5F1 (sort of) build troubleshooting issues

[Howlin' Mad Mac]

I've recently gutted my "Champion 600" reissue amp & built myself a 5F1 reusing the chassis, cabinet, transformers & speaker with plans to replace the speaker in the near future. My build follows the 5F1 circuit closely however I could see no great benefit using a 5Y3 rectifier, using 4 1N4007 diodes instead & the Chinese iron has no center tap. I also have deviated from the original by using the original 18/3 power cord (properly grounded of course), deleting the .047mfd "death cap" & twisted my heater supply lines & added 2 100 ohm resistors running from each leg of my replacement pilot lamp to ground. Upon completion of my project turning the volume control above 3 yielded a "farting" kind of distortion. Researching the problem I found several suggestions so I opened it back up & added a 25mfd/50vdc cathode bypass cap across the 1.5K resistor from pin 3 of the 12AX7 tube & eliminated the 22K negative feedback resistor, both of which seemed to help a great deal however now when I turn the volume above 9-10 I get some sort of oscillation that kind of sounds like a motorboat & hope somebody can assist me in eliminating the problem. So far my thoughts on this have been:

1.) try adding another 25mfd/50vdc cap across the 1.5K resistor from 12AX7 pin 8
2.) reverse the red & blue wires going to the OT (red now runs to A+ supply node - blue to pin 3 of the 6V6)
3.) change the values of the supply resistors from 10K/2W & 22K/2W to 1K/2W & 10K/2W respectively to increase the plate voltages a little

I kind of also remember reading somewhere about using shielded wire at 2 key spots in the circuit but I've been unable to find that article again & I didn't take notes when I 1st came across this. Can anybody help shed some light on what I should do or perhaps offer another suggestion & please if possible don't just provide what to do but a bit of an explanation as to why so I can learn & not have to be a bother in the future?

[Mandopicker]

I did the exact same thing a while back, but kept the tube rectifier. More work required and a new PT, but I liked the idea of using the tube.

It is a great amp and really sounds nice.

Champ 600 Point-to Point Rewire! The first One?

Fun project.

FWIW: Try switching the OT leads as you suggested.

[Chuck H]

+1

Motor boating IS an oscillation. Something is talking to something else with PFB. But... This could also be a power supply or grounding issue. For reasons I'm not educated enough to site an amp can sag itself into phase shifts in the power supply causing oscillations. If reversing the OT leads doesn't work you could try seperating the filter cap ground points (unless you have a multi can with only one ground tab) or try changing where the input and first gain stage preamp is grounded. Or both. You could also try a bigger main filter since you don't have a rect. tube. This would change the phase of the power supply under load.

Chuck

[MWJB]

"1.) try adding another 25mfd/50vdc cap across the 1.5K resistor from 12AX7 pin 8
2.) reverse the red & blue wires going to the OT (red now runs to A+ supply node - blue to pin 3 of the 6V6)
3.) change the values of the supply resistors from 10K/2W & 22K/2W to 1K/2W & 10K/2W respectively to increase the plate voltages a little"

1) & 3) will increase gain and will make the problem worse, as will eliminating the 22K NFB resistor. 2) probably isn't relevant if it only oscillates with the volume flat out...I'd expect it to do it at most settings if your primaries were reversed.

Layout is important with this chassis, post a pick of your build, we might see something relevant.

[Howlin' Mad Mac]

With so many forums out there it's hard to choose but it would seem I've chosen wisely. Thanks for the great ideas I've received so far. I will pull this thing apart over the upcoming weekend & try switching leads to the OT, even if just for sh*ts & giggles & while it's apart snap a few photos (guess I should've done that first anyway but better late than never) & will post them asap. As far as it only motorboating when flat out that's not entirely true. With an instrument plugged in it only starts to act up as I approach full volume (which seems somewhat less than it was before I gutted the original Chinese pcb which I found a bit odd as there is now no tone stack involved) however if I unplug the instrument I hear the same oscillation at lower volume. Also removal of the NFB resistor seems to be a big improvement (not the other way round) but this was done at the same time I added the cathode bypass to pin 3 so the improvement from previous may have been due more to that than eliminating the 22K resistor. Guessing as no one touched the subject of using shielded wire that's not thought to be an issue. Again thanks guys & I'll get those pics posted as soon as I'm able.

