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#1 2008-03-10 03:30:00

Diablo
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Registered: 2007-12-01
Posts: 139

60 cycle hum

After playing my V4 for about an hour today, the amp started to produce a 60 cycle hum (tone between A# and B). I tore into the head and started looking for signs of anything wrong, but couldn't find any obvious problems. I had just recapped the amp about a week ago and it was working perfectly up to that point. So I started to measure the resistor values in the circuit and compared to the schematic. I found a half dozen that had drifted low by about 20-30%, and one (R34) that was suppossed to be 4.7K ohms and measured only 32 ohms. I removed this one resistor and tested it at 4.7K. So the resistor is fine. The resistor bridges a capacitor (C14) that's connected to the impedance switch and secondary coil of the output transformer. I assumed that the C14 (.001 mfd) must be leaking current around the resistor. All I had was a .0022 mfd capacitor to use as a replacement. I soldered it in and the amp sounds great - very quiet...no hum.

Can anyone tell me if I should sub in the correct value (.001 mfd) of capacitor? What effect will the larger capacitor have on the electronics and or tone?

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#2 2008-03-10 19:58:49

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 60 cycle hum

C14 is in the feedback circuit,  and allows higher frequencies to pass unimpeded while lower frequencies hit the resistor. 

I would not expect this to cause your problem with the 120cycle hum.   

resistors don't always test correctly when in the circuit. because often they are in parallel with other things.   In this case R26 (33ohms) and the output transformer secondary gives you a very low impedance path to ground.

if the problem comes back up take it back to the guy who did your re-cap.  it sounds like a powersupply or grounding more than anything else.

-steve

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#3 2008-03-10 20:25:05

paully
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From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

Diablo wrote:

Can anyone tell me if I should sub in the correct value (.001 mfd) of capacitor? What effect will the larger capacitor have on the electronics and or tone?

>>>>>>>>>>>>>>>>>>
Most amps don't even include this cap in the fb circuit. I'm guessing it's a lo-pass filter to block any transformer-based hi frequency oscillations back into a previous stage. Could also be a phase inverter of some sort. I'd have to check some old diagrams. My guess would be the former(a shelving filter). Cap value is not critical, but if you have the right value or something close, stick it in. Small value caps in this range are also used in power supplies to ground any generated cycling over a certain frequency. AAR, a near-spec cap shouldn't effect your tone at all.


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#4 2008-03-10 22:01:27

Diablo
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Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Thanks Steve and Paully for the information. Since I did the recap job myself, I can't take it back and complain about the work. Originally, I thought I might have a power supply cap that went bad. I have no way to test though. Besides replacing the cap in the negative feedback, the only other thing I did was to move two wires away from one of the preamp tubes - perhaps these wires were picking up a stray signal? Anyway, I'm glad the hum is gone for now and I'm keeping my fingers crossed that it doesn't return.

I did some reading about negative feedback and how some modern amps switch out the feedback to get more overdrive. I'm going to try to mod the V4 to install a pot to control the amount of feedback.

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#5 2008-03-10 22:17:19

paully
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From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

If you're talking about the same feedback circuit(with the 4.7K resistor), I would suggest that you leave that resistor in place and put your variable pot in series with it before re-connecting back to the .001 cap. You need a minimum resistance at this point. Increasing resistance with a pot might prove interesting, or if it is indeed a tuned hi-f filter could render it ineffective. I usually don't try to second guess the design guys. I'll have a look at the old books and try to figure out what a parallel RC circuit does in this case.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#6 2008-03-11 01:15:22

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Thanks Paul. Yes, I'm talking about the 4.7K resistor and capacitor in parallel for the negative feedback. There are two ways I can modify the circuit. Both would leave the resistor and capacitor in place. I can put a simple on/off switch in the lead that connects the impedance switch 8 ohm tap to the 4.7K resistor on the circuit board. Or I can put a pot in that same lead. I'm thinking about a 15K pot.

I also played the amp again for another hour and it's still dead quiet.

Last edited by Diablo (2008-03-11 01:20:00)

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#7 2008-03-11 03:45:28

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

I just finished with the modification. I used a 10K linear pot because I couldn't get a 15K pot at Radioshack. The resistance of the pot measured 0 to 10.3K in one turn. I removed the second line output jack and installed the pot in the hole. Fired the amp up and it works just like it should. Cranking in resistance on the pot decreases the negative feedback and the amp gets louder and has a beautiful tone. I can't really say it's better because being louder can fool the ear into believing it sounds better. But the guitar certainly sounds more "alive". I can get the original Ampeg sound by cranking the pot down to zero ohms.

If anyone is interested in this mod, cut the white wire that runs from the impedance switch (8 ohm tap) to the circuit board (R34), and connect the two ends to the potentiometer.

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#8 2008-03-11 14:38:04

paully
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From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

Diablo wrote:

If anyone is interested in this mod, cut the white wire that runs from the impedance switch (8 ohm tap) to the circuit board (R34), and connect the two ends to the potentiometer.

