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Re: @Chris - Re: Zener Diodes in a 5F1 Champ to lower B+

11/28/2004 1:46 PM
Wild Bill	Re: @Chris - Re: Zener Diodes in a 5F1 Champ to lower B+ If you only need to drop a little bit resistors can be a good choice. I just wrapped up a PA amp conversion. I had been given for free an old Japanese import PA amp that used 4 EL84/6BQ5's, a pentode/triode PI and an SS preamp stage. The PT was considerately marked with the current rating for two 6.3v filament windings and there was enough there for a pair of 12AX7's on one winding and the 6BQ5 winding could feed another 12AX7 as the PI. I used 2 1/2 12AX7's to make a plexi style with PI tail resistor values swiped from Bruce's VoxyLady. The problem was the B+. It used a voltage doubler on a 130 vac winding and with my line voltage of 122 volts I ended up with 370 vdc for the plates, even with the 4 output tubes drawing about 38 ma each, for a total of 152 ma. I wanted to drop the B+ about 20-25 volts to give the tubes a break. My junkbox turned up a pair of 300 ohm 10 watt sandbox ceramics that would parallel down to 150 ohms. Some quick calculations showed that worst case current draw was much less than 10 watts so I figured I'd try 'em. I lost 22.5 volts at idle and after wailing the amp for a while the resistors were no more than comfortably warm. I also picked up some "synthetic sag" like a tube rectifier. With the lower drive requirement of an El84 the amp is a real crunch monster. Tons of sustain and real easy to get psychedelic feedback. At my local weekly blues jam there's one guy who always plays Hendrix's "Watch Tower" and another who sometimes has a few too many beers and puts down his slide to launch into a Ramones tune. I figure next week I should please both of 'em! So my point is that sometimes if you don't need much the simpler solution is enough. ---Wild Bill

11/28/2004 1:46 PM

Wild Bill

Re: @Chris - Re: Zener Diodes in a 5F1 Champ to lower B+

If you only need to drop a little bit resistors can be a good choice.

I just wrapped up a PA amp conversion. I had been given for free an old Japanese import PA amp that used 4 EL84/6BQ5's, a pentode/triode PI and an SS preamp stage. The PT was considerately marked with the current rating for two 6.3v filament windings and there was enough there for a pair of 12AX7's on one winding and the 6BQ5 winding could feed another 12AX7 as the PI. I used 2 1/2 12AX7's to make a plexi style with PI tail resistor values swiped from Bruce's VoxyLady.

The problem was the B+. It used a voltage doubler on a 130 vac winding and with my line voltage of 122 volts I ended up with 370 vdc for the plates, even with the 4 output tubes drawing about 38 ma each, for a total of 152 ma.

I wanted to drop the B+ about 20-25 volts to give the tubes a break. My junkbox turned up a pair of 300 ohm 10 watt sandbox ceramics that would parallel down to 150 ohms. Some quick calculations showed that worst case current draw was much less than 10 watts so I figured I'd try 'em.

I lost 22.5 volts at idle and after wailing the amp for a while the resistors were no more than comfortably warm. I also picked up some "synthetic sag" like a tube rectifier.

With the lower drive requirement of an El84 the amp is a real crunch monster. Tons of sustain and real easy to get psychedelic feedback. At my local weekly blues jam there's one guy who always plays Hendrix's "Watch Tower" and another who sometimes has a few too many beers and puts down his slide to launch into a Ramones tune. I figure next week I should please both of 'em!

So my point is that sometimes if you don't need much the simpler solution is enough.

---Wild Bill

2/7/2005 2:35 PM
john	where to put dropping R bill, where would you put this resistor (150 ohm, 20W)? from PT center tap to ground? thanks, john

12/3/2004 3:20 AM
Peter	DO-5 Zener - Re: Zener Diodes in a 5F1 Champ to lower B+ >In other words, you bolt it to the chassis & connect >the B+ center tap to the solder lug on the top of the >DO-5 & you're done- nice & neat & it'll hold up. Do these DO-5 Zeners come with the necessary nut to screw them to the chassis - I couldn't find this info an Mouser's website? I am asking because in Germany I cannot buy inch based nuts. Peter

12/3/2004 3:20 AM

Peter

DO-5 Zener - Re: Zener Diodes in a 5F1 Champ to lower B+

>In other words, you bolt it to the chassis & connect
>the B+ center tap to the solder lug on the top of the
>DO-5 & you're done- nice & neat & it'll hold up.

