New build: Princeton Reverb with mods - advice wanted!

[klooon]

Time to build a new amp! After building the 5G9 Tremolux, which is still going strong btw, it’s time for another low wattage tube amp. A head with both tremolo and reverb, and inspired by again some of the forum members here at the forum (e.g. tubeswell), it will be a Princeton reverb clone and the idea is ‘going back in time’. Starting from blackface and add some “old school” features:
1. cathode/fixed bias switch
2. cathode bypass cap switch in cathode bias mode
3. NFB pot
but also
4. add mid pot
5. reverb dwell knob
6. multiple speaker impedance 4, 8, 16 Ohm jack
7. tone stack bypass
8. fat switch

Here is the schematic that I made with the mods. This is the first time that I actively tweak a current design (after building kits and some amps from only the layout) – a nice challenge. I have been reading a lot and I think this might work, but please please do comment!!!!

[loudthud]

For a mid pot you want something between 10K to 25K and bypass the 6.8K resistor.

[klooon]

Thanks! Of course, it doesn't make any sense to keep the 6.8k here. Or, what do you mean here with bypass? The pot in parallel with the 6.8k or just get rid of it?? I would like to get rid of the scooped mids (at times) and have a more flat EQ like the tweeds. Would the 25k get me there???

What about the other values, do they make sense? For instance, the NFB pot value: would this be enough to get zero NFB? And, what about the bleeding resistor values for switching that I use (100k) and are they needed? Is this only against plopping or for safety reasons, to limit current rush?

[loudthud]

Replace the 6.8K with a short. If you use a 100K Audio taper pot, you will have useful mid adjustment over the first half rotation and the second half will remove the scoop. The Bass and Treble controls will be less effective however. See if you can find a schematic for the Kendrick Texas Tea control. The New Joy Zee amp has it. It's another pot that creates a shunt across the treble cap. Another thing to try is a pot in place of the "slope" resistor. Duncan's Tone Stack Calculator will help you visualize what changes can do.

[klooon]

OK! I can just omit the 6.8k now (I am doing a new build) and I will try the 100k Audio Taper, thanks for the explanation!

I have added this tea control to the circuit, see below, which could be a nice addition looking at what I like to achieve ("tweed EQ" next to BF). Is it much different from the mid control btw? Should I use this instead of the mid-control or is the mid control bringing anything more/different to the tea control?

Is the tonestack bypass switch still functional now?

For the slope resistor: I will try a few different cap values (next to 100k, I can try e.g. 47k or so) and see what it does, maybe I will hardwire this instead of another pot then).

Is the following circuit OK; did I understand the tt control correctly?

[loudthud]

The only potential problem I see is the fixed/cathode bias switch. When it is in-between positions, things can go crazy. There might be a big voltage applied to the cathode bypass cap, the grids might be floating or currents in the output tubes might spike very high. KOC did it a little differently, I don't remember if his version allowed the addition of the bias wiggle tremolo. I think he basically just shorted out the cathode bias resistor OR shorted the bias to ground. So when you moved the switch, the in-between position gave you the negative fixed bias voltage AND the cathode resistor which is safe for the tubes. If you only throw the switch when the amp is in Standby, you avoid the problem.

[Enzo]

I never think of the fixed/cathode switch as something one does on the fly, one should turn the amp off or at least to standby when switching modes. However, we converted a Peavey Classic 30 to switchable a while back, and flipping the switch didn;t seem to cause any issues, even while playing.


I think our notes are still available over at Blueguitar.org


That is what we did, we shorted the cathode resistor for fixed mode, and we switched the lower ends of the grid return resistors be tween bias supply and ground as appropriate. We made the bias adjustable, not just for the customer, but also so we could set the bias to more or less match the cathode bias level, so the switch only changed the dynamics, not also the levels.

[klooon]

Thanks for pointing that out loudthud and Enzo! And, I will definitely make the fixed bias adjustable. @ Enzo: Is the 'new' situation in the picture below the right one now?

[klooon]

Here is the new adapted schematic. I have changed the fixed/cathode bias switching.
Still a few questions:
1. Fixed/cathode bias switch: is this okay now? Did I understand you right, Enzo?
2. I saw that KOC put a 10k bleeding resistor in his cathode capacitor bypass switch. I assume this is fine here as well?
3. I put the FAT switch in V1: is this the right place? And is 100k fine for the bleeding switching resistor?
4. Is the Tonestack bypass (ground lift) still effective/functional in combination with the Texas Tea control?
5. Is the 50k pot for the increase of NFB enough to get me into the 'no NFB' zone?
6. I couldn't find an example of how to wire multiple output impedance jacks with NFB. Is this fine or are there other ways to accomplish this?

I am about to finish the layout as well, so after answers to these questions, I will be able to post this.

[Enzo]

Your bias switching looks OK to me. Not how I might DRAW it, but that doesn't matter.

I don;t know what we are doing with the multitap output. I generally stick the NFB on one winding and leave it there. The speaker changes tap to tap to match impedance, but the amp makes the same signal into the transformer regardless. NFB is a voltage. Ask yourself, "why would there need to be more NFB for a 16 ohm speaker than for an 8 ohm?"

I will say this though, if those speaker jacks are two-tip contacts, that is a clever way to switch. If there are two tip contacts, one wired to the transformer tap, the other wired to the NFB, then plugging into the jack shorts the two together through the tip of the plug. That results in selecting that NFB tap.


And allow me an observation. Having done a couple cathode/fixed switchable amp conversions, what almost always happens is that the owner then likes one a little better, and the switch never leaves that setting, once he has showed off the feature to his friends. The difference is really more of aq subtle thing in feel, so it doesn;t wind up as some sort of switchable feature like a boost.


Same thing with the reverb dwell. Sounds like a new day on paper, but really, all it can do is reduce the amount of signal into the reverb drive. And the majority of owners wind up with the dwell sitting at max. And we don;t need to make a hole on the panel to have that.

[tubeswell]

Hi klooon

See attachment

[überfuzz]

Off topic, slightly...

What editors are you fellas using?

[klooon]

Off topic, slightly...

What editors are you fellas using?

MS PowerPoint. Not accurate, but just handy as a sketch book, easy to copy/paste pictures, cover parts of an picture with a blank "mask"!

[klooon]

Hi klooon

See attachment

Thanks! That's the one I used (or tried) for the v001 and v002 above as far as the switching part is concerned.

What about the values? For instance of Ck and Rk?

And further, should I adapt the tremolo circuit (or values) to "fit" both fixed and cathode bias situations?

[überfuzz]

MS PowerPoint. Not accurate, but just handy as a sketch book, easy to copy/paste pictures, cover parts of an picture with a blank "mask"!

Ok, you could try some spice ware. Most classic amps can be found on the net. They you just have to edit to test your fancy mods/improvements.