6G3 vol/tonestack

[makepeace]

Hi!

I was just wondering if anybody could explain how this control configuration works in a 5E3, possibly including some maths if you care to. Its something that's been baffling me for a while. I've implemented this in my 5E3, but I find that the bass response suffers tremendously and as things stand, its far too bright for my liking. I'd just like to try and understand how the filters work so that I can find the 'right' values for the caps.





Thanks!
makepeace

edit: i've swapped the stock 5E3 0.005uF cap for a 0.01uF as in the 6G3, which has helped matters some, but its still very midrangey and doesn't have the same loose bass as in the normal 5E3. is there any way to get around this?

[makepeace]

Hi, is this hallowed ground or something?

[loudthud]

Most people complain that the stock 5E3 has too much bass. The mod most often recommended is to reduce the coupling caps from 0.1uF to 0.047uF or 0.022uF and change the cathode cap on the input triode to 2.2uF or 4.7uF. This makes me think there is something wrong in your amp, a bad capacitor, wrong resistor or possibly a bad speaker or transformer.

The Fender tone control works by shunting highs around the volume control or shunting them to ground. The volume control functions like a voltage divider and attenuates all frequencies but the tone control allows some of the high frequencies to get past with less attenuation. This sounds like a treble boost because the highs are attenuated less than the bass frequencies. Note that when the volume control is at maximum volume, no treble boost is possible. Since the cap value interacts with both Volume and Tone controls pots and the tubes to establish break frequencies, the math is very complex. Cap values are usually established by ear.

[pdf64]

makepeace, the t/v controls are interactive so there's no one correct answer. If you could specify at particular setting of t/v then someone cleverer than me may be able to work it all out.
Are you sure that all values are correct and it's wired up correctly?
As it is, experimentation is your best bet; after all there's only 3 caps!
Are both your pots audio taper types? If tone is linear that could be your prob.
If the tone is way too bright at vol low tone high, then reduce the value of the 470pF cap.
There's nothing in the 6G3 that will cut bass, whereas there will be significant bass roll off at very low volumes on a 5E3 (as the coupling cap feeds the vol pot wiper).
In my 5E3 type amp, I've used the 6G3 t/v on one channel, just vol on the other (with a 1nF coupling cap), and 270k mixers; it seems a good arrangement, for me.
Pete.

[makepeace]

Thanks for reading!

Most people complain that the stock 5E3 has too much bass.

Yes I am well aware of this. I find it useful, tonally, well at least if its dialed in right, and with a Rangemaster on the front it gets a lot tighter and sounds great!

The mod most often recommended is to reduce the coupling caps from 0.1uF to 0.047uF or 0.022uF and change the cathode cap on the input triode to 2.2uF or 4.7uF.

Read this a lot as well, I don't have the problem with the bass and would rather not mess with those as I believe they contribute to what makes a 5E3 a 5E3 (although one could say the same for modding the TS to 6g3 standards, however I intend to put in two push-pull vol pots, one to make the 'normal' channel normal (as it stands its a TMB tonestack) and one to switch back to the standard 5E3 tone/vol configuration for the bright channel). I've also got a 'clean' switch, which swaps the V2 triodes cathode bypass cap for a 0.01uF and switches in an NFB loop.

I've eventually figured out how to use the 5E3 controls, and I really dig the trick, however I decided to make versatility the priority as this is my only amp at this stage (bearing in mind that I read that there wasn't much of a tone compromisation involved). Admittedly I am usually playing at low volume (say between 9 and 12 o'clock), which might be contributing to the lack of adjustabillity on the bass end (when I turn it full anticlockwise, the tone is midrangey with hardly any bass, and full clockwise to 12 o'clock is a bit brighter than the 5E3 config, but as I say, things got better with the 0.01 uF cap.

The Fender tone control works by shunting highs around the volume control or shunting them to ground. The volume control functions like a voltage divider and attenuates all frequencies but the tone control allows some of the high frequencies to get past with less attenuation. This sounds like a treble boost because the highs are attenuated less than the bass frequencies. Note that when the volume control is at maximum volume, no treble boost is possible. Since the cap value interacts with both Volume and Tone controls pots and the tubes to establish break frequencies, the math is very complex. Cap values are usually established by ear.

Great! Thanks for the explanation. Guess I'll have to do some more fiddling, really thought I was done! I'm happy with what the 0.01uF cap swap has done, so I'll play around with the small cap a bit.

makepeace, the t/v controls are interactive so there's no one correct answer. If you could specify at particular setting of t/v then someone cleverer than me may be able to work it all out.

Are both your pots audio taper types? If tone is linear that could be your prob.

Wouldn't want someone to work it out for me! A couple of guidelines as to the theory of the circuit would be great though. As mentioned, volume generally is between 9 and 12. My hypothesis is that the ground cap acts as a low pass sending highs to ground and the small cap is a high pass, attenuating low frequencies, both in conjunction with the resistance of the pots? Maybe a bigger ground cap and a smaller small cap would help? All pots are audio taper.

Are you sure that all values are correct and it's wired up correctly?

Positive everything is in order in this regard, have checked it over a couple of times, and besides, all I've done since it was working perfectly as a 5E3 was swap a set of wires and move a cap around in the TS. Have put so much time into this amp the past couple of months that its almost become an extension of my brain. Terrifying!

In my 5E3 type amp, I've used the 6G3 t/v on one channel, just vol on the other (with a 1nF coupling cap), and 270k mixers; it seems a good arrangement, for me.

I'm using a 220k for the bright channel and a 100k for my TMB channel as mixers, which is working well. Still compresses like a 5E3 when I turn up the TMB volume which, pretty cool.

Guess I'll be ordering some (more) caps to fiddle around with then. Anybody have any recommendations for a range for each?

Thanks for your time!
Jono

[makepeace]

could a bigger coupling cap than 0.1uF be the answer, say 0.68uF? or is that opening up another kettle of fish?

[loudthud]

Unless you are playing with the volume controls way down, 1 or less, a 0.68uF won't increase bass frequencies usefull for guitar.