So, I bought a mid-late seventies Ampeg V4 amp this summer. I am trying to get some ideas to take back to the tech in terms of troubleshooting the hum issue.
The basic issue is that there is a hum at start up that is minimally improved with the hum balance. Moving the controls on the tone stack shows the midrange affects the hum the most. With careful adjustment of the hum balance I can get the thing completely quiet if I turn the midrange all the way down. Turning it up makes it worse.
Some history is necessary here too. After buying the amp I took it to a tech #1 and had the preamp and power tubes replaced and I also had the electrolytic caps replaced as well. I was there when he powered it up, and I'm pretty sure he didn't use a variac to do it. Only after a lot of research afterwards did I discover that you should probably use a variac for this. Anyway there was no improvement in the problem.
I took it to tech #2 who found several burnt out resistors on the preamp board and they were replaced. He also found that the preamp tube closest to the transformer was microphonic. After a lot of switching of tubes he got some reduction in the hum. Not much.
Tech #2 is a good guy and wants to get this thing right. I am going to take it back to him but I wanted to get some ideas as to where he might be looking. He thinks their might be noise from the transformer on that preamp tube.
At the minimum, I think the hum pot should be replaced and I have one for him to install I got from fliptops.net. Whether the lack of variac use means the cap job is screwed, I'm not sure. Ideas about the transformer too would help. My gut says that since things are noticeably worse and better with the midrange means that it lies in the preamp circuit dealing with this control.
Here is a video of my amp doing its hum thing and a quick walk through of what I'm experiencing.
http://www.youtube.com/watch?v=79i2mcNZ ... tube_gdata
A few folks have helped me out here before. At this point I can't ship it out to another tech, I'm going to try and work with the problem locally.
Thanks in advance for any ideas.
Ampeg V4 Hum Challenge
Moderators: pompeiisneaks, Colossal
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Cochise Montana
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Re: Ampeg V4 Hum Challenge
I think the hum balance pot itself is probably fine because it seems to be doing something in the video, but it could be screwed up somehow ,not allowing the heaters get properly balanced. Either way, your tech can easily verify if it's broken with a multi-meter, and you have a new one anyhow.
Since the hum-balance does have an effect on the hum (more of a buzzzzz to me), I would assume the problem lies in the heater circuit, but it could be a red-herring. One thing your tech could try is disconnecting the heaters and running them off 6v lantern battery. It's an easy way to isolate the heaters from the rest of the amp to see if the problem is related to the sensitive heater wiring. If so, I'm guessing it would have to be that darn hum balance pot because there's not much else in the heater circuitry, and since a production amp, I doubt they would have let the amp out the door humming away.
I'd really have to get in the amp to figure it out, of course...
I also wanted to mention that it's totally fine to power up an amp without a variac with new caps. It's only necessary with NOS caps, or caps in an amp that has been sitting. The tech didn't do anything wrong. Even folks you find who do power up new caps with a variac will admit they do habitually or because they are paranoid and/or have OCD
Since the hum-balance does have an effect on the hum (more of a buzzzzz to me), I would assume the problem lies in the heater circuit, but it could be a red-herring. One thing your tech could try is disconnecting the heaters and running them off 6v lantern battery. It's an easy way to isolate the heaters from the rest of the amp to see if the problem is related to the sensitive heater wiring. If so, I'm guessing it would have to be that darn hum balance pot because there's not much else in the heater circuitry, and since a production amp, I doubt they would have let the amp out the door humming away.
I'd really have to get in the amp to figure it out, of course...
I also wanted to mention that it's totally fine to power up an amp without a variac with new caps. It's only necessary with NOS caps, or caps in an amp that has been sitting. The tech didn't do anything wrong. Even folks you find who do power up new caps with a variac will admit they do habitually or because they are paranoid and/or have OCD
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Cochise Montana
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Re: Ampeg V4 Hum Challenge
Gaz,
Thanks for the input. I will pass along the heater idea to the tech.
Thanks for the input. I will pass along the heater idea to the tech.
Re: Ampeg V4 Hum Challenge
It might be the heater circuit or the pot. Though I suspect it's something more frustrating. The pcb on these amps tend to be problematic. They get hot and glue releases the trace from the board. Soldering in new parts can be a real bitch.
Make sure all the nuts bolts and screws are tight. Spray all the pots with deoxit excersize them good.
Test the tubes and clean the tube socket pins. Find the center tap of the PT make sure it has a solid contact to ground. Remove the death cap from the power cord. Figure out which version of V4 u have and locate the correct schematic you have 7 choices.
