ANOTHER Big Bottle 5F2a Princeton

[jsnwhite619]

So, I've slowly been working on this one the past month or so, and still no cabinet started yet. But it took some changes. Transformers are getting like hen's teeth lately, or tubes - yay XYZ for "supply chain" issues" - so I got a 290CAX instead of 290AX because it was a month+ wait on backorder. CAX can't take a 5u4 - my preference - but the primary is a little lower at 315v instead of 325v. GZ34 put it a little higher than I'd like, but after trying the 275v and 315v options, the bigger tubes HAVE to get that higher voltage to really shine here. 350v B+ just wasn't getting what I was expecting on the lower primary option. Also, I started with the 300R from my previous builds for a bias resistor, but I just was never happy with it either, and went all the way back to a 10W 470R bias resistor that should be safe for 6v6 as well. Came out to 80-82% bias I think with the 5881, but I'm much happier with the sound. The 12ax7 sounds great now where it was just way too muddy before.

This clip was through my little Champ sized closed back cab with a Jensen C8R speaker inside. I loved it. Here's a cell clip. Sounds much better with headphones, but it's kinda loud. I spent about a week with it at 300R and never really liked it. Week long family vacation, came back, same thoughts....swapped to 470R and fell in love. I used a 5u4 and 12ay7 before, but with the GZ34 staying cleaner, the 12ax7 sounded better in the preamp, but a cooler bias in the power section worked better to counter it.

 

[2L man]

There is a circuit called "self split push pull" which is biased to A-class and can be driven using the same pre amp all SE power stages are driven.

"SSPP" has two advantages against SE. Smaller/lighter/cheaper OT when PP bias current cancel core saturation and PP noise cancellation. But it needs two power tubes where I dont see any problem because we love tubes

Google Image Result for https://robrobinette.com/images/Guitar/Deluxe_Micro/Self-Split_Poweramp_Schematic.gif

If you have difficulties to get traditional transformers and you are willing to experiment you could use Antekinc $44 10W 8k PP 2.5lbs toroid and $42 100VA power transformer. 15W is $55 and only 2.6lbs but its 5k impedance would be better for two "big bottles"

I don't have any experience of Antek but three different European Toroid brand OT I have used have airy clear 3D sound.

Antekinc has two 30W SE OT but they weight 6lbs and should be good for at least 45W guitar amp where power is increased increasing plate voltage and tube(s) which can hangle higher voltages.

 

[jsnwhite619]

I forgot last night to include something else I've been doing lately and the added bonus. I've separated my inputs for a while now - both have 1M input resistors, both have different value grid stoppers (33k and 68k), but no jumper between them. I grabbed my A/B/Y pedal while I was testing it out, with cords plugged into #1 and #2. Normally, on a stock Fender, both would be at the same level when two inputs are used together. Separated like this, it had three options -- #1 is still hot, #2 is still low, and click the Y/BOTH on the pedal and they are running together for a serious lead boost.

 

[jsnwhite619]

Finally had some time at home to do some experimenting with this thing. I swapped tubes around, tried to play the same riffs each time for some real A/B comparisons. At this point I have the 5k primary and a 470R bias resistor and my JJ 6v6 is at 370v B+ and 91% with a 5y3. I didn't try the GZ34 with the 6v6 because I don't see the need to run it that high when I can bump up to the 5881 if I want "more". The GZ34 and 5881 info was listed above in the first post.

I setup my test cab with my Jensen C10Q, largely just because the lower volume output of it. The amp volume was set on 10 the entire time. Cab & mic never moved while I was swapping tubes -- preamp, power, and rectifier. I also had my dB meter a foot away, but it was slightly off axis and I think it ended up with slightly lower readings because of it, but the differences were enough to note between takes. Still have a lot of clips to edit through, but I thought this may be the best to really A/B compare without too much extra in between to distract.

