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#1 2012-12-19 18:03:15

Redwood
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Registered: 2012-12-10
Posts: 19

Need help diagnosing wonky voltages at a few preamp tubes

I recapped and retubed this early distortion model V4 and it's given me all kinds of trouble.  My first mistake was that even though the amp came to me disabled, when I replaced the electrolytics I just copied how they were installed despite the fact that they didn't follow the schematic exactly.  No major mods, just didn't align all connections to the 40/40/40 the same as the schematic and it caused a few issues that took a while to straighten out.  I've now redone that exactly like the schematic and things are better.

I can now play lightly at low volume with no problem, but turn it up past 2 or hit the strings very hard and I get this aweful mechanical sounding blatty overdrive.

Voltages at the A,B,C,D,E checkpoints aren't exact but they're not too far off and consistently a bit low. 

Voltages at the power tubes seem OK too.

Bias voltage at point F is a bit too negative, about -31 vs -28.5 called for.

V1 pin voltages are OK.

V2 pin voltages are where it starts to get wonky, and V3 is even worse.

V2-1  185V  (dead on)
V2-3  1.16V  (should be 1.6V)
V2-8  4.3V   (should only be 0.6)

I've double and triple checked every cap and resistor tied to these tubes and tested the tubes too.  I just can't figure out where my problem is. 

Any hints at all would be greatly appreciated.

Here's the whole table of my current voltage readings:

    schematic V        measured V
V1-1    145        145
V1-3    8        10.3
V1-6    165        162.5
V1-8    1.4        1.35
V2-1    185        185
V2-3    1.6        1.16
V2-8    3.5        1.8
V3-1    105        93
V3-3    2.8        3.5
V3-8    0.6        4.3
V4-1    200        196
V4-2    95        44
V4-3    45        66
V4-8    3        1.9
V5-1    310        301
V5-3    190        178
V5-6    104        113
V5-8    0.6        0.7
V2-1-1    242        240
V201-2    75        68
V201-3    95        90
V201-6    138        152
V201-9    2        3.8
           
A    545        538
B    352        335
C    347        325
D    310        300
E    338        335
F     -28.5        -31

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#2 2012-12-19 21:45:05

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

Re: Need help diagnosing wonky voltages at a few preamp tubes

Did you not include any of the grid voltages for the preamp tubes because they are all 0? Or are you just assuming they are all 0?

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#3 2012-12-20 16:23:22

Redwood
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Registered: 2012-12-10
Posts: 19

Re: Need help diagnosing wonky voltages at a few preamp tubes

Sorry, that's a good question.  I only recorded the values that appear in the schematic since I don't really know what the rest are supposed to be, but I'll take a look at them when I get a chance today.  Thanks for the pointer.

Last edited by Redwood (2012-12-20 16:23:39)

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#4 2012-12-20 20:25:01

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

Re: Need help diagnosing wonky voltages at a few preamp tubes

Also,  are the tubes new?  Have you tried swapping any of them?

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#5 2012-12-20 20:58:35

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Need help diagnosing wonky voltages at a few preamp tubes

All new tubes, with the exception of the 6CG7 in the reverb section since the one Fliptops sent me didn't fit inside the heat shield.  But I actually found another fresh NOS Sylvania in my stock on hand so just plugged that in this morning and no change.  I don't have a high end tube tester but both the new and old tubes all checked out OK on my old Sencor RC-154 solid state tube tester.

I've also gone though this morning and replaced the last of all the capacitors on the whole damn preamp board but I'm still seeing anomalous voltages at several points, most notably V3-3, V3-8, V4-3, and V4-8.  Also off a bit at V201-2 and V201-9.

Should I be starting to suspect that PEC in the tone circuit?

Values in the second column are noted from the schematic.

V1-1    140    145
V1-2    4.9   
V1-3    9.9    8
V1-4    .3mV   
V1-5    .3mV   
V1-6    158    165
V1-7    0.6   
V1-8    1.3    1.4
V1-9    .4mV   
       
V2-1    179    185
V2-2    1mV   
V2-3    1.1    1.6
V2-4    0   
V2-5    0   
V2-6    216   
V2-7    1mV   
V2-8    1.7    2.8
V2-9    .4mV   
       
V3-1    95    105
V3-2    105   
V3-3    3.6    2.8
V3-4    .4mV   
V3-5    .4mV   
V3-6    196   
V3-7    12mV   
V3-8    4.1    0.6
V3-9    1.1   
       
V4-1    192    200
V4-2    43   
V4-3    65    90
V4-4    .4mV   
V4-5    .4mV   
V4-6    219    222
V4-7    0.8   
V4-8    1.8    3
V4-9    .6mV   

V5-1    295    310
V5-2    86   
V5-3    173    190
V5-4    .4mV   
V5-5    .4mV   
V5-6    112    104
V5-7    -.5mV   
V5-8    0.67    0.6
V5-9    .4mV   

V201-1    236    242
V201-2    66    75
V201-3    87    95
V201-4    .8mV   
V201-5    .7mV   
V201-6    150    138
V201-7    165   
V201-8    1.8.V AC? not really stable   
V201-9    3.6    2

Last edited by Redwood (2012-12-20 20:59:12)

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#6 2012-12-22 02:29:41

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Need help diagnosing wonky voltages at a few preamp tubes

At this point I am seriously suspecting the PEC tone module as the source of my problem.  Once I looked more closely at the grid voltages I noted that what I should be seeing at V4-2 is 95V but I'm only getting 43V.  This despite the fact that I'm getting 219V compared to the 222V I want at V4-6.  If I'm reading the schematic correctly (big leap there), it appears this tube is in sort of backwards so that 6,7,8 are the first gain stage feeding into 1,2,3. The only components between them are C10 and C11, which I've verified beyond a shadow of a doubt, and that PEC in between C10 and C11.  It's about the only thing I haven't checked and frankly I don't even know how to since I can't find a schematic that breaks out which pin is which. 

Either way, my next volley in the attack on this thing is to finally pull the control board, which I've avoided like plague until now.  I've already picked up the parts to do the discrete replacement so hopefully I'm finally going to get somewhere with this thing.

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#7 2012-12-22 03:40:15

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

Re: Need help diagnosing wonky voltages at a few preamp tubes

Have you listened to the amp via the preamp out?

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#8 2012-12-22 19:09:21

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Need help diagnosing wonky voltages at a few preamp tubes

No, I haven't tried that because frankly I find what I've read about it confusing.  can I get away with just running the preamp out straight into another guitar amp's instrument input if I keep a dummy load on the v4 speaker jack?

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#9 2012-12-22 23:34:00

Redwood
Member
Registered: 2012-12-10
Posts: 19

Re: Need help diagnosing wonky voltages at a few preamp tubes

I poked around on the web enough to convince myself that I can safely run the preamp out to another guitar amp as long as I keep a load on the normal speaker output since it doesn't appear the preamp out breaks that circuit.  I didn't try it with a speaker since I didn't want to drive the slave amp too hard so just stuck with the dummy load.  The slave amp sounded like crap, not exactly the same as the V4 with its own speaker, but very crappy weak distortion with the tone pots zeroed out, and really bad distortion if I dime the bass pot, and just squealing if I dime the treble pot.

I should have mentioned before the PEC is all waxy with no other source for such residue other than itself.  I don't know that the caps inside it are wax based but it's all the more reason to suspect it's my culprit.

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