Clean the rocker switches.
Check the ground to the new caps. It's often easy to not get a clean solder to the chassis.
Hopefully it is easy and just the heaters keep your fingers crossed!
Make sure all the nuts bolts and screws are tight. Spray all the pots with deoxit excersize them good.
Test the tubes and clean the tube socket pins. Find the center tap of the PT make sure it has a solid contact to ground. Remove the death cap from the power cord. Figure out which version of V4 u have and locate the correct schematic you have 7 choices.
Clean the rocker switches.
Check the ground to the new caps. It's often easy to not get a clean solder to the chassis.
Hopefully it is easy and just the heaters keep your fingers crossed!
My Daughter Build Stone Henge
Re: Ampeg V4 Hum Challenge
I'm a Ampeg V4(-b) owner and I love mine.
Be sure to check out ampegv4.com if you haven't.
Using a variac is not really necessary if you are replacing caps with quality electrolytics in the power section. If you were using NOS electrolytics or firing up the amp for the first time in x_years with all it's original electrolytics, then it's advisable...even then, not everyone has a variac...and if said caps blow, you know you should replace them. It's worth the cost for reliability especially in an amp like this that doesn't have the vintage value of other amps, and is a players amp for sure.
MY amp is all fixed up in glorious condition, every coupling cap was replaced (and that is advisable), whole bias network was replaced, and it hums too. Just like you said, the midrange pot, when cranked, will do it every time.
1) The Ampeg can reproduce low frequencies unlike any other amp I've played. It's designed that way, so it can reproduce that hum quite well. Find me an amp that can be so flat in the mids - let alone boost 12dBs @ 300hz or 800hz. The hum can't hid behind the fidelity & EQ of this amp like it can many others.
2) The grounding scheme is pretty piss poor. Could take some work, but it COULD help the hum. I haven't tried it with mine since I don't feel the hum I'm getting is excessive by any means.
3) relatively matched power tubes can make a big difference in hum level than old, original, drifted power tubes. Don't be afraid of New tubes, the Winged C 6L6GCs are amazing in there, and bias them good and cold (as per stock) IMO, for a great tone. This amp don't need a hot bias to sound good, like a BF Fender might!
PS) the V4 is otherwise the quietest, loudest amp I've played.
Be sure to check out ampegv4.com if you haven't.
Using a variac is not really necessary if you are replacing caps with quality electrolytics in the power section. If you were using NOS electrolytics or firing up the amp for the first time in x_years with all it's original electrolytics, then it's advisable...even then, not everyone has a variac...and if said caps blow, you know you should replace them. It's worth the cost for reliability especially in an amp like this that doesn't have the vintage value of other amps, and is a players amp for sure.
MY amp is all fixed up in glorious condition, every coupling cap was replaced (and that is advisable), whole bias network was replaced, and it hums too. Just like you said, the midrange pot, when cranked, will do it every time.
1) The Ampeg can reproduce low frequencies unlike any other amp I've played. It's designed that way, so it can reproduce that hum quite well. Find me an amp that can be so flat in the mids - let alone boost 12dBs @ 300hz or 800hz. The hum can't hid behind the fidelity & EQ of this amp like it can many others.
2) The grounding scheme is pretty piss poor. Could take some work, but it COULD help the hum. I haven't tried it with mine since I don't feel the hum I'm getting is excessive by any means.
3) relatively matched power tubes can make a big difference in hum level than old, original, drifted power tubes. Don't be afraid of New tubes, the Winged C 6L6GCs are amazing in there, and bias them good and cold (as per stock) IMO, for a great tone. This amp don't need a hot bias to sound good, like a BF Fender might!
PS) the V4 is otherwise the quietest, loudest amp I've played.
Re: Ampeg V4 Hum Challenge
The V4 is not traditonally a 6L6 amp. The fad of using 6L6 tubes came about because until a decade ago it was hard to locate replacement 7027 tubes. If you want to run anything other then 7027 tubes look into converting to run 6550.
If you want it to sound correct you need to run 7027.
Sovtek and JJ/Telsa are dependable work horse but the NOS GE, RCA, and Phillips are the true tone of the V4.
Matching tubes is always a good thing in a Push Pull. As is swapping the push side with the pull side every 6 months and rebiasing. to keep the tubes from drifting to far out of spec from each other.
If you need parts Fliptops sells replacement Ampeg parts.
