Lowering the AC voltage on the power transformer secondary

[isaac_teller]

Hello everyone. I'm building a push pull 6L6 amp and I requested from a transformer winder near me a 0-310 secondary on the power transformer(for use with a diode bridge). What I discovered after I returned home(and after I already built the amp with 450 volt electrolytics) was that the secondary is 0-325. Is there any easy way to reduce the AC voltage on the secondary w/o too much hassle? Thanx in advance.

[sluckey]

See page two...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[Latole]

I often use zener diodes stack with success;

Picture from :
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage

And more here ;

https://el34world.com/Forum/index.php?topic=25996.0

https://el34world.com/Forum/index.php?topic=17595.0

https://www.tedweber.com/v-dump

[isaac_teller]

Thank you. This helps a lot!

[tubeswell]

What I discovered after I returned home(and after I already built the amp with 450 volt electrolytics) was that the secondary is 0-325.

Was that 0-325 an unloaded voltage? (If so, it might drop a bit when you pop the tubes in)

[Latole]

Was that 0-325 an unloaded voltage? (If so, it might drop a bit when you pop the tubes in)

100 % right

[isaac_teller]

Yep. Unloaded. About 475 VDC unloaded. It will drop when the tubes are in(2 X 6L6, 2 X 12ax7). Can I leave it as is? Will the caps hold with the initial surge(~15-20 seconds) until the tubes start to conduct and lower the DC? How about adding a standby switch for the warm-up period?

[tubeswell]

Yep. Unloaded. About 475 VDC unloaded. It will drop when the tubes are in(2 X 6L6, 2 X 12ax7). Can I leave it as is? Will the caps hold with the initial surge(~15-20 seconds) until the tubes start to conduct and lower the DC? How about adding a standby switch for the warm-up period?

My guess is the caps will probably hold the surge if they survived when it was unloaded. (If they are cathode-biased 6L6s, the B+ loaded voltage may drop quite considerably)

[PRR]

> Will the caps hold with the initial surge(~15-20 seconds)

We did that a LOT. The 450V is an all-month number, it will take more for a few seconds. The cap is not harmed until it over-heats, which takes minutes.

[isaac_teller]

The amp is fixed bias. Anyway I think I'll add a standby switch for an extra margin of safety. Thanx again for all the help.

[Latole]

Before put 6L6 you must power ON amp and read the right bias voltage at pin 5 on each 6L6 socket

Minus 35 DC volts or more.

I'll always use the bulb lamp limiter for first power ON. You should use it.

The bulb limiter will limit the current demanded by the amp and thus lower the voltages
By limiting the current the amp will be protected in case of short circuit and the fuse will be saved.
You will have to take into account the voltage drop by reading the bias voltage.

Take note of how the bulb limiter works;
Power amp ON; lamp very bright at startup and immediately dims if everything is normal.
Still very bright; problem in the amp

 

[mresistor]

If you add a stanby switch maybe add it after the first (reservoir) filter capacitors like in the twin reverb AB763. Also if still concerned about the voltage surge at start up and if there is room in your build you could place two capacitors in series with charge leveling resistors as the first filter node, which is also like in the Twin Reverb AB763.


https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab763_schematic.pdf

[isaac_teller]

I'm going to try this type of standby. It should prevent a strong inrush.


http://www.valvewizard.co.uk/standby5.jpg