Component selection - PRRI rebuild

[mellotron]

Hi all, first post here. I am stripping the circuit board out of my 65 Princeton Reverb Reissue and installing a custom eyelet board with a few minor modifications. I have a couple questions before I place my order from hoffmanamps.com.

1) I will be using individual F&T filter caps for my build. Following the 40/20/20/20 convention for a cap can, since I cannot get a 40uF for the reservoir cap, would you recommend 47uF or 30uF in its place? Why one over the other?

2) It was suggested that I lower the V4A trem tube cathode bypass cap from 25uF to 5uF, to speed up how quickly the tremolo responds to knob adjustments. Doug does not carry a 5uF/50V cap. Capacitance-wise, I can use the 4.7uF/100V or the 8uF/150V in its place. Will there be any ill effect from using caps with a higher voltage rating?

3) What do you suggest in place of a brass grounding plate between the pots and the chassis? For future serviceability, I do not wish to solder buss wire to the back of each pot. Can I just run buss wire from the four eyelet board preamp locations, bend like the brass plate, and securely smash each buss wire between a pot/jack star washer and the chassis? (With my layout, this would be in line with Reverb, Treble or Bass, Volume, and an input jack.) Or does Doug sell a brass plate and I have just not found it? (A search for "brass" resulted in a couple of knobs and turret lugs.)

Thanks for the help!

[sluckey]

1) I will be using individual F&T filter caps for my build. Following the 40/20/20/20 convention for a cap can, since I cannot get a 40uF for the reservoir cap, would you recommend 47uF or 30uF in its place? Why one over the other?

I recommend 47µF because it offers better filtering.

2) It was suggested that I lower the V4A trem tube cathode bypass cap from 25uF to 5uF, to speed up how quickly the tremolo responds to knob adjustments.

That's not true. Keep the 25µF (or 22µF). Or, you could replace the cathode resistor and cap with a red LED for a stronger tremolo effect and also a pretty flashing light.

3) What do you suggest in place of a brass grounding plate between the pots and the chassis? For future serviceability, I do not wish to solder buss wire to the back of each pot.

I like to float a buss bar behind the pots and use a single connection to the chassis near the input jack. Some people just connect the buss to the ground lug of the input jack (Switchcraft type). The connections from the pots will hold the buss in place. Look at this pic for an example...

     http://sluckeyamps.com/supro/supro_02_big.jpg

[uki]

Welcome to the forum!

1 - 47uF is fine and way closer to 40 than 30.

2 - Cathode voltage is usually low, that is why caps in that position usually have 25v rate or so, 100 or 150 is not really necessary, 50v is way more than what it take. Don't use less that rated.

3 - The buss wire in the back of the pots will make things a lot easier, it does the same thing as the brass buss, probably better, less wiring going around. Just contact with between washers and pot isn't a good way, washers may get loose with time...and ground problems start to show up.

... With my layout, ....

It is better to don't get creative with layout, follow proven working layouts, they are the way they are for a good reason. Follow a known working layout.

Creative layouts usually end with lots of bad hum!!! 

Separate the preamp ground from the power section ground.

About the tremolo speed I dunno the answer. But someone with more knowledge might shine in and clear that for you.

Posting the schematic that you are following, pictures or any information about the amp, helps people to help you.

Oh I was a bit slow, Sluckey beat me on this one !

[mellotron]

Thank you both. My minor changes are really minor.

I started with Doug's Fender eyelet board layout, as well as a tweed four-cap filter board. I put the filter board to the left, then merged the eyelet board next to it. I effectively have the doghouse filter board, bias board (three components above the filter caps--PRRI trim pot remains on the chassis, along with a new 27k resistor) and main board all rolled into one.

My board is the exact width as the PRRI circuit board. In my diagram, the ten black dots indicate where the existing screw holes are, which I can reuse.

In the attached, I have circled the two circuit changes. First, I spun R17, the 56k phase inverter plate load resistor, to attach to the + end of the final filter cap, the D / Y node. The circled eyelet labeled C will be wired to the unused C / X node. To experiment with the Stokes mod, all I will have to do is rotate the 56k resistor from D over to C.

The second change was to add a 470k grid stopper before the phase inverter. Originally, I had the new eyelet (that is currently below eyelet B) below eyelet C. I am worried about inducing hum due to this change, but went with the circled option because it is closer to the V4 socket. In either position, the wire from the end of the 470k will cross over another wire at 90 degrees; it really made no difference where the new eyelet was placed. If I don't like the sound of this grid stopper, I can just move the wire back to its original location at the bottom of that 1M resistor.

The only other changes are component values, not positions.

(Edited for spelling)

[PRR]

It was suggested that I lower the V4A trem tube cathode bypass cap from 25uF to 5uF, to speed up how quickly the tremolo responds to knob adjustments. Doug does not carry a 5uF/50V cap. Capacitance-wise, I can use the 4.7uF/100V or the 8uF/150V in its place. Will there be any ill effect from using caps with a higher voltage rating?

I don't know why C2 needs changing. Go ahead and mess with it. But if C2 is too small, it will quit its wobblings at low settings of the Rate pot.

Electrolytic caps are not precision parts. The value *may* decay with age. It is fairly common to pick a high value so it will work a few years. (Less so for penny-pinch commercial amps, but that's not why you are here.)

The static voltage is given on the plan as 2.3V. This is rather tightly controlled by the available B+ (nominal 425V) and the tube Mu (nominal 100). If the cathode rises past 4.25V the tube is cut right-off; which makes cathode voltage fall.... with 12AX7 in there we can be very sure of 2V to 4.5V, and probably right around 2.3V. (A 12AU7 in the socket may float to 22V, but also lacks the gain to wobbulate, so you'd hope the user would go back to a right tube.)

So 2.3V cap spec.

There "can" be dynamic variation with wobbles, so we might double the predicted value, but that is what the cap is supposed to *prevent*. Still, if cost is trivial, we might double, 4.6V spec.

If a e-cap is run right AT rated voltage, life is less than if you over-buy. We can't afford to do this on 445V nodes (note Fender specced 500V caps there), but at few-Volt a few more Volts is not expensive. Double again.

9.2V is really plenty ample. (25V or 50V covers idiot users who leave a 12AU7 in there for hundreds of gigs.)

You can find 22uFd 16V e-caps in lots of consumer gear. That old cassette deck in the attic may yield dozens of e-caps in the 1uFd to 470uFd 6V to 35V range. If it's been in the attic for years, half the caps will be bad, but in this circuit nothing will blow-up. Just won't wobble.