I own 4 of the best sounding V4/VT22/VT40 amps ever made, all rebuilt from the ground up with NOS parts. These amps are incredible for hard rock and for the most part very well built and total bitch to work on/trouble shoot. Living in the stoner rock capital I service dozens of these amps on a regular basis. The good news about these amps it's almost never a bad piece of iron. the bulk of the issues can be lumped into old dried up capacitors, failing tubes, crud build up, bad PCB, or burned up Bias section from a power tube taking a shit.
If you have not done it I recommend you convert the fixed bias (R49 if i remember correctly) to a locking potentiometer. It's pretty elegent to mount it into the removeable plate. They are easy to replace and no actual holes in the chassis is a good thing.
I personally use a variac always, not just to reform caps but to view the amperage and know the circuit is safe to power up. I guess you could say I'm OCD or paranoid but nothings worse then flipping the power switch and blowing up something because of a stupid mistake that got over looked, where you would see a visual rise in amperage before the voltage could do damage and give you a chance to power down and search out the reason. Speaking from experience with V4's I've done the flip the power switch and it's all good thing and actually blown up a Quad of NOS 7027 it only takes a second. the bias supply was janky and was making contact threw some carbon traces I had overlooked / written off as not an issue. That mistake cost me $400 and a red face as the client was watching. luckily I had a spare set on of tubes on hand and the guy thought it was cool to see tubes fail.
attached is a very cool way to bias your V4 it allows you more then enough room to sweep and dial the tubes in. as well as leaving the stock resitor in for safety and giving you a backup safety to ground. in event of failure. Use a 2 watt potentiometer and it's bullet proof.
By bridging the potentiometer with the resistor as you turn the dial you can increase and double or cancel out each other giving you a sweep of 35K Fixed leg 2 up to ~150K.
Setting the bias - V4's left the factory running at ~ 70% due to the one size fits all fixed bias. some far exceeded 90% and had aweful failure rates as bands ran these amps loud and hard. This need for distortion led to a master volume knob.
The V4B all ran cold in an attempt to control the distortion in the power section since it was a "B" bass amp.
If you want it to sound correct you need to run 7027.
Sovtek and JJ/Telsa are dependable work horse but the NOS GE, RCA, and Phillips are the true tone of the V4.
Matching tubes is always a good thing in a Push Pull. As is swapping the push side with the pull side every 6 months and rebiasing. to keep the tubes from drifting to far out of spec from each other.
If you need parts Fliptops sells replacement Ampeg parts.
I own 4 of the best sounding V4/VT22/VT40 amps ever made, all rebuilt from the ground up with NOS parts. These amps are incredible for hard rock and for the most part very well built and total bitch to work on/trouble shoot. Living in the stoner rock capital I service dozens of these amps on a regular basis. The good news about these amps it's almost never a bad piece of iron. the bulk of the issues can be lumped into old dried up capacitors, failing tubes, crud build up, bad PCB, or burned up Bias section from a power tube taking a shit.
If you have not done it I recommend you convert the fixed bias (R49 if i remember correctly) to a locking potentiometer. It's pretty elegent to mount it into the removeable plate. They are easy to replace and no actual holes in the chassis is a good thing.
I personally use a variac always, not just to reform caps but to view the amperage and know the circuit is safe to power up. I guess you could say I'm OCD or paranoid but nothings worse then flipping the power switch and blowing up something because of a stupid mistake that got over looked, where you would see a visual rise in amperage before the voltage could do damage and give you a chance to power down and search out the reason. Speaking from experience with V4's I've done the flip the power switch and it's all good thing and actually blown up a Quad of NOS 7027 it only takes a second. the bias supply was janky and was making contact threw some carbon traces I had overlooked / written off as not an issue. That mistake cost me $400 and a red face as the client was watching. luckily I had a spare set on of tubes on hand and the guy thought it was cool to see tubes fail.
attached is a very cool way to bias your V4 it allows you more then enough room to sweep and dial the tubes in. as well as leaving the stock resitor in for safety and giving you a backup safety to ground. in event of failure. Use a 2 watt potentiometer and it's bullet proof.
By bridging the potentiometer with the resistor as you turn the dial you can increase and double or cancel out each other giving you a sweep of 35K Fixed leg 2 up to ~150K.
Setting the bias - V4's left the factory running at ~ 70% due to the one size fits all fixed bias. some far exceeded 90% and had aweful failure rates as bands ran these amps loud and hard. This need for distortion led to a master volume knob.
The V4B all ran cold in an attempt to control the distortion in the power section since it was a "B" bass amp.
