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theoman

Moth Audio 45

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Okay, I read through the audiokarma thread.

 

The one guy asked to pull the rectifier tube, (GZ34) to see if the hum/buzz went away. I recall you mentioned that you did that and the hum went away.

He mentioned that if the hum went away, that it's possibly a wiring/grounding/layout issue. Judging by the way the amplifier is laid out and wired, it's a bit of a mess and wouldn't doubt that the wiring/grounding could be the problem.

 

I can't recall if the Moth SET amplifiers of the day had kits, like Welborne labs. It appears to be wired like a kit, or other owners may have added different parts. (The Jupiter coupling caps were not around when this amp was new I'm guessing.)

The output tubes, (2A3) are indirectly heated with DC, using some sort of voltage regulation. I have no idea if the pots are for hum nulling or not. Did you happen to see if those pots changed in the millivolt range when your turned them?

 

Sometimes triode output tubes get a bit noisy getting to the end of their life as the emission lowers, that's why I asked if you had a way to test them. Even the driver tubes and rectifier could be tested. A guy can test them in circuit by confirming proper voltages/current, but one will have to probe around in a powered amplifier.

The amplifier is a bit older, and there may be a dried up cap or two, but I don't really think that's the issue...maybe part of it...

 

Without a schematic or some sort of manual be it assembly or operating, it's a bit of a chore to figure out the circuit from pictures of underside wiring, how it works, etc...

 

There a forum member named Eric Mandaville (I hope I spelled his last name correct) who I'm pretty sure has a Moth audio amplifier...maybe it's pre-amp. He'd be the guy with most knowledge about Moth audio around here I would think.

 

Maybe find the schematic or a manual and befriend someone locally who happens to be into tube audio, either a tech or enthusiast who can look over the amplifier.

You can learn quite a bit that way.

 

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Here's from Craig at Eddie Current:

All Moth amps had manuals with schematics. If you got the amp it should have been sent in a wooden crate with the manual. That amp is 17 years old and it is likely the electrolytic caps are dried out which would account for the buzz. I don’t have any documentation on the Moth amps, and don’t work on them.

You need to measure the bias resistor to see what the bias voltage should read. The 45 tubes need about 36mA, so if the resistor is 1 ohm the meter should read 36mV. Make sure you have the correct tubes, I have not seen a 45 tube that looks like that.

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1 hour ago, mike stehr said:

Okay, I read through the audiokarma thread.

 

The one guy asked to pull the rectifier tube, (GZ34) to see if the hum/buzz went away. I recall you mentioned that you did that and the hum went away.

He mentioned that if the hum went away, that it's possibly a wiring/grounding/layout issue. Judging by the way the amplifier is laid out and wired, it's a bit of a mess and wouldn't doubt that the wiring/grounding could be the problem.

 

I can't recall if the Moth SET amplifiers of the day had kits, like Welborne labs. It appears to be wired like a kit, or other owners may have added different parts. (The Jupiter coupling caps were not around when this amp was new I'm guessing.)

The output tubes, (2A3) are indirectly heated with DC, using some sort of voltage regulation. I have no idea if the pots are for hum nulling or not. Did you happen to see if those pots changed in the millivolt range when your turned them?

 

Sometimes triode output tubes get a bit noisy getting to the end of their life as the emission lowers, that's why I asked if you had a way to test them. Even the driver tubes and rectifier could be tested. A guy can test them in circuit by confirming proper voltages/current, but one will have to probe around in a powered amplifier.

The amplifier is a bit older, and there may be a dried up cap or two, but I don't really think that's the issue...maybe part of it...

 

Without a schematic or some sort of manual be it assembly or operating, it's a bit of a chore to figure out the circuit from pictures of underside wiring, how it works, etc...

 

There a forum member named Eric Mandaville (I hope I spelled his last name correct) who I'm pretty sure has a Moth audio amplifier...maybe it's pre-amp. He'd be the guy with most knowledge about Moth audio around here I would think.

 

Maybe find the schematic or a manual and befriend someone locally who happens to be into tube audio, either a tech or enthusiast who can look over the amplifier.

You can learn quite a bit that way.

 

I will get the all tubes checked out Friday. Do you think the Amp is worth saving? if not I still can sent it back to the estate. I paid 1,100 for it.

here's the deceased owner thread on the amp

http://audiokarma.org/forums/index.php?threads/moth-s45-arrives.1079/

 

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Theo, if you want, I would be glad to buy it for the $1100. I was the one who strongly recommended you buy it, so I’ll back you up.

 

there are a couple tube amps here in St. Louis for sale that can put you in a happy spot, both private and listed at Gateway Audio Society.

check your private message here.

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From Craig at Eddie Current (moth)
 
You need to measure the bias resistor to see what the bias voltage should read. The 45 tubes need about 36mA, so if the resistor is 1 ohm the meter should read 36mV. Make sure you have the correct tubes, I have not seen a 45 tube that looks like that.
 
I don't understand what he's talking about!

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2 hours ago, theoman said:
From Craig at Eddie Current (moth)
 
You need to measure the bias resistor to see what the bias voltage should read. The 45 tubes need about 36mA, so if the resistor is 1 ohm the meter should read 36mV. Make sure you have the correct tubes, I have not seen a 45 tube that looks like that.
 
I don't understand what he's talking about!

The picture posted at 10:07pm yesterday shows a 2A3 tube in someone's hand. Does Craig at Eddie Current think you're using 45s or 2A3s?

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Where do you live? Close to LA? If so I can look at it. Criag of NOS valves could too if he is closer. Either way, we can pop the top and figure out whats going on.

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