5e3 Phase Inverter Hum (solved)

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pleasantbullet
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5e3 Phase Inverter Hum (solved)

Post by pleasantbullet »

:wink: So, I have a 5e3 amp that i built from spare parts.

And whenever I put a 12ax7 in the phase inverter, it makes the amp hum.
The hum is present with the volume turned all the way down. It goes away as your turn the volume up to about 2 or 3, But then returns around 4.
It sounds kind of like bad filtering. But i've tried bumping up the filter caps, and it doesn't seem to help.

If I put a 12au7 in the phase inverter, it goes away. Actually, with a 12ay7 in V1, and 12au7 in V2, its a super clean little amp. But i do want to be able to crunch it up, and get the rocks tones.


I made the amp to 5e3 specs. with a few mods.

-one input through 68k resistor
-V1 is permanently wired in paralell, with the plates tied together with a single 220k resistor, cathodes together like stock
-no tone pots, just a volume. so from the 220k plate resistor from the first gain stage, its goes to a 100n coupling capacitor to the 1Ma volume pot, to the input of next gain stage.

Then the rest of the amp is stock.

any ideas as to what is causing the hum when the 12ax7 is plugged in?

thanks!
Last edited by pleasantbullet on Thu Apr 27, 2017 2:36 am, edited 1 time in total.
tubeswell
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Re: 5e3 Phase Inverter Hum

Post by tubeswell »

pleasantbullet wrote: Wed Apr 26, 2017 1:39 pm... And whenever I put a 12ax7 in the phase inverter, it makes the amp hum.
The hum is probably not due to the PI tube, but more to do with to the other pre-amp tube
pleasantbullet wrote: Wed Apr 26, 2017 1:39 pmThe hum is present with the volume turned all the way down. It goes away as your turn the volume up to about 2 or 3, But then returns around 4.
This confirms that it is not the PI tube.

You say you bumped the filter cap(s). Did you take voltage readings at eacb plate, cathode and HT supply node?
He who dies with the most tubes... wins
Rnb716
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Re: 5e3 Phase Inverter Hum

Post by Rnb716 »

60hz or 120hz?
Do you have a heater center tap?
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pleasantbullet
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Re: 5e3 Phase Inverter Hum

Post by pleasantbullet »

I have a virtual center tap on the heaters, 2 - 100 ohm tied to ground.

The reason i suspected it to be the phase inverter, was because the hum is still present when i pull the first preamp tube out.
And the hum goes away with the phase invert tube out.

I'll get some plate / cathode voltages tonight.

thanks for the help so far.
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pleasantbullet
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Re: 5e3 Phase Inverter Hum

Post by pleasantbullet »

So it's 120 cycle hum.

And my voltages are as follows.

B+ 376v
After 4.7k resistor = 324v
After 22k resistor = 262v

V1 anode = 46v (v1 is parallel, with 220k plate resistor)
V1 cathode = 0.74v

V2a anode = 179v
V2a cathode = 1.2v

V2b anode = 219v
V2b cathode = 41v

I disconnected the heater virtual ground to see what kind of noise it made, and it does seem to be related to the buzz you get from the heater.
tubeswell
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Re: 5e3 Phase Inverter Hum

Post by tubeswell »

are your 6V6 (octal) heater socket pins wired in parallel or did you cross over the Pin 2s with the Pins 7s on alternating sockets?
He who dies with the most tubes... wins
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pleasantbullet
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Re: 5e3 Phase Inverter Hum

Post by pleasantbullet »

Parallel
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pleasantbullet
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Re: 5e3 Phase Inverter Hum (solved)

Post by pleasantbullet »

Star grounding was the culprit.
Noob move on my part. I feel ashamed.

Changed my grounding so all power grounds went to one, and all signal grounds to another... And voila.

Silence.

Thanks to all for your pointers. It helped.

Cheers.
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Re: 5e3 Phase Inverter Hum (solved)

Post by Stevem »

Good to hear!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

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Rnb716
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Re: 5e3 Phase Inverter Hum (solved)

Post by Rnb716 »

Good to hear! Hows it sounding?

Just thinking here... A single 68k grid resistor would give you a traditional low input level. If you use a single 33-39k, you would get the high input level. With a 1M input resistor tip to ground on the jack.

Congrats finding your hummmm!
Rnb716
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Re: 5e3 Phase Inverter Hum (solved)

Post by Rnb716 »

Thinking again.

with v1a, v1b parallel. your config is same as guitar in hi on one channel and junmper to the other channel.... I would have to experiment with that. Maybe clip in another 68k in parallel?

k... I'll stop thinking now :D
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ToneMerc
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Re: 5e3 Phase Inverter Hum (solved)

Post by ToneMerc »

Rnb716 wrote: Thu Apr 27, 2017 1:34 pm

Just thinking here... A single 68k grid resistor would give you a traditional low input level. If you use a single 33-39k, you would get the high input level.
Those 68k or 33k grid stoppers don't control input signal levels, at this location in the circuit path its main purpose is to form a low pass filter to attenuate RF signals at the grid input.

TM
Rnb716
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Re: 5e3 Phase Inverter Hum (solved)

Post by Rnb716 »

So incorrect thinking is still thinking. Right..

I agree we don't need to hear voice of America through the amp :D . And without the second 68k r to form a voltage divider there would be no signal attenuation. I still am used to seeing this value dropped to33k-47k or less on a single input circuit. Thanks
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pleasantbullet
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Re: 5e3 Phase Inverter Hum (solved)

Post by pleasantbullet »

It sounds really good. Such a clean amp.

I like tone of the parallel preamp tube, especially with my jazzmaster.

I'm going to experiment with some cap values to make it more humbucker friendly.
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