Attn. Thorsten

Ive just drawn a rough sketch of that heater bias which should be an attachment (couldnt do it via pm for some reason). Is this right?

 

Hi,

Not quite right.

The 10uF/160V needs to connect from where you have drawn it to ground, the 330k Resistor connects to the +B Rail (around 430V I'd think) and not inbetween the capacitors.

BTW, for diodes, your chocie at RS is probably good. I find RS too expensive, I generally use International Rectifier (IR) Schottkies and Fairchild "Stealth" series softswitching rectifiewr where I need more than 100V PIV rating.

Ciao T

PS, you might want make a complete thread where you cover modding the Amplifier, it maight help others... Feel free to quote from my PM's where needed.

 

It was the first cap that I was unsure about. Im glad you cleared up the B+, I had a good look last night and it didnt quite look right but now I understand! I would start a thread, I'll see if theres any interest.

 

That, and the ritual flogging you'll receive for using the wrong symbol for your resistors

 

Interest duly registered.

 

me too

 

Erm, from now on thats the symbol to use, I have spoken.

 

How about this (I know the last cap is across the resistor but Ive just got peach juice on my 1st drawing, so Ive had to do another quick)

 

Hi,

That is just peachy.

BTW, to explain briefly to the interested public the why of this....

All Valves in TBS's amp are AC heated. In any valve a "parasitic" diode exists between heater and cathode. If the heater becomes positive with respect to the cathode a certain amount of AC current is conducted into the cathode. By making the cathode more positive than the peak voltage of the Heater (but within safe limits stated in the datasheet for heater/cathode voltage) this parasitic diode is reliably reverse-biased and thus shut off.

It is a common failing in modern chinese Amp's to leave this off, it results in unneccesary intermodulation, even if tons of NFB push the actual level of the noise sufficiently far down to not be audible as hum.... It was not often used in the "golden days" of audio, but back then a -60...-70db Hum & Noise spec was considered State of the Art, so no-one bothered to go beyond this....

Ciao T

 

^ what he said

 

TBS, what's your amp?

 

Its the El Cheapo Dynavox VR-70e, was widely available on ebay for £200 but its seems to be just a certain affordable valve amp dealer selling them these days. Very popular in Germany though were they are around 300-400 euro's depending on spec (hence my desire to translate German text). A fun amp to play with and so easy to work on.