JTM 60 tubes and diode questions

[Guitar-Sam]

I was recently given a 90s JTM 60 1x12 2ch combo. Pretty cool little amp,gotta say the cleans are about as nice as I've heard outta a marshall. The overdrive is Ok but the amp tends to have a boxy sound I'm assuming this is the diode clipping. Could simply swapping diode type help or is it more the circuit itself.?? All so the amp is gonna need a retube,anyone have recommendations in these JTMs?? Also what are the pre amp tubes "jobs" in this I know v4 is PI,are V1/2 gain stages and V3 tone stack and CF like others or is there a verb tube?

 

[ampmadscientist]

I was recently given a 90s JTM 60 1x12 2ch combo. Pretty cool little amp,gotta say the cleans are about as nice as I've heard outta a marshall. The overdrive is Ok but the amp tends to have a boxy sound I'm assuming this is the diode clipping. Could simply swapping diode type help or is it more the circuit itself.?? All so the amp is gonna need a retube,anyone have recommendations in these JTMs?? Also what are the pre amp tubes "jobs" in this I know v4 is PI,are V1/2 gain stages and V3 tone stack and CF like others or is there a verb tube?

The preamp has diode clipping, look at the schematics here:
Marshall Schematics

Swapping the diodes won't really help a lot.
Taking out the diodes, and tweaking the preamp may help more.

The reverb on my schematic is solid state.

Many of the players liked Chinese 12AX7 better because it's crunchier.

 

[dreyn77]

SO a boxy sound is something to do with 5mm components turning Off and ON.

really?


I don't think so.

If you point the speakers at your ears you'll never hear the backwards pressure wave sound.

SO you need to lie down on the floor to hear the amps true sound OR you find a way to tilt the amp and point in directly at your ears OR you keep doing what you do and accept the fact that marshall has made the amp work.

so now all you do is set it for how loud you want the sound.

You DON'T take out the diodes. you wanted the marshall distortion sound and it's got the right amount built into the amp.

Start messing with taking out diodes and you're a mad professor and shouldn't be giving complex instruction to the NON professors.

maybe you actually don't even want a marshall. it's all about free stuff for you. free advice too.

 

[Guitar-Sam]

The preamp has diode clipping, look at the schematics here:
Marshall Schematics

Swapping the diodes won't really help a lot.
Taking out the diodes, and tweaking the preamp may help more.

The reverb on my schematic is solid state.

Many of the players liked Chinese 12AX7 better because it's crunchier.

Thanks for the schematic stuff I will check it out. The amp has Chinese tubes now the ones with the silver shield around the plates. I used to have a bunch of vintage tubes to play with but sold um all and don't have the cash flow expire ament on this one.so maybe I'll stay Chinese and buy a couple cp pres to play in v1

 

[Force235]

The amp should sound great, not sure what you mean by boxy, please explain if you can. I just retubed my JTM60, I used Electro harmonix in the preamp and had new Grrove Tubes #4, so I used them in the power section. If the amp is still sounding flat after a retube then it probably needs a cap job as these guys are 20 yrs old at this point.

This amp is full of tones, try cranking (diming) the clean channel volume and setting the master on like 3 or 4 to your volume liking, you should get that classic JTM break up, zep, ac/dc sound, you can adjust this high or low to your liking.
The distortion channel should have great flexibility and you should be able to dial in a JCM 800/900 sound.
I really like the stock G12 spkr, sounds excellent to me.

 

[Guitar-Sam]

The amp should sound great, not sure what you mean by boxy, please explain if you can. I just retubed my JTM60, I used Electro harmonix in the preamp and had new Grrove Tubes #4, so I used them in the power section. If the amp is still sounding flat after a retube then it probably needs a cap job as these guys are 20 yrs old at this point.

This amp is full of tones, try cranking (diming) the clean channel volume and setting the master on like 3 or 4 to your volume liking, you should get that classic JTM break up, zep, ac/dc sound, you can adjust this high or low to your liking.
The distortion channel should have great flexibility and you should be able to dial in a JCM 800/900 sound.
I really like the stock G12 spkr, sounds excellent to me.

By "boxy" I mean the boost channel seems to have a boxy hard clipped sound,kinda 900'y my 2554 I had did it also(but worse yuck!)my 2210 which has diode clipping is MUCH smoother.That's why I was wondering if swapping diodes would change that or if the clipping circuit itself was the culprit. But yea I discovered that cranked clean channel so I may be able to work around it.

 

[ampmadscientist]

By "boxy" I mean the boost channel seems to have a boxy hard clipped sound,kinda 900'y my 2554 I had did it also(but worse yuck!)my 2210 which has diode clipping is MUCH smoother.That's why I was wondering if swapping diodes would change that or if the clipping circuit itself was the culprit. But yea I discovered that cranked clean channel so I may be able to work around it.

Yeah, I think you may be onto something.

Well you are right. Maybe I was hasty...and you made a good point.
Marshalls have 3 different types of diode clipping circuits.
#1 is two diodes
#2 is four diodes
#3 is four LEDs
Maybe you should try the LEDs. Right now you have #2 I think.
LEDs are like JCM 900s, and other models.
There is a difference in the sound of these.
But the circuit board can be damaged by soldering, so be careful.

Now in the clean channel, I take the diodes out entirely. Then I have more clean headroom.

 

[iron broadsword]

Usually when people call a combo boxy, it's more to do with the speaker/cab size. Plug it into a 4x12 and then make a decision compared to the other amps you mentioned.

Led's get great amp-like tone apparently, and germaniums I know to be fantastic. Silicon can definitely sound gross in some circuits.

