c30 tweed bassman mod. leak in normal channel

[severthee]

Good day,

its been years since the blueguitar mods, there were many other mods and tweaks after that. but until this day it has never failed to give life to the stock c30 new and old.
i ve had the c30 for a decade now. many amps came and went during my upgraditis phase, but this is the only amp that stayed with me for sentimental reason. this year i thought since im gonna be stuck with this amp for the rest of my life, might as well mod it. this is only my 3rd amp to mod. im no electrical engineer whatsoever
i did the following mods
-fender input with 68k, i am still confuse which is right 68k or 33k, which is it and why?
-tonestack upgrades with orange drop
-boost switch with silvermica
-tweed bassman cathode follower
-cathode bias/adjustable bias switch by enzo

all these Steve Ahola (still cant thank you enough for your contribution) mods were done by so many others in the past. nothing new
first thing i noticed is how open sounding the amp was even with the stock crap blue marvel. i cant wait to try the greenback on this.
soft picking dynamics can be heard. it has the right amount of treble without being too overly bright like before e.g. strat chime and quack at position 2 and 4.
i did these mods all at the same time, we all know how PITA to work on this 3pcb design, thats why i dont know which had the most effect. was it the fender input? the tonestack ? or the cathode bias?

i encountered minute problems, the first one is the tweed bassman channel, i cant seem to enjoy this at bedroom or house level. the 500k i put in v3 just jumps so bad, and it gets loud so fast too. ive got only a little taste of the cranked pre-knob when i braved the volume for a minute. either i ll open it up and try a 250k or 100k? or put a master volume? what would be the best?

the second problem is the normal channel. for the longest time ive never played much on the od/dirt channel of most amps since all my effects go in the front anyway. so basically im a clean/normal channel guy. i was amazed how much the mods improved the normal channel. but when i toyed with the OD/tweed bassman channel, i left the pre-knob cranked all the way 5oclock. when i returned to the normal channel, i realized it was not as clean anymore. you can hear a slight saturation. dont get me wrong, this leak actually sounded good. as if the normal channel was breaking up slightly. when you turn the the pre-knob WHILE on normal channel, it goes back to this very clean tone with lots of clean headroom. thats when i realized the leak can be controlled by the pre-knob. its nothing to worry about since i did like how it changed the tone. my question is, is this normal after converting to cathode follower? or did i do something wrong with the relay? or was it the silver mica cap prior after v1? what should i check to stop this leak if i want this normal channel to be absolutely clean?

thanks much for any suggestions

[Enzo]

I don't venture into this section as much, because I never know what a person has or has done to it. To get any idea what all was done here, I will have to go research the mods you refer to.

A help would be a complete schematic changed to the configuration you now have.

68k or 34k? Neither is right, neither is wrong. The typical Fender input jack circuit has a 68k series resistor to each jack. But the jack cutouts are wired so that if you plug into the high gain jack, both 68k are in parallel, making 34k. If you plug into the low gain jack, your 68k is in series, with the other 68k to ground from the grid. That makes a 6db pad. So which jack is "right?" The stock 47k is right between the two.

If memory serves me, and all that stuff I wrote was YEARS ago, the cathode bias mod was more about dynamics than tone. It ran the power stage a little hotter than stock. But we also made the bias adjustable in that amp, so even the fixed bias mod was usually run hotter than stock.

Cathode bias in the power amp will have nothing to do with the operation of the preamp channels. OOps, you said cathode follower, not cathode bias. Nevermind.

Just a note, "V1" would be a tube. "VR1" would be a control, as in volume control.

So the pre control, VR2, is interacting with the clean channel? This is the difficulty. I am looking at the factory schematic. I see that all the way up, VR2 would result in a couple 470pf caps in series across the signal path. SO more or less the same as putting a 235pf cap there. That would roll off some highs. I don;t recall it being very noticable. But not knowing what circuit you now have, it is hard to predict interactions. On the other hand, you don;t describe rolloff, you describe distortion. First thing to my mind there is crosstalk. Dues to lead dress, component proxximity, or something else, the signal from the input stage is fed to both channel circuits, so even in clean channel mode, the dirt channel is still amplifying. That distorted signal is radiating and the clean channel is picking some of it up and mixing it in. I could be wrong, but that's my guess.

The silver-mica-ness of a cap is not going to cause channel interactions. I'd be very surprised if it acted any different with a ceramic cap there.

