Heavily used and modded V4 noise issues

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Heavily used and modded V4 noise issues

Postby ThisLifeILead on Tue Sep 30, 2014 2:13 pm

Schematic: http://www.ampegv4.com/images/schematics/V4.jpg

Back ground story.
A buddy of mine a few years back bought 3 ampegs from a guys collection. Had been in storage for 10-20 yrs (dont remember how long).
an early non master V4, a late distortion model and a V2
The early non master is the one i was able to snag. Within a few weeks of playing, all 3 ampegs started popping fuses. Caps had dried out and the flyback diodes were fried.So i recapped just my early V4 along with new PS resistors, Power tube 3.6ohm plate resistors, grids, etc and did buddies distortion model later. The V2 was traded away.

My V4 was apparently part of the Goddo guitarist, Gino Scarpelli's touring rig.Pretty cool. It was also modified by Gar Gilles of Garnet before storage with a gain boost on one channels sensitivity switch and a master volume on the back in one of the ext amp plugs.
This amp smells of cigarettes still and is filthy :-)

Now the amp played good for the last few years until recently it started humming again.
There is a weird coil and cap assembly mounted to the AC primary input and a cap/coil configuration installed on the input of both channels on the old shielded wire. my guess is they were experiencing some dirty power at some venues on the road.All new preamp tubes made no difference in the residual back ground hum. i prob needed to do that anyways.

Pulling the Phase inverter (v4) eliminates the hum entirely and pulling the 12dw7 (v3) the hum is reduced but still present. I checked all grounds and found them to be good.
So by deduction, correct me if im wrong here, the hum is originating between the ouput of V3 and the input of the Phase inverter V4. Looking at the schematic its just the C10 coupling cap and maybe the 1 M grid leak resistors on V4(12au7). I gotta bring the amp home from the jam space to get in there and have a real look but i thought id confirm my suspicions with the pros.
I appreciate any input :cheers:
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Tue Sep 30, 2014 7:09 pm

Removing V3 do not remove all the hum you write .

IMO there a small hum all the way or somewhere from input jack to V4.

It could be a noisy tube(s) too

You must check all wires going to each tubes's grid , from V1 to V4 . Bad wiring or leaky caps could make some hum .

Any picture inside the amp ?
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Tue Sep 30, 2014 7:42 pm

I am bringing the ugly ol'workhorse home tonight after jams. will get some gut shots.

If i pull V1 or V2, V201 (6K11) the hum doesnt change.
V1 is for channel 1 and V2 is for channel 2. they both share the 6K11(V201), 12dw7(v3) and the 12au7 P.I(V4) pulling V3 the hum goes away slightly and If you pull V4 the hum is gone.

I forgot to mention, looking in my amp at Pin6 of V3, the traces are severed between C10 and the plate. a 0.1 cap off the plate(pin6) goes to the master volume control and returns to C10. I am going to try removing the MV and see if thats my problem. ( i dont normally use it anyways.)
also after looking at the schematics the NFB is hooked to the Cathode of V3 pin8... and If i remember correctly i was not getting the measured 4.7K needed for the NFB,i was getting like 400ohms or soemthing. maybe the 0.001cap is leaking on the NFB. ( i did a bunch of measuring at the jam space a few months back at the start of the hum)
Few things to look at i guess...
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Tue Sep 30, 2014 8:49 pm

Hum could come from bad filter caps.

Master volume pot is at wrong place , it should be after signal capacitors C11 and C13 and you need to put other capacitors before the tubes .
In other word, the MV pot must be between 2 caps each side . One for isolate the 200 volt like C11 will do and another one to isolate from the bias grid voltage coming from R35 R36.

Like that it make short in phase inverter tube ( V4) plate voltage. And not good for a pot to have 200 volts dc

I understand this amp use a printed circuit not point to point ?

Wires to or from volume control to tubes plate could act like antenna and make some hum. Shielded wires remove hum .

I don't how this amp look inside , it is a must to help you . Post some picture .
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Tue Sep 30, 2014 9:43 pm

All electrolytics are new. I agree with the mv thing. Its a pre phase inverter style with a single ganged 1 meg pot. I am going to remove the master for now and get it operating quietly. Then put in a post-p.I.
Its pcb for the most part. Ill get pics up tomorrow or later tonight.
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Tue Sep 30, 2014 10:21 pm

Pre phase inverter style wired in post phase inverter !!!!
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Wed Oct 01, 2014 1:58 pm

PICS!
Image

Image

Image

Image

Image

Its a filthy beast with a bunch of weird shit going on. I think returning it to stock would be a good idea.
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Wed Oct 01, 2014 3:03 pm

Thank's for pictures

IMO this amp need to see a qualified tech to fix hum/ noise .

All you can do is to try new good tubes.
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Wed Oct 01, 2014 3:31 pm

Its a mess I know. Thats most likely part of the issue. But, the tubes have all been replaced. All the electrolytics have been replaced. Bias supply has been upgraded and rebuilt.
Its a symptom that has started recently. All the work was done almost 2 yrs ago by myself and has been fine since. I didnt touch the preamp as it was quiet. I guess its time for a preamp re work.
The plan is to return it to stock and rework it from there.
I plan to replace all the dirty old shielded wire and see if I can trace down some leaky coupling caps. I checked all the grounds with my dmm and they all "seem" ok.

Maybe even a tube socket reflow wouldnt hurt. I was actually thinking of getting out the isopropyl and cleaning up the flux residue and grime.

I dunno whats going on with that primary ac wiring. Anyone seen something like that before?
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Wed Oct 01, 2014 3:46 pm

With so many wire loose if you move some wire you may stop the noise or you may do more more noise.

Did you work in the amp before it make this noise ?
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Wed Oct 01, 2014 4:15 pm

Yes and it was noise free for nearly 2 yrs.
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Re: Heavily used and modded V4 noise issues

Postby stratele52 on Wed Oct 01, 2014 5:56 pm

ThisLifeILead wrote:Yes and it was noise free for nearly 2 yrs.



My question was ; "Did you work in the amp before it make this noise ?

You answer Yes .......

So I understand it was no noise before your work on and after you finish it was noisy !!!! You make the noise :laff:

I think this not what you mean.

Sometimes It is so tough to fix a amp in front of me , to fix by a forum , no amp in front .......... :dumass:
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Re: Heavily used and modded V4 noise issues

Postby jaywalker on Thu Oct 02, 2014 2:41 pm

So, the amp has been noise free since it's last service two years ago and it recently started making noise.
I think you have the right plan. Also check your input jacks.
I'm guessing those wires are coiled for noise rejection(???). I'm not really sure about that but I have some vague recollection of seeing that in the past. Maybe they just had extra wire to dispose of :blah:

btw, what tune is it humming?
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Re: Heavily used and modded V4 noise issues

Postby Unit_1 on Thu Oct 02, 2014 6:02 pm

man, what a rats nest

as racing would say "looks like mr. fixit has been busy"
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Re: Heavily used and modded V4 noise issues

Postby ThisLifeILead on Sat Oct 04, 2014 12:45 am

Ya its a friggin mess. Ive been cleaning the circuit board with alcohol as im working my way through it. So much smoke "Mojo" :jam:

jaywalker wrote:btw, what tune is it humming?

No clue haha i remember something about bee and baaa

I tightened all the jacks, removed the master volume, removed the coils and caps on the primary input, and channel 1 and 2 signals to board.
Its still there. I got as far as V3 with the scope and had to leave, havent gotten back in.

Now as i turn the vol 1 up a buzz is introduced (on top of the background buzz) which is similar to the background buzz.
So i am def changing the shielded wiring. just gotta order more. i also cleaned the hum balance pot to no avail. I dont think its a heater issue but ive been wrong before. The backgrpound buzz/hum has to be between V3 and V4.

Hopefully this weekend i can find some time to get through it. (going tomorrow to pick up a 1976 Sunn Model T 2nd Gen O)))))
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