the bright cap (blues jr or any other amp w/one)

[wagdog]

Greetings,

Been experimenting a bit w/my blues jr.

I usually play strats/teles, keep the master dimed (12'ed?) and add vol w/the channel vol control as needed.

Have modded the tone stack (.1uf bass cap), jumper on mid pot and toned the bias on the power amp down a bit. Amp sounds very nice.

Still found the amp a bit thin at low vol so I started looking at the 100pf bright cap on the channel vol pot. I hate the bright cap on my twin, so I figured I'd clip it out and see how it sounds. Worse case scenario I'd pull the board and put it back.

Not surprisingly, the amp sounds much fuller, especially at low vol. It's quite a significant difference and I'm pretty surprised how much of the tonal character of the amp is/was tied up in that lone cap.

Also wondering about an adjustable bright cap. Anyone seen a design like this? Maybe a 100pf cap and a 50k pot in series across the top of a vol pot.

[pdf64]

'keep the master dimed (12'ed?) and add vol w/the channel vol control as needed'
A bright cap will have most impact at low settings of the volume control it's bypassing, reducing to zero impact at max setting.
For low volume use, it may have suited you better to set both the master and channel vol about halfway; if still too bright, increase the channel vol to 3/4.
I can't see any benefit in fitting a brightness control, as you already have the method described above available (or did have).
Bright caps are one of those features that seem to polarise opinion; putting them on a switch may be best, but a master volume also allows some control of its impact.
Pete

[wagdog]

A bright cap will have most impact at low settings of the volume control it's bypassing, reducing to zero impact at max setting.
For low volume use, it may have suited you better to set both the master and channel vol about halfway; if still too bright, increase the channel vol to 3/4.

Thanks for the reply.

On the blues jr, the "channel" vol really just controls the gain of the first pre-amp sections. When you start turning it up, you get pre-amp distortion pretty quickly, and imo, it's not real nice distortion.

So I tend to play w/the master dimed in a (vain) effort to get as much clean headroom out of the blues jr as possible. And yes, I realize that clean headroom and blues jr don't belong in the same sentence, lol. I like the tone of the amp though, and at reasonable volumes it stays sort of clean w/just enough grit to really sound nice.

Bright caps are one of those features that seem to polarise opinion; putting them on a switch may be best, but a master volume also allows some control of its impact.

Agreed. I clipped it 'cause I'll be doing some other mods to the amp shortly anyway, so I figured I could always put it back, or some version of it back (such as maybe a 47pf cap instead of the 100pf that was there), or put it on a switch, or make it adjustable.

Also, I did find that the amp didn't sound that good w/a dirt pedal before, and now it really takes nicely to them.

[Jazz P Bass]

I had luck putting a 12AU7 in V2.
It helps with 'headroom'.

[wagdog]

Will give it a try Jazz.

Just for fun, tonight I installed a On-Off-On switch. One side has the stock 100pf cap, middle nothing, 45pf (100+82 in series).

You CAN definitely hear the difference between 100pf and off. The off->45pf is more subtle and I like it. The 45pf seems to work well with dirt pedals as well. The 100pf gets a bit tinny/buzzy w/an overdrive or distortion pedal.

[soundguruman]

It should be 150 pf, and silver mica, which will tend to produce less hum...

[wagdog]

It should be 150 pf, and silver mica, which will tend to produce less hum...

The stock value is a 100pf ceramic.

I tried silver mica caps as bright caps before in my twin project, and imo, didn't noticed huge difference. I think they tend to sound a little smoother, but I didn't have it on a switch so it's hard to tell.

[soundguruman]

The stock value is a 100pf ceramic.

I tried silver mica caps as bright caps before in my twin project, and imo, didn't noticed huge difference. I think they tend to sound a little smoother, but I didn't have it on a switch so it's hard to tell.

The stock bright cap is 100 pf
We change it to 150 when we beef the amp up...
Change C1 to .0033
Change C2 to 150 pf
Change R6 to 68K (raise frequency of tone stack higher, because you have a small speaker...)
Change Middle control to 50K
Change R5 to 220K or 270K
Change C35 to 500 pf
Change -15 bias to aprox. -17 the output tubes tend to be under biased...
Replace the output tube sockets w/ ceramic
Replace stock power supply filters to F&T brand, Make C15 100uF instead of 47
I put 5-7 pf 1000V silver mica across V1A, plate to grid...this reduces microphonics of input stages...

It's really not a bad amp when it's tweaked a bit. I have a friend who loves it, he put mercury output transformer in his...

[soundguruman]

The stock value is a 100pf ceramic.

I tried silver mica caps as bright caps before in my twin project, and imo, didn't noticed huge difference. I think they tend to sound a little smoother, but I didn't have it on a switch so it's hard to tell.

If you use silver mica, the reduction in 60 cycle noise is measurable.

[Rick Erickson]

[If you use silver mica, the reduction in 60 cycle noise is measurable.]
Well - I just read it on the internet - so it must be true...

[soundguruman]

[If you use silver mica, the reduction in 60 cycle noise is measurable.]
Well - I just read it on the internet - so it must be true...

ESPECIALLY on the bright cap, installed across the volume control. It's true. A regular ceramic disk installed in that position can contribute a lot of 60 cycle hum. It's measurable.
The disk cap is connected to the preamp grid. The cap acts like an antenna, and is very sensitive to external AC fields. The cap will pick up 60cps from the heaters, introduce the 60cps into the audio path, and the 60 cps will be amplified.
Using a better cap will reduce this 60cps greatly. Depending on the design of the amp, you may see a reduction of 50% in hum, at the output.

Cheap capacitors act like inductors at different frequencies. Using higher quality parts, that are designed to reject external noise, there can be a major improvement.

Of course, the major source of 60cps IS the guitar Pickup.
So stop blaming the amp....
YOU GUYS who "insist" on using single coil pickups are HOPELESS.