GZ34 in 5f4 build

[scole]

whats the best way to handle the higher plate voltage if i put a gz34 in? do i lower the plate voltage or just change the bias resistors? wich bias resistor should be changed the 6.8k or the 56k? or both?

[tubeswell]

I thought that zenering the CT of the High Voltage winding with reverse biased diodes was best way. Aiken has an article on this.

http://www.aikenamps.com/BackBiasing.html

I believe that the other thing you have to watch-out for is how much current a GZ34 draws compared with less-powerful rectifiers, and whether your PT is sufficiently rated to cope. I had to worry about this once when a tube salesman tried to sell me a GZ34 when I needed a 5Y3GT. The former draws higher current than the latter.

[Bruce / Mission Amps]

whats the best way to handle the higher plate voltage if i put a gz34 in? do i lower the plate voltage or just change the bias resistors? which bias resistor should be changed the 6.8k or the 56k? or both?

The 56K resistor needs to be bigger for more negative voltage... more importantly... try rewiring it with a 50K trim pot and a 27K resistor in series so it can be an adjustable bias supply.
I run a GZ34 in one of my 5F4 amps and it makes about 35 watts.
All my 5F4 amps have an adjustable bias supply and can use any octal based tube rectifier I want.

[scole]

so the 56k comes out and the trim pot and the 27k go in its place? similar to this ceriatone 5f4,
http://www.ceriatone.com/images/layo...one_311207.jpg

[Bruce / Mission Amps]

so the 56k comes out and the trim pot and the 27k go in its place? similar to this ceriatone 5f4,
http://www.ceriatone.com/images/layo...one_311207.jpg

Sure, you can do the bias supply like that.

It will work fine but I wouldn't be surprised to hear it has a little tiny but annoying hum that is difficult to rid.
Do you know anyone who has built it with that exact layout?

[scole]

no i dont know anyone who has built the ceriatone 5f4. what would cause the hum?

[scole]

weber says to replace the 68k with the 4.7k and the 56k with the pot? so what should it be?
https://taweber.powweb.com/store/5f4_schem.jpg

[Bruce / Mission Amps]

weber says to replace the 68k with the 4.7k and the 56k with the pot? so what should it be?
https://taweber.powweb.com/store/5f4_schem.jpg

That would be 6K8 not 68K.
The Weber method is OK, but not the way I drew and do them... and will not hum much but it shows the lack of experience with real amps, real hands on circuits and customers with "twiddle syndrome". If you turn the 50K pot all they way down (max idle current) the power tubes can blow up because all the bias voltage will be shunted to ground.
You should use a pot with a resistor under it to ground so that you can never turn the bias voltage so low that the tubes loose their bias voltage.

[scole]

what schematic could i look at to see what you mean? do you have a sketch i could look at?

[Bruce / Mission Amps]

It's so simple I hesitated to draw it up....

[scole]

thats easy for you to say. looks like a typical blackface/ab763 bias circuit.

[Paul P]

looks like a typical blackface/ab763 bias circuit.

I'd say more like a Marshall. The main difference is that bias voltage is not
taken off the pot's wiper as it is in most Fenders. Apparently bad things
happen if you lose your bias voltage.

Paul P

[scole]

looks easy enough, now where do i find the bias pots? everybody has those little pc mount kind. is there any difference between a vol/tone pot and a bias pot?

[Paul P]

I think something like a ten-turn pot would be more precise and keep its
setting better. I used single-turn Bourns 3852 cermet pots :

http://www.bourns.com/pdfs/385126.pdf

which seem to work ok. There's hardly any current involved.

Paul P

[scole]

so i assume theres no physical and or machanical difference between a basic 1/2w vol/tone control pot and a trim pot. now, what determines the values of the components, reisitors, pot, bypass cap of a bias circuit?