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  •  » Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

#1 2008-02-06 00:11:48

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

New to the forum, experienced tech but relativly new to tube amps. My guitar player plays a V4, went to test it out before band practice and it was extremely hummy with harsh clipping at low volume. It sat idle for about 3-1/2 months before this.sad I am afraid of the inevitable full recap. Replacing the tubes would be easier but I did that about 6 months ago and it biased fine and worked great afterward. Now it doesn't so I broke out my test equipment and a camera for you guys. For the following pics I am using a funtion generator @ 1.5V - 2.5khz into volume1 input(signal at the bottom half of scope) and an 8ohm dummy load with the scope across it. The tone controls have been tuned to get as close to a sine wave as possible measured out of the pre.

Pic#1 Power output with Master @1/4 and Vol1 @1/4 Measured 0.3Vp-p out.
http://i6.photobucket.com/albums/y210/rockopusahol/powerout1sm.jpg
This has significant clipping, should be clean and more voltage at these settings.

Pic#2 Power output with master @1/2 and vol1 @1/4 Measured 0.3Vp-p out.
http://i6.photobucket.com/albums/y210/rockopusahol/powerout2sm.jpg
Even more clipped, wave amplitude the same. Not sure why the tubes are saturating so early with hardly any output.

Pic#3 Preamp output with master @0 vol1 @1/4 Measured 7Vp-p.
http://i6.photobucket.com/albums/y210/rockopusahol/preout1sm.jpg
This is more what I would expect.

Pic#4 Preamp output with master @0 vol1 @1/2 Measured 15Vp-p.
http://i6.photobucket.com/albums/y210/rockopusahol/preout2sm.jpg
Distorted, maybee too much?

At this point I am assuming the preamp is OK so I decided to check some voltages at the output and power supply stages:
Bias Voltage @ -65V (Spec says -62V) and -63V @ all control grids on all 4 tubes.
OK
Plate Voltage @ 533V (spec says 540V)
Hmmm.
Screen Grid Voltage @ 538V (spec says 530V)
....damn. Isn't the plate supposed to be more positive than the screen grid?

Continuing,
Plates of V4 phase splitter:
Pin1 @ 149V
Pin6 @ 90V ...crap! The spec says these should be at 200V! Havn't checked those resistors yet but this could be the problem. I am getting adequate voltage @ the test point D on schematic. It says 380V, I got 386V.
I am also getting about 25V more than I should on test points C and E.

Then I noticed unusual heat coming off tubes, took a snapshot then turned the amp off.
http://i6.photobucket.com/albums/y210/rockopusahol/badtubeglow1sm.jpg
The plates for those 2 tubes on the right look like they are about to melt!


So basically, unless I am missing something very obvious or just missing something, I figure I need to replace the power filtering caps(and while I'm at it I might as well replace all the caps), test/replace the phase splitter plate resistors(or phase splitter), and buy another quad of 7027s. Anything else you guys can think of?

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#2 2008-02-06 13:53:58

nricco
Member
Registered: 2007-07-26
Posts: 20

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

stupid question: did you have a speaker load on the amp while testing? the tubes look like mine did after i was"testing the amp" after a little repair- without a load. they get really hot! really quickly. I'm not a tech, but have put a screwdriver to everything I've ever owned.

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#3 2008-02-06 19:32:36

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

replacing the plate resistors is a good idea. 
The filter caps look like they've been changed.  and your DC voltages are measuring well within spec (with the exception of pin 6 of the phase inverter)
you might try swapping the power tubes to see if they always glow on the right side... or if the problem follows the tubes.

as for Pin 6,  measure R31 make sure it is the correct value.  then you might replace c11 and c13.   sometimes those can start leaking DC voltage to the power tubes causing some unpleasing results.   for good form you might test some of the other resistors in that area.  (R27-29, R32 and R33)

let me know how that goes!

Nricco,  I would be sure to always have a load on the amplifier when testing.   its not just the tubes you could hurt,  your output transformer could fry too.


-steve

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#4 2008-02-07 06:48:11

rockopusahol
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From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Yeah, I used an 8ohm dummy load, not speakers. I tried using speakers once to bias but thats no good as you have to have to have the volume up quite a bit to do it right. Friggin' LOUD! I was afraid walls were going to start collapsing. Dummy load's the way to go.

I will definatly try swpping those tubes, and will fix the phase inverter. I'll get back to work on it this week-end and keep you guys posted on how it goes. Thanks for the tips Steve!

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#5 2008-03-04 11:18:16

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

I finally got around to working on it again. Swapped the current Sylvania 7027A with some older burnt G.E.s and........Voila! the amp is putting out the same signal going to the grid. That was the major problem. I still need to fix the phase splitter probably because its clipping quite early in volume on the negative peaks. The distorted, low power signal I was getting before on the output was making worry that it was the output trannie, but fewwheeeeew, it was just the stupid power tubes.

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#6 2008-03-06 08:51:33

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Removed the plate resistors and R30 measured @ 63K and R31 measured 1.2M! For now I'm going to say that's my smoking gun on the negative peak clip. Ordered some new high temp., metal oxide, 5%tolerance, 2W 47Ks to replace along with some new JJ 7027As. Should be sounding good as soon as they get here. smile

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#7 2008-03-06 09:44:47

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

that would explain your problems.   I'm glad we got it worked out.   
let us know when you get the parts replaced and try the amp out!
-steve

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#8 2008-03-06 22:38:16

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

rockopusahol wrote:

I tried using speakers once to bias but thats no good as you have to have to have the volume up quite a bit to do it right. Friggin' LOUD!!

>>>>>>>>>>>>>>>>>>>>
rockopusahol,

Glad you got a handle on the problem, but I have to question this method. Bias setting requires a quiescent circuit, not one with a signal present. If anyone's interested, here's a good tutorial. 

http://www.lynx.bc.ca/~jc/bias.html

Best, Paul

Last edited by paully (2008-03-07 15:48:17)


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

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#9 2008-03-07 00:21:53

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Paul, 

I'm guessing that when you addressed "Steve"  you did not mean me.  and you meant to address rockopusahol.   

I agree with you, biasing an amp by scope does not give accurate results.   I didn't think much of it when I read it because the V-4 doesn't have a bias adjust pot.  and rockopusahol didn't mention replacacing the bias resistors.

glad you posted that link though.  it is a good one.

-steve

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#10 2008-03-07 10:20:11

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Yeah, sorry. I didn't mean to say that I biased the V-4 in my first post, and I did not replace any bias resistors. I meant that I checked for the cross-over notch with the scope when I replaced the tubes a while ago. I guess I figured that if there was no notch then the amp was biased correctly. But after reading that article it seems that with a scope you can only look for the "lower biasing limit" or underbiasing. JC Maillet in the above article says: "the only way to roughly infer individual bias of tubes is to measure total current in each leg of the output tranny, then swap the inner two tubes and make the same two measurements - you can write down two sets of linear equations and isolate for each of the four terms to infer per-tube current..." I think I see what he's saying but how would I do this in the V-4? Since the current going through the plate resistors is combined into each side of the primary of the output transformer, I could just add them for each side and make sure they're equal right?

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#11 2008-03-07 15:49:22

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Sorry Steve, all fixed. Too much beer.

Personally, I wouldn't even worry about bias adjustment on an Ampeg as there is none. A good quad set of matched tubes from Mesa(or the like) is all that's needed, as long as the circuit components are within spec.

Here's some interesting reading and a REALLY cool video by someone we can trust. The Weber meter link is also listed. They are a real time saver. The third link is to suggested 'drop-in' replacement tubes for the original Ampeg output tubes and some blunt commentary LOL.

http://www.eurotubes.com/euro-Generic-Bias.htm     biasing vid

http://taweber.powweb.com/biasrite/br_page.htm    weber plugin bias kits

http://www.triodeel.com/7027.htm    tube substitutions

Best, Paul

Last edited by paully (2008-03-07 17:25:29)


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

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#12 2008-03-19 14:59:12

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

OK, so I replaced R30, R31, C11, C13 and replaced the power tubes with JJ7027s. I cranked it on and baked it out for about 10min or so and then turned standby off. On the power tubes I measured -62.4 grid bias on all pin5s, 539-540V on each plate of the output tubes, and on the phase splitter I got 218V on pin1 and 217V on pin6, cool!
It looked good so I put a signal on the input and watched the output:
http://i6.photobucket.com/albums/y210/rockopusahol/1output-crossoverdist.jpg
Darn. Crossover distortion. I don't know why the input signal was getting so distorted when I would turn the volume up passed a certain point. Interferance in the oscope mabee?

The output from the phase splitter looked alot better though:
http://i6.photobucket.com/albums/y210/rockopusahol/7gridAC-C13-3.jpg
It looked good on both sides with near perfect 180degree phase shift.

Wildly guessing, I tried swapping tube 2 and tube 4 but got the same results with alot of crossover still.
Then I measured all plate resistor voltage drops and calculated the current going through each:
R39=15.56mA
R46=16.17mA
R40=16.86mA
R45=16.11mA
Since tube 3 was the highest and tube 1 was the lowest(giving me a difference of 1.24mA/side of the transformer) I decided to swap tubes 2 and 3 to try and get a decent balance. After the swap I got:
R39=15.55mA
R46=15.64mA
R40=16.5mA
R45=15.5mA
The 3rd tube position was still quite high but this gave me a difference of 0.81mA, which, to me, seems like hardly any difference. I should mention that all the voltages I measured were moving around quite a bit, usually about +/-4-5mV, so all of my calculations are based on very rough averages of plate resistor drop. Also, I don't know how long those resistors have been in there but I doubt they are all exactly 3.6ohms. smile
I checked the output again and:
http://i6.photobucket.com/albums/y210/rockopusahol/Outputaftertubeswap.jpg
I guess I'm satisfied with that. I'm going to plug my ESP-400 Viper into it and hook it up to a full stack later but it's much too early in the morning for that. heh.

Also, I figured I'd share the master volume mod that a tech previous to me did:
http://i6.photobucket.com/albums/y210/rockopusahol/mastervolumemod.jpg
And here is some pictures:
http://i6.photobucket.com/albums/y210/rockopusahol/mastervolumemod2.jpg
http://i6.photobucket.com/albums/y210/rockopusahol/mastervolumemod1.jpg
It's not the way I would've done it but after I replaced the horribly scratchy pot, it seems to work OK. If it were my amp I would take it out though.

Last edited by rockopusahol (2008-03-19 16:42:14)

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#13 2008-03-19 16:31:33

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

I gotta go back to your original post, and ask why your running a 1.5 V signal into the input. I'd start over again with a signal a little more inline with an instrument level. You could just be clipping the pre due to high input.

Other than that, I'd decouple the pre from the driver and test both pre and power sections seperately. Easier to trace that way. BTW, the link in Photobucket with [img] on both ends will deliver the image here.

OOPS! And magically the pics showed up LOL.

Best, Paul

Last edited by paully (2008-03-19 16:34:09)


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

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#14 2008-03-19 16:38:50

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Yeah, I realized that the amp was responding better with a voltage of around 0.8V or so, so I used that when I tested it this morning. The post above should be fixed now.

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#15 2008-03-19 16:49:38

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

When I did the master volume additon to my V4, I simply added the pot directly after the V3 cathode, with one leg straight to ground. No caps needed. Worked fine.

About that test level, you're still near line level. Knock it down to .08 V and you should get better results.

Best, Paul


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

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#16 2008-03-19 16:56:29

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

Really that low??? I had no Idea! Damn I feel like an idiot now. HAHAHA!

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#17 2008-03-19 17:05:53

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

You can verify where you should be using your scope. Plug a guitar directly into the scope and play some different notes. Lows have more energy, highs less. Set the display accordingly to some mean level. Then set the generator to roughly approximate the note display level. That'll get you in the ballpark. Let us know what you find. Yikes, I missed lunch! Gotta run.

Best, Paul


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

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#18 2008-03-20 04:11:54

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

just to get an idea,  you could crank your volume,  leave your Eqs all at the 12 oclock position,  and then turn up your signal generator until it starts to clip.   the signal will be rather low.

what are the peaks of the output looking like when you're getting that crossover distortion?   

I've found that most V4s I"ve worked on tend to bias up really nice... but you might check the grid current of your tubes... maybe yours would like to see a change.

I don't trust biasing to the scope as much as I do biasing to the tubes.   know what I mean?

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#19 2008-03-20 09:13:02

rockopusahol
V.I.P.
From: Portland, OR
Registered: 2008-02-05
Posts: 9
Website

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

I'll try your way next time but when I did it this morning I used the input pot and eq's to get as clean and as big of a sine wave as possible out of V3. Granted I didn't have to turn the input up very much but as long as it's clean coming out of the pre-amp as you can see in the phase splitter photo above, I figure I'm alright.

When I was looking at the ouput from the power amp I would slowly bring up the master and watch the peaks, which remained clean, curved and un-squashed long after I saw the initial crossover notch.

Checking the grid current like I did on the plate resisistors seems like a good idea and I'm kinda bummed I didn't check that earlier. That could be the source of the mismatched plate currents as well.   

Yeah, I guess I was trained to test and troubleshoot everything with a scope, so I'm just going with what I know. And I guess i get a certain amount of satisfacton with being able to trace a signal through an amp and see what each component does to different frequencies. At this point i realize it's unnessisary but to me it's part of the fun!

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#20 2008-03-21 01:35:41

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

Re: Help! V-4 Multiple issues. Possible recap/retube? PICTURES!

yeah.. check the grid current,  and adjust the bias circuit as needed.

-steve

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