Princeton volume at correct level?

[Keyboy]

Hello, I'm winding up an AA964 Princeton ground up build. All 22 schematic voltage check points are on target  and I confirmed proper plate dissipation. When testing, the amp sounds good (the "vibrato" is nice) and clean. The thing is, I'm not 100% sure I'm getting all the volume I should be getting out of it. I swapped out the preamp and power tubes...exact same results.

I know some folks lower the two 68K resistor values coming immediately off the inputs to get a little more volume, but I'd be grateful to hear any other ideas. I feel like I shouldn't be complaining, but it's nagging me. 

[shooter]

All 22 schematic voltage check points are on target

once you confirm DC, Ac is next, but AC really wants a sig-gen and an oscope

[Keyboy]

Thank you. I have a nephew with the equipment you mention. I should give him a shout. Thanks again, Glenn

[sluckey]

I suggest a Showman for your next build.   

[pdf64]

… I'm not 100% sure I'm getting all the volume I should be getting out of it.

By ‘volume’, do you mean the amp has less gain that you’d expect, such that the sound is clean but not loud?
Or it has the expected amount of gain but lower power output than expected (such that it’s overdriving before it’s got loud)?

I know some folks lower the two 68K resistor values coming immediately off the inputs to get a little more volume …

Maybe, but I suggest to discount such reports. If it makes any difference then they’ve wired it up incorrectly. As in regard of the #1 input socket, there’s no audio signal loss between socket and grid.

[tubeswell]

Well you've got two pre-amp 12AX7 triodes cascaded through a tone stack (same preamp as a blackface champ), and a unity gain cathodyne splitter into a pair of fixed bias 6V6s. Should be about 12W output (which is 2 x as loud as a 1W amp). If you want the BF Princeton to be louder, you could throw another gain stage in there (like a BFPR).BFPR is a little louder at about 15W, or 18-20W if you goose up the PT and OT.

You could make a louder amp with the same preamp, but using a Long Tail Pair inverter driving some fixed bias 6L6GCs idling at 460V, but you'd want a 50W PT and 50W OT (and speaker) for that. But it would be a lot louder.

[bmccowan]

I suggest you compare it to another amp. Most people when they finish building an amp are surprised that its louder than they expected. I think you want to make sure its working correctly before you start modding things. Since you checked voltages and swapped tubes - it likely is, but if the lack of volume is "nagging" you I would still search for a gremlin.

[Keyboy]

Thanks so much for the input! I did mean to say the amp has less gain than expected - sounds great, it's just not blowing my socks off. It's looking like my expectations were just too high - everything seems to be working correctly. A minor detail...even though the AA964 obviously had a circuit board, I assembled the circuit with a genuine point-to-point approach - can't say why...it's just fun. No terminal strips, turret boards, etc. I will try to attach an in-progress photo, all comments welcome.

and...a Showman project sounds great!

[sluckey]

I assembled the circuit with a genuine point-to-point approach - can't say why...it's just fun. No terminal strips, turret boards, etc. I will try to attach an in-progress photo, all comments welcome.

That style may have been fun to build but it won't be much fun if you need to replace a component. And those long unsupported leads could make the components prone to vibration which can lead to some unusual issues. Not very practical for load in and load out stresses. Should be fine for sitting on a night stand in the bedroom. 

[Keyboy]

Thanks Slucky – and I hear ya! I’m familiar with replacing components in this amp because I’ve done it several times . It really becomes an issue down the road with a tech who’s not familiar with P2P wiring. The old-school wiring is layered on several levels to avoid the issues you mention as much as practicable. If you haven’t built a P2P-wired amp, I would encourage you to - don't put it on a nightstand though!

[punkykatt]

You did not mention what speaker you are using.

[Keyboy]

Hi punkykat, I checked the circuit with a Jensen C10R. I was planning to ultimately use a Jensen P10R, but I haven't wired that up yet. Both speakers are 8ohm. Happy to hear your thoughts.

[bmccowan]

If you haven’t built a P2P-wired amp, I would encourage you to

It is fun and satisfying. But it works out better in a smaller chassis with tubes and pots closer together. Filmosound and the smaller Hammond organ chasses fit the bill. That way the short component leads support the weight of the components. I also have rebuilt a lot of old P2P amps from the 40s & 50s. The cap and resistor leads back then were much thicker/stronger. Replacing them can be an adventure.

[bmccowan]

Back to the gain issue. Two ideas:
A raw control - either a switch or pot. I like the 250K pot approach I copied from Sluckey's Rocky, as you can dial in the "rawness."
And/or increase the NFB resistor. The 6G2 has about 1/2 the NFB.

[Keyboy]

Thanks so much, I will take a good look at both options. I will probably have a question or two to follow up as well. thx again

[Jalmeida]

I am far from being seasoned Tube amp builder or technician. So take that for what it is…

I see solder splices and wires extending Resistor and capacitor leads. I would check for resistances and intermittent connectivity. Any components or connections moved for repairs or component replacement could have induced a problem. Maybe chopstick around and push on connections and see if you sniff it out that way.

[Keyboy]

Hello bmccowan,

You mentioned increasing the NFB resistor, and that the 6G2 has about 1/2 the NFB. The AA964 has a 2.7k resistor for NFB - would a 4.7k resistor be a good test candidate to replace the original spec per your note?

Thanks again for your help!

