5F1 - hum and speaker crackle

[loudthud]

The first place I would look is the feedback around the output stage. Disconnect the 22K resistor from the speaker to the 12AX7 to see if that kills the oscillation. One thing to try is simply increasing the 22K to 33K. You can also experiment with the taps used for connection to the speaker.

One thing that worries my about grounding is the connection to the chassis at the output jack. You might want to experiment there as well.

[electrochronic]

Ok Amp is back on the bench , I'm going to find this problem no matter what.

MWJB

Myth voltages - Ok , I just don't have a real Fender champ to get reallife comparison voltages. Good to know. I won't press this for now.

No , I'll try including this change with the coupling cap on this round of bench mods.

Loudthud

I have tried isolating the output jack - no changes , which surprised me. I even ran the ground to point "A" on your schematic

While doing the later, I also tried different primary tap impedances on the OT,
but in each case no significant changes.

I'll fiddle with the 22K resistor and report back.


Interesting article on star grounding found at Aiken amps www site. even mentioned potential oscillation cause by grounding techniques where the preamp, output stages and B- are mixed improperly. Maybe I should check into this as I might have a unforseen problem.

Also for curiosity I took the plate voltage 410 and the bias current and came up with approx plate wattage or 410 vdc * .043ma = 17.63 watts on the plate of the 6v6. My RCA book says 12 watts max for 6V6, but maybe this is a conservative number I don't know. I guess having more B+ just gives me a louder amp. Don't know if this is good for my OT , which is max rated at 8 watts. The correlation between the two may not mean that I'm actually throwing ~12 or higher watts of output power thru the OT. (Now, If I were an Analog EEngineer I might actually make some sense out of this)

[loudthud]

Aiken's site is a good read and he occasionally posts comments here. I wanted to mention that the original Fender amps had 4 ohms speakers. So if the taps you have selected are predicated on an 8 ohm load, that would change the feedback factor in the output stage. Most builds don't seem to be sensitive to this but that's why I suggested the 33K change. Don't be shy about trying other values.

One more thing that just came to mind is the absence of any grid stoppers. A 1.5K in series with the grid of the 6V6 and/or a 470 ohm 1W in series with the screen grid are pretty common in push pull amps but you don't see them much on low powered single ended amps like the 5F1. Certainly worth a try if nothing else works. They should be installed close to the 6V6 socket for maximum effectiveness.

[electrochronic]

The speaker is a 4 ohm jensen. No worries here. Although, I did mesaure the resistance of the speaker and it's around 3.2 ohms, notwithstanding, my OT transformer has taps for 3.2 ohms as well. In any case I would assume its a 4 ohm speaker because that is what is stamped on the speakers metal frame.

Currently running a primary selectable impedance of around 6400 ohms on the OT which is my starting point.

I kept my grounding bus wire as before but terminated it at the first 16uf filter cap, which is then wired
directly to the HT center tap wire then chassis. Its really just reversed from the diagram in my first post with the addition of the wire to HT C-Tap.

I suspect I have do not fully have my grounds as clean as I may think I have. I say this because the Output stage and preamp stage are still tied into this bus ground. I may be mixing unwanted ground currents which
may be the casue for the oscillation. Not sure yet, but I'll break them up a see what happens

I may also need to isolate my input jacks and take a dedicated ground wire to preamp ground.

I'll try the grid stopper too.

[electrochronic]

MWJB,

The cap .02 and 220K resistor , If I'm following you correct , should these
be moved closer to the 6V6 socket ?

[electrochronic]

Forget dropping the B+, even a Fender built champ won't run the voltages on the schem, your chasing a myth.

Have you relocated the coupling cap that runs from V1, pin 6 to the 6V6 grid (and the 220K load resistor at the 6V6 end of this cap?

It is not a myth to get the correct voltages if you go with a PT with
different B+, But it certainly is a myth if you go with a 325-0-325 B+
which is listed as a champ replacement PT - nonsense that these are
being sold to unknowing new builders.

I tried a PT with the following specs and it gave me nearly spot on
voltages throughout the circuit. Runs quiet and cool. tubes will last longer.
270-0-270 120ma (540vct)
3.5-0-3.5 3.5a (7 vct) requires needs a 1 ohm dropping resistor for 6.14vac
5 - 3a (5.4vac)

The Fender schematic shows voltages in the power supply to be
340,295,250. I measured mine at 350,299,260. If you use a so called
Fender replacement transformer you will get 410-430, 360, 300. No one will
tell you this before you buy one.

