Troubleshooting help on Fender 61 Vibrosonic 6G13-A

[Bill McKenna]

Morning  Tube Amp Experts

I have a 61 Fender Vibrosonic on the bench from a friend. 6G13-A.  It had been massively modded and butchered in the 70s-80s someone tried to turn it into a Boogie I think, anyway. I stripped it and put it back to original Fender specs. Replaced all parts, so much had
been done it was impossible to untangle.

The amp powers up reads good voltages as per the schematic, plays fine on both vibrato and normal channel but is noisy and the vibrato doesn't work. I have been over it 50 times and Fixed a number of small problems but still no luck.  I tried the technique of unplugging preamp tubes and narrowed the noise down to 7025 5 next to the driver, I think.. but this is observation only the noise could be coming from other places. Without the driver in it is dead quiet so think the PS is ok.
All new Sprauge Atoms PS resistors and Rectifier board rebuilt. 
Tubes have all been checked on a Hickock tester.

I am guessing I have overlooked something simple (as is often the case) but could use any troubleshooting tips that you might have or
any explanation of how to trouble shoot these brown vibrato circuits. It doesn't have the photo oscillator like BF amps.

My troubleshooting skills in the signal path are weak.. as it mostly seems to be PS and B+ issues that I come on when repairing old amps.
My next thought was to disconnect the Vibrato section completely and connect the Vibrato channel directly to the Driver as the normal channel is. See if this at least proves the noise is in the Vib.

I can upload pics or links to the schematic 

Thanks
Bill

[Willabe]

Here's a link to very good way to double check an amp build/rebuild;

http://el34world.com/Forum/index.php?topic=17701.0

[sluckey]

Look at page 12 of this pdf for an explanation of how that vibrato circuit works.

      http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

V5 is in the signal path for the vibrato channel. I've had trouble with that tube. Grab a handful of 12AX7s and start swapping in until you find one that is not noisy in the V5 socket (I have a 5751 in my circuit). If the vibrato has no effect then V3 (oscillator) would be a likely suspect. V3 needs to be a strong 12AX7. The three caps between the plate and grid as well as the cathode bypass cap are all suspect for a dead oscillator. Check voltages. An oscillator that is working properly will have a big voltage swing (at the speed of the oscillator) on the plate. You can see this smooth swing easily using an analog meter or scope. Digital meters just appear erratic. If the voltage is a steady DC level, then the oscillator is not working.

The phase splitter (V4) is another suspect, although it's not as suspicious as V3 circuit.

[Bill McKenna]

Thanks Sluckey and Willabe,
I will study and report back. Great docs btw. Will print these out and save for sure! Lots of good help in these!
Thanks
Bill

[Bill McKenna]

Well as usual you guys got me sorted! 

I went through the amp again with the highlighter and schematic testing from top to bottom and found a couple issues. After correcting and testing some of the tube changes Sluckey suggested it started working!  I have a couple minor substitutions i wanted to run by you..they
seem to work but I still still have issues.

The main sub is that I didn't have an 8.2M resistor so I used a 10M This is between the .02 and goes to the Cathode of V3 to the Intensity pot. I will order the correct value if this is critical. Seems to work? 

I also changed the 15K for a 5K in the bias between the two 8uf @ 150v. The amp was starting to red plate with the 15K that was called for.
I have the amp on a Variac with a digital AC volt meter in line to make sure I'm right at 120 AC. Have had issues in the past with over voltage causing red plating.

My remaining issue to correct is a is strong AC background noise in the amp. The amp is open on the bench and I have florescent lights but this has never been an issue on other amps I've worked on really. The noise persists even when the light is out. Almost sounds like I have a 2 prong plug that is reversed. I've checked this and did the updated AC connect for fenders suggested in your document Sluckey.

My other fenders are dead quiet so must be something dumb that I am not picking up on?

Thanks
Bill

[sluckey]

The main sub is that I didn't have an 8.2M resistor so I used a 10M This is between the .02 and goes to the Cathode of V3 to the Intensity pot. I will order the correct value if this is critical. Seems to work?

It's not critical. Larger value means less vibrato intensity. Smaller value will increase the Vibrato intensity. You could probably go as low as 2.2M, or 3.3M, or 4.7M. You may want to experiment to see which sounds best to you. Of course, the vib sounds great with the original 8.2M.

I also changed the 15K for a 5K in the bias between the two 8uf @ 150v. The amp was starting to red plate with the 15K that was called for.

