Super Bass to JTM (Filtering)?

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revolver1
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Super Bass to JTM (Filtering)?

Post by revolver1 » Mon Apr 26, 2021 2:48 pm

Hi All,

I wanted to get some opinions about some changes I'm thinking of making to my Super Bass.

So it's currently set up similar to the last of the JTM 100'S, so Dagnall Transformers. The only difference 's being I'm using the high voltage secondary's so depending on the bias I'm getting 515 to 520v on the plates and I've got 2.2k on the tube sockets for the screens.

I first had some XF2 Mullards in there but unfortunately one of them let off a big plume of brown smoke and gave up the ghost. Luckily no damage, I'm not in the money at the moment so looking for current production alternatives I ended up getting a set of Gold Lion KT77. I actually think they are not bad which I guess says alot since they are replacing the Mullards.

The KT77 are obviously not quite as sweet as the Mullards but I was pleasantly surprised. So I tuned the circuit component wise to a point where I'm really quite happy the only complaint I could make is its perhaps a bit hard edged. On a positive It instantly remind's me of Malcom Youngs tone which is not a bad thing at all, its definitely a bone crusher but I was maybe looking for a bit more of a softer edge a bit more Koss, Clapton, Hendrix. It does the Malcom thing to a T but really if I'm looking to cop some AC/DC I'm into Angus live tone from If You Want Blood and my 2203 with the Shaffer replica does the job for that just fine.

It doesn't really knock my socks off when I hit it with fuzz, I was hoping when I hit it with a Tone Bender to cop a bit of early Beck or with my other fuzz's to get that mid to late 60's thing going on (groovy). On it's own I was kinda hoping for something in the ball park of Fleetwood Mac live at the BBC (Oh Well) that type of thing.

The other day I realised I'd made a mistake by putting a dual 32uf for 64uf on the P.I. so I've ordered up a dual 16uf to put in there. I realise it will be slightly different than a JTM because I'm using dual cans instead of radial's but essentially the values should be consistent with the late 67 ie. 50uf mains, 16uf screens, 32uf Phase Inverter, and 32/32uf preamp.

I'm intrigued to see if this will give the amp a little more give and a slightly more what I'd call a vintage vibe or feel.

Taking it further this has got me thinking, what if I reduced the filtering deeper into JTM territory? Not sure if this will be so good ghosting wise on the high voltage tap but then again the Dickinson amp would probably be higher voltage, I guess in the 560v range.

I could try a few JTM combinations here and if I mount a dual 16/16uf on the inside side wall of the chassis by the P.T I could get the Phase Inverter down to 16uf.

So it could be 32uf mains, 16uf Screens, 16uf or 32uf P.I (not sure which is best?), and 32/32uf preamp.

I could also switch the H.V secondarys to the lower voltage ones and put some Winged C EL34 in and swap out the 2.2k screens for 1k.

I do like the high voltage power if I could just soften it up a bit and get the fuzz pedals singing and dancing. Then again I'm not sure if getting it back to EL34 with a more JTM vibe is worth pursuing.

Having read Shakti's JTM100 Black Flag build thread (awesome by the way) it was mentioned that single note lead lines were a struggle. I'm not sure if the filtering is the culprit for this?

Any opinions or discussion would be welcomed.

Cheers. :thumbsup:

danman
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Re: Super Bass to JTM (Filtering)?

Post by danman » Mon Apr 26, 2021 6:49 pm

16 uf on the screen node was very popular around here for awhile. It helps to loosen things up a bit and gives the amp a bit of an elastic feel IIRC. 32 uf on the mains may be a bit to low and cause some ghosting but it couldn't hurt to try it if you already have it.

revolver1
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Re: Super Bass to JTM (Filtering)?

Post by revolver1 » Tue Apr 27, 2021 4:39 am

Hi Danman,

Yeah I think your right can't hurt to try it.

