Ampeg V4 60 Cycle Hum - Pronounced When Reverb is Turned Up

LowHeadRoom

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Hello all,

Hope you are doing well!

I was wondering if there are any Ampeg Enthusiasts out there who might help me out. I have a V4 VT-22. It sounds great and everything works fine. I just put in new capacitors and a three prong cord. However, when I max out the reverb, I get a 60 cycle hum. The hum balance pot does not help. When I disengage the reverb circuit with a foot switch or turn the reverb down, the 60 cycle hum goes away. It sounds as if the power transformer hum is being amplified.. which I guess is what a 60 cycle hum is anyway.

Anyhow, any ideas on this?

Thank you in advance!
 

PhoenixBill

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Is the reverb tank plugged in properly (in and out not inadvertently swapped), are the wires to and from the reverb tank ok, are the reverb tank contacts clean? I will call up the schematic to see which tube is the reverb driver and recovery. Also has the heater (filament) wiring been disturbed or moved, that’s a possible source of hum.
 

PhoenixBill

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Oh, if it’s a head then it’s a V4, if it’s mounted in the combo with the speakers then it’s a VT22 model, other than that it’s the same circuit (a V4). Is this an early Magnavox model, I assume.
 

PhoenixBill

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If it’s the early Magnavox era model like mine, then the 6CG7 is the reverb driver and V203, which is a 12AX7, recovers the reverb signal. Some 12AX7s are more susceptible to picking up hum than others. I would check the cathode bias cap on that half of the tube, C211 on my schematic, it’s an electrolytic and should have been replaced with all other electrolytics.
 

LowHeadRoom

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Is the reverb tank plugged in properly (in and out not inadvertently swapped), are the wires to and from the reverb tank ok, are the reverb tank contacts clean? I will call up the schematic to see which tube is the reverb driver and recovery. Also has the heater (filament) wiring been disturbed or moved, that’s a possible source of hum.
Hi there, thanks for the reply!

Yes indeed, the tank is plugged in properly. I reversed the leads just now to test it ( just in case )... surprisingly the tank still worked with the leads reversed! I was not expecting that. But the buzz persists.

I've ensured that the solder joints are good and that the capacitors and resistors in the reverb circuit are good. Most of them are new. It is not an over powering buzz, but it is louder than it should be I am sure.

Yes, you are correct. It is a 1970 Magnavox model.

The heater wires should still be where they were at the time of manufacturing. I haven't messed with them when I recapped it, but I'll doublecheck.

It sounds almost like a ground loop.
 

LowHeadRoom

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If it’s the early Magnavox era model like mine, then the 6CG7 is the reverb driver and V203, which is a 12AX7, recovers the reverb signal. Some 12AX7s are more susceptible to picking up hum than others. I would check the cathode bias cap on that half of the tube, C211 on my schematic, it’s an electrolytic and should have been replaced with all other electrolytics.
This is really helpful. I'll do this and report back.

The tubes are the originals as far as I know. My Dad never changed them. Pretty incredible. I think one of the 12AX7's may be microphonic. I'll swap it real quick and report back. I'll also check C211. Thanks again.
 

LowHeadRoom

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C211 was changed today.

I swapped V203 but the buzz did not leave.

I appreciate your advice.

You own one of these, correct? What does yours sound like with the reverb all the way up? Do you hear any noise or is it pretty hush?

I'm starting to wonder if that's just the way this amp is. It is dead quiet otherwise.

I forgot to mention that the buzz persists even when the tank has been removed. Not sure if that will help.

Thanks,
 
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PhoenixBill

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I will try to fire mine up later, but no promises that I can get to it tonight…I don’t recall maxing out the reverb on mine since there’s plenty there even turned low. Oh, did you try without the reverb footswitch? That’s connected directly to the circuit, no isolation there, so it could be a source of hum.
 

