Improvements for Normal & High Treble channels

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koffo
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Improvements for Normal & High Treble channels

Post by koffo » Mon Aug 24, 2015 8:17 am

Hi,

I finished building my JTM 45 amp about one year ago. The amp sounds really good, it is basically stock, the only thing I added is a PPIMV. The questions I have for the experts are the following:

1) The high treble channel sounds amazing, it is fat and creamy with a vintage style guitar (in my case a Gibson SG) and becomes somehow aggressive when pushed with higher output pickups (almost VH territory!). Also cleans are nice especially when you mix both channels together (in order to give more body to the guitar). The problem I have is that this amp is terrible with pedals! I tried to set it super-clean (volume at 2), and if I try to put a distortion pedal in front of the amp, the result is a swarm of bees! Then I try to dial in a bit of the normal channel, it becomes almost boomy with a lot of basses and highs, but the sound is "hollow" with no mids. I thought it could be caused by the "bright cap" on the hight treble channel, mine is 500pf, what's your experience with that?

2) The normal channel is unusable, it sound muddy and muffled. Is there any way to have it just slightly darker than the high treble one?

3) Last but not least: I replaced the power amp tubes with a pair of Genalex Gold Lion KT66, since then the amp behaves in a strange way. Basically, when the amp is cold it sounds amazing, while when it heats up it becames really boomy (high treble channel, no jumpering, bass set to 0, mid and treble at 5). Could it be a biasig issue? Currently the BIAS current is set at 45mA (I don't remember the plate voltage)

Thanks in advance for your suggestions, best regards,
Andrea

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Re: Improvements for Normal & High Treble channels

Post by danman » Mon Aug 24, 2015 5:52 pm

The normal and bright channels are identical with the only difference being the bright cap on channel 1. If you added a bright cap of the same or different value to the other volume pot it would give you the extra treble you want. Experiment with different values to fine tune it to your liking. Can you show us some pics of the wiring around the tube sockets and pots? You may be experiencing some form of oscillation from your lead dress that is causing the problems you described. Have you experimented with different tubes in V1 to see if that helps any?

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Re: Improvements for Normal & High Treble channels

Post by koffo » Wed Aug 26, 2015 4:30 am

I didn't experiment different tubes for V1, in this moment I have three Mullard RI in V1, V2 and V3. I'm going to open up the amp and take some pics.

Thanks,
Andrea

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Re: Improvements for Normal & High Treble channels

Post by neikeel » Wed Aug 26, 2015 9:25 am

danman wrote:The normal and bright channels are identical with the only difference being the bright cap on channel 1. If you added a bright cap of the same or different value to the other volume pot it would give you the extra treble you want. Experiment with different values to fine tune it to your liking. Can you show us some pics of the wiring around the tube sockets and pots? You may be experiencing some form of oscillation from your lead dress that is causing the problems you described. Have you experimented with different tubes in V1 to see if that helps any?
Don't forget the mixer bypass cap on Channel 1.

Peersonally I would try a different Rk/Ck combination on V1. I suggest try a 4k7uF caapacitor (instead of 330uF) if that works stop there, if not swap the 820R for a 1k5 resistor.

Use 100pF cap on each volume pot.
Neil

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Re: Improvements for Normal & High Treble channels

Post by danman » Wed Aug 26, 2015 3:37 pm

I knew I was forgetting something simple Neikeel...my age must be catching up to me...lol! :thumbsup:

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Re: Improvements for Normal & High Treble channels

Post by koffo » Thu Aug 27, 2015 4:53 am

neikeel wrote:
danman wrote:The normal and bright channels are identical with the only difference being the bright cap on channel 1. If you added a bright cap of the same or different value to the other volume pot it would give you the extra treble you want. Experiment with different values to fine tune it to your liking. Can you show us some pics of the wiring around the tube sockets and pots? You may be experiencing some form of oscillation from your lead dress that is causing the problems you described. Have you experimented with different tubes in V1 to see if that helps any?
Don't forget the mixer bypass cap on Channel 1.

