Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on September 22, 2010, 01:59:18 pm

Title: Bias Sensing circuit on SVT Classic.
Post by: phsyconoodler on September 22, 2010, 01:59:18 pm

Does anyone have any info on the bias sensing circuit in the SVT Classic amps?I have one that has an issue.The amp works fine and the current is correct,but the sensing LED's won't work properly on one side.The green one won't light up.The circuit is very complex and I've changed out the TLO72 and TLO74 IC's but it still isn't sensing properly.
  I'm not that great with solid state stuff which doesn't help much.
If anyone has experience with this issue I'd sure like some insight.
Thanks!

Title: Re: Bias Sensing circuit on SVT Classic.
Post by: PRR on September 22, 2010, 07:19:48 pm

Link to schematic?

Title: Re: Bias Sensing circuit on SVT Classic.
Post by: scarlett amps on September 23, 2010, 02:58:49 pm

be sure to check the 1 watt 10 ohm resistors. They occasionally die and then the leds dont work. if you need the complete schematics for this amp i have them, just send an email addy.
These amps are a complete pita to work on! i have fixed a dozen or more of them and they are never simple.
paul

Title: Re: Bias Sensing circuit on SVT Classic.
Post by: phsyconoodler on September 23, 2010, 03:59:03 pm

Thanks for the insight Mr.Scarlett
 
PRR,
  I can E-Mail you the schematic if you like.

Title: Re: Bias Sensing circuit on SVT Classic.
Post by: phsyconoodler on September 24, 2010, 02:49:59 pm

I found the problem!!
  On the output tube PC board there are some 220 ohm 1 watt resistors. R7 was open even thought it looks fine.Nasty to work on but easier once you just grin and bear it!
 

Title: Re: Bias Sensing circuit on SVT Classic.
Post by: scarlett amps on September 26, 2010, 04:51:20 pm

yup those also blow often enough if a tube dies. What a pain to service! :cry: