• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12B4A Linestage

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#81
Frank,

No fireworks tonight!!!😉

OK, I've obviously missed something here. With the 375-0-375 transformer, and 1n4007's, you think the above circuit will work?

I just want to get this right, as I dont want to BLOW anything else up.

Ground or float the CT, no difference.
Ground the PS after the rectifier.
Will the caps handle this voltage? They are 370VAC???

No fireworks...?

Bryan
 
#82
Hi,

You just have the wrong xformer for the job it seems.

Either a 0-300VAC or a 150-0-150VAC would do.
With the latter you tie the CT to the midpoint of your two diodes and ground that, together with the rest of the negative as shown on you drawing.

Too bad you probably fried the new Crees.:bawling:

When in doubt just ask.🙂

Cheers,😉
 
#84
Hi
I like very much Frank's idea to use only a buffer in a preamp unit.
I'll try to use 12b4A as a buffer.Can anyone draw a design?
I haven't a scanner to saw you what I'm thinking.And I'm not sure if it works well.
And i believe that Zout will be lower!
I'll try it the next days.
 
#85
Hey Guys,

Couple more questions now....

I hooked up the circuit with half of the 370-0-375, in the interim while the new iron is being shipped (hopefuly this is not going to hurt anything???). The circuit worked, the VR tubes lit up, and all seemed good. I even got a VERY nice slow start from the 12AX4's.

First slight problem, I hear some BUZZZZ, which I am 95% sure is coming from the first choke. What is the cure for this?

Secondly, I am still a little unsure about the rating of the dropping resistor in place directly before the VR tubes....

Lets say I have 300V B+, and a stack of 0D3 and 0A3, which should regulate at 225 volts. The CCS will be set at 25mA, and I want to run 20mA through the VR. With this in mind, I calculate the value of the resistor as:

V=IR
75=0.045 x X
X= 1644R

Is this correct, and if so, how do the tubes strike, that is would they ever see the approx 260 volts necessary to strike?

Thanks Guys,

Bryan
 
#89
Frank,

I had thought about that as a possibility. To tell you the truth, I cant tell for sure if it is the choke or the power tranny. I am bringing home a stethoscope this evening to help track it down...but I still think it is from the choke.

I've checked some values, and I think I may be pulling excessive current through the VR tubes, as the buzz starts the exact moment they strike. My B+ was a little higher than expected, and the dropping resistor behind the VR tubes is a little low. If I'm correct, this should cause the VR tubes to draw excess current, which may explain the problem.

Also, I am currently simulating the circuit load with a 10K 25 watt power risistor, but this should be fine as I have 255 regulated B+, and I will be running about 25-30mA through the 12B4A.

When you pull excess current through a choke, would this saturate the core, and thus cause BUZZZZZ?

Thanks,

Bryan
 
#91
Frank,

I just measured the current throught the circuit.

45mA total from the PS, with 25 being drawn after the VR tubes... So this seems correct. The chokes are rated at 100mA😕

The bolts are tight.

Any other possibilities?

Bryan
 
#92
Hey guys,

It seems that every time I try something, a new question is born! Here is the latest one.

I have the Pimm BBMCCCS, set to about 28mA, feeding the plate of the 12B4A. I have a 390R cathode resistor, which gives me about -11 volts grid, Right? What confuses me here, is that when I measure the plate voltage, I am only getting about 48 volts. I have about 180 volts going into the CCS. If I look at the plate curves, it seems as though it should be between 90 and 100volts. What do you think could be leading to this scenario?

Any thoughts/ideas?

Thanks,

Bryan
 
#93
Bryan -

Not all chokes are suitable for use as a power supply input choke. The Hammonds are nice cheap and useful pieces of iron, but will buzz when used in a choke input supply.

If you want something that will certainly work in this application, look for a surplus choke identified as a "swinging choke" whose inductance will vary depending on the current.

