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Yesterday, 01:48 PM
(This post was last modified: Yesterday, 01:52 PM by Steve Ward.)
Hi,
I'm looking for some technical help.
So I am in the UK and shipping costs from Canada are quite expensive, so when I bought SV1 and RBX for an amp some time ago I also bought SV3 & VCK having a cathode biasdd amp planned.
I finally got around to building that cathode biased 4 x el84 amp and after getting it all working correctly, I fitted the SV2–M36 and VCK.
The amp is based on a trainwreck rocket which was based around an ac30, but I’ve put my own slant on it also adding a few extra features and enhanced cathode adjustment/separation.
I’ve got the SV2-M36 working , but I’m seeing a fairly large voltage loss at full Power Scale and wondered whether this version needs tailoring for the amp’s B+.
Raw B+ at M2 drain is about 336V. At PS fully clockwise, M2 source/output is only about 281–285V, so M2 is dropping around 55V. Gate-source is about +4.7V. PS1/PS2 at full CW are about 33.2/33.3V.
The scaling otherwise works properly, although the 1M pot sweep gets to minimum voltage at less than half of its rotation, so I inserted a 500k trim pot and now the rotation is perfect. SV2 will take the output stage down to about 1V (I haven't rechecked this since inserting the 500k trimmer, having set by ear instead), and VCK is holding the downstream B3 supply very close to its original voltage.
I’ve checked R1 and R9, replaced D1, repaired a poor R23 joint, and the FQU2N90S M1 tests normally. M2 is running around 80°C at full PS, versus about 50°C for M1.
Is ~55V drop at full PS normal for the SV2-M36 at this B+, or should the QP4/QP5 control network be adjusted? If it should be lower, which resistor values would you recommend changing for a raw B+ of roughly 335–340V?
Also I pulled all the power tubes and retested, still getting the large voltage drop so can confirm its not load related.
I found somewhere that there might be some component values that could be tweaked to overcome this but not sure which ones and what values to try, any help would be greatly appreciated.
many thanks
Steve
I'm looking for some technical help.
So I am in the UK and shipping costs from Canada are quite expensive, so when I bought SV1 and RBX for an amp some time ago I also bought SV3 & VCK having a cathode biasdd amp planned.
I finally got around to building that cathode biased 4 x el84 amp and after getting it all working correctly, I fitted the SV2–M36 and VCK.
The amp is based on a trainwreck rocket which was based around an ac30, but I’ve put my own slant on it also adding a few extra features and enhanced cathode adjustment/separation.
I’ve got the SV2-M36 working , but I’m seeing a fairly large voltage loss at full Power Scale and wondered whether this version needs tailoring for the amp’s B+.
Raw B+ at M2 drain is about 336V. At PS fully clockwise, M2 source/output is only about 281–285V, so M2 is dropping around 55V. Gate-source is about +4.7V. PS1/PS2 at full CW are about 33.2/33.3V.
The scaling otherwise works properly, although the 1M pot sweep gets to minimum voltage at less than half of its rotation, so I inserted a 500k trim pot and now the rotation is perfect. SV2 will take the output stage down to about 1V (I haven't rechecked this since inserting the 500k trimmer, having set by ear instead), and VCK is holding the downstream B3 supply very close to its original voltage.
I’ve checked R1 and R9, replaced D1, repaired a poor R23 joint, and the FQU2N90S M1 tests normally. M2 is running around 80°C at full PS, versus about 50°C for M1.
Is ~55V drop at full PS normal for the SV2-M36 at this B+, or should the QP4/QP5 control network be adjusted? If it should be lower, which resistor values would you recommend changing for a raw B+ of roughly 335–340V?
Also I pulled all the power tubes and retested, still getting the large voltage drop so can confirm its not load related.
I found somewhere that there might be some component values that could be tweaked to overcome this but not sure which ones and what values to try, any help would be greatly appreciated.
many thanks
Steve
