First question on my build

[Chuck H]

I'll send good vibes and hope it's less than predicted. A foot and a half of snow in a single storm is "snowed in" where I live. The cities around here share plow trucks rather than own the expense individually. It's bit us in the back side a couple of times. I like to cook when I'm stuck indoors due to weather. Sometimes I have to resort to the fireplace or grill coals when the power goes out. But nothing gives that comfortable solace you need at such times like cooking a meal. If I hear about such weather ahead of time I provision just for it.

[J Luth]

OK Chuck, thanks. We got between 18" to 2 feet, a lot has melted already but they're predicting another possible storm next week. OH well.

Also, if your up to it. The parts came in and installed in the amp. Couldn't get a bias pot, out of stock, but it appears to be working now. May have been the bad cap I found, one of the lead was loose inside.

So if you want, let me know what you need me to check.
As of now it still has the hum issue with the 100R and the 220K connected.

You mentioned about putting a mVAC signal into the amp, what would be needed? Would this work?


I took a voltage reading across the 1ohm resistor on the power tubes and got the following.
One one pair 31.5mv, 30mv on the other pair 31.9mv, 26.2mv. The second reading doesn't look good. Also the low reading follows the tube if I move it.

[Chuck H]

As of now it still has the hum issue with the 100R and the 220K connected.

Wait, what? Does connecting the 220k resistor also cause hum? Or is that only in conjunction with the 100R?

You mentioned about putting a mVAC signal into the amp, what would be needed? Would this work?
[ATTACH=CONFIG]47597[/ATTACH]

Is that a small oscillator? The pic is blurry. What frequency range does it cover and what is the VAC output (you can measure that with your meter).

I took a voltage reading across the 1ohm resistor on the power tubes and got the following.
One one pair 31.5mv, 30mv on the other pair 31.9mv, 26.2mv. The second reading doesn't look good. Also the low reading follows the tube if I move it.

26.2mA (that's the current indicated by the voltage reading across your 1 ohm resistor) to 31.9mA is actually a pretty big spread. Almost 5mA. I wouldn't sweat it for function but if this was supposed to be a "matched quad" of tubes at some point I think that spread is too large and could be analogous to some other imbalances tube to tube. And THAT could cause humming at full output because the push/pull circuit isn't cancelling the hum (It's supposed to do this because either side is out of phase with the other, but the hum present on either side is the same). So, try this:

It looks like you wired your push/pull typical of any four tube amp. So if you were to run only two tubes you would install them one at left and one at right (typically done as inner two tubes or outer two tubes). Try just two tubes at a time and connect the 100R resistor. First one pair and then the other so that all tubes are tested, but only two at a time. What I'm looking for is one tube that may not be balancing with the other three, and therefor not cancelling hum as the circuit should. So if one pair is hum free and the other is not then we'll know that one of the tubes in the hummy pair is suspect.

[J Luth]

One of these days I'll figure out how to do the quotes the way you do.

Right now both the 220K and the 100r are connected. I don't remember if I tried it with just the 220K and no 100r. But will tomorrow if needed.

The picture is of a small audio generator, it goes from 20HZ to 1.5kHz, has a x1 / x100 switch, a switch for 0 or -20dB it can put out square wave and sine wave and an amplitude pot. I can try measuring any AC with the meter I have.

Yes they were supposed to be a matched quad but I guess they weren't.

Yes the amp is wired like a typical four tube push/pull.
I'll do the tube test tomorrow. Too late here now, don't need the wife throwing something at me.

[J Luth]

Chuck,

Tried it with two tubes and used every configuration I could to check them all and no real difference. Maybe slightly lower due to it only having two tubes.
Also with the 220K connected and no 100R, no hum.

Without the 100R if I listen real close to the speaker there is a bit of hum but very little. With the 100R connected it gets quite a bit louder. Don't know if that helps you.

[J Luth]

Try this again. I also have an oscilloscope here if that will help.

[Chuck H]

Hey Jim. I've just been busy, but ruminating on the matter. The "test in pairs" was to determine if you had a power tube imbalance issue that was causing hum. I do think we're still in the same boat as before. That is, I think the amp is making considerably more power with the 100R in place and that is allowing you to hear the hum that is always there analogous to output power. In other words, it may be that the hum is always there and there is just much less of it when the amp is making much less power (ie:100R not connected). To test this you would need to determine output power of the amp with and without the 100R in place. Then you measure the noise level in both states as well. That would give us a ratio of peripheral noise relative to available power. If the two match, then you have a hum issue that is not related to the 100R in or out and it's audibility is related to overall output potential rather than THAT resistor.

