B+ how much is too much?

[Ty_Bower]

The data sheets for the EL84 spec a design center voltage of 300 volts max. The DIY ST-35 shows a nominal voltage of 364 volts in its design. I've read several times of people running their tubes as high as 380 volts, and claim it's OK. I hear statements that the EL84 is rated very conservatively; don't be afraid to push the voltage (as long as plate dissipation is kept down).

Well, my 330-0-330 transformer seems to give me a B+ of around 390 volts. This seems just a hair more than the highest voltages I've heard mentioned. I'd like to bring it down a bit. I'm considering swapping out the 50 ohm resistor in the power supply for a 220 ohm part. If what I see in PSUD works out, the voltage should work out perfectly.

I'm a little concerned with the heat that will be generated in that 220 ohm resistor. The original resistor was dropping about 10 volts at 150 mA of current. That's 1.5 watts. The 220 ohm part I'm considering will end up dropping almost 40 volts, and dissipating over 6 watts. That's a fair amount of heat, especially in a small space so close to the circuit board. I've seen some old high voltage power supplies with some nasty blackened boards.

Any thoughts? I'm not ready to go the route of a C354 choke with a modified first cap. I don't have the choke, I don't have a good place to mount it, and I'm not keen on replacing the caps. Replacing the resistor seems easier to me, as long as the heat isn't going to damage anything.

Or am I OK with 390 volts B+?

[erichayes]

Hi Ty,

My 1773s run with 380 volts on the plates, and I'm using 1Ω cathode resistors--so there's really 380 volts going through those tubes.

You start messing around with dropping resistors in the power supply, the next thing you know your bass response has gone to Hell. If you want to change anything, try an inrush limiter with a slightly higher hot resistance. That way you're not sacrificing regulation as much as you would with the resistor. Just make sure you don't size it so your tubes' heaters are getting less than 6.0 volts

[dhuebert]

A couple of things:

Those ratings are aimed at a 10,000 hour lifespan and are generally very conservative.

If you're worried about too much voltage buy a set of cheapies and run them and see what happens. I can buy Sovtek 6BQ5s at the local music store for $9 a piece. At that price, roast 'em, toast 'em, who cares.

Something else you can do is to increase the size of the screen grid resistors. My understanding of pentodes and beam power is that excessive voltage/current on the screen grids is a major killer. Reviewing the schematic, I see Shannon does not actually have screen grid resistors in this design. It is a tribute to good layout that it dosen't show a scrap of instability! Anyways, a 1500 ohm 3 watt resistor soldered to the screen can help tube lifespan.

I have lowered B+ by puting IRCLs in the primaryof the power transformer, it works. As opposed to bigger resistors in the power supply which severely compromises your amps ability to respond to those high power transients.

Hope this helps

Don

[Ty_Bower]

Hmm... line voltage today is somewhat less forgiving. I'm seeing 121.9 VAC at the wall. B+ is right at 405 V. Under these conditions, even if I put in a 5AR4 and replaced the first cap with 40 uF, I'd still be twenty volts higher than I'd like. Funny thing is, the heater voltage isn't too high at all. I see 5.9 VAC.

Maybe it's time for a new meter. Or maybe a new power transformer...

[TomMcNally]

Ty -

This may seem like a silly question - but are you measuring
the B+ under load ? Is the amp biased normally, per
Shannon's specs ? If you are running it lightly, the B+
will tend to be higher than you want.

[Ty_Bower]

Under load? The inputs are shorted, speakers are connected to the outputs. Tubes are in their sockets. Combined current draw for the four output tubes is around 130 ma - a little less than spec. I've got them biased a little low to keep the wattage down a bit. If you turn out all the lights, and get real close, you might start to see the slightest hint of red glow at the plates. The screens look OK, as well as I can see them. I've got one tube with a lot of blue glow, but I'm going to swap him out for another.

With no tubes installed, I saw over 420 volts on the B+ terminals (J6-3 and J6-4, Rev B) on a day when the line voltage was a relatively low 117 VAC or so.

Overall, the amp doesn't sound bad at all, but I don't have a whole lot for comparison. The bass might be a little too solid - bordering on heavy - but I'm not accustomed to listening in the room where I'm currently set up. Maybe the speakers are in a bad spot.

I'm tempted to do as Don suggests - cook 'em until they cry. :)

[TomMcNally]

If you bias them harder, the B+ will drop down some.
That's why I was asking.
Why would you want to keep the "wattage down " ?
To make the tubes last longer ?

[Ty_Bower]

If I believe PSUD, getting the bias up another 10 mA should drop my B+ about 2.5 volts. That's certainly in the right direction, but not nearly enough to make a substantial difference.