Peersonally I would try a different Rk/Ck combination on V1. I suggest try a 4k7uF caapacitor (instead of 330uF) if that works stop there, if not swap the 820R for a 1k5 resistor.

Use 100pF cap on each volume pot.
That's a great suggestion, I'm not an expert, but I saw from the schematics that those two components (the resistor and the cap) could have a great role tonewise. What are the effects expecting lowering the cap value (it is the one on the cathode of V1 right?) on both channels?

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Re: Improvements for Normal & High Treble channels

Post by danman » Thu Aug 27, 2015 12:27 pm

Neikeel will be able to explain it in more detail, but if you lower the value of the bypass cap on the v1 cathode resistor, you will cut some of the lower frequencies from the signal. The large 330uf cap is passing the entire frequency range which can result in a muddy sound. Lowering the value to 4.7uf as Neikeel suggested will cut some of the lower frequencies and help to eliminate some of the muddy sound you hear. If you look at a lead circuit with a split cathode arrangement, you will notice that the normal channel uses a very large (220/330uf) bypass cap and the bright channel uses a much lower .68uf which helps to give it the distinctive brighter sound. Your amp uses a shared cathode arrangement, so any changes made to the bypass cap will affect both channels.

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Re: Improvements for Normal & High Treble channels

Post by neikeel » Thu Aug 27, 2015 4:59 pm

danman wrote:Neikeel will be able to explain it in more detail, but if you lower the value of the bypass cap on the v1 cathode resistor, you will cut some of the lower frequencies from the signal. The large 330uf cap is passing the entire frequency range which can result in a muddy sound. Lowering the value to 4.7uf as Neikeel suggested will cut some of the lower frequencies and help to eliminate some of the muddy sound you hear. If you look at a lead circuit with a split cathode arrangement, you will notice that the normal channel uses a very large (220/330uf) bypass cap and the bright channel uses a much lower .68uf which helps to give it the distinctive brighter sound. Your amp uses a shared cathode arrangement, so any changes made to the bypass cap will affect both channels.
Pretty much that. If you go to 1uF or so the amp will be relatively bright and may lose some of the qualities you like in the JTM tones, 4n7 is a good place to start, of course you can go lower to 2.2uF but lower than that you will actually start to actively boost the upper mids - i.e. make it too bright. The 1k5 cathode resistor with s 25uF cap is a Fender type value, but even then the low frequencies are still pretty much all there (look at the Duncan Tone Stack calculator and you can see where the low frequency roll of is:http://www.duncanamps.com/tsc/.
A 25uF passes everything above 20Hz so trimming dow is a good idea. Of course if you up the resistor to 1k5 you also get a little less gain but less fizz too.
Neil

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Re: Improvements for Normal & High Treble channels

Post by koffo » Fri Aug 28, 2015 3:39 am

Ok, thanks for the suggestions.

Now I'm going to:

1) try to lower the bright cap on the High treble channel from 500pF to 100pf
2) add a 100pF bright cap to the Normal channel
3) the first two are the easiest to try (also because I already have the components :thumbsup: ), if this is not enough I'm going to try a different cap for the cathode bypass. One question here: 4k7uF stands exactly for....what? :help:
4) if this is not enough I'm going to change also the resistor, but I'm worried that it could take away some of the gain... don't get me wrong, I know that this amp should be vintage voiced... but I like the way it becomes mean!

I'm going to update this topic with pics and the result of the mods.

Thanks again to everybody!

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Re: Improvements for Normal & High Treble channels

Post by neikeel » Fri Aug 28, 2015 4:24 am

It is 4.7uF. The stock sizes are:

1uF, 1.5uF, 2.2uF, 3.3uF. 4.7uF, 6.8uF and 10uF

You do not need high voltages just a cap that will reach accross the turrets! So 150v or 200v are the size you need.

