mark2boogie wrote:Hi All,
I made some measurements on the LSS (S/N° LS-002445, export model 230VAC) that I have here for "fuse cure" issues... Well...
1 - I have no schematic of the LSS, nor found one via the web, unfortunately (it's a pity... It would be a great help...).
2 - this is the 2nd time that I see this amp for "fuse cure" : the 1st time, power tubes were dying or dead, and the valve was flashing, when a bigger fuse was put for test purposes, because the normal fuse kept on blowing too quickly. This was in March 2012. The set of tubes has been replaced and I put an 1.25A fuse instead of the 1A (export version). After 3 hours of use and no issue, I considered the amp OK for duty.
3 - the amp is back in late June 2012, fuse blown, in nearly the same situation : the valve is dead, 2 of the EL84 are out (one is half the normal current, the other one tends to thermal runaway, tubes 1 and 4, from L to R, rear facing). All these tubes were new in March, and the voltages in the amp were a bit higher than the tube's specs, but nothing surprising nowadays in a guitar amp.
4 - So I put the amp on a Variac to have the exact required voltage (with a DMM for monitoring it), and found on 30W position, at idle current, vs GND, with a set of checked/paired EL84 tubes (42/48ma) and a 5U4G valve instead of a 5Y3:
HV PSU = 356VDC, stable
Vplate = 340-358V, so strangely floating, but this may be due to a special design of the power stage vs my measurements points (?)
Vscreen = 352V, stable
Vkathode = 12V1, stable
So nothing really dramatic, no tube glowing red, even if I found them somewhat "pushed" hot, at least to my taste...
5 - So I made HV total current measurements, at idle current (real serial measurement with a DMM ammeter) :
I30W = 315mADC - here the whole power tubes are powered via the diodes
I15W = 175mADC - here only the outer valves are powered via the valve
I5W = 175mADC - same situation as 15W, but only one tube would be driven in signal, not the other one (t's my supposition).
6 - In the same condition as above, I measured the plate to cathode voltage on the EL84/6BQ5s :
Vpk30W = 337VDC
Vpk15W = Vpk5W = 316VDC
Nothing unacceptable here.
7 - I calculated the plate+screen (= total) dissipation of the tubes, considering that the rest of the preamp tubes could absorb 25mA (which is large, but possible thanks to unseen local voltage regulators or bleeders) :
D30W = ((337*(0.315-0.025))/4) = 24.5W per tube !
D15W = ((316*(0.175-0.025))/2) = 23.7W per tube !
So & indeed...
8 - considering the EL84/6BQ5 specs :
Vpkmax = 300VDC
Dtot = Dp+Ds = 12+2 = 14W max total dissipation
Ikmax = 65mA
9 - considering the 5Y3GT specs :
Ikmax = 110-125mA (peak max = 375mA)
Vpmax = 400V
10 - I am not surprised at all if the Lonestar Special has some reliability problems under those working conditions !
Of course, I may miss something in the process because I do not have the schematic to understand clearly the idiosyncrasies of the LSS...
Any of you has made such investigation, or have additional information that I do not have about ces usines à gaz de LSS ?
Thanks & A+!
I'm not really sure what you said but you're clearly smarter than me so I'm going to go agree.
FWIW I have an LSC that had some issues with 6L6's but I switched to EL34's and the issues have stopped.