New build - voicing question

[überfuzz]

I'm designing an amplifier and when I simulate it I'm not getting any good base response in the EG. The simulation isn't exactly 100% true to the circuit, but it gives a rough idea. Here I seem to miss out on the base, not sure though. It's a AB763 tone stack, some alterations to fit my stash and some circuitry from different amps.

If anyone can help me and see if I cut the base somewhere. Please feel free to point out any other issues you find!

Edit: It's supposed to function as a guitar amplifier.

[Jazz P Bass]

I cannot say for all of them but the coupling caps look like there values are made to limit the bass response.
As compared to a 'typical' Fender circuit.

[überfuzz]

I cannot say for all of them but the coupling caps look like there values are made to limit the bass response.
As compared to a 'typical' Fender circuit.

Ok, their values are from some guitar amps I borrowed circuitry from. Are there any in particular I should try and alter?

[pdf64]

Get rid of C8, 3n3F feeding the tone stack. That will be rolling off low end.
Are R5&6 attenuators for the sig gen?
The feedback control needs a series resistor, else it will likely oscillate at higher settings.
R20 seems very high at 22k.
Pete.

[Tage]

The grid reference resistors at your phase inverter are only 100k, these should be 470-1meg, I have reduced there value before and found that the bass response suffers, not 100% on the technical reason for this, but I assume it has something to do with loading down the preceding stage. You should also increase c9 or even remove it as C20 is blocking any DC from that area. C10 could be doubled to decouple the grid at all frequencies giving better bass response. Not sure about R15 and C7, seems like that would boost bass?

The coupling caps should still deliver all the bass you need, especialy for guitar, in fact you could reduce the second coupling cap to 10nf and still be rollin in bass

[überfuzz]

pdf64 - Thanks for pointing it out. I changed to the fender styled EQ after that coupling cap was placed.

Tage - I have to read up on phase inverter before I alter them.

[Chuck H]

Ignoring other technical issues for the moment... These are the circuts causing poor bass response. Also, I notice that the pot values indicated in your SPICE are .99, .7, etc. So would that be .99 out of 10 ?!? That would be less than 1 on the knob. But I don't know how your simulator denotes.

[überfuzz]

Ignoring other technical issues for the moment...

Oh, so we do speak the same language after all. :-)

The pots I use in LTspice are scaled from 0 to 1, 0 = min (zero) and 1 = max (the value set by user). Also, I couldn't find any switches hence the bright-switch with out the switch.

I've been checking my stash to opt for stuff I have. The amps is close to air speed now, some alteration and I think we'll have a lift of. Oh, some circuitry from the last version are simply scrapped in this version, here's an update.

The current version might need a little more treble, not sure. I'm running tests, as I write, to see. I think I'll try some high pass dumping in the NFB loop, to see what that might bring.

[Chuck H]

If it's too bass heavy, I find that preamp cathode bypass caps are an especially good way to tune/balance BF topology amps. This amp has A LOT of gain (except for the 10k load at the input). more than you might think. Certainly too much bass in the circuit for an amp with this much gain. One of my early builds was similar in that it was just a BF type preamp straight into a PI. But in the BF amps you have additional loads created by the trem, reverb and mixing circuits. I ended up with a 15k load resistor where you have R8 (your value is 100k) just to tame it to typical BF gain levels. At that level I expect the amount of bass the circuit has might be OK. But if you want to get dirty it'll be a flabby mess. I usually end up with preamp cathode bypass caps of 1uf, 2.2uf or 3.3uf. I try to use 3.3uf of the first stage as it seems to trim just the useless low end and doesn't mess up the TS function like a really small bypass cap. Then on the TS recovery stage I might use something small like 1uf. If the amp needs more fullness on the higher notes I'll change that cap to 2.2uf. But that's my humble experience. Results may vary.

[überfuzz]

If it's too bass heavy, I find that preamp cathode bypass caps are an especially good way to tune/balance BF topology amps. This amp has A LOT of gain (except for the 10k load at the input). more than you might think. Certainly too much bass in the circuit for an amp with this much gain. One of my early builds was similar in that it was just a BF type preamp straight into a PI. But in the BF amps you have additional loads created by the trem, reverb and mixing circuits. I ended up with a 15k load resistor where you have R8 (your value is 100k) just to tame it to typical BF gain levels. At that level I expect the amount of bass the circuit has might be OK. But if you want to get dirty it'll be a flabby mess. I usually end up with preamp cathode bypass caps of 1uf, 2.2uf or 3.3uf. I try to use 3.3uf of the first stage as it seems to trim just the useless low end and doesn't mess up the TS function like a really small bypass cap. Then on the TS recovery stage I might use something small like 1uf. If the amp needs more fullness on the higher notes I'll change that cap to 2.2uf. But that's my humble experience. Results may vary.

Gain
I've been playing around with U10 and R10 to see if I can find any sweet-spot. You're saying I should nerf R8 too? I'm planing to add a BF styled tremolo circuit to this amp, draining signal trough R8 in series with an optobug. Do you know the resistance in an optobug has when the tremolo is OFF?

Bass
I'll tinker some with the cathode caps. I have to look around in my shelf to see if I have any smaller caps to the cathodes.

Edit 1, Turns out I have heaps of 3.3u and 4.7u caps, I'll try some of them.

Edit 2, R1000 and R1001 are there to make it possible to use the wav file.

[Tage]

Yeha the lower bypass caps will help out a lot once things are getting overdriven. I took a closer look at the PI and it has pretty low input impedance, around 200k, which is gonna prevent the PI from being driven very hard, but that may be something you like. Might have to be something you test out IRL, once the build is done.

[überfuzz]

Yeha the lower bypass caps will help out a lot once things are getting overdriven. I took a closer look at the PI and it has pretty low input impedance, around 200k, which is gonna prevent the PI from being driven very hard, but that may be something you like. Might have to be something you test out IRL, once the build is done.

I sneak peeked on an amplifier in which I like the power stage. Not much break up but a lots of power coming out of 4 EL84s. Would you mind explaining how to calculate or pass me a link where I can read about it?

Cheers!

[Tage]

I got my calculations from Merlin's book on designing Preamps.

Valve Wizard - How to design valve guitar amplifiers

An excellent resource, well worth the investment.

[Chuck H]

The feedback loop isn't the place to "adjust" gain IMHO. Though gain is affected by the loop. But you only want enough feedback to tighten things up to where your happy with the tone. After that all you get is sterility and flatter, blander tone. The loop basically nullifies chaos. No chaos is generally a bad thing even for clean tones.

If your planning to add the typical trem circuit there will be a load replacing R8 in your simulation of 50k via the intensity pot. On an actual Fender there is also some small degree of voltage division @ frequency due to the mixing circuit resistors and coupling caps.

[überfuzz]

The tone stack is from a simulation found over at the LTspice forum. . It's not quite the same as the schematics... Here's a png of the tonestack.

Is this a well known version of the AB763 tone stack, or simply wrong?

Edit, funked up attachment!? Trying to attach in next post...