Hi,
I have recently purchased a k4000 amp which was working, I have given it an overhaul, remade some bad joints and tidied up the wiring and made some Velleman and Maplin suggested mods to improve sound quality and reliability.
I have now come to bias the tubes 3 pairs of tubes I can bias ok to the initial 0.2v level. The last pair when I try to bias the first tube I can almost get it to 0.2v and then a slight turn more on the preset and it jumps to about 0.6v and backing the preset all the way off again has no effect, it just sits at 0.6v until I power off and back on.
I have swapped pairs of tubes over and phase splitter tubes between channels and can bias all tubes in other sockets just not these 2 sockets.
Any suggestions would be most appreciated.
Many thanks
Rich
I have recently purchased a k4000 amp which was working, I have given it an overhaul, remade some bad joints and tidied up the wiring and made some Velleman and Maplin suggested mods to improve sound quality and reliability.
I have now come to bias the tubes 3 pairs of tubes I can bias ok to the initial 0.2v level. The last pair when I try to bias the first tube I can almost get it to 0.2v and then a slight turn more on the preset and it jumps to about 0.6v and backing the preset all the way off again has no effect, it just sits at 0.6v until I power off and back on.
I have swapped pairs of tubes over and phase splitter tubes between channels and can bias all tubes in other sockets just not these 2 sockets.
Any suggestions would be most appreciated.
Many thanks
Rich
Hi Alan,
I have already added a mod as suggested on the Adrian Kingston site (http://www.adrian-kingston.com/K4000.htm). Which is to add a 1 Meg resistor across the wiper of each pot and the -V to mitigate the wipoer going open circuit.
I have also added a 1uF capcitor across R10 as suggested on the above site.
On the left cannel I can bias both pairs of valves without issue.
On the right channel I can bias V1 & V2 without issue.
V3 & V4 are the issue; if I can get V3 to 0.2v which is tricky without it jumping to 0.6v and then proceed to try and do V4 I cant get anywhere near 0.2v before that jumps to about 0.6v, if I then go back and measure V3 again it is showing about 0.8v.
This is only the initial stage of bias setting as the instructions say to leave the amp for 10 mins to warm up and then set all the biases to 0.4v - not a chance the way this is.
I do not have any speakers connected so cannot tell if any oscillation is occuring, I have a pair of 8Ohm 100w wirewounds connected across the speaker terms.
Many Thanks
Rich
I have already added a mod as suggested on the Adrian Kingston site (http://www.adrian-kingston.com/K4000.htm). Which is to add a 1 Meg resistor across the wiper of each pot and the -V to mitigate the wipoer going open circuit.
I have also added a 1uF capcitor across R10 as suggested on the above site.
On the left cannel I can bias both pairs of valves without issue.
On the right channel I can bias V1 & V2 without issue.
V3 & V4 are the issue; if I can get V3 to 0.2v which is tricky without it jumping to 0.6v and then proceed to try and do V4 I cant get anywhere near 0.2v before that jumps to about 0.6v, if I then go back and measure V3 again it is showing about 0.8v.
This is only the initial stage of bias setting as the instructions say to leave the amp for 10 mins to warm up and then set all the biases to 0.4v - not a chance the way this is.
I do not have any speakers connected so cannot tell if any oscillation is occuring, I have a pair of 8Ohm 100w wirewounds connected across the speaker terms.
Many Thanks
Rich
Yes I see, I'm catching up with you now. I've had a quick read up.
Off the wall thoughts:-
Is C17 and or C18 leaking?
Have you measured the voltages round RV3 and RV4 and at pin 5 of V3 and V4? Compare results to the good channel.
Do all of the bias supplies marked -V come from the same place or do the ones to RV3 and 4 follow a different route? Or maybe a tracking problem on the PWB?
Are the voltages on the anodes (pin 3) of V3 and V4 the same (very similar) as V1 and V2? Same for the screens (pins 4) too?
Alan
PS,
Out of interest what voltage is on -V and what sort of voltage is there on the grids (pins 5) of the good channel?
A
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Because this was sold as a kit some 20+ years ago and thus was only put together as good as the person building it, which did leave a bit to desired. Also there were some mods to be made to improve reliability particularly that to stop the cathode resistors turning into a fireball.
I also found that when the power supply had been recapped some years ago the person who had done it had put some film capacitors in parallel with the electrolytics in the psu, the problem being these were in the 450v lines and were only rated at 400V.
I cannot say that the amp was properly biased when I got it as the previous owner of a couple of years was not technical and had not rebased the tubes.
Just because your car is running ok doesn’t mean you shouldn’t service it or do preventative maintenance until it breaks!
I also found that when the power supply had been recapped some years ago the person who had done it had put some film capacitors in parallel with the electrolytics in the psu, the problem being these were in the 450v lines and were only rated at 400V.
I cannot say that the amp was properly biased when I got it as the previous owner of a couple of years was not technical and had not rebased the tubes.
Just because your car is running ok doesn’t mean you shouldn’t service it or do preventative maintenance until it breaks!
