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Problems with London City Bulldog / Ashton VP100 tube amp
Problems with London City Bulldog / Ashton VP100 tube amp
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Old 26th March 2020, 07:30 PM   #11
Ricotjuh is offline Ricotjuh
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Sorry but I'm not following you anymore. At the moment there is a 12AT7 in place of V4. This is straight from the factory. Are you saying I can swap it for a 12AX7 without any changes?
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Old 26th March 2020, 07:33 PM   #12
Ricotjuh is offline Ricotjuh
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Since there is a 12AT7 in from the factory, I will check whether R93 differs from the schematic.
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Old 26th March 2020, 10:39 PM   #13
Ricotjuh is offline Ricotjuh
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Quote:
Originally Posted by Ketje View Post
You can't just put an 12AT7 in for V4 without changing R93 (470Ω).
That tube needs ~1k2 for R93.
I guess the common cathodes are ~45V above ground if all is well.
Mona
Just checked and R93 is indeed 470 ohms. You say that this is not good anyway. So should I replace this tube for a 12AX7? An AT version does not belong here?
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Old 27th March 2020, 01:50 PM   #14
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Ricotjuh View Post
Just checked and R93 is indeed 470 ohms. You say that this is not good anyway. So should I replace this tube for a 12AX7? An AT version does not belong here?
No, I checked, it's ok too.
The 1k2 was to get the same anode currents as with the 12AX7.
The 12AT7 can go much lower with the Va, the extra drop on the Ra (higher Ia) is no problem.
Draw some load lines to be certain. It's with Rk=1k for the 470Ω case anode 2k4 for 1k2 since the resistor is common to the two triodes.
To see if the hum is from the V4, try without it, if still hum it's the final stage.

Mona
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Old 27th March 2020, 07:47 PM   #15
Ricotjuh is offline Ricotjuh
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Jep, also hum without tube V4. So I will have to look in the final stage.
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Old 27th March 2020, 09:59 PM   #16
Ricotjuh is offline Ricotjuh
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Just taken measurements with an oscilloscope. The measurement results have been put in the schematic below. What strikes me is that the preamp does what it should do. Although I think it is strange that when the level knob is turned up, the signal is cut off. This is not the case at a lower level. I don't think this is right?!

Then measured further after the Master potentiometer. Here we get to see a nice sinewave again. But then after V4 it seems to go wrong. The entire bottom of the sinus is cut off. And this is reflected in the output.

I also turned the master all the way back to 0. Then zoomed in on the signal and then there still seems to be a 100Hz hum. I expect it is also this one that I hear.

Please send your feedback on my measurement results.

Click the image to open in full size.
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Old 27th March 2020, 10:32 PM   #17
Ketje is offline Ketje  Belgium
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Doesn't look very nice.
Problem is, the feedback is trying to correct the result (or is suposed to), so it could be the feedback misbehave.
If you disconnect R102 (47k) there is no FB, gives more insight where it goes wrong.
Or is it the inductor formed with the IC4 (evt a source of hum), disconnect C65 (10F) to put the IC4 out of the circuit.
Mona
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Old 28th March 2020, 12:08 PM   #18
Ketje is offline Ketje  Belgium
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Here some voltages you can check.
Mona
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Old 28th March 2020, 05:58 PM   #19
Ricotjuh is offline Ricotjuh
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Just measured the indicated points. Note, I mirrored V4 in the drawing. In practice it turned out to be correctly mirrored. Does not matter for the calculation and operation.
But the measurement deviates after R95 and R94 from what you indicate. So the DC settings on these points already seem to be wrong.
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Old 28th March 2020, 06:40 PM   #20
Ricotjuh is offline Ricotjuh
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Maybe also interesting to show. I also took a measurement after C61 & C62. Then you can clearly see the differences
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