Steel preamp

Lynxtrap

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Another evening of digging around in this thing, and I am pretty happy with it right now. Updated schematic with voltage readings below.

I was sure this thing was ready to go now, but I descovered a new puzzling thing when taking final voltage readings:

I haven't done any changes to V2B, but now the plate voltage is down to around 55V. It actually fluctuates between 54 and 55. Last time I measured it was 80V, which seemed to make sense given the 1.2k cathode resistor. Or..? Could the plate bypass cap do this if it's going bad?

The cathode still hovers around 3V, which is also a bit strange considering the apparent change in plate voltage?

@andrewRneumann I was thinking the same thing so I actually put a 2.2k there. Cathode voltage went from 2.5 to 2.2, plate from 194 to 207 with the new 100k/2.2k combo.

I deleted the ground lift from the schematic, it can be installed later if needed. Didn't have the components for it, otherwise I would have installed it while I was at it.


Steel Pre schematic rev5.png
 

andrewRneumann

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I haven't done any changes to V2B, but now the plate voltage is down to around 55V. It actually fluctuates between 54 and 55. Last time I measured it was 80V, which seemed to make sense given the 1.2k cathode resistor. Or..? Could the plate bypass cap do this if it's going bad?

The cathode still hovers around 3V, which is also a bit strange considering the apparent change in plate voltage?

Measure the actual resistance of Rl and Rk. And can you get a voltage reading in tenths? Also measure voltage at the grid and verify it is 0V.

I'm not sure anything is wrong, but it is strange how the voltages shifted. I don't suspect the plate bypass cap. But voltage sneaking past the 0.022 coupling cap from previous stage could cause an issue. That is why you should verify 0V on the grid. You can also figure out if something is up in this area by observing if voltages change when you adjust the "level out" pot.
 

Lynxtrap

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Measure the actual resistance of Rl and Rk. And can you get a voltage reading in tenths? Also measure voltage at the grid and verify it is 0V.

I'm not sure anything is wrong, but it is strange how the voltages shifted. I don't suspect the plate bypass cap. But voltage sneaking past the 0.022 coupling cap from previous stage could cause an issue. That is why you should verify 0V on the grid. You can also figure out if something is up in this area by observing if voltages change when you adjust the "level out" pot.

This is what happened just now:

1. Turned it on, the plate read 80V, more stable than yesterday. It settled on 79,8 with the cathode at 2,8. Poked around with chopstick, took about ten readings, could not get it to go down. Turned it off.

2. Turned it on again. Put the probe on the plate pin and watch the voltage go down to 55, while cathode rose to around 3. Poked around with the chopstick, took about ten readings, could not get the voltage to return to 80. In the meantime, all other voltages in the amp as before. Turned it off.

3. Measured plate resistor as 99.4k and cathode resistor 1.18k.

4. Turned it on again. Plate was back to 80V, cathode 2.8V. Proceeded as in 1. Turned it off. Reflowed solder joints on plate resistor for good measure.

5. Turned it on, measured, turned it off. Repeated this about 5 times. 80V every time.

No voltage on the grid or after the .022 cap. When the plate was at 50V, I did temporarily get a fluctuating reading of about 0.004V at the cap, but that seems very close to nothing.

Not sure I can trust it staying stable. But what could be happening? The tube? Poltergeists?
 

Lynxtrap

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A couple of hours of more testing and measuring last night. I could not get it to go down to 55V.
Since it is going to a friend, I will wrap it up now and if it starts acting up or sound different I'll just have to go through it again.

It sure would be nice to know what that voltage drop was about, but as long as it behaves like it should, it's hard to even guess what it might be.

I'm quite happy about the way this turned out. The topology with the mid control between two gain stages and separated from the tone stack is something I'm going to explore further.

Before I added more attenuation, the overdrive and the way it was affected by the mid control sounded very promising for guitar!

It's hard to tell what the 6SL7 actually brings to the table, but I definitely don't rule out trying them in future projects.
 

Lynxtrap

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Some extensive testing by my friend and there are two issues.

With the Output Level (master volume) pot turned all the way down, there is a loud hum. Is this because of the V2B grid becoming grounded? Would a grid stopper solve this? I don't have the amp at the moment so I'm not able to test.

And, the recent changes to the bias of V1A and possibly lower value grid stoppers (less attenuation from the voltage dividers) did not land well, so I'll revert it back to revision 4...
 

andrewRneumann

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Some extensive testing by my friend and there are two issues.

With the Output Level (master volume) pot turned all the way down, there is a loud hum. Is this because of the V2B grid becoming grounded? Would a grid stopper solve this? I don't have the amp at the moment so I'm not able to test.

And, the recent changes to the bias of V1A and possibly lower value grid stoppers (less attenuation from the voltage dividers) did not land well, so I'll revert it back to revision 4...

I’m wondering if the best place for the anode bypass on V2B is anode to ground bus, not anode to B+. Have you tried that? Give that a whirl and see if it helps with the hum.
 

Lynxtrap

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I’m wondering if the best place for the anode bypass on V2B is anode to ground bus, not anode to B+. Have you tried that? Give that a whirl and see if it helps with the hum.

Will try, but I don't really understand why that would create hum specifically with the master volume on zero?

Although I don't see why the lack of grid stopper or perhaps a grid leak would be required either, grids are usually fine with being grounded (even in this very amp)...
 

andrewRneumann

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Will try, but I don't really understand why that would create hum specifically with the master volume on zero?

Although I don't see why the lack of grid stopper or perhaps a grid leak would be required either, grids are usually fine with being grounded (even in this very amp)...

Yeah, it's a dart, and probably won't change anything.

Maybe you need a ground lift after all? Once you move the volume to 0, the previous tube's signal is causing a bit of AC current to return to ground--and possibly it's picking up hum and feeding back when the volume is at 0.
 

andrewRneumann

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A small resistance in the ground circuit is usually enough to interrupt hum. Instead of going all in with the Merlin circuit, just try adding 100R of resistance between the output jack sleeve and its chassis ground.
 

Lynxtrap

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Any luck fixing the hum with output volume on 0?

I had a go at it tonight. I put a 22k grid stopper on V2B, and there is no hum.
Unfortunately I forgot to test it before I did that. But if it was humming when my friend had it, I can't imagine it magically disappearing when I put it on the bench in my workshop, even if such things are known to happen 😄

My theory was that it had something to do with the grid to cathode potential, mainly because I can't think of anything else having that particular effect, except for maybe a faulty pot. I hope my theory holds up and the hum doesn't return.

Thanks for taking an interest in this project!

I changed V1A back to 220k/4.7k from 100k/2.2k. Voltages remained almost exactly the same, remains to see if it does what he wants to the sound.

He thought "rev. 5" sounded "colder" than "rev. 4". The only difference besides the V1A resistors are a 250k volume pot instead of 500k with a 180k across it, some tweaked voltage dividers and the bright switch. Can't imagine any of it leading to a colder sound (with the bright switch off) except for the V1A setup.
 

Lynxtrap

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Back on the bench...
There is a strange problem that I don't understand.
With both volume pots on zero, there is a faint overdriven signal coming through. It sounds almost like the volume was dimed, as if the signal was running hot through the tubes. The signal is affected by the tone controls. It's like there is a ghost signal path in the circuit, but Halloween is over 🤔

Does anyone have a theory?
 




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