World-Designs-Forum  

Go Back   World-Designs-Forum > WAD > WAD General
Register FAQ Members List Calendar Gallery Today's Posts Search Today's Posts Mark Forums Read

WAD General For discussions re World Audio Design

Reply
 
Thread Tools Display Modes
  #1  
Old 16th September 2017, 11:09 AM
Simon Simon is offline
Junior Member
 
Join Date: Sep 2017
Location: St Aulaye, France
Posts: 3
Default WAD 300bPP Screeching

Hi

After many years of service one of my monoblocks produces a deafening screech on power on. It lasts 5-6 seconds and then seems to operate normally, though I think the sound quality has dropped too. I have swapped out all the valves in turn with some spares I have, and I have checked the circuit for leaking caps or anything loose, but nothing seems amiss. Any ideas where to start?

Simon
Reply With Quote
  #2  
Old 16th September 2017, 02:12 PM
John Caswell John Caswell is offline
Super Moderator
 
Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,487
Default Re: WAD 300bPP Screeching

Hi Simon,
Funnily enough I have just had the same problem with some 300B pp units of another make.
Traced to poor 300B valve base contacts, see if you can tighten them up in any way.

John
Reply With Quote
  #3  
Old 21st September 2017, 08:35 AM
Simon Simon is offline
Junior Member
 
Join Date: Sep 2017
Location: St Aulaye, France
Posts: 3
Default Re: WAD 300bPP Screeching

Thanks John, I cleaned up the contacts and tightened them gently, however C15 has now blown and a quick measurement showed the input voltage at 185v, way above where it should be. The VR1 resistor glows red too, so something is amiss.
Reply With Quote
  #4  
Old 21st September 2017, 11:46 AM
John Caswell John Caswell is offline
Super Moderator
 
Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,487
Default Re: WAD 300bPP Screeching

Hi Simon,
If C15 has blown and also VR1 getting hot then the associated valve V4 300B is drawing too much current.
First thing is remove both 300bs power up and check the voltage on the grids ie on R21/26, of the 300Bs - should be approx -50V dc. If it is, then it is likely you have a knackered 300B - how old are they? and they would need to changed for a matched pair.
If it is lower than that ie say -5 V I would suspect C13 - 0.47F as being leaky.
Power down disconnect C13 and power back up again grid should read - 50V if so then C13 is leaky - change it!.

Let me know how you get on

John
Reply With Quote
  #5  
Old 22nd September 2017, 08:47 AM
Simon Simon is offline
Junior Member
 
Join Date: Sep 2017
Location: St Aulaye, France
Posts: 3
Default Re: WAD 300bPP Screeching

Thanks John C13 was indeed the issue, it is an Audio Note copper PIO, so I have swapped it for an orange drop temporarily. Given their cost I might swap the lot out for polypropylene alternatives, any thoughts on a good fit?
Reply With Quote
  #6  
Old 22nd September 2017, 09:41 AM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 4,718
Default Re: WAD 300bPP Screeching

Hi Simon and John,

I've owned your amps and have had a similar PIO electrical leak problem with a Leak ST20. Grid bias goes high and before you know it the output valve is red and into thermal runaway.

I would no longer use a PIO type cap in a critical position of DC blocking such as this. Check a new one on high DC and you'll probably find it leaks some mV already. Years ago Noel warned against such with the original 300BPP (where a leak could take out the interstage TX) and his suggestion was a polycarb iirc.

I would swap out all C11 and C13 for plastic film. I've tried polyester, polyprop, teflon and various PIO inc AN Alu and Tin (tho not copper) for sound quality and, for me, the polyprops that Matthew sells as Soniqs SAX are a good balance,

http://www.world-designs.co.uk/spares.htm
Reply With Quote
  #7  
Old 22nd September 2017, 05:52 PM
bikerhifinut bikerhifinut is offline
Senior Member
 
Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 984
Default Re: WAD 300bPP Screeching

Yep the Soniqs are good 630V rated and reasonably priced. I put them in my KT88 amp and there they sit doing their job. There seems to be a slight improvement over the standard fit polyester grey ones but in any case I rather like that they have nice decent length wire ends for easy soldering onto tags.
I have to say (and may i be forgiven) that I rather like the inexpensive "orange drop" polywotsits, I believe Bob O does too.

A.
Reply With Quote
  #8  
Old 22nd September 2017, 06:52 PM
John Caswell John Caswell is offline
Super Moderator
 
Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,487
Default Re: WAD 300bPP Screeching

Hi Simon et al,
Well that is good to hear.
I suggest that you change the associated "cathode" capacitor C15 and resistor R29 as well as the hum bucking pot VR1 and the two Rs 27/28 as they will have been overloaded, I would also venture to suggest that the 300B may be damaged (Ouch!!!!!)
I have to admit being surprised at how good "Orange drop" capacitors are, considering how cheap they are to make and buy. The WD Soniqs capacitors are very good and as Richard says are a good balance. The usual problem is that you can spend a fortune on capacitors to find out that you don't like them.

John
Reply With Quote
Reply

Tags
300b push pull amp, screech, screeching

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 12:21 AM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright World Designs