I've found the plates of my output tubes begin to take on a faint red glow when the total wattage in the tube (plate voltage minus cathode voltage, times idle current) starts to exceed the specs. In my case, I've got (405 - 15) * 0.033, or about 12.9 watts. This seems to be right on the edge of faint red glow.

I'm fairly comfortable about knowing when I'm exceeding the dissipation rating of the tube. I can measure the voltage and current, and calculate the wattage. I can visually see when the watts get a little too high.

I'm a little less comfortable about the voltage rating of the tube. I know the datasheets say 300 V. Well, I past that a long time ago. :) I've heard some people run theirs at 380 V. I'm past that too. I don't see any fireworks, or smell any smoke. How do I know when I've crossed the line? Will it sound really bad? Will my tubes burn out after a month? I dunno. Maybe I just have to fry up a couple and see how they taste.

[paart]

I suspect that some tube plate voltage ratings are picked at random. RCA's specs on the 6BQ5 are 300 volts on the plate, but jump to 400 volts for the 7189 which has essentially the same ratings for everything else! Certainly the plate distances from the other electrodes didn't change, as the construction looks to be visually identical, although it's possible that the internal spacers could be made from different materials. Either Mullard or Amperex (I can't remember which) rates their EL84 for 525 volts!
Before I came to my senses and bought a variac, output plate voltages on my Stereo 35s would often run 400 volts or even a little more, and ran happily for years like this! Perhaps though, this could be the reason that I had to replace a can filter after twenty years! Yellow_Light_Colorz_PDT_16
I wouldn't worry about high plate voltages on these tubes.

[Ty_Bower]

Thanks for the reassurance. I'm beginning to accept that whatever changes I make to the power supply in an attempt to lower the B+, will probably result in other undesirable side effects. Best case is to just leave it alone.

I've been doing a little research on the types of tubes I can run in this guy. I've got some Russian tubes, 6P14P, most likely made in Saratov at the Reflektor factory. These are the standard grade tubes, and seem to carry a rating of up to 400 volts on the plate, and 300 on the screen grid. The plate curves show data up to 300 volts.

There is a higher rated part (military spec?) which seems to be rated for up to 500 volts all around. I'm going to try to put my hands on some of these, and try them out for peace of mind. The standard grade tubes I have in there now do exhibit a small amount of "blue glow" in a totally dark room. I don't recall these tubes doing this when I ran them at a lower voltage (in a different amp).

Here's a link to the specs on the standard and -EV versions of this tube:
http://www.tubes.ru/techinfo/HiFiAudio/6p14pev.html

[EWBrown]

I've seen the "blue glow" in other EL84s (as well as EL34s, 6550Cs, 300Bs, etc...

The 6P14P-EV tubes are definitely heftier construction, if anything, the glass is thicker, and the inner elements are supposed to be more robust.
These were intended for military and industrial use, so they had to be rugged and dependable.

Last time I made an bulk order of 16 6P14Ps from "anthonywest" on e-bay he sent me the EVs at no extra charge.

I don't know how the pricing is today, byut just a year or two ago, a batch of 16, including the shipping, was often as low as $40.

At prices this low, trying to maximize tube life by cutting down on performance doesn't calculate...

/ed B in NH

[erichayes]

When I was starting to design the 1773, I intended to use the Sovtek military EL84s. A Peavey dual 100 rack mount amp came into the shop with a noise problem. This thing had 16 EL84s, all original non-military Sovteks with 425 volts on the plates. After I fixed the problem, I fired it up. Those tubes, even after bein' rode hard and put away wet, still managed to put out 100 WPC. That's when I decided that the non-military Sovteks were good enough for me.

[Ty_Bower]

Back to the original question... how much is too much?

I thought the idea of biasing a tube was to place the idling point somewhere in the middle of the linear portion of the curve on a graph of grid voltage vs. plate current. But, you need to keep the idle power below the melting point of the plates. Assuming the max power is 12 watts, at 200V plate you can afford to idle as high as 60 mA, at 300V you're at 40 mA, and at 400V you're down to 30 mA.

If I look at the grid voltage vs. plate current graphs on the 6BQ5 datasheets (I'm using the set from Standard Telephone and Cable, but they're all basically the same) there is no line for plate voltage = 400V. But, I can read the numbers off the plate current vs. plate voltage graph (page 9) and draw my own line.

It seems that as the plate voltage goes up, you need to keep moving the idle current down to stay under the 12 watt rating. For relatively high voltages (400V), the idle point is pretty far away from the linear portion of the curve. This doesn't seem like a good thing to me. What am I missing?