To test for output power you'll need to know the AC output of your little signal generator under the load of the amplifier input. That's easy to measure with any multi meter. The amp doesn't even need to be on. Just connect the signal generator to your input however you have to and then measure the AC at the input jack. see if you can get 70mV to 100mV with the frequency set to anything between 400Hz and 1000Hz. Then you would need a dummy load. A purely resistive load of your amps preferred output impedance and capable of withstanding 100W. This would be wired so it can be connected to the amp instead of a speaker load.

[Dave H]

Tried it with two tubes and used every configuration I could to check them all and no real difference. Maybe slightly lower due to it only having two tubes.
Also with the 220K connected and no 100R, no hum.

Is the hum acceptable with the 100R installed and the volume pot turned to zero? If it is then it's not the output stage or PI (or any stage downstream of the vol pot).

[J Luth]

Dave,

I would not call it acceptable. It is not extremely loud but loud enough to be heard and enough that I wouldn't use it playing out and that is with the volume at zero.
If the hum was less noticeable I would leave it alone.
If I remember correctly, I would have to check again to be sure on this, with the volume at zero the hum is present, that part I know, I would have to check where the hum increases when the volume is turned up it may be in the 1/2 to 3/4 range. Again I would have to check that again.

[Dave H]

I would not call it acceptable. It is not extremely loud but loud enough to be heard and enough that I wouldn't use it playing out and that is with the volume at zero.
If the hum was less noticeable I would leave it alone.

OK, Try shorting the input of the PI (junction of C13 and R15) to ground. With the 100R in place is the hum level acceptable now? If it is the PI and output stage are OK. I'm trying to establish where the hum is coming from. We seem to have been going round in circles for four pages

[J Luth]

Junction of C13 and R15 shorted to ground, the hum is cut by about half. It's slightly more than I think would be acceptable. Also if this helps or is expected, the gain appears to be reduced by more than half.

We seem to have been going round in circles for four pages

Yup but probably due more to my old brain.

[Chuck H]

Junction of C13 and R15 shorted to ground, the hum is cut by about half. It's slightly more than I think would be acceptable. Also if this helps or is expected, the gain appears to be reduced by more than half.

"gain appears to be reduced by more than half" is confusing in context. Do you mean the gain is reduced when the 100R is removed or that the gain is reduced when the C13 R15 junction is grounded. I assume you mean when the 100R is lifted because there should be NO gain with the PI grid grounded. Running with that...

If it sounds like the gain is reduced by more than half it probably means that it's reduced by a lot more than half. Do you have or can you get a big honkin resistor for a dummy load?

[J Luth]

Below is what I did, but as a side note, there is a reduction in gain when the 100R is disconnected but it does not pertain to this test it was connected.

The reduction in the hum is with the ground wire connected to C13 and R15 and the volume knob on the amp at zero. The 100R is always connected when I try what is suggested unless told to disconnect it.

The gain reduced by more than half, was with C13 and R15 grounded and the following done.

After I connected the ground wire to C13 and R15 and noticed that the hum decreased some I wanted to see if the reduction was due to the ground wire getting rid of the hum, just masking it, reducing gain or whatever I could possible find in case I was asked. I put the amp on standby, disconnected the ground wire, connected up the small oscillator to the input set it to a 1k signal and set the volume on the amp at 3. Took the amp off standby and turned the amplitude on the small oscillator up to get a decent level. Then put the amp back on standby and reconnected the ground wire no changes were made to the oscillator or amp volume took it off standby and that is when I noticed the reduction in volume.

Hope that clarifies it a bit more, I'll try to be more descriptive.

I have a small dummy load here. It has 2, 50 watt 4 ohm resistors mounted to a plate. It is switchable between 4 and 8 ohms, 4 ohms through 1 resistor, 8 ohms through both wired in series. It works I just can't turn the amp way up for a long time, they get too hot. Can't get anything larger around here.

[Dave H]

The gain reduced by more than half, was with C13 and R15 grounded and the following done.

After I connected the ground wire to C13 and R15 and noticed that the hum decreased some I wanted to see if the reduction was due to the ground wire getting rid of the hum, just masking it, reducing gain or whatever I could possible find in case I was asked. I put the amp on standby, disconnected the ground wire, connected up the small oscillator to the input set it to a 1k signal and set the volume on the amp at 3. Took the amp off standby and turned the amplitude on the small oscillator up to get a decent level. Then put the amp back on standby and reconnected the ground wire no changes were made to the oscillator or amp volume took it off standby and that is when I noticed the reduction in volume.

It is confusing. I would expect very little if any audible hum with C13, R15 junction shorted. Does it still hum with the PI tube removed? With the short in place it's as Chuck said the gain is zero to the input signal so what gain is reduced by a half and how is it being measured?

[J Luth]

I will recheck it again later today if I get a chance and try to explain it better.

I have a couple guitar jobs that I need to finish up today.