Wow, even lowering my voltages in the circuit did not cure my oscillation problem at high gain settings.
However , it did not introduce any new problems either. I think the tubes will like the new lowered voltages
in the circuit , overall sound just as good until oscillation shows up at higher gain.

I tried relocating the cap and 220K , I found no significant changes with my overdrive, oscillation. Its still
there when the amp is pushed at higher gain settings.

[electrochronic]

The first place I would look is the feedback around the output stage. Disconnect the 22K resistor from the speaker to the 12AX7 to see if that kills the oscillation. One thing to try is simply increasing the 22K to 33K. You can also experiment with the taps used for connection to the speaker.

One thing that worries my about grounding is the connection to the chassis at the output jack. You might want to experiment there as well.

I tried all the above and no significant changes to the oscillation problem.

[electrochronic]

Aiken's site is a good read and he occasionally posts comments here. I wanted to mention that the original Fender amps had 4 ohms speakers. So if the taps you have selected are predicated on an 8 ohm load, that would change the feedback factor in the output stage. Most builds don't seem to be sensitive to this but that's why I suggested the 33K change. Don't be shy about trying other values.

One more thing that just came to mind is the absence of any grid stoppers. A 1.5K in series with the grid of the 6V6 and/or a 470 ohm 1W in series with the screen grid are pretty common in push pull amps but you don't see them much on low powered single ended amps like the 5F1. Certainly worth a try if nothing else works. They should be installed close to the 6V6 socket for maximum effectiveness.

Tried all the above and no significant changes to oscillation overdrive problem at high gain stage.

Added a 470ohm 1 watt across pin 4 and 6 on 6V6, also added 1.5K ohm 1/2W between pins 1 and 5 on 6v6.
No luck , problem is still there.

I have replaced every component in the amp except x2-8uf caps and the OT transformer. I'm thinking of trying a OT with higher wattage rating maybe around 15W or so. Maybe I'm saturating the current one which is a Hammond 125C rated for 8 watt and 60ma DC.

Gutted the amp 3x and tried different ground layouts including elimination of bus ground. All 3 of the new ground layouts worked fine when related to reducing hum and buzz ( the amp is completely quiet even at full volume, just a bit of hiss, thats all) , but the oscillation is still there and unchanged at high gain settings.

Lastly, I must tell anyone that a center tap for the heater is absolutely necessary in getting the amp to
really be quiet. Either type will work , a real center tap or 2x 100 ohm resistor are equally good.

[MWJB]

"It is not a myth to get the correct voltages if you go with a PT with
different B+, But it certainly is a myth if you go with a 325-0-325 B+
which is listed as a champ replacement PT - nonsense that these are
being sold to unknowing new builders." OK, so what are the "correct" voltages - you tell me, based on your experience? I can tell you, that if you have a vintage Fender that reads the voltages that are printed on the 50yr old schem - IT IS BROKEN! THE SCHEMS ARE WRONG! 350-0-350 is typical for a champ PT running a SE 6V6...there are slightly lower voltage Tweed Champ PTs available but a shade over 400vdc after the rectifier is not necessarilly a bad thing - SF champs have often run like this with the same power tube for years, even decades.

Look again at the PT spec - 325-0-325 @ 70mA - are you drawing 70mA from the B+ rail. When I put a 50-60mA draw on my champ PT I get 390vdc on the plate with a GZ34 rectifier, 390/1.3 = 300-0-300VAC. Be aware that final B+ depends on current draw, wall AC at that time of day, type & brand of tube, how that tube is biased & that PTs are subject to the same manufacturing tolerances as anything else. You are fixing a "problem" that you simply do not have, you should be concentrating on getting rid of your oscillation.

270-0-270 may sound fine to you, if so, no need to change it, but it will not give you typical champ voltages.

You simply do not need a 15W OT for your amp either, your problem is not likely down to your transformers, but your compact layout & wire routing. Please post update pictures including tube socket wiring.

Isolated jacks are not necessary either.

[electrochronic]

"OK, so what are the "correct" voltages - you tell me, based on your experience? I can tell you, that if you have a vintage Fender that reads the voltages that are printed on the 50yr old schem - IT IS BROKEN! THE SCHEMS ARE WRONG! 350-0-350 is typical for a champ PT running a SE 6V6...there are slightly lower voltage Tweed Champ PTs available but a shade over 400vdc after the rectifier is not necessarilly a bad thing - SF champs have often run like this with the same power tube for years, even decades.

