Mojotone NC3015 Debugging help

[_ej_]

EDIT: Per a suggestion further down in the thread I've decided to expand this a bit so I can have everything in one place.

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

• I deleted the Normal channel. This was accomplished by grounding the grid of V1A and removing the inputs and volume pot from the circuit.
• I tweaked the grounding scheme. I disconnected the bus bars from the back of the pots and grounded the can cap and PT centertap to the end of the bus.
• I shielded the wires going from the input the V1 and from V1 to the volume pot. I also moved the grid stopper closer to the tube socket.
• I replaced the amber lamp with a blue LED (https://www.amazon.com/dp/B01F1SDE4E?re ... title&th=1).

I have gone through the amp twice and verified it's wiring matches the layout with the exception of the modifications above. I've powered up the amp and it sounds fantastic however I have almost no headroom. From 0-1 on the volume knob I get clean volume but beyond that I just get overdrive. I've checked the voltages and everything is reading low when compared with the schematic. I've confirmed the values of the voltage divider resistors in the B+ circuit are correct.

My question is do we think the schematic voltages are correct and I have an issue somewhere or is the schematic wrong

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

V2a:
Plate - 114V
Cathode - 0.8V

V2b:
Plate - 187V
Cathode - 117V

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

V4/5
Pin 3 - 11.3V
Pin 7 - 338V
Pin 9 - 312V

B+ Circuit Voltages:
XA: 351V
XB: 312V
XC: 264V
XD: 186V

I've attached the schematic below.

NC3015.png

Here are gut shots:

[Stevem]

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 these same lines how does that cathode voltage read at that resistor, more then the 13.7 the schematic shows?
If so then the output tubes are drawing too much current which will pull down the power supply voltage.

If it reads more then 2 volts above that 13.7 then I would change out that 130 ohm 5 watt to a 150 ohm, or add a 20 or 25 ohm 5 watt in series with that 130 ohm.

If all of these check out I would then purchase another 6CA4 / EZ81 rectifier, and I would buy a N.O.S one from Antique electronics .
Even NOS there only 25 bucks.

[Astronomicum]

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.

[_ej_]

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 was the case. I calculated the current draw for the first voltage drop and it would be like 0.4mA across that 2.2k resistor.

What are some good rules of thumb for current draw? Just look up the specs for the tubes and assume they're drawing full current?

[_ej_]

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 these same lines how does that cathode voltage read at that resistor, more then the 13.7 the schematic shows?
If so then the output tubes are drawing too much current which will pull down the power supply voltage.

If it reads more then 2 volts above that 13.7 then I would change out that 130 ohm 5 watt to a 150 ohm, or add a 20 or 25 ohm 5 watt in series with that 130 ohm.

If all of these check out I would then purchase another 6CA4 / EZ81 rectifier, and I would buy a N.O.S one from Antique electronics .
Even NOS there only 25 bucks.

Heater voltage was a little high last time I measured it, around 6.7VAC.

I'll check the AC rectifier voltage.

All the tube voltages were low compared with the schematic. Around 1/2 the voltages written on the schematic for the preamp, less so for the poweramp but still low.

Why would the rectifier be an issue with the voltage coming off the rectifier is correct?

[Stevem]

Simply because the recto may be weak.

[Helmholtz]

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.
This adds up to 9mA.
Adding the EL84 screens currents of 5mA (11% o plate current) each makes 19mA.
So the voltage drop should be roughly 19mA x 2.2k = 42V.

[_ej_]

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.
This adds up to 9mA.
Adding the EL84 screens currents of 5mA (11% o plate current) each makes 19mA.
So the voltage drop should be roughly 19mA x 2.2k = 42V.

Do you think the schematic is wrong? I asked the Mojotone tech line about my low voltages and they said the schematic is correct.

The way the amp is operating now I have basically 0 headroom. From 0-1 on the dial I get clean volume but beyond that it gets no louder just more distorted. It doesn't seem as loud as AC15s I've played on the past, I can comfortable stand on front of my 412 with this thing cranked.

I'm going to try and see if any tube is drawing excess current and dropping my voltage.

I get what you're saying about the screen voltages looking better, but the voltages at the plates and cathodes of the 12ax7s are about half the values listed on the schematic.

[Helmholtz]

I asked the Mojotone tech line about my low voltages and they said the schematic is correct.

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 calculate the lower preamp voltages.
You could increase preamp voltages by lowering dropper values but the most effective measure would be replacing the 2.2k dropper by a 3H or 5H choke.

Do you have a scope and sig gen?

[_ej_]

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[_ej_]

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 calculate the lower preamp voltages.
You could increase preamp voltages by lowering dropper values but the most effective measure would be replacing the 2.2k dropper by a 3H or 5H choke.

Do you have a scope and sig gen?

Not at home but I have access to some very nice ones at work.

Don't AC style amps typically have chokes anyway?

So I calculated my current draw based off my measured voltages.

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

Does any of that seem out of the ordinary? You mentioned the EL84s should only be drawing 5mA.

I'd like to be confident that the amp is working correctly before I go modding. I've been down the path of modifying to fix issues and it made debugging even harder.

[Helmholtz]

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 cathode resistor.
Using schematic voltages I get V1A+V1B = 1.3mA, V2A : 0.9mA, V2B : 3.7mA, adding up to 5.9mA for XD.
So if schematic voltages would apply (which they don't), the 22k should drop 130V.
(remember 1mA drops 1V across a 1k resistor)

XC supplies XD plus around 1.4mA for V3.

XB supplies XC and XD plus the power tube screens.

You mentioned the EL84s should only be drawing 5mA.

Yes, around 5mA per screen.
EL84 plate currents supplied from XA.

[_ej_]

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 cathode resistor.
Using schematic voltages I get V1A+V1B = 1.3mA, V2A : 0.9mA, V2B : 3.7mA, adding up to 5.9mA for XD.
So if schematic voltages would apply (which they don't), the 22k should drop 130V.
(remember 1mA drops 1V across a 1k resistor)

XC supplies XD plus around 1.4mA for V3.

XB supplies XC and XD plus the power tube screens.

You mentioned the EL84s should only be drawing 5mA.

Yes, around 5mA per screen.
EL84 plate currents supplied from XA.

Understood. I calculated the draw at each node using the voltage drop and resistors between the nodes. So from XC to XD I measured a 78V drop, using the 22k resistor I get 3.5mA. so that's the wrong way to do it? I really need to actually read the theory book I bought.

So the values you calculated seem correct for the circuit? Would any of them point to an issue? Like I said it seems like the amp isn't quite operating correctly

[Helmholtz]

Please measure plate and cathode voltages of all ECC83s.

[_ej_]

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