Unfortunately I have no way of heating the tubes to 120C and also do not have a variac, but is not a problem because I've still got a set of FU50.
Already changed the bias to 0.45 but it is too late to be able to increase the volume and see if is distorting (I do not want to wake the neighbors)
Already changed the bias to 0.45 but it is too late to be able to increase the volume and see if is distorting (I do not want to wake the neighbors)
They look identical to the GU50 but I could not say what material they have or what power handling.
Is this usual for GU-50 tubes to start off a little gassy? I've heard about (and owned) many that seem to be like this - and my GU-50s are the only tubes I own with visible getter usage.
Any ideas where the gas comes from (manufacture, storage seepage, other?).
It is usual for any NOS tube, especially kept in wet environment.
Bake it in 200C oven for couple of hours, then try.
Can't edit the post, adding.
Originally the tube was designed by Telefunken Deutschland, manufactured for German military and government during WW-II under the name LS 50.
After the war it's envelope was redesigned. It was manufactured by Soviet plants for Soviet military and government under the name ГУ-50. At the same time Eastern German RFT plant manufactured a version without an aluminium cap, under the name P-50.
Swiss BBC manufactured own version of LS-50, called P50-1. Despite of name similar to RFT version it had Telefunken type envelope. P50 and ГУ-50 had identical envelopes.
Slightly modified version, compatible with LS-50, was called by Swiss BBC P50-2.
Eastern German tubes P50/1 and P50/2 were different! They shared the same envelope with Soviet one. P50/2 was redesigned P50 to serve as a sweep pentode in TV sets.
Later Germans made EL152: 6.3V filament version in a different envelope.
ГУ-50 tubes we buy today were released from military warehouses. That means temperature changes, humidity changes, many times, during long period of time. Imagine Siberia where temperature may change from -40C in winter to +40 in hot summer! No metal can have the same temperature coefficient as a glass, so microscopic gaps between legs and glass exist. That's why they need to be baked before first usage, otherwise molecules of gases would eat cathode coating instead of getter flash.
That's why I was asking about Chinese Fu50: do they use identical materials, or geometry only? ГУ-50 can dissipate 100+ Watt shortly, without any damage. If Chinese tubes are the same, it is a good sign! That means, I may expect to see a tube of my dream, like on the picture:
Originally the tube was designed by Telefunken Deutschland, manufactured for German military and government during WW-II under the name LS 50.
After the war it's envelope was redesigned. It was manufactured by Soviet plants for Soviet military and government under the name ГУ-50. At the same time Eastern German RFT plant manufactured a version without an aluminium cap, under the name P-50.
Swiss BBC manufactured own version of LS-50, called P50-1. Despite of name similar to RFT version it had Telefunken type envelope. P50 and ГУ-50 had identical envelopes.
Slightly modified version, compatible with LS-50, was called by Swiss BBC P50-2.
Eastern German tubes P50/1 and P50/2 were different! They shared the same envelope with Soviet one. P50/2 was redesigned P50 to serve as a sweep pentode in TV sets.
Later Germans made EL152: 6.3V filament version in a different envelope.
ГУ-50 tubes we buy today were released from military warehouses. That means temperature changes, humidity changes, many times, during long period of time. Imagine Siberia where temperature may change from -40C in winter to +40 in hot summer! No metal can have the same temperature coefficient as a glass, so microscopic gaps between legs and glass exist. That's why they need to be baked before first usage, otherwise molecules of gases would eat cathode coating instead of getter flash.
That's why I was asking about Chinese Fu50: do they use identical materials, or geometry only? ГУ-50 can dissipate 100+ Watt shortly, without any damage. If Chinese tubes are the same, it is a good sign! That means, I may expect to see a tube of my dream, like on the picture:
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Thanks for the tip - I didn't know my cathode as at risk.
The aluminum cap looks like it has seen some water or dampness so I expect they were stored in some damp warehouse for a while. I'll bake my other 6 GU-50 tubes (I ordered 8 of them).
I love the whole history and toughness of the GU50 which is why I ordered the GU50 version of the amp, I have a suspicion that are better than all the other big output tubes, at any price.
The FU50 _look_ identical to the GU50, but I cannot say if they are, they sound the same too as far as I can tell
What is the tube you show - that's too tall for a GU50! Looks like it should be a GU75.. it's photoshop isn't it?
The aluminum cap looks like it has seen some water or dampness so I expect they were stored in some damp warehouse for a while. I'll bake my other 6 GU-50 tubes (I ordered 8 of them
).I love the whole history and toughness of the GU50 which is why I ordered the GU50 version of the amp, I have a suspicion that are better than all the other big output tubes, at any price.
The FU50 _look_ identical to the GU50, but I cannot say if they are, they sound the same too as far as I can tell
What is the tube you show - that's too tall for a GU50! Looks like it should be a GU75.. it's photoshop isn't it?
What is the tube you show - that's too tall for a GU50! Looks like it should be a GU75.. it's photoshop isn't it?
No, it's free Irfanview software.
My point is, if Chinese plant still make them, they can make such a killing tube. 50% longer electrodes would magically convert it from RF to the best AF output tube ever made.
Hello Globulator,
During yesterday I was listening more carefully and noticed that something was wrong.
After checking the voltages again, the voltage at thermistor was 0V, but between the two black wires had 5.9V, seems to me therefore that the thermistor is not ok.
This may have been caused by me, when changing the tubes I inadvertently put one 6N3 in that position.
