Rectifier ideas....voltage dropping, etc...

[David Root]

A resistor does cause "sag", just like a rectifier tube, when inserted in a SS generated DC supply. When you crank up the amp, it draws much more current (in an AB1/AB2 design), which causes the voltage drop across the tube or resistor to increase, lowering output power, and, depending on the VA capability of the PT, stressing it to some extent.

The zener diode drops a fixed voltage, it resists voltage sag under load, lessening the effect. You could put in in-line after the first filter cap, like the resistor, but it will dissipate more power. Conversely you could put the resistor between the CT and ground too, it will work but not like the zener.

In fixed bias AB amps, which usually derive the bias from the plate winding of the PT, putting the zener in between the PT CT and ground shifts the bias more negative, possibly pushing the tubes into cutoff if the zener drop is very large. So a bias adjustment is necessary to restore your desired setting.

This use of the zener, and the caveat about fixed bias AB, is covered in Kevin O'Connor's TUT2, pages 2-45 and-46, which is where I first found it.

[IaXy]

Well, try it and just put the resistor in parallel with the standby switch. That will give you your B+ you want. For the test you won't have the standby switch.

David, I understand what your saying but the sag the rect tube will induce, what 50 votls? This in series with the 20 volt drop on what he wants to do, you are saying that it will sag the 20 volt more than what it would normally? 70volts? hmmm. I don't think that resistor would have that effect on the circuit. I don't think the resistor sags as that of a rect. tube. I could be wrong.

Bob

[dehughes]

Basically, I'm looking for the simplest way to drop about 20v out of my AC30-ish amp's circuit, to get the amp running closer to what it would have "back in the day"...

[skyboltone]

Basically, I'm looking for the simplest way to drop about 20v out of my AC30-ish amp's circuit, to get the amp running closer to what it would have "back in the day"...

Ok, here goes. Please Please just go to http://www.duncanamps.com

Then click on the software section. Then download the PSU designer II software. ITS FREE!!! Then model your power supply using the outstanding tools available. Figure about 1 ma per pre amp stage and a couple or three ma of for screen idle current. Tubes vary on idle plate current but 80 to 110 will be close. LOOK IT UP in a tube manual or tell me the tube you are using and I'll look it up. The PI about 1-2 ma in idle. Then go to town! It's easy as can be. Try it you'll like it.

Twenty volts difference in plate voltage for output tubes really isn't a big deal. What you need to get right is plate voltage and cathode voltage on the pre tubes. Just dropping the whole string 20 volts in my humble opinion will not make you happy. They tell me that substituting a resistor for the choke will make tonal differences. There ain't no way to predict what that will be without trial and error. Thats where the switch comes in. Close the switch and the resistor goes away, open it and it's back in the circuit.

[David Root]

I was just using 50V as an example, nothing more. I have done it myself with a 50V Zener, back when I didn't know enough to match my PT more closely to my application!

The whole point of using a Zener instead of a resistor is that the Zener clamps the voltage, i.e. it is acting as a one-way device, which the resistor cannot do. This reduces the corresponding current swing amplitudes. Zeners figure in a lot of voltage regulator designs because of that.

[MarkB]

Well, try it and just put the resistor in parallel with the standby switch. That will give you your B+ you want. For the test you won't have the standby switch.

David, I understand what your saying but the sag the rect tube will induce, what 50 votls? This in series with the 20 volt drop on what he wants to do, you are saying that it will sag the 20 volt more than what it would normally? 70volts? hmmm. I don't think that resistor would have that effect on the circuit. I don't think the resistor sags as that of a rect. tube. I could be wrong.

Bob

A rectifier sags because of its resistance. There's no difference between the resistance of a rectifier tube and that of a resistor. Ohm's law applies in either case.

[dehughes]

Basically, I'm looking for the simplest way to drop about 20v out of my AC30-ish amp's circuit, to get the amp running closer to what it would have "back in the day"...

Ok, here goes. Please Please just go to http://www.duncanamps.com

Then click on the software section. Then download the PSU designer II software. ITS FREE!!! Then model your power supply using the outstanding tools available. Figure about 1 ma per pre amp stage and a couple or three ma of for screen idle current. Tubes vary on idle plate current but 80 to 110 will be close. LOOK IT UP in a tube manual or tell me the tube you are using and I'll look it up. The PI about 1-2 ma in idle. Then go to town! It's easy as can be. Try it you'll like it.

