[Collins] 75A-4 Power Xfmr

[Carl]

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: "Carl" <km1h at jeremy.mv.com>
Cc: <collins at mailman.qth.net>
Sent: Wednesday, April 13, 2011 4:29 PM
Subject: Re: [Collins] 75A-4 Power Xfmr


> But the 75A4 has a choke input filter so the low drop and hard conduction 
> of the silicon rectifier doesn't stress the transformer. He says that too. 
> Tubes aren't as happy with the high peak currents of a capacitor input 
> filter and silicon rectifiers pass that stress on to the transformer. Good 
> transformer vendors rate a plate winding at a significantly greater DC 
> current with a choke input filter than with a capacitor input filter 
> because the peak currents to the capacitor input filter are much higher 
> and of much shorter duration.


Agree altho Collins wasnt known for sourcing the best transformers in those 
years. Since not all receivers use choke input I thought that article would 
be of more general interest and not model specific.


> I count on a transformer of typical construction have about 4 to 5% 
> impedance, and only part of that is resistive making heat. But 10 watts 
> less load on the 5 volt winding is 10 watts less load on the primary and 
> should allow 10 watts more load on the 6.3 volt winding without changing 
> the primary current or the total transformer heat dissipation. And since 
> part of the voltage drop is in the primary and part in the secondary for a 
> particular load I expect the 6.3 volts to stay up a bit better when 
> slightly overloaded by having removed the 5 volt load.


This is mostly conjecture


>
> Of course one can confuse the whole matter by using the freed 5 volt 
> winding as a buck winding in series with the AC line to the radio. And 
> gain in energy dissipated in saturating the transformer core as well as 
> lowering the operating voltage through the radio.


That is done often, especially with consumer type sets. Hams seem to be very 
slow on adapting the bucking concept to counter the high line voltages.

>
> Point is, unless run with way to high line voltage, transformers are 
> resilient and not quickly damaged with a fairly large overload. 
> Transformers designed to run at low line voltages and designed to be the 
> most economical of construction (minimum copper and iron) will heat the 
> core rapidly as well as the primary winding from the effects of being 
> driven too far into saturation by the high line voltage which cause high 
> peak primary currents.

Hallicrafters are the most prone to transformer problems; Ive measured 
several models that are already starting to saturate at 123V. A few 
Nationals are similar...unfortunatey they are the top of the line models.


If one looks at the transformer primary current
> with an oscilloscope, the maximum proper line voltage shows up easily as 
> the current peaks rise much more rapidly than line voltage rises.


If you use a dual trace scope and also monitor the secondary you will see 
the effect of line spikes that arent pre filtered. With todays line 
pollution there are sometimes spikes well above what would be considered 
safe. For that reason I use the 3 capacitor line bypass ( .01 both sides of 
line to ground and up to .1 line to line, leakage current wont affect GFI's) 
plus another at the secondary.  National used one at the secondary on a few 
models to counter noise on 3rd world power lines. Unfortunately paper caps 
deterioated and took out transformers.

SS diodes also have their own switching signatures that can create noise. A 
single .01 across each string and another to ground at the output eliminates 
it.

I prefer the vacuum tube rectifier plus the bucking transformer, when not 
pushed hard they have an amazing life expectancy. Switching a 5U4 to a 5R4 
reduces filament load by 1A, has a higher PIV and drops a little more 
voltage. Except for filament current they also are a good swap for the 5Y3 
and other 2A octal rectifiers.  I use the regular ST shaped GY's in most 
radios and the WGB potato mashers in several transmitters.....prices are 
still well below the 5U4 and 5Y3. Some rebase them and use in place of the 
5Z3.

Carl
KM1H



>
> On 4/13/2011 2:54 PM, Carl wrote:
>> This chap would disagree about the benefit of going SS with the rectifier
>>
>> http://www.w8ji.com/power_transformer_stress.htm
>>
>> Ive built full bench bucking transformers into the station; one large
>> transformer takes care of all the 120V line dropping for each bench, old
>> or new gear. Using unpopular oddball voltages at high current found
>> cheap at hamfests and surplus dealers I run 110-114V to everything.
>> Various consumer radios, mostly high tube count wood consoles and table
>> models, have a small transformer either under the chassis or in a small
>> box that includes a HB line filter and outlets that feed several radios.
>> A simple 12.6V 2A secondary will handle up to a 240W load; I run mine at
>> about half their ratings.
>
> The transformer won't smoke run at nearly twice their ratings but the 
> voltage regulation won't be very good.
>
> Most classic radios had no line filter, just a couple .05 mfd caps from 
> each side of the line to chassis which often puts enough voltage on an 
> ungrounded chassis to tickle and draws enough current when the chassis is 
> grounded to trip a GFCI.
>
> All that is good for your radios and will go a long ways to get a very 
> long transformer life.
>>
>> National for one commented in QST that the early HRO sensitivity was
>> better with 180V B+ than 250V which is only needed for full speaker 
>> volume.
>
> In the 75A4, the 6AQ5 runs from the first filter capacitor at about 220 
> volts while the rest of the radio is supplied through another choke and 
> capacitor combination at about 180 volts. With only 4 pins on the power 
> supply connector, National limited the possibility of running different 
> voltages on different stages.
>
> 75S varied over their lifetime from running 120 volts up to about 185 on 
> the output stage and the 6BF5 ran hot the whole time, hotter in the later 
> radios at higher voltage, but they did add a bit of unbypassed cathode 
> resistor for tube protection from a failed bias supply and distortion 
> reducing inverse feedback in the latest versions. A 6AQ5 draws less 
> current but at the low voltage and wrong impedance plate load has more 
> distortion and lower power output.
>>
>> With 2 or 4 tube audio amps in the consumer sets, I hear no volume
>> difference at the lower line input; they still rattle the timbers!
>
> The simple baffles of the console radios make for very efficient speakers, 
> unlike some modern "hifi" speakers that absorb all the back wave to get a 
> more uniform response vs frequency.
>>
>> Carl
>> KM1H
>>
>>
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.