12 thoughts on “26 DHT Preamp Gen3 (Gyrator version)”
Hello Ale, I used as Rod coleman suggested B+120V & -9V across the 8 Sic diodes, I measured to know how mA draws the tube DVM shows 0.023V across R5 10 ohms so that the tube is only drawing 2.3mA. As I see in your schematic you use 156V for B+. Have I to increase B+until reach the 7mA as per your schematic?
TIA & best regards
Felipe
Hi Felipe, with SiC bias you will fix grid to cathode voltage so you will need to increase Anode voltage with the Gyrator trimpot to achieve target anode current.
Hi Felipe. If you can adjust the channels means (in principle) that the gyrator board is working and isn’t damaged. You can replace the entire board with a resistor to rule out that isn’t any other thing outside the gyrator that isn’t working. Best to use a scope and a function generator so you can trace the signal.
If this doesn’t work send me an email with the full diagrams, voltage measured and pictures of the build. Can’t advise more with such little information.
Hi Ale, fuse 150mA for B+ sharing both channels burned, replaced the same value fuse, now left channel is ok but right channel can’t adjust the voltage (always full voltage and can’t adjunt to 150V. Looking at the schematic I made an error for C1 in place of use 100nF I used 220nF without the 220nF output cap.
HI Felipe. Please don’t post comments on this thread for support. There are hundreds of people subscribed which likely aren’t interested on this problem debug. Please send me an email as discussed
Without the output cap you killed the gyrator MOSFETs. You will need to replace the pair of cascoded FETs. It may also be that the LND150 pair CCS is damaged but you can find that out after replacing the FETs and if the voltage regulation doesn’t happen when you adjust the trim pot.
Hi Ale, OK I understand, if I have problems I will contact you by email.
Hi Ale,
Only to let you know your boards are working OK, great stuff, thanks for support.
Best
Felipe
Ale today connected the preamp, the SQ is excellent, thanks for your work.
Cheers
Felipe
Glad to hear Felipe that it worked out in the end. Would be great if you share in DIyaudio thread your experience to encourage other builders.
The new board (Rev08) has some protection built in to protect the Top MOSFETs in case of the accident/misconception you had.
Enjoy the preamp!!
As Rod Coleman suggested I used 120V for B+ and -9V bias. to know how much current the tue draws I measured voltage across R5 and is 0.023V divide by 10 ohms gives only 2.3mA. Have I to increase B+ to 156V as per your schematic to reach the 7mA?
Hello Ale, I used as Rod coleman suggested B+120V & -9V across the 8 Sic diodes, I measured to know how mA draws the tube DVM shows 0.023V across R5 10 ohms so that the tube is only drawing 2.3mA. As I see in your schematic you use 156V for B+. Have I to increase B+until reach the 7mA as per your schematic?
TIA & best regards
Felipe
Hi Felipe, with SiC bias you will fix grid to cathode voltage so you will need to increase Anode voltage with the Gyrator trimpot to achieve target anode current.
Thank you Ale. Please delete the other reply because is duplicated.
Hi Ale, adjusted both channels to 5.6mA each but no sound, there is a way to check it?
Thks & rgds
Felipe
Hi Felipe. If you can adjust the channels means (in principle) that the gyrator board is working and isn’t damaged. You can replace the entire board with a resistor to rule out that isn’t any other thing outside the gyrator that isn’t working. Best to use a scope and a function generator so you can trace the signal.
If this doesn’t work send me an email with the full diagrams, voltage measured and pictures of the build. Can’t advise more with such little information.
Hi Ale, fuse 150mA for B+ sharing both channels burned, replaced the same value fuse, now left channel is ok but right channel can’t adjust the voltage (always full voltage and can’t adjunt to 150V. Looking at the schematic I made an error for C1 in place of use 100nF I used 220nF without the 220nF output cap.
HI Felipe. Please don’t post comments on this thread for support. There are hundreds of people subscribed which likely aren’t interested on this problem debug. Please send me an email as discussed
Without the output cap you killed the gyrator MOSFETs. You will need to replace the pair of cascoded FETs. It may also be that the LND150 pair CCS is damaged but you can find that out after replacing the FETs and if the voltage regulation doesn’t happen when you adjust the trim pot.
Hi Ale, OK I understand, if I have problems I will contact you by email.
Hi Ale,
Only to let you know your boards are working OK, great stuff, thanks for support.
Best
Felipe
Ale today connected the preamp, the SQ is excellent, thanks for your work.
Cheers
Felipe
Glad to hear Felipe that it worked out in the end. Would be great if you share in DIyaudio thread your experience to encourage other builders.
The new board (Rev08) has some protection built in to protect the Top MOSFETs in case of the accident/misconception you had.
Enjoy the preamp!!
As Rod Coleman suggested I used 120V for B+ and -9V bias. to know how much current the tue draws I measured voltage across R5 and is 0.023V divide by 10 ohms gives only 2.3mA. Have I to increase B+ to 156V as per your schematic to reach the 7mA?
TIA & kind regards
Felipe