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

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

HOT Iron ?
HOT Iron ?
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
Thread Tools Search this Thread
Old 1st November 2006, 07:48 PM   #21
HollowState is offline HollowState  United States
diyAudio Member
HollowState's Avatar
Join Date: Aug 2006
Location: Taxland, New Jersey
Hi Ed,

If this were my amplifier and I wanted to measure output tube current, here's what I would do.

I'd install a low value carbon comp resistor in each tube's cathode to ground. By low value I mean around 4.7 ohm ( watt) or something close to that. I could then measure the small voltage drop across that resistor and use simple ohms law to calculate the current. (I=E/R or voltage divided by resistance)

For a current of 50mA, you would measure 0.235 volts across 4.7 ohms. And you could even make that cathode point available with a feed-through standoff on the chassis near the tube for future measurments. And do not be concerned about adding a tiny resistance in the cathode cirucit. Pragmatically it will have no effect on the sound or circuit performance.

If you'd prefer not to alter the amp in any way, then you could make an adaptor socket to place under the tube that would make the cathode and resistor available for your test probe. It they're octal base tubes it's easy. Just use a defective tube's base as your plug with a socket mounted on top.

  Reply With Quote
Old 1st November 2006, 08:31 PM   #22
Vespasian is offline Vespasian  United States
diyAudio Member
Join Date: Sep 2006
Thanks - Got it!
  Reply With Quote
Old 1st November 2006, 10:53 PM   #23
Vespasian is offline Vespasian  United States
diyAudio Member
Join Date: Sep 2006
Default Proximity?

The way this thing is laid out, the output tubes are right next to the transformers. On some other amps I see from the same period, they are not nearly as close, like Fisher x100's or even Scott's 299d. One opt has got the rectifier next to it also. The problem is compounded a bit also because the replacement JJ 7591's are larger than original stock (jj's on left) and can actually TOUCH the laminations on the transformers if tilted. Perhaps Tubelab is correct and this is simply a radiant heat thing. After 20 or so minutes the far sides of the tranformers are much cooler than those facing the tubes.
Click the image to open in full size.
  Reply With Quote
Old 2nd November 2006, 12:36 AM   #24
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
Tubelab_com's Avatar
Join Date: Jul 2005
Location: West Virginia panhandle
The tube current can be measured without adding any resistors.

With the amp off, and everything discharged (I would measure it after it has been off overnight) measure the DC resistance of each half of the OPT primary. Write down the numbers. Turn the amp on, with no signal applied, measure the voltage across the same primary halves. Take the measurements after the readings have stabilized, but before the transformers heat up, the resistance will change slightly. You can use ohms law to compute the plate current. The screen current can not be easily measured, but can be assumed to be 5 to 7 mA per tube.

It is normal for the dc resistances of each transformer half to be slightly different. It is also normal for the currents to be slightly different. Adjust the balance pot for minimum hum.

I set a black RF attenuator on top of the electrolytic cap on one of my SimpleSE boards, This puts it about 1/4 inch from ONE 6L6 tube. It was too hot to touch after about an hour.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote


HOT Iron ?Hide 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
Gun or Iron? mashaffer Tubes / Valves 13 29th December 2007 01:22 PM
Appropriate Iron for 211 Triode SE wrenchone Tubes / Valves 50 7th December 2007 03:38 AM
Best Use For My Sansui Iron? chrisvw Tubes / Valves 13 27th May 2006 08:28 PM
soldering iron jaudio Parts 15 17th March 2006 07:22 PM
For those of you in the UK who need a new soldering iron 5th element Parts 2 26th March 2004 10:58 AM

New To Site? Need Help?

All times are GMT. The time now is 03:20 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio