Amity Amp Nutshell Audio

[desperateaudio]

Has anyone built the Amity Amp from Nutshell Audio? I am building it now with 2A3's and I have one complete and on the attached schematic if you look at the 48v at the CT of the filament transformer for the pair of 2a3's i am getting 150v to ground across the 420 ohm and not 48v. I have changed the tubes, changed the filament transformer and checked and rechecked the circuitry i have wired up and it matches the attached exactly. Can anyone shed light on why they think this could be happening.

Thx

[Gordy]

I've no experience with it, but I would start by checking the high V to see if it's the 320V stated on the diagram. Also, if you inset a 1 R resistor in the high V line you can use it to see if your output is drawing the stated 115 mA current (I = V / R).

Also, we've all mis-read resistors, so it's worth a quick check to make sure it really is 420 Ohms.

[DougL]

You are using the Angela 385-0-385 transformer?
What is the plate voltage?
Is the Secondary of the interstage grounded?


You might also PM Lynn Olson, who is a poster here.

[Palustris]

There is something very wrong with either your measurements or the circuit. There is no way that a pair of 2A3s can pass 357mA. They would self destruct instantly and would need a B+ of over 600V to achieve the momentary burst of annihilating current.

I suggest that you retest the cathode resistor for resistance to ground (i.e., make sure it is really grounded), the B+ voltage and the Vk-p of the output tubes.

[desperateaudio]

Originally Posted by Gordy

I've no experience with it, but I would start by checking the high V to see if it's the 320V stated on the diagram. Also, if you inset a 1 R resistor in the high V line you can use it to see if your output is drawing the stated 115 mA current (I = V / R).

Also, we've all mis-read resistors, so it's worth a quick check to make sure it really is 420 Ohms.

resistors all checked twice including dropping resistor.

[desperateaudio]

Originally Posted by DougL

You are using the Angela 385-0-385 transformer?
What is the plate voltage?
Is the Secondary of the interstage grounded?


You might also PM Lynn Olson, who is a poster here.

no on the angela transformer but I have the proper plate voltage so that should not matter.

yes the secondary is grounded.

[desperateaudio]

Originally Posted by Palustris

There is something very wrong with either your measurements or the circuit. There is no way that a pair of 2A3s can pass 357mA. They would self destruct instantly and would need a B+ of over 600V to achieve the momentary burst of annihilating current.

I suggest that you retest the cathode resistor for resistance to ground (i.e., make sure it is really grounded), the B+ voltage and the Vk-p of the output tubes.

my measurement is being taken from the top of the 2a3 cathode resistor at the center tap of the heater winding and the resistor that i have been using for the mock up is a 560 ohm 25 watt resistor so i should be passing less not more. I have changed the resistor twice and problem still remains.

i will check the others thanks,

[Palustris]

Originally Posted by desperateaudio

I have the proper plate voltage

If you have the proper plate voltage then there is no way that the tubes are conducting 357mA. You say you have checked twice, but have you really done full resistance and voltage measurements from, say, pins 1 and 4 of both 2A3s to ground? Have you checked from the ctr tap of the filament transformer to ground (not to the ctr tap of the IT, but to ground)? The point is that there are many components that are interrelated in that circuit and just slapping an ohm meter across the cathode resistor only tells a small part of the story.

[desperateaudio]

Originally Posted by Palustris

If you have the proper plate voltage then there is no way that the tubes are conducting 357mA. You say you have checked twice, but have you really done full resistance and voltage measurements from, say, pins 1 and 4 of both 2A3s to ground? Have you checked from the ctr tap of the filament transformer to ground (not to the ctr tap of the IT, but to ground)? The point is that there are many components that are interrelated in that circuit and just slapping an ohm meter across the cathode resistor only tells a small part of the story.

i will recheck and report back thx

[rdf]

Also try lifting the 40 uF Ultrapath cap, it might be passing DC.