Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

Sony Vfet Illustrated build guide
Sony Vfet Illustrated build guide
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th May 2018, 01:44 PM   #531
sledwards12375 is offline sledwards12375  United States
diyAudio Member
 
Join Date: Oct 2008
Location: North Carolina
I proceeded on to Part 4, and all WAS going good. Started with the left channel only, and voltage across R32 and DC offset at output were low. Started the incremental increasing and all was going well. Around 80 mV across R32, I accidentally started to change P3 instead of P1 as I was supposed to. Now things went bad. Started to change P3 and P4 to compensate, along with P1 and P2. Decided to start over but maybe it was too late. Turned P3/P4 fully counterclockwise and rebiased front end. But now I could only get 6 mV across R32, no matter how I adjusted P1 and P2. The VFETs were cold. I think I cooked a pair of VFETs. Was there a proper procedure for starting over when things go wrong with the VFETs installed?

I went on to bias up the right channel and everything went fine. It is running with 100 mV and about about 5 to 10 mV offset at the amplifier output. Question about this working channel. R5 and R6 drops are about 1.43V each. However, the DC offset on the front end (T18 to Gnd) is now about 6.3 volts. Is this acceptable and just the result of biasing up the VFETs? Do I dare adjust P3/P4 at this point?

Please advise what I should do from here. A one channel amp wont bring very fun listening.

Steve
  Reply With Quote
Old 11th May 2018, 03:29 PM   #532
sledwards12375 is offline sledwards12375  United States
diyAudio Member
 
Join Date: Oct 2008
Location: North Carolina
UPDATE:

Had the assistance of Anand (nycavsr2000) on the phone and he diagnosed my problem after measuring a few voltages: regulator Q13 was blown. Replaced it and all is well!

My question remains, after everything is stable, are there any voltages I should check and make further adjustments if they are out of range? Looks like R5 and R6 may have dropped a bit from 1.5V during the VFET biasing. And is the DC offset at T18/GND still relevant?

Steve
  Reply With Quote
Old 11th May 2018, 05:14 PM   #533
LUX35 is offline LUX35  France
diyAudio Member
 
Join Date: Jan 2011
Location: Bretagne Rennes
T18 < 200mv allways
Chris
  Reply With Quote
Old 11th May 2018, 06:15 PM   #534
sledwards12375 is offline sledwards12375  United States
diyAudio Member
 
Join Date: Oct 2008
Location: North Carolina
Chris:

T18/GND, with everything else in range I am getting 2.5V and 6.2 V. If these need to be less than 200 mV, how do I correct it now that the VFETs are installed and biased correctly?

Steve
  Reply With Quote
Old 11th May 2018, 07:11 PM   #535
Dennis Hui is offline Dennis Hui  Canada
Hoopy Frood
diyAudio Member
 
Join Date: Aug 2002
Location: Toronto, Canada
Sony Vfet Illustrated build guide
Hi Steve,

What are you getting for voltages across R5 (T6-T7) and R6 (T8-T9) (and what
are the corresponding T18-GND values)?

Dennis
__________________
We are fearless amplifier builders. A little heat doesn't scare us at all.
- NP 2002
  Reply With Quote
Old 11th May 2018, 10:08 PM   #536
sledwards12375 is offline sledwards12375  United States
diyAudio Member
 
Join Date: Oct 2008
Location: North Carolina
Hey Dennis:

My T-18-GND values were about 2.5V and 6.2V, while R5 and R6 were about 1.48V. I have now figured out how to correct the T-18-GND values. I first reduced both of the P1/P2 pots a bit. Then I snuck up on 1.5V by tweaking P3/P4. The big issue is that those two pots are extremely sensitive. So sensitive that just barely touching them with the screw drive blade can change the values by 50 to 100 mV! I have 3 meters, 5 would have been better.