>>>>>>>>>>>>>>>
Diablo,

Small correction(sorry) to the instructions. To be variable, the pot has to be connected to an end lug and the wiper(center lug). Otherwise it's just a fixed resistor. I'm sure that's the way you did it, but the instructions might be interpreted differently.

Be careful using this mod, as the OT is only capable of delivering so much current. Otherwise, glad you got it working.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#9 2008-03-11 23:11:52

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Thanks Pauly for the correction, one of the wires has to be connected to the center of the pot to get variable resistance. As to the effect of no negative feedback on the output transformer, the volume increase is maybe 10-20% louder. I'm not playing the amp more than 1/3rd volume at any time - otherwise it's just too loud. My theory is that folks that use the hot plates are the ones smoking their transformers.

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#10 2008-03-12 02:43:09

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 60 cycle hum

lets make one thing clear.  hot plates do not smoke transformers.   you'll have to  present some evidence to support your "theory".

I will admit that running an amp with a hot plate runs it hotter than if you had a master volume... but no more so than the engineers at ampeg intended. 
most people who blow their transformers are not taking care of their amps,  running them at the wrong impedance,  and not replacing the filter caps.

anyway,  I'm curious Diablo,  how does the amp break up now?   do you find that the break up is more gradual?   do you notice much difference in frequency response?

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#11 2008-03-12 22:33:57

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

hangman wrote:

anyway,  I'm curious Diablo,  how does the amp break up now?   do you find that the break up is more gradual?   do you notice much difference in frequency response?

I haven't played it enough to determine how it breaks up - seems like I get more breakup. The frequency response is slightly different - sounds more "alive". The only recommendation I can make is to try the change yourself to see if you like it. I think it's an easy and inexpensive way to get a different sound out of the amp, and it increases the versatility of the amp since you can change the feedback to any setup you like.

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#12 2008-03-18 03:20:37

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Here's an update on the feedback control knob. Removing the feedback increases the hiss a little bit, so my WAG is that one of the purposes of the original Ampeg RC feedback circuit was to reduce hiss.

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#13 2008-03-20 03:59:44

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 60 cycle hum

Diablo,  that makes sense.  As I mentioned the capacitor was allowing higher frequencies to pass unimpeded,  while lower frequencies hit the resistor.   
The higher frequencies are then "fed back" to a greater degree,  causing a reduction in volume for those frequencies.

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#14 2008-03-20 16:30:13

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

I'm sticking with the 'lo-pass' circuit to block high frequencies. Take a look here,
http://en.wikipedia.org/wiki/Low-pass_filter
and scroll down to the section called "active electronic realization". See if the diagram doesn't look familiar. Adding or decreasing resistance via pot will just reset the x-over's notch point, and is probably why hiss is being allowed back into the circuit.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#15 2008-03-20 22:46:18

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

paully wrote:

I'm sticking with the 'lo-pass' circuit to block high frequencies. Take a look here,
http://en.wikipedia.org/wiki/Low-pass_filter
and scroll down to the section called "active electronic realization". See if the diagram doesn't look familiar. Adding or decreasing resistance via pot will just reset the x-over's notch point, and is probably why hiss is being allowed back into the circuit.

Best, Paul

Thanks Paul and Steve.
I went to the link, and here's what I concluded.
Please tell me if you think this makes sense.
The Ampeg original feedback circuit is a resistor and capacitor in parallel. It operates like a current divider. At low frequencies all the current goes through the resistor, and at higher frequencies the capacitor starts to conduct more of the AC signal. At high frequency the capacitor has very low impedance so more of the signal passes through and it acts like it has no resistance. The result is that at high frequencies more current is sent as feedback and this reduces the gain and reduces the hiss. When I added the potentiometer in front of the RC parallel circuit, the added 10K resistance reduced all of the feedback current, increasing gain and some hiss.

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#16 2008-03-20 23:20:10

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

Hi Diablo,

Whenever I try to answer a question and there is doubt, debate or confusion, I throw the question out to trusted sources. I did this with the RC network in question. You can read what I've received back so far. Maybe their responses will explain better than mine what this RC filter does. IMHO, it should not be tampered with.

Myles Rose:
http://forums.musicplayer.com/ubbthread … /fpart/143

RodC:
http://messageboard.tapeop.com/viewtopi … 877207f0e6

Best, Paul

Last edited by paully (2008-03-20 23:20:54)


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#17 2008-03-21 02:10:57

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 60 cycle hum

Paully,   

I'm pretty sure we're saying the same thing from different angles. 
I was describing what the capacitor in the feedback circuit does  (allows high frequencies to pass to a greater degree).  you are describing The Overall effect  which is high frequency attenuation/low pass.  Which we agree about.

