Single Channel 1983 JCM 800

[gtrjunior]

1) Anybody know if it is possible to plug into both the high and low inputs simultaneously (using an A/B box) to essentially make the single channel into a 2 channel? If not, if there a mod that can accomplish this?

2) Also, is the V3 slot as sensitive to which tube is put there as my JVM 210h is? In the JVM it is a higher voltage circuit and therefore needs a tube that can handle more voltage than the average Russian tube can handle. Most recommend a Chinese non-spireal filament tube for this slot. Just wondering if this is also the case with the JCM 800 series. ( I know, I know...wrong thread for tube questions...)

 

[plexilespaul]

1) Anybody know if it is possible to plug into both the high and low inputs simultaneously (using an A/B box) to essentially make the single channel into a 2 channel? If not, if there a mod that can accomplish this?

2) Also, is the V3 slot as sensitive to which tube is put there as my JVM 210h is? In the JVM it is a higher voltage circuit and therefore needs a tube that can handle more voltage than the average Russian tube can handle. Most recommend a Chinese non-spireal filament tube for this slot. Just wondering if this is also the case with the JCM 800 series. ( I know, I know...wrong thread for tube questions...)

1)no you can't
2)v1 is the most sensitive on the single channel jcm 800

 

[damienbeale]

No. They are not two channels. The hi input goes into an extra gain stage before going into the lo input. If you have anything in the lo input, the stage before (anything going through the hi-input) is cancelled out.

Besides which, the volume difference between the two different gain stages is so huge that you would also require a second master volume switchable by relay. It can be done, but requires a bit of fiddling, an additional relay, and is a little invasive.

What you are referring to as sensitive in the JVM is the cathode follower position, which is v2 in the 2203/2204. It is also a bit of a tube killer, if you have a weak one, but it is not quite as crucial. In all honesty, I don't believe the JVM is as bad as it is made out to be either. There was a bad batch of Tung-Sol reissues in the late 2000's/early 2010's which would die very quickly (almost instantly) in many cathode follower stages. The Russians did react to this and sorted it out. I'm not thoroughly convinced the problem still exists, but then I don't get to play a JVM day in, day out to confirm.

 

[gtrjunior]

Thank Guys!
I'm a little disappointed to hear that there isn't an easy mod to the 2 channel dilemma, but it is what it is.
As far as the JVM Cathode Follower goes, I believe certain Russian tubes, provided that don't use a spiral filament can work fine. Svetlana 12ax7's are one of them. They are also capable of handling much higher voltages than say, a JJ ecc83s. I accidentally put a JJ in V3 recently and, of course it failed on me just before a gig. Right now I have a TAD Premium in there which is a Chinese tube recommended by some of the JVM forum members...so far so good...

 

[FutureProf88]

No. They are not two channels. The hi input goes into an extra gain stage before going into the lo input. If you have anything in the lo input, the stage before (anything going through the hi-input) is cancelled out.

Besides which, the volume difference between the two different gain stages is so huge that you would also require a second master volume switchable by relay. It can be done, but requires a bit of fiddling, an additional relay, and is a little invasive.

What you are referring to as sensitive in the JVM is the cathode follower position, which is v2 in the 2203/2204. It is also a bit of a tube killer, if you have a weak one, but it is not quite as crucial. In all honesty, I don't believe the JVM is as bad as it is made out to be either. There was a bad batch of Tung-Sol reissues in the late 2000's/early 2010's which would die very quickly (almost instantly) in many cathode follower stages. The Russians did react to this and sorted it out. I'm not thoroughly convinced the problem still exists, but then I don't get to play a JVM day in, day out to confirm.

Nicely said, Damien. Don't use your average Russian tube in the cathode follower stage, still. I tried a Sovtek LPS in my Superlead clone and it lasted all of about half an hour. There is a video out there interviewing a guy named JD Simo and he says that in his experience it is more important that the cathode follower and phase inverter WORK, rather than having sweet sounding tubes in them. I tend to agree with that and I actually run JJ ECC83S's or Mesa tested/branded 12AX7STR's in those stages. V1 is where you want to put your good preamp tubes.

 

[damienbeale]

The vast majority of the Russian 12ax7 tubes do not have a spiral filament, but I honestly do not think that is the problem either. It is not the heater filament that fails in the CF position. It is all to do with the differential between the heater to cathode voltage differential. Your average Marshall design cathode follower is VERY hard on any ecc83 type in this respect, which is one of the reasons that big benefits can be had by elevating the heaters. The other being noise benefits in many designs.

Though the Chinese to tend to make more robust tubes, but on the other hand, they are usually guilty of varying the specs and build away from the original tube specifications. For example, the EL34B isn't really an EL34 in the true sense at all. But then again, neither is the JJ E34L, which has been around for an awfully long time now. Since Tesla's final years, in fact.

 

[FutureProf88]

The vast majority of the Russian 12ax7 tubes do not have a spiral filament, but I honestly do not think that is the problem either. It is not the heater filament that fails in the CF position. It is all to do with the differential between the heater to cathode voltage differential. Your average Marshall design cathode follower is VERY hard on any ecc83 type in this respect, which is one of the reasons that big benefits can be had by elevating the heaters. The other being noise benefits in many designs.

Though the Chinese to tend to make more robust tubes, but on the other hand, they are usually guilty of varying the specs and build away from the original tube specifications. For example, the EL34B isn't really an EL34 in the true sense at all. But then again, neither is the JJ E34L, which has been around for an awfully long time now. Since Tesla's final years, in fact.

Damien, have you heard of the tube replacers? I'm not sure exactly what the technical name for them is but Jet City makes a line of them. They are solid state analog devices that are supposed to mimic (to an extent) the operation of a 12AX7. I have been wondering how one of those would fare in the CF slot.

 

[damienbeale]

Damien, have you heard of the tube replacers? I'm not sure exactly what the technical name for them is but Jet City makes a line of them. They are solid state analog devices that are supposed to mimic (to an extent) the operation of a 12AX7. I have been wondering how one of those would fare in the CF slot.

Probably fine, since many of them have long guarantees... But I'll be damned if I want to shell out that much money on something that may just sound poop. There are a few companies doing these, but I just haven't been tempted. I'll let somebody else do the 'sperimentin on this one.

 

[paul-e-mann]

Thank Guys!
I'm a little disappointed to hear that there isn't an easy mod to the 2 channel dilemma, but it is what it is.
As far as the JVM Cathode Follower goes, I believe certain Russian tubes, provided that don't use a spiral filament can work fine. Svetlana 12ax7's are one of them. They are also capable of handling much higher voltages than say, a JJ ecc83s. I accidentally put a JJ in V3 recently and, of course it failed on me just before a gig. Right now I have a TAD Premium in there which is a Chinese tube recommended by some of the JVM forum members...so far so good...

Whats your opinion of a good OD or distortion pedal through your amp set relatively clean in the high input (amp gain set to 2-3 then step on pedal for dirt)?

 

[gtrjunior]

What I have been doing is plugging into the low input for a clean tone and using a variety of pedals for different favors of dirt. IMO the Blackstar Dual pedal KILLS! It is extremely marshall-like and pretty damn versatile. But I also have a tube screamer and a klon centaur clone pedal which are dialed in at different gain levels. This allows me to have several different sounds available. If you still plan on plugging into the high input I would think there are probably tons of OD/distortion boxes that will get the job done. Try a tube screamer, OCD...something like that...try out a ton of pedals till you find the right one for you...I'm sure if you asked 10 people you would get 10 different answers as to which pedal is best!!!