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1964 Vibroverb AB763

2/5/2006 9:33 PM
Tim C.	1964 Vibroverb AB763 I have an original 1964 Vibroverb that I have been strugglimng with for over a year. I hope someone can help. When I play the amp at 5 or above on the volume it exhibits ghost notes about an octive below the note you are playing in both channels. I have recapped it and fixed the filament wiring. I also replaced the output transformer. I am at a loss. Please help. I hate to gut it and rebuild it but I am just about to that point. Tim C.

2/5/2006 9:33 PM

Tim C.

1964 Vibroverb AB763

I have an original 1964 Vibroverb that I have been strugglimng with for over a year. I hope someone can help.

When I play the amp at 5 or above on the volume it exhibits ghost notes about an octive below the note you are playing in both channels. I have recapped it and fixed the filament wiring. I also replaced the output transformer. I am at a loss. Please help.

I hate to gut it and rebuild it but I am just about to that point.

Tim C.

2/7/2006 8:55 AM
KB	Tim you may want to go to the general discussion page and check out Ray Ivers post on Listening To The Power Supply. Theres some ghost noting sitings going onn there. KB

2/7/2006 11:17 AM
Ray Ivers	Ghost Noting Tim, IMO the most likely causes of ghost-noting are, in order of likelihood: a) speaker problems ("cone cry", etc.) b) power supply/grounding problems c) guitar problems I'm sure you've already tried many different guitars and probably speakers too, so that leaves b). Last Friday I spent all day on a ghost-noting problem, and my final diagnosis was that at full power the B+ ripple greatly increased, while at the same time one side of the phase inverter clipped before the other, unbalancing the output stage and reducing its inherent push-pull ripple rejection. The dynamics of this PS/PA behavior vary from amp to amp, but IME it happens to some extent in most guitar amps, although it may not always produce audible ghost-noting. I won't bore you with how to completely eliminate it on a design level, as I don't think you'll want to redesign/rebuild the power supply and driver stage of an original '64 Vibroverb, nor would I recommend such a thing. But there are some things you can try, if you're open to making some minor/easily-reversible mods, which it seems that you are. I would try the following changes, in order: 1) Jumper across the phase inverter 'feedback tail' resistor (4700Ω in a 6G16, 47Ω in the AA763 & AB763 Vibroverbs) with a clip lead to eliminate the negative feedback loop, and see what effect this has on the ghost-noting, if any (this is more of a diagnostic aid for the moment, although you may end up liking and keeping this tweak - but leave this jumper in place for all subsequent modifications so you can clearly hear what they do or don't do). Notice that I didn't recommend lifting the feedback wire from the speaker jack, or the series feedback resistor (either 10K or 820Ω ) from its eyelet. Then... 2) With the NFB removed, try different phase inverter tubes and see if you can hear any differences in ghost-noting between them. You may get lucky and find a PI tube that's imbalanced in just the way you need it to be. 3) Bite the bullet and replace the GZ34 rectifier tube with a solid-state replacement, then greatly increase filter capacitance at schematic points A, B, C, and D (don't try this with the tube rectifier, you'll smoke it). If it were my amp, I would use 100uF or even 200uF of effective filter capacitance at point A (let me know which Vibroverb circuit you have and I'll recommend some specific caps), and at least 50uF/500V caps at points B, C, and D. Common Internet 'wisdom' says this will make your amp "stiff" and "sterile"; you be the judge. One thing's for sure - if the ghost noting is power-supply-ripple related, this will kick its ass. Again, this can be done (carefully! I shocked the sh*t out of myself on Friday!) with clipleads. 4) Install an AC-balance control on the phase inverter. Try the other things first, and then if you want to try this I'll work out the numbers for you. If the PS ripple is reduced and the PA balance improved at full power, you should achieve a very noticeable reduction of the ghost-noting, perhaps to inaudibility. Ray

2/7/2006 11:17 AM

Ray Ivers

Ghost Noting

Tim,

IMO the most likely causes of ghost-noting are, in order of likelihood:

a) speaker problems ("cone cry", etc.)
b) power supply/grounding problems
c) guitar problems

I'm sure you've already tried many different guitars and probably speakers too, so that leaves b).

Last Friday I spent all day on a ghost-noting problem, and my final diagnosis was that at full power the B+ ripple greatly increased, while at the same time one side of the phase inverter clipped before the other, unbalancing the output stage and reducing its inherent push-pull ripple rejection. The dynamics of this PS/PA behavior vary from amp to amp, but IME it happens to some extent in most guitar amps, although it may not always produce audible ghost-noting.

I won't bore you with how to completely eliminate it on a design level, as I don't think you'll want to redesign/rebuild the power supply and driver stage of an original '64 Vibroverb, nor would I recommend such a thing. But there are some things you can try, if you're open to making some minor/easily-reversible mods, which it seems that you are.

I would try the following changes, in order:

1) Jumper across the phase inverter 'feedback tail' resistor (4700Ω in a 6G16, 47Ω in the AA763 & AB763 Vibroverbs) with a clip lead to eliminate the negative feedback loop, and see what effect this has on the ghost-noting, if any (this is more of a diagnostic aid for the moment, although you may end up liking and keeping this tweak - but leave this jumper in place for all subsequent modifications so you can clearly hear what they do or don't do). Notice that I didn't recommend lifting the feedback wire from the speaker jack, or the series feedback resistor (either 10K or 820Ω ) from its eyelet. Then...

