Dirty sounding Audio Note 300B kit 1 amp. Suddently gone dirty.

[rv295]

Hi folks,

I’d appreciate some advice on a now very dirty sounding Audio Note Kit 1 300B amp that I bought recently.

The amp was purchased with the original valves which have never been changed, it’s an earlier kit 1 (I believe built around 2004 give or take).   Circuit diagram below if it helps.

I budgeted for a complete new set of valves and I’m happy to replace them all if needs be.  I haven’t gone out and bought new valves because up until today it has sounded phenomenal, I fell in love with it as soon as I heard it and I’ve probably used it for around 40 hours since buying it.

Earlier today I put it on and after about 30 minutes I noticed the sound changed, it was like someone pushed a distortion peddle on a guitar.  You can hear the distortion and I don’t fancy just playing Nirvana for the rest of my life so I’d like to get this sorted.

I also noticed that the transformer directly behind the rectifier was exceptionally hot even though the others hadn’t even started to warm up.  I haven't had it long enough to really be sure if this is normal or not.  It has been getting hot after a long, long session but nothing that has worried me until today.

I started it back up after letting it sit for about 6 hours.  It sounded fine at first and then went very distorted again.  I also noticed that when I turned the power off - while leaving the source playing it briefly went back to sounding perfect again but then faded out as the power reserves were used up.

Should I just start buying replacement valves and start swapping them out to see when it improves?  Should I start with a particular valve(s) or could it be something else completely?

Any suggestions or info gratefully received.  

Here is the amp: 

I believe this is the correct circuit diagram:

 

[Valvebloke]

Was the fault on both channels or just on one ? If it was just on one channel then does it follow the valves if you swap them between the channels ?

VB

 

[rv295]

Was the fault on both channels or just on one ? If it was just on one channel then does it follow the valves if you swap them between the channels ?

VB

Hi Graeme,

I "think" it was both channels but please let me check it again in the morning.  If it is one channel I'll swap the 300B's around and see if it moves. I'll report back.  

Thank you!

 

[Valvebloke]

I should say that if the mains transformer really is getting very hot then I wouldn't run the amp too long in this condition. Transformers are pricey and (often) not economically repairable if they fail.

VB

 

[rv295]

I should say that if the mains transformer really is getting very hot then I wouldn't run the amp too long in this condition. Transformers are pricey and (often) not economically repairable if they fail.

VB

Hi Graeme,

I've run the amplifier today and after 15 minutes of it running perfectly (no sign of heat in any of the transformers and no distortion) the left channel suddenly cut out completely.

When I bought the amp the previous owner said a connection to one of the binding posts needed looking at (the same channel that has gone off).  I thought this would be just about within my DIY skill-set and on closer inspection they had come slightly lose (probably weakening a solder joint).

When the left channel cut out I touched the binding posts and it came back on... then went off again and hasn't come back.  Just out of curiosity I swapped the 300B valves around but the same channel worked. 

I will take the bottom off later on and have a look at the state of the binding post connections.  (keeping my hands well clear).  If there's anything obvious I'll post a pic.

Strange one this, I wonder whether the issues could be related but can't imagine how.

Regards
Ross
 

 

[Valvebloke]

In general it's not a good idea to run a valve amp with the speakers disconnected (this is a consequence of having an output transformer, which is why it's not usually an issue for solid-state amps). Doing so can do permanent damage to the output transformer, and to other parts too if you're even more unlucky. This fault could be the same as your other fault if the loose binding post sometimes makes high-resistance/intermittent contact, so you get some music signal through but it's distorted. This can be compounded if the amp uses negative feedback and takes it from a point after the faulty output connection. Otherwise the two faults could be entirely unrelated.

VB

 

[rv295]

In general it's not a good idea to run a valve amp with the speakers disconnected (this is a consequence of having an output transformer, which is why it's not usually an issue for solid-state amps). Doing so can do permanent damage to the output transformer, and to other parts too if you're even more unlucky. This fault could be the same as your other fault if the loose binding post sometimes makes high-resistance/intermittent contact, so you get some music signal through but it's distorted. This can be compounded if the amp uses negative feedback and takes it from a point after the faulty output connection. Otherwise the two faults could be entirely unrelated.

VB

Thank you Graeme, I did have the sense to turn it straight off after the binding post issue.

The binding posts are fixed and the amp is up and running perfectly once again.  I'm running it now and all previous issues seem to have gone, there is no noticeable distortion and the transformer feels cool.

Here are the insidesI don't understand it but it's fascinating to see how simple it is)

And here is the offending binding post.  (the one on the right)

In the past it has been tightened and the tag had spun around so it was touching the paintwork on the base of the chassis.  I think the last time I pushed it - the post must have finally worn through the paint and shorted to the chassis, I think I'm lucky no major damage seems to have been done.

So far so good, I've re-soldered that joint and tightened all the binding posts up so it doesn't happen again.

 

[rv295]

Very sad news... after about 40 minutes it happened again.  The transformer got hot and there was distortion from both channels.

I'm going to get it down to Graeme to work his magic.

 

[ESK]

Sounds like a problem I had a few years back with a valve preamplifier - distortion from both channels.  It needed a new mains transformer.

 

[rv295]

Sounds like a problem I had a few years back with a valve preamplifier - distortion from both channels.  It needed a new mains transformer.

Oh dear, that sounds expensive!

I'm hoping it's an easy fix but I want to get it sorted one way or another. 

 

[Juancho]

i sorted one of these recently which had similar symptoms. It was the Audio Note coupling caps, a very common problem with them when they moved over from Jensens. The paper insulator just gives up due to chemical action over a number of years. I think it's unlikely to be a transformer problem. Could also be a shorting main smoothing cap

 

[rv295]

i sorted one of these recently which had similar symptoms. It was the Audio Note coupling caps, a very common problem with them when they moved over from Jensens. The paper insulator just gives up due to chemical action over a number of years. I think it's unlikely to be a transformer problem. Could also be a shorting main smoothing cap

Thank you for the info David!  That fills me with a bit of hope. :^