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I need some help to raise the gain on my Driver PCB for my VTA-120. It's the low gain board that I use all 12AU7's but there is not enough gain and I have to turn my preamp up 2x higher to get the same volume level as before.I don't crank the system but I don't want to turn up so high on the preamp. Can I reduce the value of the Plate resistor to up the Plate voltage slightly to increase gain without causing any other issues?
The plate voltage in this PCB is suppose to see a 135Vdc but I know that can vary depending on what type of Rectifier you are using. I'm using a GZ34 right now but I do have the Weber Copper Can than WZ68 which will give more voltage than the GZ34. I will try before doing any changes in the circuit.
I was looking to increase the plate voltage to about 150-160Vdc from 135Vdc. would this solve my gain issue? I don't need a whole lot of gain like the High gain PCB did before with the 12AT7's.
Also is there a 12AU7 substitute that may have a slightly higher gain that I can plug right in?
Any advice would be greatly appreciated.
Edits: 09/27/16Follow Ups:
I will just have to live with the gain level with this preamp/amp combo. These two sound so nice together too, I just needed a tad more gain.
Thank You to everyone for their input/ advice.
If only a smooj of gain is all you need... maybe, try a 12AT7 and see if you can tell a difference. You may not. If you do... try the 12AT7 for a bit.
Good luck!
I'm guessing you've already thought of this but is there an unbypassed cathode resistor on this tube?
Otherwise, personally I'd change the circuit to use a 6922 variant if you have 6.3V available for the heater.
Naz
I like the sound of these Gold Lion 12AU7 driver tubes, just need a tad more gain. nothing crazy to work better with my BAT VK-30 preamp.
Is to have the lowest overall system gain that still satisfies your volume spl requirements. Listening at "10" will give you better sonic nirvana than listening at "3" because you are not throwing away so much of the signal.
I having to put the volume control now at the half way point. I would only go as high as 10 before. My speakers are 91db. I guess I just need to get going on building the 2 way horn speakers now.
The only problem would be if you cannot turn it up loud enough to satisfy you. If that is the case, you probably need a preamp that has some gain. A lot of people have a problem with the 12AU7 tube, they have a "bias" against it more than just its gain structure. I don't seem to think it is that big of a sonic penalty.
Since it has double the current output than the GZ34. This should fit my need. Just 1 or 2db increase would fit my need. When I listen to Charlie Byrd material it's just perfect in volume level but when I go to other material it's just too low for me.
Edits: 09/27/16
"...when I go to other material it's just too low for me."
Is the volume to low for you even when you turn the volume control all the way up?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
My preamp all the way up. Out of all my amps used with this preamp which is a BAT VK-30. The preamp has new 6H23's in it too. I could turn it all the way but never had to in the past or even anywhere close to all the way up. This is the first amp I will have to push the preamp close to all the way up and never had to cross past the half way point to get a good satisfactory volume level.
It's not really bad, just need maybe a 2db gain to put we're it was before with my other amps. The sound of this amp is wonderful and I'm really enjoying it. If I had 93-94db. Efficient speakers,this would not be an issues.
So far from the responses I'm getting, I will just have to live with this.
What I was really getting at was this, when you said "...when I go to other material it's just too low for me." what did you mean?
(edit, I see now that you meant compared to the way it was)
If the sound volume in the room is too low, then just turn up the volume control.
My point is, it's not to low for you if you can turn up the volume control and get the volume you want.
It's only "too low for you" when you can't get the volume in the room you want even with the volume control turned all the way up.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Many of todays sources will drive a power amp to clipping without a preamp. So many preamps operate at less than unity gain. This isn't such a smart thing.
So lets see what your source and preamp are operating at when the amp is playing at normal listening levels. If the preamp is operating at a gain of 1/2 to 3, no problem. If it has to be 5, 10, or more then, and only then, would I consider modifying the power amp. Of course this somewhat depends on the preamp
Oh, and do this with something other than a turntable as a source.
Was this unit originally designed to take a 12AU7 input tube?
Thanks!
nt.
A 12AT7 has more gain than the 12AU7. Your ideas for raising the gain won't work, and its not that great an idea to reduce GNFB to get the gain either!
So you could try the 12AT7. The problem there is that the tube has different operating points and probably won't sound the best until the circuit is optimized for the tube. So that's not exactly an easy task unless you have soldering skills and know your way around Ohm's Law to say the least.
If I were you I would either deal with the fact that you have to turn up the volume more or talk to VTL and see if they have any solutions. Like putting the old board back in.
Hi Ralph, Thank You for the reply and info.
Why would I need to talk to VTL? The amp is not a VTL and the PCB is the original pcb that came with the kit. So there is no old board to put back in.
There are two versions of pcb's for this VTA-120 amp the first was a high gain and the newer one is a low again. I actually would need the gain some where between the low and high gain.
I mis-read! Never mind about VTL...
Do you have a schematic of the circuit board?
DIY Tube forum. Here's the link. It's an earlier version but looks to be the same as mine.
Edits: 09/28/16 09/28/16
You are overlooking GNFB. When open loop gain is sufficient, overall gain is controlled by the NFB resistor pair. Increasing stage gain will increase the amount of NFB, but not the net gain.
Eli D.
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