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VVR problems
VVR problems
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Old 12th November 2019, 04:10 PM   #11
razorrick1293 is offline razorrick1293  United Kingdom
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Ok so I got it working (blown zener) but it's still being weird. The amp works perfectly fine when the vvr is set to minimum to 150v, but then it starts oscillating and humming again, up until 270v where turning it up more causes it to jump down1 to 245v it then works as normal. Is it just the pot?
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Old 12th November 2019, 04:23 PM   #12
GKTAUDIO is offline GKTAUDIO  United States
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Quote:
Originally Posted by Gnobuddy View Post
You can blow a MOSFET simply by picking it up in your fingers. You need to be wearing a grounded wrist-strap, and working on a grounded conductive surface.


... But those same two items - grounded wrist-strap and work surface - are deadly for working with valve equipment, where they hugely increase your chances of being electrocuted.

... The problem is, the engineering is bad. You have the parts crammed tightly together, output valves right next to input valves, with a rats-nest of wires threaded randomly through the mess, with absolutely no attention paid to layout or its consequences. -Gnobuddy

When I first attempted to install VVR into my guitar amps I had failure after failure of the MOSFET due to static that I initially didn't eliminate properly with a grounded anti-static pad. I had the strap and pad, but didn't ground the pad properly. Once I learned that expensive lesson, my installations went without problems.

Great caution about using the strap and pad with powered up tube gear.

Another problem I had with my first installation attempt was noise caused by plaiting the wires connecting the MOSFET instead of keeping the leads shorter and parallel. The maker of the VVR kit caught my mistake in photos.

The amps I have built have been higher gain TW and other circuits that are quite sensitive to wire length and layout. Less wire is good, in my experience. Who needs antennas and stray capacitance inside their a amps?
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Old 12th November 2019, 04:26 PM   #13
razorrick1293 is offline razorrick1293  United Kingdom
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What were the symptoms of the failure?
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Old 12th November 2019, 04:27 PM   #14
GKTAUDIO is offline GKTAUDIO  United States
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Hmm. It's been years, but as I recall, no sound, and very low output with buzz.
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Old 12th November 2019, 04:29 PM   #15
razorrick1293 is offline razorrick1293  United Kingdom
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See my output is fine except for on 1/4 of the pot. I've got a new pot on the way. I'l see if that works
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Old 12th November 2019, 04:38 PM   #16
GKTAUDIO is offline GKTAUDIO  United States
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Disconnect the pot and measure it with your DMM to see if you have a problem with the track.
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Old 12th November 2019, 05:29 PM   #17
tristanc is offline tristanc  United Kingdom
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If you haven't seen it, follow some of the tips here: VCB/VVR installations - cathode biased only

and here: InfoCentre - VCB/VVR kit (cathode biased amps)

I assume you're building it like this but with a differing MOSFET?

Click the image to open in full size.
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Old 12th November 2019, 07:23 PM   #18
razorrick1293 is offline razorrick1293  United Kingdom
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It's not the pot, voltages are normal with it disconnected. It's just at 150-245v.
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Old 12th November 2019, 08:54 PM   #19
GKTAUDIO is offline GKTAUDIO  United States
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The ones I have used were NTE2973 - much more capacity (and $$$) than the ones you are using, yet they still get HOT.

I have seen these recommended as less expensive but with lower capacity. They may be similar to yours, but you can check.
STW15NK90Z

Last edited by GKTAUDIO; 12th November 2019 at 08:56 PM.
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Old 14th November 2019, 10:17 PM   #20
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by razorrick1293 View Post
It's just at 150-245v.
My guess is it's the same problem that's been plaguing you for this entire thread: unwanted oscillations due to problematic layout and wiring.

The voltage gain of a vacuum tube changes with the applied B+ voltage. As you twiddle B+ with your VVR, each gain stage in your circuit experiences variations in gain, output impedance, and frequency response.

That being the case, it's my guess that it just so happens that the whole circuit bursts into oscillation for some range of B+ that you're dialing in.

There's a simple way to test my hypothesis: disconnect the VVR output from your amp. Put a dummy load (resistor) on the VVR output instead, something that draws roughly the same current as your amp would. See if the VVR behaves properly with the dummy load.

If the VVR is fine with the dummy load, and the problems come back when you connect the amplifier instead of the dummy load, then you know the problem is the amp, and not the VVR.


-Gnobuddy
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