How does this work?

[ricach]

I can't get my head wrapped around how a circuit like this works.... specifically how the signal at Pin 8 got amplified, I understand the tone circuit.

(after clicking on attachment, you'll probably have to click on the picture, than click once more to zoom in)

[Hardtailed]

I can't get my head wrapped around how a circuit like this works.... specifically how the signal at Pin 8 got amplified, I understand the tone circuit.

(after clicking on attachment, you'll probably have to click on the picture, than click once more to zoom in)

I'm guessing this is the classic cathode-follower config (I don't see how the anodes are connected, but I'm assuming).

The signal is not actually amplified, but rather buffered... in a way

Here's the long explanation:
The Valve Wizard -Cathode Follower

[Wilder Amplification]

One of the reasons I could see where that would be confusing is because they cut off the plate resistor on the gain stage that drives the cathode follower. But there is a plate resistor there.

That's the standard DC coupled cathode follower tone stack driver circuit found in the Bassman as well as the majority of Marshalls.

[J M Fahey]

Agree and add:
Pin 1 has a load resistor to +B, usually around 100K; that first (left) triode amplifies around 50X if a 12AX7.
That signal is direct coupled into the second (right) triode, suffering no loss (nor amplification).
About 95% of it appears on pin 8, at a much reduced impedance (a couple KiloOhms) instead of the around 50K it would have otherwise.
You might ask: what's the purpose of that cathode follower, given that a typical unbuffered Fender stage drives properly a tone stack very similar to this one?
Reason is, it drives it "better", specifically on very high and very low frequencies; net result is the amp sounds beefier.
The tone control is lossy, being passive, so the volume control gets around 10 to 25% of the signal present on pin 8, yet the whole stage, including the triodes, has a net gain of around 10X.
Of course that gain depends on frequency and on pot position, that being the function of tone controls.

[ricach]

Thanks. On this particular amp the gain pot drives that tube, and what shows up on pin 2 is an excellent, healthy looking signal. however, the signal coming off of pin 8 is garbage - pretty enemic. I'm focusing on the problem being in the tone stack. The signal coming off the stack does not respond to the tone controls, and the volume works opposite - 10 being off. (the volume pot has been switched out already).

btw: this is an egnater rebel 30 amp i'm working on - they were reluctant to give me a schematic, but did give me one that has no values on it. because of their reluctance, i didn't think i should post the whole schematic.

[Hardtailed]

Make sure all caps in the tone stack are ok. These are not just for voicing, they block DC. If they start leaking, they can screw up with the 2nd triode (the cathode follower) bias.

The treble cap is often a Silver Mica and those often get leaky under high DC potential.

[ricach]

I figured the same thing, I replaced it already. C19 is a .00039u 1KV ceramic disk. I jumpered from the high side of the treble pot to the wiper of the volume, hoping to bypass the entire stack, and I get nothing, no sound at all. makes no sense.

[Hardtailed]

I figured the same thing, I replaced it already. C19 is a .00039u 1KV ceramic disk. I jumpered from the high side of the treble pot to the wiper of the volume, hoping to bypass the entire stack, and I get nothing, no sound at all. makes no sense.

Try lifting the ground from the middle pot (that sort of take the tone-stack out of the loop... sort of)

Make sure you have no DC on any pot (volume included) as referenced to ground.

[ricach]

I'm getting -10mV on the backside of the caps. Is that significant?

[J M Fahey]

Hi ricach, go step by step.
1) Start by lifting one leg of C19 and R22; that disconnects the tone stack.
2) Measure DC voltage on pin 1 (plate), you should have between 140 and 200 something V there. Now that you are there, post values of components shown on the partial schematic.
3) On Pin 8 you should have around 2V DC more than on pin 1. Be careful.
4) Inject around 100mV AC on pin 2, you should get around 5 V AC on pins 1 and 8.
5) Turn off, discharge, etc.
Re-solder free legs of C19 and R 22.
If your signal dies, C19 is probably shorted or *very* leaky.
Post results and we'll go on.
PS: don't understand the need for "mystery" on tube amps, they are all the same, with minor tweaks on some part values, and maybe an extra tube stage, which is just the others.

