1970s Ampeg V4 Strange Input Stage Behavior

[dubulup]

I’ve been reading a lot on this forum the last couple days and must say you guys are awesome!

I typically work on hi-fi and vintage equipment but I’m helping out as the local amp tech left town suddenly.

the guitarist says one day, something abruptly changed and now has distortion about 10 o’clock on both channels and his volume doesn’t increase past noon…he plays with the master knob maxed (pair of 7027A’s and 16ohm cab). also noted the power lamp started flickering about the same time.

being that it’s both channels I figured it could be PSU related…after checking power tubes. Plates are 535V, screen 475V, and bias -50V (amp has been cares for and has had a lot of replacement parts, 3-prong, adj. bias mods…I removed the polarity and death cap). I threw a set of 6L6’s and the issue remained.

checking with my scope, with 0.333mV 400hz signal, V1-1 displays a perfect sine wave until 10 o’clock. After that point the positive half distorts and the negative half of the signal increases perfectly. Looking at the cathode waveform the negative looks almost flat. Cathode resistor measures 3.2k (1.6-2.4V for all cathode voltages)…I checked the ultra-high switch and sensitivity switches, all appear to work properly and both channels behave exactly the same (at least within tolerance of each other).

the plate on V1-6 is the only measurement that is really high compared to the rest; 243V vs spec’d 195V. V2-6 is 191V.

chasing the AC signal thru the 12AX7s (V1/V2)…the .333 input amplifies to 58 and 64V respectively at 12noon on the dials. Seems extremely high according to the service manual; which doesn’t say where to put the channel volumes but says everything else should be at noon. New 12AX7s tried no difference, pulling one channel at a time…no difference.

I’ll be back at the bench once the kids go to sleep…thank you so much for reading if you made it this far. Love to hear some thoughts. Sorry for typos…typed on my phone without proofing.

[The Dude]

Firstly, can you verify that this schematic matches your amp? I'm not aware of a V4 that has only 2 7027A's.

[g1]

Screens seem quite low, usually they are quite close to plate voltages, maybe 5 or 10V less (check R55). Bias sounds hot, usually more negative than -50, which may be why plate voltage is also low. With bias mods someone probably set it to run hot. Normally with stock bias and modern line voltage, we see more than 545V on the power tube plates. If only 1 pair of power tubes is installed, plate voltage should be even higher.

Schematic attached.

[g1]

Note that sensitivity switch is shown in -9db position, which I think should apply to the DC and AC volts measurements for V1.

[dubulup]

Firstly, can you verify that this schematic matches your amp? I'm not aware of a V4 that has only 2 7027A's.

Correct that is the schematic on the top cover. He only runs a pair…seems appropriate for his 16 ohm cabinet.

[dubulup]

Screens seem quite low, usually they are quite close to plate voltages, maybe 5 or 10V less (check R55). Bias sounds hot, usually more negative than -50, which may be why plate voltage is also low. With bias mods someone probably set it to run hot. Normally with stock bias and modern line voltage, we see more than 545V on the power tube plates. If only 1 pair of power tubes is installed, plate voltage should be even higher.

Schematic attached.

Also correct, I told him it was hot…but I have dialed it back as he was happy with it. I’ll dial it back and see if the screen voltage comes up. But I assume it was hot when it used to be clean…I can’t say something else didn’t fail causes it it run hot now. Obviously nothing moved the pot while the amp was assembled.

[dubulup]

Note that sensitivity switch is shown in -9db position, which I think should apply to the DC and AC volts measurements for V1.

I’ll move the to -9dB and remeasure.

much appreciated for the replies

[The Dude]

Low B+ may also indicate poor filtering. Has the amp been recapped? In recent years, I've rarely (if ever) worked on one of these that didn't need caps.

[dubulup]

Low B+ may also indicate poor filtering. Has the amp been recapped? In recent years, I've rarely (if ever) worked on one of these that didn't need caps.

Yes, newer caps throughout; shiny blue cans. Bias is now -62V and power tube plates / screens are about the same.

couple mods I’m going to try and post pictures of.
A couple of 33uF/450V caps series with resistors (+) side measures just shy of B+ (528V) tied in R55/R42/R47 junction and the (-) -0.767 tied into a brown wire that leads under the board and one end of the stand-by switch and negative side of the filter caps…these voltages are with reference to ground.

then coming off the board to a terminal strip are individual 51V-5W zener diodes dropping the screen voltage…makes sense why the screen voltage didn’t change. Funny part is the two empty sockets the zener measures 9 and 12V respectively while the occupied sockets show 51V drops.

well I learned a great deal about the output stage…still need to sort out the asymmetrical distortion of the input stage, haha!!