LowHeadRoom

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I will try to fire mine up later, but no promises that I can get to it tonight…I don’t recall maxing out the reverb on mine since there’s plenty there even turned low. Oh, did you try without the reverb footswitch? That’s connected directly to the circuit, no isolation there, so it could be a source of hum.
Thanks PhoenixBill, much appreciated. No rush.

I tried unplugging the foot switch but the buzz continues. Thanks for the advice. It's cool to talk with another vt-22 v4 owner.
 

PhoenixBill

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Okay I did fire up my V4. I’ve only played this a handful of times since I recapped it back around 2000 but it may be time to recap it again, I have a hint of hum at idle that won’t diminish any further despite tweaking the hum balance pot. Anyway, cranking the reverb all the way up did not add any additional hum. Past noon on the reverb knob was getting into serious reverb land anyways though. I got no additional amp-based hum turning either volume knob up either. Too bad the amp and a 4x12 cab weighs more than I care to tote around, it does sound mighty good. Of course it’s also way louder than I need too. So the weight and the ridiculous volume potential means it doesn’t get used like it should be. I’ve had it since the 1980’s so I’m not quite ready to get rid of it yet though.

You might want to see how much AC ripple you are getting at idle on the plates of V203. Very carefully, clip one end of your meter to ground, set it to AC volts, and (being very careful to to touch B+ anywhere) measure the voltage (on the plate resistor leads would probably be easiest to get to). I can tell you from experience the B+ on these things packs a heckuva wallop, I still recall inadvertently getting bit while inside it sometime in the mid 1990’s. Oh, and check all the DC plate voltages while you’re there too.
 
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LowHeadRoom

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Thanks PhoenixBill! I appreciate you taking the time to do that. Glad to hear your's is in good working order. They really do weight a TON. I struggled to get the 4x12 cabinet over to my bench (for testing) earlier this week.

I will check the voltages. It looks like there should be 115 on the plate. In your post you said to set it at AC, do you mean DC or AC? Thanks in advance!

1660773588830.png

Okay I did fire up my V4. I’ve only played this a handful of times since I recapped it back around 2000 but it may be time to recap it again, I have a hint of hum at idle that won’t diminish any further despite tweaking the hum balance pot. Anyway, cranking the reverb all the way up did not add any additional hum. Past noon on the reverb knob was getting into serious reverb land anyways though. I got no additional amp-based hum turning either volume knob up either. Too bad the amp and a 4x12 cab weighs more than I care to tote around, it does sound mighty good. Of course it’s also way louder than I need too. So the weight and the ridiculous volume potential means it doesn’t get used like it should be. I’ve had it since the 1980’s so I’m not quite ready to get rid of it yet though.

You might want to see how much AC ripple you are getting at idle on the plates of V203. Very carefully, clip one end of your meter to ground, set it to AC volts, and (being very careful to to touch B+ anywhere) measure the voltage (on the plate resistor leads would probably be easiest to get to). I can tell you from experience the B+ on these things packs a heckuva wallop, I still recall inadvertently getting bit while inside it sometime in the mid 1990’s. Oh, and check all the DC plate voltages while you’re there too.
 

PhoenixBill

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For ordinary plate voltage readings, DC. So with your meter set on DC somewhere between 100 and 125 volts would be good. Ripple though is an AC voltage that rides on the DC so you can set your meter to AC to read ripple. I would expect readings in the millivolt range, at that point in the power supply.
 

LowHeadRoom

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For ordinary plate voltage readings, DC. So with your meter set on DC somewhere between 100 and 125 volts would be good. Ripple though is an AC voltage that rides on the DC so you can set your meter to AC to read ripple. I would expect readings in the millivolt range, at that point in the power supply.
Well, I went ahead and tested the plate voltage and all is good. Everything is normal. I replaced some out of spec resistors. The only thing that cuts off the hum is to remove the reverb recovery tube. I have shotgunned every single capacitor and out of spec resistor related to v203.