Maybe its oscillating. Check you have the transformer connections to the plates and screens the right way round, and you have a 8R on the output.
Alan,
With the amp warmed up for 15 mins I have the following:
On the left (good) channel, biased up correctly to 0.4v I have near as damn it -34v on all grids.
On the right channel it’s -46v on all grids. I have the all pots turned to minimum on this channel but now all 4 valves on this channel are showing a bias voltage of between 0.5 & 0.65v.
The -v into the left channel board is -47.4v and the right is -47.7v so nothing there. The +v3 is 364v on the left and 374v on the right, this is the 350v feed. The +v1 is 425v on the left and 424 on the right, this is the 450v feed.
On the pots I have between -23 & -26v on the ends connected to the gnd (0v) resistor and -v ends are all -47v. The wipers are all at -35v on the left, correctly biased channel and at -47 on the right channel with them all turned down.
I have attached a schematic of the psu board incase it is any help.
One possibility I have to try is to swap over the left and right channel boards and see if the fault moves to the other side.
Many Thanks
Rich
With the amp warmed up for 15 mins I have the following:
On the left (good) channel, biased up correctly to 0.4v I have near as damn it -34v on all grids.
On the right channel it’s -46v on all grids. I have the all pots turned to minimum on this channel but now all 4 valves on this channel are showing a bias voltage of between 0.5 & 0.65v.
The -v into the left channel board is -47.4v and the right is -47.7v so nothing there. The +v3 is 364v on the left and 374v on the right, this is the 350v feed. The +v1 is 425v on the left and 424 on the right, this is the 450v feed.
On the pots I have between -23 & -26v on the ends connected to the gnd (0v) resistor and -v ends are all -47v. The wipers are all at -35v on the left, correctly biased channel and at -47 on the right channel with them all turned down.
I have attached a schematic of the psu board incase it is any help.
One possibility I have to try is to swap over the left and right channel boards and see if the fault moves to the other side.
Many Thanks
Rich
Attachments
It’s on pcbs and the wires are all colour coded or straight jumpered on solid links so all the connections are definitely the right way round.
Check the EL34 grid voltages directly on pin 5, that may give you a clue.
Here is the original booklet; https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k4040.pdf
Here is the original booklet; https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k4040.pdf
Now that all four valves in the same channel show the same symptom after warming up, I have to suspect that this channel oscillates. Is the OT hooked up the right way? Do you own an o'scope?
Best regards!
Best regards!
My initial thought is that you have a problem with the R18 - R21 resistor grounding on the bad side. Or grounding generally?
If you look at the diagram RV1 and R18 (and the 3 others) form a divider across the -47 volt line. So as they are both 100k ohms their mid point has to be half of -47 so -23.5 volts with respect to ground. This is what you get on the good side. (-23 and -26v.)
Please check you have -23 or so volts on the bad side.
Other thoughts are
Are the 1 meg resistors actually 1 meg on the poorly side?
Have you tried measuring the voltage on pin(s) 5 of the faulty channel with the valves out? Does that alter the result?
Alan
Sorry did not explain very well. Just wondered if it was bursting into oscillation at HF when you wind the bias on. If you remove the feedback R13 and C11 and recheck.
Jon, the grid voltages were all measured directly on pin 5 to the ground rail for the particular channel and all 4 tubes were -34v on the good channel and -46v on all tubes in the bad channel.
Kay, the OT is hooked up the right way as all the wires are colour coded and have the colour printed on the board at the terminal. I can borrow a usb scope from work if needed.
Alan, the voltages of -23 to -26v at measured at the ground resistor connection to the pot were for both the good and bad side, there was an even mix between sides, nothing stood out. While I haven’t actually checked the 1M resistors for value I installed them new only the other day and they are 0.5w metal oxide. I am away for the weekend but will do a continuity check between the ground rails on both channels to verify when I get back.
Baudouin, I’ll try removing the feedback and see what happens after the weekend.
Many thanks everyone for your help.
Rich
Kay, the OT is hooked up the right way as all the wires are colour coded and have the colour printed on the board at the terminal. I can borrow a usb scope from work if needed.
Alan, the voltages of -23 to -26v at measured at the ground resistor connection to the pot were for both the good and bad side, there was an even mix between sides, nothing stood out. While I haven’t actually checked the 1M resistors for value I installed them new only the other day and they are 0.5w metal oxide. I am away for the weekend but will do a continuity check between the ground rails on both channels to verify when I get back.
Baudouin, I’ll try removing the feedback and see what happens after the weekend.
Many thanks everyone for your help.
Rich
I tried removing the feedback but this made no difference, so I took the plunge and swapped the left and right channel boards over, so in effect swapped the power and output transformer over and the problem magically disappeared. I do notice that with the pots all turned down the channel board that was giving the issues does read higher than the other board but can still be successfully biased and no longer jumps about.
Many thanks
Rich
Many thanks
Rich
And the other board still works perfect at the other set of trannies? So you might have solved a cold solder joint issue.
Best regards!
Best regards!
You swapped the two output transformers. You didn't swap an output transformer with a power transformer. You would know if you had. Smoke everywhere.
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