Question on Cornet --- voltage

Laudanum · « **on:** 16 Aug 2004, 09:52 pm »

I've had my Cornet up and running for several months now, no problems. Just went to check the filament voltage for the 12AX7 tubes ... I admit, I didn't check after build

Anyway I noted the voltage at 5.7 volts (dc), not the specified 6.3 volts.
Voltage at that 2.4 ohm power resistor is 7.7 volts dc with 5.7 volts dc leaving that resistor (same as at the filaments). Not being very good at this stuff, I assume that the 2.4 ohm resistor serves to knock the voltage down to 6.3 volts for those preamp tube filaments? Should I change the value of the 2.4 volt power resistor to raise the filament voltage? If yes, what value to try? .... or, what else to check first? (please be specific).

If it matters, line voltage around here is 116 volts and that's fairly steady .... not sure if that has anything to do with it.

Thanks

GRD · « **Reply #1 on:** 17 Aug 2004, 05:15 pm »

I'm no expert, but if I apply Ohm's law to your readings I calculate a current draw of 0.83 amps for a voltage drop across the 2.4 ohm resistor of 2 volts (7.7 to 5.7). With the same current, a 1.8 ohm resistor provides a voltage drop of 1.5 volts, giving 6.3 volts (7.7-1.5). I would think there would be a small effect (a little more current draw) with a higher voltage, and that would drop the final voltage a little bit.

However, before I would change out the resistor, I would determine why the circuit was operating differently than specified so I would be fixing the right problem. Are you running the 270BX or the 370BX? I ask becouse 1.8 ohms is the specified resistor with the 370BX and 2.4 ohms is the specified resistor with the 270BX (at 120v).

Laudanum · « **Reply #2 on:** 17 Aug 2004, 07:22 pm »

Hi GRD

Thanks very much for the reply.
I'm running the 270BX. I'm assuming that the voltage should be higher coming into that 2.4 ohm resistor. If we stay with .83 amps with a 2.4 ohm resistor then you expect a 2 volt drop which would mean one would expect 8.3 volts or so at the 2.4 ohm resistor. But I'm not sure what the current SHOULD be. And isn't that 2.4 ohm resistor basically right off the secondary tap of the tranny? .... I don't even know what the voltage is supposed to be off the tranny there????
I'm also wondering if everything is supposed to be as is at 120 volts and if the fact that I have 116 volts here could make the difference???

I admit, I'm befused ....

GRD · « **Reply #3 on:** 17 Aug 2004, 08:29 pm »

The heaters on the 3 tubes should be about 300ma each for a total of around 900ma. The 2.4 ohm resistor comes after the bridge rectifier and a few filter capacitors so the DC voltage will be higher than the incoming AC. The 270BX is spec'd at 115 volts and Jim's design assumed it would be operated at 120v, so all voltages from the transformer would be about 4 - 5% higher. Just based on operating conditions, your voltages would be about 3% lower than the design (116/120). You're running closer to 10% low on the filament current.

Long shot - Is the 2.4R really 2.4 ohms? Also, you could measure the voltage drop across the (confirmed) 2.4R and get an accurate current reading.

Laudanum · « **Reply #4 on:** 17 Aug 2004, 09:30 pm »

Hi GRD

Yes, I measured it at 2.4 ohms as claimed.

Well, right now ...
Voltage drop across the 2.4 ohm resistor is right at 2.0 volts (7.63 at the resistor and 5.62 after the resistor) .... so, following ohms law, the current is right near 833ma ....

Things change a little bit with different tubes and rectifiers but noot by that much. Swapping the rectifier and all tubes to new JJ's the drop is 2.1 volts which works out to 875 mA

Either way .... voltage after the 2.4 ohm resistor is about the same as it is on the filaments and that's right around 5.5 - 5.6 volts ..... that's it.

Oh and right now ... or when the above measurements were taken, line voltage was at 114 volts.

Laudanum · « **Reply #5 on:** 17 Aug 2004, 09:34 pm »

one more thing that may help ..... I have 6.8 volts AC on the green filament taps before that diode array and those cap banks ..... if that helps

GRD · « **Reply #6 on:** 17 Aug 2004, 11:12 pm »

I'm out of my territory by now.

The 2.4R value is correct and the current draw is about right. So everything to the "right" of R21 seems to be OK (no extra current draw).

The line voltage is 5% low (114/120) versus design but the filament voltage is 12% low. I'm only guessing, but I would also measure the B+ to see if that is 5% low (313v versus 330v) or also 12% low.

However, I'm wondering if it's just the line voltage difference. The 270BX at 115v has the same specs as the 370BX at 120v and "R21" in the 370BX at 120v is 1.8 ohms. I'm stumped by the 12% lower filament but I could be missing something obvious.

Any other ideas from the master builders out there?

Laudanum · « **Reply #7 on:** 17 Aug 2004, 11:42 pm »

I appreciate all your help, GRD!

