Need advice for simplified blackface style 6V6 amps

[imaradiostar]

I'm in the process of building a pair of identical blackface-style amps. I have a pair of PT's and OT's that are approximately the size, ratios and voltages of a Blackface Princeton. I want to build some small simple little amps that I can use in place of some of my larger equipment that's currently serving this purpose. I want the minimum of components and weight while still achieving a Blackface sound. The reverb needn't be a surf verb sound (not too much drive) but I'd like to have a decent reverb volume.

I have two questions- one revolves around reverb drive and blending, the other around phase inverter design. I intend to use three dual triodes, two 6v6's and a 5 volt tube rectifier. My chassis (already built) are 11.5" x 5" by 1.5" and I'd prefer to use them if I can so simpler is better! If this fails I'll upgrade to a 13" hammond enclosure but I want to keep it as small as possible.

I want to simplify the reverb to reduce my tube count and eliminate the 3.3M resistor- I don't know why but it always bugged me! I am planning to use a blackface preamp followed by some sort of resistive mixer into the phase inverter. I'll drive the reverb from the standard 500pf cap into a 1meg pot to allow verb drive adjustment. I am still considering using a mosfet verb driver. I would like to use a 12DW7 low-gain side as the reverb driver. I was considering using it like a Magnatone or Ampeg with a largish plate cap driving a high-impedance reverb tank. I have a few tanks laying around the should work well for this.

I'm most concerned with reverb return. I was thinking that I could use a 12ax7 in the LTPI for a little extra gain after reverb recovery. The single triode (12ax7 or 12dw7) I use for verb recovery will be setup for highest possible gain. I'm considering a simple 1M linear pot panning between the output of the preamp and reverb recovery stage, possibly with extra series resistance as needed to make the control usable. I don't need to turn off verb with a knob because I'll mute it with a remote footswitch activating a relay or LDR on the send or return circuit. I'm hoping to have more than enough gain for pretty sounding verb while retaining a tone similar to the "normal" channel of a BF amp but with a little less gain.

Of course...this assumes I use a LTPI. Can anyone out there convince me that the Princeton circuit would be a better starting point? What would be the clean volume difference of the two? My PT and OT are only good for about 15 watts, similar to a BF Princeton.

Thanks, hope everyone is enjoying their Satuday night!

jamie

[Don Moose]

I'll mute it with a remote footswitch activating a relay or LDR on the send or return circuit.

Like the way the original fender reverb mute works, mute it at the recovery stage - that's where all the noise is going to come from if you try muting at the drive end.

Your other concerns ... a MOSFET reverb driver will reduce your tube count, and may let you add a DRIVE control in a practical way. I think the Fender external reverb unit calls this a DWELL control.

That 3M3/XXpF cap as a reverb mixer always made me cringe, too. I like the idea of injecting the recovered reverb at the 'other' input of the PI. I do sort of wonder if you're going to wind up with a ton of extra gain from that third triode without that divider of 3M3/(470k+REVERBPOT).

Hope this helps!

[imaradiostar]

Thanks for the reply Don- I think I wasn't clear! I don't plan on having a third triode as a verb recovery/blend triode so I was more worried about having too little gain, not too much.

I'd read somewhere here that the signal change through that 3.3m resistor driving the resistors in the verb circuit essentially makes for a -27dB drop. The recovery stage after it (assuming a gain of 100) is going to gain 35dB if I'm doing my math correctly. Why should I throw away all of that gain and then just amplify it again? The first verb recovery tube feeds a voltage divider that kills about 70% of the signal and this isn't accounting for the lowish reverb settings most non-surf musicians use.

I've built stuff like this before without trying to refine it very much and I think I could make it sound really decent with a little bit of tweaking. The PI has an exceptionally high input impedance so I can hopefully use this to my advantage. With careful gain staging I should be able to have almost equal gain to the BF circuit but without the added recovery tube. Most people use their reverb on 2-4 anyway so why do I need all of the gain of the BF circuit?

If this fails there's always the option to add a little tiny circuit with a jfet and a zener diode for reverb recovery and use the normal high-gain triode blender. Maybe I'll find that that goofy big resistor really makes all the difference.

Would you use a single mosfet for drive or a jfet-mosfet cascode?

jamie

[Wakculloch]

If you need convincing of the superiority of the concertina phase inverter, let me be the one to try.
Gain will be 10dB higher.
Output impedance is significantly lower.
You wont need to fudge anode loads to retain balance.
Almost 9V more drive capability into a 100k load before clipping.
There can be bass rolloff issues, but if you can afford to loose some gain then using an 12AT7 or 12AU7 and directly coupling the two stages should take care of that.
As you may notice i'm a great fan.

[imaradiostar]

So...I can expect more drive using a concertina type if I'm using a 12at7 or similar tube? I once read that this type PI works much better and can drive cleaner using a triode with a lower plate impedance like a 12at7 or au7. What about that 12ax7 replacement with the larger plates? (can't recall the number)

I think it would be neat to build a blackface princeton circuit but use a 12ax7 for the front end, another for the verb recovery and mixer stages and some higher power dual triode as the verb driver and phase inverter. This would keep me inside my 3 preamp tubes size limitation while avoiding extra stuff that I don't want like excessive verb dwell and trem.

what's the sound difference between the ltpi and the concertina?

jamie

[Wakculloch]

Using the valve complement that you have, I feel changing from an 12AX7 may loose you as much gain as changing to a LTPI, and any extra drive capability wont be needed on a pair of 6V6's.
Feel utterly inadequate to comment on the subjective nature of the sound of different topologies. I'm sure someone out there has done an A-B comparison.
I always use the concertina arrangement in my builds, after having tried all the usuals (and some unusuals).
I presume Fender decided to use it in the Princetons to provide extra gain in the power amp.

[imaradiostar]

I'm still not sure what to do with this project. I'm half way through building a simplified princeton reverb prototype on an old chassis I have laying around but I had a thought- I may build a standalone stereo spring reverb unit since I have a bunch of random small PT's laying around. This would allow me to build an ultra-simple Fender Deluxe style amp with one preamp tube, a phase inverter and two power tubes. I'll probably stick with a tube rectifier but might make it switchable for a little more headroom when needed. This way I can use my tiny little chassis without trouble.

If I were going to build a standalone stereo spring verb it sounds like an excellent opportunity to make a stereo Magnatone type tube vibrato circuit. I've always though it would be a neat thing to build in stereo.

jamie