JTM-45 build help

[mteresko]

I got this completed today, and it worked great for 10 minutes! I started off at lower volumes, and then I turned it up to about 7, turned up the guitar vol, and it suddenly went dead with a blown HV fuse. I went back, removed all the tubes, re-checked my wiring, which seems to be fine. I went back through putting in the rectifier, checking voltages, putting in the pre tubes, and checking voltages again. Everything looked good at this point. I put in the power tubes, and when I hit stand-by, I got a very loud hum. I quickly shut it down, but the mains fuse had blown. More or less I am following the Metro instructions, with Heyboer iron. Any advice would be appreciated.

[img854]http://s19.postimg.org/6p74yym6r/jtm45_6.jpg[/img]

[img854]http://s19.postimg.org/8eb69wvkj/jtm45_4.jpg[/img]

[img854]http://s19.postimg.org/6xznrrsnn/jtm45_5.jpg[/img]

[xtian]

Out at a gig and can't offer advice but I can guarantee that it didn't fail because it's ugly.

Nice build, and welcome to TAG.

[Reeltarded]

Your amp does look very nice.

I can't see very well. I can't see anything out of place. There is a chance you have lost one of the pair of output tubes. That will make it hum pretty loud and depending on the failure it will take the fuse in a blink.

[pdf64]

Yes, nice work!
Suggest that the power tubes are removed and the voltage survey on p20 testing part 1 of the metro instructions is attempted.
Note them down and report back.
The JTM45 standby arrangement of hot switching the reservoir cap on to the rectifier is horrible, putting unneccessary stress on to the rectifier.
Best just to ignore standby, leave it in the operate mode and let the GZ34 do its thing and ramp the VB+ up slowly and smoothly.
Had you checked that the bias was suitable for the power tubes, ie what cathode current were they drawing at idle?

[Bob S]

What the guys above said. Yes.
Beautiful build. She'll be screaming in no time.
Oh & put a jumper wire across that standby switch.

[martin manning]

First time through you checked all voltages and idle bias current? Then after the failure everything looked good until just before the power tubes were reinstalled, i.e. you were able to switch from standby to run? If so, I'm with Reeltard on a shorted power tube.

[Littlewyan]

Beautiful build mate, well done.

I'm with Miles and Martin on this one. Failed Output Valve. As a test you can check the pins on the power valves to see if any are shorted using a meter. Another test if to turn every control to 0, put one of the valves in, turn the amp on and see if it still hums. If it does turn it off immediately, then try the other valve.

[mteresko]

First time through, I checked all the voltages, but didn't check bias current. Bias voltage seemed fine, so I left it all the way cold and couldn't resist giving it a try.

I will get some more fuses later, and recheck the voltages. As I only have the one pair of KT66s, would subbing 6L6s or EL34s pose a problem?

Thanks guys!

[mteresko]

I think you're right...power tube. I'm getting anything from 200 ohms to a dead short between pins 2 and 3 on one tube depending on how I tilt it.

[Littlewyan]

Thats the heater shorted to the anode. Odd. Is this only on one power valve? Definitely explains the humming.

Its the Anode Current you want to look at by the way not the bias voltage.

[martin manning]

You should be ok substituting 6L6GC's, but just to be sure what plate and screen voltages did you get, and what is the primary impedance of your OT?

[mteresko]

I was getting 420-422 on the plates and screens of both without the power tubes in place. Primary impedance is 8k.

[martin manning]

OK, you are fine using 6L6's, but I recommend miss-matching the OT by one click on your impedance selector, i.e. for an 8-ohm load, set the selector to 16 ohms so that you are running an effective 4k ohms. Bias them to about 45-50 mA.

EL34's will also work with the same miss-match to show them 4k. Bias them at ~40 mA.

[romberg]

Since both fuses have been taken out, I would fire it up with a light bulb current limiter when trying new tubes for a bit of insurance:

https://tubeamparchive.com/viewtopic.php?t=20341

Mike

[pdf64]

I think that sino KT66 tend to need a lot of bias voltage, so just setting it max cold might be insufficient.
Yes to the light bulb limiter; better late than never.