Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baby Thomas on May 26, 2020, 02:20:49 am
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Hello!
I made a simple one channel clean amp with somewhat strange output section - SE with 6AN5. I also installed a VVR circuit (first time for me) to alter the B+ for the output tube (acting as an additional Volume Control). The circuit is working as expected, changing the voltage for 6AN5's anode and screen between ~122vDC and ~32vDC. However the output volume doesn't change at all, it's pretty much the same. On the scope I can view the clean output power changing from ~0.3W to ~0.03W, but to my ears there isn't any volume drop. The only thing that happens is I loose headroom as soon as the pot is in the lower 25-30% of it's range.
Is this normal? Is it because the power is already too low? Is it the tube type I used? Is it because it's SE?
Any help will be highly appreciated. Thanks!
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You can't trust your ears from so low power.
Look everything is working.
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I have a 1W SE amp (EL91) with VVR. I scale the EL91 anode and screen only, not the pre-amp. Works fine however you do need to reduce the drive to the output stage otherwise it will distort early. You're getting that effect, as you say you're losing headroom. Hence I think your VVR is working but at such low power and only chaging from 0.3 to 0.03W it's hard to hear it.
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Latole & JB gived you the correct answer
Maybe this can help to understand easily
(https://i.imgur.com/m8chrre.jpg)
Franco
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Gain and power are two different things.
The gain of a tube amplifier changes very little with supply voltage.
The maximum power does change with supply voltage.
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I'm somewhat familiar with power vs. volume relationship, but I thought if the power goes down 10 times (0.3W -> 0.03W) I'll have half the volume.
Yesterday I didn't have the schematic with me, I'm attaching it now.
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VVR reduces headroom, not volume. Although maximum volume will of course decrease as you turn down the voltage.
You describe that you get a reduction in headroom, so it looks like it works as expected.
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I'm somewhat familiar with power vs. volume relationship....
Gain and power are two different things.