Still no break-up. 2x 5F1's.

[Sea Chief]

Is this your schematic, I don't see any Cathode bypass caps?
http://el34world.com/charts/Schemati..._5f1_schem.pdf
If it is not please give me a link to the correct schematic.

There is not a fender link specifically to a 5F1 with these bypass caps afaik, there is only the older version online. They are an addition by fender on the original schematic so some of their 5F1's have them, some dont.

Here is a schematic of a 5F1 with bypass caps. It is all the same as any 5F1 but is just a more 'modern' lets call it 5F1 with the 2 additional caps. Most builders afaict use this version/ probably Fender's own reissue 5F1 too I'd bet.

C6, C7 https://taweber.powweb.com/store/5f1_schem.jpg

[big_teee]

There is not a fender link specifically to a 5F1 with these bypass caps afaik, there is only the older version online. They are an addition by fender on the original schematic so some of their 5F1's have them, some dont.

Here is a schematic of a 5F1 with bypass caps. It is all the same as any 5F1 but is just a more 'modern' lets call it 5F1 with the 2 additional caps. Most builders afaict use this version/ probably Fender's own reissue 5F1 too I'd bet.

C6, C7 https://taweber.powweb.com/store/5f1_schem.jpg

I like this one.
http://el34world.com/charts/Schemati...-Schematic.pdf
It adds a bypass on V1b, and should have more OD.
Where is the voltage that is 100v too high you referred to earlier?

[Justin Thomas]

So, if I underwtand you correctly, this is the only amp you've got to work with at the moment and need to tweak it to suit your needs? That's a valid position then. Well, I can at least answer this: if it's 100V too high, I wouldn't worry about it if your caps are up for it. SF Champs run at 420V all day long AND have a lossy tone stack too, yet they distort. I highly doubt that the too-high voltage is the source of your lack of distortion. Is anything going to fry from it? Probably not, if your caps are up to it and you have a good 6V6GTA or JJ. But will you be able to lower it to the "proper" 5F1 level of 320V? Probably not without a new PT. It sounds like the PT is already spec'd for a SF anyway - is this the amp with the MM PT?

You've done everything you can, without drastically modding/rebuilding the amp. There's nothing more you can do to it, since it only has one knob to begin with, without "radical brain surgery" to the amp in question. Get your hands dirty. Since you have all the parts and it won't cost anything extra, why not just rebuild it from input to output? It's what, 20 parts? Takes time, but no money. It&s that or a pedal out front, which I know you've already excluded. Did you try disconnecting the -ve feedback yet? And IS the Volume pot the right taper? I know the value is right. And no, my point was that MOST of the Actual Champs or clones I've played whether vintage or not are rather deficient (to me) in the distortion department, and too clean, even on 10.

I understand your frustration - I'm sitting on a 66 Jet with the same damn problem.

Justin

[Tom Phillips]

Sea Chief,

…I will remove the feedback resisitor maybe as a last go...

Did you ever try lifting the 22kΩ resistor? It is R13 on the version of the schematic that you attached to post #113. As I see it, the suggestion is just a diagnostic test at this time so don’t think of it as a permanent change or as a possible fix for your primary problem. Another important step is to verify that the R13 installed in your build is really 22kΩ. (Within ±20% anyway). The resistor will be easy to measure when it is lifted. You only need to disconnect one side of the resistor. Either side will do so just pick the side that is easiest to disconnect. If the actual value is way below 22kΩ then the amp will be badly affected because there will be excessive negative feedback. You should also verify the actual value of the phase inverter cathode resistor. It is R6 on the posted schematic and should be approximately 1,500Ω. Don’t just look at the markings on the resistors. Do an actual measurement with your Ohmmeter and post the results. It’s also OK to play the amp with R13 lifted and tell the group haw the amp performs in that condition.

Note: To get a valid measurement of R13 one end of that resistor needs to be disconnected from the circuit.
You can measure R6 while both ends are still in the circuit.

All these resistance measurements should be done with the amp turned off and caps drained of all charge.

Regards,
Tom

[Chuck H]

Sea Chief,
To answer your question about the high voltage I can only speculate that the PT is a "vintage correct" model right down to the 110V primary. Or at least has the HV set up to run higher voltages as if it were on a 110V primary. Either would increase voltages over an original plugged into 110V by about 45V. If the amp in question also uses a diode rectifier, as I know at least one of your amps does, that will also increase volts by another 45V. So... 90V too high.

If this is the amp with the low PT voltage, yet has a higher than normal B+, I'll speculate that the rectifier circuit is "bridge" type. Producing a lot more voltage than a more typical full wave type rectifier circuit.

Justin mentioned that for him most Champ type amps are "too clean". Your descriptions of your amps behavior have been clear enough. I just want to say that what I'm sure Justin meant was that they aren't dirty enough. Not necessarily that these amps are clean, as in don't distort or clip.

Once you get the sine wave generation figured out you can run some signal voltage tests with the amp open and you and we will be better able to determine if the amp is behaving normally. It won't even take very long once you figure out how to put a steady signal of a known voltage into the input. You will need to post reports between tests and wait for responses to do further tests, but overall the amount of time should be small. Just take it as it comes and try to hang onto some patience and faith.

[Sea Chief]

Ok thanks for this input chaps.

