Noob's first build !!!!

[bereanbill]

First I have to thank everyone who gave me input on this project...especially firechief who talked me through cascading gain stages. Everyone...you rock !!!!
I fired up my first build tonight and I have to call it a sucess ! If you are familiar with it, it is based on a Fender 5C3 Deluxe...with a few departures. First I changed the grid leak bias to cathode bias...mostly so I could safely use a stompbox in front. Then, with firechief's help, I cascaded the two volumes together. the first volume goes back into the second half of the first 6SC7 preamp tube, then into the second volume which goes into the second preamp tube. The only problem so far is some sort of oscillation when the first volume is up over 3/4...too much gain or too hot of an input? I have a 68K resistor between the input and the grid (pretty standard for a guitar amp) but no resistor to bleed off signal to ground. Most commonly that's a 1 meg. Maybe tomorrow I'll try that...too late tonight and I'm a bit google-eyed from 5 straight hours behind the iron.
Thanks again to all who gave me input...I'll post again when I try a couple things out. Any suggestions???

[Iain McNeill]

congrats on your first build!

Can you post a link to your previous thread so we can see the schematic (or post the schematic). Stability problems are always lurking in amp design - half of it is layout and wire routing the other half is topological. The schematics' a good start to debug.

[bereanbill]

Hi..not able to scan in photos of this amp right now, but schematic for the Fender 5C3 Deluxe is available at Schematic Heaven. I have no experience drawing schemoatics, but I will try to detail the alterations to the original that I have done.

[bereanbill]

Ok...after a little thought, I remebered I had eliminated the NFB loop from this amp in an attempt to get earlier break-up. The resistor was on the board, but it was not wired to the output jack. I wired it up and it improved a bit, but I still had a sort of throb when the first volume was up over 7. ( almost like tremelo )I thought: A- a bad bypass cap ? or: B- too much gain in the first gain stage. I took out the bypass cap, and then the amp squealed at about 5 on the first volume. So I replaced the bypass with a 100uf@25v and VOILA !!! No more oscillation anywhere. Now all I have to do is eliminate some hum and I will be 100% satisfied. ( Right now the satisfaction meter is at about 85% ). The filaments are wired in series in this circut...would wiring them in parellel help eliminate some hum? Should I run balanced 100 ohm resistors to ground on the filiments? ( I saw this in a mod book somewhere once?? ) Anyway...AGAIN THANKS TO EVERYONE WHO OFFERED THIS NOOB ADVICE !!! I'm incureably bitten now...already planning a single-ended Champ clone with an extra 12AX7 for MORE GAIN !!!!

[Iain McNeill]

Yeah, changing values in the feedback network is a sure way to get instability.

Mr. Nyquist showed us that if the phase shift passes through180° while the gain is greater than 0dB you will howl. Breaking the loop to measure gain/phase is hard to do so most people set it empirically (push gain/bandwidth higher and higher til it squeals then back off a bit) or by some indirect method e.g. inject a signal into the output and see where the amp O/P fails to attenuate it (or more scarily - amplify it!).

Hum is an insidious demon. It could be inductive coupling of diode-capacitor pulses into circuit loops, capacitive coupling of rail noise from adjacent cabling. Wrong grounding scheme (search ground loops for PAGES of discussion! )

PSU wiring is a particular rathole. Make sure the charging current from rectifier to caps don't share wires with amp power feed. You also need to consider ground here.

Filaments can couple 60Hz into the amp - I don't think this is likely but its not my specialty. But if you've wired them in series then you have enough volts to regulate them in parallel - or do you not have enough amps?

[jjman]

All your tubes are 6.3volt filaments so you cannot run any of the heater “in series” if you are using a 6.3volt heater winding on your power tranny. I’m thinking that you are considering changing to a “center tapped” heater setup as apposed to the stock 5C3 “one side grounded” setup. Making this change may or may not reduce hum. I personally would make the change.

And yes, you would need to use the “artificial center tapping resistors” if you change to the center-tapped heater strategy. However, if your power tranny has it’s own real center tap wire (perhaps green with yellow stripe,) then you could/should ground that center tap wire and NOT use the “artificial center tapping” resistors.

[bereanbill]

Yeah...I've played the NFB game quite a few times with Fender amps. The 60s models all had the same basic scheme...from output back to pre-PI...just different values of feedback resistors. Most of the time I was able to just eliminate the loop without causing any instability. With the 6G2 circut, I had to put it back to 1/2 the value. This amp now has stock value (1 meg ) and I'm happy as heck with the tone.
I'm thinking the hum is from the PT being too close to the output tubes...one is about 1/4" from it...is that possible? The chassis is from an old Allied phono/mic amp that had the right number of controls and sockets, but the PT I used is bigger than the original. That coupled with using 6L6s ( plates were too hot for 6V6s) took up space? Could the tube too close to the tranny cause hum? I know with preamp tubes it can.
Overall, I'd call it a sucess for my first build...I was just a bit surprised that the larger value bypass cap cured the instability. But I do recall reading that with the higer value cathode resistor ( 2.2K) that you should use a lager cap as well. With no formal electronics training ( I'm a carpenter for crying out loud !) I use a lot of trial and error technique. Thanks for coddling the noob, Iain !

[bereanbill]

Thanks jjman...my mistake calling it series....it is the one side grounded. There is no CT for the filaments, so I would need to make the artificial CT. Since I opted not to use a pilot light ( I used that hole for the input jack) where would I put the resistors? On the first tube off of the tranny? ( That would be the first power tube)

[Iain McNeill]

Originally posted by bereanbill
I use a lot of trial and error technique.

nothing wrong with that at all. How else do you learn?


P.S. and yes, the PT 1/4" from output circuit could quite easily induce hum from transformer leakage flux. Try rotating it coz the leakage flux is not a smooth sphere around the TX.

[jjman]

Originally posted by bereanbill
Thanks jjman...my mistake calling it series....it is the one side grounded. There is no CT for the filaments, so I would need to make the artificial CT. Since I opted not to use a pilot light ( I used that hole for the input jack) where would I put the resistors? On the first tube off of the tranny? ( That would be the first power tube)

The physical location of the CT resistors is not very important as long as they are schematically correct. Some connect the junction of the resistors to the cathode of a 6v6 instead of ground. This will "float" the heater circuit to the positive DV voltage on that cathode which many say further reduces heater-hum. If you went that route the best location would be on one of the 6v6s sockets.