EAR834 - component questions...

[Jessica_K]

Adding a resistor 0.22R in one side of the heater wire will bring the 6.3 volts down if you wish although 6.5V is in tolerance.

The B+ is acceptable. But again depending on the combination of the zeners in the PSU swapping one for a slightly lower breakdown voltage would drop the 15 or so volts you need to be perfect

Robert

[netphreak]

Swapping just a random one of the 4 diodes?

Also - I noticed there is a slight variation in voltage: 298VDC to 300VDC, and 6.4VDC - 6.5VDC, but I assume that's expected. I'll try a resistor on the heating wire.

[Chris65]

You can also just replace one or both resistors in the heater supply on the PSU board to drop the heater voltage a little.
Two 0R33 or one 0R47 5W should work, assuming you have two 0R22 there now.

[netphreak]

There's two 0.22 resistors on heater supply, yes. Will try to replace them with a single 0.47, and see what difference it makes

There's 2 x in5373, and 2 x in5374 diodes - suggestions on which one to replace (with what) to get 300 down to 285 would be greatly appreciated! To be honest I'm slightly concerned to play around with such currents with my limited knowledge...

Thank you so much for sharing your wisdom in here, I think it's quite unique!

[Chris65]

There's two 0.22 resistors on heater supply, yes. Will try to replace them with a single 0.47, and see what difference it makes

You want an extra 22Ω or so resistance, replace both the 0R22 resistors with 0R33 or just one of them with 0R47. Then total resistance is increased to around 60-70Ω.

[Jessica_K]

Replace one of the in5374 with a in5373 if that is not enough replace the other. I run with all in5373's and I get 284V

Robert

[netphreak]

Just for the record - both in5374's replaced with in5373's  - which gave me 284VDC as well. Decided to leave the 6.5VDC - 0.2VDC is rather insignificant after all.

As soon as I receive the RCA connectors I ordered, we'll see if there's any audio in it

[Albertfifties]

Trying to follow: Is 0.000120 correct? Are you referring to the 110pF caps, and it should be 0.000110? Assuming this, here's some math:

R1: 2M resistor + 3.3M resistor = 5.3M resistanse (5 300 000)
R2: Feedback resistor = 750K (750 000)
C1: 330pF (0.000330)
C2: 100pF (0.000100)

Formula: (R1 x R2) / (R1 + R2) x (C1 + C2) = Time Constant in uSec

(5 300 000 x 750 000) / (5 300 000 + 750 000) x (0.000330 + 0.000100) = 318

I see the math but I am puzzled.
Both 2M + 3.3M resistors in front of the cathode follower are 'hidden' in a feedback loop. They are not what the feedback caps 'see'. So they would not work for the 'real' 318 uS pole.
It is funny (serendipity?) the outcome looks like an RIAA pole.

For me the feedback wordk as follows:
The feedback unit 110pf//750K+330pf go against the combined resistance of the input stage and loading by the grid resistor (Ra=220K//Rp'=300K//Rg=2M) calculated as something like 120k for hf and the coupling cap increases this a bit for the lf making an extra pole that reduces the gain at very low freq. [Where Rp' is the effective plate resistance calculated incorporating the cathode resistor of the first tube.]
So it is only the 2M Rg of V2 that the feedback 'sees'.

[steinkare]

Is it important to use 3w resistors for the 3,3Mohm. I am building a EAR 834 with pcb from ZeroZone. They are recommended 3w resistors for the 3,3Mohm and 2MOhm resistors, and also for some of the other resistors. Why can I not use 1w resistors. There are almost "no" power over these resistors, maybe 0,1-0,3 watts?

[Albertfifties]

as far as I know, 
- the higher value resistors (2W) are also higher voltage resistors, and inherently have a lower (mechanical) noise: the contribution to the noise floor is less, and even if they are within the feedback loop this should be as low as possible because the spectrum is wide and this gives problems precisely in the feedback because of the limited bandwidth and phase changes in that feedback, specifically when using tubes.
There is quite some voltage over them so it helps a lot.

[steinkare]

Thank You!