From what I read ghosting can occur mildly or heavily on a bit of an amp by amp basis, I'm fairly confident of my grounding scheme so may as well give it a whirl.

I've got the 16uf on the screens and have it on other amps, I like it alot. What I'm now interested in finding out is what 16uf on the P.I does, I've never taken it that low before.

Theres that JTM 50 filtering that goes 32uf screens and 16uf P.I which seems like the wrong way round to me but I think Roe said he'd seen them in both configurations but more commonly 32uf screens and 16uf P.I.

I think he said there was some squish and compression to be had.

I know it's been a little quiet which is why I like go get a little conversation going, hoping some of the guys who built JTM clones might give some insight.

danman
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Re: Super Bass to JTM (Filtering)?

Post by danman » Tue Apr 27, 2021 7:01 pm

It's been very quiet here lately. I try to check in once a day and see if anything new has been posted but it's been eerily silent here for a few months. Hopefully things will pick back up again soon.

revolver1
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Re: Super Bass to JTM (Filtering)?

Post by revolver1 » Wed Apr 28, 2021 3:09 am

I was checking out some Johan Segeborn clips on YouTube yesterday the 67 ones. Great demo's, and really useful because he showed the inside of the amp. It looks like 32uf mains, 16uf screens, 32uf P.I and 32/32uf preamp.

Great sounding amp and I think I can hear the effect of the filtering on the circuit as compared to another clip a fella had on there of a 67/68 1992 Superbass that I'm guessing has the 50uf on the mains.

Johan's amp sounded rawer for want of a better term, a bit rougher round the edges. Sounds very good and I'm keen to try those specs but I do have a gut feeling it won't be quite as versatile.

The other 1992 seemed to have the sound I'm looking for, with a Les Paul it instantly reminded me of Clapton, Koss, Beck so I'm feeling hopeful as my transformers are most likely the same.

Big difference at the moment I guess is mine is on the H.V secondarys and the KT77. I do quite like those KT77 but I dare say the only reason they sound O.K is the high plate volts. There is a bit of a mid 70's Page vibe, I sort of guess his metal panel Super Bass would have higher voltage on the plates. Sounds great with a Superlead taking care of the upper mids and top end and the Super Bass filling in the meat.

Just waiting on a couple of parts then will make changes one at a time to narrow it down. At least I think I'm on the right track now and feel quite hopeful. If I'm not playing I'm always happy when I'm learning and fiddling about with my amps.

danman
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Re: Super Bass to JTM (Filtering)?

Post by danman » Wed Apr 28, 2021 6:39 pm

Please report back with your findings when your'e done.

revolver1
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Re: Super Bass to JTM (Filtering)?

Post by revolver1 » Fri May 21, 2021 1:20 pm

Finally got a chance to give this one a whirl after making some changes and it's real nice. The high voltage and KT77's were quite cool but it didn't play so nice with my FX and was a bit one trick. It was cool just on it's own but its alot more versatile now.

So the changes really make it like the very last incarnation of the JTM Black Flag amps.

I changed the high voltage tap over to the lower pair and installed some winged C, I'm now getting around 470vdc on the plates.

I changed the filtering to 2x dual 32uf for 32uf mains, 1x dual 16uf for 32uf P.I, 2x dual 16uf for 16uf Screens and a dual 32uf on the preamp.

I changed the negative feed back to the 16 Ohm tap so 27k on 16 Ohm and changed the 10k preamp dropping resistor to a single 8.2k 2w.

Well I'm really very pleasantly surprised with it. It definitely has that Koss, Clapton, Hendrix vibe going on definitely more like what I was hoping to get from this amp. There is maybe the slightest bit of flab on the low E when the volume is backed off the guitar but nothing to really worry about. I could probably sort it by upping the P.I to 48uf but I have a feeling 32uf will play nicer with the 16 Ohm NFB. It may well settle with more break it.

I also didn't notice much in the way of ghosting and I ran it well cranked up also with fuzz and a few other boost/drive pedals and so far I didn't really notice anything untoward.

