12AX7/12AU7 Preamp Supply Voltage Lowered

[Intergalactic Tourister]

Hello.

I have an old stand-alone preamp that originally accepted 220v B+ from the power amp. But I don't have the power amp - so I put a small xformer inside (primary:117v, secondary:120v). I was hoping to understand the relationship between the extreme decrease in voltage relative to the other components which were left unchanged.

For instance, does the plate see this lower voltage as a an increase in plate resistor value and therefore more gain? Or simply just a decrease in voltage?

Also, what would be the best route to get it closer to being biased with this low voltage?

12ax7
plate resistors: 100k/pin 6 and 100k/pin 1.
cathode resistors: 2.2k/pin 8 and pin 3 is wired straight to GND.
grid leak: 22k

12au7
plate resistors: 100k/pin 6 and 22k/pin 1.
cathode resistors: 3.3k/pin 8 and 2.2k pin 3 (.0039uf bypass cap)

The mains at the wall measured 123v... and the secondary wires actually measured about 148v before I connected it to the bridge rectifier.



Thanks

[tubeswell]

Here's an exercise for you. Download this spreadsheet http://www.freewebs.com/valvewizard2...inePlotter.xls

and input the relevant voltage supply*, resistor and cap values in the 'parameters' worksheet, and then click on the relevant tube graph sheet to see the respective load line plot etc

* you'll need to measure the DC voltage you get after rectification and filtering, before you can populate that field in the worksheet

[Chuck H]

If the new Xformer has the mA rating you could wire it up with a voltage doubler rectifier. Otherwise it's lower cathode R's and higher bypass caps. At lower plate voltages the lower cathode R biases the tube closer to ideal and the final cathode R value is the impedance that determines the knee frequency of the bypass cap value. The tube data sheets for the tubes you choose can be your guide for bias or you can just wing it (like me). If it's a guitar amp then some color on the wave form could actually be a good thing. No need to be exact. You never know until you try. There are no data sheets or guide lines for good sounding distortion, just parameters for how to eliminate distortion. That's one reason guitar amps are as much art as tech. There's no school for it even though it's a desireable result.

[Intergalactic Tourister]

Wow, thank you!

That looks exactly what I've been looking for but been unable to find

I've looked at Duncans (and others) calculators and it seems to need parameters I can't really give without tearing up the innards. It asks for plate currents, etc., and this amp is old with components heavily wrapped and nicely soldered.

I'm glad I waited.

thank you.

[Intergalactic Tourister]

The secondary is 40MA D.C. I'll check out the voltage doubler rectifier but it is a small chassis and difficult to fit new items inside.

And I actually really like the sound its making with the low voltage except it has some unwanted "features"... like its clipping to soon (or early) on the negative (or positive) side. Not sure which its doing but I'm sure its one or the other. Seems if I can simply nudge it toward or away "optimal" bias I'll at least know where I'm at.

I'm going to try the program Tubeswell sent me... but in general you say I should decrease both cathode resistors from 2.2K to maybe 1.2K? And increase the .0039uf bypass cap on the 12au7?

I kinda winged it, I guess. I really just wanted to get the preamp running. But it sounds pretty neat like this and I think I'll leave it alone after I fine tune it.


Thank you again

[Enzo]

You mentioned low voltage a couple times, but WHAT B+ voltage is the thing seeing?

If you really are getting 148vAC, rectified and filtered that yields 209vDC, pretty close.

A useful rule of thumb for me is most 12AX7 triode sections conduct about a milliamp or so. Some more some less, but that is a good place to start. Then real world circuits will tell you more precisely.


before you start changing a bunch of resistors and caps, find out what you are working with.

[Intergalactic Tourister]

I'm sorry, I'm getting about 138vac (not 148vac) measuring only the secondary when completely disconnected from the preamp.

This is what I measured:

Measured 183v at the bridge rectifier to gnd.
Measured 123v at the last filter cap to gnd.

12ax7
Pin 6 to gnd = 91vdc
Pin 1 to gnd = 93vdc

12au7
Pin 6 to gnd = 48vdc
Pin 1 to gnd = 98vdc

[tubeswell]

I'm sorry, I'm getting about 138vac (not 148vac) measuring only the secondary when completely disconnected from the preamp.

But what is the rectified DC voltage at the reservoir cap (with tubes loaded)? And have you got a schematic?

[Intergalactic Tourister]

I edited my previous post right when you replied.

With the tubes in:
Measured 183v at the bridge rectifier/first filter cap to gnd.
Measured 123v at the last filter cap to gnd.

[Intergalactic Tourister]

I don't have a schematic. But its a very simple preamp circuit from the 50s or 60s.

I could maybe drawout the section you need to see and upload a picture?

[Intergalactic Tourister]

Ok. I used the Load Line Plotter and was hoping someone could help me interpret it.

V=182
Ra=100000
Rk=2200
Ck=0
Rg=0
Cf=0
Rl=10000000
Co=15

I haven't made measurements in years so... did I measure the B+ correctly as stated in my previous replies? I'm not sure if I employed a shunt method or not.

I didn't fill in Ck, Rg, or Cf. I'm just using the input part (or first stage) of the 12ax7 tube, correct?
*I don't have a input grid resistor.
*I don't have a cathode bypass cap on the 12ax7 input tube.
*And I don't have a input grid to plate cap.

[tubeswell]

I didn't fill in Ck, Rg, or Cf. I'm just using the input part (or first stage) of the 12ax7 tube, correct?
*I don't have a input grid resistor.
*I don't have a cathode bypass cap on the 12ax7 input tube.
*And I don't have a input grid to plate cap.

Ck is for the cathode bypass cap value. If you are using a bypass cap here, you need a value.
Rg is for the following load resistance (after the coupling cap), and is needed to properly work out the AC load on the stage in order to work out the frequency response. If you assume a 'maximum gain' condition, then enter 500,000 or 1,000,000 value here

[Intergalactic Tourister]

Ok. thanks

I used Rg=1,000,000 and the high freqs went way down.

After the coup caps is my 1M volume.

[tubeswell]

Oops I see again that he has 'Rg' as the grid stopper for the stage, whereas I was thinking (without having looked at it for a wee while) that Rg was the following stage's grid leak resistance (which is typically 470k to 1M, which he has marked as 'Rl'). So don't put 1,000,000 in for 'Rg'. If you are not using a grid stopper, then leave it alone

(BTW "Cf" is a feedback capacitor, if you're using one (which most people don't))

[Intergalactic Tourister]

I'm not using Cf.

On the graph, my 12ax7 bias point is between -1v & -1.5v. The X-axis is Va=125v and the Y-axis is Ma=.55

Where does the point need to be to be considered correctly biased?