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Old 8th January 2013, 02:09 PM   #11
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Quote:
Originally Posted by Rod Coleman View Post
With heaters on guitar amps, the high gain means that the first stage can have some sensitivity to heater hum. this is the reason that some Marshalls have a much higher value of cathode bypass cap - 330 - 470uF instead of the usual 22uF.
Surely you would want a smaller cap, as the cathode bypass increases gain over the frequencies it passes.
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Old 8th January 2013, 02:15 PM   #12
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The heaters are capacitively coupling 50/60Hz to the cathode. This is not the same as an anode signal - where current flows from anode to cathode.

The cap can help by diverting the noise current to 0V. Otherwise the noise current flows in the cathode resistor, where is DOES mix with the cathode current, & generates hum.
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Old 8th January 2013, 02:22 PM   #13
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Quote:
Originally Posted by Rod Coleman View Post
The heaters are capacitively coupling 50/60Hz to the cathode. This is not the same as an anode signal - where current flows from anode to cathode.

The cap can help by diverting the noise current to 0V. Otherwise the noise current flows in the cathode resistor, where is DOES mix with the cathode current, & generates hum.
Dual function cathode caps. I like it.
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Old 8th January 2013, 04:59 PM   #14
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The first stages are in an old low-noise 7025 with lower gain. It was noisy when bumped, but cleaning up the socket and pins seemed to work. The next stage is also supposed to be a 7025, but they're hard to get, so it's a 12AX7.

Rod, thanks for the list, it's all things I would consider but might mis-prioritize. The stock bridge has a small disc cap directly across each of the four diodes, obviously an attempt to suppress switching noise.

This amp is an ultralinear with 6 6L6GC, and I use it only for a big/clean bright guitar, big/clean baritone guitar, or big/clean bass guitar (but not quite SVT big). The output section doesn't have a great breakup mode, and I'm not going for any compression, sag, or sustain with this one...I have other amps for that. So unless I install a switch on the screens, the ultralinear output section doesn't have a great breakup or compression mode anyway; its strength and unique sound is its dynamic range and clean bright punch. So there's really nothing to destroy by adding caps to this one.

This Fender has no power supply chokes. Yet. Fender usually only uses small chokes farther down the ladder.

I have a 10Henry 300ma Edcor, their biggest closed bell-end standard choke, with 72 ohms DC resistance. But at 500+ volts it won't handle quite enough current, so to run everything thru it at 500+ volts, I would need to bypass it with a resistor. That actually might work, and dampen any resonance from the big coil. Then a cap. Then I could add 1 or 2 smaller chokes farther down the network. The correct ideal really big choke would work well in this amp because there's already dropping resistors before the wire to the output trans center tap, so I can add a choke right at the main filters without affecting the output voltage, by just adjusting the values of the existing resistors to achieve the same original voltages out. I was really hoping to use it in a different 50-watt amp.

Then I also have a medium-size (physically smaller, still a closed bell-end type) 88mHY choke with less than 1/100 the inductance value, but which can handle 5 amps and only has 1 ohm DC resistance. It's probably really more appropriate for a power supply for a solid-state amp, but it can handle the high voltage, and it only has 1 ohm DC resistance. At 500+ volts, that means it can handle like 2500 watts. But will 88mH do anything? So the Edcor is overwound with too-thin wire and this one is underwound with too-thick wire.

I was inclined to cut holes in the existing capacitor can and bolt on some capacitor clamps, and put in some bolt-terminal cans. Not dual-section, but bigger cans instead of the standard axial-lead caps.
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Old 8th January 2013, 06:47 PM   #15
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...I mean cut holes in the existing standard Fender capacitot tray, and use mounitng-clamp rings to mount big can electrolytics with bolt terminals...
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Old 8th January 2013, 07:20 PM   #16
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Is that the Studio Bass 180W (Rivera-era Amp with UL 6x 6L6)? I converted one of these from UL to pentode mode, in part to solve the noise and hum problem. With UL, the Beam Tube's advantage of being able to create a low noise endstage is very difficult to realise.

If you add more capacitance, please check with PSUD2 that the rms current from the mains transformer is not substantially increased, or the clean power will actually be reduced, and the trafo & rectifier overstressed.

The long wiring from the transformer via the rectifier to the cap box is so extreme on those Fender models that a quiet outcome will be very tough. Increasing the C value will raise the peak pulse current, and make it worse.
I used series-connected 220uF 400V TSHA caps in mine, and mounted them right next to the transformer, on some tag-strip.

88mH will not help, sadly. Values in the Henry range are required.
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Old 9th January 2013, 02:00 AM   #17
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Geeez..
DC heaters Work Fine. Countless examples exist.
Why? the 'if Granpa didn't do it' attitudes?
One can implement snubbers to tame AC heater hum.
But a simple standalone DC heater supply can give a silence that can be strikingly odd to diehard tubies.
Although only IF your design circuit isn't otherwise poor :-)
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Old 9th January 2013, 03:43 PM   #18
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I think there's consensus DC heaters work well on high-gain preamp stages, but do they help on the output tube stages? That was where most thought DC heaters didn't help.
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Old 9th January 2013, 03:53 PM   #19
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Rod, the Studio Bass, Super Twin, and Super Twin with Reverb are all very similar except the obvious (like reverb, or the super twin not-so-super distortion knob). Except the Studio Bass uses a lower B++ (they all use the same 500V B+ for 6 6L6GC) and different caps in the rotary graphic EQ.

What's PSUD2, simulation software? Wouldn't those have to be some pretty leaky caps to draw more current (except momentarily at start-up of course). If they made that much difference that would kind of prove how badly they were needed.

So some caps right at the rectifier; more right at the load.
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Old 9th January 2013, 05:54 PM   #20
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So give me a very general estimate for sizing the "current spec" for a choke that I run everything thru, including B+, for a big tube guitar amp. (I realize the choke might be enormous, or else it would drop B+ or have too little inductance to be useful, making a retrofit more complicated) I have never even measured the wall draw, so I'm just guessing at the inefficiency, based on the amount of heat but I have less feel for how a tube amp averages over time or momentarily with regards to signal. For a tube amp with 180 watt output, I conjecture it might occasionally want to draw roughly 300 watts from the power transformer secondary; at 500 volts that would be .6 amps = 600 ma I would want a choke to handle. Am I in the ballpark, and how does this conjecture relate to reality?

Of course I realize I need to simuate or measure and do the math as some point...or I'm a hack having fun (a definite possibility).
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