New build 5F1 & AA764 champ, 2 in 1

[nokaster]

i made up my mind and my first amp build will be a champ.

i've been reading a lot about the tweed 5F1 and the backface AA764 and i want both. and it is do-able with a few switches.

i already decided to get a 3PDT to bypass the AA764's tonestack and 2uF/25V cap on pin 8 of 12AY7.

i'm gonna try the negative feedback mod, SPST disconnecting the 2k7/22k resistor to the speaker.

but i still have some questions...
which is the best way to go on a 5F1/AA764 in 1 for the power supply and filtering?
should i use the 5F1 16uF/8uF/8uF and 10k/22k?
or the AA764 20uF/20uF/20uF and 1k/10k?
or should i mix them up?
or is there another ultimate sollution, not involving switching?

in gerald weber's article on a permanent AA764 to 5F1 conversion he changes the AA764 2k7 feedback resistor to a 22k and the AA764 10k power resistor to a 22k. so i guess this wouldn't be a bad start for my 2 in 1 amp?

and than there are 2 more components on the AA764 that aren't in the 5F1 schematic and about which i have found info yet:

- what does (and should i do) with the 25uF/25V elco on the first triode of the 12AU7? it's in the AA764 schematic, not in the 5F1. i guess it's there for gain, like the 2uF/25V on the second triode?

- last one is the 47 ohm resistor in the AA764. 2nd triode, connecting the 1k5 and 2uF to ground. again... what does it do and what should i do with it in my 2 in 1 build?

my last question is on bias... what about fixed bias in champs?
i can't find much info on the subject.
can it be done? is it "interesting"?

i did search and read a lot on this and other forums, so the questions that remain or vilid i think. and maybe usefull for others...

hope someone will have some answers!
thx in advance!

[MWJB]

"i already decided to get a 3PDT to bypass the AA764's tonestack and 2uF/25V cap on pin 8 of 12AY7." You mean 12AX7? If you want to do a "switchable" hybrid I'd suggest keeping the NFB loop from the BF champ, just changing values. E.g. the 5F1 has a 1.5K Nfb load and a 22K dropping resistor. You could use the stock BF 47ohm NFB load with an 820ohm dropper, rather than the 2.7K fitted as stock. This would minimise the effect of the 12AX7 pin 8 cathode bypass cap shunting AC to ground when switched to 5F1 mode (a bypass cap will work here in 5F1 mode to a degree but not exactly ideal). Have the preamp bypass caps switchable.

The 25/25 cap at 12AX7 pin 3 on the BF schematic bumps up gain, this cap WAS actually fitted to most (if not all) 5F1 as well. 5F1 schematic is wrong/outdated.

Filtering - go 30-40/10/10 or try 30-40/20/10, 10's will sub for 8uf with no change in sound. No reason to go as low as 20 for the first stage.

As far as screen & preamp dropping resistors go, you might want to test by ear, perhaps go in at 5K/22K to split the difference, with a view to fine tuning? It's not unusual for tolex champs to run 6V6 screen voltage in excess of plate voltage with a 1K dropping resistor. Preamp voltage is obviously a factor, but so is the amount of dropping between 6V6 plates & screens, your ears will really tell you what you need, aim for 200v-ish on the 12AX7 plates as a start. Don't think about resistor values per se, think about voltage at the various stages & just use the resistors that give what you want.

47ohm at 12AX7 pin 8, folowing the cathode resistor - covered in earlier paragraph. It's the NFB load resistor for the BF rather than the 1.5K in 5F1.

Fixed bias is easily installed, or even a fixed/cathode bias switch (I have one). Fixed tightens response, less compression, less grindy harmonics. Won't make the amp vastly different but worth having due to the simple installation.

Other simpler route might be to go 5F2A with perhaps a switchable 500pf cap on the tone control, with the cap switched out & the tone pot up 5F2A becomes similar to 5F1. With the 500pf cap out the circuit and the tone pot at settings lower than flat out, you'll get a more "vintage budget" amp, darker, thicker sound. You can get a reasonable BF sound out of a stock 5F2A by playing with the controls, it's a pretty flexible design.

