Lonestar Special Critically Ill

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knotts
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Re: Lonestar Special Critically Ill

Post by knotts » Fri Jun 29, 2012 11:04 pm

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 :shock: :shock: :shock: & :?: :?: :?: 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 ! :twisted:

Of course, I may miss something :roll: 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 :mrgreen: ces usines à gaz de LSS :mrgreen: ?

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.
Knotts
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Chester
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Re: Lonestar Special Critically Ill

Post by Chester » Sat Jun 30, 2012 1:05 am

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 :shock: :shock: :shock: & :?: :?: :?: 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 ! :twisted:

Of course, I may miss something :roll: 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 :mrgreen: ces usines à gaz de LSS :mrgreen: ?

Thanks & A+!

WOW - good work

So, given that blown rectifier tubes is a common complaint, is your assessment that the LSS has a "design issue", or that this particular unit is not performing to spec?

My Matchless Indy never misses a beat, which is a good example of how tubes can run hot in Class A without burning out rectifiers. After 1 hour of playing you can bake a pizza on the thing, but it's a well thought out circuit built with bomb proof overspec'd components :mrgreen:
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mark2boogie
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Posts: 268
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Re: Lonestar Special Critically Ill

Post by mark2boogie » Sat Jun 30, 2012 2:48 am

So, given that blown rectifier tubes is a common complaint,
It cannot be from another way if my measurements are correct...
is your assessment that the LSS has a "design issue", or that this particular unit is not performing to spec?
You point it, chester : since I have no other precise information about the LSS (particularly the schems), I cannot confirm that there is a real design issue, but I am akeen to think there is (I service amps since 1980), and this amp is faulty about that.

For me, this amp is running way too hot and too much over the tube specs... So without any further info, the only thing that I can do is to "tame" the plate voltage and change the rectifier for a 5R4GY (best) or a 5U4G (way more available, including at MESA), in order to improve reliability in time, with an acceptable power output sacrifice...

A+!
Only one venerable Boogie MKIIA S/N° 4915 from March 1980 (dixit Daniel "igfraso" - thanks !), in Tolex Cab with a 1978 ALTEC 417-8H Series II speaker, and personal circuit improvements.

guilhemamplification.jimdo.com

mark2boogie
Mark III
Posts: 268
Joined: Thu Jun 02, 2011 4:00 pm
Location: France

Re: Lonestar Special Critically Ill

Post by mark2boogie » Sun Aug 12, 2012 11:57 am

For me, this amp is running way too hot and too much over the tube specs... So without any further info, the only thing that I can do is to "tame" the plate voltage and change the rectifier for a 5R4GY (best) or a 5U4G (way more available, including at MESA), in order to improve reliability in time, with an acceptable power output sacrifice...
That's done. The amp now works correctly since my last post on the subject. His owner is very happy, the difference in loudness is not critical, and it has a "brownier" sound...

Over.
Only one venerable Boogie MKIIA S/N° 4915 from March 1980 (dixit Daniel "igfraso" - thanks !), in Tolex Cab with a 1978 ALTEC 417-8H Series II speaker, and personal circuit improvements.

guilhemamplification.jimdo.com

rfrakes
Mark II
Posts: 100
Joined: Thu Jun 07, 2012 11:49 am

Re: Lonestar Special Critically Ill

Post by rfrakes » Thu Sep 20, 2012 4:15 pm

http://www.filestube.com/m/mesa+boogie+lonestar+special

http://www.google.com/imgres?imgurl=htt ... w&dur=4543

I hope these help. I guess the second link is useless, as it is more of a flow chart.

ja22y
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Posts: 360
Joined: Fri Oct 05, 2007 3:51 pm

Re: Lonestar Special Critically Ill

Post by ja22y » Fri Sep 21, 2012 3:10 pm


rfrakes
Mark II
Posts: 100
Joined: Thu Jun 07, 2012 11:49 am

Re: Lonestar Special Critically Ill

Post by rfrakes » Sat Sep 22, 2012 3:14 pm


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