#124 Build Questions

[lgehrig4]

Also, I left it on fo a while and the PT felt warmer than I could ever remember another PT feeling. Not hot, but quite warm. Should I take temps of the tubes and PT just to see if either are out of safe range? No smells or burn marks on the tubes. Their glow looked normal.

I'm using a Heyboer PT APD 8004-H

Orange (shield) and red/yellow (CT B+ 0V) are both grounded.

[martin manning]

If two pin 8’s are tied together and each has 1R to ground you have two tubes drawing current through 0.5R.
For 88 mA you want 0.088A*0.5R=0.044V at the test jack, same as if you were measuring one tube using 1R.
I would knock that down 5% since you are measuring Ik which includes screen current. That would make the target 0.044 *0.95=0.0418V

If I'm understanding correctly you set 44 mV on the test jack.

The PT running hot at that current doesn’t sound good, but if you can keep your hand on the PT, it’s not too hot. Something is probably not right, though, like perhaps you have some oscillation going on. Try disconnecting the negative feedback from the speaker jack and see what happens. You may have to swap the OT primary leads.

[lgehrig4]

To simplify things and remove some of the chassis clutter I removed the test jacks and left the 1 Ohm resistor connected to all pin 8's. I bypassed my variac and plugged into the wall which is a little hotter than what I had the variac set to. So now I have 422V and biased to 39mV

As for the issue discussed above, I removed the neg feedback site there definitely is a slight change in the tone, but not volume. I can literally dial up the master and volume to 2:00-3:00 and its not much at all. Clearly not what it should be. That said, what little volume it has does not come on until you hit approximately 2:00 on both the volume and master.

The good news is that the PT didn't get hot. It was warm but nothing that would raise an eyebrow this time. My concern is the volume and the not much happens until you get to 2:00. A majority of the dial does very little between 9:00 to 2:00.

Does this still sound like something that it worth swapping the OT primary leads for?

[martin manning]

If the primaries were reversed, you would have heard a big increase in volume with the feedback disconnected. I think we can rule that out. It sounds like you have a wiring error, an open connection, or maybe a missing ground. Do you have the same issue with or without the OD engaged?

[sluckey]

Is this resistor 820Ω? Looks like 120Ω.

[lgehrig4]

Is this resistor 820Ω? Looks like 120Ω.

Its 820Ω grey|red|brown

[lgehrig4]

If the primaries were reversed, you would have heard a big increase in volume with the feedback disconnected. I think we can rule that out. It sounds like you have a wiring error, an open connection, or maybe a missing ground. Do you have the same issue with or without the OD engaged?

Yes. Clean or OD is the same.

I'll test this now with my 4x12, take more time with the knobs and come back with a more detailed report.

[lgehrig4]

Ran it quickly through its paces. 4x12 and a Les Paul

Normal or FET channel. Overdrive or clean. At best the volume is blues Jr, but not even as fulll. This is with volume and master at 3:00. Seems the volume barely moved until after 2:00 on the master and volumes. Once you hit 4:00+ you start to get some howling. The FET channel in the OD setting is all hair with no body.

It sounds like the OT primaries have been ruled out.? I built this slowly and carefully. The boards were done over a month ago and I checked the wiring multiple times both during and after.

I haven't checked the preamp voltages yet so I guess I'll do that next

[martin manning]

You have some tweaking to do for sure, but it's all solvable. The fact that you get howling at higher gain means there is a tendency to oscillate, which is most likely to be related to lead dress. We can prove that the OT primaries are connected correctly. First, I see a big drop from B+4 to B+5. Something seems wrong there since that should be ~10V or less. Get voltages on the preamp tube pins 1, 2, 3, and 6, 7, 8.

[lgehrig4]

You have some tweaking to do for sure, but it's all solvable. The fact that you get howling at higher gain means there is a tendency to oscillate, which is most likely to be related to lead dress. We can prove that the OT primaries are connected correctly. First, I see a big drop from B+4 to B+5. Something seems wrong there since that should be ~10V or less. Get voltages on the preamp tube pins 1, 2, 3, and 6, 7, 8.

It was the pi tube! As soon as I switched that I got my volume!

I will go back and check the voltages again. Where could I find what the preamp voltages should be? I searched here but didn't find anything.

edit: I did find this discussion on preamp voltages but I was curious if there was something more official to what the original 124 had or what was ideal for this amp.

https://ampgarage.com/forum/viewtopic.php?t=28407

[martin manning]

ok, great! If it's not howling now the OT primary leads are connected properly.

Plates on V1 should be ~180V, V2 maybe 5V higher than V1. Grid and cathode voltages should be reasonable for cathode biased gain stages, grids at zero, cathodes 1-1.3V.

Get voltages at the top and bottom of the 10k drain resistor on the FET board so we can see where it's biased. And BTW what is the value of your source resistor?

[lgehrig4]

And BTW what is the value of your source resistor?

2k

[lgehrig4]

ok, great! If it's not howling now the OT primary leads are connected properly.

Plates on V1 should be ~180V, V2 maybe 5V higher than V1. Grid and cathode voltages should be reasonable for cathode biased gain stages, grids at zero, cathodes 1-1.3V.

Get voltages at the top and bottom of the 10k drain resistor on the FET board so we can see where it's biased. And BTW what is the value of your source resistor?

Here are the voltages. I mentioned above that the source resistor is 2K

IMG_3449.jpg

[martin manning]

The resistor feeding the B+5 node on the filter board is a 22k… should be a 2k2. See if correcting that fixes the V1 voltages. If so, remeasure the top and bottom of the FET drain resistor.

[lgehrig4]

The resistor feeding the B+5 node on the filter board is a 22k… should be a 2k2. See if correcting that fixes the V1 voltages. If so, remeasure the top and bottom of the FET drain resistor.

Nice catch! I must have screwed something up when I measured pins 3 and 8 the first time because the voltages I had written are completely different than what I have now.

B+5 = 276
FET drain resistor | top = 10.8, bottom = 19.42

V1
P1 = 176
P2 = -
P3 = 1.48
P6 = 174
P7 = -
P7 = 1.52