JTM 45 won't quite come alive

Wescaster

Gold Supporting Member
Messages
94
I am building a Tube Depot jtm 45 kit. So far I have two problems.

1-Two inputs don't work.

2- Very low output volume
The voltages in the power tubes are good. The output tranny is reading good. Current to all tubes is good. Actually a little high.

Also the power tubes are biased to 29 and 32 ma. The pot is maxed so I will have to replace the 47k resistor with ...something..

Any suggestions would be appreciated. I've re checked my work many times and all the resistor and cap values are correct. http://s971.photobucket.com/user/wesaasness/library/?sort=3&page=1
 

garret

Member
Messages
1,002
In the assembly manual there is a power on start up checklist. Are all the voltages correct?

I built one of these kits as well. My voltages all read just a little higher than those stated in the instructions.

I've got a few of the mods installed as they suggested. Also installed a PPIMV instead of the MV. It's a very, very loud amp.

Can you get better pictures around the input jacks?

Also, the screen resistors are only 2 watts, from memory. They should be at least 3 watts.
 
Last edited:

zenas

Member
Messages
8,759
Two inputs for one channel? A dead triode in V1 will do that and so will a host of other things. Shouldn't be to hard to nail down.

Low output could be only one power tube working. (Or other things) Make sure you have proper voltage on the power tube sockets AND make sure the pins are making contact with good tubes.
 

Wescaster

Gold Supporting Member
Messages
94
In the assembly manual there is a power on start up checklist. Are all the voltages correct?

I built one of these kits as well. My voltages all read just a little higher than those stated in the instructions.

I've got a few of the mods installed as they suggested. Also installed a PPIMV instead of the MV. It's a very, very loud amp.

Can you get better pictures around the input jacks?

Also, the screen resistors are only 2 watts, from memory. They should be at least 3 watts.
All my voltages are a bit high, but not bad. I will probably install the PPIMV as well. I just thought I'd build the kit with what they gave me first and modify it from there. I'll order up a couple of 3 watt screen resistors. Thanks


http://s971.photobucket.com/user/wesaasness/library/inputs?sort=3&page=1
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,057
Old test---as old as audio amplifiers at least---- that gives you your voltages AND determines if there is amplification in the various stages. Start out the output. When you test for voltage on the plates, you should hear a small Pop!. IF you are making your voltage chart, record the voltages. Then, move to the driver tube....the pops should be a little louder. Each stage you test on your way to the input preamp stage/s should yield louder pops than the previous test. The input preamp stage should yield the loudest Pops! When you fail to hear a louder pop in comparison to the stage you just left, there is a problem in that area.
Mods...there is no reason to think about mods until a circuit is fully and correctly functioning, imho and ime.
 

garret

Member
Messages
1,002
I'm also wondering why the power tubes won't bias to 38ma. My bias pot has plenty of range and have had three different sets of tubes in it that did require adjustment. That's a bit weird.

Also never see both tubes bias the same, always a off a bit by a 2-3ma.

The stock build has a very loose and wooly bottom end.

 

Wescaster

Gold Supporting Member
Messages
94
Nice work Garret. Looks pretty much like my build pics.. I think I even have the same carpet. :)
I emailed Tube Depot for the plans for a ppimv. It looks like I'll have to get a stacked pot.

Still no luck in my testing around with this. I have an old friend that used to work for NASA that was helping me trouble shoot. He's stumped at this point. He wanted to put in a balance pot to bring the two power tubes to match before biasing, but I don't think I want to do that.
 

zenas

Member
Messages
8,759
My first ex father in law worked for NASA. Trust me that doesn't mean the guy knew a dammed thing about vintage style tube amplifiers. More likely to tell you what an obsolete piece of crap it is and redesign the whole thing into something like a 60s solid state Silvertone. :)
 

garret

Member
Messages
1,002
Yeah, with start up after build its usually a small detail that could be the issue. You just have to go through every connection to verify all is correct.

Assuming you tried swapping out V1.
 

Wescaster

Gold Supporting Member
Messages
94
It's alive!!! As "Hacksaw pointed out".. The first resistor on the board was supposed to be 820 ohms and not the 820 k I had in there. I found a couple of other resistor mistakes. I think the main culprit was the the resistor across v 2. I now have plenty of output. I'll bias it tomorrw and continue to try to get inputs 1 active. Seems like the bass control is not especially effective. I removed the MV and ordered up a PPIMV kit from triode.
 

Wescaster

Gold Supporting Member
Messages
94
Update on the JTM 45+. I've had it running for a while now. Sounds great. I added a 22k resistor to the 47k at the bias and that put me right it the ball park. Installed the PPIMV. That seems to work pretty well, but when I turn it all the way up it sorta drops dramatically in volume at the last 5% or so. There are somewhat different layouts for this MV. The Tube depot version does not call for the 2.2m resistors. I did install those.
As for now I am using the amp and it's working out well. I'm sure I'll do some experimenting with 6l6s,kt66s etc. 34s for now. I got the two #1 inputs going. I just simply missed a wire. I'm not sure I'll have any need for those inputs. Very high treble content. Thanks for every ones help.
 

robrob

Member
Messages
409
Congrats on getting your JTM45 running.

Here's the cure for your loose bottom end:

https://robrobinette.com/5F6A_Modifications.htm#Lead_Channel

Since the only difference between the two channels is a bright cap it makes sense to voice the channels differently. This mod mimics the Marshall 1987 preamp lead channel and tightens up the overdrive tone. It also makes the channel more pedal friendly especially for gain, reverb and delay effects.
 

garret

Member
Messages
1,002
On this Tube Depot kit I installed a shared/split cathode switch, number 9 on the mods list in the assembly manual.

I also changed the bright channel coupling cap as stated in mod number 2. The bass was wooly and loose with the .022 cap.

Also modded for 2204 gain as stated in the schematic on page 34 of the assembly manual.

I would not recommend voicing the two channels too differently as it will have an effect on the tone with the channels jumped together.

I know opinions do vary on 4 holes but the tonal options with jumped channels is quite amazing.

It's a great sounding amp. With the 3 possible negative feedback options, SS or tube rectified and shared/split cathode all on switches you have even more tonal options.
 

robrob

Member
Messages
409
I would not recommend voicing the two channels too differently as it will have an effect on the tone with the channels jumped together.
It's a good effect, almost like having a third channel in between the "lead" and "normal" channel.
 

garret

Member
Messages
1,002
Rob, have any sound clips? Been to your site many times and appreciate all the work you've done.

I've got a couple of those mods you talk about but not all.
 

robrob

Member
Messages
409
No, no sound clips. I don't play well enough to post sound clips. I keep hoping to get a good player to come over so I can get some clips of all the amps and mods to post.
 




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