Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on October 14, 2016, 05:28:19 am

Title: couple of Vox questions
Post by: Leevi on October 14, 2016, 05:28:19 am

I'm wondering couple of thing in the VOX AC100 poweramp.

1. What is the idea of the 100R plate resistors. E.g. VOX AC50 which is very close to AC100
has those resistors on the screen and no plate resistors at all.

2. Then there is a 18K ground resistor on the screen. I think this resistor will run hot?

http://el34world.com/charts/Schematics/files/Vox/Vox_ac100_poweramp.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_ac100_poweramp.pdf)

/Leevi
Title: Re: couple of Vox questions
Post by: sluckey on October 14, 2016, 06:00:14 am
The plate resistors are used to prevent oscillations. You see them in some big parallel/push/pull amps. Ampeg SVT is another example.

Think of the 18K resistor as part of the power supply. There's 450V across it so it is dissipating 11.25 watts. I would expect it to be hot if the power rating is a reasonable 25 watts.
Title: Re: couple of Vox questions
Post by: Leevi on October 14, 2016, 10:01:43 am
There are couple of other differences between AC50 and AC100 power amp,
The feedback across the PI is taken in AC100 through 1M resistor  instead of 470K which
makes the amp even more louder. Also the bias resistors are 330K instead of 220K.


/Leevi


Title: Re: couple of Vox questions
Post by: sluckey on October 14, 2016, 10:49:33 am
If you're gonna list the differences between those two amps you gonna need a big piece of paper.   :icon_biggrin:
Title: Re: couple of Vox questions
Post by: Leevi on October 14, 2016, 11:01:30 am
Quote from: sluckey on October 14, 2016, 10:49:33 am
If you're gonna list the differences between those two amps you gonna need a big piece of paper.   :icon_biggrin:


You are right.


I'm currently building a 100W VOX based amp and should decide if I build an AC50 with two extra power tubes,
pure AC100 or a combination of those. The ultimate goal is to get an amp with very high headroom.
Tips are welcome.

http://el34world.com/charts/Schematics/files/Vox/Vox_ac50_1.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_ac50_1.pdf)

http://el34world.com/charts/Schematics/files/Vox/Vox_ac100.pdf (http://www.drtube.com/schematics/vox/ac100jen.gif)

/Leevi
Title: Re: couple of Vox questions
Post by: Willabe on October 14, 2016, 11:12:26 am
Quote from: Leevi on October 14, 2016, 11:01:30 am
I'm currently building a 100W VOX based amp.....

Quote from: Leevi on October 14, 2016, 05:28:19 am
1. What is the idea of the 100R plate resistors.

Don't leave those plate R's out!

Pompeiisneaks (Phil) built an AC100 not long ago and chased a bug for several months(?) on a thread here that was many pages long. He tried everything to fix it.

Only to find out in the end he had left those R's out. Installed them and the amp was now stable.
Title: Re: couple of Vox questions
Post by: Leevi on October 14, 2016, 11:20:55 am
Quote
Don't leave those plate R's out!


Thanks for the tip. I thought that this is the first thing I'll leave out.


/Leevi
Title: Re: couple of Vox questions
Post by: Willabe on October 14, 2016, 12:01:51 pm
Treat them like you would a grid stopper, with as little of a lead between the output tube plate pin and the 100R.

Mount them right on the output tube socket.