Grit with my grind

[dobbhill]

I've finished my amp and am experiencing a quality that I just don't like:
The high treble frequencies sound really rough and not nearly as creamy and smooth as the mids and lower freq's. I thought that different types of treble tone caps may make a difference and was curious as to the types you guys had good (and bad) experience with.
Ceramic disc
Silver mica
film: polyester, polyrop., polystyrene, polywannacracker, etc.....
I have a polystyrene in there now and have always had good luck with them in Fender and Marshall type clones.
Also, I'm using polypropelyne coupling caps; a mix of Wima and Xicon.

What do you think?
Thanks, D

[doctord02]

a lot of folks like to use the Mallory 150's to warm things up - they may roll off a bit of the high end... I like the 150's and similar styled caps in my JTM45 clone and in the wreck that Paul Ruby built for me.

I find the average Orange Drop caps are kinda brite/harsh for my ears.

Other than that I try and avoid ceramics and use silver mica for the pico farad ranges...

[glasman]

a lot of folks like to use the Mallory 150's to warm things up - they may roll off a bit of the high end... I like the 150's and similar styled caps in my JTM45 clone and in the wreck that Paul Ruby built for me.

I find the average Orange Drop caps are kinda brite/harsh for my ears.

Other than that I try and avoid ceramics and use silver mica for the pico farad ranges...

I agree, the polyprops would not be a good mix for the Wreck style amp, they are too articulate and let every once of top end through the amp. They work well in "darker" voiced amps. The mallorys have the right amount of roll-off to control the top end.

The SOZO's would probably be another good choice.

Are you using the 50pf across the phase inverter, this can help to control some of the top end as well.

Gary

[Wayne Alexander]

Gary, please explain the 50pf "across the phase inverter." Exactly where? My A1A sounds great but it's hissy with either of the bright caps switched in, and I get a high pitched, almost ultrasonic, squeal on top of the notes when the treble is turned way up.

[loverocker]

If you look at almost any Marshall, you'll see a 47pF/50pF cap across the PI's two plates.

[Wayne Alexander]

Thanks. I'll try that. I've built a bunch of amps, but not a "big" Marshall, so the reference was going right past me. It's like a "cut" control from a Vox amp, but without the pot to make it adjustable. If I'm trying to phase-cancel out only the highest of high frequencies, presumably I'd use the smallest cap possible, like a 10pf?

[Jackie Treehorn]

I really like polypropylene 716 orange drops in my build. I've got something around 75 pf across my PI plates, though. What preamp tubes are you using?

[Wayne Alexander]

I have Sozos in my A1A, it sounds spectacular tonally, one of the most amazing sounding amps I've heard. I'm currently using NOS Mullard preamp tubes and NOS Hammond (RCA?) 6v6's with the low-voltage tap off the power transformer, leading to plate voltage on the power tubes around 334, and the high-impedance tap on the output transformer (6600 instead of 5200 which I tried first - didn't make much difference). Overall the amp is a bit dark sounding, however, unless I switch in one or the other of the bright caps (100pf or 500pf, standard bright switch in A1A), and when I run it with one of the bright caps switched in there's a lot of background hiss, and there's a high, almost to ultrasonic range overtone that I need to tame for my bandmates to be happy. I haven't yet tried the cut cap across the PI plates, that'll be this weekend.

[Robert]

I used OD 715s due to reading a reference by someone with a real Twreck who had those in his. It can be a bit grainy on the high end and it was especially so before a got all the bugs worked out. The lower the bias current, the more grainy, thin and trebly the tone. However if I bias the amp so that pin 5 is -30 as recommended by KF in the Twreck pages in the archive, the harshness goes away and it really sounds its best in many respects. I know that is pushing the limit of 20Watts max for EL34s, so I probably won't often run it that low. I used a 100pF across the PI plates.

[dobbhill]

I changed all the caps from Wima film/foil & Xicon metalized PP to metalized polyester types(150's) and the treble cap to a silver mica. Not a huge change, but a bit in the right direction. I'm starting to think that my problem is an oscillation problem due to very poor layout. The "buzz" seems to ride on the top of the notes when I reach a certain level volume/gain. This amp has plenty of gain, and sounds really good with the treble control on "2". The bass control seems to work like it's supposed to, but the amp is just plain bright "as the dickens!"
I've tried some of my old Telefunkens and Mullards, but they really don't change the "buzzing" quality. The amp really sounds and responds well at low volumes, but I just don't like the way it distorts. What a shame, as thats what I built the thing for............
Wayne-if you're using the low-voltage tap, what are the plate voltages of your preamp tubes? Are you using the same power supply architecture, or adjusting the dropping resistors?
Thanks everyone for your replys. I'm seriously considering a re-build with extreme attention to layout. Mom always said liffe would be easier if I did what I was told.............
D

[Wayne Alexander]

D: I haven't changed any of the A1A power supply architecture, I just used the 260-260 v taps off the Heyboer multitap power transformer after trying it originally with the 300-300. I had to rebias, but didn't do anything else. I think I like the sound better, it's less "hard" sounding.

[dobbhill]

I tried the lower B+ and have to say that I like it better. I worked on my layout a bit and the distortion quality that was bothering me is much better. The amp is still extremely bright. I can play it with the treble turned off and it still has a good bit of high frequency content. I guess I just like a dark sounding amp. My JTM 45 is not near this "top-endy." Can you think of anything else I should look at that may be causing excessive HF content? Also, what is the "correct" value for the coupling caps between the PI and output tubes: 0.22 of 0.1 uf ??
D

[Robert]

I used 0.022uF coupling caps per the A1a schematic. I still have work to do to tweak out the treble content as well and round out the tone more. I really noticed a difference when I took my amp to the music store and played through a marshall (Celestion G12H) and boogie (Celestion V30) cab. I had read KF voiced the Express for the G12H. Remember too his design for this amp was for a cutting lead amp. No doubt it will do that in a band, but its not really that necessary for a studio type application. I think now is when the fun begins and the originality and tenacity of the builder are key to a satisfying amp in the end.

Robert

[Wayne Alexander]

Dobhill, what guitar(s) are you using and through what cab? I've played mostly a Les Paul 57RI with Fralin 8k/9k pickups, and a 335 59RI with WCR Crossroads pickups through mine, and I'm using a Mojave 2x12 cab with one G2H and one Red Fang. It's fairly dark, I need to switch in one or the other of the bright caps to have enough sparkle. I have a Tele and a Strat, but I haven't tried them with it yet, my guess is that they'd be significantly brighter.

[dobbhill]

I have a '74 Marshall 4 x 12 with Rola/Celestion G12M-25's. It's pretty bright. I understand the G12H to be a darker speaker. I have been using a mahogany Tele thinline with Fralin's for testing, but I pulled out my hollow body Heritage with Seth Lover's last nite. The sound was much richer and not nearly as bright. The amp isn't so bright after all. I'm playing with a switch that changes the treble cap from 500pf to 50pf and I like what it does. I still haven't nailed down the distortion abnormality yet.
D