new champ build aa764

[scole]

http://music-electronics-forum.com/a...4&d=1180230529
i noticed in that schematic the first filter cap is 40uf, i used a jj 40 20 20 20 cap can. should i switch to the 40 to get rid of the hum? im not using a choke but i could put one in if necessary.

[Bruce / Mission Amps]

Yes. I can't remember if I drew that one but the idea was to use the choke input filter in order to lower the B+.
With a choke input, the B+ will only be around 90% of the rectified DC out of the GZ34 rectifier. Or in your case, the SOVTEK 5Y3GT would be OK now.
So if your power transformer is pushing, say 360vac into a rectifier, then the B+ out of the choke at at the first filter cap would be around 324vdc.
Note that there is NO filter cap connected to the rectifier.
This is a very good filter and will reduce AC hum better then a filter cap on the rectifier and no choke.
In your application a simple 3Hy-5Hy choke for a Twin Reverb would probably be OK.

[scole]

champ update

i installed a 470k 3w between pin 8 on the rectifier and the first node, i moved to the 40uf cap on the cap can wich is a 40/20/20/20. heres what i have now, 341v pin3, 350v pin 4, 22.1v pin 8, 22.13v across the 460 cathode resistor. looks like the tube is at about 15 watts. the amp is still loud as hell but it overdrives at about 6 on the vol. it doesnt make that crackling sound anynore. the sound looser now not as stiff, the hum, is almost gone. it sounds very fendery but not as beautifull as a real fender. ill try a better tube and maybe a speaker. i have a new jensen 10 inch alnico in now.

[TD_Madden]

I don't know about anyone else, but I'm finding this thread very interesting and educational!

[scole]

thats what its all about t. im wondering, shouldnt i have 350 volts on pin 3 and 341 on pin 4? in other words should the screen voltage be lower than the plate voltage?

[MWJB]

"in other words should the screen voltage be lower than the plate voltage?" It often so on BF/SF champs & Princetons, the 1K 'B+ to screen dropping resistor' doesn't drop enough voltage when you also consider the much lower current draw at the screen.

[scole]

so now the 1k has to go up? to 2k2?

[MWJB]

And/or add a screen grid resistor between power supply rail & pin 4.

Or, if you are building a tolex champ "clone", live with the voltages you have. Yes, it's not the ideal from a design point of view, but most Champ/Princeton owners didn't know they had a problem until they got the DMM out.

Experiment and go with what sounds best.

[scole]

And/or add a screen grid resistor between power supply rail & pin 4..

i added a 2.2k 2w at pin 4, its getting better. now i have pin 3 345v, pin 4 344v, pin 8 21.76v im still at about 15.3 watts dis on the tube. the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away. the overdrive is still doesnt come untill about 9 or 10 on the vol scale if it went to 12 lets say. i want the gain to start sooner and be alot smoother. so do i keep adding resistance to the b+ rail or rebuild with a choke as in bruces sketch?

[Bruce / Mission Amps]

i added a 2.2k 2w at pin 4, its getting better. now i have pin 3 345v, pin 4 344v, pin 8 21.76v im still at about 15.3 watts dis on the tube. the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away. the overdrive is still doesnt come untill about 9 or 10 on the vol scale if it went to 12 lets say. i want the gain to start sooner and be alot smoother. so do i keep adding resistance to the b+ rail or rebuild with a choke as in bruces sketch?

Are you still using that high value cathode resistor?
I think most builders would like it if the amp doesn't come on until the volume was up 2/3 or more.
What is the plate voltage of the preamp tube now?

[scole]

the cathode resistor is still the 470 (460) actual, im running the rca 5y3. my plate voltage at v1 is 201.4 on pin 1, 201.6 on pin 6.

[MWJB]

"the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away."

"the overdrive is still doesnt come untill about 9 or 10 on the vol scale"

Neither of these symptoms seem normal to me, I don't think either are related to your screen voltage specifically. Sounds like a wiring/layout issue.

[Bruce / Mission Amps]

"the amp sounds better but it makes noise when the vol is up all the way, if i drop back about one notch it goes away."

"the overdrive is still doesnt come untill about 9 or 10 on the vol scale"

Neither of these symptoms seem normal to me, I don't think either are related to your screen voltage specifically. Sounds like a wiring/layout issue.

Yeah me too.

FOLLOW PROPER SAFETY issues so I don't have to keep repeating myself.


<<<Turn the amp off and unplug it form the wall>>>

Unsolder the grounded lug of the volume pot and the ground end of the .005uF cap on the tone control.

No plug your guitar in and turn it's volume control to ZERO.
Turn the amp on and slowly turn the guitar vol up and see if it sounds radically any different.
Get it so it sounds good with the guitar volume and then shut the amp off again.
Solder the volume pot ground back on and try again.
Your volume will drop but you should be able to get it way back up again by turning it up all the way.
Now try grounding the tone cap again.
Either the tubes are dogs, the OT is bad, you are using a mis-marked part, made an error in reading the difference between a 15K resistor and a 1500 ohm resistor (etc.) or something in the wiring is not right.

[scole]

im double checking everything. i noticed something, what is the bypass cap for v1 pin 8. is that 2 uf. i put a 25/25 there.

[Bruce / Mission Amps]

im double checking everything. i noticed something, what is the bypass cap for v1 pin 8. is that 2 uf. i put a 25/25 there.

If you are using the bf tone stack then the 22uF/25v cap on both triodes is what you want.