Princeton Semi-Clone Output X-Fmr?

[Rob's Radio-Active]

Hi, all, I'm new here and this is my first post.
I don't play guitar myself (maybe strum a chord now and then), but I love music, am a vintage electronics enthusiast, and tube amps are fascinating to me.

This is my breadboard prototype, similar to the old brownface Fender Princeton. It follows that basic topology, but there are some significant changes, too, so I can't really say it is a clone. Anyway, my friends playing though it like it, but one thinks a touch more base might be good. So....I think I want to upgrade the output transformer from the current 15 watt, 8000 ohm plate to plate, to a 25 watt, 7600 ohm plate to plate to try to get the lows out into the 12" speaker a little more, before building the permanent chassis. Also, the guy I am building it for is thinking about using an alnico speaker rather than the current ceramic magnet unit. Because alnico speakers are somewhat less efficient, I am not sure if that will undo any possible bass response gain.
Thoughts anyone?

Cheers,
Rob

The clipboard/breadboard:


Simplified output stage:

[Justin Thomas]

Can you post an entire schematic, with all the changes, to represent the amp as-is right now? There may be much easier and cheaper solutions than a new OT... it nay be that all it needs is a different speaker, not both. Try it through a different speaker to see.

Justin

[The Dude]

IMO, I wouldn't start at the OT. Firstly, different speakers sound......well, different. It's hard to say what his alnico speaker will sound like, but if that's the route you're going, I'd start there and see what happens tonally. That may be all that's necessary as ceramic speakers are generally described as more "edgy" than alnico. Again, this is a generalization. It doesn't mean ALL ceramic speakers are brighter than ALL alnico speakers. You'll have to try it. Also, if we're talking about new speakers, there is a break in period for speakers. Most will sound "edgier" until broken in. Cabinet type and size are also going to matter (volume, closed or open back, etc.)

[Rob's Radio-Active]

Thanks for the input...yeah the speaker is now in an open back cardboard box.
Here is the complete, as built schematic right now. I will likely remove the bias switch that allows user switching between cathode and adjustable grid bias, and just go with adjustable grid bias. That control is set up well within safe limits for the tubes, maximum to minimum bias, but does allow a choice of tone. It will also have two inputs.

[Jazz P Bass]

That tone stack is strange (to me, Fender wise)

Have you run that through Duncan's Tone Stack?

Princeton schematic:

[Rob's Radio-Active]

It's not a Fender tone stack. It seems to work fine the way it is...it's just that (and I can't hear it very much) the client thinks it sounds a wee bit farty on the lowest end when cranked up.
I figure the output may be saturating a little, since the 120VAC powered B+ is considerably higher than the original amp at 110VAC mains voltage. I dunno.

[pdf64]

The input impedance looks to be a bit low, and may damp down the instrument's treble resonance if plugged straight in.
Connecting the 2nd stage directly to the input stage like that, without any attenuation / grid stopper, may make it prone to instability and bias shift / blocking distortion.

It works out so much better to have a gain control between the input and 2nd stage.
Gain pots at the input will tend to degrade the signal to noise ratio and frequency response, hence are almost never used in this application.

[Mick Bailey]

Your speaker will likely not give the best response as it stands - wait until it's in a proper cabinet (or get hold of one to test) before coming to any decision.

+1 on the input impedance being too low - this will certainly kill the sparkle (unless that's intended). A volume control right at the input is also more sensitive to sounding scratchy, depending on the particular tube.

[Jazz P Bass]

A friend of mine has a Fender Princeton that he paralleled a 15" speaker onto the amp apeaker.

I had already cut back the first stage bypass cap to 10uf (to remove the farty sound when cranked).

Whew. With that 15 attached. Talk about bass response!

[ThermionicScott]

Thanks for the input...yeah the speaker is now in an open back cardboard box.
Here is the complete, as built schematic right now. I will likely remove the bias switch that allows user switching between cathode and adjustable grid bias, and just go with adjustable grid bias. That control is set up well within safe limits for the tubes, maximum to minimum bias, but does allow a choice of tone. It will also have two inputs.

I love seeing switchable cathode/fixed bias whenever I see it. One comment on that, though: 1-watt resistors are fine at quiescent (I calculate 0.69W), but under full power, you may be over the dissipation limit. I'd bump them up to 2-watt units at a minimum and give them plenty of breathing room.

Tangentially from there, I'm a big fan of using a separate cathode resistor and cap on each tube to allow unmatched tubes to find their ideal bias point, but to keep the switching, you'd need a 3PDT switch, I think. Tied together as they are here, you might as well use a single 340Ω and 100uF.

Hope this helps. I like a lot of what you're going for here, and I think you've gotten some great feedback from the others.
- Scott

[uneumann]

Minor point... Do you really want that cap from mains to ground? It's a bit of a hazard that way. You'll get much the same line-noise suppression if you connect it across the primary and if it leaks or shorts (forbid) your chassis isn't impacted.

[Mick Bailey]

Whew. With that 15 attached. Talk about bass response!

I have one of Roy Blankenship's amps in right now - 1x15" (looks like an Eminence) and 2xKT88 in a cab not much bigger than a 1x12". Just about the most profound bass out of any guitar amp I've ever heard, without being farty at all. Also crystal highs. Come to think of it, every 1x15" I've heard sounds superb.

I wonder why we don't see more?

[Justin Thomas]

One of the greatest-sounding amps of all time that I personally played was a brown Pro, with the 3-tube trem. I <love> 15s for guitar. They're so "round," and still have crisp highs. Hmmm... Maybe I shall make a 15" baffle for my Concert... come to think of it, none of my amps have 12"s...

Justin

[Rob's Radio-Active]

Thanks for the ideas, all!
uneumann, the death cap is an archive of design ideas. It was never installed and I will leave it off.
Scott, I still calculate 1.7 watts split between the two at full average power, but you are right, that is too close. I might leave them off and go with grid bias only.
I'm too tired to make much sense right now (chemo day) so I will give all this a better reading and thinking tomorrow.
cheers
Rob

[ThermionicScott]

Thanks for the ideas, all!
uneumann, the death cap is an archive of design ideas. It was never installed and I will leave it off.
Scott, I still calculate 1.7 watts split between the two at full average power, but you are right, that is too close. I might leave them off and go with grid bias only.
I'm too tired to make much sense right now (chemo day) so I will give all this a better reading and thinking tomorrow.
cheers
Rob

Hoo-boy. Hope the chemo does the job! Nasty stuff, from pretty much anyone I know who's experienced it.