[grantlack]

Finished building one of these last week...sounded quite decent but tonight it's developed a high pitched tone tonight....any ideas what this could be?

might be a microphonic tube.  sound like this?
http://www.youtube.com/watch?v=QM2qWZYbInI&feature=player_embedded

skip to like a minute in and that's a pretty characteristic tone.

[JayDubrek]

Just checked that clip and it's definitely not that...just gone back to the comp...the tone is at about 16-17khz when audio is being passed. When the audio stops a low and noisy oscillating sound creeps in. Bloody annoying....one week of silent operation them this happens!!!

[pucho812]

check connections for colder solder joints.  try different tubes

[grantlack]

Just checked that clip and it's definitely not that...just gone back to the comp...the tone is at about 16-17khz when audio is being passed. When the audio stops a low and noisy oscillating sound creeps in. Bloody annoying....one week of silent operation them this happens!!!

DISCLAIMER: i think i get this circuit, but somebody should jump in and hit me with a rolled up newspaper if i say something stupid here.
i'd still do a tap test to rule out microphonics, and possibly number and swap them.  if there's a bad tube at V1, the introduced noise will be amplified by the 'good' gain stages, so whichever configuration gives the most noise should dictate swapping of V1.  if low-freq problems are still persistent, i'd check your grounding of the grid leak R's (13, 14, 21, 22).  if these aren't grounded properly, your filtering from the RC filter formed w/ the coupling caps (c2-c5) will be compromised.

[JayDubrek]

Thanks for the advice chaps...I'll try tap testing and swapping tubes first...

[gary o]

Mine isnt microphonic tube noise make horrible ocillation ...I cant get any decent sound of of it....Ive made lots of vari mu comps never had this much trouble....might leave it couple of days and go over it again.....i build a very similar VK1 that sound great!

[grantlack]

Mine isnt microphonic tube noise make horrible ocillation ...I cant get any decent sound of of it....Ive made lots of vari mu comps never had this much trouble....might leave it couple of days and go over it again.....i build a very similar VK1 that sound great!

is it high frequency or low?  assuming i'm not an idiot (BIG assumption  ) and understand the filter behavior here, the corner frequency of the coupling/grid leak HPFs for the 6BC8 is 1/(22n*1M) = 45.454545... Hz.  might be too low of a 3db point to cover up low-freq instability, in which case decreasing the cap value would bump up the corner frequency and help attenuate motorboating.  it'll eat the program material too though, so it's a response trade-off.

hi-freq might be a transformer issue, in which case a zobel RC network is your friend.

[grantlack]

Thanks for the advice chaps...I'll try tap testing and swapping tubes first...

just for clarification, swap with *new* tubes of the same type, don't just put the various tubes in different gain stages.  as you well know from buying them, they aren't all the same.  also, i've been looking at the altec circuit the whole time  so now i'm trying to sort the ACTUAL circuit.  idiot status confirmed.

[JayDubrek]

Luckily I bought several of each type....once again thanks for the advice. The world of valves is very new to me!

[alalcoolj]

I bet it's the 6bc8 tube.  I had the exact symptom on a drip175 build

[gary o]

I suspect my 6BC8....but only have the one ... .......think il try a pin compatable tube even if doesnt work properly it may confirm the suspicion.....

[EmRR]

pull the SC rectifier and see if it gets better.

[eskimo]

Just to be a dick, are you sure it's the tube? I seem to recall a photo of your insanely awesome prototype birdnesting. 

[gary o]

I tried removing the 6AL5 on my one EMR thanks ....but no joy I disconnected it & also input & then the 50 k pot......cant see what else.....

Eskimo I happen to have insanely wired birds nesting too Im embarrsed to post a picture  haha .....doesnt help matters I no but its not the cause of problem for me....i still suspect my input tube.....

[eskimo]

Oh, my comment was targeted right at your lovely desk!

[gary o]

Haha oh it was me Eskimo ....yeah I forget people seem my scary desk....no yr right its a bit hap hazard......Iv tidied it a little honest...just swapped all tubes around still same its bugging me usually get it sussed by now ...constant spose its a square wave low i can change its tone by feeding higher or lower B+ or adjusting threashold pot or adjusting the balance pot.....when I feed my voice in from a pre amp, my heavily distorted voice replaces the square wave sound works bit like a ducking setup

[MagnetoSound]

Sounds like you may have set up some positive feedback path somehow between the output tube and input stage for the threshold pot to have an effect. Check your filter caps on supply rail again to begin with.

[gary o]

I say the threashold po effects the osclilation it can change its tone .....if i completly disconnect the side chain I still have the oscillation tho...

Before i go on i should say to those who dont no me Im more of a solder n see kinda DIYer I dont have the knowledge that lots of you have here.....but would like to borrow it....so thanks for so far.

My power supply as i said isnt the altec design....its one i made earlier quite simple its has various B+ outputs via bleed resistors something like 100v 200V 300V.....I use it on my VK1 it works fine

however it doesnt output the exact B+ the so called RS124 needs according to schematic but I thought like all rest of my projects it would work with a ball park B+

Im away from my infamous desk at the moment so i cant see the volts out but basically Iv observed the lower the B+ I feed it the lower the square wave sounding oscillation is.......would make a great analog synth....

Im wondering how critical the B+ is in this case ??    the very similar VK1 doesnt bloody care it will take what you give it.....

[gary o]

had a look at my voltages Im feeding 162v to the junction of the  2x33K res & the 39K... so Im 20V down fair enough......but at pins 1 & 6 of input tube instead of getting 65V or less cos Im 20V down, I have a wapping 121 V......that cant be right......

the other side of the 39K the schematics says 13V I have a huge 160V

Pins 3&8 should be 1.2V but I have 0.6V each

[mitsos]

had a look at my voltages Im feeding 162v to the junction of the  2x33K res & the 39K...

Is that a typo? did you mean 390K?

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