So, this clip has a 12ay7 throughout, but the rectifier and power tubes get swapped around... and I've got to admit that I can't tell a real hell of a lot of difference between any of them. In the room with the extra volume and bass through the floor and all the goodness that comes from "live music" it seems much more noticeable. But, blind test with a mic, I wouldn't bet the farm on picking each one correctly each time. I'll come back later and post the order and descriptions in a follow-up post just in case anyone would like to try and guess which is which. Both riffs are in the same order. A, B, C is the same tube combo both times - the rhythm clip and the blues riff. So here's three combos of 5y3/GZ34 & 6V6/5881.

It's bridge then neck rhythm 3 times, then the blues riff three times.

 

[jsnwhite619]

The db meter was handy in this too. Again, I think the position may have slightly lowered the measurements, but I wrote it all down. Also, I have a pretty heavy NFB value - 2.7k and 47R tail like a Princeton Reverb, so I'm sure all these numbers could go up a bit with that changed or disconnected. Then, I went to http://www.hometheaterengineering.com/splcalculator.html and used it to get a rough idea of the output using the meter and the SPL of the Jensen C10Q. The numbers actually worked out mostly like they should. If double the power gains 3db, then looking at the chart here, it plays out correctly. 2.5w to 5w (the meter on measures to .5 increments) was 2.5db; 4.5w to 8w was 2.5db; 3.5w to 6w was 2.5db. Finer measurements probably would have been spot on.

5Y3	6V6	12AY7	108.5db	2.5W
5Y3	6V6	12AX7	111db	5W
5Y3	5881	12AY7	109.5db	3.5W
5Y3	5881	12AX7	112db	6W
GZ34	5881	12AY7	110.5db	4.5W
GZ34	5881	12AX7	113db	8W

 

[NTC]

Great data. I love these types of experiments. Basically, going from a 6V6 and 5Y3 to a GZ34 and a 5881 is not even 1 louder - only 2dB more. The off axis measure won't affect the DIFFERENCE between measurements.

 

[jsnwhite619]

Great data. I love these types of experiments. Basically, going from a 6V6 and 5Y3 to a GZ34 and a 5881 is not even 1 louder - only 2dB more. The off axis measure won't affect the DIFFERENCE between measurements.

It just occurred to me that the difference in the ay7 & ax7 output matches closely to their specs as well. A 12ay7 has a gain factor of 45 and 12ax7 of 100, a little more than twice as much gain. In each comparison above between V1 in each setup, the 12ax7 was roughly 3dB louder each time...doubling the preamp gain in this situation added roughly 3dB to the output.

 

[Mongo Park]

I use old radio transformers and they mostly have a 5y3. To get a little more out of them they can run a 5V4 in a amp like yours.

https://frank.pocnet.net/sheets/093/5/5V4GA.pdf

 

[jsnwhite619]

Since we've got some space now, I'll reveal the order of the clips above.
5y3 & 6v6
5y3 & 5881
GZ34 & 5881

I swear the more I listen the less difference I can tell between them.

 

[Kevin Wolfe]

Try a used or NOS 5y3.

 

[jsnwhite619]

Try a used or NOS 5y3.

These are both JJ's and the 5y3 dropped the B+ by 30v-35v.

 

[Kevin Wolfe]

I’ve swapped out JJ’s for used 5y3’s and dropped even more voltage. Even had to re-bias on one occasion. The old ones are very tough and not expensive. Rather, they weren’t expensive.

 

[jsnwhite619]

Finally got around to building a head cab for this thing. I don't think I've done anymore tweaking to it since I last posted. But I finally played my '72 RI Tele through it today, and I loved it! The extra punch over single coils, but the lower output than my Les Paul's is a perfect middle ground for this amp. This is with a 12ay7, 5881 & GZ34 inside. I've changed the pots on the guitar out to 500k, can't remember what caps I ended with, and it's 50's style wiring inside, but the pickups are the stock WR humbuckers from mid-2000's. C10Q reissue in the test cab.

 

[jsnwhite619]

Got the finishing touches on the tweed done! Looks weirder now without the chassis installed, but the big hole on the right is for the cord and speaker cable.