Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tiny_Daddy on October 01, 2011, 10:57:55 am

Title: VOX AC50 woes
Post by: Tiny_Daddy on October 01, 2011, 10:57:55 am

Got a VOX AC50 with a pile of wax under both transformers, so much that they were glued to the board that goes under the chassis. Then there was the copious amount of hot-melt glue on a bunch of replacement capacitors because someone had added extra caps in series for higher voltage even in the bias supply which was wired wrong anyway.
I punched a hole in the chassis and installed a dual 50uF / 500V can for the first two filter sections. This avoids having bare high voltage wires on top of the chassis. All seemed well except the output transformer was shorted. Replaced that and the B+ soared to 520V, exceeding the rating of the dual 50uF.
I am thinking a solution is to connect the negative from the dual 50uF to a 1000uF 50V capacitor and add a resistor divider to cause 35V to be on the 1000uF.

Title: Re: VOX AC50 woes
Post by: phsyconoodler on October 03, 2011, 12:02:54 pm

I have an AC50 head that needs attention too.I really don't like the way the filter caps have those open wires and attach to the chassis with those isolation grommets!
   Mine is working but all the electrolytics need changing.It hums a LOT. But it's a project for another day.

Title: Re: VOX AC50 woes
Post by: kagliostro on October 03, 2011, 01:50:41 pm

Hi Tiny Daddy

What about the efficiency of the brimistor ?

the original project uses 2 x 32uF 500v

what about to put in series one other 50uf+50uf as to obtain a 25uf+25uf 1000v cap ?

Kagliostro

Title: Re: VOX AC50 woes
Post by: Tiny_Daddy on October 03, 2011, 05:53:41 pm

The mod worked and biased both 50uF caps up to a 535V rating without taking up a lot of space. The 1000uF/50V fit easily under the chassis on a terminal strip, and has a low AC impedance, necessary for the 2-section filter to work properly.
One other thing: The Brimistor is there. Also a tube rectifier. My 1965 schematic shows 33+33uF with SS rectifier.
Thanks, 2X33uF on order.

Title: Re: VOX AC50 woes
Post by: kagliostro on October 04, 2011, 01:37:35 pm

Quote

The mod worked and biased both 50uF caps up to a 535V rating without taking up a lot of space.

Please can you post a schematic of your solution ?

I'm not able to figure it

Thanks

Title: Re: VOX AC50 woes
Post by: sluckey on October 04, 2011, 02:30:10 pm

Good idea Tiny_Daddy.

Kagliostro, schematic probably looks similar to this. Voltage divider resistors may be different values but should have about the same ratio.

Title: Re: VOX AC50 woes
Post by: kagliostro on October 04, 2011, 02:39:12 pm

Many Thanks Sluckey

I agree with you, Tiny Daddy had a Very Good idea

If I understand well, with the divider the ground reference for the 50uf cap is at 35v, so they see only 500v instead of 535v, the 35v are entrusted to the 1000uf 50v cap

Kagliostro

Title: Re: VOX AC50 woes
Post by: sluckey on October 04, 2011, 02:59:43 pm

You got the idea but the numbers are wrong. The 50uF has 35 on one end and 520 on the other end. That means there is only a difference of 485v across the 50uF cap. These numbers are specific to this circuit. This is a cheap/good way to extend the voltage rating of a cap and comes in handy when space may be a factor.

Title: Re: VOX AC50 woes
Post by: Tiny_Daddy on October 04, 2011, 05:39:19 pm

I ran about 1mA through the resistors. Move the divider to the other side, to the screen voltage because it's a lot quieter.

Title: Re: VOX AC50 woes
Post by: kagliostro on October 05, 2011, 12:53:50 am

Very interesting discovery for me

Thanks friends for sharing this thing

Kagliostro

Title: Re: VOX AC50 woes
Post by: The_Gaz on October 06, 2011, 02:26:43 pm

Awesome, just when I thought I knew all the tricks!