selenium bridge rectifier replacement question

[bzuidgeest]

Hello all,

I want to replace a selenium bridge rectifier on a tube amp. Now this has been written about many times before and I know I need to compensate the difference in voltage drop.

However the geloso amp (G260) I'm working on uses the selenium rectifier for the heaters and has the positive DC side connected to ground and the negative to the caps and such. Do I still connect the resistor to the cathode (plus side) or should I connect it to the anode (neg side) this time?

I've include a picture of the relevant schematic part below. showing old and new version.

http://imageshack.us/photo/my-images/716/rectifierreplacement.png/

[Tubeamp]

Hi bzuidgeest,

However the geloso amp (G260) I'm working on uses the selenium rectifier for the heaters and has the positive DC side connected to ground and the negative to the caps and such. Do I still connect the resistor to the cathode (plus side) or should I connect it to the anode (neg side) this time?

Which resistor do you mean? R36?

[battradio]

Add a 20 ohm 5 watt resistor to the negitive terminal of the bridge rectifier to the junction of C25 & L2 replacing the jumper wire . then mesure the 12.6 volts to the 12SL7's , if the 12.6 volts is higher than 13 volts DC the 20 ohm resistor should be in creased in value , if the voltage is about right leave well enough alone .

[bzuidgeest]

@tubeamp. Because I was not certain where to place the compensating resistor this time. I left it out of the schematic. battradio does however describe it perfectly.

@battradio. That is most usefull info. It confirms my thought that it should be placed at the negative side this instance. As an extra question. Do you know why in most descriptions people measure the voltage on the heater? Why not on the bridge + restisistor? Seems more accesible to me to measure on the bridge...

[battradio]

Because that , if the voltage on the heaters is correct , all three of the voltages is correct , the bias will be correct and the voltage at the rectifier bridge will also be correct without having to do any math .Be sure that all the the tubes are pluged in when measuring the heater voltage . A test fixture is available that plugs into the tube socket and then you plug the tube into it and it has 8 test points around the adapter so the voltage can be measured top side .

[bzuidgeest]

Since I have no such test socket, I'll just have to measure from the other side. Fortunately I do have test leads that clamp onto the test points, so I don't have to wiggle around with the pins or stick my hands in. Just going up slowly with the variac. and monitoring the volt meter.

I measured the voltage on the dc leads of the selenium bridge before I removed it. If I measure the new bridge + resistor to the same voltage, does that not also imply the other voltages are correct, without doing any math?

I do see where measuring on the heater is usefull if you don't have a pre replacement voltage. Also It gives one a check of the circuit. So I'll do it that way. after all you have most likely been doing this a lot longer than me:)

[bzuidgeest]

I replace the old bridge with a new one and connected a 500 ohm 5 watt pot between it and the rest of the circuit. I then tuned it to the proper resistance value by measuring the V1 heater. It came in at about 16 ohm.

I then replaced it first with a conservative 22 ohm 5w fixed resistor and measured both V1 and V2. V1 was about 12v and V2 about 11.3v. I tried again with 15 ohm which gave me 12.8 over V1 heater and 11.8 over V2 heater. (all voltages after heating up a few minutes)

V2 heater seems rather low, but looking at the schematic I do not see how to get both heaters to an equal voltage value in the current schematic. Also audio seems fine using the lower 11.8 value....

Any comments?

[battradio]

Clean the tube sockets and resolder the heater pins connection on the tube sockets 8 & 7 , if that doesn't help swap places of the two 6SL7's if the voltages folow the tubes leave well enough a lone .

[Tubeamp]

Here is an article as regards tubes heaters...


"Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and should be avoided. Running them at lower voltages, within reason, will increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation. "

[bzuidgeest]

@battradio. The voltage difference follows the tube. Although switching them around did increase the voltage on both tube a little. This let the higher voltage heater reach 13v. I switched them back and the previous voltages returned. I'll just leave it be as it sounds fine.

@tubeamp. Thank you for the info, the lower voltage heater is closely matching that 10% deviation, but it seems ok.

Since all is working fine I consider the matter solved. Thanks to everyone for helping out.