Twenty volts difference in plate voltage for output tubes really isn't a big deal. What you need to get right is plate voltage and cathode voltage on the pre tubes. Just dropping the whole string 20 volts in my humble opinion will not make you happy. They tell me that substituting a resistor for the choke will make tonal differences. There ain't no way to predict what that will be without trial and error. Thats where the switch comes in. Close the switch and the resistor goes away, open it and it's back in the circuit.

Yeah, I downloaded that software a couple of weeks ago, but I admit that I'm a bit uneducated in regard to all of this. Granted, I just gave it a quick look-over, and then inserted various values for the resistor....it seems like I'll need something along the lines of 200 ohms or so to get started. I get the general idea, so I'll just have to wire things up and then try out the various resistor values that I have here at home. I'll see where the rest of the voltages land, and see how the amp reacts to it all, and go from there. Still though, I'm going to wait until the speaker cabinet arrives, as then I'll have the proper context for any further tinkering.

[dehughes]

Well, I FINALLY got around to wiring up the switch and installing a 10w, 130 ohm resistor in line between the recto pin and the filter caps (actually, it is connected to the recto pin and a fuse...but you get the idea...). It works FANTASTICALLY! I just happened to have a 130 ohm 10w resistor lying about, and figured, what the heck, try it! It took the voltage on the first cap from around 347 down to about 306, and the EF86 plate is now seeing about 97 or 98 volts as opposed to 111-ish. VAST improvement on gain tonality, clarity and feel across the board. VERY COOL. So, with the switch "on" the amp is seeing no resistor, and has voltages in line with current wall voltages given this circuit. With the switch off, the resistor is in effect and the voltages are much closer to what they would have been back when an AC30 was plugged into a real 110v wall socket. Super cool...

Thanks guys...! MANY THANKS!

One last question: I noticed that the resistor gets REALLY REALLY HOT...is this something to be concerned with? I mean, I know things get really really hot inside an amp, and tubes get really really hot too, but is there anything worth doing to keep all things well? I've wired up the resistor so it hangs between the fuse and the recto pin, with plenty of clearance all around it, and it is located towards the end of the chassis where there are vent holes for heat dissipation. I'll post a picture...

[jaysg]

One last question: I noticed that the resistor gets REALLY REALLY HOT...is this something to be concerned with?

Yes. Which way is up? If the heat rises onto that socket, that will cook it extra over time. If the chassis hangs down, it's less of an issue, though if it rises into wood, that will bake over time. A steel emc sheild will help dissipate a hot spot.

[dehughes]

One last question: I noticed that the resistor gets REALLY REALLY HOT...is this something to be concerned with?

Yes. Which way is up? If the heat rises onto that socket, that will cook it extra over time. If the chassis hangs down, it's less of an issue, though if it rises into wood, that will bake over time. A steel emc sheild will help dissipate a hot spot.

Yeah, the socket will be above the resistor, partially, at least. Should I move it somehow? I'll have to find another resistor, if so...

The chassis is aluminum, which I've noticed really dissipates heat, so perhaps some of the heat will be carried away by the chassis?

[jaysg]

Yeah, the socket will be above the resistor, partially, at least. Should I move it somehow?

If you had a small terminal strip, there might be room to mount the socket end in between the socket and the nut with the big washer. A wider strip with a couple of feet would allow you to move it off the fuse holder and not run wires directly underneath it.

[skyboltone]

How many ways to skin the cat?

Here's one. Put a little thermal grease under it and bolt it directly to the chassis. Put it where you want it and use ordinary hookup wire to accomplish the job.

http://www.vishay.com/docs/30201/rhnh.pdf

Available from mouser for a small fee.

Good luck
Dan

[dehughes]

Interesting ideas...very cool.

So the consensus is that I should move the resistor, correct?

[dehughes]

How many ways to skin the cat?

Here's one. Put a little thermal grease under it and bolt it directly to the chassis. Put it where you want it and use ordinary hookup wire to accomplish the job.

http://www.vishay.com/docs/30201/rhnh.pdf

Available from mouser for a small fee.

Good luck
Dan

Alrighty. I purchased one of these resistors and am ready to mount it in. My brother had some silver thermal grease that he was using for CPU heat sink installs, so I'll use that and see how it works out.

THANKS!

[dehughes]

Well, the resistor is bolted down and thermal greased and soldered in....and works really well. THANKS GUYS! YOU ALL ARE A GREAT HELP!!!!