New build questions - bright cap values?

[JeffH]

I just finished my JTM 45 kit and it works and sounds great except:

The bright channel is really bright - is there a consensus of what to use on the bright channel volume pot to tame it slightly? Also, the normal channel is dead quiet (when not playing anything), but there is some hiss on the treble channel. I'm assuming that changing the cap will probably change the amount of hiss also, right? I built the kit with sozo caps and standard tranny's ,resistors and tubes. I used the extra filtering on the internal cap can.

I finally got the chance to jumper the channels, plug into a 4x12 and give it a workout and had the most fun playing I've had in a long time. My wife said it shook the house. This is my first Marshall-style amp (except for a couple Traynor YBA-1's), nearly all my previous experience has been with various classic Fender amps.

This is my second amp build - I've previously built a 5E3 kit, and some pedal kits. The instructions were very thorough and it went together very well. I haven't had a chance to get any pix yet. I'm planning on getting the 50 watt plexi kit next.

[gutpile]

I used a 100pf on just about all of my builds... I think the kit calls for 500pf... try a 100 or 250... if still too bright, try removing it! easy enough change to tinker around!

Mike

[Roe]

discussing bright caps without taking into account volume settings makes no sense

[JeffH]

Thanks - I actually use different volumes when I play the amp. I realize the cap has less effect as the volume is increased, so let's say about 4 to 5 on the individual channel volume or jumpered.

I noticed there are different (extra) caps included with my parts and was thinking of the 100pf. I use mostly Gretsch and Fender guitars. I suppose the lower value cap will help decrease the hiss, too? Like I said, the normal channel is dead quiet.

[Roe]

more treble=more hiss but also more cut.
here's my suggestions regarding volume and cap size respectively
2/3: 100pf
5/6: 500pf
6/9: 5000pf

[ThunderOne]

The best way to go about it is to first try your amp without any bright cap. If you like it that way, you're done! If not, try 100pf and go on up from there until you find "your" sound. Also remember that the amp and speakers break-in process will change your perception of "brightness", so just give your gear alot of play time...

[tbwrench]

I started at 500pf and it was so shrill on my bright channel JTM45, 1960AV cab.. I took it off completely and love it. I play a lespaul, tele and strat. The tele on the bridge pu with the 500pf was unbearable earsplitting shrill. This was with the amp on 3 or 4.
I will probably leave the bright cap out for good.

[JeffH]

I've been trying to find time to play it, and when not playing I've been letting it idle to see if that helps to "break in" the components. I'll probably try a 100 pf. I've used it with a Hiwatt 4x12 that has two greenbacks and two Fanes, and also with an open-back 1x12 with a Tonker. The Tonker was brighter, and strangely seemed a bit louder.

I've probably got about 10 hours on it so far and will go in and re-check the bias soon.

[billstets]

I first built mine about a year and a half ago with the 500pf and pretty much immediately changed it to 100pf. I had it that way for over a year and then tried 250pf. A couple of weeks ago I pulled both caps (vol pot and mixer) to make it PA specs, like Udo Pipper's JTM45, and I think I prefer this way the best. It's the most balanced across the frequencies. No harsh highs. Bass is still too much, but that's another issue. I also tried bass spec, only the mixer cap, and I thought it was a little more flubby and bassy, which the JTM45 certainly doesn't need more of! You can try all these options as well as the different vol pot cap values. They all have advantages and disadvantages.

[JeffH]

Thanks for all the advice. Honestly, I'm having too much fun playing the amp to go back in now, but I'll probably try the 100pf soon. I'm really only posting so I have an excuse to post a picture



I have to try to get two knobs to fit better - I saw the tip about using a 1/4" drill bit to open it up a little.

Rechecked bias yesterday and it's still where I set it, approx.42ma.

[Blues Rocker]

I'm a little late on this post, but after experimenting with the 500pF bright cap and no cap, I settled on the 100pF cap. I find it is a reasonable compromise on at most volumes and sounds great with my ES-335. I cannot imagine any use for a 500pF as it was piercingly shrill on the treble...and that with humbuckers. With the 100pF a side benefit was that it brought down the High Treble channel hiss noise to not even worth talking about. I like that.

[arledgsc]

I'm currently running a 100pF bright cap with no mixer bypass cap. No mixer cap makes the normal channel more balanced like a PA spec by not rolling off the highs and the 100pF bright cap adds a little sparkle to that channel. No icepicky highs and lots of warmth.

To balance the low frequencies I am going to try adjusting the PI plate 0.1uF coupling caps value down. Going down to .022uF will roll the bass off about 1dB at 100Hz. Not much but not looking for dramatic changes. I have a couple of 0.039uF mustards to try first.

Increasing the NFB resistor will narrow the frequency extremes slightly plus add more gain. I get so much cream out of my JTM45 with 100K that I may never lower it.

I am so glad I chose to experiment with my amp because I am slowly dialing in a sound that I really like. Plus it is easy to put back if you don't care for the changes.

Scott

[PCollen]

I'm currently running a 100pF bright cap with no mixer bypass cap. No mixer cap makes the normal channel more balanced like a PA spec by not rolling off the highs and the 100pF bright cap adds a little sparkle to that channel. No icepicky highs and lots of warmth.

To balance the low frequencies I am going to try adjusting the PI plate 0.1uF coupling caps value down. Going down to .022uF will roll the bass off about 1dB at 100Hz. Not much but not looking for dramatic changes. I have a couple of 0.039uF mustards to try first.

Increasing the NFB resistor will narrow the frequency extremes slightly plus add more gain. I get so much cream out of my JTM45 with 100K that I may never lower it.

I am so glad I chose to experiment with my amp because I am slowly dialing in a sound that I really like. Plus it is easy to put back if you don't care for the changes.

Scott

If you use a split cathode V1 set-up, and perhaps the .0022uF coupling cap off the bright channel V1 plate, you may not need a mixer bypass cap at all..it all depends upon the tone you want and if you're happy with the results you achieve. I use the 500pF bright mixer bypass cap , but I also have my NORM V1 Rk/Ck set like a Fender BF @ 1.5K/22uF.

[TheOuvs]

I'm a little late on this post, but after experimenting with the 500pF bright cap and no cap, I settled on the 100pF cap. I find it is a reasonable compromise on at most volumes and sounds great with my ES-335. I cannot imagine any use for a 500pF as it was piercingly shrill on the treble...and that with humbuckers. With the 100pF a side benefit was that it brought down the High Treble channel hiss noise to not even worth talking about. I like that.

Thanks for this post, I changed my 500pF for the 100pF and got the same results as you did and I am a lot more happy with the tone. I play a variety of guitars too and found the 500pF to be just too high especially with the single coil guitars.