Looking at VR3, assuming you mean that rather than V3, the stock 10k is effectively in series with the 1 Meg R7. They form a voltage divider Increasing VR3 to 500k would dramatically change that. However, I assume this is no longer stock. In any case, volume jumping up fast at the start may be a matter of changing the pot taper. Part of the reason for that voltage divider is to prevent the dirt channel from being WAAAY louder than the clean Or was..

What exactly did you change in between the first and second triodes?

[severthee]

Thanks for your reply, here are the things i did with the amp. Its basically the tweed bassman mod i found in b
blueguitar.org
c4 originally 470pf is now changed to silvermica 680pf
c7 originally 470pf is now removed as per tweed bassman mod.
R9 2.2m parallel to where c7 used to be is changed to 1M
r7 1M was taken out as per bassman mod
vr3 is changed to 500k

i think its best to redraw the schematic, i ll upload it in a bit.

You are correct vr2 is interacting with clean channel. Theres no rolloff. But an introduction of distortion. Very soft yet audible distortion. I assume it is crosstalk now that you mention it. Your description/guess is spot on. It may be the dress or proximity.
The first problem i had with this was an arcing of voltage across jumper 707 (screen 320vdc)
to one of the holes of r63(-36volts). The holes were so close. But i guess ive to blame my.dirty soldering and not the design. I cleaned up the pcb and burned part with lighter fluid and the amp worked.

I ll get back with the schematic showing what i did on my c30. Thanks

[severthee]

pardon my schematic, i only edited in MS paint. i only drew the fender input and the tweed bassman mod. i didnt draw the tonestack, boost section i did in mine, plus the cathode bias/fix bias part
http://farm8.staticflickr.com/7246/7...3cbb8668_b.jpg



also i include some pictures exhibiting my crap wiring skills
http://farm8.staticflickr.com/7267/6...7e1bf3fe_b.jpg
http://farm8.staticflickr.com/7230/6...34eb873f_b.jpg
http://farm9.staticflickr.com/8008/7...cb18b9ed_b.jpg
the last picture would show instruction 19. v2b cathode series with .01uf connecting to r13 and v3 500k pot
i use a shielded cable as seen in the picture. but i installed the .01uf by replacing one of the jumpers as suggested JP607. so basically jp607 is now a .01uf. im not too sure if this cap is touching the chassis. i may have to take a look at it again

let me know if i need to enlarge the schematic

thanks for looking

p.s. credits go to Steve and David for the tweed bassman mods. these are not my mods

[severthee]

i think the attached file would show up bigger

[tubeswell]

I experimented with the 'bassman' mod years ago. Soundbyte attached (coupelo CF mod chords) I think I used 820R Rk on the inverting stage and a 56k load for the follower for this soundbyte IIRC. While this provided a certain level of grunginess with everything dimed, it wasn't the vibe I was looking for, so I undid it again and devised a mod of my own (C30 'SLO100' mod - soundbyte also attached), which Steve put up on his site (and which gave a better OD as far as I was concerned- , but at the expense of the more-scooped fendery cleans in the 'clean' channel).

With this type of amp it isn't possible to get radically different voicings in each 'channel', because the 'channels' both rely on the same input stage, and so you either shape it for fendery cleans or more marshally mids, but you cant practically do both without a whole lot of grief. The C30 isn't really made for modding (understatement of the year). So if I was to (ever) do the SLO mod again, I would also drop the extra grid load resistor on V2A (R9 in the attached schematic) because it sucks too much gain when using single coil guitars.

[JoeM]

Is there something inherent in the original C30 that it needs modding?

[tubeswell]

Steve Ahola's tonestack mod was the most useful mod for that amp from my point of view, and its probably the only C30 mod worth keeping. (But if you are inclined to make a rod for your back, then the C30 is an ideal modding platform ;-) )

[severthee]

Thanks for the soundclips!
the cathode follower chords.mp3 confirmed my mods. mine sounded almost the same, a bit harsh mids and brittly treble(specially when using single coil).
but nothing that the eq cant control. i just turn the mids and treble down and i get a more spongey, face melting tone. i just wish i can bump up the gain just a wee bit. the mod does remind me more of a blackface(or was it the silverface that was too bright?)

The SLO mod mp3 sounds great, awesome playing too! did you use a looper? i like the mid raunchiness and the sagged highs on your clip. but i just really love my cleans now and not too sure if i'm ready to part with it hehe. not to mention open the d*mn amp again. if ever i want to shape the tone this c30 again, i ll just roll the tubes and swap speakers. mess with the fx loop.. anything that doesnt involve taking out the chassis
i guess i am putting the last nail(screw) in this amp and finally move on to a different amp. 5e3 is calling me. and also jcm800 build...