JTM60.... Can we make it better????

[TIMBO]

I guys, Thought I'd start another project  . I can find mixed reviews on this one and they are from one end of the scale to the other.

Most say that the one thing that is a major problem is the overheating. Also the OVERDRIVE channel is not too everyone's liking and the clipping diodes are bypassed for a more natural overdrive.

The clean channel is the big plus as most rave about it,so that's good.

After a lot of rebuilds using a pair of EL84 and just finding that I can't quite hit the spot sound wise, the JTM30 (had one when they first came out) tweaked my interest awhile back.
So I rummaged through my cupboard of crap and was able to scrape together a build.

I trimmed to SS stuff away to find the basic circuit and it looks good for a platform to build on. 

A PSU from another amp that didn't fly is my power source and should provided about 400v to the Tung Sol 6L6g's.
A second transformer will power the heaters @12v and relays @ 12vDC.

So i'll start with the schem and see how it sounds as is, I'll even put the diodes in for kicks. Thanks

[TIMBO]

Hi guys, Had no problems with start up and adjusted the power tubes cathode resistor to a good 24w/tube.

Very quiet at idle which is a good start and took voltages and nothing came up out of the ordinary.

Found this  for things to try, but at the moment the only thing I have changed is the grid resistor on V1 and put it in series to feed V1a with 33k and V1b with 66k and sounds great. Also changed the bypass caps on V1a&b to 2.2u as I did not have any 1uf caps  
http://forum.metroamp.com/viewtopic.php?f=4&t=13499&start=30
Just a couple of things......
After a short (5 minutes) I noticed a burning smell and found that the 5w resistor on the power tubes cathode was heating up rapidly,i replaced it knowing that 5w was a bit small for this and paralleled a 470 and 360 ohm 10w resistors to give me 203ohms@20w, just for a bit of overkill, this also heated up and got very hot to touch,i don't think this is right????

I'm thinking that I'm close to having a good sound and its very loud   and I'm going to add a ppimv although it already has a pre PI master volume.
Does having a premaster effect how the PI responds to the amount of signal that it sees and if removing it, will it blast the PI and therefore distorting the signal too much that all the ppimv will do is pass on an overdistorted sound to the power tubes
In other words is it better to have both volumes one adjust the gain and the other to adjust the overall volume.Thanks  

[sluckey]

...203ohms@20w, just for a bit of overkill, this also heated up and got very hot to touch,i don't think this is right????

There's 29V across that resistor. P = E²/R = 29²/200 = 4.2W. That resistor is supposed to be hot.

Does having a premaster effect how the PI responds to the amount of signal that it sees and if removing it, will it blast the PI and therefore distorting the signal too much that all the ppimv will do is pass on an overdistorted sound to the power tubes
In other words is it better to have both volumes one adjust the gain and the other to adjust the overall volume.

If you remove the preMV you still have a volume control in the preamp that will control the signal going to the PI. But having a preMV and a postMV will give you more versatility when controlling PI and PA overdrive. I'd keep both if you have the room. I'd keep the PPIMV if I had to give up one or the other.

[TIMBO]

Thanks Sluckey, I just didn't think a 20w resistor would get that hot.

No room on the front panel, so i'll put the PPIMV on the back. 

[sluckey]

Thanks Sluckey, I just didn't think a 20w resistor would get that hot.

It's got to deal with the same amount of heat as the 5 watter does. It just dissipates the heat better. It might still put a blister on your finger whereas the 5 watter might steal your fingerprint. 

[TIMBO]

 

[TIMBO]

Hi guys, The metroamp forum is about the only info that I can find that has any mods that can be done. The layout of the build I have makes it easy to do mods without pulling too much apart to do them.

One of the mods is changing the plate and cathode resistors on the tone stack driver V2 (plate 220k and cathode 2.2k) is this not going to have the same effect as a 100k/1k combo???? and checking some voltage in tube data some of the jcm800 are close to what I have.

Just as need to know the tail resistor on the PI is 22k and most of the marshalls are 10k, what effect does this have and is it worth changing???

Last one, I see that some amps don't have grid resistors on the power tubes, good/bad???

I have tried some combos with the clipping diodes and I can say that they sound fine and yet a lot have said that they were CRAP  Thanks
 

[darkbluemurder]

One of the mods is changing the plate and cathode resistors on the tone stack driver V2 (plate 220k and cathode 2.2k) is this not going to have the same effect as a 100k/1k combo???? and checking some voltage in tube data some of the jcm800 are close to what I have.

My guess is that the higher values will yield more compression. 100k/1k is the classic combination.

Just as need to know the tail resistor on the PI is 22k and most of the marshalls are 10k, what effect does this have and is it worth changing???

The lower value is classic Marshall (copied from the classic Tweed Bassman). The higher value limits the drive to the power tubes somewhat by decreasing the headroom of the PI slightly.

Last one, I see that some amps don't have grid resistors on the power tubes, good/bad??? 

Bad IMHO. EL84s are susceptible to oscillations so I would add grid resistors of at least 10k for each tube. And BTW check that the screen grids are at least 1k for each tube.

It seems that the mods are basically a return to Marshall's classic circuit values.

Cheers Stephan

[TIMBO]

Hi guys, Pick through the list of mods and most made little or on difference at all. That's Great  that means that I don't have to pull parts.

The overdrive needs a bit of work but its in the ball park for sound. The diodes are a keeper just need to settle on a combination, 4x1N4007 are at the top of the list so far. Thanks

[TIMBO]

Hi guys, after going through the list of mods, I found that the amp was pretty good as it was. Although it was suppose to be a 60w amp I didn't have any EL34s so the next best was 6L6s so it is now looking more like a JTM50. As I do, I was not going the just settle on this as the finished product, so I started to compare the JTM60 with the other JTMs and the 45 being a stellar amp I found that the clean channel was close to the 60 who would have thought, Also the 45 had some voltage readings at the PSU nodes. The PA was similar as well.

So at the almost finale of the build I have a JTM45 with an OD channel.

OD sounds great but only when the gain is at 2 on the dial, so some tweaking needs to be done.

I used the earthing buss and again there is no hum issues at all. This is working out to be a very good system to build by and it is very easy to do mods. Thanks

[TIMBO]

MORE

[TIMBO]

Hi guys, This build is turning out great, but I thought I'd play around with the tone stack driver. As is the 220k/2.2k combo is good but it at times maybe overdriving things a little as the clean channel is OK up to a point but gets a bit ratty at higher volume. I am only guessing that this is the cause as the normal combo is 100/1k.

Seeing a bit of a sequence happening and never seeing a 150k/1.5 combo, is this a doable combo 

[TIMBO]

Hi guys, This one IS done. I changed a few things to be more like a JTM45/Bluesbreaker with an OVERDRIVE channel.Thanks

[TIMBO]

A couple more..............

[TIMBO]

I'll put this one in as well, it took a bit of doing. A roll of duct tape came in very handy.Thanks