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My Daughter Build Stone Henge
Re: Ampeg V4 Hum Challenge
As a counterpoint:
7027s were probably re-pinned, high quality 6L6GCs with a rating of 35W rather than 30W. The Ampeg didn't make use of the 'extra pins' 6L6GC's so they're a direct swap on that level. JJ pins and brands some as 7027s if you MUST have them...even eurotubes says just use the 6L6GCs. Or, pay the price for NOS...
6550's will run the power transformer hotter with the large increase in heater current. Screen resistors should be increased as well. People do it, some say Ampeg offered it as an 'upgrade.' You won't get significant if any more clean headroom though, just a different tone.
The schematics (not talking about the master volume or distortion versions) for the V4b and V4 are identical in the power section. I have a hard time believing that the 75k bias resistor in there would run any 7027s or 6L6GCs hot.
The bias range between 35k and 150k is a lot - and potentially could fry some nice tubes if set wrong by mistake or slip.
I had a bias pot trimmer in there, and set/tweaked it by ear more or less on multiple occasions. When I measured where I had ended up with the bias value, it was so close to 75k in resistance it was frightening. I put a fresh 75k resistor in and am leaving it. I have 68k and 82k resistors on hand if I ever need to sway one way or another.
To each their own.
Again, just a counterpoint.
7027s were probably re-pinned, high quality 6L6GCs with a rating of 35W rather than 30W. The Ampeg didn't make use of the 'extra pins' 6L6GC's so they're a direct swap on that level. JJ pins and brands some as 7027s if you MUST have them...even eurotubes says just use the 6L6GCs. Or, pay the price for NOS...
6550's will run the power transformer hotter with the large increase in heater current. Screen resistors should be increased as well. People do it, some say Ampeg offered it as an 'upgrade.' You won't get significant if any more clean headroom though, just a different tone.
The schematics (not talking about the master volume or distortion versions) for the V4b and V4 are identical in the power section. I have a hard time believing that the 75k bias resistor in there would run any 7027s or 6L6GCs hot.
The bias range between 35k and 150k is a lot - and potentially could fry some nice tubes if set wrong by mistake or slip.
I had a bias pot trimmer in there, and set/tweaked it by ear more or less on multiple occasions. When I measured where I had ended up with the bias value, it was so close to 75k in resistance it was frightening. I put a fresh 75k resistor in and am leaving it. I have 68k and 82k resistors on hand if I ever need to sway one way or another.
To each their own.
Again, just a counterpoint.
Re: Ampeg V4 Hum Challenge
6L6 are not intended to exceed 500V it's a gamble using them in a V4.
The Iron in the V4 is over spec'd and has no trouble running 6550. I prefer the 7027 since it is the tone of the amplifiers design but the 6550 would be my second choice since they can handle the high voltage.
If the amount of swing in the pot scares you use a smaller value potentiometer. The major advantage of this design is that you have multiple backups. Where the stock resistor gives you only that resistor and a traditonal potentiometer gives you only a potentionmeter this give you both plus a back up to ground. The bias pot should be the locking type to prevent drift and set by a qualified service tech.
The schematic and the parts used where not always the same. Keep in mind Ampeg modified the V4 line over a dozen times during it's production the schematics floating around in free use domain are not always the ones that directly reflect the version you may own. I've had V4/VT22/VT40/V4B all come across the bench with bias resistors other then 75K set by the factory still with red piant on the solder joints.
The Iron in the V4 is over spec'd and has no trouble running 6550. I prefer the 7027 since it is the tone of the amplifiers design but the 6550 would be my second choice since they can handle the high voltage.
If the amount of swing in the pot scares you use a smaller value potentiometer. The major advantage of this design is that you have multiple backups. Where the stock resistor gives you only that resistor and a traditonal potentiometer gives you only a potentionmeter this give you both plus a back up to ground. The bias pot should be the locking type to prevent drift and set by a qualified service tech.
The schematic and the parts used where not always the same. Keep in mind Ampeg modified the V4 line over a dozen times during it's production the schematics floating around in free use domain are not always the ones that directly reflect the version you may own. I've had V4/VT22/VT40/V4B all come across the bench with bias resistors other then 75K set by the factory still with red piant on the solder joints.
Last edited by selloutrr on Mon Sep 26, 2011 7:57 pm, edited 1 time in total.
My Daughter Build Stone Henge
Re: Ampeg V4 Hum Challenge
Same gamble as running 350v maximum plate voltage rating 6V6s in deluxes and silverface champs, no?selloutrr wrote:6L6 are not intended to exceed 500V it's a gamble using them in a V4.
