JTM-45 build help

[mteresko]

I've got EL34s in now, impedance selector at 16, and not blowing fuses.
[img:667:162]http://s19.postimg.org/p0ldx05hv/voltages.png[/img]

These numbers seem pretty good to me. However, at 16 ohms into an 8 ohm speaker, I get an extremely fuzzy and low amplitude signal out. I biased to 38mA, which is as high as it would go (replace the 68k fixed resistor with something smaller?). Switching the impedance to 8 ohms restores the normal tone and volume, although with more hum than I would like. I got the same results with 6L6s.

A light bulb current limiter is on my to do list...

[xtian]

You could easily have burned up your screen resistor(s) when the power tube(s) went.

[martin manning]

3V drop from pin 6 to pin 4 on V4 looks ok, but an increase of 2V on V5 does not. Are those measurements correct?

4k should be a very reasonable primary impedance at 400V. Are you sure you have the OT's secondary taps identified and connected correctly?

[mteresko]

Checked again and V5 drops 2.5v from pin 6 to pin 4.

In checking the impedance switch wiring, I noticed that I was getting .5 ohm or less between each of the 3 switch terminals and ground. Here's the wiring diagram I'm working from:

[img1054]http://s19.postimg.org/sgdi6k2qb/HTS_70 ... M45_OT.jpg[/img]

[img896]http://s19.postimg.org/5g6uu84wj/impswitch.jpg[/img]

It's hard to see in the pic, but the center terminal is completely clear of the terminal behind it.

I pulled the wires off the switch and I've got .4 ohm or less across each of the 3 secondary windings. Is that normal? It seems low.

[martin manning]

Yes, the low resistance of the secondary windings is normal, as they are a relatively few turns of heavy wire. Your connections look correct as long as the center terminal of the impedance switch connects to the terminal with the brown wire on it when you select 16 ohms. One possible explanation for the low volume and distorted sound is that you have an oscillation when using the 16 ohm tap.

[JazzGuitarGimp]

Yes, the low resistance of the secondary windings is normal, as they are a relatively few turns of heavy wire. Your connections look correct as long as the center terminal of the impedance switch connects to the terminal with the brown wire on it when you select 16 ohms. One possible explanation for the low volume and distorted sound is that you have an oscillation when using the 16 ohm tap.

Or perhaps the 8R brown-green tap is toast.

[mteresko]

Got it working tonight! I contacted Heyboer asking about the possibility of OT damage, and they informed me today that the wiring diagram I was supplied with had the brown and green wires reversed, so at the 16 ohm setting it was bucking the other windings! Put in the new KT66s I got today and everything checked out great. Sounds awesome! Thanks to all for your help.

[JazzGuitarGimp]

Congrats! (Imagine a thumbs-up emoticon here)

[mteresko]

Thanks, Lou...btw, should I be concerned about the high heater voltage? It's down slightly (3.4) with the new tubes in place.

[JazzGuitarGimp]

I'm probably not the best person to answer this question, but I _think_ I've read +/-10% is acceptable. So 6.93V max and you're at 6.8V. I suspect this is okay. But you could always add a 1% resistor in each leg of the heater string. If you can tell me the type and quantity of tubes and pilot light you're using, I could help calculate the resistance and power rating of the resistors to add.

[martin manning]

I'm in the habit of checking the phasing and voltage ratios on new transformers because wire color code errors do happen sometimes.

How does it sound with an 8-ohm load on the 16-ohm tap (a 4k primary impedance)? 8k is quite high, and will result in higher plate voltages and screen currents being produced in the output tubes. Perhaps that contributed to the tube short you experienced. In the long run using the 6k6 primary taps would be safer in my opinion.

I would not worry about the heater voltage if it is inside the +/- 10% spec.

[ValveStorm]

Interesting! Given that they amp worked for several minutes, do you guys think the mis-wired OT (and impedance settings) could have caused the tube short?

Got it working tonight! I contacted Heyboer asking about the possibility of OT damage, and they informed me today that the wiring diagram I was supplied with had the brown and green wires reversed, so at the 16 ohm setting it was bucking the other windings! Put in the new KT66s I got today and everything checked out great. Sounds awesome! Thanks to all for your help.

[martin manning]

I don't think the 16-ohm tap was used until I suggested miss-matching the load to get a 4K primary impedance for 6L6's. If that is the case then it wouldn't have had any effect.

[mteresko]

I don't think the 16-ohm tap was used until I suggested miss-matching the load to get a 4K primary impedance for 6L6's. If that is the case then it wouldn't have had any effect.

That's correct. The tube failed using the 8-ohm tap before the 16-ohm was tried.