Nobsound EL34 Single-ended Class A tube amplifier

If anyone could steer me in the right direction , I purchased a Nobsound EL34 Single-ended Class A Tube Amplifier and has been working beautifully until today... I switched it on and the large center tube (rectifier) sparked inside and now no tubes light. Would this rectifier tube likely be the problem and need replacing .. Iam not familiar with tubes?? Thanks
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I switched it on and the large center tube (rectifier) sparked inside and now no tubes light. Would this rectifier tube likely be the problem and need replacing ..

If it was just a bad rectifier tube that blew then the answer is Yes , it is the problem

However , if something has gone wrong 'inside' that caused the tube to blow then, No

I'm not qualified to say how to test to find out which , sorry
 
If it was just a bad rectifier tube that blew then the answer is Yes , it is the problem

However , if something has gone wrong 'inside' that caused the tube to blow then, No

I'm not qualified to say how to test to find out which , sorry
Thanks … I just found the fuse but don't have a spare so I'll have to wait until it arrives and see if everything is working or not. I have noticed that sometimes I see an arch in the Rectifier tube when powering on. :idea:
 
you cannot turn tube amps on and off quickly. this blows components.

if the tube rectifier arcs, check the value of the first cap right after against
the datasheet for the rectifier tube for maximum values. if the cap exceeds
the maximum stated value then lower the cap value with replacement.
 
I have always even waited for the tubes to warm a little before turning even the volume up and never powered up again until tubes cooled down. The Chinese company is going to send me a new tube in a couple weeks... after trying to verify the cap value got me lots more questions than answers. The rectifier tube in there is 5U4G and on their site it says it's a 5Z3P. Then looking at datasheet for 5U4G it says filter cap =40uF.. looking at other schematics they show a 4uF being used? pin 8 going to +of 100uF 450v. pin 2 going thru 10ohm resistor then + of 100uF 450v. when I get the fuse and new tube it might work fine ..the only thing was about 1 in 10 times I powered it on I noticed an arch in the rectifier tube. My 5U4G tube only has 4 pins 2 & 8 4 & 6 ..what thru me off was they are using unused pins on socket to wire up other parts of circuit.
 
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first off, fast cycling of tube-rectifier-based amp causing failure is different than too much first
stage capacitance causing failure. good that we've eliminated the first as the root cause.

next. the Chinese 5z3 is not the same as the old American 5z3 - the later is a
4-pin base and the former is an octal base and mostly equivalent to a gz34/5ar4.

do note that some octal rectifier tubes do not come with all 8 pins of the octal base
populated so some folks think it's a 4-pin base. count the populated/non-present
holes and check for a guide pin with a locator tab in the middle of the tube. I am
assuming the tube manufacturer takes liberties with their designs.

from memory the Chinese 5z3 is indirectly heated (like the 5ar4) and only because it
is indirectly heated that there's a slow ramp of B+, and to some people, good for less
stressful start-up operations.

the GZ34/5AR4 is designed with 40-60uf max at the first cathode tap of the tube.
this number varies depending on the manufacturer. it would wise to use the lowest
in case your future replacements cannot tolerate the upper end of this first cap range.
then if you really need large capacitance values in the amp, then use as much as
you need after the first cap stage. IOW shift anything over 40ufd from first to second
stage in the PS.

in all cases, anything close to 100uf will cause a short on power up, simply the
nature of caps as they charge, the larger the value, the higher the current draw
the higher the probability of an arc. think crowbar across auto battery.

replace that 100ufd cap with 40UFD (less is OK), then use it elsewhere. repeated
use (power on) will cause eventual failure. I replace those e-caps with a 10ufd film
cap but that's me.

lastly, and most important - you mention the use of pin 1. I would research this
before anything else. on GZ34/5AR4s pin 1 is i.c. internally connected and
I would not be happy if, in an amp the designers did not design for American/European
GZ34/5AR4, they decided to use a Chinese tube with or without this pin1 feature.

between the tube difference, pin1, and first cap value, I'd think about changing the
PS circuit to use the more common rectifier tube (I recall there was no Russian
equivalent so to use a Chinese 5z3 you are limited to only the Chinese 5z3).

with more (descriptions, links perhaps, pictures, schematics, and even
a datasheet for the Chinese 5z3 even if it's in Chinese) the PS can be
changed to use American/European GZ34/5AR4 rectifier tubes which
will give you more choices, cheaper costs, and perhaps better reliability
and SQ. there are numerous threads on the SQ of the 5u4, GZ34/5AR4,
and others regarding SQ. your unit joins the big boys threads like the ST70, etc.

if you decide to go down this route, I'd highly recommend looking at the
Dynaco ST70. very classic. and depending on the Chinese 5z3 heater
draw (which in turn tells you the transformer current for the rectifier tube)
then you can even use the 5u4 variants.

an alternative to modifying the PS circuit for use with GZ34/5AR4 and possibly
the 5U4, is to convert to SS, either by way of using a SS-rectifier equivalent
in the rectifier tube 8-pin socket, or simply to wire SS diodes on the socket
pins. More research is mandatorily required because the SS diode output
is higher than the tubes and may exceed voltage ratings.
 
Thanks for your input. pin 8 going to +of 100uF 450v. pin 2 going thru 10ohm resistor then + of 100uF 450v.
My 5U4G tube only has 4 pins 2 & 8 4 & 6 ..what thru me off was they are using unused pins on socket to wire up other parts of circuit.

