I have an older Yaquin MS-300B, in desperate need of a bucking transformer. The power transformer is designed to work on 110V,
and when fed 120V, B+ is 490v with 470v on the 300B plate...
Meanwhile data sheets say 450 should be the maximum....
Id like to extend the life of my tubes, so please don't say just leave it if it sounds good...
Feeding the amp with 110v still has B+ at 460v...
I have read resistors before the 5U4G might be the best way to reduce the B+ a little besides (in addition) to a bucking transformer....
Whats your take, and is there anything Im missing ?
and when fed 120V, B+ is 490v with 470v on the 300B plate...
Meanwhile data sheets say 450 should be the maximum....
Id like to extend the life of my tubes, so please don't say just leave it if it sounds good...
Feeding the amp with 110v still has B+ at 460v...
I have read resistors before the 5U4G might be the best way to reduce the B+ a little besides (in addition) to a bucking transformer....
Whats your take, and is there anything Im missing ?
Line voltage where I live has recently topped 127. I run my amp with expensive and rare tubes on a small Variac and dial in the voltage I want.
I have another amp that I run on a battery backup device for computers and that's good as well.
Variance with a digital voltage readout or separate readout is the safest way to assure you are at the right voltage is the smart way to go. Line voltage is all over the place during the day. This shows you actual voltage and let’s you adjust it on the fly. Maybe $200 total invested.
I guess the first question is what is the actual plate voltage across the tube - the answer will differ depending on whether this amplifier uses fixed bias or cathode bias.
I run with fixed bias and the full supply voltage is across the 300B, in my case typically 395 - 410V depending on line voltage. The filament circuit for each tube is referenced directly to the PSU GND through a 1 ohm resistor I use to measure the plate current.
In the case of a cathode biased amplifier about 70 - 100V will be dropped across the cathode resistor depending on tube and supply voltage. In your case with cathode bias you would be well under the maximum plate voltage rating of the tube, with fixed bias you might be in marginal territory.
I use a quartet of EML 300B in my amplifiers.
Try a 5R4 or 5AR4 rectifier if you want to drop the plate voltage a bit. (The quality of modern 5AR4 is pretty iffy - put a 1N4007 or similar in series with each plate lead if you do. In the case of the 5AR4 you may not see much of a reduction, but its delayed warm up characteristic is useful.)
I run with fixed bias and the full supply voltage is across the 300B, in my case typically 395 - 410V depending on line voltage. The filament circuit for each tube is referenced directly to the PSU GND through a 1 ohm resistor I use to measure the plate current.
In the case of a cathode biased amplifier about 70 - 100V will be dropped across the cathode resistor depending on tube and supply voltage. In your case with cathode bias you would be well under the maximum plate voltage rating of the tube, with fixed bias you might be in marginal territory.
I use a quartet of EML 300B in my amplifiers.
Try a 5R4 or 5AR4 rectifier if you want to drop the plate voltage a bit. (The quality of modern 5AR4 is pretty iffy - put a 1N4007 or similar in series with each plate lead if you do. In the case of the 5AR4 you may not see much of a reduction, but its delayed warm up characteristic is useful.)
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High heater voltages too.
This is a class A amp. PS load is constant. Build a line reducer using a 25W resistor for $2 plus the cost of a metal box, cord and recepticle. $15, tops. Choose the value of the resistor based on the line amperage and the voltage you want to drop. Example: 1.5A and 12v = 8R.
I suggest to move from capacitor input to filter after rectification to choke input. Filter input capacitor voltage is rougly 1.4 times ACV RMS. With choke input this goes down to 0.9 saving power and extending transformer and rectifier lives.
Bucking transformer is your best bet unless you want to choke the amp with series resistance in the B+ circuit. BUT it's a tube amp so 20% plus minus voltages are not a problem.. according to tube guru Tim de Pavaricini
Cathode resistor right now is 1k, and at
110V in, B+ 459, The OPT drops it to 440 at pin 2 on the 300B, 1k cathode resistor has a 74 volt drop
115V in, B+ 471, The OPT drops it to 448 at pin 2 on the 300B, 1k cathode resistor has a 78 volt drop
120V in, B+ 494, The OPT drops it to 472 at pin 2 on the 300B, 1k cathode resistor has a 84 volt drop
The OPT is about 4K and none of the load lines look too good for a long lasting tube... The 300B-XLS certainly might fit better, but that's another $800+
I will keep this in mind, if even for the warmup!
