A #102 Attempt - First amp

[Vertigo]

I did tweak the OD trimmer back to about 25% last night after playing it a bit more. When the clean volume gets much past 4 or 5 on the dial is where I start to lose note separation in chords and even when playing clean it gets a bit murky.

I did not spend much time with the FET boost on the SC guitars. I'll explore that a bit more tonight.

The power indicator will be rewired. I should have gone with my gut and done it while I was building the amp but lacked the confidence to off "off recipe"

Thanks for the troubleshooting tips. I'll try them out this evening and will get new voltage readings.

I tweaked the PI trimmer so that the output is about 6v less than input...somewhere in the back of my memory I read that. When I powered it up, it had a 9v difference and I can't honestly say that the slight tweak made a noticeable difference in the sound to me.

Brook, my chassis from Amplified Nation says "Drive" and "Volume" on it for the OD section and I went with that even though I thought that I should probably use Level and Ratio. I think I'm going to try to remove the print, cut some vinyl masks and label them how they're supposed to be labelled.

Thanks for all the help!

[martin manning]

I wouldn't sweat the control labels. Both conventions were used.

On the PI trimmer, It's the plate voltage difference you are referring to, correct? The plate voltage on the input stage (the inverting side, which has the lower value plate resistor), should be about 5V higher than the non inverting side, assuming a PI tube with matched sections. From there you can tweak it by ear, or by balancing the output signals, which is a bit more involved.

[Vertigo]

Finally got back to monkeying around with the amp after being out of commission for a week and waiting for a Mouser order. Ayan's 470p/470K peaker mod was added and it really cleared out the mid range gunk on the OD channel, but at the expense of the sensitivity and easy feedback generation at low drive settings. It's more musical now, so I'll put that in the win column.

The voltages have changed a bit since the last reading

V1
1: 202
2: 0
3: 1.587
6: 192
7: 0
8: 1.754

V2
1: 198
2: 0
3: 1.730
6: 198
7: 0
8: 1.759

V3
1: 269
2: 39.2
3: 64.9
6: 262
7: 36.3
8: 64.9

B+1: 446, B+2: 444, B+3: 424, B+4: 322, B+5: 315


Incoming voltage from the wall is 119V
Plate voltage on the power tubes is 438/439

Now that V1 is closer to where it's supposed to be I'll try out a few tubes in V2 to see what kinds of changes occur.

[Vertigo]

I wouldn't sweat the control labels. Both conventions were used.

On the PI trimmer, It's the plate voltage difference you are referring to, correct? The plate voltage on the input stage (the inverting side, which has the lower value plate resistor), should be about 5V higher than the non inverting side, assuming a PI tube with matched sections. From there you can tweak it by ear, or by balancing the output signals, which is a bit more involved.

I think it's likely that I've been reading voltage at the wrong place. I was reading directly off the legs of the trimmer. If I read the voltage directly off the plates, I get 268V(V3-1)/264V(V3-6) when the trimmer is cranked full left. When it's full right it reads 271V(V3-1)/259V(V3-6). I cannot get V3-6 to read higher than V3-1. Isn't that contrary to the popular recommendation? IIRC, I've read that somewhere between 3-7V difference between the two with V3-6 being the higher of the two. I measured some exact resistance values when building this amp, but don't recall if I specifically measured these...is it likely that the tolerance is on the high end of the spec for one of them and that's the issue? Should I be trying more tubes? I'm open to the possibility that I'm going about this entirely the wrong way too.

[martin manning]

The input side plate voltage (pin 6, if your amp matches the layout), should be higher, and this is just as measured to ground. You already have a built in a difference in the fixed plate resistors of 10k, and apparently the sections of the tube are miss-matched enough that even another 10k difference won't bring the input side higher than the other one. If the sections were perfectly matched, the input side should be about 5V higher with the trimmer centered. I'd try another tube.

[Vertigo]

After five tubes, I found one that allows the 3V difference on the plates. The trimmer is turned all the way down, but it works and sounds pretty good when I use a guitar with humbuckers. A buddy just let me know that EuroTubes is nearby so I'm going to pick up a few of their E83CC tubes with "fully balanced triodes" to see how they do.