The other voltages seem well:
P in FU 50 393V,
B + in the preamp / driver 371V,
6N3 4.47 V / 4.24 V
ECC82 5.72 V / 5.71V
What I find strange is the amp still play, despite sounding out with less definition, less separation of instruments, and distortion that seems to be evident mainly in metals.
Can you please give me your opinion?
Cheers
During yesterday I was listening more carefully and noticed that something was wrong.
After checking the voltages again, the voltage at thermistor was 0V, but between the two black wires had 5.9V, seems to me therefore that the thermistor is not ok.
This may have been caused by me, when changing the tubes I inadvertently put one 6N3 in that position.
The other voltages seem well:
P in FU 50 393V,
B + in the preamp / driver 371V,
6N3 4.47 V / 4.24 V
ECC82 5.72 V / 5.71V
What I find strange is the amp still play, despite sounding out with less definition, less separation of instruments, and distortion that seems to be evident mainly in metals.
Can you please give me your opinion?
Cheers
Sounds like the thermistor is not doing anything then - or it's too big - do you get any switch-on flash on the ecc82?). That device should not affect the sound though.
Bit confused as to why you only have 5.9V on your filaments - in my Sweet Peach the heater voltage was bigger than the HT voltage (in proportion) and would be 6.3V at 230Vac input. What is your mains Vac voltage?
5.9V is still within specs for the tube however - you can check in the tube datasheet.
Maybe there is a problem somewhere and that's why you seem to run a hotter mains transformer (If it is hotter!).
If all filament voltages, anode voltages and cathode voltages are fine, there probably isn't anything bad happening - can you have another listen?
Bit confused as to why you only have 5.9V on your filaments - in my Sweet Peach the heater voltage was bigger than the HT voltage (in proportion) and would be 6.3V at 230Vac input. What is your mains Vac voltage?
5.9V is still within specs for the tube however - you can check in the tube datasheet.
Maybe there is a problem somewhere and that's why you seem to run a hotter mains transformer (If it is hotter!).
If all filament voltages, anode voltages and cathode voltages are fine, there probably isn't anything bad happening - can you have another listen?
Hello,
At lunch break I had time to remove the thermistor, this was the biggest problem the sound immediately improved but I still had some distortion.
I gradually increase the bias to 55mA and the difference is huge, seems to me that this amp does not like low bias.
Filament voltage is now at 6V.
It seems that now everything is fine but I need to hear more, the tubes are still new.
Thanks for the help Globulator
At lunch break I had time to remove the thermistor, this was the biggest problem the sound immediately improved but I still had some distortion.
I gradually increase the bias to 55mA and the difference is huge, seems to me that this amp does not like low bias.
Filament voltage is now at 6V.
It seems that now everything is fine
but I need to hear more, the tubes are still new.Thanks for the help Globulator
Hi, sounds like you have better lunch breaks than me
It looks like the thermistor was dropping 0.1V then - I wonder why it had a reading of 0V? Was your multimeter still on a high range at the time?
Interesting it has an effect on sound. More interesting is that the lower bias had such an effect, I had sort of noticed it sounded better around 55mA but I did not look into it.
This indicates that the loud bits will still be non-linear - even at 55mA. Which means my next mod should be another big jump in sound.
One thought I had about the transformer heat was to use a small toroid as a filament heater (provided I could find somewhere to put it) this would take quite a load off the main transformer. It needs to supply about 12.6V for the GU50s and 6.3V for the driver tubes, I need to look up the power budget too.
It looks like the thermistor was dropping 0.1V then - I wonder why it had a reading of 0V? Was your multimeter still on a high range at the time?
Interesting it has an effect on sound. More interesting is that the lower bias had such an effect, I had sort of noticed it sounded better around 55mA but I did not look into it.
This indicates that the loud bits will still be non-linear - even at 55mA. Which means my next mod should be another big jump in sound
.One thought I had about the transformer heat was to use a small toroid as a filament heater (provided I could find somewhere to put it) this would take quite a load off the main transformer. It needs to supply about 12.6V for the GU50s and 6.3V for the driver tubes, I need to look up the power budget too.
Does some Chinese amp ever exist with right transformers, or they are too heavy to ship overseas and get some profit?[/QUOTE]
I think it depends on what temperatures you have ambient, and what you are happy with. Mine takes a good long time to heat up (Summer in the UK - about 20C outside) and the most it gets is still OK to touch but you can tell it's warm! The OPTs never get warm at all and have loads of bass and a clear treble - even with my current super low feedback mod - so I think the OPTs are really quite good.
The mains transformer is potted by the way - so the outside temperature is probably close to the inside temp. and therefore nothing is technically getting too hot - unless up to 50C is too hot. I suppose it could be 10% bigger - but this is a cheap cheap amp remember, it's never pretended to be anything else which is why it's nice getting some really good sound out of it - and far cheaper than a kit even if I bought a bigger mains transformer for it
I think it depends on what temperatures you have ambient, and what you are happy with. Mine takes a good long time to heat up (Summer in the UK - about 20C outside) and the most it gets is still OK to touch but you can tell it's warm! The OPTs never get warm at all and have loads of bass and a clear treble - even with my current super low feedback mod - so I think the OPTs are really quite good.
The mains transformer is potted by the way - so the outside temperature is probably close to the inside temp. and therefore nothing is technically getting too hot - unless up to 50C is too hot
. I suppose it could be 10% bigger - but this is a cheap cheap amp remember, it's never pretended to be anything else which is why it's nice getting some really good sound out of it - and far cheaper than a kit even if I bought a bigger mains transformer for it - Status
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