 

[dreyn77]

Guys! there's NO difference between a JTM30 and a 5010 as far as marshall grind sound. sure there's a tone difference but not as far as the gain sound is.
SO diodes aren't making much difference, i'm here to say.

You can swap the whole amp and still not have any difference in the sound other than an extra bass sound.

I've also done the whole amp swap with the big valve amps too. and it's not real noticeable either. it's a tone swap for sure, and a tiny gain difference but that's cause one amp was modern and the other was vintage.

 

[iron broadsword]

Having heard the difference between tubes and diodes in my jcm900 DR, and having built a dr boogie pedal which is a mesa dual rec's lead channel built into pedal form using transistors, and comparing it to an actual dual rec, there IS a difference for sure. But that difference is very small when it's going through a tube power section, which still lets it have the tubey punch we all love. The solid state versions however fail slightly in that it is a little muddier, and a little more harsh to the ear. Check out this vid where I compared the before/after in my jcm900.. in the pre-mod clip the notes are far less defined, but in the post-mod clip you've got the string seperation.

[ame]www.youtube.com/watch?v=dOyCfyDcVHQ[/ame]

If all else is the same, that is the difference. It can be lived with for sure.

 

[dreyn77]

I think if you designed the amp from scratch you could get celestion to slightly tweek the speaker construction to takeout that harshness which they can and do do and have done in the past.

 

[mickeydg5]

There is a link at the C105 position. You see it. Remove it and try anything resistance wise below a 5k resistor there.

Or if you want more distortion on the upper frequencies try something a small value capacitor around .002uF (2nF) or lesser value.

Try different values to see what you like best.

 

[Murray]

I've worked on a few of these over the years. Great, compact tube amp with stellar cleans & usable od sound for sure.

I've kept one that currently is moderately modded & is currently running JJ ECC83's for pre & PI duty and KT77's in the power sockets and I think it is sweet. Lots of tube choices out there. Down the rabbit hole you go.

Point of interest that is critical in these amps (IMHO) is bias. The amp design blows because the filter caps & bridge rectifier sit directly over the horizontal power tubes. Heat trap shortens lifespan. Some folks bias these pretty hot, which exacerbates the problem. I like to set the bias at @65% of max dissipation so that things stay cooler. Sounds better to my ear too. But that's just me. If aged electrolytic caps are still in place a cap job makes a world of difference.

Some people install fans or 1/8" aluminum plate between the tubes & chassis but I've never seen the need. But if it gets pushed hard & heats up, these are easy options.

THIS has some good info on the amps circuit. Also some good mod threads on this site, as I recall.

The amp has 3 ckt boards: power, pre & mains, with a lot of cables interconnecting. The schematic is not the easiest to follow, but it's all there. These share design with the JTM600 head. I believe that Marshall drawings J600-61-02 is a JTM600 w/o the NFB loop and JTM6-61-02 is the JTM60 with NFB. Might help in a search. Most components are 1/4w and the pcb itself is very easily damaged by irons.

Pretty sure the tube layout is V101 Clean Pre, V101B, Boost pre, V102 Boost pre #2&3, V103 Common pre #1&2 and V104 the PI.

Hope this helps. I love these amps. Hope you enjoy it. Sorry for the wordy post.

M

 

[Guitar-Sam]

Thanks Murray,good info. I heard bout the overheating issues.My amp shows signs of heat on the back faceplate it's more brown near the power tubes than gold.evan the front is slightly darker gold on the one side. Also what KT77s are you using? I was thinking of trying gold lions.also tempted to try eh 6ca7

 

[Murray]

Thanks Murray,good info. I heard bout the overheating issues.My amp shows signs of heat on the back faceplate it's more brown near the power tubes than gold.evan the front is slightly darker gold on the one side. Also what KT77s are you using? I was thinking of trying gold lions.also tempted to try eh 6ca7

The KT77's I've got in there now are JJ's. The amp likes 'em & so do I. Articulate, if I had to come up with a descriptor. Gold Lion's would be an excellent choice, IMO. Worth the extra $$$? Dunno. Depends on what your ears tell you (tempered by what you can afford). I've not tried any 6CA7's so I can't comment, but I know some folks like these a lot. Certainly worth a test drive.

 

[Guitar-Sam]

So we have a sick amp on our hands as I found last night.Well I got an old Bel 12ax7 so I decided to try it out. After tryin it in couple spots I stuck it in V1 and jammed for a while. The amp seemed noisy so I decided to try the old tube and I realized it was a tube socket contact issue. Now to make sure I left the amp on without any input plugged and master dimes. Was quiet for a bit then it started making this intermittent electrical noise that's when I noticed one of the el34s red plating along the seem of the plates. So I turned it off swapped the two el34s and the same tube red plates in either socket. I'm assuming amp is Ok and the one tube just drifted out of spec,sound right??? I do not know if this thing was biased befor but it has Ruby/Shuaguang el34s so I know they've been swapped.

 

[mickeydg5]

Make sure pin #5 on that tube is clean, to bare metal.

If it still gives problems it is safe to say that it needs to be discarded.

 

[Guitar-Sam]

Is there a way to check the bias on this thing with a multimeter? I opened it up and it is a friggin nightmare in there. Does it have a 1ohm resistor already and were should I test from,it's crazy trying to get to the sockets. I can't tell on the schematic but I'm guessing no resistor. And it looks like a big job to get the board out to install one? For the hell of it I tried the outer pair I have Laying around from my 100w amp and one red plates in that pair too and also follows the tube not the socket,could it be bias is just too high?? I found 471V on pin 3

 

[mickeydg5]

There are no bias monitoring resistors. Your best bet is to have a bias probe tool capable of measuring both plate voltage and bias current.

 

[Guitar-Sam]

Yea that's what I figured. Thanks