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

Question for Heathkit W-5M owners
Question for Heathkit W-5M owners
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 16th October 2007, 02:39 PM   #1
ltarini is offline ltarini  United States
diyAudio Member
Join Date: Sep 2007
Default Question for Heathkit W-5M owners


Two schematics exist for the W-5M, one has R30 listed as 15K and one has it as 6.8K. The 15K schematic specifies plate voltages of 255 on the 2nd 12AU7, while the schematic with 6.8K does not list plate voltages for the 12AU7.

I am very interested to know which version is older and what are the 12AU7 plate voltages for the 6.8K version.

If you have a W-5M amp I would appreciate it if you would let me know the serial number and which R30 resistor you have. If you don't recall the color code for resistors off hand, just give me the sequence of colors with the silver band last.

If you happen to know your plate voltages on the 2nd 12AU7 that would also be of interest.




The motivation is: I am trying to solve a bias balance problem on a W-5M. All resistors are within spec and matched where indicated, but V3 consistently gives at least 10 mA more current than V4. The closest I can come to zero balance is .4VDC, that's with the entire resistance of the pot on the V4 side. My 12AU7 voltages are 295, not 255, so I'm wondering if the 15K R30 would bring them down. Next step is probably to take out the 1 uF signal caps and check them.
  Reply With Quote
Old 16th October 2007, 03:28 PM   #2
Tom Bavis is offline Tom Bavis
diyAudio Member
Tom Bavis's Avatar
Join Date: Jun 2002
Location: Macedon NY
I suspect that R30 was increased to avoid a low-frequency oscillation, as it drops the cutoff frequency of the RC filter by half. The higher value will lower the voltage on the phase splitter.

Bias is unrelated. The coupling caps certainly could be the problem - disconnect one end of each and recheck bias balance - if it has changed, caps areresponsible.
  Reply With Quote
Old 17th October 2007, 02:03 AM   #3
ltarini is offline ltarini  United States
diyAudio Member
Join Date: Sep 2007
It makes sense that 6.8K is the older version of R30, because my amps also have the older transformer, the 16458. If anyone has a 16309 unit with a 15K resistor in R30 position, that would be consistent with the timeline, if not actually conclusive. Should I upgrade to the 15K resistor, then?
  Reply With Quote
Old 17th October 2007, 03:35 AM   #4
Trout is offline Trout  United States
diyAudio Member
Trout's Avatar
Join Date: Nov 2004
Location: Midwest Madman
I just looked at Mine and R30 is 15K, My original manual/book has a date of 1959, But I beleive my units are 1961.



BTW, Mr Bavis is correct on suspecting the coupling caps. I had the coupling caps in both units cause serious bias issues. After replacing them it was smooth sailing.

  Reply With Quote
Old 17th October 2007, 03:48 AM   #5
ltarini is offline ltarini  United States
diyAudio Member
Join Date: Sep 2007
I found the following info. I'm going to do all the changes except the surgistor.

Look at:


"W-5M Service Bulletin

The Heathkit W-5M Amplifier, a "standard" in our audio amplifier kit line for many years, has been discontinued. Style changes and new concepts in circuit design plus the increased trend towards stereo, contributed to the discontinuance of the W-5M. During the production of the W-5M, a few minor changes were made in its circuitry to improve its performance. These changes are listed below:

Page 19, Step 12 - Change "15k ohm, 1 watt resistor (medium body, brown-green-orange)" to "15k ohm, 2watt resistor (large body, brown-green-orange)" R-28.

Page 19, Step 13 - Change "6.8k ohm, 2 watt resistor (large body, blue-grey-red)" to "15k ohm, 2 watt resistor (large body, brown-green-orange)" R-30.

Page 20, Step 12 - Change "22k ohm, 1/2 watt restitor (small body, red-red-orange)" to "22k ohm, 1 watt matched pair of reststors (medium body, red-red-orange)" R-10.

Page 20, Step 13 - Change "22k ohm, 1/2 watt resistor (small body, red-red-orange)" to "remaining 22k ohm, 1 watt matched pair of resistors (medium body, red-red-orange)" R-9.

Page 21, Step 1 - Change "47k ohm, 1 watt resistor (medium body, yellow-violet-orange)" to "47k ohm, 2watt resistor (large body, yellow-violet-orange)" R-8.

Also a Surgistor was added to limit the high voltage surge that may occur when the amplifier is first turned on. This will help prolong the life of the components used in the amplifier, where line voltage or transformer voltages may be somewhat higher than normal but still within their rated specifications. An information sheet regarding the installation of the Surgistor is enclosed with this component."
  Reply With Quote


Question for Heathkit W-5M ownersHide 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
A simple question for the Aleph 3 owners JEKYLL Pass Labs 6 24th May 2004 07:59 PM
Question for DCX2496 owners - Volume Control Branwell Digital Line Level 6 3rd March 2004 02:46 AM

New To Site? Need Help?

All times are GMT. The time now is 12:51 AM.

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