Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on June 20, 2022, 02:45:49 pm

Title: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: joesatch on June 20, 2022, 02:45:49 pm

Got my slo preamp very close there where i want it. lotta gain. Initially i was experiencing some low mudiness and a combination of changing the first coupling cap in this diagram to a .01uf and a .47uf cathode bypass cap has helped alot (along with preamp tube swaps).  i am still lacking the very high presence i prefer. I have the tone maxed but since there is no presence control on this circuit i cannot boost that.  What would i change on this early part of the circuit to get more presence?

Also there is a plate bypass cap on V2b. If i lower this value will that open it up?

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: PRR on June 20, 2022, 03:24:09 pm

> there is a plate bypass cap on V2b. If i lower this value will that open it up?

Got snips?

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: joesatch on June 20, 2022, 03:25:39 pm

you mean clip the bypass cap?

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: Esquirefreak on June 20, 2022, 04:04:41 pm

That cap acts as a LP filter, so I'd try and just lift one side of it to allow all treble to come through.

Higher value cap - lower crossover/more low frequencies bled to ground. A lower value cap will only allow the highest frequencies through.

If no cap turns out too bright for you, then adding a cap will help attenuate highs. I'd try 470pF, 390pF or even 250pF.

On a side note, you could make a "fixed" presence mod with a couple of resistors and a cap, to form the filter that a presence control ultimately is.

/Max

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: nandrewjackson on June 20, 2022, 04:27:08 pm

I've built 3 or 4 of this circuit over time, heck maybe 5 of them.


The 39K cold clipping R makes things "dark". On one test version, I wired a 1KR and 50K pot in series there, and adjusted the pot to find the sweet spot (to my ears).  Going slightly over the marshall 10K gets into the SLO range, 38K is too far. 20K to 30K sounds more "alive".


The plate bypass cap is too large, or not needed at all.


I usually settled on 0.0068 uF coupling caps everywhere before the CF.  0.01 was way too boomy.


Also, try 100K grid leak before V3b. That should tighten things up. On one version I had 68K grid leak there, that was the best one.


I recommend looking at the high gain channel of the carving legacy Steve Vai amp. And have a look at dumble overdrive special schematics.


To me, the SLO probably needs to be a real soldano, pushing a 4X12 at way too high dB to sound correct. Other amps can get that sound at lower volumes.

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: thetragichero on June 20, 2022, 06:39:25 pm

good suggestions here. also when it comes to figuring out treble peakers this comes in handy: http://www.muzique.com/schem/filter.htm

when i had the jet city jca50 combo (it's basically a slo) i found the 'overdrive channel' just couldn't do it for me (just sounded overly compress to me), but with some tweaking the clean channel was fantastic after i made a bunch of mods to it. at some point i'm planning to build ~30w version of just that channel

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: joesatch on June 20, 2022, 08:29:07 pm

thanks folks. i removed the plate cap not a big difference but slight. reducing the grid leak lowered the gain a bit so i left it i believe its at 300k. The cold clipper was 39k i replaced with a 33k. I tried 25k but it was too much gain.  I'm satisfied for now.

https://youtu.be/wUDfIsIBE6A (https://youtu.be/wUDfIsIBE6A)

the preamp tubes made a huge difference in tightening it up. For some reason everyone recommends JJ's in these, they were nothing but mush. V1, V2 are EH  V3 is a tung sol (bright tube!), V4 is a sovtek lps

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: nandrewjackson on June 21, 2022, 12:42:31 am

Which JJs ? Did you use the ecc83? Or did you use the ecc803S  with long plates?


I did a 12ax7 shootout a couple years ago for my own amusement. The winner for clean tones was genalex gold lion. The top for high gain was the new mullard, but every one I had was microphonic. Runners up definitely included EH and tung sol ✅✅✅✅

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: joesatch on June 21, 2022, 06:25:40 am

Quote from: nandrewjackson on June 21, 2022, 12:42:31 am

Which JJs ? Did you use the ecc83? Or did you use the ecc803S  with long plates?


I did a 12ax7 shootout a couple years ago for my own amusement. The winner for clean tones was genalex gold lion. The top for high gain was the new mullard, but every one I had was microphonic. Runners up definitely included EH and tung sol ✅✅✅✅

ecc83S

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: thetragichero on June 21, 2022, 09:00:52 am

my build philosophy is to make an amp sound good with JJs in it so that regardless of tube it should sound good

Title: Re: Help voicing OD channel. Treble peaker / Bright cap mods
Post by: nandrewjackson on June 21, 2022, 04:06:11 pm

To my ears, the JJ ecc803s is thumbs down


And the shorter plate JJ ecc83 is thumbs up.