JTM45 reissue after modded for KT66 going back to 6L6

[Ye Auld]

Hi guys, I'm new on this forum and I need some help if possible.... I own a Marshall JTM45 reissue ( 2002 ) that was mod to take valve arts KT66, a tech that no longer is around did only this part of the mod:

((( Lord Valve's Heads-Up on Using KT-66 Tubes With the JTM-45

Lord Valve's field reports indicate that using the Valve Arts (Chinese) KT-66 tubes in the JTM-45 (reissues and originals) can be problematic. The amp design is prone to parasitic oscillation that may kill your tube. The solution: put 5.6K swamp resistors in series with pin 5 on each power tube socket. First unsolder the wire connected to pin 5, then solder the 5.6K resistor directly to the pin socket. Re-connect the wire to the other end of the resistor. The goal is to make the leads on the resistor as short as possible.

Lord Valve also suggests putting 1K/5W screen resistors across pins 6 and 4. Here pin 6 is used as a tie point for the screen supply. These may already be present on a factory JTM-45 or Bluesbreaker reissue.)))

After hearing my friends JTM45 with 6L6s, I think I like the sound of 6L6Ge better, so I swaped the KT66 with Groove tubes 6L6GE ( 30 watts tubes ) and did the bias with a weber bias rite, so plate voltage reads on both tubes 417, and one tube is 35.5 and the other 36.7, since I'm not a tech I'm worried if these readings are correct between proper range, and also if the mod described above will affect somehow on the long run the performance of the tubes, OT, etc etc, any help will be greatly appreciated !!!!

 

[jack daniels]

The mod is really for the modern Valve Arts KT66 due to its limitations when used in the amp. Search out a NOS GT Saratov KT66 pair. Or go with a Gold Lion KT66 pair here...

www.thetubestore.com - Genalex - Gold Lion KT66 Audio Tubes

The KT66 is a 25 watt tube as where your 6L6 GE is a 30 watt tube so you'd have to adjust your bias setting accordingly between KT66 and 6L6 tube sets. Yes, there is a variance between some matched tube sets, sometimes around .05 percent. You are swapping a 25 watt tube for a 30 watt tube, AND your mod was done for the VA KT66.

 

[jack daniels]

John Templeton - The Tube Store - "The compression in the Gold Lion tubes really seemed focused on the highs and upper mids, producing a smooth, singing sustain as each note bloomed. Chords compressed very nicely without loss of individual note definition or mushiness. By comparison, the Shuguang tubes were more compressed in a slightly lower frequency band, leaving the highs a little more raw sounding. While both sounds are very useful, I feel the Gold Lion tubes are better for those that want that smooth Santana sustain and the Shuguang tubes are the way to go if you want the raw sound of reverend Billy Gibbons."

 

[jack daniels]

Or possibly the Tung Sol 5881 biased as a 25 watt tube ala Gary Moore.

 

[Ye Auld]

thanks a lot Jack, as I said I think I really like the sound of 6L6s on this amp, right now I was playing the amp with the voltage plate at 417 and the tubes are bias at 35.5 ma, so do you think since my amp was modded for KT66 should I bias the tubes like around 30ma or 32-33ma? because I´m planning to stick with the 6L6s , thanks a lot !!!

 

[Ye Auld]

I also have a 59 reissue Fender Bassman and when I run the JTM45 and the bassman together in stereo they sound fantastic, those amp complement each other real nice !!!

I used to have a JCM900 DR ( really nice amp too ), but honestly for me the best marshall is the JTM45

 

[Ye Auld]

So please let me know what you think about the bias, thanks Jack !!!!

 

[Ye Auld]

I just follow your advice and also read what a guy on this forum was saying ( mickey I think ) about biasing this amps in the low 30s so I did and now the amp sounds great !!!

