you have mains that varies by 20% or something!).

I find regulators of this kind terribly noisy, and that won't help!

[Rod Coleman]

170mm is not really too far between valve and regulator - provided the leads are twisted. But 30-50mm is better.

[Richard]

Hi Rod,

I have gone with a wooden chassis as my metal working skills are poor and I lack the tools. But will add some metal mesh for looks

Is there a preferred value for this feed through cap? I suppose a ceramic cap will do. I’ve had good experience with those Russian PETP caps and there is a whole thread here on them. Is one limited with cap choice?
Petp Capacitors-one Of The Best?

Here is a pic of my high voltage psu.

Rich out

[Sheldon]

Your circuit diagram is wrong. You show a capacitor in series with the supply output. This will block DC. A feedthrough capacitor would be represented by a small cap from the output to the chassis, at the point where the supply passes through the chassis. A feed through capacitor is a wire that is surrounded by a layer (one or more) of dielectric that shunts RF from the wire to the chassis. DC current passes through the wire from one side of the chassis to the other.

Here's a description: What is Feed through Capacitor - Electronics Schematics Components Symbols

Look at the symbol at the top of the page, and imagine the leads from either side of the symbol connected to the chassis, and the through wire in line with your power supply lead.

Sheldon

[Rod Coleman]

Originally Posted by Richard

Hi Rod,

I have gone with a wooden chassis as my metal working skills are poor and I lack the tools. But will add some metal mesh for looks

Is there a preferred value for this feed through cap? I suppose a ceramic cap will do. I’ve had good experience with those Russian PETP caps and there is a whole thread here on them. Is one limited with cap choice?
Petp Capacitors-one Of The Best?

Here is a pic of my high voltage psu.

Rich out

Rich, the K73s will be a better choice than feedthroughs if you have a wooden box. With 1mH ferrite choke, 1uF or more will be useful.

FEeedthough caps are very effective at frequencies that leaded parts can't do much with |(>10MHz), but without a metal enclosure the HF/VHF/UHF may couple electromagnetically from the source of noise (eg rectifier recovery pulses) to the low level audio circuits.

Running the power from the trafo/rec/caps to the preamp via shielded cable can help, when a metal enclosure is hard to do.