It must have something to do with the grounding scheme. It's not so pronounced that it ruins the experience of using it, but it just doesn't seem like it should have left the factory that way.
 

arlum

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There aren't many forums with members who can come up with this type of information. PhoenixBill should be both complimented and investigated for his knowledge on this subject. I bow to you, Sir. I used to sell Ampeg V4 heads with different Ampeg cabinet choices at Hamilton Music at Northwest Plaza in St. Louis County, MO. during the years '71 & '72. Ampeg was a bit of a different animal from our other amp offerings. In Ampeg's case it was volume. If you wanted to fill an outdoor concert venue with sound Ampeg was the choice to make. Still ..... it had a weird tube compliment and you had to understand how to use the switches above the knobs on the face of the amp to get breakup in the amps voice. Once you figured it out it was one of the best outdoor venue amps ever made. Sadly .... in today's world of venues most "sound guys" would toss their headphones into the pit and walk away. These amps weren't designed to be mic'd and fed into a house PA system. They were designed to deliver amp tone to tens of thousands of people at an outdoor festival or an open outdoor arena like a baseball stadium or what ever. Used in that manner they couldn't be beat. Again .......... PhoenixBill should be complimented for his knowledge of Ampeg amp builds. TDPRI is Killer.
 

PhoenixBill

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You’re welcome! My dad was an electrician and I started fiddling with electrical stuff back when I was a kid. Then sometime around 1980 while I was in college getting my engineering degree I tried repairing my mom’s old tube stereo, though not successfully because there wasn’t the information readily available to learn from. No Internet, libraries didn’t carry books on amplifier basics. But I started dabbling with guitar electronics, using info from The Guitar Handbook and along the way I started trying to figure out what stuff really did. I got my Ampeg V4 sometime in the mid-80’s but it had an intermittent problem where it would play fine then cut out when warmed up. At least it had the schematic on the lid of the chassis! Eventually by the late 80’s I got a whole bunch of technical books and among them was a text from the 50’s called Elements of Radio which actually explained power supply, how a triode amplifies a signal, etc and now I could make sense of the schematic. Somewhere around then I also picked up a copy of The Tube Amp Book. Though I still didn’t get heavy into amp repair by about 2000 not only had I repaired the Ampeg (coupling cap) I had acquired a signal generator and an oscilloscope and dummy load and made a phono preamp; a couple of other amps later and a few more books I could actually ask intelligent questions when talking to an electrical engineer at work who fiddled with making electronic circuits. By 2005 or thereabouts I found myself getting various old stereos from thrift stores and fixing them up…not just by randomly replacing parts, but actually making measurements and following the signal path on the scope. This continued in earnest in 2012 after my wife passed and not only did I need something to occupy my thoughts, but I hooked up with a guy who sold vintage stereos and whenever he had an amp he couldn’t fix, he would bring it to me. I was living in Nashville then and I recall fixing other tube Ampegs, an old Bassman, an old Echoplex, a bunch of Peavey amps both tube and SS, and restoring a bunch of tube stereos and radios. Oh I restored an old Epiphone Zephyr amp, had the chassis rechromed and built a new eyelet board from scratch. Now I am retired in Phoenix and have a dozen amp projects waiting for the time and money…so while I am not necessarily a true Ampeg expert at least I have a decent idea what to look for.

As far as the really old Ampegs, the book Ampeg: The Story Behind The Sound talks about its founder, Everett Hull, and how he hated distortion in his initial amp offerings. He wanted a clean sound—the Amplified Peg for the upright basses was intended to provide a pure volume increase. So the philosophy of the amps was a clean, well-designed audio circuit. By the mid 60’s though Ampeg had trouble appealing to guitar players and the company was experiencing financial difficulties; they wound up owned by Unimusic then Selmer/Magnavox.
 