I think maybe a question should be ..... with 6.8 volts AC available measuring right at the green filament taps of the transformer (where they are soldered to the board) what should the DC voltage be at the 2.4 ohm resistor? IOW ... 6.8 volts AC should be ?DC? after the diodes and caps and at the input of the 2.4 ohm resistor? ...... anyone? somebody?

Thanks

Laudanum · « **Reply #8 on:** 18 Aug 2004, 09:13 pm »

GRD ... If you're still around.

I checked B+ and it's currently at 288 volts .... line voltage is down to 111 or just over. I have 289 volts AC off the transformer primaries where they solder to the board.

I think this is all directly related to line voltage just looking at the trends going from 116 V down to 111V on the line. Right now, filament voltage is down to 5.4 volts .... 7.4 at the 2.4 ohm resistor and the 2 volt drop is pretty consistent. I could drop that resistor from the looks of things but that's not going to help anything else but the filament voltage so ....

Not much I can do about it I guess as everything looks to be down with the lower line voltage out here. I've never measured it at lower than 114V and like I said, it's usually at 116. But, it's hot and it's peak time right about now and I'll be it stays like this with everyone running the AC at full tilt.

Any more thoughts, please chime in. If it sounds like I'm onto the problem here, let me know, confirmation would also be appreciated.
If I'm off base, let me know that too so I can figure it out if I haven't already.

Thanks

GRD · « **Reply #9 on:** 18 Aug 2004, 10:40 pm »

Yup, the line voltage makes sense. If you are running the 270BX at the rated 115 volts, it's essentially the same as running the 370BX at 120 volts. I would pick up a 1.8 ohm resistor for R21 and two 3.6k resistors for R2 (left and right channel) and substitute those. Those are the 370BX values. That's it, assuming 115v is the nominal supply voltage in your area (might be worth a call to your power company to confirm). Maybe pick up a couple of values close to those to fine tune if you want - resistors are cheap.

Best of luck.

Laudanum · « **Reply #10 on:** 19 Aug 2004, 12:52 am »

Will do - will call the power utility and check first, if I can get an honest answer that is.

Thanks for all your help, GRD. I appreciate it greatly

hagtech · « **Reply #11 on:** 22 Aug 2004, 08:50 pm »

Wow, 114Vac is awfully low. Is that in the USA?

I guess there is nothing you can do about that (realistically). So just assume you have maybe 115Vac nominal and we can adjust resistor values from there. You can lower resistances for B+ and heater (the first R in the RC filters) until you get voltages specified in the schematic.

I would say that 5.7V on heaters is too low. Transconductance changes and affects the EQ. Basically, the unit will sound a bit softer and more rolled off, similar to a tube end-of-life situation. With the higher heater, you'll hear a bit more punch and top end. My theory is that you have to get the tube operating in it's sweet spot. And heater has to be good for the rest of the design to work. B+ is not quite as sensitive in this design.

The construction, core size, wire size, ratios, between the 270BX and 370BX are all different. Hard to just say one is 4% more or less than the other. On the plus side, at 115Vac, your tranny will run a bit cooler than most!

jh

Laudanum · « **Reply #12 on:** 23 Aug 2004, 06:49 am »

Hi Jim

Thanks for the reply.
Yes, I'm in florida.
Voltage was down to 110 volts here late afternoon a few days ago. On hot afternoons when it doesn't rain, around 4pm - 6 pm voltage has been 110 - 113 volts consistently as I've been keeping track. Unbelievable isn't it .... I have a 2hp 4 gallon compressor that I picked up cheap to use as an air duster and it won't even kick on on the hot, late afternoons when the voltage drops out. All it does is "motorboat". I guess demand is really high with everyone running their AC but it doesn't seem right. But I have been keeping track of line voltages the last few days to geta handle on the trends.

Anyway, you must have missed part of the thread here because earlier in the day and later at night, line voltage is up right at 120 volts or a little above. The voltages in the Cornet are just about right on when the line voltage gets up there. So, it's a catch 22 I guess. If I drop R values I'll be higher than spec early and later at night and if I keep values where they are now, Cornet voltages are low, late afternoon. It's probably better to leave them alone as line voltage is up for a good bit more of the day and night than it is low.
I was thinking of maybe a line conditioner/stabilizer/regulator in the near future but not sure if the affordable ones are any good .... ie. Tripp-lite units in the $200 range.

Any thoughts? Thanks

hagtech · « **Reply #13 on:** 23 Aug 2004, 05:54 pm »

That is a problem. Only way around it is line conditioning. Probably relatively expensive. There seems to be a number of new machines on the market now other than the PS Audio. I think even Monster Cable has one.

jh

Laudanum · « **Reply #14 on:** 23 Aug 2004, 09:40 pm »

Jim, thanks for your input.

Regards

Salectric · « **Reply #15 on:** 29 Aug 2004, 07:30 pm »

It appears that it would be wise to use a regulator in the heater supply, so that the heaters can see the right voltage all the time. Even on a hot afternoon in Florida!