2) With the NFB removed, try different phase inverter tubes and see if you can hear any differences in ghost-noting between them. You may get lucky and find a PI tube that's imbalanced in just the way you need it to be.

3) Bite the bullet and replace the GZ34 rectifier tube with a solid-state replacement, then greatly increase filter capacitance at schematic points A, B, C, and D (don't try this with the tube rectifier, you'll smoke it). If it were my amp, I would use 100uF or even 200uF of effective filter capacitance at point A (let me know which Vibroverb circuit you have and I'll recommend some specific caps), and at least 50uF/500V caps at points B, C, and D. Common Internet 'wisdom' says this will make your amp "stiff" and "sterile"; you be the judge. One thing's for sure - if the ghost noting is power-supply-ripple related, this will kick its ass. Again, this can be done (carefully! I shocked the sh*t out of myself on Friday!) with clipleads.

4) Install an AC-balance control on the phase inverter. Try the other things first, and then if you want to try this I'll work out the numbers for you.

If the PS ripple is reduced and the PA balance improved at full power, you should achieve a very noticeable reduction of the ghost-noting, perhaps to inaudibility.

Ray

2/8/2006 9:17 AM
R.G.	By the way, Ray, one thing you can do that will drastically change the power stage's effects on the pre/PI sections is to put a 1N4007 in series with the first resistor Pi-section. This keeps the power amp from draining back the voltage from the preamp on peaks. In the PI, you can do something similar by using a diode in series with the choke, but you have to have a diode reverse biased to ground on both sides of the inductor to let the inductor clamp to ground in case the current ever goes discontinuous. It keeps the power amp current drain out of the PI's knickers.

2/8/2006 9:17 AM

R.G.

By the way, Ray, one thing you can do that will drastically change the power stage's effects on the pre/PI sections is to put a 1N4007 in series with the first resistor Pi-section. This keeps the power amp from draining back the voltage from the preamp on peaks.

In the PI, you can do something similar by using a diode in series with the choke, but you have to have a diode reverse biased to ground on both sides of the inductor to let the inductor clamp to ground in case the current ever goes discontinuous.

It keeps the power amp current drain out of the PI's knickers.

2/8/2006 9:30 AM
Ray Ivers	R.G., I've been meaning to try that diode thing! It seems like it would work really well in isolating the PA's PS-loading effects from the sensitive preamp & PI circuitry. How about large-value series resistances to ground before/after the choke, instead of diodes? It seems like the reverse-biased diodes might be conducting small spikes to ground quite often at high output power if the series diode does its job, and might eventually fail; just a thought. Ray

2/8/2006 9:30 AM

Ray Ivers

R.G.,

I've been meaning to try that diode thing! It seems like it would work really well in isolating the PA's PS-loading effects from the sensitive preamp & PI circuitry.

How about large-value series resistances to ground before/after the choke, instead of diodes? It seems like the reverse-biased diodes might be conducting small spikes to ground quite often at high output power if the series diode does its job, and might eventually fail; just a thought.

Ray

2/8/2006 11:34 AM

R.G.

quote:
"How about large-value series resistances to ground before/after the choke, instead of diodes? "

Bad idea. The inductor will do whatever it has to to keep the current flowing. The current will be whatever was flowing in the inductor when the attempt at interruption was made. The induced voltage will be the pre-interruption current times the resistance and that can be large indeed. The diodes make a low resistance clamp, so the voltage is clamped to ground instead of hundreds of volts minus.

quote:
"It seems like the reverse-biased diodes might be conducting small spikes to ground quite often at high output power if the series diode does its job,"

They would, about onece per attempt at interruption.
This is a standard use of diodes in switching power supplies, to give the inductor a clamped place to stand when the current is interrupted. In such applications, the diode clamps once per cycle at 50kHz to over 1MHz.

Note that the reversed diode is not breaking over, it's forward conducting. The diode needs to be high enough voltage not to break over reverse.

quote:
"and might eventually fail; just a thought."

I don't think so. If it's just forward clamping, the current is low, just the sum of the PI and preamp plate currents. A 1A diode like th 1N4007 has a repetitive surge rating of around 100A peak, so the current spikes won't kill it. You do need to arrange to keep the reverse voltage under 1kV, but the reverse direction voltage (that is, the normal B+ supply voltage, which is in the reverse direction to the diode) is limited by some biggish capacitors on each side of the inductor, so the inductor would have to overfill those capcitors to overvolt the diode solidly. Instantaeous overvoltages would be clamped by the caps.

2/8/2006 3:47 PM
Ray Ivers	R.G., Oh, OK, I never thought about the negative spike, only the positive one - and as you said, that would be sucked up by the caps. Whenever I think of a reverse-biased diode from HV to ground, I always think of the Music Man 3kV OT-protection diodes, which apparently are performing the same function; I replaced so many bad ones that I figured they were being used in a sacrificial reverse-breakdown manner, which would be pretty hard to do in a 3kV diode, come to think of it. Ray.

2/8/2006 3:47 PM

Ray Ivers

R.G.,

Oh, OK, I never thought about the negative spike, only the positive one - and as you said, that would be sucked up by the caps. Whenever I think of a reverse-biased diode from HV to ground, I always think of the Music Man 3kV OT-protection diodes, which apparently are performing the same function; I replaced so many bad ones that I figured they were being used in a sacrificial reverse-breakdown manner, which would be pretty hard to do in a 3kV diode, come to think of it.

Ray.

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