68 custom vibrolux Mod to reduce Early Breakup

[Arbutt]

Hi all
I have a customer who has 68 custom vibrolux. He uses both channels, But states he would like to have the break up later.
Complete re tube has been done with all new tubes Bias is cool 60%. Changing V1 and V2 for 12AY7 was done
with Mixed results. im going to see if a 5751 might cut it , But i don't think it will please him
Looking at the map i can see that custom and vintage channels both go through V4 ( 12AX7) before on to the PI
My little brain thinks by changing V4 for a Lower gain tube Might make some changes to the break up volume

Im hoping that a few of you experts on here have done a few mods with good results
looking forward to your expert advice
Mr A
'68 Custom Vibrolux Reverb Schematic.pdf

[pdf64]

Just reduce the value of R45 (lower leg of the potential divider prior to the power amp, in place of the normal channel mix resistor) until he's happy.
The classic Fender value for R53 (power amp negative feedback resistor lower leg) would be 100R, maybe try that first.

[Chuck H]

He uses both channels, But states he would like to have the break up later.

As always... Knob settings are arbitrary. There will be no difference in clean volume whether the amp breaks up with the volume knob at four or with the volume knob at nine. The clean volume will be the same.

I think it's important to determine what your customer actually wants with this request. If he just wants the amp to break up later then you can keep reducing gain until you get it where he wants. Or you could change the volume control pots to a shallower taper to keep the gain the amp has but reduce gain at a given setting. Niether of these will make the clean tone louder.

If what your customer ACTUALLY wants is for the amp to be as loud as it is at, say, 7 but still clean (which is what I suspect) then what he's asking for is more power. There's not much to be done for this. You could try more efficient speakers but this is harder to come by with 10". He could acquire a 2x12 cabinet and put in a pair of efficient 12s, bypassing the stock speakers. That should make a notable difference. It won't change where on the volume knob the amp breaks up but it will be a bit louder.

[pdf64]

Yes, swapping the volume pots from the stock 30% to a standard audio 10 or 15% taper would move the clipping point to a higher number on the dial / provide better resolution for lower SPLs.
But sourcing pots that fit is more bother than tweaking a resistor or 2

[Helmholtz]

What does EARLY break up refer to: available clean sound level or just low pot setting?
What exactly was the result with 12AY7s?
What output power does the amp measure?

[Justin Thomas]

My experiences with preamp tube swapping have been largely futile at this goal. As far as knob settings, it works. But it also reduces volume, sometimes significantly.

Swapping V4 for a lower gain tube will also affect your reverb. There are tubes that are half 12AX7 & half 12AU7. JJ makes them, the ECC832 & ECC823. The difference between them is which side is the low gain triode; depending on how the amp is wired determines which one you'd want. I find them to be quite useful!

Swapping a 12AT7 or 12AY7 for a 12AX7 makes my amp stay clean up to 9 on the Volume knob onstead of say, 7. Using a 12AU7 makes it not distort at all but puts out less volume too.

Try the negative feedback first because it's easy & cheap, but I suspect the same as Chuck - more power. I don't know if a Super Reverb will cut it, and it's unfortunate they don't make a BF Vibrolux Reverb RI. Otherwise if they want to stick with Fender, it's either a Twin Reverb or (horror of horrors - I JEST!) one of the Ultralinear models of the late 70s.

Jusrin

[g1]

Bias is cool 60%.

What makes you say that? I consider 60% fairly hot for a Fender amp. The number given on the schematic equates to around 45% so you are way over that.
Hotter bias than stock means earlier breakup than stock.

[Helmholtz]

Hotter bias than stock means earlier breakup than stock.

I don't think bias makes a significant difference. Probably less than 1dB regarding gain. Clean output limit doesn't depend on bias anyway.

[pdf64]

The trem probably needs bias to be set on the cool side for decent depth to be achievable.

[Helmholtz]

Don't get me wrong, when Fender specifies a cathode voltage of 60mV I'm sure there's good reason.

[g1]

I don't think bias makes a significant difference. Probably less than 1dB regarding gain. Clean output limit doesn't depend on bias anyway.

For any given signal level you can adjust the bias and watch the output level increase/decrease on the scope. I don't think he is looking for a change in the output limit, rather the typical 'want it to start breaking up at a higher number on the volume control'.
I agree that the db difference may not be huge, but if you want to make the breakup later, why start by changing the bias to make it break up sooner.

[Justin Thomas]

Edit: I'ma dumbis. Just saw the schematic & see there's no bug in this. Can anyone tell me how to delete a post?

Even on a roach trem? I've heard this about the bias wiggle, but not standard BF/SF.

Just curious, not instigating.

Jusrin

[Arbutt]

Morning all. Many thanks for your comments. He is bring the amp back too me on monday and has agreed to leave it with me for a bit
I really appericate the help on this subject
many thanks again
Mr A

[Arbutt]

For any given signal level you can adjust the bias and watch the output level increase/decrease on the scope. I don't think he is looking for a change in the output limit, rather the typical 'want it to start breaking up at a higher number on the volume control'.
I agree that the db difference may not be huge, but if you want to make the breakup later, why start by changing the bias to make it break up sooner.

Thanks G1 this what he wants really is for a later break up.

[Arbutt]

As always... Knob settings are arbitrary. There will be no difference in clean volume whether the amp breaks up with the volume knob at four or with the volume knob at nine. The clean volume will be the same.

I think it's important to determine what your customer actually wants with this request. If he just wants the amp to break up later then you can keep reducing gain until you get it where he wants. Or you could change the volume control pots to a shallower taper to keep the gain the amp has but reduce gain at a given setting. Niether of these will make the clean tone louder.

If what your customer ACTUALLY wants is for the amp to be as loud as it is at, say, 7 but still clean (which is what I suspect) then what he's asking for is more power. There's not much to be done for this. You could try more efficient speakers but this is harder to come by with 10". He could acquire a 2x12 cabinet and put in a pair of efficient 12s, bypassing the stock speakers. That should make a notable difference. It won't change where on the volume knob the amp breaks up but it will be a bit louder.

Thanks Chuck I don't believe that he will want to change speakers or even a ext cab is on the card