JCM2000 DSL 50 low volume

[Ken Hall]

Hi,

I'm new to this forum so please be kind! I have a DSL50 which has low output volume. Any voltages that I have checked seem OK and the output tubes are OK. I get no volume plugging my guitar into the effects in jack. I have plenty of medical electronics experience but little tube experience. Any suggestions are welcome.

Thanks, Ken

[Enzo]

V4 is your phase inverter tube. That is the input to the power amp proper. The FX return is a good first test point in the middle of the amp, but let's go to the PI now.

Pin 7 is the input grid. With the amp running, stick your meter probe on pin 7. Leave the other probe not grounded, we want the meter probe to be a hum antenna. You could also use a small screwdriver or something. Does hum come out the speaker when you touch?

The signal to that grid comes in throough C28 from pin 1 of CON11. Touch the probe to CON11-1. Same result? You could pull the cable out of CON11 in case it is loading things down. Now probe the naked pin1 there. Hopefully this will tell you whether the rest of the power amp is working or if the problem indeed is further out into the power amp.

Have you tried a different 12AX7 in V4?

Your power tubes may be OK, but are the voltages OK on them? Do you have B+ voltage - 450-500VDC - on BOTH pin 3 and 4 of each socket? Power off - measure resistance to ground from pin 8 of each socket. SHould be 1 ohm. And you could also measure that from each oputside pin of the three bias setting test pins - the end pins are connected straight to the power tube cathodes. We want to make sure those 1 ohm cathode resistors are not damaged.

Likewise the PI tube V4. You have some sort of B+ at pins 1 and 6? And substantial voltage at the cathodes pins 3,8?

Now some low voltages. Your op amps run on +/-15VDC. Those two rails are provided by zener regulation. ZD1, ZD2. Look for a pair of 250 ohm 5w power resistors, the dropping resistors for the zeners. Got 15v on one end of each?

Look on the control board drawing upper right. See where it says MUTE between RL2b and RL3b contacts? Now look lower left on that page for JS1 (the input jack) and it says BEAD near it. See the cutout contact on the sleeve pin is also labelled MUTE? RL2b selects which POST control VR2 or VR4 controls your volume. When nothing is plugged into the input jack That MUTE point up by the relay contacts is grounded by the jack.

SO plug something, anything, into the input jack out front before plugging a guitar into the FX return. Just to be sure. And just in case, when your output is low, check with an ohm meter if that MUTE point is grounded up between the relay contacts.

[Ken Hall]

It was V4. I should have tried subbing it sooner, but when I traced the signal, it stopped there. New tube, all is well. Thanks for your detailed response. You certainly know your stuff.

Cheers, Ken

[plec.22]

Have the same thing amp works but not sounding as it should..fresh pair of power valves and set up to 40mv per side as did not want to run them at 20w per valve anyway..

The thing is, it's not as loud plus all preamp volts are ok, ht is 460dc pin 3 and 5 show good volts. on the clean channel you have to turn things up almost 6 per gain and volume using sine 1k at 100mv input it's doing say 18.23 volts out and watching things on the scope i did notice the bottom part of the wave almost rounding off but the top half showing good wave with out clipping. the tone stack setting are all sitting at mid point.

so I put amp in to s/by and plug the guitar in and off s/by but it does not sound loud as have to put the clean channel levels up to six...it's just lacking that grunt...

even checked the sent/return jack...

feel free to add comments

regards.