That's a good candidate for a simple mod to make the bias adjustable if that sounds interesting to you. Put the 15K back in and replace the 56K resistor with a 50K trim pot in series with a 22K resistor. That will allow you to adjust hotter or cooler than the original circuit.

My remaining issue to correct is a is strong AC background noise in the amp.

Try to narrow down where the noise is coming from. Is it both channels? Is it affected be either or both volume controls? You can pull little tubes to see if doing so kills the noise. Pulling V1 thru V5 will eliminate both preamps, leaving only the PI and output tubes in circuit. Does that make it quiet?

[eleventeen]

Are you sure you have a CT, real or synthetic (synthetic is actually better) on your 6.3 winding? A tube amp without a heater CT will in 90+% of case hum really badly. And a super good HVCT ground connection near the PT? And good hum practice throughout the amp?


If those things aren't right they cannot be fixed any other way. Just saying.



Assuming it is not e-caps, can you locate the AC hum by pulling tubes starting from the PI and moving back towards the preamp?

[Bill McKenna]

Thanks guys,

That's a good candidate for a simple mod to make the bias adjustable if that sounds interesting to you. Put the 15K back in and replace the 56K resistor with a 50K trim pot in series with a 22K resistor. That will allow you to adjust hotter or cooler than the original circuit.

Funny I had that exact thing in there and ripped it out as part of the restoration, just didn't want any thing in the amp that was old or unknown quality. I will put it back in now. its an easy mod I've done to other Fenders.

Try to narrow down where the noise is coming from. Is it both channels? Is it affected be either or both volume controls? You can pull little tubes to see if doing so kills the noise. Pulling V1 thru V5 will eliminate both preamps, leaving only the PI and output tubes in circuit. Does that make it quiet?

Yep have been doing this. It seems more less noisy everywhere. Its a bit louder on the vibrato channel than the Normal.
I already checked to see that all the heaters are wired on the same side. This is something I've had to fix before that helps with noise.

Are you sure you have a CT, real or synthetic (synthetic is actually better) on your 6.3 winding? A tube amp without a heater CT will in 90+% of case hum really badly. And a super good HVCT ground connection near the PT? And good hum practice throughout the amp?

Transformers are original fender, everything is wired as stock as much as I can, remind me again about heater CT? I have vague memory of having to fix this on some amp along the way but its been a few years  .

Let me dig around on the old threads and schematics and see if I an find a CT post.

Thanks Gents.

Bill

EDIT... untangled quotes

[Bill McKenna]

clearly I need to learn to use quotes :)

Anyways... I checked it out and there are Center taps on the fender Transformer.  I also tried the 2 100 ohm resistors trick just for kicks without any noticeable difference.

Then I went through the tube removal process again. All noise stops when V5 is removed. This is the tube just before the driver which collects the 2 signals from the vibrato as I learned from your cool pdf Slukey. The noise doesn't change at all when V2 -V4 are removed.
V1 is quiet and sounds really nice when V5 is removed.  So we know for 100% that this is a Vibrato channel issue and most likely in V5 
So.. guess I need to go back over this area. It all works so not sure why there would be noise at this level unless something was wired wrong?

Thanks
Bill

[Bill McKenna]

I'm still stumped, 

I narrowed down the noise to V5 primarily and this eve when back over and traced this area. Vib is working fine sounds great
Normal Channel sounds great.. pretty quiet with V5 removed. But the Vibrato Channel with V5 installed has nasty A/C noise and hum.
With V4 and V3 removed the noise is the same on the Vib Channel. Also if V1 and V2 are removed its the same.

I tried disconnecting the Preamp from where it connects into V5 at the 220K and the volume of the noise went down some but
still clearly there. I tried reversing the Normal Channel with the Vib channel (they are the same). Got about the same level of noise.
Thought maybe one of the Filter caps was bad even though it is new so changed the one connecting to the B+ (blue wire) This had
no effect. They are all new Sprauge Atoms 20@500 BTW.   

Question, is there a straight forward way to test through to find a A/C noise like this in an amp? perhaps some kind of signal probe?
I don't mind buying some tools if needed. Seem to recall years ago some kind of AC signal tracer that analogue guys used
to ping out amps?

Thanks for any help. I'm stumped.
Thanks
Bill

[sluckey]

Measure voltages on V5 pins 1, 2, 3 and 6, 7, 8. Post results.

With power off, measure resistance from pin 2, 3, 7, and 8 to chassis ground. Post results.

***BE CAREFUL!***  Use a gator clip lead connected to chassis to put a ground on V5 pin 2. Then ground V5 pin 7. Then ground both pin 5 and pin 7. Does any of this eliminate the noise?

