bridge rectifier bias question

[andrew]

Can the bias circuit on an Express be modified for use with a full wave bridge rectifier? Thanks!

[Phil_S]

For the bias supply, you tap the high voltage before it enters the rectifier. It should make no difference what kind of rectifier you use.

[andrew]

Thanks for the reply. I connected the bias circuit to one of the AC in junctions of the diodes and the tubes quickly redplated. The bias reading was over 100 and turning the bias pot barely changed the reading. I disconnected the bias supply and hooked it up as a cathode bias and there was no redplating.Any help please. Thanks!

[stoo]

This may be obvious but the bias supply is a negative voltage. -100vdc on the grid should have turned the tubes off...totally. +100 vdc !!..well.... check those tubes.
Stew

[rhinson]

Can the bias circuit on an Express be modified for use with a full wave bridge rectifier? Thanks!

to take it off the hv sec in a bridge config you have to have a blocking cap.
check out this schematic of a sundown amp to see how it's done. some ampegs did this too.

http://www.schematicheaven.com/newamps/ ... d1000h.pdf

rh

[skyboltone]

Can the bias circuit on an Express be modified for use with a full wave bridge rectifier? Thanks!

to take it off the hv sec in a bridge config you have to have a blocking cap.
check out this schematic of a sundown amp to see how it's done. some ampegs did this too.

http://www.schematicheaven.com/newamps/ ... d1000h.pdf

rh

This is correct. I had a thread awhile ago about this problem. You cannot get a positive and a negative voltage out of the same winding without a center tap unless you use the ac rated blocking cap. Otherwise the diodes in the bridge provide a back track and you get a direct short on HT. Normally though, if you have the diodes biased correctly you won't red plate you just take out your bias caps and diode, or the HT winding. I had a scheme somewhere but I can't find it. I'll draw something up maybe if there's still confusion later.

[skyboltone]

Here's the scheme isolated:
[IMG:768:751]http://i34.tinypic.com/291ksvl.jpg[/img]

The .047 cap must be rated for at least 1.41, prefferably more than the RMS voltage of the high voltage winding. Normal 715 or 6PS style film caps won't do it. But Mouser has a variety of AC rated film caps called "snubber caps" that will do. I had a couple of Wima "FK" series in the drawer that worked fine. Also if you do the math on the filter network of the .047uf and 56K resistor ahead of the filter cap for the bias circuit you'll notice the odd coincedence of 60hz knee.

[andrew]

Thanks for the replies. As it relates to the Express bias circuit do I use the .047 cap before the 220k resistor or do I forget the Express circuit and use the Sundown bias circuit?

[andrew]

Thanks for the reply. I connected the bias circuit to one of the AC in junctions of the diodes and the tubes quickly redplated. The bias reading was over 100 and turning the bias pot barely changed the reading. I disconnected the bias supply and hooked it up as a cathode bias and there was no redplating.Any help please. Thanks!

[skyboltone]

Thanks for the replies. As it relates to the Express bias circuit do I use the .047 cap before the 220k resistor or do I forget the Express circuit and use the Sundown bias circuit?

You have to abandon the express circuit. Use the sundown scheme but you'll have to monkey with the values of the dropping resistors. Wait a minute......thinking. Build it just like the sundown scheme and then measure the voltage at the center of the two 220K resistors, without any power tubes in. Then get back to us with the voltage range you have there. By the way, it has to be a negative voltage so watch the polarity on the caps and diode. Shoot for a range of say 25 to 35 volts, unless you intend to run 6V6s alot, then shoot for 20 to 35 volts negative with reference to ground. Oh, forget all the monkey business between the two grid resistors and the cathode shown on the Sundwon scheme. From the junction of those two resistors it's identical to the Express.

Dan

[rhinson]

Here's the scheme isolated:
[IMG:768:751]http://i34.tinypic.com/291ksvl.jpg[/img]

The .047 cap must be rated for at least 1.41, prefferably more than the RMS voltage of the high voltage winding. Normal 715 or 6PS style film caps won't do it. But Mouser has a variety of AC rated film caps called "snubber caps" that will do. I had a couple of Wima "FK" series in the drawer that worked fine. Also if you do the math on the filter network of the .047uf and 56K resistor ahead of the filter cap for the bias circuit you'll notice the odd coincedence of 60hz knee.

i always just use a 1000v poly for this when i do it, but the ac winding here to do an express will be not all that high---say 300vac or less for the winding.
so a 630v poly would do fine. in the old sundown's i've worked on the blocking cap was just a standard cheap 630v cap and the winding was a 400v winding! ol' denny kager was cutting it a little close on those. rh

[andrew]

Thanks for the responses but I wired up the bias tap from a Fender schematic and after many blown fuses it works. The voltages are too high though: with EL34s they are 467,455,362,359,358. I have a 25watt Zener that will drop 30vts but it looks like I need to drop it a total of 60vts! If I had the money I'd get the right PT! But then again $20 for a bigger Zener or about $80 for the correct PT....

[rhinson]

which fender schematic? rh

[andrew]

Schematic Heaven - Blues DeVille http://www.schematicheaven.com/fenderam ... eville.pdf

[rhinson]

the transformer in this schematic has it's own bias winding apart from the hv sec winding-----you can either make the old standard half wave circuit, a bridge circuit, or a voltage doubler even and get the neg. voltage supply you need. we were under the impression the trannie you had had no bias winding and that you were taking it off the hv sec winding. if that's the case then you will need a blocking cap coming off the winding if you your going to roll your own from the hv sec winding. rh