• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Modding a Velleman K4040

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#65
Or this...
Screenshot (170).png
 
#67
I ordered the parts for the current mirror alternative since they were cheap and locally available. I hate to drag this project out any more than I have to and this way I should be able to get the phase splitters operational one way or the other very soon. Thanks again for all the help.
 
#69
I finally got around to inputting some music and found that I some significant motorboating at output tube sockets pin 5. I don't have the output tubes installed yet as I wanted to make sure I had clean audio out of the phase splitters first. I have the motorboating without any input also. Do you think that changing C4 (and C3 ?) to 100nF would fix this?
 
#71
It may be its oscillating at VHF which you don't see but causes the motorboating. Try a grid leak resistor in series with the grid on U3 1K. If you have the output tubes unplugged then there is no method of LF oscillation I can see other than if there's also HF instability.
 
#73
baudouin0 said:
It may be its oscillating at VHF which you don't see but causes the motorboating. Try a grid leak resistor in series with the grid on U3 1K. If you have the output tubes unplugged then there is no method of LF oscillation I can see other than if there's also HF instability.
Click to expand...
I don't completely understand, if I add a 1K in series with U3 grid, wouldn't that be in series with R4 (1 meg)? I thought (although probably wrong) that grid leak resistors were from grid to gnd. (?)

I was wondering if the motorboating was being caused by the 10M90S's (?).

Thanks for the help, baudouin0 & gerrittube.
 
#74
Sorry grid stopper resistor in series with all else to grid. The word leak means grid to ground I used it wrong.
https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

Yes I saw on another thread here that the 10M90S can oscillate with a LTP. This is HF oscillation but can causes all sorts of problems from hiss to motorboating to picking up radio.

I don't think that circuit can just motorboat without the output valves and FB connected. If its just an LF problem it should show up in LTspice anyway.
 
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#75
OK, thanks. I think I will try a bc574b based current mirror as per your previous schematic for one channel and see if that fixes the problem. I had the motorboating with the output valves not installed and even unhooked the global feedback even although I didn't think that should matter with the outputs not installed. The audio was clean coming out of the 12AX7's but not at the 12AT7 plates.
 
#80
I finally found time to sort things out. Once I hooked up my scope I found that I did not have motorboating at the outputs of the phase splitters. It seems that the oscillation/motorboating was not the amp but caused by a defect in my ancient Heathkit IT-12 signal tracer which was amplitude related. :-( Both the 10M90S & the BC547B based current supplies work fine. I ended up using the current mirror version for both ch's.. I installed the KT88's, set bias and found the amp to sound fantastic hooked to my Acoustat speakers. The only problem left is getting the global feedback to work. As soon as I connect it very loud motorboating occurs. Even without the feedback the amp sounds better than it did before however. At this point I am very happy with the results. Thanks to everyone who has contributed to this project, especially baudouin0. 🙂
 
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