Bright cap on Blues Junior?

[Mark]

I was working on a mate’s version 3 Blues Junior and I wondered why one of the mods on them isn’t to put the 100pF bright cap on a switch?

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The other thing I was wondering about was the 10K grid stopper resistors, I’ve heard they are there to stop RF etc getting into the amp, but I was wondering if increasing the valve from 10K to 220K or 470K bypassed with a 470pF cap might minimise blocking distortion?

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As shown in the first picture the circled 130K resistor is there to stop the amp going into low frequency oscillation when volume, master and bass controls are dimed.

I put the 2.7K cathode resistor in for some Marshall goodness, I have found there is slightly less output (audibly heard) and the distortion is slightly different. If I didn’t have it on a switch I wouldn’t have noticed it.

I have noticed that Fromel have an interesting replacement valve board. I mentioned it as it uses a 12DW7 in lieu of a 12AX7 to use a cathode follower powering the tone controls, it allows the output stage to be made cathode bias which may or not be a good thing, the heaters are elevated via the cathode resistor of the output stage I assume.

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Any thoughts?

[Stevem]

You could do that which is like a bright switch on a BF and SF Fender, but when it’s wired that way as the volume get opened up more and more to 50% the bright effect is reduced until after 50% point it does not take place at all.

If your ok with that then have at it.

[Mark]

You could do that which is like a bright switch on a BF and SF Fender, but when it’s wired that way as the volume get opened up more and more to 50% the bright effect is reduced until after 50% point it does not take place at all.

If your ok with that then have at it.

I typically wouldn’t use the bright switch on a Fender amp unless I had a particularly dull sounding guitar. That’s what prompted me to mention it.

[Mark]

I end up using a three way switch with the 100pF and 250pF value. I liked it.

[stratomaster]

There's also a "dull" cap at the end of the preamp right before the master volume. I tend to pull both out and replace them with a plate bypass cap on the final stage of about 680pF. Gives a more natural high end in my opinion.

[Mark]

There's also a "dull" cap at the end of the preamp right before the master volume. I tend to pull both out and replace them with a plate bypass cap on the final stage of about 680pF. Gives a more natural high end in my opinion.

Thanks for your comment, the dull cap wasn’t in the amp. It may have been removed in a previous mod.

The 250pF cap sounds well suited to funk tones on a Strat using the 2 & 4 positions.

[nuke]

Which version of the Blues Junior III?

There's two versions of the III. One has the "sparkle" mods that make it even more bright.

The bright cap on the BJR, has been there in every version, from the original to the present second-revision of the IV. It really doesn't do very much. As the volume (pre-master) is turned up, the bright cap does very little and most players tend to turn up the volume for some distortion and control loudness with the master.

The one place it makes a difference is working the amp to get a clean sound by turning the master way up, and the volume very low. In that case, you get that Fender bright-clean tone and it kinda works well for that.

[Mark]

Which version of the Blues Junior III?

There's two versions of the III. One has the "sparkle" mods that make it even more bright.

The bright cap on the BJR, has been there in every version, from the original to the present second-revision of the IV. It really doesn't do very much. As the volume (pre-master) is turned up, the bright cap does very little and most players tend to turn up the volume for some distortion and control loudness with the master.

The one place it makes a difference is working the amp to get a clean sound by turning the master way up, and the volume very low. In that case, you get that Fender bright-clean tone and it kinda works well for that.

I disagree that the bright cap doesn’t do much. I could definitely hear the difference when switching it in and out. Agreed the switch does less when the volume/gain is increased.

[nuke]

I think the best mod is to change bypass cap for the “fat” switch to about 2.2uf instead of 25uf. That rolls the bass down a bit in that stage so it doesn’t get so flabby, and then a bigger cap value going into the PI stage. It adds some bass back in. The net effect is the flubbiness is reduced in the “fat” on condition and you actually get some bass too.

Also, pull the tube board out and remove all the solder on the three pins of each EL84 that are no-connections. Clean thoroughly and use epoxy to glue them to the board, top, bottom and through the hole.

If you catch them early, you can ward off the broken traces problem on the EL84 pins. Also if you have a cracked foil to the active pins, don’t slag solder all over it. Run a proper jumper wire, neatly and securely following the board layout. Get the wire down into the hole and solder it well to the tube socket pin.

I’ve caught them early and epoxied these pins in a number of Blues Junior amps with very good long term results.