Zener diode to drop B+
Moderators: pompeiisneaks, Colossal
Zener diode to drop B+
I understand that a high wattage zener diode can be used between the HV center tap and ground to drop B+ whatever the breakdown voltage is for a fullwave center tapped HV style rectifier. I was thinking the zener can be used the same way with a bridge rectifier if placed between the bridge ground and chassis? Does anyone have any experience with this or know if this is correct? I need to drop about 30V or more from a marshall 100W bridged B+ supply as I'm using it to power a 6-EL84 liverpool type amp and the marshall PT and bridge are giving me about 385V where I would like about 355V or less before the plates. Thanks for any clues on this.
Re: Zener diode to drop B+
Yeah I use a 25 volt one. It's 25 watt stud.
It's on the center tap of the HT winding.
They can be expensive depending where you buy them, I think I paid $16 locally.
But it does work.
The only thing I can suggest is trying to calculate your tube load on the HT.
With mine, I ended up adding a tube and this PT is kind of soft and it pulled the voltage down quite a bit.
I could probably take the zener out now but it's still there.
It does get pretty toasty too!
But that 100 watt PT probably won't sag much.
Does your PT not have a CT?
When I was researching my problem, I don't recall the info saying I could put it between the bridge rectifier and the ground.
It may be do to loading properties and could be hard on the diodes.
Dunno, but I'm sure somebody here will.
It's on the center tap of the HT winding.
They can be expensive depending where you buy them, I think I paid $16 locally.
But it does work.
The only thing I can suggest is trying to calculate your tube load on the HT.
With mine, I ended up adding a tube and this PT is kind of soft and it pulled the voltage down quite a bit.
I could probably take the zener out now but it's still there.
It does get pretty toasty too!
But that 100 watt PT probably won't sag much.
Does your PT not have a CT?
When I was researching my problem, I don't recall the info saying I could put it between the bridge rectifier and the ground.
It may be do to loading properties and could be hard on the diodes.
Dunno, but I'm sure somebody here will.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Zener diode to drop B+
It has a center tap, but this 100W marshall PT was designed to run a bridge, i.e, it is only 175-0-175 secondary. Running them with a regular full wave recto only gives about 245volts. So, the dilemma is to run the bridge but not run 385VDC on the plates of EL84s which I dare so is too high for many modern or even not so modern EL84s. I would feel much better getting it down to 350V or less. Maybe it has to be reversed and placed right in the B+, I don't know, that's why I need to find out before I try this any further. I thought the bridge ground was analagous to the center tap ground, so that is how I came up with that. Mouser has these for under $10.Structo wrote:Yeah I use a 25 volt one. It's 25 watt stud.
It's on the center tap of the HT winding.
They can be expensive depending where you buy them, I think I paid $16 locally.
But it does work.
The only thing I can suggest is trying to calculate your tube load on the HT.
With mine, I ended up adding a tube and this PT is kind of soft and it pulled the voltage down quite a bit.
I could probably take the zener out now but it's still there.
It does get pretty toasty too!
But that 100 watt PT probably won't sag much.
Does your PT not have a CT?
When I was researching my problem, I don't recall the info saying I could put it between the bridge rectifier and the ground.
It may be do to loading properties and could be hard on the diodes.
Dunno, but I'm sure somebody here will.
Contrary to common misconception, a bridge rectifier gives a much higher output voltage than does a regular full wave recto on the same PT. At least that has always been my experience. A bridge generally gives close to double what the full wave recto gives for output voltage. So I thought the Zener would be the perfect solution.
Yes, they get quite hot, so you use heatsink compound and mount them to the chassis and maybe even add a SS type finned heatsink/cooler. I'm sure it can work, I just need to figure out exactly how it is supposed to be installed with a bridge. The zener doesn't care where the current comes from, it will drop it's breakdown voltage and things will be OK. I got a 25W zener, so I'm pretty sure it will take the current for a 50-60W amp fine.
Last edited by Rick on Thu May 06, 2010 4:34 pm, edited 1 time in total.