The amp in Q (just one for now) seems to have excesively high B+ at 392v. The PT, so I was told (for this is why I bought the amp innitially, just for this unit > to swap into amp1, but as its actually rather a nicely made amp it makes little sense now so I keep amp2 as it is: a plain jane 5F1) was a 110v Champion 600 afaik that has been rewound specifically to Champ specs by an amp Co. I Q'd the seller on this 3x and he is still adamant it was made to (EU) Champ spec.

Maybe -just maybe- he has made a SF Champ spec'd PT. Though Im pretty sure he would have made to the ubiquitous 5F1 Champ as he knew it was for a DIY self build by the seller, who he knows very well indeed. That would account for the high B+. Whether or not that then accounts for my OD issue/ lack of.. is speculation on my part only I guess.

I will lift R13 certainly and see how this affects things & post results. As to remodelling the amp to the AA764.. blimey I dont think I can cope with that tbh. Besides Im so pressed for space (its a 600 width chassis/ tiny!) that I dont think this is a viable plan tbh.

The HV caps are reated at 475v (TAD germany- usually vg quality).

The 6V6 is a 6n6 (this IS a 6v6gt) russian military rugged looking thing. I tried a EH 6v6 too (sounded the same/ acted the same).

R6 (furthest RHS on board) with its 25/25 bypass cap, indeed does measure bang on 1.5k.

Thanks SC.

[Sea Chief]

Chuck H-

I do have the tone generator thing page that goes ooooooooooo! But further than that I have no idea how it is used. How it is even conected to my amp is like an exercise in existential engineering, unless I'm shown how! maybe its fairly simple.

[Sea Chief]

The Vol pot is a CTS 1Meg, and I sure is the correct taper even if 'AUDIO' not on it, as the vol doesnt suddenly jump between 7-10 say. Its even enough dialing/ turning for me to think its correct.

[frus]

linear volume pot would jump suddenly between 0 and 1, and very gradually between 1 and 10

[Sea Chief]

IT GROWLS!

GODAMMIT IT GROWLS!

I lifted 22k fdbk r leg & it measures 28.7k (but its colours/ I checked all on rcving, say its defo a 22k). It might be a bit hard and nasty sounding and it still needs dialing to 7 of 10 to start breaking up, which is damn loud as its 6w.. & now I have a persistant hum introduced which needs addressing.. but a result even so.

Shivver me timbers stone the crows etc. Persistance pays off!

Thanks chaps- really grateful for the continual help! Sea Chief.

[Dave H]

Single ended amps do hum unless the power supply is well filtered. You could reduce both hum and B+ voltage by adding a PI filter to the power supply. It's only two components, a 1k 10W wirewound resistor and a 47u 450V electrolytic capacitor (both from Maplin). 1k will reduce the B+ by about 50V.

[Sea Chief]

Single ended amps do hum unless the power supply is well filtered. You could reduce both hum and B+ voltage by adding a PI filter to the power supply. It's only two components, a 1k 10W wirewound resistor and a 47u 450V electrolytic capacitor (both from Maplin). 1k will reduce the B+ by about 50V.

Dave H-

So wouldnt I be killing 2 birds with one stone with this suggestion? IE stop the hum (wasnt there before I did the 22k lift) and get the B+ down too? 50v less would mean Id be down from 392v to 342v: bang on the (fender orig) B+ 5F1 schematic figure.

There's always a caveat.. it cant be that straightforward/ I wait for it then.

[Jazz P Bass]

You may want to try different values for the feedback resistor.
You could use a 50K pot as a test.
Somewhere there may be that spot where the hum is reduced & the distortion is still there.

I had a look at the Fender Champion schematic & there are a few subtle changes that were made to the 5F1 circuit.
The B+ is stated as 360Vdc.

One: the pi filter was added.

Two: the feedback resistor value is retained as 22K but there is an added 47 ohm 'tail resistor' at V1B cathode.
Not too sure what this does. Lifts the ground maybe?

Three: they added a screen resistor

Four: there is an added grid resistor to the 6V6.

Five: they dumped the 22K dropping resistor for the 12AX7 source voltage.

http://music-electronics-forum.com/a...lmanual-1-.zip

[Chuck H]

Since we're talking mods...

A bit conscientious, but a twisted filament circuit with a filament center tap lead to the top of the 6V6 cathode circuit can virtually eliminate hum in these amps IMHE.

I'm all for adding a "sag" resistor as part of a Pi filter as per Dave H.

Raising the preamp voltages, as per Jazz's post will also provide more drive for the power tube grid, increasing clipping.

The added sag resistor and power tube grid resistor will also reduce the harshness.

Another idea might be a "conjunctive filter". Which is just a fancy term for a resistor and capacitor circuit across the OT primary leads. The right component values can really smooth out the tone without making the amp dull sounding.

Fine tuning these little amps can be confusing for a novice. We've done this crap for years. If you're up for it there's every possibility of idealizing the tone for clipping.

[Dave H]

I had a look at the Fender Champion schematic & there are a few subtle changes that were made to the 5F1 circuit.
The B+ is stated as 360Vdc.

One: the pi filter was added.

Two: the feedback resistor value is retained as 22K but there is an added 47 ohm 'tail resistor' at V1B cathode.
Not too sure what this does. Lifts the ground maybe?

They added an RC filter for the screen. That won't reduce the B+ voltage. I was thinking of a pi filter before the OT like the one in the 5D2 Princeton.

The 47 ohm resistor is the bottom resistor of the feedback divider. It will have more gain like that (less -ve feedback). The 5F1 uses the 1k5 bias resistor as the bottom resistor of the feedback divider.