Building Euro21 801A tube preamp

[euro21]

Without knowing anything about this R-core (for example p-s capacitance), it's hard to say anything.
In #16 the old EI may be adequate, if the secondary DCR enough low, and you use appropriate schematic (C-r-CMC-C or coke input with low DCR choke).
You should decide what you want.

 

[merlin el mago]

Could I measure the capacitances with a DVM meter?

 

[euro21]

Yes, and measure the DCR parameters too.

 

[merlin el mago]

EI measurements

 

[merlin el mago]

EI capacitances GND to GND

 

[merlin el mago]

EI measurements

 

[merlin el mago]

R-Core primary

 

[merlin el mago]

R-Core secondaries

 

[euro21]

140-160pF enough good (EI splitted bobbin).

 

[merlin el mago]

R-Core secondaries

 

[merlin el mago]

I guess with these power tx can't do input choke, only can do C-L-C or C-CMC-C, wich one better?

 

[euro21]

I now measured some 24V transformers (2.5...5A) with my -good- LCR meter (DVM gives only informative values).
The secondary DCR varies 0.28 ...0.1 Ohm.

I'm afraid, that this 24-0-24 EI core transformer with 2x0.9R secondary won't be good.

The R-core voltage is may enough, the p-s capacitance acceptable, must to try it.

I suggest to try PT-diodes-C-L-C with appropriate resistor as load: series connected 8R+8R 30-50W (later used as filament bias resistor).
For example 22V raw supply output as goal and 16R load =1.375A

If the raw supply PSU fulfil it (or greater), suitable for this LT power supply.
If the output voltage collapsing, must to search better transformer.

 

[merlin el mago]

I guess the LL1694 only can be wired in parallel for 3A so can't do input CMC with 1,5A right?

 

[merlin el mago]

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.

I guess the TME power tx isn't enough with 1,35A to power the 1,25A required for filaments!!!

Are you sure the RS power tx can be used to power the two filaments, have separate bobbin parts between primary & secondary also 50VA each secondary?

I'm afraid the two R-Core are too big for mine PSU box...

 

[euro21]

LL1694 as CMC capable 1.5A (2.4A saturation) after first capacitor as C-CMC-C filter.

If you use your old R-core 2x21V 120VA, this raw supply may enough good.

Try to set raw supply output voltage such level, which enough for R.C. regulator.
If you use V9, ask Rod for necessary headroom.
This sample schematic use 5V headroom for V7, V9 requiring lesser.
With varying the two small resistor value the necessary voltage may setting.

The first filtrating capacitor (C2) must be high ripple capable.

 

[merlin el mago]

I have R. C. V8

 

[merlin el mago]

Received by R. C. email about rules for filament bias:

Here are the Rules:

MAX V(out) = BIAS + 4.5V
MIN V(out) = BIAS + 2.5V

FilBias R = BIAS / (1,25 + Ia)

 

[euro21]

So 0.5-0.66R for R3 and R4 in #35 is OK.

 

[merlin el mago]

Today I will check if the PSU box have enough space for the two R-core.

 

[euro21]

You have two independent 21V AC in 120VA R-core transformer, so even one transformer is enough.