Iam assuming pin 2 & 8 are the filament? Also would having (2) 100uF on each side affect the total capacitance of this in half? =50uf

I can't see them selling these for years without ever figuring out how out spec it is. weird
 
now, with more input, we have additional problems.

1. are you saying the exact 5u4G tube you are using has only 4 usable pins AND NO PIN 1?
2. any other components in the PS path (this 10R is brand new information. take and post pix.
3. download datasheet for Chinese 5z3p and verify pin 1 is NOT available, not pinned to tube
socket, and not internally connected. verify by peering into Chinese 5z3. post link or better
post datasheet, and take picture of inside of Chinese 5z3P and post large picture (take
close-up with good natural lighting (no flash, no midnight shots), crop but keep resolution
and post.
3. draw picture of tube connection OR take and post picture, so many things are left out
so the picture is worth ten thousand words.
4. pins 2 and 8 are the heater and because it is designed as a directly heated tube, these
two pins also act as the cathode in a rectifier tube. the B+ voltage can be pulled off either pin.
5. each side? not sure what exactly you mean - each side of cathode pins? draw
schematic - I can't visualize - not enough information. if you cannot take a picture
then describe what each cap is tied to from positive and negative side
6. if the two caps are not in Parallel then it cannot be 50uf. and if the designer intended this
instead of using a single capacitor he needs to review capacitors 101. week one, lecture one.
7. need schematic or picture of PS before condemning the amp.
8. if you used a tube with an active pin 1 (assuming the Chinese 5z3 does NOT) then
there may have been some damage and thus explaining the problem(s) and it is no longer
the problem of the rectifier tube but the designer's active circuit use of empty pin 1 as
a tie point that needs to be fixed. and it needs to be fixed to existing schematic that will
only allow the continued use of ONLY the Chinese 5z3p. IOW do not use any other
rectifier until you verify all the points made earlier and here and now
 
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pin 8 going transformer pin 2 going to transformer and joined with 10ohm on unused pin 5 thru 10 ohm to unused pin 1 then to + side off 100uf.
pis 4 & 6 going to transformer
Pin 8 is not directly to the other + 100uF .. I read a short from it going there but it's going thru
the transformer
 
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tube 5U4G … don't have a 5Z3P And the amp was working fine for a couple months
the 5Z3P data also shows only 4 physical pins 2,8,4,6
5z3p_sock.png

5Z3P

5z3p.jpg

my tubeupload_2019-1-30_13-0-23.png
 
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long story short pin 2 going + of the 100uF on right side of pic and thru 10ohm to the other 100uF on left side of pic. Via unused pins as tie downs.
 
Cold not find schematic but Iam assuming it's like this minus the diodes.
with the 100uF's one going thru a resistor ?

12SL7-KT88-Push-Pull-Tube-Amp-Power-Supply-Schematic.png
 
what changed from working several months to failure? any tube pulls? or ABSOLUTLY nothing
changed (which means failure/cause is totally different)? anything other piece of new information?

schematic please, the wording doesn't match pictures and is confusing to me (maybe just me -
others can jump in and take over) and I need the schematic to un-confuse, make sense of
the wording and relate to the pictures.

I cannot picture second cap as part of PS circuit. it's either
1. the second part of a voltage doubler
2. other side of a bipolar PS (not really possible or necessary for a tube amp that's simple)
3. decoupler for another stage or channel.
 
that KT88 is a SS PS and completely different than a tube rectifier PS. there are 6 caps in that
and we don't know how many in your PS.

one is a bridge and the other could be full-wave or voltage doubler. again need actual schematic.
 
I thought there was discussion here about the stock rectifier tube being a bit of a weak link and that some owners replaced them. Am I remembering correctly?
 
possibly. the 5z3 can pass enough current to power a push-pull el34 amp like the Dynaco ST-70.

if the tube has weakened due to long use and high draw then a replacement 5z3p would work.

one of the many issues here is the use of an American/European GZ34/5AR4 or equivalent
that has an internal connection using pin1 in an amp that uses a tube that does not have
a pin1 and uses this particular pin to tie other components.

one scenario I can think of is the tube arc'd over (due to repeated overstressing of super
high first cap) somewhere inside the tube (assuming pin1 is the typical shield - but that's
a guess on my part) and took out some components on the outside (tied to pin 1)
and now is the weak link and will continue to show earlier failures.

but without a schematic I am stuck. I cannot square the word descriptions to the pictures
and rationalize them into a schematic that I (or others) can apply some screening/diagnosis
to infer what happened, and what to do to prevent future failures.
 
Just wanted to thank you for your input... I have got some 2amp fuses and replaced it and everything seems to be working. I powered it on about 6x or so time from yesterday and today and have not noticed any arching in the rectifier tube. They are sending me a replacement rectifier tube on the slow boat from China. It really is a beautiful sounding amp and puts all my ss amps to shame.
 
Well .. I did notice an arch again but the fuse held … After much researching I think your right about that 1st 100uF which is too high. Iam going to replace it with a 47uF... Also heard others changing the 5U4G to an 5AR4 .. I'll wait and see if that's needed. I wonder if the 47uF will alter the sound at all? Also I found a schematic that quite possibly be mine .. someone else made dictations on it.


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