[MWJB]

"Guessing as no one touched the subject of using shielded wire that's not thought to be an issue." Well 5F1 were built without shielded wire, but they had a proven layout, even if I build one today, I'd still use sheilded wires to pins 2 & 7 of the 12AX7. It's the sort of thing that's more practical for the small scale/home builder...a factory would get away with it if they can (time is money & all that).

[Howlin' Mad Mac]

Originally Posted by MWJB

"Guessing as no one touched the subject of using shielded wire that's not thought to be an issue." Well 5F1 were built without shielded wire, but they had a proven layout, even if I build one today, I'd still use sheilded wires to pins 2 & 7 of the 12AX7. It's the sort of thing that's more practical for the small scale/home builder...a factory would get away with it if they can (time is money & all that).

Big thanks there guy! I knew I had read that in one article or another somewhere but when I tried to find it again I didn't have any luck. I doubt that alone will solve all my issues with this build but I know it's one of those things that'll never hurt, inexpensive to do & just shows everyone peeking inside this amp was built with love. Gives me warm & fuzzies all over... LOL

[Howlin' Mad Mac]

Well I finally got the chance to open this pig up & make some changes over the weekend. Unfortunately I forgot my camera at work so I couldn't snap some pix while I had her open. Anyway I simply swaped out some shielded wire for the unshielded I used in my original build (connected to pins 2 & 7 of the 12AX7) & that got rid of the oscillation thankfully. Now what I've got sounds pretty darn good (& a damn site better than the stock Champion 600 did) except you can only hear it if it's turned all the way up to 12 & anything below 9 or 10 sounds like it's not amplified at all. Also there is almost no overdrive or "brown sound" at all (but using a Boss DS-1 pedal, gain at max, tone about 1/3 & distortion at 1/4 or a lot of boost/very little fuzz) I get very nice tone just much weaker than I feel it should be & not as powerful as stock. Again my build is a solid-state rectified 5F1 w/ NFB removed, 25mfd/50v cathode bypass cap across 1st gain stage using brand new Sovtek 12AX7 & JJ 6V6, original Chinese trannies, original 6" POS speaker (I know... I gotta get a Weber.) Any thoughts or ideas my more knoweledgable friends?

[Howlin' Mad Mac]

P.S. Also wanted to mention in my build I also swapped out the .022 "Orange Drops" coupling caps for a couple of .022 "Mojo Vitamin T" oil & foil caps. I can't imagine why they would cause an amp to lose all its balls but figured it's worth mentioning. Thx.

[Chuck H]

The cap (most likely) isn't the problem. Unless the cap is faulty. If you have the tools you should do a signal trace to find out where the signal drop out is. That amp should be able to crunch up just fine. If it doesn't something is miswired. Probably a "Oooh, how'd I get that wrong" kind of error, but it's there. You could do it with any AC signal appropriate for the input (keyboard, tuner signal, signal generator, etc.) and a volt meter. Just get some kind of fixed signal running through the amp and check the AC volts along the signal chain. You'll find a dropout where it shouldn't be somewhere.

Chuck

[Sir Cuitous]

+1 on what Chuck said.
You can also check your voltages and post them here. If anything isn't right someone here will know.

[Howlin' Mad Mac]

Originally Posted by Chuck H

The cap (most likely) isn't the problem. Unless the cap is faulty. If you have the tools you should do a signal trace to find out where the signal drop out is. That amp should be able to crunch up just fine. If it doesn't something is miswired. Probably a "Oooh, how'd I get that wrong" kind of error, but it's there. You could do it with any AC signal appropriate for the input (keyboard, tuner signal, signal generator, etc.) and a volt meter. Just get some kind of fixed signal running through the amp and check the AC volts along the signal chain. You'll find a dropout where it shouldn't be somewhere.

Chuck

I'll try that soon as I get a few moments to open 'er back up. Great idea about using the reference tone from a tuner. I don't have a lot of equipment I'd really like to have (signal generator, oscilliscope, etc.) like back in electronics school all those many moons ago & I would've never thought about using the tuner "beep". I'm sure you're right about it being something simple but I've looked it over until my eyes have gone fuzzy & still can't see what I could've missed. Only other things I can think of would be to lower the supply filter resistor values to increase voltages a bit (which I wouldn't mind trying to much) or swap out the stock transformers for new ones (due to the price of transformers something I really don't wan't to do) as this is just a POS amp anyway.