It plays nicely with my FX in general and was very pleasing to play. Its softened up a little and sounds pretty sweet, it definitely nails that 60's vibe. It's not quite as brutal as it was but can still rock out I have other amps to cover that and this makes for a great addition to the collection.

Lastly I hooked up my 1987 and ran them both together, all in all it was alot of fun I'll be keeping it like this for a while I think. Knocked out a bit of Humble Pie, for a moment there I was Rockin' the Filmore. :shred: :thumbsup:

danman
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Re: Super Bass to JTM (Filtering)?

Post by danman » Fri May 21, 2021 6:26 pm

Happy to hear that the filtering changes got the amp headed in the direction you were hoping for! :rockon:

revolver1
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Re: Super Bass to JTM (Filtering)?

Post by revolver1 » Fri Jun 04, 2021 10:00 am

I've done a little more playing around with filtering and tried a few things I couldn't find others had given insight on so I thought I'd post my findings.

This kind of ties in so first off was my 1987 50w. I pulled the mains and Screens/P.I filters so I could alligator clip them in and out for comparison.

While I was there I also did the same with the tone stack cap and the .0022 coupling cap so I could finally make a decision which way to go.

For the tone stack cap I had a 500pf RS silver mica and a SRC 470pf dogbone. I settled on the 500pf silver mica, I think for me it was close but the S.M felt a little more versatile when playing with the Les Paul controles.

I then played around with the .0022 cap but I went back to a .0047. I know I've said it a few times before but I really like this in a 50w circuit. It lets just a bit more bottom end through which makes it feel a bit more like a 100w, it sounds a bit more like an older Plexi to me but it doesn't totally loose the cut. For instance my wah sounds better but doesn't loose the skwall when toe down. I guess it's a bit fuller and interesting.

So now to the filtering. I had 100uf mains, 50/50uf P.I/screens and 32/32uf preamp.

I first tried 64uf mains and 32/32uf P.I/screens and liked this alot more.

I then tried a 32/16uf, 32 P.I and 16uf screens. I really liked this, the sound just opened up a bit more and sounded more harmonicaly rich.

Finally I tried a 32/16uf for 48uf on the mains. The sound got a bit softer and was quite nice for certain things but ultimately I felt when driven hard some of the frequencies were missing. It didn't seem quite right, like maybe some mids or top were missing so the definition was slightly lacking. Also it was a smidge noisier at idle, not much to worry about but when boosted the noise was considerably louder and not really acceptable for me.

So in conclusion for me 64uf mains, 16uf screens, 32 P.I and 32/32uf preamp.

This sort of ties in with my 67 build because of my findings regarding the mains filtering.

So after reading some posts from Roe I wanted to try 22/22uf on the preamp. I really liked this, it is the closest yet to the vintage vibe I've been striving for but with one draw back, the ghosting is now quite noticeable.

I was noodling around with Framptons lead lines in Four Day Creep and the amount of ghosting lead me to believe the filtering isn't quite right for Rockin the Filmore tones.

So now I'm considering upping the mains back to 50uf. I dropped Roe a P.M or at least I think I did, to get his opinion because I think he has a switchable mains set up with 50uf mains and 22/22uf pre amp with diode rectification.

The 22/22uf is probably letting a little more bottom in and I think that is making the ghosting more prominent. In theory upping the mains might tighten the bass up and allow it to work.

Alternately I could put the 32/32uf back on the preamp because it was pretty good like that but I'm slightly reserved because like I noticed with the 1987 I think there may be something slightly missing with the lower mains filtering.

Really this amp should be 50uf mains, 16 or 32uf P.I, 16uf screens and 32/32uf pre but I'm still interested in that 22/22uf pre amp it sounds more like that BBC Interview with Clapton where he is playing the S.G and demoing the Wah. Also one of my faves Humble Pie.

Maybe I'm completely wrong but still a bit more experimenting to be done.

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