[ricach]

Hi ricach, go step by step.
1) Start by lifting one leg of C19 and R22; that disconnects the tone stack.
2) Measure DC voltage on pin 1 (plate), you should have between 140 and 200 something V there. Now that you are there, post values of components shown on the partial schematic.
3) On Pin 8 you should have around 2V DC more than on pin 1. Be careful.
4) Inject around 100mV AC on pin 2, you should get around 5 V AC on pins 1 and 8.
5) Turn off, discharge, etc.
Re-solder free legs of C19 and R 22.
If your signal dies, C19 is probably shorted or *very* leaky.
Post results and we'll go on.
PS: don't understand the need for "mystery" on tube amps, they are all the same, with minor tweaks on some part values, and maybe an extra tube stage, which is just the others.

C19 had been replaced already.
Lifting both C19 and R22 I get
V1 - 188.1V
V8 - 189.0V

also
V2 - -11mV
V3 - -11mV
V6 - 288.3V
V7 - 188.7V

I can only 'inject' a guitar strum.. On pin 1, excellent signal, but nothing on Pin 8. That's a new development since what I used to get was a horrible enemic signal coming off of 8 when the tone stack was engaged.

Legs reattached, still nothing. I'll get the component values and update the schematic.

***Update. probe not making the best contact on pin 8, i do get a signal, but still not good.****

[ricach]

Updated schematic attached.

[DIY_Guy]

PS: don't understand the need for "mystery" on tube amps, they are all the same, with minor tweaks on some part values, and maybe an extra tube stage, which is just the others.

I think Bruce is more concerned about hiding his 'Tube Mix' and wattage control circuit.

[J M Fahey]

Hi ricach

C19 had been replaced already.
Lifting both C19 and R22 I get
V1 - 188.1V <-as expected
V8 - 189.0V <-as expected

also
V2 - -11mV <-not as expected ("0") but acceptable
V3 - -11mV <-NOT as expected, ,should have 1.2 to 2.5V, measure again, check for bad contact, etc. Does *not* match the normal reading on V1
V6 - 288.3V <-as expected
V7 - 188.7V <-as expected

On pin 1, excellent signal, but nothing on Pin 8.

Are you measuring with C19 and R22 still lifted? I guess not, please confirm.
Please fill in the tone stack values.
With no signal and C19 soldered in circuit, measure DC voltage on its right pin, the one that connects to the top of VR7.
Measure with VR7 on "0" and then on "10". Any change? Post both DC voltage values.
Thanks.

[ricach]

Hi ricach


Are you measuring with C19 and R22 still lifted? I guess not, please confirm.
Please fill in the tone stack values.
With no signal and C19 soldered in circuit, measure DC voltage on its right pin, the one that connects to the top of VR7.
Measure with VR7 on "0" and then on "10". Any change? Post both DC voltage values.
Thanks.

Yeah, I was just thinking I needed to relift them and recheck. I should add that I did already check voltages with the pots on 10 and 0, and did not get the expected 0volts with all pots at "0". Nor did I get any change in voltage as I dialed down the pots. Suspecting bad ground, I grounded the volume pot to various places in the circuit with no change. Checking the resistance of each pot from the wiper to either side, they all appeared correct except for the mid pot. It shows zero resistance at the "0" and "10" positions which tells me the pot is shorted from top to bottom. I should probably remove it from the circuit and see if its the pot or the circuit that is shorting it.

I also should verify the schematic with the actual pcboard.

The nagging thing that keeps haunting me is that this channel did work properly for a few seconds when I 'fiddled' with the original volume pot. So I replaced it, but obviously didn't change a thing. Suspecting a possible bad board trace, I removed the pot altogether and moved the output to the treble wiper so in effect it now acts as the volume control. But no change. Of course I should have updated the schematic to reflect this.

I will recheck all this again tonight and update the pot values. Thanks.