Marshall JTM30 mystery

[Pedro Vecino]

the clean channel sounds great and loud, but the boost channel has very low output even on 10 its prolly equal volume to the clean on 4.. it didn't use to be like this and i have no idea what caused it..

The memory that I have of the JTM30īs (fifteen years ago or more) is that in addition to a complicate bias adjust (not adjustable, zener diodes, location with difficult access..) are amplifiers very ineffective (or at least relatively ineffective) in relation to his power amp (2x5881 or 2xEL34).
You talk about the boost channel with volume at 10, but says nothing of the gain.
Adjust the overall master to 10, adjust the gain of the boost channel to 8, for example, and compare volumes between channels.
If you find balance, then you can consider it as good. At least in that aspect.

[Steve A.]

oh wow i had no idea a different switch had to be used...i thought all it took was wiring in series and moving some wires that go to the switch around..

With 230VAC you have two hot leads (120VAC to ground) so for safety reasons you would want a double pole switch.Practically all of the switches I have are rated for 250VAC so I don't think that is the issue.

I have the 1- 12" version of this amp and there are fuses on the PCB to protect the power transformer. One potential problem with these amps is that the tubes are mounted horizontally. When I got a Marshall Power Brake the first thing I did was set it to the lowest volume and set all of the controls to 10. After 10 minutes of some incredibly rude sounds it died.One of the power tubes shorted out and took the HT fuse out, too. The power tubes were a particularly short 6L6*** and I could not find any tubes short enough to fit behind the expanded metal tube cover so I whacked it with a hammer until it did fit. Bad choice- I didn't realize that you could buy expanded metal at a hardware so I should have bought some and made THAT fit while leaving thd factory tube cover intact. D'oh!

Steve A.

*** I am pretty sure that they were not 5881's although they would have been a good no-hammer-needed replacement...

[J M Fahey]

complicate bias adjust (not adjustable,

Agree that adjusting something not adjustable can be a bit complex
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sorry, couldn't resist ......... said the 1k 5W ceramic

[jtm310]

Hey Pedro I tried the gain at all levels but the volume drop is still there..it used to be as loud as the clean channel.. Did some chopstick poking today but can't locate a cold joint or anything.. Really stumped at this point..

[jtm310]

I was trying to measure bias after replacing the 9.1v zd with a 15v nor as many have suggested..can anyone tell me what to do after these steps?

1) measured plate voltage (pin3 to ground) - 425V
2) when amp was off, took resistance between pin 3 and centre tap - 73.5ohms
3) max plate diss of the 6l6wgc - 70% Of 30w = 21W
4) 21/425 = 0.0494 (49.4mA)
5) voltage drop across pin3 and centre tap - 2.9v and 3.15v for the other side

As you can see I'm avoiding measuring any currents as I'm not confident enough to not blow out my multimeter, but I do take all precautions and drain all caps every on/off cycle..thanks!

[Pedro Vecino]

In my experience 50mA is a excessive dissipation at idle on that model. The ventilation capacity is very limited. Better to use a value not exceeding 35mA. to prevent premature problems.
I've never used the shunt bias metod. For me is best to use a socket to read the current through the cathode or plate. You could also (this is more delicate) inserting an ammeter between the pin 3 and the wire connected to the transformer repeating the process on both tubes to check the symmetry.
In relation to the lack of volume in lead channel, this circuit is made around the first 12AX7 (both triodes). Check the condition of the V1 12AX7, plate resistors, potentiometers and connections.
The clean channel starts in V2 first triode and after that, a relay selects one of the two signals to inject to V2b. Thereafter, the tone stack and driver circuit.
Exclusive to the lead channel is only the first 12AX7.

[jtm310]

thanks for the advice Pedro! I too think that 50mA is pretty high but i could only get 30w 6l6wgc's..which makes the max plate dissipation 21w..

i found the page i read about the transformer shunt method: 50 Watt biasing technique

If i understand it right, I have all the readings I need? Voltage drop needed for max plate dissipation: 0.0494 X 73.5 = 3.63V

Current voltage drop across center tap and plate: 2.9 and 3.15.. meaning i have it underbiased a little?


Also, not much luck with the volume drop after swapping preamp tubes and contact cleaning all jacks and joints..guess might have to blindly reflow suspect-looking joints..

[jtm310]

found this The Last Word On Biasing

and i think i may have found the safest and fairly accurate method..

so basically i take the voltage drops i measured (2.9 and 3.15V) and divide them respectively by the ~75ohm resistances i measured and get ~0.04A or 40ma per tube...

Am i understanding this right?

[Jazz P Bass]

40 ma's is crazy for a 6L6 tube.

[jtm310]

is that the norm though for jtm30 owners who replaced the 9.1V zener with a 15V? Since from the factory they were biased too hot..

[Amp Kat]

Imo the best way to bias as also mentioned above is to use a precision resistor of 1 ohm across the tube socket pin 8 to ground and read across it. A bias meter is also a very good investment to make that will eventually pay for itself and get one that reads current and preferably wattage. I like Randall Aikens work but 70% is not written in stone and can be excessive for that amp. I like to bias my amps just out of crossover and as low of a percentage as possible to save them as much as I can. If you use powertube distortion there could be benefits to a hot biased amp but in compromise to burning up the tubes faster. If you are using preamp distortion or playing clean there is no advantage in tone to biasing them hotter.

Do whatever you have to and get the negative grid bias voltage range so you can bias it wherever you want if that calls for a larger zener then by all means do it.

[Steve A.]

As you can see I'm avoiding measuring any currents as I'm not confident enough to not blow out my multimeter, but I do take all precautions and drain all caps every on/off cycle..thanks!

I damaged a Radio Shack DMM about 15 years ago by measuring B+ to ground. At least I think that was what caused the problem. After it worked okay for most of the other ranges but was way off in measuring B+. (I think it had to do with the AC audio signal on the tube plates confusing the meter.)

[felixq78]

Hi all,
I bought a JTM30 2x10 recently from eBay and though the seller seems reliable the amp came DOA. My repair person says it's the transformer but the seller maintains that it was working before it left his house.

Here is what it is doing: Power lights up on front, small glow in the power tubes, no tubes will get warm at all, no sound whatsoever.

The question is, could it be something other than the transformer causing these symptoms and could it have happened in transit like the seller states?

If it turns out this way, can anyone recommend where I can buy a reasonable priced replacement transformer for the JTM30?

Thanks,
scr@tchy
URL="http://www.daughtersandsons.net"]www.daughtersandsons.net[/URL] -hear my band!

Same as the JTM60 the JTM30 will just keep blowing power transformers, they have an inherent design fault; NO BIAS POT. This makes them run hot then BOOM ! But don't let this perturb you . There is a simple mod that will clear up this problem. My local amp tech "Valve Heaven guitar amp repairs Sydney" added a bias pot and did some other trick mod and I'm still getting that magical Marshall tone without replacing power transformers every month.