Replacement rectifier.

[kalee20]

Originally Posted by paulsherwin

The resistor is to mimic the voltage drop of the original selenium rectifier and limit the inrush current. It isn't essential but fitting one is good practice. It would normally go between the rectifier +ve connection and the reservoir electrolytic.

I'd actually suggest to put it between the transformer and the rectifier! In series with one lead (or if you really want, a resistor of half the value in each lead).

The reason? It won't make a scrap of difference normally, but if the rectifier ever fails, the resistor will limit the current and hopefully burn-out before the transformer does. Whereas if the resistor is on the DC side, the failed rectifier will be straight across the transformer, which is then almost guaranteed to burn out.

[turretslug]

My thinking, too- a conservatively-rated silicon bridge should be reliable but a common failure mode is shorted junctions. It's possible to find power film resistors (2,3 etc. W) that are designed to go fssst, smoke, O/C without drama or fire hazard under over-load.

[vidjoman]

Sorry I misread the 'off' as of. Often happens with predictive text throwing in extra letters or just typing on a touch screen adding more letters than needed.
Yes it could be the screens connected together to the HT.

[exlecy1]

The pin no on the 4x EL84 is no 9, the amp model no 36196-A, used in several models. The rockola wallmount 430 1484.

[exlecy1]

the amp is from model rockola 430 serial no 36196-A, the valve pin no is 9.
the selenim rec is still in place so i applied 240 v to check the DC output which
is 198v, the silconn rec has output of 216v i don't know if this info is useful or
is this where the resistor needed

[paulsherwin]

198V sounds plausible for a selenium rectifier in good condition given that silicon gives you 216V. It probably doesn't need changing.

[vidjoman]

Just found a Rock-Ola circuit with EL84 outputs which shows the HT line as 310v going to pin 9. Fed from a transformer winding to a bridge rectifier but can't make out the voltage shown (could be 235) - very poor photocopy. It may not be the same amp as yours as it has no model numbers on the diagram. Most of their amps seem to use 6L6 or 6973 output valves.

[Craig Sawyers]

Originally Posted by paulsherwin

198V sounds plausible for a selenium rectifier in good condition given that silicon gives you 216V. It probably doesn't need changing.

I agree. In fact as I mentioned I could probably have left the original selenium rectifiers in place in the Lorenz SZ42.

Provided they are not running hot and smelling of garlic (Selenium oxide), in which case take the unit outside because of toxicity, and also the house can stink for many days.

Craig

[exlecy1]

does the difference in output voltages create problems in the amp ? i may do as Paul suggests and reinstate the selenium ,is there any way to check it, i have placed an attch in the management section of the amp schematic but not sure how to attch here

[paulsherwin]

The small increase in HT probably doesn't matter, but it will largely disappear if you fit the resistor as suggested.

Selenium rectifiers of this type normally fail gradually over several years, dropping more and more voltage and running hot. Occasionally they can fail catastrophically emitting noxious and extremely smelly gas, but this is very rare with audio equipment.

[Craig Sawyers]

I would be surprised if the additional ten volts or so would be a problem. That is about ten volts in about 300, or 3%. Mains voltage is 230V nominal +10% - 6% and so the selenium voltage drop is swamped by the allowable variation in mains voltage.

There is a good write up about selenium rectifiers, and replacement strategy, here http://yarchive.net/electr/selenium_rectifiers.html

Craig

[exlecy1]

if i decide on the silicon rec and place a resistor in series with + dc output say 150 ohms what wattage should the resistor be,

[paulsherwin]

3W or 5W should be fine. 2W will probably be OK too if you have one to hand.

[exlecy1]

thanks to Paul and all for help and info, it must be hard trying to explain to
someone with no electronic savvy, cheers

[ms660]

The OP said the amp is a 36196-A, the schematic that the OP posted is for a 41710-A.

Looking at another schematic, for the Rockola 434, it's output stage looks very similar to the one in the schematic that the OP posted, the cathode voltage and resistors for the EL84's are shown as the same so far as I can make out (10v and 130 ohms) The 434 schematic shows the HT winding as 235v and the HT as 315v, the OP has reported the measured HT as 198v (selenium) and 216v (silicon):

http://arcarc.xmission.com/PDF_Jukeb...atics/434a.pdf

?

Lawrence.

[paulsherwin]

I did think the voltage seemed a bit low for an amp, but as I said I don't have any experience with juke boxes. The voltage from the silicon bridge does correspond to the selenium voltage though, so if there's a fault it's somewhere else.

[exlecy1]

hi Lawrence
amps are the same different numbers for two models and years but same
schematics there are 3 different model no's but same amp

[ms660]

According to your measurements it looks like the HT's low then, measure the AC voltage across the mains transformers HT winding, then across the AC input at the bridge.

Lawrence.

[vidjoman]

The schematic for the 434a as linked by ms660 is the same as the copy I have which gives 315 as the HT.

[exlecy1]

the output on the trani is 244v same at the rectifier ac legs ,,attch
schematic of pwer supply + amp