1970s Ampeg V4 Strange Input Stage Behavior

[Helmholtz]

interesting about the zener. I get that it doesn’t do it’s jam with no current flowing, what’s funny is all 4 sockets have the same voltage on pin 4 regardless if they are occupied. I agree a series zener won’t improve regulation…my understanding is someone wanted the screen to always be 51V lower than the plate. I’d have to think long and hard about this and running -50V bias to decipher what their intention was; I’m guessing this isn’t a common mod for V4s.

Your meter resistance will cause a small zener current even without a tube inserted.
It is a good idea to use seriers zeners to lower screen voltage when above 450V.

[Helmholtz]

I see it first in the input second stage (12AX7) after the volume control gets a hair beyond 10 o’clock …to tell you the truth I believe I even see it on the grid (pin 2); but the plate on pin 6 is a perfect sign wave. Let me get some scope pictures.

So I guess it's caused by grid conduction at pin 2. What is the peak-to-peak voltage here?

[35L6]

Did you say your input amplified a third of a volt to 58V ? That's a gain of 160 or so . Your input isn't 3.3V is it ?

[g1]

Does the manual specify RMS or peak-to-peak signals?

attached is the service manual with the specified voltages. see post # 3 for schematic.
I'll note that errors are not uncommon for voltages listed on ampeg schematics, usually in preamp. Sometimes you have to do a lot of backchecking of plate vs cathode vs supply node voltages.

[dubulup]

So I guess it's caused by grid conduction at pin 2. What is the peak-to-peak voltage here?

I was remembering wrong, pin2 grid is clean even at MAX channel volume...looks about 20Vp-p. I've got 8 scope pictures, but can't seem to upload them here (size issue) and I don't have a photo hosting site. I'll see about signing up for a free one.

Did you say your input amplified a third of a volt to 58V ? That's a gain of 160 or so . Your input isn't 3.3V is it ?

yes, at max channel volume the RMS reading is 50+V. The input measures 0.333V RMS at the input jack.

attached is the service manual with the specified voltages. see post # 3 for schematic.
I'll note that errors are not uncommon for voltages listed on ampeg schematics, usually in preamp. Sometimes you have to do a lot of backchecking or plate vs cathode vs supply node voltages.

Those are the voltages I was hunting.

next I should be investigating the shared anode resistor and the coupling cap for this stage...they are common and would cause both channels to get out of whack. The DC measures 244V here...

[Helmholtz]

yes, at max channel volume the RMS reading is 50+V. The input measures 0.333V RMS at the input jack.

Not clear where you're measuring that voltage.
The manual specifies a V1 pin 6 signal of 10Vrms (corresponding to 28Vpp) with an input of 0.3Vrms.
This corresponds to a gain of 33.

[dubulup]

Correct, I get that at 10 o'clock...when I MAX out the channel volume it skyrockets. anode resistor resistor measures 68k...coupling cap is quite old (maybe original) but couldn't see that causing an issue at the plate of the 12AX7. Think it's worth changing?

[Helmholtz]

Correct, I get that at 10 o'clock...when I MAX out the channel volume it skyrockets. anode resistor resistor measures 68k...coupling cap is quite old (maybe original) but couldn't see that causing an issue at the plate of the 12AX7. Think it's worth changing?

Again, where do you measure what voltage?

The channel volume cannot increase the V1 pin 6 (plate) voltage.
And the signal at V1 pin 2 cannot be larger than the signal at V1 pin 6.

[dubulup]

Shit sorry...you are right. I get 7V on pin V1-6 (channel 2 is 9.2V) and dialing the volume knob to 10 o'clock I get the .5V on pin V1-2 and 5.8V on pin V1-1; basically spot on for the service manual.

12 o'clock noon on volume I get 38.8V on pin V1-1 (channel 2 is 43.3V)...maxing it out goes beyond 50+V.

This may be 100% normal for this amp...should the amplfication skyrocket so quickly. I mean I know the pot is linear but damn, it gets distorted very early in the stages at 12 noon with 0.333V input signal.

Resized the images and figured out the upload tab...titles should explain what is going on.

[dubulup]

shoot...the names aren't visible after posting (edit...damn they didn't show up in posted order either; hovering over them the title does show up. Such a bonehead)

2 - input at jack
7 - V1-6 after C3
1 - V1-2 (10 o'clock)
4 - V1-2 (MAX)
3 - V1-1 after C7 (10 o'clock)
5 - V1-1 after C7 (2 o'clock)
8 - V1-3 (10 o'clock)
6 - V1-3 (noon)

thanks for bearing with me! The #'s above are the order in which they appear on my screen

[g1]

Not sure what is going on with the photo numbering and pinouts, but 2nd stage of V1 has around 2V at the cathode. That means at around 2V pk at the grid (pin2) it will start clipping. Your photo # 4 shows far more than that.

edit: maybe pics 4 and 6 are reversed?

[dubulup]

Ah yes, thank you for that…I plotted the schematic values on the 12AX7 chart and swinging more than -2Vp we hit E voltage way before +V swing would stop amplifying.

This leads me to believe the amp is still functioning properly and something else in his signal chain is either hotter now than before or maybe a speaker is damaged in his cabinet???

Again his complaints,
1- distorts earlier on the dial than previously
2- volume doesn’t increase any more (beyond say noon) with channel knob (master volume maxed)

[Enzo]

I haven't given up on my phase inverter notion. I wasn't suggesting the 12AU7 was bad, but more that the circuit around it was funny. And half a phase inverter might still work reasonably well up to some moderate level before going wonkers.

[Dave H]

love to hear some thoughts on the asymmetrical distortion I get in the input stage. Maybe this amp is still perfect and his cabinet is at fault or the pedals feeding it.

he wants more headroom and says the volume doesn't increase like it used too.

My thoughts:

I notice V1,1 plate is directly connected to V2,1 plate without any mixing resistors. This means V1,1 plate has to drive the output impedance of V2,1 plate (and vice versa) making clipping very asymmetrical as you can see from the scope screen shots. It could be working “as designed” like Enzo said.

The vol pots are linear taper which makes the volume come up quickly with pot rotation and there may not be much volume increase after 12 noon compared to a log pot. I'd set the pre-amp clean and check the power output at the onset of clipping into a dummy load.

[Helmholtz]

What is the voltage drop across (between the ends of) R13 (68k)?
That voltage determines the positive clipping level of the plate signal (V1 pin 1).