18Watt.com

Hi guys, would appreciate your expert opinions here. I have completed this 36W Build and am going through my initial startup process. It is basically a Ceriatone 36W layout with GDS Transformers. Before plugging in the rectifier tube I was double checking continuities and I have continuity between the tip and sleeve of every output jack (!?!?!?). I have quadruple-checked everything in the build, gone through the schematic and layout with a highlighter twice each, including the NFB circuit which is connected to the 16 Ohm lead. I suspect a faulty OT. I was checking through my emails with Graydon and remembered something odd -- when I was inquiring about the secondary wire colors (which I believed was Black = Common, Green = 8 Ohm, Yellow = 4 Ohm, Orange = 16 Ohm and Graydon sort of confirmed that by his memory but not definitively), and at that time, when I was first installing the OT onto the chassis, I had the following readings when I tested against black: Green 0.4 Ohm, Yellow 0.4 Ohm, Orange 0.4 Ohm. That was with the primary leads soldered into the circuit and the secondary leads all free. This seemed odd to me at the time but I forgot about it until just now. Assuming my wiring otherwise is correct, would this indicate a faulty OT?

Thanks in advance!

Here is a picture of the speaker jacks wiring. I have yellow to 4, green to 8 and orange to 16. Black goes to common which is the bare wire across on the chassis side of the cliff jacks.
The green wire on the right side of the common goes to my star ground.

That's not really an indication of anything. I have a brand new GDS 18W OT, and I did the same test. I got around .9Ω on all 3 (not connected to anything).

The one bad OT I ever had, tested as expected on primary resistance. So that was of no help. Continuity testing, especially when related to transformers, is often not helpful at all.

Did you have a cable plugged in when you did your continuity test?

JMPGuitars wrote: ↑

Fri 08/07/20 4:12 pm

Did you have a cable plugged in when you did your continuity test?

Same result with cable in any of the 3 jacks and also with no cable plugged in at all.

crgfrench wrote: ↑

Fri 08/07/20 4:13 pm

JMPGuitars wrote: ↑

Fri 08/07/20 4:12 pm

Did you have a cable plugged in when you did your continuity test?

Same result with cable in any of the 3 jacks and also with no cable plugged in at all.

Verify the jack wiring, and plug cables into all jacks at once. If it still happens, then disconnect the OT from ground and check again. It's possible it's similar to testing CT'd heater wires when it's not on. You'll get continuity to ground, but obviously should not test that with it powered on.

Nevermind -- sorry for the stupid question, I'm seeing 0.3 Ohms and that is normal. Thanks for the feedback!

Craig, I have a question about them 3 inputs wired separately to each tap. On my TMB tremolo I did the same with only the 4 and 16 ohms taps. 2 inputs... but now I have built my 2x10 extension cab at 16 ohms I was afraid of having mismatched impedances. I guess when you plug 2x16 ohm load in parallel ut becomes a total load of 8 ohms. So I rewired it with the 2 inputs in parallel onto a dpdt switch with on 1 side 8 ohms and the other side 16. So I just have to switch between the 2 , depending on running with or without extension cab.
How will you run an extra cab? Or 2? Is it healthy for the OT to feed 2 or 3 sec. windings simultaneously? I'm a noob on that subject. Thats why I made sure my wiring is correct the way I know it...

Note: I wired the cab for 16 ohms because my modern classic has the 3 way rotary impedance switch.

Bieworm wrote: ↑

Sat 08/08/20 4:27 am

Craig, I have a question about them 3 inputs wired separately to each tap. On my TMB tremolo I did the same with only the 4 and 16 ohms taps. 2 inputs... but now I have built my 2x10 extension cab at 16 ohms I was afraid of having mismatched impedances. I guess when you plug 2x16 ohm load in parallel ut becomes a total load of 8 ohms.

The point of the wiring method he used is to avoid a switch. He's still only going to use one impedance jack at a time.

Speaker load nominal resistance works the same way any resistance is calculated. 8 + 8 in parallel = 4Ω. 8 + 8 in series = 16Ω.

Thanks,
Josh

JMP is correct -- this is going in a 1x12 combo with an 8 Ohm Celestion. Theoretically I probably should just hardwire the 8 Ohm tap directly to the speaker. My general preference would be a 3-way rotary switch with a main and an external jack. But this chassis and faceplate had the 3 individual holes already there and labeled. So in the spirit of Laze, I left it that way and will simply run the speaker wire from the 8 Ohm jack to the speaker. The next combo I will probably use speakon connectors though for extra protection of the amp.