The AM Forum
March 28, 2024, 09:06:25 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Brute-force bias supplies  (Read 6406 times)
0 Members and 1 Guest are viewing this topic.
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10057



« on: January 29, 2005, 02:04:20 PM »

Quote from: N8ECR
I have for sale: 2 machine/industrial control transformers rated for 1.5KVA(1500VA) made by Westinghouse, have a different core disign than usal...They have two windings on one side, and a single on the other I think designed for 220/440 to 220/110 depending on how you wire them up.


I have a couple of these too, but mine are rated at 150 va.  About the size of your fist.  Also made by Westinghouse.

Does anyone know exactly what these were intended to be used for?

I plucked one of mine out of the BTA-1R I parted out.  Picked up the other one, new in the box,  at Dayton.  They make excellent transformers for brute force bias supplies for class-B modulators (and slopbucket leeenyars).  You can set them up for 110 or 220v each side of midtap, and easily pull 500 ma of bleeder current using a few hundred ohms of wirewound rheostat or slidetap resistor to pick off the appropriate bias voltage.  The class-B grid current is a small enough percentage of the total bleeder load on the bias supply, that good regulation is maintained without resorting to electronic voltage regulation.

My HF-300 rig uses a pair of 5Z3's for rectifiers.  Of course, this circuit lends itself very well to solid state rectifiers, since you don't need diodes with very much peak inverse voltage rating in this application.

Why not use electronic regulation and a lighter weight voltage supply?  The problem I had with mine was due to a.c. line voltage variation.  The bias voltage remained fixed by the regulator, but the modulator plate voltage varied with the line voltage.  The modulator tubes effectively amplify the relative difference between bias and plate voltage, so that with a slight change in line voltage, the resting plate current varies big time, and I found myself constantly fiddling with the bias control to maintain the proper value.  With a simple brute-force supply, the static plate current varies at about the same percentage as the modulator plate voltage, which is the way it should be.  

The reason this works is that when the line voltage drops, the plate voltage drops, but so does the bias voltage at the same percentage, causing less drop in static place current.

You may have noticed that most tube-type broadcast transmitters use this simple bias supply circuit instead of electronic regulation.  These transformers are a real find for this applacation, since low voltage, high current plate transformers are very rare.  UTC and a couple other companies made such transformers specifically for bias supply applications, but these have become extremely rare and hard to find.  Industrial control transformers seem to be plentiful.
Logged

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

- - -
This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
John K5PRO
Member

Offline Offline

Posts: 1033



« Reply #1 on: January 30, 2005, 11:37:16 AM »

They are used to step down bigger AC line voltages for things like numerical controlled milling machines, to run the controllers. Also, in commercial DC power supplies, which run off 220 or 480 VAC primary. The interlocks, the relay controls (ladder string), etc. By having a standard rating transformer, from 75 VA up to many KVA in 120/240/480 windings, there is a decent market for these things.

I used one for a bias power supply, which has been very successful in my workplace. About 10 years ago I was faced with expensive tube failures which were unexplained.A pair of 4CX250,000B tetrodes are used as a plate modulator and had -350 VDC for cutoff bias. Autopsied tubes indicated that the electron beam was being focused by the particular bias and plate voltage(30 KV) with the screen at zero (during idling), and was cutting slots into the copper anode, liberating ions and causing lots of trips.

I couldn't find a suitable long term supplier of transformers (Stancor made the original bias transformers for these), so i found a nice 500 VA GE 120/480 machine tool transformer, which did the job perfectly, with a C input filter. 10 years later and it is still giving long tube life, around 30,000 hours for these big tubes.
Logged
N8ECR
Guest
« Reply #2 on: January 31, 2005, 11:44:26 AM »

These I have for sale are about the size of a Heathkit HG-10 vfo they should be good for about 15amps, and weigh about 25 or 30 lbs.  They have a strange core which it looks like some one took a stack of laminate iron and bent it into a U and inserted them into the coil to make up two oval donuts.  If nothing else these would make up excellent isolation transformers.  I picked them up with it in mind of all the fet/E class builders out there, maybe they would be of use in some straping Class E rig.

A conversation with Bob/W2ZM, he thought these might make good Modulation transformers???  Don't know about fidelity here, but if you are on a budget, here you go!! cheep mod iron

Treminals are marked H1-H2-H3-H4 and X1 and X2
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.032 seconds with 18 queries.