High B+ GZ34 deluxe reverb

[7xa12]

What do you recommend for deluxe ab763 when B+ seemed to be bit too high.
What voltage drop I can expect after installing all tubes?
my mains 235VAC @ B+ = 456 VDC (hammond 291 BEX with JJ GZ34 rectifier)

[Amp Kat]

Obviously they were designed to run about 410 volts but with todays higher voltages even the originals will run around 430 to 450 depending on the wall voltage. They will drop some more voltage when all of the tubes are in and certainly even more as you start to crank it but I can't give you a number because they are different from Country to Country and international trannies could vary somewhat over a domestic. You can try a resistor in the choke pi filter to control it maybe a little more as heat dissipation in the resistor or possibly a back -bias diode from center tap to ground to drop a tad more.

[7xa12]

They semeed to be design for 240VAC mains, Might be a problem with rectifier? do you recommend JJ 5u4GB in my amp?

[Amp Kat]

Not really, I think you are fine where you are. The original FD tranny is only 10 volts less that that one. What is the actual voltage going into the rectifier tube ?

[7xa12]

from 235 to 240

[7xa12]

I will take a pic with voltages, give me a minute

[7xa12]

here just gz34 rectifier installed ( 457VDC) and at the same time 229VAC at mains.

[Amp Kat]

[ATTACH=CONFIG]25279[/ATTACH][ATTACH=CONFIG]25280[/ATTACH]here just gz34 rectifier installed ( 457VDC) and at the same time 229VAC at mains.

K what I really meant was AC going into the rectifier tube so with that calculation it is probably right about 325 VAC which is pretty much spot on so by the time you get all the tubes in and everything settled down it should be dang close or close enough for Gov. work.

[7xa12]

thanks Amp kat
AC going into rectifier with no tube installed is changing from 340VAC to 350VAC

[7xa12]

so my GZ34 rectifier tubes rising the voltage more then 110V. but I I understand with my voltages scould be fine
why you dont recommend the 5U4GB?

[7xa12]

I am thinking about more reliable and safer for PT in ab763 circuit. but if there is no big difference. I will leave the gz34.

[Bruce / Mission Amps]

All those unloaded AC voltages are all nearly meaningless until you install the rectifier tube, the preamp tubes, both power tubes and with all tubes hot and turned on ...IE: Not in standby.
Then see where you are with respect to changing anything other then testing and resting the idle current of the power tubes to around 9-11 watts each.

And yes, there is no reason what so ever that if you want lower B+ voltages, you can not use a 5U4GB rectifier with that PT. It will work fine too.
Just reset the bias voltage of the power tubes with that combination of B+ on the plates of the power tubes and a 5U4GB so it now is around 9-11 watts.

[7xa12]

Thank you Bruce
I am not very experienced with amps and this is my first amp build.
this forum is great, and I appreciate all your help.
From the point I am now - the transformer and rectifier seemed to be ok. and the first cap as well.
would it be right If I continue testing with all the tubes installed at once ? or just a tube one by one?

[Amp Kat]

Thank you Bruce
I am not very experienced with amps and this is my first amp build.
this forum is great, and I appreciate all your help.
From the point I am now - the transformer and rectifier seemed to be ok. and the first cap as well.
would it be right If I continue testing with all the tubes installed at once ? or just a tube one by one?

If you did everything correct there is no reason to fear installing all of the tubes and powering up. If you are fearful that there is a problem get a light bulb limiter and use it which is a good idea on a new build anyway because there can be surprises sometimes. If there is a problem it's a good chance the fuse will tell you but going one by one will only isolate it if there is a problem but would still yield the same results. As Bruce said and I that you won't know until you get everything all biased up where those voltages are going to sit. From my guess you may be 20 volts higher than the standard which isn't crap to a tube. I like the way the GZ-34 sounds as a personal preference but nothing wrong with the 5U4.

[7xa12]

Yes you are right, I did a test with bulb limiter and.... B+ drop to 414V and ( REctifier and power tubes @wal voltage 233V )
then all tubes installed with speaker and spring reverb connected :
B+ drop to 354V. the bulb stay dim.
Power tubes
Plates 357V (pin 3)
Screen Grid 353V (pin4)
Control grid (-42V) (pin 5)

Preamp tubes plates (pin1)
V1- 186V
V2 181V
V3 - 353V
V4 -219V
V5- 345V
V6 - 183V
V5- 345V
V6 - 183V
I just tested it with bulb limiter 100W bulb , I know that that bulb limiter will drop B+.
but is the test meaningles too? How much the 100V limiter drop the B+? I will do the next readings without the limiter.