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

Instruments and Amps Everything that makes music, Especially including instrument amps.

OT question
OT question
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 18th September 2019, 10:38 PM   #21
jimmy74 is offline jimmy74
diyAudio Member
 
Join Date: Nov 2009
Yes there are resistors from the UL taps to the screens. My meter is working fine, all preamp tubes warm up and yes the heaters all work properly. I'm not sure why all the cathodes are 0v but I do suspect that the volume control on channel A acts as some sort of bias because if I turn it up full I get very low output but also I can hear the power tubes glass clinking like when tubes warm up or cool down
  Reply With Quote
Old 19th September 2019, 01:10 AM   #22
kodabmx is offline kodabmx  Canada
Spreading triode love.
diyAudio Member
 
Join Date: Oct 2011
Location: Toronto
OT question
Quote:
Originally Posted by jimmy74 View Post
...if I turn it up full I get very low output but also I can hear the power tubes glass clinking like when tubes warm up or cool down
That should never happen? By the looks of your measured voltages, I think whatever connects the preamp tube cathodes to ground is shorted.

Also, instead of Photobucket, just upload the pictures to this forum... It works much better
  Reply With Quote
Old 19th September 2019, 02:53 AM   #23
jimmy74 is offline jimmy74
diyAudio Member
 
Join Date: Nov 2009
Well some news....when I plug a guitar cable in the input I do get voltage on the preamp cathodes so not too sure what's going on there. 1 or 2vdc. Biased it to around 33ma. I still think it's the OT but at the same time channel B not working at all has me in doubt. There are 75ohm resistors on the UL taps.

20190918_141656.jpg
  Reply With Quote
Old 19th September 2019, 08:15 AM   #24
Tesla88 is offline Tesla88  Italy
diyAudio Member
 
Tesla88's Avatar
 
Join Date: Dec 2011
Location: Italy
Quote:
when I plug a guitar cable in the input I do get voltage on the preamp cathodes so not too sure what's going on there
What happen to preamp tubes plate voltages ?

OK, let's assume that V1 to V4 are working since their plate voltages are lower than B+ voltage .I guess V5 is drving output tubes and it's a long tail phase inverter, can you confirm? having 531V on plates and nothing on cathodes and grids ,i would check components / connections around V5 This would also explain low power at output

If you want to check OT (IMHO it's fine) With the amplifier disconnected from mains, remove output tubes, apply low AC voltage (6 ... 12 VAC should be fine) at output of OT or speaker output jack if provided (with speaker unplugged) and measure AC between plates wires at output tubes sockets.

If OT is a 3500R to 8R the voltage ratio is around 20, so applying 6V AC at output should give you around 120V AC between plates, less voltage between screens terminals.
  Reply With Quote
Old 20th September 2019, 06:48 PM   #25
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
jimmy74,

You said:
508V Screens
512V Plates
75 Ohm resistor from UL tap to Screen

512
-508
-----
4V

Even if the DCR of the complete primary winding was 0 Ohms, then you have:

4V/75 Ohms = 53mA Screen Current!

Which one of the above figures is incorrect?

Last edited by 6A3sUMMER; 20th September 2019 at 06:50 PM.
  Reply With Quote
Old 21st September 2019, 02:54 AM   #26
jimmy74 is offline jimmy74
diyAudio Member
 
Join Date: Nov 2009
Ok I fed 9vac through the OT secondary and am getting 129vac across the plates, 52vac across the UL windings and 29vac CT to ground
  Reply With Quote
Old 21st September 2019, 08:51 PM   #27
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
Why 29V CT to ground? Sounds like the CT is not tied to the B+ cap to ground (of course with B+ powered down, and discharged).

UL taps 40%.
  Reply With Quote
Old 21st September 2019, 09:07 PM   #28
jimmy74 is offline jimmy74
diyAudio Member
 
Join Date: Nov 2009
I had unsoldered the CT from the board... its giving me around 64vac to the plates
  Reply With Quote
Old 23rd September 2019, 08:43 AM   #29
Tesla88 is offline Tesla88  Italy
diyAudio Member
 
Tesla88's Avatar
 
Join Date: Dec 2011
Location: Italy
OT is fine so I will check splitter / driver stage, check for open resistors.

Quote:
29vac CT to ground
If CT was unsoldered it's fine, with CT connected to B+ (with unit unpowered) you should read 0Vac

I would trace the amp schematic, not that hard and really helpfull

Last edited by Tesla88; 23rd September 2019 at 08:47 AM.
  Reply With Quote

Reply


OT questionHide 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
Phase inverter bias question and also an impedance question Illinest Instruments and Amps 8 27th December 2015 08:31 PM
Quickie woodworking question! With bonus shellac question... head_unit Construction Tips 13 12th September 2010 05:38 PM
do i REALLY need a crossover?? and an amp question. and an ohm question legendaryfrog Multi-Way 4 29th October 2006 01:39 AM
newbie with input tran. question and wiring question imo Solid State 0 18th January 2006 10:10 PM
old speaker question, bullet midrange question & link to speaker project wallijonn Multi-Way 10 5th November 2002 06:03 AM


New To Site? Need Help?

All times are GMT. The time now is 05:03 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki