Understanding rectifier tubes

BloozeGit
January 26th, 2010, 08:22 AM
Posted this in the Stromberg-Carlson thread with no response, maybe it got lost in there.

I'm trying to expand my education on rectifier circuits here, hope someone can help...the Stromberg-Carlson schematic (http://www.theused.com/manuals/stromberg/stromberg_au-29.pdf) gives the 5Y3 plate voltages as 340VAC each, does this imply a 340-0-340 PT secondary? If so, for a specified output of 335VDC wouldn't this be overkill?

I'm basing my deduction on the 5Y3 chart showing DC current output vs RMS plate voltage. A single 6L6GC at say 300V plate voltage draws 55ma, the 6SJ7s would draw say 3ma each to give a total of 61ma total. From the chart that should give close to 400VDC to the filter input compared to the spec of 335VDC.

What am I missing out here?

muchxs
January 26th, 2010, 09:38 AM
Posted this in the Stromberg-Carlson thread with no response, maybe it got lost in there.

I'm working on two Stromberg-Carlson projects. One is a hi-fi amp that's getting loaded into a tweed Super chassis. The other is billed as a "hi-fi" amp by S-C, it has the correct tube lineup for a tweed Harvard or I'm going to bust it up and build two amps with the parts. The OT is roughly correct for another mutant tweed Super.

Neither one is like yours.

I'm trying to expand my education on rectifier circuits here, hope someone can help...the Stromberg-Carlson schematic gives the 5Y3 plate voltages as 340VAC each, does this imply a 340-0-340 PT secondary? If so, for a specified output of 335VDC wouldn't this be overkill?

Where do you get that? I'm seeing V-4 indicated at 350 volts. Plate voltage isn't indicated on the schem unless I'm not seein' it.

For a quick rough estimate of your supply voltage simply multiply the measured voltage on either side of the CT (5Y3 pins 4 or 6 referenced back to ground) and multiply it by 1.2. That's a rough estimator specific to your power supply with a 5Y3. That means if the PT has a 300-0-300 secondary your raw voltage at pin 8 should be around 360 volts. Everything changes under load, it's a just quick way to figure out if your power supply is hitting the numbers. If it's way off look for a fault or a short before things start to smoke.

I'm basing my deduction on the 5Y3 chart showing DC current output vs RMS plate voltage. A single 6L6GC at say 300V plate voltage draws 55ma, the 6SJ7s would draw say 3ma each to give a total of 61ma total. From the chart that should give close to 400VDC to the filter input compared to the spec of 335VDC.

What am I missing out here?

Grid 2 current?

You also might want to consider that if you drop your supply voltage your plate current increases. Your tube has to make up the power somewhere. It's drawing more like 80 ma down at 250v.

The schematic shows 350v at pin 8 on the rectifier. Do you measure 350v? What do you measure at pins 4 and 6? Measurements are reality. Beyond that the question is "Does it work?" If you answer "Yes." the next quesiton is "What's the problem?"

BloozeGit
January 26th, 2010, 11:04 AM
Actually I'm still in the planning stage so measurements aren't an option...still looking to get the right PT. The voltages are given in a table on page 2...misprint perhaps?

muchxs
January 26th, 2010, 12:19 PM
Oh.

You probably know more about transformer availability in Singapore than I do. Lots more! I thought you had the actual amp and you wanted to make something out of it. You may have to balance what you want against what's available. 6SJ7s? Look at 5C1 Champ.

Here's my pick: 300-0-300 @ 100ma, 5V 3A, 6.3V 3A.

Low enough for 6V6s, high enough for 6L6s. It's better to have more a little more current available, keeps your options open. Same with the 5V 3A filament. Filament current is cheap and three amps accomodates a variety of rectifiers. That's also something to think about with your 6.3 volt filament. KT66s draw a little more than 6L6s and EL34s draw twice as much. If you've got the current available you can swap 6V6, 6L6, KT66, EL34 and a variety of other tubes. Keep your options open!

Too much voltage is a common pitfall. Transformer winders think they're doing you a favor by giving you more. You'll see 350-0-350, 375-0-375, 400-0-400 or more. Those just complicate your life. The high voltage is way too high, your power supply design gets to be a hassle. Those big ol' boat anchors were designed to be used in a regulated power supply with a glow discharge tube to bleed off the excess voltage. Regulated power supplies are seldom seen in tube guitar amps. Gibson used them in a couple amps. The closest thing to a regulated power supply was the original 5F6 Bassman (not the 5F6A) with the 83 tube. An 83 is a mercury vapor rectifier that performs like a regulated power supply, it doesn't "sag". Don't worry about any of that. Build it! :lol:

limbe
January 26th, 2010, 12:50 PM
Actually,the high voltages goes back to the old Fender amplifiers.They nearly always had voltages way over the specs for the tubes to get a couple of extra watts from the amplifier.That philosophy is still with us today.The big difference is that back then the tubes could take it.

Natstrat79
January 26th, 2010, 01:40 PM
Great answers but I still don't know that we have answered this question. This is one that I've always wondered about too. I think it boils down to this...

If I am custom building an amp and want a specific plate voltage is there a way to get a rough prediction in order to know what transformer to purchase.

So that I know that a 340v - 0 - 340v with a 5y3 rectifier would give me roughly ??????

or if I used 375v - 0 - 375v with a 5AR4 that would give me????

ThermionicScott
January 26th, 2010, 02:17 PM
Great answers but I still don't know that we have answered this question. This is one that I've always wondered about too. I think it boils down to this...

If I am custom building an amp and want a specific plate voltage is there a way to get a rough prediction in order to know what transformer to purchase.

So that I know that a 340v - 0 - 340v with a 5y3 rectifier would give me roughly ??????

or if I used 375v - 0 - 375v with a 5AR4 that would give me????

The answer is, you have no way of knowing without taking current draw into account. If you're truly *designing* the amp, you'll have a rough estimate of how much voltage and current you'll be swinging to achieve the target power output.

Build a Firefly with any tube rectifier, and the current draw will be so low it'll act like a solid-state job. Plug a 5Y3 into a high-power Tweed Twin and the voltages will sag impressively before the tube dies. :lol:

- Scott

big-daddy-59
January 26th, 2010, 02:18 PM
quick ,simple, rough, and dirty method

desired plate voltage DIVIDED by 1.2 with a 5y3 rectifier= supply voltage
or desired voltage divided by 1.3 for a 5ar4 or 1.4 if using a solid state rectifier.

muchxs
January 27th, 2010, 12:09 PM
quick ,simple, rough, and dirty method

desired plate voltage DIVIDED by 1.2 with a 5y3 rectifier= supply voltage
or desired voltage divided by 1.3 for a 5ar4 or 1.4 if using a solid state rectifier.

+1, those are your estimators.

If you're not an engineer you can always be a reverse engineer. Find an application similar to yours and use it. If you can't find a schematic that uses a 5Y3 in a high powered amp there muse be a reason, no?

limbe
January 28th, 2010, 12:21 PM
Thatīs very good advice muchxs and if we use your example and then look up the 5Y3 in a tube manual we will see that the maximum current a 5Y3 i can deliver is 125 mA which isnīt enough for a big amplifier by a long shot.

Natstrat79
January 29th, 2010, 02:43 PM
In amps that use multiple rectifiers does that effectively double the maximum current provided by the rectifiers. For example if we know that the maximum current provided by a 5y3gt is 125ma then if I use two of them do I get 250ma.

Then my question is in amps like the lower power tweed twin that uses two 5u4gb we can induce more "sag" by removing one of the rectifiers and reducing the maximum amount of current that can be provided, but does removing one rectifier change the B+ and thus the plate voltages?

ThermionicScott
January 29th, 2010, 04:05 PM
In amps that use multiple rectifiers does that effectively double the maximum current provided by the rectifiers. For example if we know that the maximum current provided by a 5y3gt is 125ma then if I use two of them do I get 250ma.

Then my question is in amps like the lower power tweed twin that uses two 5u4gb we can induce more "sag" by removing one of the rectifiers and reducing the maximum amount of current that can be provided, but does removing one rectifier change the B+ and thus the plate voltages?

Tube rectifiers have a high internal impedance that acts sort of like a resistor. A resistor will drop voltage based on how much current is being drawn -- more current, more drop. When you put two rectifier tubes in parallel, it's like paralleling two resistors, so the impedance (and voltage drop) is then cut in half for a particular current draw.

Lots of higher-powered amplifiers back in the day used several rectifier tubes in parallel, not only to deliver enough current, but to help preserve B+ voltage.

- Scott

limbe
January 29th, 2010, 07:53 PM
Yes,Natstrat.Two 5Y3gt can handle 250mA and you get half the sag that you would get with one 5Y3gt.As Scott S points out old amplifiers,for example old Bassmans like the 5E6A uses two 5U4GAs not because that one 5U4 couldnīt deliver the current needed(5U4 can deliver 225 mA) but to halve the sag/voltage drop in the power supply

celeste
January 31st, 2010, 04:11 PM
In amps that use multiple rectifiers does that effectively double the maximum current provided by the rectifiers. For example if we know that the maximum current provided by a 5y3gt is 125ma then if I use two of them do I get 250ma.

Then my question is in amps like the lower power tweed twin that uses two 5u4gb we can induce more "sag" by removing one of the rectifiers and reducing the maximum amount of current that can be provided, but does removing one rectifier change the B+ and thus the plate voltages?

It is not good practice to use parallel active devices at rated current, unless you use some method to ensure power sharing, so it would not be safe to get 250ma from a pair of 5Y3's unless you used some series resistance, after them , but before you combine their outputs, to ensure one does not get over used while the other just loafs along.

Do not use a part at m ore then rated maximum unless you are willing to live with the increase of failures, so if pulling one 5Y3 means the other is now passing then 125ma you have to except that it will not have a normal life span.

limbe
February 3rd, 2010, 11:37 AM
No,it might not be good practice to use two rectifier tubes in parallel as Fender has done in several amplifiers.Still,the answer to Natstrats question is,yes it will double the max current compared to what one rectifier can deliver.In the example I mentioned you can pull one 5U4 in the 5E6A Bassman since the amp only needs something like 80-90 mA which one 5U4 easily can handle.What I am trying to say is:You can pull one of the rectifiers in an old amp if you are sure that one tube alone can handle the current without problems.