Building Euro21 801A tube preamp

[merlin el mago]

Building Bela's aka euro21 801A preamp https://www.diyaudio.com/community/threads/801a-tube-preamp.395883/

I will use Ale's Moglia aka Bartola Hybrid mu-follower as anode load.

I will need to mod the Vout LT PSU for filaments, I have a couple of Lundahls Filament Current Choke LL1694 that can be configured as serial or parallel also common mode choke and choke input. Could be configured as choke input CMC?

More questions will follow.

TIA

 

[euro21]

Like this?

Con: relatively high LT voltage transformer needed, because LL1694 coil DCR is -relatively- high.

 

[merlin el mago]

Thanks Bela, I only own 2 chokes LL194 so can't do togheter choke input & CMC
30 VAC 100VA could be enough for the power tx?
I have CMC 14mH 27mΩ that I use for 26, could I use for 801A?

 

[euro21]

"30 VAC 100VA could be enough for the power tx?"
Yes, about 23-24V raw supply voltage is expected, if you use paralleled LL1694 coils as input choke.

 

[merlin el mago]

Not cheap https://es.rs-online.com/web/p/transformadores-de-montaje-en-pared/0504101
Better paralleled LL1594 coil as input choke than CMC Bela?

 

[euro21]

"Not cheap https://es.rs-online.com/web/p/transformadores-de-montaje-en-pared/0504101"
It's 2x 30V/50VA, so even enough for both channel.

CMC is good for common mode disturbation (generates rectifiers both on ground and B+).
Input choke generates less stress on secondary, less stress on first capacitor, so it's another animal.

 

[merlin el mago]

Isn't necessary two separate power tx for filaments?

 

[euro21]

More important is that primary and secondary isolation be realized.
See this Indel transformer:
https://www.tme.eu/es/details/ts40_041/transformadores-con-sujecion/indel/ts-40-041/

I usually use this type transformers with separated bobbin parts.

 

[merlin el mago]

For these I need two right?

 

[euro21]

Yes, you need series connected 15V secondaries for one filament, so two transformer necessary.
If the transformer -sales by RS- has enough good pri-sec separation, can be used to drive both channel.

 

[merlin el mago]

RS power tx has Isolation 4kV RMS & Efficiency 87%, is enough?

 

[euro21]

These are irrelevant values.
Ask RS for primary and secondaries disposition (is it has separated bobbin or not) or capacitance between pri-sec.

 

[merlin el mago]

OK thank you

 

[merlin el mago]

RS only have datasheet that I attach but I didn't see the info https://docs.rs-online.com/5556/A700000008747406.pdf

 

[merlin el mago]

I will look for in my transformers stash to see If I have something I can use.

 

[merlin el mago]

I have 2 R-core 2x21V 120VA and 2 EI core 2x24V 2A

 

[euro21]

The one on the right would be good (24-0-24 with two diode) ... but 24VAC not enough for this choke input, 22V DC output raw supply.
If you use this PT as there (C-r-CMC-C) will be OK.

p.s.
If you use 24VCT with very low (for example 0.1R) DCR input choke (and R.C. V9), the raw supply output voltage criterium accessible.

 

[merlin el mago]

I have others RS power tx from same brand and are double bobbin

 

[euro21]

That can be good too.
If you choose the one you want to use, try to measure secondary DCR.

 

[merlin el mago]

Bela will better to buy RS new one or use old R-core or EI?