Replacing Selenium Rectifier (Fisher TFM-300)

[artsybob]

Hello all, I'm trying to replace the selenium rectifier in a Fisher TFM-300 tuner. I bought an 8 amp 400V full wave bridge rectifier as a replacement. Would this be a drop-in replacement or would I have to solder in additional resistors? Thanks for the help!

Here's the schematic:
http://www.fisherconsoles.com/non console manuals/fisher tfm300 beg 10000 sm.pdf

 

[century tek]

Why? Is the original giving you problems?

 

[artsybob]

Why? Is the original giving you problems?

Don't know, I haven't tested it yet (I've only replaced the electrolytic capacitors). Could I just leave it in there and only replace it until it gives me problems?

 

[DaveVoorhis]

Don't know, I haven't tested it yet (I've only replaced the electrolytic capacitors). Could I just leave it in there and only replace it until it gives me problems?

Bad idea. The failure mode is clouds of noxious smoke and possible damage to the amplifier.

See this article for further information, including instructions for calculating the appropriate value of series resistor if you decide to use one: http://w3hwj.com/index_files/RBSelenium2.pdf

 

[century tek]

1) I would probably replace it for reliability and safety if you were to use it on a daily basis for 4 hours or more especially if you normally turn it on and leave it on through out the day.

2) I wouldn't replace it if it still works fine especially if you were to use it sparingly or at least stay in the room while it's on. I am still using a 50 year old tube amp with its original can capacitor and its holding up fine so far but I never go off and leave it on like go to the store or something. Even a so called called quick trip could be a disaster.

 

[artsybob]

Here's the replacement rectifier I bought: https://www.alliedelec.com/nte-electronics-inc-nte5314/70215806/

Is this a suitable replacement or should I use something else? It was only $3.71 so it's not a huge loss if it's not. I've never replaced a selenium rectifier before so any help is appreciated!

 

[DaveVoorhis]

Here's the replacement rectifier I bought: https://www.alliedelec.com/nte-electronics-inc-nte5314/70215806/

Is this a suitable replacement or should I use something else? It was only $3.71 so it's not a huge loss if it's not. I've never replaced a selenium rectifier before so any help is appreciated!

It should be fine.

 

[quaddriver]

dunno about the specs of your 300, but on all the american tubers, generally am/fm sets, I replace the Se rect with silicon and I replace the (typically) 22ohm inrush limiting resistor (tends to be in the 2-3watt range) with a 120-150 ohm cement in the 10w range to keep my resulting plate voltages around the same.

The Se rect has a large internal resistance.

 

[arts]

If the operating voltages are still close to specified (as they age,selenium rectifiers usually output less) measure the voltage drop across R5. This will allow you to calculate the units current draw.When you replace the rectifier,use the new values to calculate the required replacement resistor.

 

[artsybob]

dunno about the specs of your 300, but on all the american tubers, generally am/fm sets, I replace the Se rect with silicon and I replace the (typically) 22ohm inrush limiting resistor (tends to be in the 2-3watt range) with a 120-150 ohm cement in the 10w range to keep my resulting plate voltages around the same.

The Se rect has a large internal resistance.

The TFM-300 is an interesting unit being both solid-state and tube. It's got 16 transistors and three tubes (one 6HA5 and two 6CW4 Nuvistors). Don't know if that affects what resistors I put in/

 

[DaveVoorhis]

The TFM-300 is an interesting unit being both solid-state and tube. It's got 16 transistors and three tubes (one 6HA5 and two 6CW4 Nuvistors). Don't know if that affects what resistors I put in/

Nope. The intent of the resistors is to limit the voltage from the silicon rectifier + resistor combination to that which was emitted by the original selenium rectifier, which has a higher voltage drop than a silicon rectifier.

 

[arts]

The TFM-300 is an interesting unit being both solid-state and tube. It's got 16 transistors and three tubes (one 6HA5 and two 6CW4 Nuvistors). Don't know if that affects what resistors I put in/

The three tubes are used in the very first RF stages of the tuner,and will govern the overall sensitivity and selectivity (the ability to separate stations from each other) of the entire unit. Therefore the quality (ie strength) of these tubes is critical to overall performance.If any are weak,the tuner will suck.

Fisher used several different designs of these tube-type RF stages over the years.But regardless of the tube lineup,design or number of tuning gangs,I still believe that Fisher had one of the best front-ends in the business.

And although I am not a fan of solid state anything,I must say that this line of hybrid Fisher tuners,when properly rebuilt and aligned,can easily compete with any other tuner,regardless of age,price or complexity,in both RF capabilities and sound quality.

(full disclaimer: I love Fisher tuners)

 

[quaddriver]

Nope. The intent of the resistors is to limit the voltage from the silicon rectifier + resistor combination to that which was emitted by the original selenium rectifier, which has a higher voltage drop than a silicon rectifier.

yeah what he said.

somewhere, there is on the schematic the path where you get your plate voltages (the top 'line' of the tube in the SCM). still being ignorant of the 300, in my tube radios this figure is 90% of the time between 90-110v.

now depending on how old the set is it is based on: RMS of AC line voltage, half or full wave, smoothed by a cap then minus some voltage drop of a resistor.

the (typical) 22ohm inrush limiting resistor was place in front of the se rect to impede current heading to that discharged cap, which appears as a dead short. the cap would survive, the Se rect would not. since the rectifier on my sets was in the 100ma range, any modern 1A silicon diode with an 'x' amp surge, will handle it, BUT, the inrush acted in concert with the natural resistance of the Se rect to give 'some' output.. at any rate, 2 things work against you,

a) silicon has far less resistance and a very low voltage drop and
b) line voltage today sits around 125 when typically in the past it was 117. so not only do we worry about the extra 8 volts scurrying across the heater string, it throws the plate voltage off.

empirically I found that 120-150 ohms in huge watt numbers (10) take the place of the inrush plus the Se rect internal resistances. YMMV

on the fisher board, those guys are fairly obsessed any might have the parts subs already at hand.

and one last, if not replaced, your main filter cap, which may or maynot be multi sections or multi discreet units, tended to sit around 150wv. with everything I have found today 160v stuff is the lowest I would use, 200v might even be safer.

consult your schematic.

and unlike amps where guys drive plate voltage to lethal limits (for both humans and tubes) in the FM section, such behaviour might not be optimum