Re: Ampeg V4 Hum Challenge
In fender amps, from what I've seen the new production 6v6 don't last long in vintage circuits. if played regularly 8months to a year. NOS for the most part are fine, other then a bad tube you can get years even decades. It was a different manufacturing practice back then.
If you like to gamble and spending money on tubes more often then you might need to, go for it.
From a purest point of view. The circuit was designed for the 7027. The 6L6 was used as a means of keeping the amps running while the 7027 were not longer in production. Since the reproduction of the 7027 by companies such as Sovtek/New Sensor and JJ/Telsa it's not longer necassary to modify or bastardize the circuit. which due to the fragile PCB is a very good thing. I've had to fabricate a PCB set because the traces came off while resoldering in components. From my experience and other players they have welcomed back the stock design with 7027 output tubes. However that's the nice part about music it's subjective each to thier own.
If you like to gamble and spending money on tubes more often then you might need to, go for it.
From a purest point of view. The circuit was designed for the 7027. The 6L6 was used as a means of keeping the amps running while the 7027 were not longer in production. Since the reproduction of the 7027 by companies such as Sovtek/New Sensor and JJ/Telsa it's not longer necassary to modify or bastardize the circuit. which due to the fragile PCB is a very good thing. I've had to fabricate a PCB set because the traces came off while resoldering in components. From my experience and other players they have welcomed back the stock design with 7027 output tubes. However that's the nice part about music it's subjective each to thier own.
My Daughter Build Stone Henge
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LeftyStrat
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Re: Ampeg V4 Hum Challenge
I wish I still had my VT22, which was the V4 in a 2x12 combo.
One thing you mentioned is that the middle control affects the hum. That could just be the EQ, but the middle control uses a tapped inductor that could be picking up hum. You may try to temporarily shield it to see if it affects the hum level.
It's been a while, but the last VT40 I serviced had a bad hum and it was a 12dw7. Are you sure the first tech changed all the tubes?
Man, I really miss the EQ in those Ampegs. I may have to start looking for one.
One thing you mentioned is that the middle control affects the hum. That could just be the EQ, but the middle control uses a tapped inductor that could be picking up hum. You may try to temporarily shield it to see if it affects the hum level.
It's been a while, but the last VT40 I serviced had a bad hum and it was a 12dw7. Are you sure the first tech changed all the tubes?
Man, I really miss the EQ in those Ampegs. I may have to start looking for one.
It's never too late to have a happy childhood.
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Cochise Montana
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- Location: Montana
Re: Ampeg V4 Hum Challenge
selloutrr,
If there was a smiley that was bowing down in thanks I'd use it here.
There isn't so this will have to do
I plan on cutting and pasting these ideas to a word document and giving it to my tech so we can go over these things. I'm hoping on trying least invasive methods first and then moving forward.
Fack I hope it's just the hum pot.
Liquids, you and I have chatted before on AmpegV4 boards. Thanks for your insight too.
Thanks everyone for that matter.
If there was a smiley that was bowing down in thanks I'd use it here.
There isn't so this will have to do
I plan on cutting and pasting these ideas to a word document and giving it to my tech so we can go over these things. I'm hoping on trying least invasive methods first and then moving forward.
Fack I hope it's just the hum pot.
Liquids, you and I have chatted before on AmpegV4 boards. Thanks for your insight too.
Thanks everyone for that matter.
Re: Ampeg V4 Hum Challenge
have the tech test the tubes. Just because they are new does not always mean they are good. It would be the easiest fix. If he can't test the 6K11 PM me I'll test it on the amplitrex free of charge just cover shipping. I work on so many V4 I had a special test adaptor made.
You have a great amp!
You have a great amp!
My Daughter Build Stone Henge
Re: Ampeg V4 Hum Challenge
V4 modifications
http://www.rickonslow.com/Content-TubeA ... 32-56.html
http://www.lynx.bc.ca/~jc/v4mod.html
http://www.ampegv4.com/forum/viewtopic.php?id=301
http://stonemarmot.com/rants/?p=39
http://www.rickonslow.com/Content-TubeA ... 32-56.html
http://www.lynx.bc.ca/~jc/v4mod.html
http://www.ampegv4.com/forum/viewtopic.php?id=301
http://stonemarmot.com/rants/?p=39
My Daughter Build Stone Henge
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Cochise Montana
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- Location: Montana
Re: Ampeg V4 Hum Challenge
Cool. Thanks. Looks like the last one is a no go, as mine has a distortion control and the circuit is different, but that Rf elimination mod is intriguing.