Re: Zener diode to drop B+
Yeah, on my tube buffered FX loop I used the Fender Stand Alone Reverb PT.
It is rated at 306v secondary.
I used a bridge on that.
I have a 47uF cap and choke type Pi filter and the voltage at the node on the other side of the choke is 342v.
It is rated at 306v secondary.
I used a bridge on that.
I have a 47uF cap and choke type Pi filter and the voltage at the node on the other side of the choke is 342v.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Zener diode to drop B+
I first just used some big 300 ohm power resistors and that dropped the B+ enough. It made so much heat that the chassis would heat up and finally heat up the PT and that just wasn't the way I was going to do it.
So the zener dropping 30V seemed like a very good idea and I could live with 355V on the EL84s that would probably draw it down further with a moderately warm cathode bias since there are 6 of them. I have each pair biased with a 130 ohm cathode resistor. It's a wild high gain amp, so it is worthy of getting it straightened out and has awesome power.
Maybe I should have just replaced the PT with a more suitable voltage and avoided the whole problem. Weber VST used a Marshall 100W PT on their 6- EL84 6M50 amp design, that is how I got into this whole mess. I used that PT as an example and I had it, so I tried it but ran into this HV problem. They run the plate voltage way too high in my opinion. EL84s are not EL34s, simple as that. I even contacted their tech on this and he seemed to think it was OK, I do not. To be fair, they used the high voltage PT with a regular full wave recto while I'm using the low voltage PT with a bridge, but they end up giving pretty much the same thing, so that isn't really the source of the problem. So much for taking other people's ideas, it often works, sometimes not so much.
So the zener dropping 30V seemed like a very good idea and I could live with 355V on the EL84s that would probably draw it down further with a moderately warm cathode bias since there are 6 of them. I have each pair biased with a 130 ohm cathode resistor. It's a wild high gain amp, so it is worthy of getting it straightened out and has awesome power.
Maybe I should have just replaced the PT with a more suitable voltage and avoided the whole problem. Weber VST used a Marshall 100W PT on their 6- EL84 6M50 amp design, that is how I got into this whole mess. I used that PT as an example and I had it, so I tried it but ran into this HV problem. They run the plate voltage way too high in my opinion. EL84s are not EL34s, simple as that. I even contacted their tech on this and he seemed to think it was OK, I do not. To be fair, they used the high voltage PT with a regular full wave recto while I'm using the low voltage PT with a bridge, but they end up giving pretty much the same thing, so that isn't really the source of the problem. So much for taking other people's ideas, it often works, sometimes not so much.
Re: Zener diode to drop B+
Hi Rick
Does your PT have a 5V winding? If so, then you could use a hybrid ss-Tube rectifier. That way you will get a voltage drop from the tube.
(Pics courtesy of Merlin Blencowe)
Does your PT have a 5V winding? If so, then you could use a hybrid ss-Tube rectifier. That way you will get a voltage drop from the tube.
(Pics courtesy of Merlin Blencowe)
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Re: Zener diode to drop B+
Hi, I already thought of that. In fact I ordered in a 2A 5V power supply to do just that and already wired in an octal socket for the recto tube, but the Zener idea involved much less equipment, wiring, and space since the Marshall PT has no 5V supply and none of them do to my knowledge. Marshall went with SS rectification right away to avoid the probem of insufficient tube rectifiers for hi currents needed, so that is why the lack of 5V fil. supply with them (my guess anyway).
Another shortcoming is that GZ34 only drops around 17V maybe a bit more, but that is not really enough. Other recto tubes with more drop like 5U4GB would drop 44V or so and that would be fine, but then you need a 3A 5V filament supply which is bigger and would cost more, it's just not very good to have PTs with no 5V fil supply. I need to carve that into my desk so I don't order any more without one.
Another shortcoming is that GZ34 only drops around 17V maybe a bit more, but that is not really enough. Other recto tubes with more drop like 5U4GB would drop 44V or so and that would be fine, but then you need a 3A 5V filament supply which is bigger and would cost more, it's just not very good to have PTs with no 5V fil supply. I need to carve that into my desk so I don't order any more without one.