Questions/Feedback- Modified Princeton Reverb Build

[Colinblakely]

Hi All,
I would be grateful for advice from those more knowledgeable than I!

Several years ago I built a modified Deluxe Reverb. I added a Type 2 PPIMV, PPI Treble cut and Robinette's lead channel mod. I really love the sound and range of the amp- from classic DR cleans to really nice preamp overdrive at reasonable volume levels. Now, I'm interested in creating a Princeton Reverb with a similar range. However, with only one channel, I'm trying to incorporate the lead channel mod via switching.

I'm attaching the schematic and layout I've worked on. I ended up removing the tremolo so that I didn't have to cram in the other mods. A few questions:

• Keeping the reverb but deleting the tremolo, how do I go about calculating the value of the voltage dropping resistors? I'm still working on understanding the math behind all this. Also, are there other missing modifications I need to make to account for in deleting the trem?
• I have limited experience with switching and want to make sure I've set things up correctly in my schematic and layout.
• Any general feedback on the concept and design of the circuit would be appreciated.

Thanks!!
Colin

[dogburn]

I can't comment on most of your switch mods, but I think the raw switch looks good. But what's that 1M resistor on the switch for the V1a cathodes?

Since you deleted the tremolo, you might want to do a one-tube reverb here and that way you can eliminate one whole tube.

[passaloutre]

I don’t care much for the Vox cut control, but some people do, so go for it.

I think your preamp gain switch will be a nice second voice.

On “Raw” switching, I’ve had better luck, rather than completely eliminating the ground reference for the tone stack, adding a big resistor in series with the 6k8. 100k is enough for me but you could try a 250k or 1M. Variation on this theme is to make it a potentiometer, so you can dial in your rawness.

[Colinblakely]

Thanks for the quick replies! Dogburn, I am not familiar with a one tube reverb design, but I will do some research. I would love to drop a triode so I'm not leaving one wasted. I never get past 3 on my DR reverb (even with a 12AU7 driving it), so a single tube reverb might work even better.

I found information suggesting the 1M resistor would keep the capacitors on both sides of the switch charged and thus reduce the likelihood of popping when I switch over.

Passaloutre, I have a 250K raw pot installed in my DR in lieu of eliminating the ground reference, but I find I'm only ever using it at 1 or 10. I'll experiment with a fixed resitor instead.

[dogburn]

Here's a thread discussing Lectroid's Princeton, including schematic:
https://el34world.com/Forum/index.php?topic=32694.msg362944#msg362944

I can share the layout I created for my project later in the day.

[dogburn]

Here's the DIYLC file, based on Rob Robinette's layout file and modified for one-tube reverb, no tremolo, NFB pot, Mid control, plus a few other tweaks.

[Colinblakely]

Thanks for providing that, dogburn. You changed the second voltage dropping resistor from 18k to 10k. Does that give you the correct C and D voltages without trem or the 2nd reverb driver? How did you calculate that?
Thanks Again,
Colin

[tubeswell]

How did you calculate that?

Ohms Law:
Voltage = Current x Resistance

Which, rearranged for Resistance is:

Resistance = Voltage/Current


Or for Current is:


Current = Voltage/Resistance.


Work out the cathode current for each gain stage (this tells you the DC going through each stage from the High Tension supply to the ground return)

Add up the current being pulled through each HT supply resistor.

If you know the current being pulled through each supply resistor (and knowing a resistance of each resistor), you can work out the voltage that will be dropped across each resistor.

See the example diagram at the bottom of the page on this link:

https://valvewizard.co.uk/smoothing.html

[dogburn]

I should have mentioned that I built this in a Fender Super Champ X2 chassis and cab I got via ebay and it came with an unidentified vintage power transformer. The seller had been planning to use all that to do a Tweed Deluxe project but abandoned it.

So I had to measure the transformer output and then figure from there what to do regarding dropping resistors and choice of rectifier tube. Of course, I ended up changing things along the way - I wasn't sure if the PT could handle a 5U4, and so was planning for a 5R4, but eventually went with a GZ34/5AR4, which was just right.

Anyway, the dropping resistors I ended up with may not be ideal for you if you've got a standard PR power transformer. Otherwise, the rest of the circuit should be suitable. I tweaked the reverb section to be more deep/intense than others, since that's what I prefer.

[Colinblakely]

Thanks for the explanation and link, tubeswell. I have several amp books that I have not been able to get through because they move way too quickly explaining the theory. Merlin's explanation is more accessible than any others I have seen. I just ordered his book.

dogburn, I noticed you changed many of the capacitor values in the reverb circuit. Is that specifically to account for the decrease driver gain or just modifying the circuit to taste?

[dogburn]

Modified partly for taste, like the 2.2M mixing resistor was to allow for more reverb compared to Lectroid's schematic. The 220uF bypass cap was for cutting down on hum in the reverb circuit (a suggestion I saw elsewhere on this discussion board). If you are interested in reverb that is more tame than the Fender circuit, go with Lectroid's values.