Replace both of the 100K/5% CC resistors that connect to the V5 pin 1 and 6 (also connect to that blue wire from the cap board). Use metal film resistors. Does this eliminate the noise?

Replace the 4.7K resistor and 4µF cap connected to pins 3 and 8. Use metal film resistor. Cap can be any value between 4µF and 25µF. Does this eliminate the noise?

Replace both 1M grid leak resistors with 1M metal film resistors. I'm talking about the 1M resistors that connect from pins 2 and 7 to ground, not the ones that connect back to the VIB phase splitter. Does this eliminate the noise?

[Willabe]

Do what Sluckey said 1st.

Then if you still have trouble here's a link to Doug's signal tracing amp.

http://el34world.com/Hoffman/tools.htm

[Bill McKenna]

Thanks for the tips Guys
Will do all you suggest and report back
Thanks
Bill

[Bill McKenna]

Evening Guys
Got down to the shop today and worked on the Vibrosonic
I tried all the things you suggested.
Here are the Voltages and resistances.
Voltages
Plates P1/P6   345/340 
Grid   P2/P7   .05/.08    1Mohm to ground
Cath  P3/P8  3.8           5.3K to ground

replaced all requested parts without any change in noise levels (these were all new parts checked before install)
Clip to ground on the grids yields cut of noise  Low freq noise cut on P2 High AC type noise on P7 cut.

Guess I know that this is something coming from the upstream signal path. I unplugged V4 V3 V2 (no change in noise)
Guess this limits the noise to something right around the signal path Perhaps the .005, the 220K, the .00025 or the .01?
These are all new parts as well prechecked before install but I can pull and replace again if you think this is the next right
logical step?

Thanks for the tips and looking forward to next ideas.

Thanks
Bill

[sluckey]

I've got a couple suggestions...

First, lift the end of the cap as indicated in the pic. Leave it dangling for a while. This will kill the guitar signal coming from the V2 preamp. Does the noise go away? If so, we need to look at V2 circuit. I highlighted components of interest in green. We'll deal with that in another message.

If the noise does not go away then ground the three points (one at a time) as indicated. Does the noise go away when any of these points is grounded?

PS... Please attach some high rez pics of the board. I need to be able to see resistor color bands well enough to tell the value of the resistors.

[Bill McKenna]

Have been following up today on your suggestions Slukey,

First, lift the end of the cap as indicated in the pic. Leave it dangling for a while. This will kill the guitar signal coming from the V2 preamp. Does the noise go away? If so, we need to look at V2 circuit. I highlighted components of interest in green. We'll deal with that in another message.

I did this and narrowed down the noise to V2 circuit as you suspected. As a double check I disconnected the signal bridge wires from Normal and Vibro channel, and jumped the Normal Chanel to both 220ks and this works pretty well. Minimal noise.
Tried reversing these and noise is coming from the Vibro channel as best I can tell.

The gain seemed bit hotter on Normal vs Vibrato so I checked the voltage and resistance of V1 and V2 and found that the 1M Volume pot on the vibrato channel producing about 2x the resistance as the V2 pin 7 grid   250K vs 125K on the Normal Channel. Replaced the pot now they are about the same (this was in no way part of the noise issue). Just a pot issue I think.

Noise is still present and here are the voltage and resistance from V2 and from V1.

Pin    V2           V1                   V2 ohm      V1ohm
1       217          205                    0            0
2          0               0                 36K         37K
3          1.5            1.6           1600        1500
6       223           213                   0             0
7            .006          .002         140K        142K
8          1.7            1.7             950          924

All this seems about right.

I replaced all the parts you colored on the diagram and also replaced most of the rest of the V2 parts. Excluding the Cathode cap and resistor.  Using new parts and metal film resistors. Orange Drop Caps etc.  I tried swapping tubes in V1/V2.
Noise persists Everything is working properly but the AC type noise and lower end hum is present. 

I tried doing the grid to ground trick on V2 and even with both grounded there is still some noise. Thinking the Cathode cap and resistor are the next targets for swap out on Pin 3 since Pin 7 on V2 shares this cap and resistor with V1.

What do you think?
Thanks
Bill

[sluckey]

I did this and narrowed down the noise to V2 circuit as you suspected.

OK, you're saying that with the end of the cap lifted, you have no noise? And at some point you said there is still noise when V2 is pulled. That only leaves the four highlighted components, or the wiring between V2 plate and the end of the cap that you lifted, or possibly grounds.

The gain seemed bit hotter on Normal vs Vibrato

It should be considerably hotter. Maybe even too hot. V2 uses a split resistor plate load resistor (100K and 10K) and the output is taken across the 10K, so it's output is only about one tenth as much as the output from V1.

