Need help understanding & tweaking 5F4 Super tonestack

[RobBozic]

I can't follow the 5F4 type tonestack, ie: how do you tweak the mids, treble & bass etc..?

My 5F4 has a lot of lower mids & low end which could be from using Ragin Cajun speakers, it seems like the tone isn't balanced with these speakers, sort of a lopsided frequency response if that makes sense.

Anyway can someone explain the tonestack in the 5F4.

Thanks in advance
Rob

[RobBozic]

Actually,

I just foundthat the Duncan Tone Stack Calc. has an "E- Series" plot which I didn't know existed. I never got past the Fender & Marshall plots!

[loudthud]

Wow! I never saw the E-Series in TSC either. The range of the controls is really wide.

In general I would start with the fixes usually applied to 5E3s. At the volume controls the .02uF could be reduced to .01uF or maybe as low as .005uF. Generally any cap in the signal path might be used to reduce bass. But when you change caps within a feedback loop, the results can be not what you expect. The 5E3 has no feedback loops. You try to reduce bass but the feedback trys to make up the difference and you get distorted bass. In TSC reducing C3 to 20nF gives a little bass rolloff but it's in a feedback loop so it might not sound good. Same with the .1uFs at the 6L6 grids.

[RobBozic]

Hey "Loudthud", it's amazing that so many people didn't know that the 'E Series' exists on the Duncan Tone Stack Calculator.

Anyway, I changed the treble & bass pots down to 250KA, the plots now resemble the typical TMB Marshall / Fender style. If you keep the treble & bass at "5" it's close to a Fender Blackface , but then at "8" it gets close to the 5F6A. The amp also lost some volume, not a bad thing with those Ragin Cajun speakers.

I've learnt so much in two days with regards to the 5F4 type circuit. One of the things is that Leo Fender probably just chose the 1M pots because it meant it was easier & cheaper for his stocking arrangement.

'Loudthud', I'm thinking of changing the PI to the 5F6A long tail type. From the treble pot wiper it would go straight into the existing 0.022 cap & the rewire the rest of V3 to suit, do you see any issues with this?

Rob

[loudthud]

The 5F6A PI will work fine. Note that there will be a strong tendancy to get the phase reversed causing the amp to oscillate the first time you turn it on with feedback connected. If you need more overall gain, increase the 4.7 meg to 10 meg or higher.

[RobBozic]

The 5F6A PI will work fine. Note that there will be a strong tendancy to get the phase reversed causing the amp to oscillate the first time you turn it on with feedback connected. If you need more overall gain, increase the 4.7 meg to 10 meg or higher.

Loudthud, that's exactly what happened, each time I turned the amp it would start howling very loudly. So I disconnected the NFB & it was OK, the amp sounded great, but one minute later I saw some smoke & shut the amp off. One of the 470R screen resistors smoked themselves.

I'm a bit confused, firstly how do I correct the oscillating feedback? Secondly why would the 470R burn out?

I also moved the OT centre tap before the choke as per 5F6A.

Thanks
Rob

[loudthud]

To correct the phase you can reverse the connections to the output transformer at the 6L6 plates (pin 3). That's usually the easiest unless one wire is too short. You could also reverse the wires at the speaker jacks. Another alternative is to reverse the wires going to the 6L6 grids (pin 5).

The 470 ohm burning might indicate a bad 6L6 or just be a symptom of the oscillation. If the resistor measures less than 600 ohms or so, I'd be tempted to at least try the 6L6s after fixing the phase to confirm the tube is bad. See if the tube(s) bias normally. If the resistor is burned badly or way off value, replace it and the tube(s).

Sidebar: What kind of load was connected when the amp oscillated? If the frequency of oscillation was high and speakers were connected, screen current could go quite high burning the 470 resistor without totally destroying the tube. A non-inductive dummy load would have provided more protection against high screen current. The tube may not be good as new but still have some life left in it, at least until you get all the bugs out of the amp.

[RobBozic]

Thanks Loudthud,

The speakers were hooked up all the time (hoping I didn't burn them out as well), one of the 470R still measured at 470R but the other one didn't measure anything at all. Once I replaced it I could literally see it was burnt.

I've replaced the 470R resistor but haven't turned the amp on as yet. I'll swap the OT primary wires to the 6L6 plates first.

I need to invest in some test equipment as I really love building amps, I've still got a lot to learn though.

Thanks again
Rob

[tubeswell]

Hi Rob

(I haven't looked back through the posts in detail so excuse if you covered his but) out of curiosity, what power rating was the 470R screen resistor you were using?

[RobBozic]

The 470R resistors were 2W metal oxide. I've since replaced them with 1K 2W because that's what I had lying around.

Rob