 

[Ye Auld]

More pics

 

[Ye Auld]

more pics

 

[Ye Auld]

Here you can see Lord Valves mod on photo 1 and 2......... the white resistors (or whatever they are I´m not a tech ) soldered to the tube sockets, those are not the originals values, those are the ones put to be able to take KT66 valve arts !!! and now I´m back to 6L6s

 

[Ye Auld]

and here are the tubes ( power and preamp )
2 6L6 GE premium
1 national 12ax7
2 mullards reissue

 

[jack daniels]

thanks a lot Jack, as I said I think I really like the sound of 6L6s on this amp, right now I was playing the amp with the voltage plate at 417 and the tubes are bias at 35.5 ma, so do you think since my amp was modded for KT66 should I bias the tubes like around 30ma or 32-33ma? because I´m planning to stick with the 6L6s , thanks a lot !!!

As for biasing, I will bias anywhere between 65 to 70% static dissipation, that is if the new modern tubes can take it. As Ken mentioned in another post, he does it by ear. He sets to a certain range and then varies (by ear) up or down from this point.

 

[jack daniels]

and here are the tubes ( power and preamp )
2 6L6 GE premium
1 national 12ax7
2 mullards reissue

GOOD MAN! You've got the situation well in hand..........and he's off

 

[ampmadscientist]

Hi guys, I'm new on this forum and I need some help if possible.... I own a Marshall JTM45 reissue ( 2002 ) that was mod to take valve arts KT66, a tech that no longer is around did only this part of the mod:

((( Lord Valve's Heads-Up on Using KT-66 Tubes With the JTM-45

Lord Valve's field reports indicate that using the Valve Arts (Chinese) KT-66 tubes in the JTM-45 (reissues and originals) can be problematic. The amp design is prone to parasitic oscillation that may kill your tube. The solution: put 5.6K swamp resistors in series with pin 5 on each power tube socket. First unsolder the wire connected to pin 5, then solder the 5.6K resistor directly to the pin socket. Re-connect the wire to the other end of the resistor. The goal is to make the leads on the resistor as short as possible.

Lord Valve also suggests putting 1K/5W screen resistors across pins 6 and 4. Here pin 6 is used as a tie point for the screen supply. These may already be present on a factory JTM-45 or Bluesbreaker reissue.)))

After hearing my friends JTM45 with 6L6s, I think I like the sound of 6L6Ge better, so I swaped the KT66 with Groove tubes 6L6GE ( 30 watts tubes ) and did the bias with a weber bias rite, so plate voltage reads on both tubes 417, and one tube is 35.5 and the other 36.7, since I'm not a tech I'm worried if these readings are correct between proper range, and also if the mod described above will affect somehow on the long run the performance of the tubes, OT, etc etc, any help will be greatly appreciated !!!!

Lord Valve is a self absorbed egotistical idiot. He is no authority on anything. Quite simply, a hot air balloon with 2 legs.
I frequently refer to him as: "Lord Toilet Valve..."
And many techs refer to him as: (we can't say that here...)



But anyhow, 470 ohm is the correct screen resistor. (For: KT66 and 6L6.)
Parasitic oscillation is cured in a completely different way, mostly at V1a preamp tube....sometimes in the effects loop. Sometimes in the power supplies.
KT66 when set up properly, is not prone to any such problem.

KT 66 and 6L6 will give you a similar result, and any oscillation has nothing to do with using KT66 specifically.
If there is oscillation, it's caused by a different problem, not the KT66.
Changing the screen resistor will not cure oscillations. Somebody is pulling your chain.

However oscillations in the preamp of Marshall amps has occurred regardless of "what" output tube. It's in the preamp.
And Marshall has specific ways to stop it from happening.

Another Problem:
Some older Marshall amps had no screen grid resistors...
And you must add those resistors.
But this is because the screen draws too much current, not because of "oscillations."

The older tubes like EL34 were changed to be more efficient.
This design change caused the screen to draw more current.
Then the 1K needs to be added, to use NEW EL34 - (VS. OLD EL34) !
Because the higher screen current needs to be limited.
NOT because of "oscillations !"

And this advice comes from the Marshall Factory...not "lord" toilet valve.

