Pre-Amp Build

[paul_b]

Looks good!

 

[Wntrmute2]

Any opinions as to switching the plate resistors from 47K to 33K by adding a 110K resistor in parallel? Allowing different B+ voltages to be utilized by just trying different VR tubes? 210V vs 150V.

 

[paul_b]

Higher B+ will allow you to use a higher impedance plate loading resistor. This will increase the circuit's open loop gain and decrease distortion a little bit. With 210V of B+, the plate loading resistors could go from 33K to anything between 47K and 53K and you'll end up with about the same cathode bias voltage. Power dissipated through those resistors will rise a bit, so a 47K/5W resistor would be my choice.

 

[Wntrmute2]

Sonically, do you think switching in a parallel plate resistor is a negative?

 

[Deke609]

I've been waiting patiently for Paul B. to suggest putting a CCS on the plate/cathode of the 76 tubes. I was sure that was one of his planned mods. Since you're now contemplating bigger loading resistors, why not try CCS? I'm no help there, but I'm sure Paul knows the necessary component values off the top of his head.

cheers, Derek

 

[paul_b]

I don't see any issues with paralleled plate loads and switches. Sure, a current source could be used as well.

 

[Wntrmute2]

In revisiting my calculations and @paulbottlehead's recommendations, I think I need higher wattage resistors than I used. Metal film resistors in 47K @ 5W are impossible to find. I can easily source Mills wirewounds in 5W as well as 12W but I believe I received advise to use the metal film resistors. I can parallel 3W 100K and 91K resistors to come up with approximately 47K. The watts needed to be dissipated by the plate resistors is about 3.5W.
Input?

 

[Deke609]

For longevity, I'd be inclined to have a combined wattage rating that is at least 3 X the actual dissipation. If you want to stick with metal film, why not 4 parallel resistors each rated for 3W: 2 X 200K and 2 X 180K.

cheers, Derek

 

[Wntrmute2]

I am not sure I need that much power capability. I redid my calculations and came up with about 0.3W.
Voltage drop 120V
Resistor value 47K

I can source Mills 5Watters easily enough. What do you think about non-inductive wire wound resistors or metal oxide?

 

[Deke609]

What is the wattage rating of the resistor(s) in there now? At 0.3W actual dissipation, 1W would be (barely) sufficient, and anything above 2W gives you plenty of excess dissipation capacity.

 

[Wntrmute2]

Paul specified 1 watt resistors but I sourced 2 watt ones cuz I always try to spend more money than I need to.

 

[paul_b]

Use a pair of 100K/3W resistors in parallel. 50K and 47K are certainly close enough to work properly.

Revisiting the calculations, I must have thought that the plate loads were already specified at 3W or something, as 1W is still adequate for these.

 

[Wntrmute2]

I'm building another of Paul's masterpieces to replace my George Wright little preamp. It will be like the first but with switchable B+ (I hope).
I'd also like to put in a second set of output jacks as I'd like the ability to run a sub-woofer or even just to try a different set of output capacitor flavor. Can I just wire the second set of RCAs in parallel?

 

[paul_b]

Yes, you certainly can.

 

[Wntrmute2]

Changes made in the red circled area to add a second set of output jacks. Do I need to run the feedback loop differently? Otherwise does this look OK?

 

[paul_b]

You don't need the extra coupling caps and resistors, just wire the second set of jacks in parallel.

 

[Wntrmute2]

So I need some help regarding cap choice in the feedback loop. I have the below caps available as well as a pair of used Hoveland Musicaps.
Thoughts or recommendations?
I'm using a pair of Ft-3 0.1uf as coupling caps between stages

 

[paul_b]

The K71 might be fun.

 

[Salectric]

If you’re going to use a cap in the feedback loop, I would connect it to the input side of your output cap rather than the output.

 

[Wntrmute2]

The K71 might be fun.

Did you mean the K75?