Tube Amp Archive

I can hear the "raggedness" you're talking about. It's minor, but I hear it. However, where it comes from.... could be anything. Could be the speaker, or the cabinet hardware, something in the amp construction, a tube that is microphonic, or some form of exotic distortion or oscillation. Heck, the ...

Been following your build. I think the clip sounds good, but you might be hearing something that is bothering you that I am not focusing on. Let me preface this by saying I am not a huge fan of Mojotone's layouts or choices. The voltages they show on the NC3015 schematic seem very high to me. I bui...

Finally got my Mojotone NC3015 build cleaned up and working well. I am hearing a distortion in the low end that isn't pleasant. Is this a typical EL84 tone or is there something I can do about it? It's one of those things that would probably disappear in a mix but I can hear it and it bothers me. I'...

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'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 wi...

Helmholtz wrote: ↑Thu Apr 03, 2025 9:51 pm Please measure plate and cathode voltages of all ECC83s.

V1a/b:
Plate - 105V
Cathode - 1V

V2a:
Plate - 114V
Cathode - 0.8V

V2b:
Plate - 187V
Cathode - 117V

V3a/b:
Plate - 199V
Cathode - 52V

And just because I measured them V4/5
Pin 3 - 11.3V
Pin 9 - 312V

Don't AC style amps typically have chokes anyway? Smaller/cheaper ones often don't. XD is supplying V1 and V2 and is drawing 3.5mA. XC supplies V3 and is drawing 1.3mA XB is supplying V4 and V5 and is drawing 12.9mA You can calculate actual tube currents from measured (!) cathode voltage divided by...

Well then he isn't a good tech. I can't say about the XA voltage because this depends on actual mains voltage, the individual EZ81 tube and PT tolerance. But the 2V drop across the 2.2k dropper is obviously wrong, so all following voltages will be off as well. Using the rules I gave above you can c...

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I like your amp voltages much better than the insanely high numbers in the schematic considering the EL84 plate and screen limits of 300V. To estimate the voltage drop across the 2.2k resistor: The typical current draw of an ECC83 gain stage is around 1mA, while the cathode follower here draws 4mA....

maxkracht wrote: ↑Thu Apr 03, 2025 2:13 pm Yes, like explained earlier. If it were mine, I would keep everything about the same, just move the two center tap wires.

Thanks man, I'll try that tonight. I appreciate it.

So you're saying connect the green/yellow to pin 3 of the EL84 sockets? Yes, though I would consider this the lowest priority of the things I mentioned. If there is noise related to your ground scheme, I would suspect the HT centertap (red/yellow) first. It should be connected directly to that cap ...

Looking at this again, it appears you have the neutral going to the fuse instead of the live. Mojotone diagrams aren't great with these kinda details... It's also a good idea to attach the live side of the fuse holder to the tip instead of the sleeve, so if the fuse holder is sitting open without a...

What is your ac heater voltage on each leg. 3.15 as shown? Do you have 6 volts ac on the rectifier? To end up with 370 vdc the PT will need to output about 420 vac, how does that check out I would also check that 130 ohm 5 watt resistor on the output tubes, does it test out less then 130? A long th...

The voltages on the schematic are not correct. Case and point, lets assume 4mA on XD for the four 12AX7s. Across a 22K, that is an 88V drop, not the 4V that is shown. So it appears your voltage drop on XD looks OK. Work the math and you may find they are OK. Yeah I was trying to figure out if that ...