How to cure JTM45/100 ghosting without altering tone?
Posted: Fri Jun 03, 2016 6:03 pm
Hi folks,
been long time away from the forum - as all of us - had lots of work, but managed to stick my head out lately and get back to playing guitar more.
I love my JTM45/100 clone and after trying out some amps and listening to a billion sound clips, i think this is IT for me and for the music i like. BUT the amp has ghosting, very audible at even moderate drive levels (from about 6 on the volume and upwards), especially neck pickup on Les Paul (single note runs).
I've been going through many threads on this. On plexi palace ****** has given very insightful explanations and various suggestions what can fix the issues. Here are some links to the relevant posts:
http://www.vintageamps.com/plexiboard/v ... 879#p46140
http://vintageamps.com/plexiboard/viewt ... 905#p18158
http://vintageamps.com/plexiboard/viewt ... 15#p961017
http://vintageamps.com/plexiboard/viewt ... 15#p961135
Without going into technical details that are boyound me, here is a short summary.
The ghosting on these circuits could be circumvented by:
- Increasing henries of the choke
- Increasing screen filtering
- 2K screen resistors on V4 and V5 while useing 1K screen reisitors on V6 and V7 (to counter the PI imbalance)
- bypass choke with capacitor
****** did point towards the fact that all of these solutions are Band-Aid type of fixes and that the main cause and best solution for getting rid of ghosting is found within solving the PI imbalance issue (in a proper way, where it does nt alter the tone of the amp - e.g. minimize even order harmonics, etc.)
zaphod_phil added in this post (http://vintageamps.com/plexiboard/viewt ... 30#p961985) the following: "If grid current is pushing the bias voltage off by such a big amount, doesn't that suggest that maybe 100k grid reference resistors would actually be a better idea than the usual 220k? Maybe that's what the non-band-aid cure should be. As a comparison Hiwatt power stages use 100k grid reference resistors, and you never hear of grid voltage imbalance problems with them. They also use 22k grid blocking resistors, which also helps to control grid current. So that may be worth trying in Marshall amps as well, instead of the usual 5k6."
Also Phil suggests in his post that selecting slightly unmatched power tubes could have similar effect.
Guys, please help me to find the most suited solution, if i want to keep the amp's tone as unaltered as possible, but want to get rid of the ghosting - what has worked for you, what would you suggest?
I really want to solve this, but don't wat to mess with the amp for unnecessary experiments.
Thank you very much for your input!
VintageCharlie
been long time away from the forum - as all of us - had lots of work, but managed to stick my head out lately and get back to playing guitar more.
I love my JTM45/100 clone and after trying out some amps and listening to a billion sound clips, i think this is IT for me and for the music i like. BUT the amp has ghosting, very audible at even moderate drive levels (from about 6 on the volume and upwards), especially neck pickup on Les Paul (single note runs).
I've been going through many threads on this. On plexi palace ****** has given very insightful explanations and various suggestions what can fix the issues. Here are some links to the relevant posts:
http://www.vintageamps.com/plexiboard/v ... 879#p46140
http://vintageamps.com/plexiboard/viewt ... 905#p18158
http://vintageamps.com/plexiboard/viewt ... 15#p961017
http://vintageamps.com/plexiboard/viewt ... 15#p961135
Without going into technical details that are boyound me, here is a short summary.
The ghosting on these circuits could be circumvented by:
- Increasing henries of the choke
- Increasing screen filtering
- 2K screen resistors on V4 and V5 while useing 1K screen reisitors on V6 and V7 (to counter the PI imbalance)
- bypass choke with capacitor
****** did point towards the fact that all of these solutions are Band-Aid type of fixes and that the main cause and best solution for getting rid of ghosting is found within solving the PI imbalance issue (in a proper way, where it does nt alter the tone of the amp - e.g. minimize even order harmonics, etc.)
zaphod_phil added in this post (http://vintageamps.com/plexiboard/viewt ... 30#p961985) the following: "If grid current is pushing the bias voltage off by such a big amount, doesn't that suggest that maybe 100k grid reference resistors would actually be a better idea than the usual 220k? Maybe that's what the non-band-aid cure should be. As a comparison Hiwatt power stages use 100k grid reference resistors, and you never hear of grid voltage imbalance problems with them. They also use 22k grid blocking resistors, which also helps to control grid current. So that may be worth trying in Marshall amps as well, instead of the usual 5k6."
Also Phil suggests in his post that selecting slightly unmatched power tubes could have similar effect.
Guys, please help me to find the most suited solution, if i want to keep the amp's tone as unaltered as possible, but want to get rid of the ghosting - what has worked for you, what would you suggest?
I really want to solve this, but don't wat to mess with the amp for unnecessary experiments.
Thank you very much for your input!
VintageCharlie