Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on May 29, 2014, 12:09:36 pm

Title: Sag Resistor
Post by: dude on May 29, 2014, 12:09:36 pm

I've been searching info here and elsewhere on using a sag R with a SS rectifier.

I like the slight sag a 5V4 gives, 5Y3 too much for me. The amp I'm converting is a Crate VC 20 to a 18 watt lite w/6v6, two 12ax7's and two 6V6's. The PT is stock VC club, B+ of about 320v.

Doing some math seem I need a 10 watt sag resistor, I only have a "5 watt 200 ohm R" which is what the schematic calls for in an 18 watt lite schematic but that amp is using el84's.

Will that 5 watt sag resistor be on the verge of blowing with 6V6's?  I think it might...?

I have two 10 watt wire wound R's, 300 ohms each, in parallel that would be 150 ohms @ 20 watts but space is very tight and I'd have to crowd them in some where.  :dontknow:

Some advice I searched says the 5 watts is OK, other say 20 watts at least. Maybe I did the math wrong.

I'll post progressive pictures of the conversion when I'm done.

Thanks as always,
al

Title: Re: Sag Resistor
Post by: sluckey on May 29, 2014, 12:34:22 pm

Quote

I only have a "5 watt 200 ohm R"

Half the 5W rating to 2.5W for a safety margin then do some math. A 200Ω resistor would need 112mA flowing thru it to dissipate 2.5W. (It would need 158mA flowing thru it to reach 5W dissipation.) I doubt two cat biased 6V6s will even come close to that. You should be fine.

However, just connect the resistor in the B+ line. Then measure the voltage ACROSS the resistor. Use this formula to determine the actual power dissipated by the resistor...

   P = E²/R

PS... I just put a 250Ω 5W resistor in an AC-15 amp. Voltage dropped across the resistor was 25V. That calculates to 2.5W dissipated by the resistor. I doubled the wattage for safety margin. The resistor gets hot (It's supposed to!) so give it some breathing room.

I bet you would find this thread interesting...

     http://el34world.com/Forum/index.php?topic=17125.0 (http://el34world.com/Forum/index.php?topic=17125.0)

Title: Re: Sag Resistor
Post by: dude on May 29, 2014, 02:41:08 pm

Yeah, I read that thread and that switch is pretty cool.

I could switch the sag in or out, for a brown tone when the sag is in. I just ordered from Doug so next order I'll get some.

Since I do a lot of conversions that switch is great for stealing the standby/hole for a VVR. 

This is off my own thread but it's mine anyway, ha:

Below is a layout for a SS cathode follower in an 18 watt lite, simular to what I'm building, has anyone every used one of this Mosfet IRF-820 for a solid state cathode follower to the tone stack? If so, any thoughts?

thanks as always,
al

Title: Re: Sag Resistor
Post by: Jack1962 on June 05, 2014, 03:47:31 pm

I would , as Sluckey suggested give it plenty of room , and if possible attach it to a heatsink