Vibrolux Reissue Problem

[J Luth]

I haven’t been on the forum for quite a while due to a few serious health issues. Its been a long 2 years or thereabouts. Hopefully it’s behind me,

The question is on a Custom Vibrolux Reverb Amp Professional Tube Amp, well that’s what fender calls it . Main circuit board is printed with 1990 , 02. It has a few issues, filter caps, some microphonic tubes and a general cleanup. The problem I am not sure of the cause is on both channels, with the volume controls turned all the way down, there is still a significant amount of volume. It’s not just a small amount of bleed, you could actually play a guitar or whatever through it as if you were practicing.

Before I get into it, any ideas would be appreciated

Schematics attached Vibrolux1.pdf Vibrolux2.pdf

J

[Chuck H]

First, nice to have you back. And I'm glad this was something that can potentially be "behind" you.

Since it happens with both channels I'm going to guess it's a ground issue. Perhaps some ground connection analagous to both volume pots (and probably more) is compromised. I've never been in the reissue model of this amp so I don't know what sort of connections could be responsible, solder or board plug lead, etc. It there are any non soldered ground dependent connections I would try cleaning those first. If that doesn't fix it I would go to inspecting soldered ground connections.

You should also analyze whether this is actually the normal signal chain or if it's some kind of bleed from some different issue. That is, do all the controls work normally with this signal bleed? Tone controls, reverb, vibrato? There's a possibility that if it's a bad ground these controls could also be affected. But please report on this anyway.

[J Luth]

Thanks Chuck, I wont know for a few more month's

Little bit of a repeat of the above post with a bit more info.

With the amp powered on and nothing plugged in there is just a slight bit of noise nothing really unusual. With both volumes all the way off, plug a guitar into either channel and it is loud enough to play. The treble and bass controls do not work with the volume turned down. Turning the volume up on either channel; the volume slowly and evenly drops off slightly until the knob reaches between 2 ½ and 3 at which point it starts to increase as normal. The treble and bass controls on both channels work as expected as soon as the volume is turned up. I also checked the treble and bass incrementally between off and 2 ½ and they work properly.

Turns out the owner states the volume's have been like that for a long time.

[Chuck H]

It's been my experience with these amps (well, that is, any BF design Fender) that the volume drop off is consistent with both channels operating simultaneously. There is a negative feedback effect that occurs because the channels are out of phase. This reduces volume to somewhere under five on the controls after which one channel will dominate and you can get a bit louder. I'm still suspecting ground connections or maybe a failed modification to the circuit.

That the controls don't do much with the volumes down and signal bleeding through could indicate that the ground for the volumes and tone controls is the same, but with the volumes up the channel NFB interaction helps the tone controls to work somewhat. It's really hard to say at this point.

[loudthud]

There seems to be a black wire that goes along the front panel between boards and then grounds at each end. Could that be the problem ? Do the pot bushings ground to the front panel ? The housings are usually connected to ground on the circuit board.

[Helmholtz]

Agree with missing ground connection.
Use your Ohmmeter to check.

[J Luth]

There seems to be a black wire that goes along the front panel between boards and then grounds at each end. Could that be the problem ? Do the pot bushings ground to the front panel ? The housings are usually connected to ground on the circuit board.

I checked all of those black wires and all are good between every board and all are grounded.
Doing a tap test, tapping the Volume and tone control PCB no noticeable change, tapped the input PCB and the jack to PCB wires a definite change in the noise. Took a chance and removed the plastic input jacks and installed a switchcraft, no change. Still the same.
Still checking all the ground connections.

[52 Bill]

Based on your descriptions, I would check the filter caps for the preamp section. The signal is bleeding or leaking through without going through the controls.

Try paralleling a known good cap across the existing filters and see what happens.

[J Luth]

Based on your descriptions, I would check the filter caps for the preamp section. The signal is bleeding or leaking through without going through the controls.

Try paralleling a known good cap across the existing filters and see what happens.

Appears to fix it. The caps do need to be replaced, I put a new F&T on and BINGO all good so far.

Ordered all new filter caps.

[SoulFetish]

Edit: i think my post was incorrect, at least as to why there is interaction between both channels.
Lemme think on this…

[27613rdu]

I have found with Blackface and Silverface Fenders that certain tubes in the V1 or V2 position will cause this effect. I believe that its due to crosstalk between the two triodes in the tubes. The 2 triodes operate out of phase and the 2nd one is after the volume control. As you turn it up the signals from the 2 tubes begin to mix and cancel each other out. The the 1st triode dominates as it is turned up more and the amp works properly. I have found that the JJ ecc803s tubes, which are long plate 12AX7s will do it consistently in the V1 and V2 positions. Unless it's annoying you unbearably I wouldn't worry about it. If you can't live with it ,changing your V1 and V2 tubes, maybe even to a different brand, will probably get rid of it.

[J Luth]

Update on the Vibrolux
Most everything is good, I have one question at the end of this.
The problem with the amp being loud with both volumes turned all the way down has been fixed, it was the filter cap. Thanks 52 Bill
Found a few other problems, most preamp tubes are microphonic and the power tubes were no good, fixed. Also found the positive lead of the 47uf 350v cap on the main board loose. When I clipped the lead off the cap it fell out of the board. I was replacing that cap but what a pain lifting that board. The lead had very little if any solder on it and I saw none on the board.

Question, the owner complained that the amp gets real loud way too soon when it is turned up. After I replaced everything I put it on the scope and with a 1K tone. turning the volume up on either channel the sine wave top and bottom start to flatten just before 3 on the knob and and completely flat by 4.
Also tested with a guitar and it is over driving a lot by at about 4 on both. Much higher than that and it is ridicules.

Anyone know if there any way to calm the volumes down?

[g1]

with a 1K tone. turning the volume up on either channel the sine wave top and bottom start to flatten just before 3 on the knob and and completely flat by 4.

What was the level of the 1K tone at the input?
To me that sounds pretty much like the standard 'clean til 3 or 4' for classic Fender amps.
Of course the tone controls settings will also have an impact.

[Chuck H]

What was the level of the 1K tone at the input?
To me that sounds pretty much like the standard 'clean til 3 or 4' for classic Fender amps.
Of course the tone controls settings will also have an impact.

This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^

A typical Fender amp with that preamp topography is clean to about four WITH SINGLE COILS. If you're using humbuckers you might not get the volume knobs past two and a half.

I've never measured the volume pot taper on any actual Fender amps I've worked on but my understanding from on line reading is that the volume controls were 20% taper. Some think the schematic markings of A-35 indicate a 35% taper but I think it may be a different indicator since 35% would be a really nit picky taper value. Fender did use 30% (J taper) pots for some things here and there. Anyway...

You can try changing the pots to a 10% audio taper. Bourns makes some good ones and that should give more range and control in the adjustment without changing anything about circuit function. Ten would still be ten and the bright cap (bypass cap) will still be acting over an equal resistance for any given gain level. It would just change the slope of the adjustment so the gain comes on more gradually.

[J Luth]

g1
I used 1k at 100mV, tone controls at noon.

It does seem to get a lot louder faster than any other amp I have tested.

Other players that have used it, have also said that it is a lot louder. They are good seasoned players not rookies.

He might just have to live with it.