Do these DO-5 Zeners come with the necessary nut to screw them to the chassis - I couldn't find this info an Mouser's website? I am asking because in Germany I cannot buy inch based nuts.

Peter

12/3/2004 7:17 AM
Ed	Peter- Yes, they come with all the necessary hardware. It even icludes a plastic insulating nut & a mica washer for isolating the zener from the chassis. Uou don't use those in this application. But has the nut & a lock washer, which you do use. Mouser's web site is pretty lame, getting better, but still a nightmare to locate parts a lot of times. Good luck. -Ed

12/4/2004 5:03 AM
Peter	Where to install the DO-5 Zener for low hum? Hi, if you look at the 5f1 layout diagram, where would you install the DO-5 chassis mount Zener for low hum? The only place, where there is enough room, is left of the 5Y3. Is this o.k.? Or will the heat from the Zener damage the 5Y3 tube in the long run? About the other grounds: As this is a Mojo kit, I don't have a brass plate. Most of the grounds are connected to the chassis using a terminal lug on the bottom right power transformer screw. Other grounds are soldered to the input jack and volume pot. So is putting the DO-5 Zener left of the 5Y3 o.k.? Peter

12/4/2004 5:03 AM

Peter

Where to install the DO-5 Zener for low hum?

Hi,

if you look at the 5f1 layout diagram, where would you install the DO-5 chassis mount Zener for low hum?

The only place, where there is enough room, is left of
the 5Y3. Is this o.k.? Or will the heat from the Zener damage the 5Y3 tube in the long run?

About the other grounds:
As this is a Mojo kit, I don't have a brass plate.
Most of the grounds are connected to the chassis using a terminal lug on the bottom right power transformer screw. Other grounds are soldered to the input jack and volume pot.

So is putting the DO-5 Zener left of the 5Y3 o.k.?

Peter

12/4/2004 1:45 PM
Ed	Peter- Wherever you have enough room. The closer to the tranny & wherever the B+ center-tap is grounded now, the better. The body of the DO-5 is about 1" wide, so you should give yourself an area 1-1/2" wide or bigger, if possible. The heat shouldn't be a problem, and I find that the most heat is generated on the nut side (outside) of the chassis, so you won't have a lot of heat buildup inside the chassis & as long as there's ventilation inside the cab, you should be fine. And again, like Bruce said, you're not dropping excessive voltage with a 15 watt amp, so it's not going to generate a ton of heat. -Ed

12/4/2004 1:45 PM

Peter-
Wherever you have enough room.
The closer to the tranny & wherever the B+ center-tap is grounded now, the better.
The body of the DO-5 is about 1" wide, so you should give yourself an area 1-1/2" wide or bigger, if possible.
The heat shouldn't be a problem, and I find that the most heat is generated on the nut side (outside) of the chassis, so you won't have a lot of heat buildup inside the chassis & as long as there's ventilation inside the cab, you should be fine.
And again, like Bruce said, you're not dropping excessive voltage with a 15 watt amp, so it's not going to generate a ton of heat.
-Ed

12/6/2004 11:27 AM
Howard	I installed mine in the left side, facing the back of the amp. This is the smooth (no holes yet) vertical surface on the left side of the chassis adjacent to the PT. In my self built cabinet, there's plenty of clearance there for the nut and it's about as far away from the signal carying part of the circuit as you can get. This leaves me more room on the bottom surface (where the tubes are mounted) for magic mini switches to do things like changing the cathode bypass and coupling cap values, NFB etc.

12/6/2004 11:27 AM

Howard

I installed mine in the left side, facing the back of the amp. This is the smooth (no holes yet) vertical surface on the left side of the chassis adjacent to the PT. In my self built cabinet, there's plenty of clearance there for the nut and it's about as far away from the signal carying part of the circuit as you can get. This leaves me more room on the bottom surface (where the tubes are mounted) for magic mini switches to do things like changing the cathode bypass and coupling cap values, NFB etc.

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