[dubulup]

I’ll move the to -9dB and remeasure.

V1-1 = 38.8
V1-2 = 3.79
V1-6 = 7.0
V1-7 = 3.3

V2 is similar…

so the second stage (1,2,3 triode) is still getting overdriven but the mu seems appropriate.

Leaving the 1st stage with 7 volts, according to the service manual I should see less than .5V but I’m getting 3.79V. I’m not seeing any DC on pin 2.

no audio difference between ultra hi 0 & -…+ is slightly louder.

[dubulup]

Ok, so at 10 o’clock channel 1 volume…I can match the service manual values all the way through the 6K11 tube. When I get to the 12DW7 tube I’m in the mV range in the 1st triode…so this is where the master volume comes into play and my parts for the dummy load aren’t here yet; so it’s bed time so I don’t wake the kids.

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.

i think running 4 power tubes with an 8ohm cab will get him that. Thoughts on that?

oh and this amp sounds badass with my bass and 4 ohm cab on the 2 ohm setting…at least before my house starts to rattle.

[Enzo]

dub, may I suggest some general advice. You seem to know whereof you speak, and you came from the hifi world. One thing to never lose sight of is that these are guitar amps and not hifi amps. They are not trying to be hifi. They are designed to add their own character to the sound, not meant to be neutral and accurate.

SOme thoughts in no particular order.

DO both sides of your phase inverter have plate voltage? Is signal looking similar on both plates?

Note the signal for the lower phase inverter triode grid comes from a voltage divider between the two plates, the resistors have different values. COnsidering this amp is 50 years old., those values may have shifted. If the say input stage has a plate resistor way off value, it will still work. But this needs to have a pretty close ratio. DOn't ask me how I wasted an hour or so assuming my 820k rising to 1 meg didn't matter. Check those two resistors for value.

[Helmholtz]

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.

Asymmetrical large-signal distortion is quite normal with single-ended stages (not only tubes).
Where do you see it first?
Best post a scope pic with scope settings.

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

then coming off the board to a terminal strip are individual 51V-5W zener diodes dropping the screen voltage…makes sense why the screen voltage didn’t change. Funny part is the two empty sockets the zener measures 9 and 12V respectively while the occupied sockets show 51V drops.

I understand there's one 51V zener in series with each screen. So when no tube is inserted there's no zener current and the zener can't properly zener.
Also a series zener will not improve voltage regulation.

[dubulup]

dub, may I suggest some general advice. You seem to know whereof you speak, and you came from the hifi world. One thing to never lose sight of is that these are guitar amps and not hifi amps. They are not trying to be hifi. They are designed to add their own character to the sound, not meant to be neutral and accurate.

SOme thoughts in no particular order.

DO both sides of your phase inverter have plate voltage? Is signal looking similar on both plates?

Note the signal for the lower phase inverter triode grid comes from a voltage divider between the two plates, the resistors have different values. COnsidering this amp is 50 years old., those values may have shifted. If the say input stage has a plate resistor way off value, it will still work. But this needs to have a pretty close ratio. DOn't ask me how I wasted an hour or so assuming my 820k rising to 1 meg didn't matter. Check those two resistors for value.

Thank you Enzo, I do understand these amps are intended to distort asymmetrically however, I’m going off the players quoted symptoms “one day he had distortion at 10 o’clock when it used to break up in the afternoon…on both dials”. He’s telling me something has changed. Chasing that gremlin shows a weird input stage distortion I have not seen before.

the PI 12AU7 measures fine and plays fine in other amps…I’ll check resistor values but the distortion is way upstream in the 2nd triode of both 12AX7s.

thank you again, I do appreciate anyone responding to this…as typing this out helps my brain organize thoughts…and once in a while I may come up with a new idea too, haha!

[dubulup]

Asymmetrical large-signal distortion is quite normal with single-ended stages (not only tubes).
Where do you see it first?
Best post a scope pic with scope settings.

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



I understand there's one 51V zener in series with each screen. So when no tube is inserted there's no zener current and the zener can't properly zener.
Also a series zener will not improve voltage regulation.

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.


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.