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+--- Thread: SV2-M36 Voltages (/Thread-SV2-M36-Voltages)
SV2-M36 Voltages - Steve Ward - 09-17-2026
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
RE: SV2-M36 Voltages - K O'Connor - 09-17-2026
Hi Steve
With PS set for maximum output voltage there should only be about 4V drop across the mosfet and the waste heat will be pretty low. For example, 50mA load corresponds to only 200mW of heat and a very low temperature rise.
As the threads about potentiometer tolerances, PS sweep and the compromises that result regarding possible dead spots of sweep, the issue comes down to the pot itself and the circuit voltage. The 1M pot can be 1M2 down to 800k at its 20% tolerance extremes. In spice, a 900k pot gives the nearly expected sweep in the SV2-M36 where R23=36k, R5=332k, R3=100k and D5=4v7. R3 and D5 do not effect the overall sweep, there role is to set a minimum output voltage. "Nearly" means that the high-end voltage drop is closer to 11V rather than 4V.
R5 is a power resistor in the physical circuit with a value of 330k; at 5% it can be 346k5 to 313k5. R23=36k is 1%.
With the 900k pot and R5=330k, changing R23 from 36k to 38k gets us to a 4V drop but a short dead spot of sweep covering about 20k (22% of the total pot resistance). In the simulation there is a "sweet spot" value between 36k and 37k5 where the dead sweep is reduced to zero.
If the pot is 800k, R23=37k5 works well. Most of the 1Ms I have measure around 900k, with very few lower than that.
For a 1M2 pot and stock value of 36k, maximum Vout has a 6V drop.
Back to R23=36k and a 900k pot: If R5=313k Vout drops 4V and there is a small dead sweep; R5=346k Vout loses 23V, which can be reduced to 4V with R23=39k.
To summarise the tests above, we see that the two loose-tolerance components in the divider dominate the initial behaviour. Fortunately their values are stable over time and changing the tight-tolerance R23 can produce pleasing results.
RE: SV2-M36 Voltages - Steve Ward - 09-17-2026
Many Thanks Kevin,
I thought it was something to with the PS control but couldn't work out what and where. I used a CTS good quality 1MA pot for PS and I measured a sample from the stock I have and pretty sure it was selected to be as close to 1M as I could find. I'll check the values of the resistors you mentioned. I might try a couple of different 1M pots this time at deliberately different values and if no luck I can experiment with R23. I'll report back with results!
best
Steve
RE: SV2-M36 Voltages - Steve Ward - 09-17-2026
Hi Kevin,
I made some progress tonight and the results seem to show that the excessive full-output voltage drop is not simply due to the tolerance of the PS pot or R5.
The amp is being tested with all four EL84s removed and PS fully clockwise. The excessive drop was present both before and after I added the low-end PS3 trim, so that trim does not appear to be the cause.
The originally fitted components were:
- PS pot: CTS 1MA, measured 990k end-to-end
- R5: removed and measured 326–327k
- R23: previously lifted and confirmed 36k
- Added PS3 trim currently set to 176k
As an experiment I replaced R5 with the lowest value 3W 330k resistor I had, which measures 323k, and temporarily replaced the 990k audio PS pot with a 1.1M linear pot. The linear taper was only used for testing the full-CW endpoint.
With 1.1M PS pot + 323k R5, EL84s removed and PS fully clockwise, I measured:
Point Voltage
W1-1 / B+ input 381.7V
W1-2 / OT centre tap 330.9V
Voltage lost through plate regulator 50.8V
R5 PS1 end 38.1V
PS2 / pot wiper 38.2V
M1 gate 38.18V
M1 source 31.92V
M1 drain 382.1V
M2 gate 337V
So changing from the 990k pot / ~326.5k R5 combination to 1.1M / 323k only reduced the full-CW loss from roughly 55V to 50.8V.
There are a couple of things that look significant:
PS1, PS2 and M1 gate are effectively identical at full CW, so the pot is reaching its high-end correctly and the command voltage is getting through R1 to M1.
M1 is also clearly operating: gate 38.18V, source 31.92V, so VGS is about 6.26V; its drain is essentially at B+.
M2 gate is 337V, while W1-2 is 330.9V, so the gate is only about 6.1V above the regulated plate output. M2 is therefore still dropping the entire 50.8V between its input and output.
R5 also seems to be behaving sensibly. With 381.7V at its B+ end and 38.1V at its PS1 end, the 323k resistor has about 343.6V across it, corresponding to roughly 1.06mA.
Given your simulation examples, I would have expected the measured combination of approximately 1.1M PS pot, 323k R5 and 36k R23 to be considerably closer to the desired full-output condition than this. The change in pot/R5 value has moved the output slightly in the expected direction, but clearly does not account for the remaining ~51V loss.
My next proposed experiment is to temporarily parallel the 323k R5 with approximately 10M, giving an effective R5 of about 313k, close to the value in your example that produced roughly a 4V drop. I have not done that test yet.
At this stage it looks to me as though the excessive drop may be arising further into the M1 / QP driver / M2 gate-drive section rather than simply from the PS-pot/R5 tolerances.
Do these voltages suggest a particular node or component in the M2 driver section that you would check next?
Best,
Steve
RE: SV2-M36 Voltages - K O'Connor - 09-17-2026
Hi Steve
With a 1M pot and R23=36k and R5=330k Vout max is low by about 8V.
With R5=313k Vout max is low by 4V.
If R5=346k Vout max loses 23V.
I haven't found the fixed resistors to be off by their limit, but there are a lot of parameter spreads to deal with. For one thing, I don't have a model for the FQU and have used something that is not as similar gate-wise as I think. Gate threshold voltages on mosfets have a wide spread, so you will have to tinker with the fixed Rs to get the loss at Vout max down to where it should be.
One thing you might be able to control is how hot the mosfets get. Is there airflow over the chassis position where the board is mounted? If not, add a low-speed fan to move air past the tubes and out the back of the cabinet.
6Vac >> bridge >> 10mF cap >> 8Vdc to 12Vdc fan
Have fun
RE: SV2-M36 Voltages - Steve Ward - 09-17-2026
OK thanks Kevin,
I'll continue with this tomorrow. I was intending to add a fan once I build the head shell for it later.
Which other fixed resistors would be worth trying apart from R5 & R 3? given my readings so far.?
are you also saying a different FQU would be worth a go?
best
Steve
RE: SV2-M36 Voltages - K O'Connor - 09-17-2026
Hi Steve
Just work with the fixed resistors.
Have fun