I have a couple questions about an amp schematic?

[Newmarket]

With all due respect - can I suggest that the "chat for cash" element if the thread title be removed. You are not asking for detailed design work or PCB design / manufacture etc.
Ime chatting about this stuff is what people are more than happy to do here.
Happy to be corrected if the mods disagree with my take on this.

 

[vintagelove]

With all due respect - can I suggest that the "chat for cash" element if the thread title be removed. You are not asking for detailed design work or PCB design / manufacture etc.
Ime chatting about this stuff is what people are more than happy to do here.
Happy to be corrected if the mods disagree with my take on this.

That's fine by me, maybe even move to a more appropriate section (where guitar amplifier circuit questions would make sense).

I do appreciate everyone's input so far. I have a great workbench project for guitar guys I'll put up in thanks. Just have to put together the info.

 

[Admiral-dk]

In a Fender (as the Title in the above eludes to) - you usually have RT as 250K Linear, RB as 1M Log, R1 is 68K and C1 either 250pF or 500pF (depending on model) .... Unfortunately for this, both Rin and RL are not the Fender values either - so another shift in result too ....

+ right now it is way past my bed time (as I have to get up for work tomorrow) => so good night for now.

Oh and try using the sliders - I don't expect much Bass control with the values shown above (I could be wrong) ....

Per

 

[Newmarket]

That's fine by me, maybe even move to a more appropriate section (where guitar amplifier circuit questions would make sense).

I do appreciate everyone's input so far. I have a great workbench project for guitar guys I'll put up in thanks. Just have to put together the info.

Yes. Understood. It's down to the mods to move or not. But I think that what you are asking is well within what people here are willing to help out with on a community basis. If you wish to show appreciation I'd suggest to support the site itself. All Good. Cheers.

 

[abbey road d enfer]

Pretty sure they should be grounded, the grid/cathode voltage won't change otherwise & I can't see how it would amplify anything.

+1

The op-amp driving the output transistors is correct, I think, because they're both common-emitter connected, which inverts the signal.

Correct. I had not seen that.

 

[vintagelove]

In a Fender (as the Title in the above eludes to) - you usually have RT as 250K Linear, RB as 1M Log, R1 is 68K and C1 either 250pF or 500pF (depending on model) .... Unfortunately for this, both Rin and RL are not the Fender values either - so another shift in result too ....

+ right now it is way past my bed time (as I have to get up for work tomorrow) => so good night for now.

Oh and try using the sliders - I don't expect much Bass control with the values shown above (I could be wrong) ....

Per

Hi, I grabbed the bass pot value from a Super reverb schematic (I have a 64 here that is glorious). I looked through the popular amps, and they all had that same value. Which amps were you seeing the 1m bass pot in? Thanks.

 

[vintagelove]

Pretty sure they should be grounded, the grid/cathode voltage won't change otherwise & I can't see how it would amplify anything.

The op-amp driving the output transistors is correct, I think, because they're both common-emitter connected, which inverts the signal.

+1

Correct. I had not seen that.

Hi, with regard to the cathode

There are several different "tube driver" circuits, but most of them I saw have the cathodes connected to the negative voltage. I won't be able to get into my amps chassis for a few more days as we have work going on there, but I will report back with where they go.

....

In case it was missed by anyone, the opamps are providing the gain, then the starved voltage tubes are clipping the signal. That's consistent through all the tube driver circuits.

 

[Voyager10]

I did a quick LTSpice simulation of the tone control pots.

Bass pot​

Equally-spaced (linear) intervals between 0 and 100%, with Mid and High pots at 50%:

Mid pot​

Again, equally-spaced linear intervals, Bass and High at mid-travel:

High pot​

As you'd expect...

Circuit​

LTSpice model was as follows...

 

[Voyager10]

Just to add: the coupling capacitors & 10K resistors to the tube grids give a bass rolloff. 100nF and (effectively) 20K gives ~80Hz, 47nF and 20K gives ~160Hz. That's certainly going to be audible with electric guitar, where the bottom E string is 80Hz.

I'd try using 220nF caps in both places, see if it helps.

 

[abbey road d enfer]

Hi, I grabbed the bass pot value from a Super reverb schematic (I have a 64 here that is glorious). I looked through the popular amps, and they all had that same value. Which amps were you seeing the 1m bass pot in? Thanks.

It doesn't matter much, since it's connected as a rheostat. 1Meg is supposed to give more bass than 500k, but actually the difference is minuscule. You can see on the graphs in post #28 that the LF response is pretty much cramped after mid-rotation.
Most Marshall amps have 1Meg in Bass.

 

[abbey road d enfer]

Just to add: the coupling capacitors & 10K resistors to the tube grids give a bass rolloff. 100nF and (effectively) 20K gives ~80Hz, 47nF and 20K gives ~160Hz. That's certainly going to be audible with electric guitar, where the bottom E string is 80Hz.

I'd try using 220nF caps in both places, see if it helps.

The problem here uis that usually, there are two different paths for clean and "drive" sound. For "drive" it is quite common to hipass the signal because the pick attack punctures the sound.
Here, I would say that the hipass is more or less correct for dirty sound but inadequate for clean. I would say it's pretty difficult to find a better compromise. Difficult but not unfeasible; as it involves personal taste, trial-and-error is the way.

 

[Admiral-dk]

Sorry for the late reply

I also see that my memory doesn't quite match facts - the majority of Fender Tube Amps has a A250K as a Bass Pot (still more than A100K) - the Bassman and the Deluxe use a A1M in that position. The majority of old Marshall, Mesa, Peavey, etc. use a A1M as the Bass Pot ....

I also see that there are others who already has posted full curves - and they are much closer to the old Fenders than I thought ....

Per