I like this, but for that price may be get another variac ?
Love the ideas you folks have though...
Doh! Your right! I did not measure any voltage, time to see whats happening there as well...
Thanks!!
You can put resistors in series with the power transformer secondaries to burn off some AC voltage before the rectifiers.
Did you say you're using a 5U4GB rectifier tube? If so, you could also go to a 5R4. But putting resistors in series with the 5U4 plates will do pretty much the same thing, which is increase the series resistance of the rectifier, dropping more voltage across it. That increases the impedance of the power supply, which is something most try to avoid.
If at 120VAC your B+ is 494VDC, dropped to 472VDC at the 300B plate, and there's an 84V drop across the 300B's 1k cathode resistor, that's 472-84 = 388VDC plate-cathode. That's well within the 300B's maximum ratings.
390V * 0.085A = 33.15W plate dissipation. The max Pdiss for a 300B is 40W. So you're within max ratings there too.
I think the only thing you have to worry about is whether the 300B's filament voltage is too high, or if you're using filament supply regulators, whether those are exceeding their max dissipation.
Based on the voltages measured the plate voltage should be fine. Subtract the voltage across the cathode bias resistor from the plate voltage measured and that is the actual voltage across the tube. In this case well under 450V for all cases mentioned. (Note some types will handle considerably more than this.)
My experience with JJ300B is that up to 10,000 hours at 400V across the tube and 70mA bias is possible. I have never gotten less than 6,000 hours out of a good 300B. (I've been building 300B based amps since 1999)
The filament voltage is sort of critical particularly if you are running EML tubes, and here something within a couple of % is recommended. (I recommend 4.9 - 5.1V or better. Not a bad idea to limit inrush current to 2 - 3X or less of the rated continuous current, EML does not recommend CCS heating which I do nonetheless.)
My experience with JJ300B is that up to 10,000 hours at 400V across the tube and 70mA bias is possible. I have never gotten less than 6,000 hours out of a good 300B. (I've been building 300B based amps since 1999)
The filament voltage is sort of critical particularly if you are running EML tubes, and here something within a couple of % is recommended. (I recommend 4.9 - 5.1V or better. Not a bad idea to limit inrush current to 2 - 3X or less of the rated continuous current, EML does not recommend CCS heating which I do nonetheless.)
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Heater voltage within 5% is OK too. If your primary voltage is 120v but it was made to run at 110v, it may exceed that, but 110v isn't standard. Are you sure it was made for 110v and not 115? Just check your heater voltage. If you need to drop it just get some small value resistors. R = V/I so do drop 1/2 volt say you get R = .5/1.2 or about .42R in a 2W resistor. 1.2a is the current draw of a 300B. You may be limited in the values of resistors you can find but you can get close enough.
I have no problem of 110v running at 120v. The home line is usually between 118v-120v. My amp is running with SS diodes so I add a cap and resistor to reduce the voltage based on 120v.
For the 6.3Vac filament, I added 3x1.2r/3w in parallel to reduce 7.1Vac to 6.5Vac.
5.0Vac is at 5.1Vac so i didn't have to do anything.
Yours should be easy to add a cap and resistor to reduce current since there is a lot of space on PS side.
For the 6.3Vac filament, I added 3x1.2r/3w in parallel to reduce 7.1Vac to 6.5Vac.
5.0Vac is at 5.1Vac so i didn't have to do anything.
Yours should be easy to add a cap and resistor to reduce current since there is a lot of space on PS side.
Been a long weekend, too beat to be playing near 500v.. tomorrow Im going to check the ac voltage on the heaters.. before i decide anything...
Thank you ! and stay tuned!!
A bucking transformer doesn't have to be very big because it only handles the % reduction, ie the bucking transformer secondary current is the 110V primary current. So, a 10-16V bell transformer from the hardware store is probably a quick fix. You could put it on the side of an electrical box on the end of a cord.
It is also totally unneccesary as shown above by members that understand that the max Va of a tube is the max voltage between the anode and cathode. A lot to do about a non-problem.
Jan
So in essence even with a B+ of 494 volts, the 474V on the anode and 83V on the cathode is really only a Va of 391v for the tube?
I guess thinking about it that ot makes total sense as the cathode is not at ground...
Slighy off this threads topic .. considering the cathode resistor is 1K, and my output transformer is about 4k reflected with my speakers, would the slope of a load line drawn use B+/5k ?
Still need to measure heater voltage, but thank you for the input, much appreciated...