Another Dual Lite build thread - half working

[choosebronze]

Checked all voltages. Yes, I see DC on the AC15 channel. Readings are lower than the original throughout the amp, I think bc the PT's only giving 280vac, but the voltages are all fairly close everywhere. That all looks good. Completely rewired the EF86 socket. Still nada.

Just trying to learn what kind of readings are normal, I probed Pin 9 (input) of the EF86. Got a hum through the speaker which was controlled by the Vol/Brill/Cut pots on the AC channel. Ok, that means if signal gets to the tube, it'll get through the rest of the circuit, right? I bypassed the switch and connected the input jack directly to AC15 input resistor. Still no sound. Replaced the resistor, no dice. It feels like I'm close, but I must be wrong.

[sluckey]

I probed Pin 9 (input) of the EF86. Got a hum through the speaker which was controlled by the Vol/Brill/Cut pots on the AC channel. Ok, that means if signal gets to the tube, it'll get through the rest of the circuit, right? I bypassed the switch and connected the input jack directly to AC15 input resistor. Still no sound. Replaced the resistor, no dice. It feels like I'm close, but I must be wrong.

I agree, the problem seems to be between the input jack and the grid of the EF86. Next thing to do is replace that shielded cable. I would just use a regular hookup wire to test. If the amp comes alive then replace with shielded wire.

BTW, looks like you may be using RG-174 like Doug sells? The plastic insulation for the center conductor is very fragile and will melt very quickly from too much iron heat. You have to be aware of this fact and get the iron in and out quickly and don't let the cable move until it is cool. It's also a good idea to not have any bends in the center conductor that puts tension on it while soldering. The insulation is almost guaranteed to melt and expose the wire in this case. I hate RG-174 just because of the low melting temp of that plastic insulation. I switched over to RG-216 a long time ago just for this reason. It has teflon insulation. Cannot melt it with a soldering iron.

[labb]

Steve will get you to the problem if you hang with him. If I had this problem I would build be an audio probe, plug it into the output of a CD player and use the probe to inject the signal into the circuit starting at the PI and working back to the input until I found where I was loosing the signal. P.S. one whisker from the shield of the RG-174 can ground out your signal.

[choosebronze]

Thanks for confirming Sluckey. There's a local store that I bet sells RG216. That's dual shield cable right? Do I just cut back the outer shield and leave it, and then ground the inner shield like I have been w/ the 174? Maybe it'll be obvious once I see it.

[sluckey]

Not saying the RG-174 is your problem. But you've replaced everything else around that EF86 with no joy, so it's time to replace the RG174. I would temporarily replace with common hookup wire to see if it fixes the problem. Then go get some RG-216 at your leisure. And of course you could just use another piece of 174, just be careful with it. Heck it's working fine in the other channel. And I do have lot's of 174 in earlier projects so I'm not totally poo-pooing 174. Just bragging on 216!   

This is RG-216. Same size as RG-174, but tough as nails!

[PRR]

> temporarily replace with common hookup wire

+1

Signal with a little hum or squeal beats heck out of no signal at all. And confirms/denies the bad-cable diagnosis.

[choosebronze]

You're geniuses! Connected the input directly to Pin 9 and it worked. Sounds great. There's a local store that sells most kinds of cable by the foot, so I can just go buy some RG216 and redo everything, I'd rather have the peace of mind.

On the MV wiring, I disconnected the shielding from ground, that brought everything back to full volume. I gather I wasn't supposed to have the shields grounded, though I'm not sure why. I'll let you guys know once it's all wired, but thank you, thank you, thank you, it seems like everything is on the right track finally.

[choosebronze]

The store didn't have RG216 so I tried again (carefully) with 174 and the amp is up and running!! This thing has a ton of gain. I'm excited to play some crunchy tunes. Thank you so much for all the help, especially you Sluckey. I'm off to go play guitar but I'm sure I'll be back soon if something goes wrong. 

[sluckey]

It's a fun little amp. Fun to build and fun to play! Still one of my favorites. I seem to prefer the Vox channel. Glad you got it sorted out. Hope you have fun with it.

[choosebronze]

I finally had the time to get this chassis into an enclosure and I just wanted to share the result. This was the first time I've used fabric (inspired by Benson Amps) instead of a tolex-like material. Fabric was so much easier! I don't think I'll ever go back to tolex. Thanks again Steve for this great project. It was a lot of fun and sounds great. And saved me from having to build an AC15 and an 18w =)

[PRR]

> I've used fabric

OW!! My Eyes!!!

Good thing you made the biggest 4-8-16 flag I have ever seen.