• 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.

learning, building and testing tube amp

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#41
fragman56 said:
I just shake my head at the thoughtless things I do. But, the good news--I am learning a bunch!
Click to expand...

LOL. We all go through this. I was fortunate enough to have a very learned engineer to look over my shoulder and correct me when I was learning.
Brett- I must become familiar with PSUD. I have probably fried many of the components by now. The tubes are probably ok?
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PSUD is a very handy tool and so much easier than trying to work it out longhand.
Tubes are very resilient little beasties, and will take all sorts of abuse and keep woking, where a semiconductor would have let the smoke out. Your tubes are probably fine.
I think that I can scrounge a few more of the large caps. Would paralleling caps at the filter help assuming the proper voltage rating? Parallel caps increases surface area therefore capacity?
Click to expand...
Extra capacitance is often a good thing, as it will give lower ripple on the supply rails, and therefore less hum, but certainly don't stress about it. Use Joels circuit as a guide.
Correctamundo about the increase in surface area. Caps are basically two sheets of conductor, with an insulating dielectric between them, then rolled up and shrinkwrapped or put in a can. Beware voltage rating!
I'll have to order a resistor that large. Great advice on joints, connections, 90 degree, etc.--many thanks.
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Glad to help. Get that PSU working right first, then step through the circuit again, checking joints, layout etc.

Joel's circuit is very similar to what your's should have been, so maybe just build his. Perhaps he will post or email you a pic of the layout (if he is willing and able) that you can follow to get working, as we know his works well. The power supply is different, so I'll redo it in PSUD tonight and post the details for what parts you have on hand.
BTW, any comment on the pi config for this amp? I thought about winding an inductor--if I calculated correctly I would need about 5300 feet of wire!?
Click to expand...
Pi is good, though my preference is for chokes.
Wind an inductor? Jeez you're keen. <a href="http://www.tubesandmore.com">AES</a> have a Triad 10H 90mA 270ohm for about $US17. Could you build one cheaper? I haven't calculated a choke in (many) years, so, I dunno about the wire length.

Cheers
 
#45
Power supply

Just built and tested the attached PSU as well as revised the circuit as attached.

Hum significantly decreased but still slightly audible (60hz measured). Maybe the wiring config?

I doubled the resistors because I built (on a wooden board) only one channel. Measured voltage was 250vdc on the B+ supply.

Bias measured for Rk output (6bq5) was 8.17 vdc; 1.33 vdc for the 12ax7 Rk--ok, I think.

Sound volume still low. How is output power calculated or measured? This tube should yield about 2 watts out and should have plenty of power for near field listening assuming decent speakers (93 db/1 meter)?

Please excuse the simple questions--and thanks for your help!

Aspiring diyer,
rick
 

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#49
Grid Resistor

Hi Joel,

I don't know.

I understood that the 12ax7 needed to be attenuated because it has too much gain for the output tube. I put the grid resistor in because I don't know any better...thanks for your comment. I thought the grid resistor is for bleeding off ???? Perhaps you can explain what's going on here.

Any comment on the power output question...

Thanks,
Rick
 
#53
Rick,

The fixed resistor, or a volume pot if you chose one, perform two functions. First they provide a DC path from the grid back to the cathode. All the electrodes in ANY tube must have a dc path to the cathode, period. Second, they are the load that the previous stage works into. In this case, since the first stage, this also determines the "input impedance". (yes, I'm simplifying - there are other factors, but they are small).
So, if we chose to add a volume pot, we don't need another resistor to ground, because the pot is a path to ground, AND a fixed load to the source of the signal.
The volume pot is a voltage divider. The AC signal develops across the total value, say 500k. But when you turn your volume knob to halfway for example, you are "picking off" the amount of voltage developed across only 250k to ground, and so on as you continue turning it. Eventually when you reach the end of the pot, the grid is directly tied to ground - no signal. Make sense?
 
#54
NOT THE BEST WAY.

Hi,

Not such a good idea IMHO.

You see what happens when the shunt resistance varies is that the working point of the tube varies along with it.

Ideally one should use a fixed gridleak resistor and vary the resistance in series with the signal accordingly.

This works in a far more linear way and as such it will not change frequency response with varying settings.

Cheers,😉
 
#56
NOPE.

Hi,

No,a pot will vary shunt resistance and series resistance according to the position of the wiper.

When you turn it the total R will still be the give Rtot but the divider will move its' position shifting more or less resitance from one legg to the other.

So the gridleak resistance will only be Rtot when the pot is fully open.

Cheers,😉
 
#60
EVER HEARD ABOUT....

Hi,

Just consider the leakage currents and current flow.

Now imagine the cathode is tied to ground,use a gridleak resistor of high Ohmic value and see how a negative voltage develops on the grid with respect to ground.

It is not because the cathode is lifted with respect to ground that the grid goes negative all by it self...it uses a ground return to get there.

So if you make that resistance variable....see what I mean?

I am home BTW...😉
 
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