Diablo,
you've got the idea. that is what is taking place in the feedback circuit. 

if you want some more info on feedback,  check out Aikenamps.com,   click on Tech info,  Advanced,  and you'll see two interesting articles on feedback
one entitled "designing for global negative feedback"
the other "what is negative feedback?" 
Really good stuff.

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#18 2008-03-21 02:17:21

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: 60 cycle hum

ha... I just clicked on your links paully... saw someone else recommended aiken's page.   Thats really fun stuff.

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#19 2008-03-21 13:42:25

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Thanks for the links and your thoughts Steve and Paul. All very good information and I think I understand the circuit now. The reason I put the pot in was because some folks complain that the amp is too clean, and other amps that folks rave about (like the old Marshall 18 Watt) because of their breakup don't use negative feedback. Also some amps that folks rave about(Allen) use a "raw" switch to turn the feedback on or off. I'm also trying to build my knowledge up to the point where I feel comfortable tackling a kit amp build. I want to build one of the Marshall 18 Watt clones.

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#20 2008-03-21 16:01:17

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

Diablo wrote:

some folks complain that the amp is too clean, and other amps that folks rave about (like the old Marshall 18 Watt) because of their breakup don't use negative feedback...

>>>>>>>>>>>>>>>>
Well, for obvious reasons it's not really fair to compare an 18 watt Marshall to a 100 watt V4. It may be that you have to drive the Ampeg's o-tranny to approximate the same saturation effect as the Marshall(if possible). Prob is, it'll be at a lot higher volume.


Also some amps that folks rave about(Allen) use a "raw" switch to turn the feedback on or off.

>>>>>>>>>>>>>>>>
If nothing else comes out of this thread, I hope that you realize that the feedback circuit in any amp is designed in to avoid/cancel transformer artifacts that can harm it and other components. EG: Unexpected, very  high frequency OT oscillations can be seen by a speaker as DC current, and we all know what DC does to a speaker.


I want to build one of the Marshall 18 Watt clones.

Sounds like a fun project. I've built more than a few amps from the ground up, and can only offer one piece of advice. Try to get original Marshall parts, especially the caps and OT. From what I've read, that's where guys make a mistake when repairing Marshalls(and loosing the magic sound).

Slightly off topic but relevent, I ran across this thread at another forum. I don't know who Francis Vaughan is, but he gives the most down-to-earth and informative explain of transformers that I've seen, short of a text book. Even the heavy hitters there were impressed. Good read.

http://www.gearslutz.com/board/geekslut … rmers.html

Have fun.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#21 2008-03-21 16:58:07

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

There's more than a little voodoo involved with amp design. A lot of the information is contradictory. For guitar amps, some folks claim that "good" overdrive is just power tube distortion and not saturation of the output transformer. Others claim that the OT saturates to add to the good sound. I frankly don't know myself. I understand enough about transformers to be dangerous. In my other job, I am the expert on steel used for motors and transformers. I like to call myself the "chief magnetician". I'm certainly no electrical engineer, just a metallurgist. I've never seen a list of the various guitar amp output transformers with the max flux density and type of steel. The better transformer steel grades have very high permeability and can create high flux density with fewer amp turns of winding. But it comes down to how many pounds of steel they use for the rated power output too. It's fairly simple if you want to minimize heat damaging effects and make a robust output transformer - use more steel (lower flux density) and bigger diameter wire.

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#22 2008-03-21 17:36:16

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: 60 cycle hum

Diablo wrote:

- use more steel (lower flux density) and bigger diameter wire.

>>>>>>>>>>>>>>>
That's true, but at the same time funny. I just saw a blurb that said it costs the government $.017 cents to make a .01 penny. Not much hope for thicker copper wire there LOL. Did you have a chance to read that 'Slutz' article?

Here's some stuff on a company with a great product(I actually tried one out back then) that never caught on. They obviously believed that transformer sat/overdrive was the key to great sound, as they built it in BEFORE the amp's output section. I don't know if the company's even around anymore, but I was into blues then, and this thing had a magical sound.

http://i125.photobucket.com/albums/p71/goosefat/Discription.jpg

Best, Paul

Last edited by paully (2008-03-23 17:54:52)


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#23 2008-03-21 19:00:38

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: 60 cycle hum

Yes Paul, I read the gearslutz article and it was good accurate information. Thanks for the info on the Guytron.

Something else to consider about the V4 and V4B is that Ampeg uses the same transformers for the bass amp as the guitar amp. By design, to operate at the lower frequencies of a bass guitar requires higher primary inductance for the output transformer, which requires more winding and typically a larger core than for the lowest frequencies on a guitar.  Bottom line is to prevent output transformer saturation and distortion with a bass guitar means that the output transformer is way too large to get close to saturation with a guitar. It should be darn near impossible to kill a V4 OT with a guitar.

Here's a neat link on designing OTs.

http://www.geofex.com/Article_Folders/x … former.htm

Last edited by Diablo (2008-03-22 00:14:18)

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