Look again at the PT spec - 325-0-325 @ 70mA - are you drawing 70mA from the B+ rail. When I put a 50-60mA draw on my champ PT I get 390vdc on the plate with a GZ34 rectifier, 390/1.3 = 300-0-300VAC. Be aware that final B+ depends on current draw, wall AC at that time of day, type & brand of tube, how that tube is biased & that PTs are subject to the same manufacturing tolerances as anything else. You are fixing a "problem" that you simply do not have, you should be concentrating on getting rid of your oscillation.

270-0-270 may sound fine to you, if so, no need to change it, but it will not give you typical champ voltages.

You simply do not need a 15W OT for your amp either, your problem is not likely down to your transformers, but your compact layout & wire routing. Please post update pictures including tube socket wiring.

Isolated jacks are not necessary either.

Agreed the oscillation my main concern , the transformer is not the issue but exploring the possibility and finding my voltages to be very close to the schematic sounded like a good move. I think my voltages are in a very conservative territory now with no additional problems resulting from the move to a different transformer. Had to rule this out since nothing else seems to be working in ridding the oscillation problem. I suppose I'm discovering some things along the way, which makes this even more interesting. Short of getting a turret board and blueprinted chassis this build has little circuit differences. Very puzzling.

I guess I'm not quite understanding why incorrect schematics are circulating all over the web , including
Weber versions and other kit versions with very similar voltages printed on them. Trying to make sense of this ? There are so many inconsistencies regarding this amp and its voltages , it makes it ever confusing when trying to get the build correct.

Nevertheless, I do appreciate the help and hope I can get to the bottom of my oscillation issue.

Keep in mind the oscillation issue is heavily influenced at higher gain setting ( guitar volume and amp volume cranked ) at this level I should be hearing nice distortion breakup this amp is known for,

I'll repost new photos and layout very soon. thanks

[MWJB]

"Short of getting a turret board and blueprinted chassis this build has little circuit differences. Very puzzling." It's very tempting to think that as long as "tab A" goes into "slot B" and that all the obvious connections match up, that when you get an issue that it must be blamed on this part, or that part, or a high voltage. You're certainly not the first & definitely won't be the last.

Trouble is, there are lots of factors affecting your/any build that you can't directly see with the naked eye (magnetic couplings, poor grounding & layout, etc)...until you build up a level of experience or have these things pointed out to you. Nobody is born with this knowledge it has to be learned and different individuals may have slightly differing methods to ensure a trouble free build...once you find a method that works for you, stick to it.

It's not something limited to home builders/hobbyists either...plenty of mass produced amps have foibles that folks just have to live with.

Now how about those updated photos?

[electrochronic]

Well, you can probably tell by my tone I've been up all night with the soldering
iron - no sleep yet and little progress. My build has turned into rats nest too. Looks more like a prototype build now.

time to get some ZZZZZ, then clean up the build and photos to come.

[electrochronic]

On pin 4 of 6V6 reading around 363+ volts dc with my 325-0-325 original tranformer installed. This is the B+ in front of the 22K resistor in the power supply. I have a 1/2w resistor there. I have tested it with ohm meter and it reads ~23K, but it could give trouble when the circuit is live. Obviously you cannot take ohm measurements with a DMM while the circuit is live.

I have read that 1/2Watters are not good to use past 350Vdc

Some build notes say 22K 1/2W while others say 22K 1W

With ohm's law, knowing voltage and current should tell me what wattage resistor to use. Just don't know the current since some of it is probably being dropped on the 10K 3w and filter caps as well.

Question : Should this resistor be 1W or greater ?

[MWJB]

No harm in using 1W or even 2W, they will last longer...however your 1/2W resistor will probably last years. Plenty of Fenders have survived 40+ years with 1/2W resistors being fed over 350vdc. They way to measure what the resistor is dissipating is to square the voltage drop accross it, then divide by ohms, use a resistor rated for at least twice the dissipation.

E.g. Say you had 66v dropped accross that resistor, (66*66)/22000ohms = 0.198W. Looks like 1/2W would be fine there, I would set out to build with 2-3W but I don't think that the 1/2W 22K is the cause of your issues. Run the amp for a while, then remeasure idle voltages, any significant change?

[electrochronic]

see recent post #32 for corrected diagram