JTM 45 not yeilding enough gain, nor enough bass

[csteward]

I've had my metro JTM 45 for about 3-4 years now. I've modded it over time to be more like a 50w bass amp. However, it still doesn't give a lot of bass, or enough bass I guess I should say, I play through the normal channel too because the HT channel is almost impossible to play through because of the spikey treble. It also does quite give enough gain, it's almost there, but not quite. What can I do to fix this problem, or what might be the cause of this problem?

Also I have a solid state rectifier in place of the tube. The reason this concerns me is because I know that it should have a lot more bass in it, and gain, especially considering I modded it more towards 50w specs. Now my Marshall TSL that has tubes that are 6 years old in it yields more bass, and about the same amount of gain. Idk if it's tubes or what. and I mean it could be my guitars, but I can't believe that it would be with every one of my guitars. I will say that the monster cables I've been using definitely let through a lot of high end.

[neikeel]

A normal JTM45 tends to have quite a lot of loose low end, and the gain only really kicks in when the volume (usually the high treble input).

It depends a bit on what mods you did when you converted it. I presume that it was a stock Metro JTM45 with stock Heyboer transformers and choke with KT66s and 12AX7s as supplied.

If you want tighter bass then I would suggest:

Increase the mains filter cap to 100uF (two halves of a 50uF can in parallel - like a late JMP50), use another dual 50uF can for 50uF screens and 50uF PI, increase your on board 16uF cap to 33uF (higher than that gets a bit stiff IMO).

Decrease the value of your V1 cathode bypass cap to between 4n7 0r 25uF depending on what you have lying around (25v electro is fine) keep the 820R that will be fine. Use a 100pF bright cap ob Vol 1 pot.

I am slightly worried that there might be a more fundamental issue with your amp as it sounds as if it was not quite as I would expect from the start.

As usual some clear high res pics and a description of the mods/specs would help a lot.

[csteward]

Thanks Neil, that has been a good deal of help. I will see what I can do as far as pictures of the insides. I only made a few changes really. I'll get you picks though. Yeah it has always concerned me that I can't quite use the high treble channel. I mean I should be able to get into Marc Ford tone (my target tone, or some variation on that) with no problem I would think. I always have everything on 10 on my amp, and then lower the volume with a hot plate.

Also does taking the bright cap off decrease bass response?

[neikeel]

Thanks Neil, that has been a good deal of help. I will see what I can do as far as pictures of the insides. I only made a few changes really. I'll get you picks though. Yeah it has always concerned me that I can't quite use the high treble channel. I mean I should be able to get into Marc Ford tone (my target tone, or some variation on that) with no problem I would think. I always have everything on 10 on my amp, and then lower the volume with a hot plate.

Also does taking the bright cap off decrease bass response?

You run everything on 10?

In that case bright cap makes no difference, leave it off. Bright cap just focuses the tone up to 7 on the volume dial.

I can rarely stand the presence up over 6 on any of my Marshalls (I still have some high frequency hearing left ). On quite a lot of Marshalls I find you just get more dirt over 8 on the volume and if you are unlucky just mud! Two exceptions to this are my '67 amps (JTM50 and a 45/100 - sound best with tone controls and volumes all the way up )

[csteward]

does turning the presence down affect the gain at all? I feel like it may. Maybe I am wrong though. and yeah, I'm only 20, so I definitely still have most of my high frequency hearing lol. Maybe that's even my problem, idk. Do you find that the valve arts are spikey at all in the treble? i'm also having trouble finding them to buy now, since I need to replace my tubes. I know this does affect the sound, but it wasn't right even with brand new tubes.

[toner]

...I am slightly worried that there might be a more fundamental issue with your amp as it sounds as if it was not quite as I would expect from the start...

+1

I don't have a lot of JTM 45 experience but I have played through a stock Metro 45. I can't help but think the amp would get very "mushy" and almost "fart out" with everything on 10. I would never set the bass above 4 even with a strat.

Your power tubes are probably worn out if you play very often with those settings for 3 years.

What speakers / cab do you use? That makes a HUGE difference; more than almost any amp mods.

Have you tried lowering the volume to around 7 and using a clean boost pedal? I do that a lot when I want more gain.

The stock negative feedback (27k 16 ohms) is very clean sounding. You may want to decrease NFB by moving it to the 8 or 4 ohm taps and/or increasing the NFB resistor. This will give you more PI and power tube gain for more overdrive. Be aware that it can also make the bass response "looser" so IF you have mushy bass, that may get worse.

[Smokin Tone]

The presence control will affect the gain. Try turning it up and rolling back the treble control some and the tone control on your guitar (if you have one). Try jumping the inputs. I built a stock Metro kit for a friend and it had more bass than you could ever need. I just built another one to stock Metro specs but used Classic Tone transformers and a 3H choke. This thing has as much gain as a pissed off 50 watter. I used low filtering too. Plenty of bass.

[Coot Boy]

I can't help thinking something is kaput as the OP says he has not enough bass, this should never be the fact with a 45.

[csteward]

As far as mods go, I have changed the nfb resistor to 47k and put the wire on the 8ohm tap. I've also changed the 270k mixer resistors to 470k. I also added a 330uf V2a cathode cap, maybe .68, but I'm pretty sure it's definitely 330uf. And I added the solid state rectifier. That's about it. My speaker cab is a avatar Vintage 412 loaded with slightly aged G12H30s. I've had the cab since the beginning of the year. Do the valve art KT66's tend to be a bit spikey in the highs rather than smooth? Or even lacking in bass? what about JJ ECC83s?

as far as using a boost pedal, I just bought an OCD the other day, I know that's not a clean boost, but it should help. I have been rolling the tone off more recently though because in the past couple weeks my tubes did go bad. However, like I said, this problem preceeded it. Although the treble was a tad bit nicer with brand new tubes. I do jump channels too. I like the blend of the channels.


Thanks for all your responses.

[Coot Boy]

I've done just about every mod possible to my 45 clone over a couple of year period and IMO the amp always sounds best stock, esp the NFB, 47/8 is just too loose for me and a 330uf on V2a I couldn't stand that on my 100 watters let alone the 45. So you must be trying to add more bass by the sound of things and again I'll say that shouldn't be needed.
JJ pres are cold but do the job, if you can find some decent used old stock pres you'll hear a nice improvement, the valve art 66s are decent IMO but I've never had the real deal in mine but I have no complaints with the shuggys.
Just to reiterate, the JTM45 is a real nice amp and I don't think you can improve upon it with mods, if you still think you have not enough bass then I say something is not right with the amp I would put it all back to stock with the tube recto, try n find a good old one as well for the reliability factor but put it back to stock n see how she fares, maybe a clip of the no bass sound would help as well as some good clear pics.
Unless you have bad tubes they are not the problem with the low bass issue however old pres are sort of mandatory when you can get around to it but the no bass thing needs to be fixed first. I can not see how you can have not enough bass if the amp is working right.

[csteward]

well I just got new tubes in. I decided to try the tung sol 12ax7's this time, and I just biased the amp. And it sounds 100% better. It's crazy. It never even sounded this good with a new set of JJs. Hopefully problem is solved. I can even use the high treble channel now, with no problems.

[Coot Boy]

Good to hear, now get that 330 off of V2a.