 

[jack daniels]

Lord Valve is a self absorbed egotistical idiot. He is no authority on anything. Quite simply, a hot air balloon with 2 legs.
I frequently refer to him as: "Lord Toilet Valve..."

But anyhow, 470 ohm is the correct screen resistor.
Parasitic oscillation is cured in a completely different way, mostly at V1a preamp tube....sometimes in the effects loop.
KT66 when set up properly, is not prone to any such problem.

KT 66 and 6L6 will give you a similar result, and any oscillation has nothing to do with using KT66 specifically.
If there is oscillation, it's caused by a different problem, not the KT66.
Changing the screen resistor will not cure oscillations. Somebody is pulling your chain.

However oscillations in the preamp of Marshall amps has occurred regardless of "what" output tube. It's in the preamp.
And Marshall has specific ways to stop it from happening.

Another Problem:
Some older Marshall amps had no screen grid resistors...
And you must add those resistors.
But this is because the screen draws too much current, not because of "oscillations."

The older tubes like EL34 were changed to be more efficient.
This design change caused the screen to draw more current.
Then the 1K needs to be added, to use NEW EL34 - (VS. OLD EL34) !
Because the screen current needs to be limited.
NOT because of "oscillations."

This is what I heard, and what was actually absorbed for posterity's sake....

AMS - "But anyhow, 470 ohm is the correct screen resistor.
Parasitic oscillation is cured in a completely different way, mostly at V1a preamp tube....sometimes in the effects loop.
KT66 when set up properly, is not prone to any such problem.

KT 66 and 6L6 will give you a similar result, and any oscillation has nothing to do with using KT66 specifically.
If there is oscillation, it's caused by a different problem, not the KT66.
Changing the screen resistor will not cure oscillations. Somebody is pulling your chain.

However oscillations in the preamp of Marshall amps has occurred regardless of "what" output tube. It's in the preamp.
And Marshall has specific ways to stop it from happening.

Another Problem:
Some older Marshall amps had no screen grid resistors...
And you must add those resistors.
But this is because the screen draws too much current, not because of "oscillations."

The older tubes like EL34 were changed to be more efficient.
This design change caused the screen to draw more current.
Then the 1K needs to be added, to use NEW EL34 - (VS. OLD EL34) !
Because the screen current needs to be limited.
NOT because of "oscillations."

 

[jack daniels]

I think many on the MF forum know about these "personalities" as mentioned here. It's not in one's best interest as a music store/business owner to "bash" others even if those particular people are very deserving of said accolades.

Music store owners and techs I've dealt with that do this end up being very questionable themselves. It's always good to make people aware, should the subject come up in conversation however, it's all in HOW you convey that message.

 

[ampmadscientist]

As for biasing, I will bias anywhere between 65 to 70% static dissipation, that is if the new modern tubes can take it. As Ken mentioned in another post, he does it by ear. He sets to a certain range and then varies (by ear) up or down from this point.

No, you do not do it "by ear." Legitimate technician uses "a scope."

The modern tubes NEED a screen grid resistor to limit the current.
Old amps did not have the resistor, because the tube was different.

The design of the tube was changed. The screen draws more current.
The resistor was added because of this tube design change.
The current is limited BY that added resistor.

Screen Grid Resistor:
6L6 and KT66 = 470 ohm
EL34 = 1K ohm
IF it is not installed already, you MUST install it.

 

[jack daniels]

No, you do not do it "by ear." Legitimate technician uses "a scope."

The modern tubes NEED a screen grid resistor to limit the current.
Old amps did not have the resistor, because the tube was different.

The design of the tube was changed. The screen draws more current.
The resistor was added because of this tube design change.
The current is limited BY that added resistor.

Well My My My, you have an opinion. And just like assholes, everybody got one. I don't doubt your veracity, just your intent.

 

[mickeydg5]

I am in agreeance with AmpMad concerning the screen grids. They should be 470 ohms whether KT66, 5881 or 6L6 types.