Another alternative might be to use the BF circuit & lift the ground connection for the middle resistor, treble pot becomes a tone pot, bass no longer functions & gain/mids are boosted. A 50KA middle pot in place of the BF 15K fixed midle resistor will do a similar thing when turned much past 12o'clock (measure pot taper to see what's going on).

Ignore the B+ voltages in the Fender schematics 370-410v at the 6V6 plate are more realistic, SF Champs might even hit 430vdc with the stock 470ohm cathode resistor.

If it was me I'd just go straight for the 5F2A with a 10"/12" speaker.

[nokaster]

You mean 12AX7?
You could use the stock BF 47ohm NFB load with an 820ohm dropper.
If it was me I'd just go straight for the 5F2A with a 10"/12" speaker.

thx man! good info in you post.
thx for taking the time.
excellent info on the 25uF/25V, power filtering and looking at voltages, rather than resistor values!

i did mean 12AY7 instead of the stock 12AX7.
i'm 100% sure shure i'll like the lower gain 12AY7 better. (for experience with champs build by a professional amp builder in my city.)

i thought the bf had the same 1K5 NFB load as the 5F1, but in the AA764 it's paralleled with the 2uF cap and in series there is a 47 ohm resistor to ground. reading your post, the 47 ohm would be the NFB load by itself. that still isn't clear to me.

i'm very interested in a schematic or layout on how to switch between fixed and cathode bias in a champ. i just can't find in on the www...

i have my mind set on a champ 2 on 1, after looking and reading for a long time. the princeton suggestion had crossed my mind, but i'm gonna stock with the champ.

again, thx for taking time to help me out!

[echuta13]

You might want to take a look @ this thread: More Pwr and Headroom for SF Champ

Rather than go the fixed bias route though... you may want to just stiffen up the cathode cap on your output tube. I use a 1000uf 50v cap, and it does tighten up the sound.

[MWJB]

It still doesn't sound like fixed bias though, fixed bias just needs a 100K 2W resistor a 25uf 100v cap & a 50K trim pot...and a dpdt if you want it switchable, it's so easy to do on a scratch build there's little point in halfway house measures.

Ideally though I'd do something more like a 5F6A tweed bias circuit fed from the AC recitifier winding, 100K 2W dropper, 10uf 100v, 10K 0.5W dropper, 10uf 100v, 50K trim pot for load to ground (you could add a fixed resistor between the pot & ground after a bit of testing to ensure there's always a sufficient load).

Nokaster - If you look at where the NFB loop taps into the BF Circuit you'll see that it connects AFTER the 2/25 cap & 1.5K, rather than the hot side of the 1.5K in the 5F1. So in the BF it's just the 47ohm providing load for the NFB loop.

[nokaster]

now i get it!

maybe i should stick to the 5F1 schematic and just add a switchable AA764 tonestack.
i guess this would result in a stock 5F1 champ
(including the 25uF cap on the first triode, 22k & 1k5 NFB res, 10k & 22k power res, no 2uF on second triode, no 47 ohm res, 40/10/10 powerfilter)
+ a bf champ with low gain for bedroom use...

[nokaster]

another question arose when looking at the ceriatone layout for a AA764

there are 2 100 ohm res on the 6V6
pin 7 - 8
pin 2 - 8

what are those there for?

[echuta13]

The 100ohm resistors are for the filaments artificial center-tap. Notice that they connect to the cathode bypass cap... this puts DC on the filament wires, and should help in keeping hum to a minimum.

[nokaster]

thx man!
good info!

are these 100 ohms usefull/do-able in a 5F1?
ceriatone doesn't have them in their 5F1, only the blackface.

if they are good in a 5F1, what wattage should i go for?

[tubeswell]

you only need the 100R resistors off the heater supply if the heater winding doesn't use a center tap. (If I can say that another way - if you've got a heater winding center tap connected to the chassis - you don't put the 100Rs in.)

[nokaster]

OK! thanx!

[cubhead]

In regards to the 100 ohm resistors, (this may be a dumb question) but how do you know if you have a heater winding center tap connected to the chassis?

[unparalleled]

In regards to the 100 ohm resistors, (this may be a dumb question) but how do you know if you have a heater winding center tap connected to the chassis?

You will be able to measure about 3.2 V AC from the chassis to either side of the filaments.

[cubhead]

Thanks!