AC 30'ish preamp. Common plate voltages?

[markmalin]

Hi all,


I've built up my AC30'ish preamp.  This is an experiment using a bunch of spare parts, so bear with me.  I have a hacked up schematic below to help explain.  Basically I took Hoffman's AC 30 preamp section, wired a single input jack to an A/B/Y style switch configuration, then into the inputs of each channel.  So I can run one, or both blended.  The output goes to a 1Ma master volume and output jack.


I'm searching for correct plate voltages on the preamp of an AC 30 and having trouble, so I wanted to ask you guys what is reasonable.  What I have is:


my "E" supply in the schematic is:  263 V
my "D" supply is : 276V


V1 a plate: 124v
V1 b plate: 120v
V2 a plate (pins 1-7): 167v
V2 b plate (pin 8) : 276v


Mark

[PRR]

What is "correct"? Clean? Distorted?

I think it is up to the builder/player.

[markmalin]

I guess I'm looking for a typical AC/30 to start with.  I've kind of gotten a feel for where a typical Marshall or D-Style or Fender style preamp's plates usually sit.  Then again....based on the model, I guess an AB763 can vary quite a bit can't it...  I'm wondering whether a standard AC/30's preamp plate voltages would be in this range.

[pdf64]

The point is that a stand alone preamp HT wont be modulated by signal level, it will be stiff and fixed at the idle level, whereas that of a full amp will. The latter will result in an endless ‘undulation’ of the operating points of all the stages.

[sluckey]

No help on the voltages but I wanted to say that blending the channels together is not gonna sound good because the two channels will be out of phase with each other at the point where they are mixed together.

[Latole]

On Vox AC30 schematic: boths plates; V1= 135 volts and V2 at 180v

Other schematic V1a 231v V1b 228v
                       V2 a 178,  V2b 182


Short answer :
Less voltage = less gain = less output power

[markmalin]

No help on the voltages but I wanted to say that blending the channels together is not gonna sound good because the two channels will be out of phase with each other at the point where they are mixed together.

Sluckey, is that because V1A inverts the signal for the one channel, but V1-B for the other channel inverts the signal, then V2A + V2B does only 1 inversion because it's a cathode follower?   I thought maybe each grid/plate/cathode section would do one 180 degree inversion.  So 1 in the one channel, and 3 in the other, but I must be wrong

[markmalin]

On Vox AC30 schematic: boths plates; V1= 135 volts and V2 at 180v

Other schematic V1a 231v V1b 228v
                       V2 a 178,  V2b 182

Short answer :
Less voltage = less gain = less output power

Does more voltage = more headroom, or not so much?

[sluckey]

There is no phase inversion with a cathode follower. So 1 inversion in CH1 and 2 inversions in CH2.

[shooter]

look at my reply #1 in your other thread, that should keep both IN-phase

V1 is taking a 200mVac signal to maybe 4vac.  you have to try hard to get the 1st gain stage to do anything but amplify the signal "as is".  So 80vdc, 160vdc, pretty much equal at the 1st gain stage.  After that you have to tinker 

[markmalin]

look at my reply #1 in your other thread, that should keep both IN-phase

V1 is taking a 200mVac signal to maybe 4vac.  you have to try hard to get the 1st gain stage to do anything but amplify the signal "as is".  So 80vdc, 160vdc, pretty much equal at the 1st gain stage.  After that you have to tinker 

Thanks!

[markmalin]

So shooter, basically I want to blend the 2 channels at the input of the CF, not after it.  Correct?  I'm looking at a 6V6 plexi schematic and that's what it looks like to me

[shooter]

Yep
not sure on your version, but the plexi doesn't sound very good using either "channel" by itself, but once you blend the 2, you should note an increase in smile factor as you dial it in 

[sluckey]

So shooter, basically I want to blend the 2 channels at the input of the CF, not after it.  Correct?  I'm looking at a 6V6 plexi schematic and that's what it looks like to me

Let's get some circuitry concepts straight... V2A is a gain stage. It is not part of a cathode follower. V2B is the cathode follower. The channels must be mixed together BEFORE V2A gain stage. The logical mix point is the wipers of the two volume controls. This will allow the two channels to add the two signals together rather than subtract the two signal.

I'm pretty sure we're both talking about mixing at the same point, but it seems that you look at both V2 triodes as a cathode follower. That's the point I want to clarify.

[markmalin]

So shooter, basically I want to blend the 2 channels at the input of the CF, not after it.  Correct?  I'm looking at a 6V6 plexi schematic and that's what it looks like to me

Let's get some circuitry concepts straight... V2A is a gain stage. It is not part of a cathode follower. V2B is the cathode follower. The channels must be mixed together BEFORE V2A gain stage. The logical mix point is the wipers of the two volume controls. This will allow the two channels to add the two signals together rather than subtract the two signal.

I'm pretty sure we're both talking about mixing at the same point, but it seems that you look at both V2 triodes as a cathode follower. That's the point I want to clarify.

Thanks, sluckey.  That helps.  I've incorrectly always assumed the combination of the two triodes (V2a and V2b) being wired this way constituted a "cathode follower", so I was referring to both when I was saying "cathode follower".  Right, we were both talking about mixing at the same point (before the V2A gain stage), but my misunderstanding of the fact that V2A functions as a gain stage, and V2B is the actual cathode follower made it confusing.  I appreciate the clarification.

[shooter]

now that you have the basics;

http://www.valvewizard.co.uk/dccf.html
worth reading many times 

[markmalin]

now that you have the basics;

http://www.valvewizard.co.uk/dccf.html
worth reading many times 

Many thanks!  All of you :)