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Author Topic: My new tube preamp design hums!  (Read 5960 times)

slideman82

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My new tube preamp design hums!
« on: February 01, 2011, 08:03:14 PM »
Hi! Well, I decided to use low voltage tubes in a fenderish style preamp with my all-mosfet power amp. I chose a couple of ECC189, they run really well with about 90V DC. In fact, I used dual 45V DC taken from a couple of zeners (power amp uses dual 50V supply), with 1000uF caps filtering for both lines. It's basically a 3 stage Fender preamp, with a cathode follower after first stage, and before tonestack. Every cathode goes to a resistor to -45V DC, and tonestack and volume and master pots are referred to 0V. I placed a small resistor and a 470uF cap as LPF for first tube (that's first stage and cathode follower) B+ (that's about 44VDC), but didn't make this LPF in negative supply after 2nd tube (2nd and 3rd stage). The fact is when I turn vol pot (right after tonestack, before 3rd stage) a bit of hum appears, so, it must got to do with first stage. I already changed from filament AC to DC supply, 6V regulated. Probably I'll place some small inductors in the tubes supply for better filtering. How does it sounds? Pretty cool!

Any more ideas or comments?

thanks!

J M Fahey

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Re: My new tube preamp design hums!
« Reply #1 on: February 02, 2011, 07:56:18 AM »
Congratulations on advance  :tu: but, as Kung-fu-sion said, an image is worth 1000 words, so, please, post that schematic!!!! :grr
He he, please just do. ;)
Don't forget to include some voltage readings.

rowdy_riemer

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Re: My new tube preamp design hums!
« Reply #2 on: February 02, 2011, 12:55:40 PM »
I've heard that 12au7's are suitable for supply voltages this low.

Enzo

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Re: My new tube preamp design hums!
« Reply #3 on: February 02, 2011, 04:32:24 PM »
Don't aim your laser focus at lack of filtration as your hum source.  The hum turns down with the volume control.  If there was bumpy B+, you'd still get some hum, I;d think.

First, is the hum 60Hz or 120Hz?   120Hz is power supply ripple, 60Hz is not.

Try swapping tubes around, any help?

Even if it is power supply ripple, it may be ripple currents through shared ground, rather than lack of filtration.

The volume control controls the hum level, so at the grid of the stage previous to the control, tack a ground wire from grid to the ground end of the cathode resistor.  Soes that silence the hum?  (It will also silence the signal)

This could be one instance of heater related hum.  How is your heater line grounded?  Center tap?  virtual center tap?   Have you tried elevating the heaters?

slideman82

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Re: My new tube preamp design hums!
« Reply #4 on: February 02, 2011, 07:31:24 PM »
I always do star ground, SS preamp doesn't hum or anything, and sounds great! Even with Fets! No IC's! I think zeners I'm using to regulate dual tube supply aren't doing their job, have to recalculate some things... heaters supply is DC, when it was AC was center tapped with a 500 ohms pot I had around there. I think I had less hum with AC heater supply. This hum increases a bit when I turn the volume more than 50% and when I hit the strings, with no signal, almost dissapears. Also have to try shielding tubes, they are a bit close to the power transformer (that's shielded).

Thanks for all these tips!

PS: I'm a bit lazy, that's why I haven't posted the schematic! I'll try to draw it.

slideman82

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Re: My new tube preamp design hums!
« Reply #5 on: February 03, 2011, 09:09:00 AM »
I've never seen a schematic so badly drawn!

joecool85

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Re: My new tube preamp design hums!
« Reply #6 on: February 03, 2011, 09:16:36 AM »
I've never seen a schematic so badly drawn!

I've drawn worse, no worries  8|
Life is what you make it.
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J M Fahey

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Re: My new tube preamp design hums!
« Reply #7 on: February 04, 2011, 01:07:55 PM »
Just to start with something,
1) You are cooking your Zeners, they dissipate almost 3.5W each !!!!!!!!!!!!!!!!!!!
2) that 100r+470uF filtering the top rail should have a symmetrical one filtering the lower rail too; it's not "ground" in this case
100r is too low, go for 470r or 1K
3) Maybe it's a symbol mistake, but you seem to be entering the volume pots through the cursor and leaving through the top, the opposite of normal practice.
The only one major use for that is in Jazz Bass volume contrlos, an idea by "cost conscious" Leo Fender to save on a pickup selector switch.
I'd love to see some voltage values in the schematic.
Personal opinion, feel free to hand draw on an A4 or Letter sheet and scan it.
Save as greyscale .gif; avoid .jpg if possible because it blurs fine lines and text.
As of the drawing quality, I've seen and posted far worse. ;)

slideman82

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Re: My new tube preamp design hums!
« Reply #8 on: February 05, 2011, 12:23:04 AM »
hahahaha! Thanks! But don't worryl, zeners are just fine, it's all under calculations, this zeners also feed other preamp, a ss one, down to dual 24V supply, they're quite cool (and I used 2 47V zeners in parallel). I didn't have time to fix it, I think I'll try it tomorrow or may be Sunday.

BTW, I made a mistake in the schematic with vol and master pots! They're wired ok.