[bmccowan]

I believe so. The two amps have a somewhat different NFB circuit, so I think its a matter of experimenting with that resistor. If you increase it too much you will know as the amp will become noisier overall. And that amp is known for being clean and quiet.
The raw control will do more IMO. But either/both will increase gain.
I have a 6G2, but not an AA964. From what I have read - players like to drive that amp with an OD pedal.

[PRR]

> the amp has less gain than expected -

There was a time when "rude noises" was NOT a good thing in an amp.

So gain was "just enough", not OVER-THE-TOP.

And also: if a Princeton did not do all you wanted, Fender had several fancier models with more everything.

In fact I do not remember the Princetons from the day. Everybody had a Champ from young and a Twin for playing-out. No love for the little guy in the middle.

I think the Princeton found a niche with boosters, or with PA, or in studio. It's very "honest". Also a good base for snazzy effects.

[Jalmeida]

I 100% agree with PRR. The princeton is almost exclusively used in live band settings only when mic’d into a PA. Too loud of a drummer and all bets are off. It is all about expectations. I have one I play exclusively mic’d with a clean boost. Mine has a 12” speaker which is a little louder. Without a mic and boost it won’t cut through at all.

[bmccowan]

Gain and volume are a bit of apples and oranges. I understand that Keyboy adjusted his concern to gain, rather than total volume. Anyways, I agree that Princetons are "in between" amps. I have two: a 6G2 and a SFPR. They are both too loud to play at home without doctoring the signal, and back when I used to gig, they were not loud enough as-is. And the particular amp that Keyboy built is famous for being the cleanest Prince at all volume levels. As PRR said, an honest amp. I have some other 2x6V6 amps that I like better for playing at home. The voltage is much lower than what BF and SF Fenders push out which allows them to breath more at reasonable volume levels. If one wants maximum clean volume, high B+ voltage is your friend. But if you want some grit at lower volumes, it is not. I should add, IMO.
BTW - the tweed Harvard is one of the best amps Fender ever made. I am surprised more hobbyists don't build them.

[Keyboy]

BMCCOWAN - thanks for the note back - I replaced the 2.7 NFB resistor with the 4.7 and the sound seems energized to the point where, based on the sound, I think I'm in the right place. I can appreciate what PRR and JAMEIDA were saying about the Princeton being behaved. I totally agree about needing a mic for performing. I'll cover the cab and send a couple photos. Thanks so much for all your input!

[Keyboy]

Sorry, I meant 2.7K removed for a 4.7K resistor! Cheers

[bmccowan]

Very cool - glad you liked the change. I always worry that I might be full of crap when I suggest something. Although when that happens, Sluckey, PRR, Hot Blue Plates, pdf, or someone else with better chops usually steps in and bails me out.

[PRR]

If you are not running a museum, you can up that 2.7k as much as you wish, to infinity (and beyond?).

The 2.7k makes it "polite", low distortion and not enough gain to make rude noises. Infinity can be some more sensitivity (gain) and can be "raw", though several well-loved amps had no NFB and nobody makes a fetish about it. (I was surprised how clean an Ampeg VT-40(w/distortion) was, for a simple driver and no overall NFB cleanup. OT was flat past 15KHz too.)

When I say "the 2.7k", I really mean the ratio of the NFB network. Other amps may run 47k:820r for similar gain and NFB.

[Keyboy]

Sounds good PRR! I will get everything built and spend some time making friends with the new set up, but I can see myself continuing to play with that NFB. Your term "polite" was spot on for how the amp wants to behave. I'm so used to having circuits that are more raw, this project surprised me.  Thanks again to you and BMCCOWAN for helping out!

[sluckey]

If you simply disconnect the ground end of that 6.8K resistor on the back of the BASS pot, you will see how much raw gain this amp has. If you like what you hear, then put a 250KA pot between that 6.8K and ground. Then you can dial in the amount of raw you want, anywhere from polite to wild.

[Keyboy]

I will definitely try this Slucky - thanks! It's an easy test and since I'm working with the AA964 vs the reverb, I've got space on the face plate I created to make way for just such a pot. Let's see what happens!

[bmccowan]

Hey Keyboy, that is what I was referencing below. The link to Rocky is: http://sluckeyamps.com/rocky/rocky.htm there is a downloadable schematic which includes the raw pot. I think it works great as you can adjust the rawness to suit the amp volume and the darkness of your current mood. Of course by touring his site, you may find a few other amps you want to build.

Back to the gain issue. Two ideas:
A raw control - either a switch or pot. I like the 250K pot approach I copied from Sluckey's Rocky, as you can dial in the "rawness."
And/or increase the NFB resistor. The 6G2 has about 1/2 the NFB.

[Keyboy]

Wow, holy smokes!! I disconnected the ground end of the 6.8 from the back of the bass pot and the amp really came alive just like bmccowan and sluckey advised! By unhooking the resistor the Princeton is tame no more. I will make room on the face plate for a 250KA pot as mentioned below. (the pot will be essential as the amp gets really crazy above 6-7 or so without any resistor.) Thanks a ton you guys!!!

[Keyboy]

Sorry, I meant 6.8K!

[sluckey]

It would be ideal if you could move the trem pots over and mount the Raw pot next to the bass pot.

[Keyboy]

Thanks Sluckey, Good advice - I was wondering about that myself. Might take a little jiggering, but I'll see if I can't make that happen.