Building Euro21 801A tube preamp

[merlin el mago]

Yes, you are right.

 

[merlin el mago]

Bela, R.C. told me is better to use 0R82 but I see you use 1 ohm for R1?

 

[euro21]

If you have 0.82R, use it.
R1=1/Ifilament

Some years ago I bought box of 1R Mills MRA12, so I used its instead of 0.82R, which was in the kit.
V7 was capable working at 90% potentiometer value with 1.25A.

 

[merlin el mago]

Nice resistors.

 

[merlin el mago]

Work in progress Vin R. C. 25VDC, same PSU as 26 C-L-CMC-C, PSU needs resistors to lower the Vout to 24VDC or maybe 23VDC because the attached pic is the max. attenuatio can gives the trimmer. First to try the valve I used a 50W 6R8 power resistor.

 

[merlin el mago]

Right PSU C-L-C-CMC-C

 

[merlin el mago]

Ordered a couple Rod Coleman V9 regs. for filaments.

 

[merlin el mago]

Ordered to Mouser the non inductives 8R 50W for filament bias and some more material.

 

[merlin el mago]

Just arrived the Vishay Dale NH-50 8R 50W 1% no inductives for filament bias 🤗

 

[merlin el mago]

Only to experiment, what value of anode load resistor & output tx are required?

 

[euro21]

#26 has 7k6 output impedance at 135V, 5.5mA, -10V.
2x, rather 3x anode load required.

3x:22k if R loaded, 180H if choke loaded.

 

[merlin el mago]

Thanks Bela, and for 801A: resistor, choke & OPT?

 

[euro21]

Oops, I lost the tube type.

Depends of the operating point.

8-12k or 64..95H ... but anode current is much larger (for example 25..30mA), so dissipating surface (heatsink) is large.
Size of the choke/transformer (large current+large inductance==larger core, larger current==thicker wire==larger core==larger weight) would be large too....the price would go to the sky.

In the R load case the B+ would be uncomfortably large.

Both of them has negligible PSRR, so regulated supply is needed: cost, cost, cost...

BTW when I tested it -for example- only with 100H choke/OPT as anode load, it was much more worse (higher THD, changed -not so good- FFT spectrum), than with higher impedance load (gyrator/CCS).

p.s.
If you use this tube with R or choke load mode, the output impedance will be practically unusable large (even over 5k).

 

[merlin el mago]

What about a CCS + OPT?

 

[euro21]

What's the goal of using parafeed OPT in preamp?
Here it only means disadvantage: costly capacitor, costly transformer, degrading bandwidth and distortion.

 

[merlin el mago]

Lowering the output impedance and keeping separate paths for DC & AC?

 

[euro21]

"CCS loading of output stage"
Apples and oranges....

It's totally meaningless in case of 801a (mu=8) as preamp.
You should decide what you want.

 

[merlin el mago]

You are right only for power amp.

What about for headphone amp?

 

[merlin el mago]

I'm sorry Bela you are right, low mu + OPT left the lineamp with a little gain, no gain or negative gain.🙏