Using Resistors to Change B+

[The New Steve H]

I'm looking around for a new PT for my weird project amp. I want at least 140 mA available for idle current. The design calls for 550V on the plate windings (275-0-275).

I know about a couple of PTs that are very expensive and which fit the bill pretty well, but I located a cheaper one. Problem: it comes in at 600V.

http://www.classictone.net/40-18086.pdf

I assume I can just mess with the resistors on the power supply to get me down where I want to be. Correct? If I use a 5U4, I will be able to get ample current, and according to the PT specs, the voltage will be lower than a 5AR4 even before I change resistors.

Another question: the amps designs I've seen use the highest available voltages for B+, and then the preamp and so on are powered by lower voltages farther down the line. Is there any reason it has to work that way? If I need a higher voltage somewhere in the preamp, is there any reason I can't draw the power from the point closest to the PT and then draw the B+ from a lower voltage a cap or two away?

[John_P_WI]

Steve, no time to answer in detail, but the xformer you listed will not have enough current capacity. You neglected any dissipation in heat and other loads. At a minimum, I would look at something like Hammond's 370 HX.

You do not want to use resistors to lower the B+, they will induce sag and change the dropped voltage to heat.

Also taking the power amp B+ father down the power string will not work due to the currents required by the power amp. (think of how large resistors and choke - if using a choke - would be needed plus all of the added sag).

Finally, you want the preamp voltages to be taken the farthest away from the rectifier as they will be the cleanest.

It would be wise for you to draw the string out on paper with the node loads and understand what is happening at each point. Ohms law is your friend.

[The New Steve H]

I don't care about the heat, but the sag is a concern. I want the amp to sag, but not to the point where it causes a problem.

I can't believe how much current these little tubes suck. The amp will max out at 14 watts, and I'm looking at 50-watt PTs. I've actually considered using different tubes, but the closest thing I've found is the EL844, and it sucks tons of current, too.

It looks like the PT you recommend is the best choice I've seen.

[John_P_WI]

Steve,

I have not taken time to study your schematic, let's assume you have a plate voltage after the tube rectifier of 350 v and and a cathode voltage of 15 v.

350 v - 15 v = 335 v. And given idle current from your post above of 0.140 ma =

P = VI or 335 v x 0.140 ma or 47 watts!

Your 140 ma may be off???

[The New Steve H]

I believe what I really need is 250-0-250. That probably means a Hammond 370KX, which is harder to find.

Shouldn't a Trainwreck transformer work?

[The New Steve H]

I was told these tubes (6BM8) have a plate current of 35 mA each, so 140 mA total. The PT I have on hand is rated for 100 mA.

Datasheet: http://www.tubezone.net/pdf/6bm8ecl82.pdf

Plate voltage should be 275; cathode is at about +23.

[Phil_S]

Your sizing of the tx is off and no amount of work around will compensate for that error. Remember, lowering the B+ will proportionately raise the current draw, too.

Using resistors to drop B+ only works well for small changes. Otherwise, you'll need big, high wattage, sandblock resistors to stand up to the heat generated. All that wasted heat will sap your already under-spec'd tx of mA you need for the tubes.

If you don't want to spring for an expensive replacement tx, try haunting eBay for a while. Decent transformers are sold every day for under $25 shipped.

Or figure out how to use one of these, which will give you your 250-0-250 at 400mA, solving your supply problems. http://www.antekinc.com/details.php?p=671

[The New Steve H]

What's wrong with a Trainwreck transformer? The voltage is fine.

[John_P_WI]

I am not familiar with this tube, and usually don't use smaller tubes.

First off, what is needed is what class do you plan to operate - from the current figures you mentioned it looks like you would like to run class A? Assuming push pull? What is the R a - a of the output transformer that you are using? I did not see any push pull data on the quick glance at the tube sheets.

Personally, I think you are asking for way too much current out of these little guys. Even at the reduced voltage in your post above and 140 ma, that still calculates at nearly 40 watts - I think I saw in the data sheet maximum dissipation was 7 watts. 7 x 4 tubes = 28 watts maximum. I would be very suspect of the 140 ma value given above.

I would guess a 250 - 0 - 250 @ 200 ma would get you close for the 4 tubes, that being said if you currently have a 100 ma transformer, pull one set of output tubes and check your bias current and voltages. That should be a close enough of an empirical test.

[tubeswell]

There's no reason why you can't put a separate pre-amp RC supply string off one of the 'earlier' filter cap nodes. Several amps have parallel power supply strings this way. The thing you have to be careful of is having adequate filtering to overcome any tendency of the gain stages in your string to produce 'wobble' in the power supply.

[The New Steve H]

I am trying to put a schematic together, but just learning the software is a two-day project. I'm trying. I swear.

I know I've put this up before, but people seem to need another look. This is the amp I started with. It's called a "Little Wing." It uses two 6BM8 output tubes. Supposedly, it puts out a total of 7 watts. You can see the voltages on the drawing.

I am not using the preamp or PI from this schematic, so the only relevant portions are on the right. I am using FOUR output tubes, shooting for 14 watts. I have two OTs in parallel. I am basically duplicating everything past the PI, with a larger PT driving it all.

I have been told that these tubes are going to suck 35 mA each at the plates, even at 14 watts dissipation for four tubes.

The triodes are not connected. I am only using the pentodes. The preamp and PI will be a 12AY7 and two 12AX7s, just like a 5F6A.

A Trainwreck PT will give me the right voltages, and it will drive the heaters. My only concern is the current. Since a Trainwreck has four EL34 tubes, which draw more juice than 6BM8s, I am hoping I can just use a Trainwreck-clone PT. I found one, but the current for it is not specified, so I figured I would check here before placing an order.

[tubeswell]

I know I've put this up before, but people seem to need another look. This is the amp I started with. It's called a "Little Wing." It uses two 6BM8 output tubes. Supposedly, it puts out a total of 7 watts. You can see the voltages on the drawing.

Yep that's a tubenit/10thTX amp

The triodes are not connected. I am only using the pentodes.

Wow! - waste of perfectly good triodes sitting there doin' nuthin'!. You could use them for a number of things like a CF buffer for each output stage grid, or for the inverter and something else

[Firestorm]

6BM8 only uses 35mA if you run the plates at 200VDC (200 x .035 = 7W); that's the single tube, Class A max. What is your intended B+? If you go with 275-0-275 PT, you can get a B+ around 330 with a tube rectifier like a 5U4, so at 330 on the output tube plate, your max. current (again single tube, Class A) is only 21.2 mA (330 x .0212 = 6.996W). In Class AB push-pull, your idle current for 4 tubes will be between 42.4mA and 59.36mA (depending on how hot you bias them). At max signal you should wind up drawing around 85mA. So your 100mA PT should be okay, especially if it's rated conservatively.

[Phil_S]

Steve,
You've got to make up your mind about what you really want. If you want 250-0-250, that's what you should get. You say "Trainwreck" transformer. What TW? An Express uses a 585V supply. Rocket and Liverpool are EL84 amps, not EL34, so I am thinking you mean an Express tx. That's not a 500v tx.
Phil

[The New Steve H]

Okay, so instead of ordering a PT, I have to kill somebody.

The intended B+ is 275. Right now I'm getting more like 295.

Thank you for clearing up the PT mess. It made no sense to me, but I figured people wouldn't steer me wrong.