Need someone more knowledgeable

[Howlin' Mad Mac]

Looking for someone with a little more experience to advise me regarding a current build. It's a basic 5F1 - repl. Fender trannies, tube recto, etc. Changes I've made are 1) beefed up supply caps: 40mfd/500v, 20mfd/500v, 20mfd/500v, 2) lower supply resistors for a little higher voltages 1K/1W, 10K/1W, 3) addition of 1N4007 diodes to recto inputs/outputs (ss bridge backup for 5Y3), 4) ability to switch in simple tone stack (5E2 tone control), 5) switchable RC cathode circuit at 1st gain stage (25mfd/1.5K, .68mfd/2.7K) 6) switchable NFB level which all should work with no problems I'm aware of other than 40mfd 1st supply cap is maximum a 5Y3 can handle. Regarding this I've 2 relatively simple questions. I know I can experiment & find this out on my own but... soooo... my questions...

1) since I'm using all new top quality components I no 2.7K resistors in my box & don't wanna wait 3 or 4 days on the UPS driver... Would it change things much if I substituted a 3.3K resistor or if it's thought better a 1K & 1.5K in series?

2) The stock NFB resistor value is 22K & I know omitting it increases gain but not always desireable so I was thinking of adding a switch that instead of eliminating entirely would drop a lower value. 2 ways I've considered doing this are switching in a 2nd 22K resistor essentially cutting the resistance value by half if I remember my training from electronics school so many moons ago or just switch the stock 22K out of the circuit switching in a lesser value... thinking 1/4 or 5.6K value. Can anybody suggest which would be the way to go??? Keeping in my I'm looking for a modest but noticeable increase in gain. Also is would there be any benefit to also adding a cap to ground HPF in the NFB loop? If so any suggestions as to what value might work well .01mfd, .1mfd, larger?

Any help would be much appreciated... pix of this build to follow shortly.

[loudthud]

You can use the 3.3K or the 1.5K and 1K in series. It won't make much difference either way. On the feedback resistor, going too small may cause the amp to oscillate. Try 22K and 47K or open (disconnect).

[Howlin' Mad Mac]

Thanks Loudthud. I thought it wasn't enough of a difference to matter much however I then found a pkg of 2.7K's out of place in my parts box so that's become a non-issue. Regarding the NFB value you suggested trying 22K (which is the stock value for a 5F1), a 47K (but wouldn't that cause the opposite or decrease gain but clean things up more?) or none (which I've experienced the lovely oscillation that can cause... definitely something I'm attempting to avoid in this build.) Now on the Champ AA764 schematic the NFB value is only 2.7K. Why is the value so high on a Tweed Champ & so low on a BF/SF? Is it because of the tone stack. My build is using a tone stack but not the 2 knob version like the AA764 but a single knob job (oh my!) lifted from the 5E2 Princeton. Perhaps I should use a DPDT switch so instead of 2 seperate SPDT switches so that when I defeat the tone stack I'll also switch to a 22K NFB & when I switch it in NFB will drop to a lower value such as 2.7K? I was originally thinking of switching in a second 22K in parallel halving the value but wasn't sure if I'd see enough gain increase w/o oscillation so was contemplating a drop to about 5.6K or 1/4 the 22K value yet twice the AA764's? Oh, I'm so confused... LOL

[Howlin' Mad Mac]

Tagboard layout

Here's 1st pic of my new build tagboard... it's populated except for the NFB resistor but I haven't soldered or cut my leads out to my pots & switches, etc. Let me know what ya'll think & I do love to hear those suggestions. Looking to start building more of these pups (& lap steels, Hammond restorations & piano/electric piano restorations) for clients to make a few bucks on the side & I've learned so much from reading posts on this forum. You guys rock!!!

[loudthud]

The easiest way to learn about feedback loops is to study non-inverting opamps at DC. Basically the feedback resistors form a voltage divider so the resistor to ground plays as big a role as the series resistor. In the Champ with the 2.7K, the resistor to ground is 47 ohms. The tone stack in a Champ is out of the feedback loop. Those black and silver faced Fenders just have lower gain.