JTM 45 Clone Problem

R

rgreen11

New member
Hey guys just got my JTM 45 clone out of the shop, had it retube with groove tube el 34m (Mullards)rating 7 and 12ax7, 3 again groove tubes. Now heres the problem it sounds very spongy. Does anyone else have this problem with there clone or Marshall JTM 45. My guitar is a Les Paul standard with a JB in the bridge. Now before you ask what did it sound like before I really don't know. It sounded graet in the store, so I took it to a gig and 3 songs into the first set I blew a power tube. The tubes that were in it before were GTel34 s and 12ax7, 1. It's a well built amp from a local builder with all the best parts,
so I don't know what the deal is. I choose the different tubes because I wanted to get even more headroom. Has anyone had this experience with the el 34 m or matbe even the 12ax7,1? Please help cuz I'm alittle bummed.
Thanks RG
 
Re: JTM 45 Clone Problem

Those amps were originally designed for 6l6/kt66. Did the builder do the proper circuit mods for EL34s?
 
Re: JTM 45 Clone Problem

Does it have a rectifier tube? If it's authentic clone of a JTM 45 it should have a GZ34 Rec. Tube. In general Amps w/t a rectifier tube will have a sag effect as the power goes across the rec. tube. This will sound spongy. The quick fix for this would be a Weber Copper cap. This is installed in place of the rec. tube,And the voltage sag will be gone. You also may want to have your tech look at your filter caps. The original spec. on a JTM 45 was 32x32. If that's what you have? You can have him increase the filtering values to 50x50. This will make the amp sound tighter. Good Luck with it.:D
 
Re: JTM 45 Clone Problem

It actually has a soild state rec, which I was happy about because I didn't want the sag or spongy sound but I'm still getting it.
 
Re: JTM 45 Clone Problem

It sounds to me as though it's biased a little hot. All power supplies have some sag to them, even solid state, and the hotter you set them the more sponge you get. Did the shop mention what the bias current was set at?
 
Re: JTM 45 Clone Problem

On edit:If you already have a SS rectifier some things to try:

Check the bias again. The rectifier change changes the voltage and requires a different bias setting. Too hot of a bias will result if the old bias setting is attempted to be carried over.

Go to KT-66/6L6 tubes and re-bias. KT66 will be less spongy than EL34.

Have a tech check the filtering. It could be only 16Uf on the PI cap if set up to vintage specs. I run 32/32.

I can go back and forth from KT66 to EL34 with just a re-bias on mine. I'm still running 470 ohm screen grid resistors too. If yours is acting wierd with EL34's you may need 1500 ohm SGRs on yours. Actually back in the day they often ran no SGRs at all, even with EL34's. But tubes were better then.
 
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Re: JTM 45 Clone Problem

glassman said:
It sounds to me as though it's biased a little hot. All power supplies have some sag to them, even solid state, and the hotter you set them the more sponge you get. Did the shop mention what the bias current was set at?
Click to expand...

I don't like running EL34's very hot. 65% works better for EL34's for me.
 
Re: JTM 45 Clone Problem

Lake Placid Blues said:
On edit:If you already have a SS rectifier some things to try:

Check the bias again. The rectifier change changes the voltage and requires a different bias setting. Too hot of a bias will result if the old bias setting is attempted to be carried over.

Go to KT-66/6L6 tubes and re-bias. KT66 will be less spongy than EL34.

Have a tech check the filtering. It could be only 16Uf on the PI cap if set up to vintage specs. I run 32/32.

I can go back and forth from KT66 to EL34 with just a re-bias on mine. I'm still running 470 ohm screen grid resistors too. If yours is acting wierd with EL34's you may need 1500 ohm SGRs on yours. Actually back in the day they often ran no SGRs at all, even with EL34's. But tubes were better then.
Click to expand...


Yeah I couldn't remember exactly what the changes were, I guess it could just be the screen resistors.

I also second your comment on changing the filter caps if you want a tighter feel. 50/50 is Marshall uses on the reissue version, I'm sure to satisfy more customers used to modern amps.

There is also the tone stack mod you could try. It involves changing on resistor and one cap to different values. I can't remember what those are, but the mod is often called the Jimi Hendrix mod, you can Google it.
 
Re: JTM 45 Clone Problem

Lake Placid Blues said:
I don't like running EL34's very hot. 65% works better for EL34's for me.
Click to expand...

I quit using the % of max dissipation method some time ago; it's based on a "bogey" tube and doesn't take into account the range (hot to cold) of modern tubes. I buy my tubes DSS matched; that way I have the the full bandwidth idle current of the tube already identified for me and I can select a proper starting point for my customer. I have set up my tube testers to give me the same results the DSS system does for when I have a set of "unknown" tubes come in.

65% is a very appropriate range for an EL34 amp with a middle of the road (bogey) tube. For a really hot tube, 55% might be more appropriate; 75% might work well for a cold tube. This is of course just to get a middle of the road setting; I adjust from there based on player preference. The wider range of TC for modern tubes has really changed the way I have to do things; but it has also opened up a whole bunch of tonal possibilities at the same time.

This isn't even taking into consideration amps like a Recto where a setting of 20% to 30% of max dissipation might be necessary to get the tightness players expect of those amps.
 
Re: JTM 45 Clone Problem

I don't have any tube testers or anything other than a multimeter and a calculator. What I do is use the old % dissapation to get me into the ball park, and then make minor adjustment to taste, try and get the tubes to sound like I want them to in the amp. But I just do my own amps really so if I don't have to try and please somebody else.
 
Re: JTM 45 Clone Problem

The shop did not tell me what it was biased at. However they did say that it was baised better than the old tubes, which were GT el34 s rating 6, so I don't know if that means hotter or not. I'll have to call them on Monday.
 
Re: JTM 45 Clone Problem

Strat Deluxers thread reminded me of another factor with JTM45s 45's use rather large coupling caps of .1u in the PI. This was common with bass amps of the time. However going to plexi spec coupling caps throughout of .022 (actually .0022 on the treble channel in the preamp) is another way used to tighten up a JTM45.
 
Re: JTM 45 Clone Problem

You can change the cathode cap/resistor too. Lots of guys like to use the 100 watt super lead treble channel arrangement. It's a split cathode amp, the 45 isn't, so if you use the 2 input or bridge the channels, this can change the sound dramatically.

There's tons of info on these amps at Metroamp.com BTW.
 
Re: JTM 45 Clone Problem

el34 and ss rectifier - this is not jtm45 clone! It could be 1987, 1986
+1 metroamp forum
 
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