Thanks once again for the help Martin

[martin manning]

After five tubes, I found one that allows the 3V difference on the plates.

This seems like some pretty bad luck. Picking up a tube that you know is balanced would eliminate that variable. You might want to give the Ampeg method a try, if you can get a signal source and a dummy load. I added a post with some comments to the PI trimmer thread here: https://ampgarage.com/forum/viewtopic.p ... 89#p416389

[Vertigo]

After five tubes, I found one that allows the 3V difference on the plates.

This seems like some pretty bad luck. Picking up a tube that you know is balanced would eliminate that variable. You might want to give the Ampeg method a try, if you can get a signal source and a dummy load. I added a post with some comments to the PI trimmer thread here: https://ampgarage.com/forum/viewtopic.p ... 89#p416389

I went to see the guys at EuroTubes on Friday and they picked me out a perfectly balanced E83CC so we'll see how that goes once I have time to goof around with it again. I bought a few others while I was there so I could try them out in V1 to see if I can tell the difference.

A friend of mine gave me an oscilloscope a few weeks ago and I have a dummy load resistor slated to arrive tomorrow along with the speakers I ordered. I've been reading the PI trimmer thread over and over. I actually tried doing it last week without the dummy load by feeding the signal in, but predictably no one could deal with the sound for more than a few seconds.

[martin manning]

A friend of mine gave me an oscilloscope a few weeks ago and I have a dummy load resistor slated to arrive tomorrow along with the speakers I ordered. I've been reading the PI trimmer thread over and over. I actually tried doing it last week without the dummy load by feeding the signal in, but predictably no one could deal with the sound for more than a few seconds.

Well, that will be fun, you can observe the signal balance directly. And, yea, 50W of sine wave is not pleasant!

[Vertigo]

A friend of mine gave me an oscilloscope a few weeks ago and I have a dummy load resistor slated to arrive tomorrow along with the speakers I ordered. I've been reading the PI trimmer thread over and over. I actually tried doing it last week without the dummy load by feeding the signal in, but predictably no one could deal with the sound for more than a few seconds.

Well, that will be fun, you can observe the signal balance directly. And, yea, 50W of sine wave is not pleasant!

This whole journey has been excellent, even the setbacks.

I bought a 100W 8R resistor from Parts Express and then found some threads suggesting that it's not enough resistance and will probably overheat. Just as a precaution, I'm going to pull some aluminum and copper from my drops bin in the shop to act as a makeshift heatsink, pump the sine wave in and monitor it with the infrared thermometer to see if I'll have enough time to play with the scope. If that doesn't work, maybe I'll finally buy an attenuator.

[martin manning]

For the purpose of setting the balance you only need to dissipate 50W, so it'll probably be ok.

[Vertigo]

That's good to know. Thanks Martin

[Vertigo]

That matched JJ tube I got last week seems to be a dud. One of the plates was reading 80V.

Normally Friday’s are my days to do fun stuff and I was going to get a replacement, but we staged all of our important docs and valuables last night in case we have to evacuate as the fires creep closer to us in SE Portland. Other than a trip to the emergency vet, I’ll be hiding out inside trying to keep my kids feeling normalish The smoke here for the past few days is gnarly. Hoping for some rain out here ASAP and I hope all of you are healthy and safe.

[pompeiisneaks]

Stay safe!

Up here in Washington we've got a lot of smoke/haze and smell of fire, but nothing close in the Seattle area, but our eastern side of the state is in a bad way as well.



~Phil

[Vertigo]

Stay safe!

Up here in Washington we've got a lot of smoke/haze and smell of fire, but nothing close in the Seattle area, but our eastern side of the state is in a bad way as well.



~Phil

I hope the smoke clears out for you soon. The AQI here has been well over 400 for the past few days and while it appears that we're safe, we're getting a little stir crazy. With any luck the weather reports are correct and we'll get the rain and coastal winds that will help suppress the fires and clear out the smoke