Spectral Morn
07-05-2009, 16:35
Hi Guys
Well as you may or may not know from two of my threads in this section of the forum...I have been down an amplifier;as one of its valves had gone. Thanks to Marco a set of nice Svetlana Cryo KT88 valves arrived yesterday, only slightly used.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy124.jpg
With the Music Reference RM200 you have to set the bias of the valves. In the case of the RM200 this must be between 36mv and 44mv. The Rm200 is a parallel push pull amplifier so both sets of valves must be matched...otherwise as there is only one bias adjuster per channel it would be very difficult if not impossible to adjust the bias properly. As Rodney Hanna pointed out to me this afternoon in his opinion a bit of a weakness as it really does mean the valves need to be very closely if not identical in their matching. Individual bias pots would allow for a larger discrepancy in this area (though matching is best).
I began my making sure everything I needed to know was known and the Digital Volt meter was set correctly....Black probe in Coms and Red probe in Volts/omega. I set the dial to Volts DC as per the amplifiers instruction book and then allowed the amp to warm up. In the case of the RM200 the designer says 15 minutes is enough to get things up and running. I left the amp on for that time and then located the TP points at the back of the amp. You place your Red probe into the first TP point for that set of valves and the Black into the closest 8 Ohm speaker post to that TP point.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy126.jpg
With the left channel first I did a reading for both valves using the + and - 8 ohm posts and the same for the Right channel. Without adjustment both sets of valves were to high 64mv and 75mv, so using an electricians screw driver I turned the individual pots slowly clockwise to reduce the bias, which I could follow on the DVM's screen.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy127.jpg
You will need to check the readings over the course of about an hour, just to make sure everything is nice and stable and the readings are not leaping about all over the place.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy125.jpg
Alas for me I had some problems with this...The readings for the two middle valves seemed to suggest they needed to be swapped over. After things had cooled down after the amplifier was switched off (YOU MUST DO THIS)I powered it back up and took readings again I was able to get the right channel nice and stable but not the left. The right channel stabilised at 39mv and 40mv...but the left would not. Valve one was set at 39mv and two at 36 but they kept drifting. The best I got it to, was 38mv and 40mv. However a slight noise started on the left channel and the readings went off. I ended up with 35mv and 58mv....before I switched the amp off.
It would seem that the second valve position on the left channel is compromised in some way. Either a fault exists which took out the old valve or when it failed it damaged something under the valve base. I think it to much of a coincidence for this position to be a problem again, for there not to be something wrong. I spoke to Rodney and he agrees it needs to be checked out.......great a 160 mile round trip to do so.:(
I had hoped this would be a happy little write up, and I would be able to wax lyrical about the Svetlana valves....but alas no. However I wanted to share the procedure of setting bias with you all, as its really not that hard to do. I did take a few precautions such as wearing rubber gloves(No jokes Marco I am not really in the mood ;)and rubber soled slippers and I used extra insulation tape on the screw driver handle and DVM probes. One needs to be careful as there are lethal voltages present in all electronics, but perhaps more so in valve amplifiers, and normally I really would not recommend sticking anything into the guts of a valve amp....let alone a metal bladed screw driver.
So I am back to Sand amplification again...at least for awhile.
Watch this space for the outcome of this saga....
Regards D S D L
Well as you may or may not know from two of my threads in this section of the forum...I have been down an amplifier;as one of its valves had gone. Thanks to Marco a set of nice Svetlana Cryo KT88 valves arrived yesterday, only slightly used.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy124.jpg
With the Music Reference RM200 you have to set the bias of the valves. In the case of the RM200 this must be between 36mv and 44mv. The Rm200 is a parallel push pull amplifier so both sets of valves must be matched...otherwise as there is only one bias adjuster per channel it would be very difficult if not impossible to adjust the bias properly. As Rodney Hanna pointed out to me this afternoon in his opinion a bit of a weakness as it really does mean the valves need to be very closely if not identical in their matching. Individual bias pots would allow for a larger discrepancy in this area (though matching is best).
I began my making sure everything I needed to know was known and the Digital Volt meter was set correctly....Black probe in Coms and Red probe in Volts/omega. I set the dial to Volts DC as per the amplifiers instruction book and then allowed the amp to warm up. In the case of the RM200 the designer says 15 minutes is enough to get things up and running. I left the amp on for that time and then located the TP points at the back of the amp. You place your Red probe into the first TP point for that set of valves and the Black into the closest 8 Ohm speaker post to that TP point.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy126.jpg
With the left channel first I did a reading for both valves using the + and - 8 ohm posts and the same for the Right channel. Without adjustment both sets of valves were to high 64mv and 75mv, so using an electricians screw driver I turned the individual pots slowly clockwise to reduce the bias, which I could follow on the DVM's screen.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy127.jpg
You will need to check the readings over the course of about an hour, just to make sure everything is nice and stable and the readings are not leaping about all over the place.
http://i549.photobucket.com/albums/ii364/davros124/SystemSynergy125.jpg
Alas for me I had some problems with this...The readings for the two middle valves seemed to suggest they needed to be swapped over. After things had cooled down after the amplifier was switched off (YOU MUST DO THIS)I powered it back up and took readings again I was able to get the right channel nice and stable but not the left. The right channel stabilised at 39mv and 40mv...but the left would not. Valve one was set at 39mv and two at 36 but they kept drifting. The best I got it to, was 38mv and 40mv. However a slight noise started on the left channel and the readings went off. I ended up with 35mv and 58mv....before I switched the amp off.
It would seem that the second valve position on the left channel is compromised in some way. Either a fault exists which took out the old valve or when it failed it damaged something under the valve base. I think it to much of a coincidence for this position to be a problem again, for there not to be something wrong. I spoke to Rodney and he agrees it needs to be checked out.......great a 160 mile round trip to do so.:(
I had hoped this would be a happy little write up, and I would be able to wax lyrical about the Svetlana valves....but alas no. However I wanted to share the procedure of setting bias with you all, as its really not that hard to do. I did take a few precautions such as wearing rubber gloves(No jokes Marco I am not really in the mood ;)and rubber soled slippers and I used extra insulation tape on the screw driver handle and DVM probes. One needs to be careful as there are lethal voltages present in all electronics, but perhaps more so in valve amplifiers, and normally I really would not recommend sticking anything into the guts of a valve amp....let alone a metal bladed screw driver.
So I am back to Sand amplification again...at least for awhile.
Watch this space for the outcome of this saga....
Regards D S D L