Steve
  Reply With Quote
Old 11th May 2018, 10:15 PM   #537
Dennis Hui is offline Dennis Hui  Canada
Hoopy Frood
diyAudio Member
 
Join Date: Aug 2002
Location: Toronto, Canada
Sony Vfet Illustrated build guide
Happy to hear you've solved the problem.
__________________
We are fearless amplifier builders. A little heat doesn't scare us at all.
- NP 2002
  Reply With Quote
Old 12th May 2018, 11:43 AM   #538
LUX35 is offline LUX35  France
diyAudio Member
 
Join Date: Jan 2011
Location: Bretagne Rennes
If you have at T18 : 50mV, the job is good
  Reply With Quote
Old 14th May 2018, 03:25 AM   #539
slimecity is offline slimecity  New Zealand
diyAudio Member
 
Join Date: Jan 2009
I threw a bit of a tanty trying to set this amp up a few months ago and mothballed it. Yesterday I pulled it out again to finish off one side. After attacking this amp fresh - I found these steps helped immeasurably:

1. Ensure that you do the proper DC offset setup properly and thoroughly before installing the VFETs,

2. Once the VFETs are in - be prepared for the setup to take a while (all day if need be!)

3. Use three multimeters - I had my one good one, plus 2 imported cheapies. Connect the good one to your speaker terminals , one other one to your ground-T18 point, the last one across R32.

4. Aim to set your R32 100mV level first by using P1 & P2 adjustments.

5. Check that your 1.5V across R5 and R6 are still good - these wont change markedly by the following setup procedures.

6. Tweak the adjustments using P1-P4 - but with a focus on P3 and P4. The P1 and P2 adjustments will quickly re-adjust your R32 voltage (should that drop by a few volts).

7. I dont know if this is correct - but after I dialled in the R32 voltage, I then focused primarily on the speaker terminal DC offset. Once I had this close to 0v, I then fine tuned by adjusting all pots with micro-tweaks (and taps to loosen the spring tensions) to try to keep both voltages close to 0.

8. The voltgae between T18 and ground was the hardest one to nail down. At the end however I had 99mV on R32, 18mV on the speaker terminals and 34mV on T18 - however the latter was the only one that was fluctuating. In the end I was only adjusting this reading by tweaking the pots.

9. The T18 measurement is extremely susceptible to heat changes. So at the last stages of adjustment (or preferably, sooner) do these adjustments with the lid ON - otherwise you will try to set (with the lid off) and the readings start taking off about 10 minutes after you replace the lid. ensure you have a small screwdriver that you can poke thru the lid.

I got it in the end but I will double-check and re-calibrate both sides once I have the time to get back to this amp. I had the one channel running for a while and I can tell this will be so worth the effort.
  Reply With Quote
Old 14th May 2018, 09:21 AM   #540
poseidonsvoice is online now poseidonsvoice  United States
diyAudio Member
 
poseidonsvoice's Avatar
 
Join Date: Jul 2004
Location: NC
Sony Vfet Illustrated build guide
Slimecity,

Thanks for documenting your final biasing procedures, much appreciated. My colleague “sledwards” above went through something similar and it literally took him all day. It is nice to know that even with VFET builders located several ponds away...basic physics doesn’t seem to change!

6L6/Pass/others,

That being said, I do have a question. This kit is shipped with the single turn pots. Any advantage/disadvantages to this type vs, multiturn Bourns equivalents?

Best,

Anand.
__________________
Science is not a democracy - Earl Geddes
DIYAudio is a journey not a destination!
  Reply With Quote

Reply


Sony Vfet Illustrated build guideHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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

Similar Threads
Thread Thread Starter Forum Replies Last Post
F6 Illustrated Build Guide 6L6 Pass Labs 2173 Yesterday 08:27 PM
Aleph J illustrated build guide 6L6 Pass Labs 5293 10th July 2020 06:22 PM
BA-3 Amplifier illustrated build guide 6L6 Pass Labs 325 7th July 2020 02:43 AM
F5Turbo Illustrated Build Guide 6L6 Pass Labs 598 2nd July 2020 10:30 PM
ACA illustrated build guide 6L6 Pass Labs 169 16th June 2020 03:12 PM


New To Site? Need Help?

All times are GMT. The time now is 12:36 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2020 DragonByte Technologies Ltd.
Copyright ©1999-2020 diyAudio
Wiki