I'd still like to see some pics.

[Bill McKenna]

Hey Sluckey

OK, you're saying that with the end of the cap lifted, you have no noise? And at some point you said there is still noise when V2 is pulled. That only leaves the four highlighted components, or the wiring between V2 plate and the end of the cap that you lifted, or possibly grounds.

Yes Lifting the .05 cap or removing the wire from it that bridges over to the driver side 220K stops the noise.  When V2 is pulled noise persists. I replaced the 3 parts already and all the other parts in the front Vibro channel without improvement. I suppose its down to wires, tube sockets and possibly the pots. Had thought about replacing the vibro pots with the normal pots as they are identical. Normal Quiet and vibro with noise.

I tried to get pics but they came out poorly. Also they were too big so I will try again today.
Thanks
Bill

[Bill McKenna]

Pictures of the main board.
pic 1 driver and end of V5

[Bill McKenna]

Picture 2 and 3

[Bill McKenna]

Bumping thread.. for greater visibility.
thx
bill

[Bill McKenna]

Did some more research on the Vibrosonic tonight
I am not thinking that this noise is something power section related. While the Noise gets much less when V5 is pulled or if the Vibro Channel is disconnected, it is still present. Its simply less amplified.
The only time it gets totally quiet is if the driver tube is removed V6. I also have a Master volume installed and notice that
even when its potted all the way down there is still some Humm.  Thinking this may be a bad filter cap.. as its that low hum
typical of amps with leaky caps. I replaced all the caps with yet another set of new Sprauge Atoms. 20 uf @ 450vdc.
Still with new Caps I am getting this noise which is a combo of AC and Hum.  I am beginning to wonder if there may be a problem in the Power transformer.
I completely rebuilt the Diodes board with new uprated parts. Put a 50KL and 22K resistor for Bias in as suggested.
Have replaced virtually every part of the vibrato channel. This noise is driving me nuts 

Any thoughts or help appreciated.
thx
Bill

[sluckey]

You've practically replaced every component in the signal path for the VIB channel. You've replaced all filter caps. At this point I'd have to examine layout, wire routing, and reflow all ground connections, especially that brass plate behind the pots (Need a BIG iron for the brass plate). Also retighten  all pots and jacks including the speaker jacks. I'm about out of gas if this doesn't do it.

The only time it gets totally quiet is if the driver tube is removed V6.

Have you replaced the output tubes? How is the bias set? Badly mismatched tubes can cause hum. With V6 in, disconnect the NFB wire from the speaker jack. Does this affect the noise/hum?

Are you sure you have reversed all the mods and the amp is now back to original condition (except for the bias pot mod)?

[Bill McKenna]

Thanks Slukey.. 
This thing is really turning into a mystery. I've never had such problems with noise before 

Good thoughts. I will check all of the things you mention. and report back. 

I stripped the original amp which was some kind of Boogie style bastard child of the early 80s. Down to the last bit. It had been turned into a 4 6L6 output tube Fender Blonde Twin style Power, with a master volume. (which I have disconnected). 
I put the power section back to factory specs. 2 5881s etc. 

I checked the electro harm (sovtech) 5881s with the Hickok with Mutual Conductance test to make sure they were balanced they were within a few %.
I replaced with a set of the Groove tube matched GE repro 6L6s from another amp that were known good and matched. 
When I did this one of the 470 ohm 1w Resistors smoked. I didn't have time to check the voltage before.. but it seems something is very wrong in the outputs. I also was seeing some minor red plating earlier, wondering if there may be a driver issue. I will work on this next along with what you suggest.
Thanks
bill

[Bill McKenna]

Success!!! 

I figured it out finally it was as you thought Sluckey.. but very odd.

I got the issue narrowed down to the 3 parts you suggested in the graphic earlier. I had replaced these a couple times without any
drop in noise The 10K,1 100K and .003. to Pin 6 on V2. I ended up removing the .003 and wiring up the Vibro channel the same
as the Normal Channel and all noise went away. Not sure why these parts were making the noise but even after swapping them
in and out with new parts again.. I got noise.. but with them out it was quiet. So just left them out. Amp sounds good and clean, loud etc.
I may try buying some mica .003 vs poly and see if that has any effect.. But for now the amp is working well and sounds good so at least
I'm out of deep weeds and can tinker away in self service mode againl

Thanks again for all the patient help and suggestions. You provide a great service to novice amp hacks like me!
Can't say that enough.

Thanks
Bill