Mojotone NC3015 Debugging help

[Astronomicum]

I see your power tube dissipation is much more reasonable than that given on the schematic. 13.7V across 130 ohms is 105mA and with a plate voltage of 372V, that is 19.6W per tube! Now that's cooking! Your numbers show 86mA @ 312V; 13.6W per tube. So far, I am not seeing any of your numbers indicating that your problem is caused by voltage. I would not trust any voltages on that schematic. I would cross out every voltage and record your own, and add the AC voltage feeding the rectifier, then go from there.

[B Ingram]

I've got a Mojotone NC3015 build up and running. I been doing my initial checks and the tube voltages all read low when compared with the schematic. ...

You seem to have 4 or 5 threads about this same amp. Some have various details about "scrounged transformer" and "circuit mods."

It is probably wise to keep all discussion in a single thread so the folks offering help will know whether the circuit matches the schematic, or changes have been implemented, or transformers use are not what Mojotone specified, etc.

[_ej_]

I've got a Mojotone NC3015 build up and running. I been doing my initial checks and the tube voltages all read low when compared with the schematic. ...

You seem to have 4 or 5 threads about this same amp. Some have various details about "scrounged transformer" and "circuit mods."

It is probably wise to keep all discussion in a single thread so the folks offering help will know whether the circuit matches the schematic, or changes have been implemented, or transformers use are not what Mojotone specified, etc.

Fair enough, I'll make this the running thread.

[_ej_]

I expanded the detail in the first post. I'm left with a couple questions at this point

• Does it seem like the lack of headroom an issue somewhere I've missed or is the amp working as intended? I really like the tone of the amp but I feel like its significantly quieter than other AC15's I've played. I'd like more clean volume out of it.
• Do the voltages I've listed for the preamp tubes seem appropriate? I've been trying to track down an AC15/30 preamp with voltages listed to compare to but I've had no luck.
• There is a hum that is introduced by the Top Cut knob. As you turn it clockwise it increases the top end and also adds hum. Any ideas on how to eliminate that?
• Finally there is just a constant hum when the amp is on, even with a guitar volume turned off. I tried the chop stick test but none of the leads I move affected the noise. That was before I updated the grounding so I am going to try it again. However is there any improvement to the grounding that might help reduce the noise floor.

[B Ingram]

I expanded the detail in the first post. I'm left with a couple questions at this point

• Does it seem like the lack of headroom an issue somewhere I've missed or is the amp working as intended?

I've got a Mojotone NC3015 build up and running. The circuit largely matches the schematic/layout given by Mojotone with a couple changes.

• [li]I deleted the Normal channel. This was accomplished by grounding the grid of V1A and removing the inputs and volume pot from the circuit. [/li]

... I have almost no headroom. From 0-1 on the volume knob I get clean volume but beyond that I just get overdrive. ...

Thank you for the large photos, they helped confirm the issue. You likely need only ground the unused channel-mix resistor (220kΩ) at the circled turret in the photo below.

- The stock circuit reduces the signal-level of each channel by about half due to the interaction of the 220kΩ channel mix resistors.
- The output of the Top Boost channel is normally reduced via the voltage divider these mix resistors form; the path of this divider is highlighted in the schematic below.

- When you deleted the Normal channel Volume but kept everything else, you removed the ability of the channel mix resistors to reduce signal-level.

You should be able to run a single wire form the turret to the ground buss, and reduce the excessive signal level.