G2 problem

hi everybody !

I built the G2 amplifier some weeks ago with some slight modifications

1. I put a led driver circuit in parallel with the powertransformer
2. used seperate switches for smooth and fat
3. added a bright switch (disconnecting the capacitor on the volumepot, I won't recommend this one as it didn't bring much)
4. added a switch to the secondairy of the outputtransformer switching between 2 and 3 (with 5 as common) so I can connect an 8 or 16 ohms speaker

the amplifier worked great for the first 10 to 15 hours, all voltages were
ok according to the schematic (all within 1 or 2 volts)
all mods worked fine too

then suddenly during use the power (loudness) went down and a scratching
sound accompanied the now quiter and distorted sound
this scratching sound was like a discharging electrolytic capacitor
(starts loud, gets softer and fades out after a few seconds, then comes back again but not in a rythmic way)
it was not affected by the setting of the volumepot or inputsignal, it even occured with no jack plugged into the input

i checked the voltages and noticed they had dropped quite a lot :

C10/R12 201V DC to ground
C9/R11 190V DC to ground

this made me suspect the electrolytics (C9 & C10) which I changed (I used 47uF
450V types, they are little larger but still fit in well)
the scratchy sound is gone now, but voltages did not change and sound is not as loud as it used to be and very distorted (in an ugly way, gated and without any sustain)

so now the voltages are
C10/R12 201V DC to ground
C9/R11 190V DC to ground
+ of rectifier 215V DC to ground
output (red/red) of powertransformer 186V AC
(229V AC on primairy of powertransformer)

I checked all solderjoints and connections and cannot find anything strange
after all the amplifier did work ok for the first 10 hours or so

I would be very grateful if someone could help me, after all it was a great sounding amplifier

RR

ps there is a picture of the inside in the gallery

Re: G2 problem

still no suggestions ?

yesterday I checked the voltages of the powersupply disconnected from the amplifier circuit
I took out V2 and disconnected the (blue)wire from R11/C9 to R6/R3, now the powersupply should act like on page 12 in the (german)manual
it surprised me to read the following voltages :
R12/C10 270V DC
R11/C9 270V DC
+ of rectifier 271V DC
secondairy from powertransformer 203V AC
shouldn't these voltages be the same as they were went I built the amplifier,
they were 240V DC and 190V AC (within 1 or 2 volts) altough I cannot remember if I also checked the voltage on R11/C9 (which now is the same as the voltage on the red wire from the ooutputtransformer)
when I connected the amplifiercircuit by refitting the R11/C9 to R6/R3 wire and
reinstalling V2 the voltages were as before, a lot lower as they should be :
R12/C10 201V DC
R11/C9 190V DC
+ of rectifier 215V DC
secondairy of powertransformer 186V AC

the amplifier still sounds very bad, not enough volume and very distorted
I don't have spare 6N1P and 6N2P tubes so I can't replace them
but now I noticed the powersupply without load from the amplifier gives a very high voltage I think that might be the problem

maybe somebody can help me now ?

RR

Re: G2 problem

hi again,

at last I (almost) found the problem !
while checking voltages for the millionth time I realized I had to push rather hard with my multimeters' probe on either pin #3 of V2 or R10
when touched lightly there was no voltage at all, when pushed harder it read 5V DC like it should be
the solderjoint did look good but I resoldered it anyway and this did not help
only bending of R10's leg helped to keep the voltage
now there is 5V on the cathodes of V2 all other voltages are as they should be and the amplifier works fine again
I haven't found out what exactly is at fault, R10 would seem logic but by bending its leg you also move the tubesockets' pin #3, so the tubesocket could be bad too, I will change R10 first

this being (almost) solved now leaves me with the following question:
could it be the capacitors of the powersupply damaged R10 when they broke down ?

Martin : yes I changed the capacitors
you also mention the free running voltages, I cannot find anything about them in either the schematic (dated 25-12-2005) or in the german manual (dated 07-01-2006 rev.1.0)
if you updated either of these I would like to recieve the updates, after all they could be handy for future troubleshooting

I'm very happy the amp works again like it should, it really is fun to play
I never heard a small practiceamp with such a great sound like this and the output volume is exactly right for homepracticing
only downside could be that it is rather critical on speakers, I tested several and altough with some speakers it sounds great other speakers didn't like the amp at all, it turned out I like a naylor 10" speaker best

RR

Re: G2 problem

hi,
well I will see if it is the resistor or tube socket when I change them one by one, but as the problem started with the bad caps I wonder if this had some influence on an electric component rather than a socket therefore I suspected the resistor, I will order the tubesocket and try
I still cannot find free running voltages in the schematic, although you do mention them ("zahlen in klammern sind spannungen ohne rohren") they really are absent in the schematic dated 25-12-2005 rev.1.0

RR