LowHeadRoom

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You’re welcome! My dad was an electrician and I started fiddling with electrical stuff back when I was a kid. Then sometime around 1980 while I was in college getting my engineering degree I tried repairing my mom’s old tube stereo, though not successfully because there wasn’t the information readily available to learn from. No Internet, libraries didn’t carry books on amplifier basics. But I started dabbling with guitar electronics, using info from The Guitar Handbook and along the way I started trying to figure out what stuff really did. I got my Ampeg V4 sometime in the mid-80’s but it had an intermittent problem where it would play fine then cut out when warmed up. At least it had the schematic on the lid of the chassis! Eventually by the late 80’s I got a whole bunch of technical books and among them was a text from the 50’s called Elements of Radio which actually explained power supply, how a triode amplifies a signal, etc and now I could make sense of the schematic. Somewhere around then I also picked up a copy of The Tube Amp Book. Though I still didn’t get heavy into amp repair by about 2000 not only had I repaired the Ampeg (coupling cap) I had acquired a signal generator and an oscilloscope and dummy load and made a phono preamp; a couple of other amps later and a few more books I could actually ask intelligent questions when talking to an electrical engineer at work who fiddled with making electronic circuits. By 2005 or thereabouts I found myself getting various old stereos from thrift stores and fixing them up…not just by randomly replacing parts, but actually making measurements and following the signal path on the scope. This continued in earnest in 2012 after my wife passed and not only did I need something to occupy my thoughts, but I hooked up with a guy who sold vintage stereos and whenever he had an amp he couldn’t fix, he would bring it to me. I was living in Nashville then and I recall fixing other tube Ampegs, an old Bassman, an old Echoplex, a bunch of Peavey amps both tube and SS, and restoring a bunch of tube stereos and radios. Oh I restored an old Epiphone Zephyr amp, had the chassis rechromed and built a new eyelet board from scratch. Now I am retired in Phoenix and have a dozen amp projects waiting for the time and money…so while I am not necessarily a true Ampeg expert at least I have a decent idea what to look for.

As far as the really old Ampegs, the book Ampeg: The Story Behind The Sound talks about its founder, Everett Hull, and how he hated distortion in his initial amp offerings. He wanted a clean sound—the Amplified Peg for the upright basses was intended to provide a pure volume increase. So the philosophy of the amps was a clean, well-designed audio circuit. By the mid 60’s though Ampeg had trouble appealing to guitar players and the company was experiencing financial difficulties; they wound up owned by Unimusic then Selmer/Magnavox.
Thanks for sharing your story! That’s pretty cool. I am 33 and just starting the hobby. My dad has had this amp for ages. I would like to continue to learn and progress. I am just now learning to make sense of the schematics (replacing coupling caps, etc). I’m this repair however I am moreover shotgunning components. Overall it must be related to grounding somehow, but I’ve done a bunch of ground tests and can’t figure it out. The amp had two main ground points. The input jacks and a bolt to chassis on the reverb circuit.

I’ve replaced probably every capacitor in the reverb circuit which kind of hurt me to do since it now looks like heavily modified.

But the “buzz” just keeps coming. When the reverb circuit is disengaged, bled to ground or the recovery tube is popped out , there is literally no hum or buzz.
 

LowHeadRoom

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Update:

I replaced all polarized capacitors and the hum lessened.

I replaced C16 .047 uf capacitor with a .068 uf capacitor and all of the sudden the hum reduced to a very tolerable level.

I think that's as good as we are going to get!

I think every capacitor in this amp has now been replaced. What a learning experience!

1660836394566.png



What does this capacitor do? It's attached to the rectifier diodes. Anyone know what it does?
 

PhoenixBill

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I have deleted my original response to your question, to correct what that cap actually does…

I had to look at the whole schematic again and with some help from Google, the correct answer: it’s there to divide the voltage that comes out of the secondary winding. A capacitive divider. So if you have changed the value, re-check your bias voltage to make sure it’s still the -62 volts.
 
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