- Pat
 
#94
I have the Pimm BBMCCCS, set to about 28mA, feeding the plate of the 12B4A. I have a 390R cathode resistor, which gives me about -11 volts grid, Right? What confuses me here, is that when I measure the plate voltage, I am only getting about 48 volts. I have about 180 volts going into the CCS. If I look at the plate curves, it seems as though it should be between 90 and 100volts. What do you think could be leading to this scenario?
Click to expand...

Hi Bryan,

Have you measured the voltage across the cathode resistor to verify that the CCS is delivering the expected current in circuit? If there is 28ma flowing the cathode should measure +11 volts from ground. Also verify that the grid of the 12B4 is real close to 0 volts with the circuit operating. If the tube is gassy at all it might be trying to run away and be showing a postive voltage on the grid.

If all the above checks out try increasing the value of the cathode resistor to increase the grid bias. See if the tube responds to this by increasing the plate voltage.

Also, have you tried different 12B4's in the circuit? With CCS loads the plate voltage does vary from one tube to the next.

Gary
 
#95
Hi again,

I finally got around to building a battery biased CCS into a Radio Shack project box this morning for prototyping ideas. Big knob on the front connected to a 20 turn pot, adjustment range of 5ma to 120ma. Mosfet attached to the bottom of the case for heatsinking. Ran it up to 22 watts (65ma,350 volts) for a short time during testing. Looks like the project box can handle 10 watts or so.

Slapped together your 12B4 circuit. Tested 5 12B4's to see what the plate voltage variations are. With the CCS set for 28ma here is what I got.

12B4 plate voltage at 28ma with a 390 ohm cathode resistor.

#1 138
#2 124.5
#3 147
#4 119.5
#5 138
#5 no grid connection to ground 55

I was able to duplicate the plate voltage you are seeing if the grid is not connected to ground via grid leak resistor. I was using a 100K resistor in this test.

Check your grid leak resistor.

Gary
 
#97
It's ALIVE!!!

Hey Guys,

Well, during the first half of the game last night, I managed to get the preamp up and running. It worked!😀

Currently no bypass cap on the cathode resistor.

Now, many more questions.

1. When I measure the voltage at the ground of the 12B4A, I measure about 120mV DC. At the point where this connects to the earth I have 0 volts. I can also measure this value at the grid. Any thoughts? All other voltages around the tube are correct, and the CCS is working well. Would this be indicative of a need to increase the grid leak resistor?

2. When I turned off the power switch, there was a couple loud pops as the circuit discharged. Any thoughts? The pops were about 3-4 seconds atfter the switch was turned off, and maybe included 2-3 pops.

3. The bass seems very weak. Maybe this is due to the selection of components:

12B4a:
Approx 120 volts on the plate,
30 mA,
390 rk
2.2K rg
1.0 uF coupling cap with 1M to ground after the cap.
100K audio pot

Of course, this is the first minute of use in the circuit, and all things will need to break in a bit, but the bass was suprisingly low. Thoughts?

None the less, it was AWESOME to hear some tunes!!!

Thanks All,

Bryan
 
#99
Frank et al,

Ok, I'm a little confused here....

I have a 100K pot, which feeds the grid from the input. From the grid to ground, I have a 2.2K "grid-leak" resistor.

The output is cap-coupled with 1uF, and the output-side of the cap has a 1M resistor to ground.

Is this incorrect, and would this effect the bass response?

Also, any ideas on the popping noise when I turn off the power?

Thanks,

Bryan
 
#100
Hi,

I have a 2.2K "grid-leak" resistor.
Click to expand...

That's an almost dead short to any previous stage, SS or Tube..

A gridleak resistor goes from G1 to ground, usaully this varies from, say 50K to 1M or higher depending.

2.2K is sooo low it almost shunts the previous stage to ground, why?

The output is cap-coupled with 1uF, and the output-side of the cap has a 1M resistor to ground.
Click to expand...

Seems O.K. Ignore the 1M resistor, that's just a bleeder..
The 1µF cap should be fine into a 100K load or even a little smaller.

Is this incorrect, and would this effect the bass response?
Click to expand...

Hell yes, it would.

Also, any ideas on the popping noise when I turn off the power?
Click to expand...

Try turning of the amp(s) first with the preamp muted.

Cheers,😉
 
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