In fact you probably have a 10uF spare from the build?

If you do not reduce the size of that cap you will still get a very flubby dull normal channel.
Neil

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Re: Improvements for Normal & High Treble channels

Post by koffo » Fri Aug 28, 2015 5:41 am

Yes, I have a spare 10uF cap left, I've just ordered the 4.7 one, but I'm going to try the 10uF first (just to check if it improves).
Thanks for the clarifications neikeel!

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Re: Improvements for Normal & High Treble channels

Post by koffo » Mon Sep 07, 2015 5:12 pm

Just finished testing a couple of mods and fixes on the amp. Here is what I found:

1) swapped the 500pF bright cap with 100pF on the High Treble channel, that's much better now! At low volume (around 2) I'be lost a good part of that glassy sound I had before, but I had too much previously (in order to have an acceptable sound I had to mix both channels). Now I have a good sound all over the gain range from 1 to 10, and also pedals are much better now! So, thanks a lot for the suggestions :worthy:

2) I found the problem that caused the high treble channel to get boomy after a while I was playing: it was due to a sloppy connection on the PPIMV there were two leads almost touching each other... I unsoldered everything and did a rerouting of a couple of connections and wires, and isolated them properly with heat shrink tube. Also in this case problem solved! The strange thing was that I built this amp almost 2 yars ago, and jigged with it regularly, and this problem only appeared recently

3) since the amp was opened I took my time to set the bias. My B+ is 455, and I have gold lions that are rated 25 watts. So my calculations is: (25 * 0,7)/455 which is about 38mA. I found that the bias was about 32mA previously, now the amp sounds warmer especially when pushed...did I do everything right or Am I missing something?

Now only the normal channel boomyness is left, on the next week end I'm going to try to add another 100pF bright cap and let's see...

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Re: Improvements for Normal & High Treble channels

Post by koffo » Sat Sep 12, 2015 8:46 am

This morning I tried several things. First of all I swapped the 330uF cap with the 4.7 one... definitely, now the amp sings! :)
The high treble channel now is much more like a Super Lead, brighter and more aggressive. Probably if you play blues most of the times it can be annoying, anyway, in my case, I use to play classic rock and some hard rock sometimes, and for me this has been a major improvement!
Then I added also a 100pF cap on the normal channel and now this one has improved dramatically. With the volume at around 2 it is clean with a good amount of basses but not woofy as it was before. When I raise the volume I get almost the same sound I had before the mods with the high treble channel.

So basically I'm really satisfied, now the amp sounds much more consistent, and most of all I have two different channels with different souls and more versatility.

I noticed also that after the mods, the two channels are much more interactive, if I'm playing on the high treble channel, changing the volume on the normal channel have a dramatic effect on sound (also if the channels are not jumpered!). I noticed it also before modding the amp, but the effect was more subtle. Anyway I don't consider it a defect, on the contrary, I think this is a cool feature of the amp! You can bring brightness to the normal channel, or give body to the high treble one easily now!

I attached some pictures of my amo before the today's mod.

Thanks to everybody for helping me, you guys rock! :rock:

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Re: Improvements for Normal & High Treble channels

Post by danman » Sat Sep 12, 2015 10:09 am

Happy to hear that you got it voiced more to your liking! :thumbsup:

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Re: Improvements for Normal & High Treble channels

Post by koffo » Sun Sep 13, 2015 1:27 pm

Yes, danman and neikeel,

After applyimg your suggestions is definitely better than before. Today I played around with a Plexitone pedal and added a reverb and a bit of delay, putting all the effects in front of the amp. So I used it 100% clean, attached to the high treble channel volome at 2, raised the volume of the normal channel at 1.5, master volume at 5, basses at 0' mids at 5 and trebles at 4, presence at 6.

Great sound also with pedals now!

Just one last thing: I have a zero-loss effect loop I bought form metroamp site quite a long ago. The question is simple: is it really "zero loss" or I should expect the tone to change after it is installed ?

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