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Old 10th January 2013, 07:06 AM   #31
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I'm glad that you're enjoying PSUD2! It really does save a lot of guesswork.

For normal 220uF 400V caps,assume 0.7 to 1 ohm (yes, ESR is the data-sheet value).

For the transformer resistances, just measure primary (do this by measuring at the mains plug. The mains lead is checked at the same time, even if it's 0.1 ohm - and you can also be sure it is not powered). Measure the secondary ohms with a meter, too.

The resistance for PSUD2 is the "secondary-referred" resistance:

R = Rs + (Rp x (N^2))

because the primary resistance is added when factored by the square of N (turns ratio).

So for 120V input, 360V output (N=1/3, looking backward)

R = Rs + Rp/9


Choke sounds very good!
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Old 10th January 2013, 08:00 AM   #32
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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.
I've got to digest that for a while...this is the cap from cathode to ground in parallel with the resistor from cathode to ground? The super twin uses a 750 mfd there on the first stage; that's pretty big.

I expected to see a stopper cap from grid to ground, but there are no caps anywhere from input jack to grid on the first stage. This is not like my old Alembic Fender-style circuit.

Check out the wiring of the input jacks; classy little mute of the first stage when nothing's plugged in. It doesn't mjute via grounding the input to the first stage, nor does plugging into just one bypass the gain-splitting resistors for summing two inputs; no, when nothing is plugged in it grounds the input to the second stage so any noise from the first stage is also turned off. That's kind of classy, and if I get rid of all output stage hum it would work. Heck, I'm going to change one of those 33K series input resistors for a Mesa-boogie style lower like 1K or even none at all, and have one hot input and one less hot; I don't have a stereo guitar or share an amp, so why would I want to waste any sensitiity to mix two identical inputs? Might as well see which loads a particular individual pickup better.
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Old 10th January 2013, 08:31 AM   #33
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Yes, 750uF is big - and it's there to reduce hum from heater pickup, as described. You can use a high-quality 1000uF 6.3V cap there. Please don't use higher voltage rating - the working voltage is only 1-2V. The cap will polarise faster and better, and leak less, when the working voltage is a good portion of the rated voltage.

The cap stoppers were used to control instability in some Fenders (Riveras, especially) where the wiring was so badly laid out that oscillation was assured.

Talking of oscillation - take care when reducing stoppers. The Boogies I have looked at use ferrite beads on the grids, where the stopper is omitted. These play the same role. The first stage stopper (68K in classic circuits) is a good value for most purposes. I believe that Boogie used zero so that really high-output pickups can overdrive V1 (gain set HIGH, master volume set LOW). A special kind of dirty sound. For clean sound, the stability will be more important.
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Old 10th January 2013, 08:50 PM   #34
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The ferrrite beads are probalby handy for preventing RF input, especially without caps to limit the bandwidth. Band used to practice under the local radio antenna tower.

Thanks, Rod. I'm going to return to the 'insturments & amps' forum (or whatever it's called) now.
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Old 17th January 2013, 11:57 PM   #35
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..., or supply the screens from a FET capacitance multiplier (see attached circuit). I have used this circuit reliably in many guitar amps, and it preserves the sound of the amps, while lowering hum and noise.The circuit can reduce the screen voltage of EL34 amps to 390V, which will improve their reliability immensely, without degrading the sound...
Why just the screens? It looks like an FET capacitance multiplier will handle the full current of the B+ for the output tubes, even on these huge amps.

I imagine this is similar to the London Power QuietSupply kit for high-voltage tube amps:

Tube Amp Mods - - QS-HV - Quiet Supply for High-Voltage Tube Amps

The kit is $60 compared to about $20 for the parts; a bit expensive but the printed circuit sure does make it difficult to get anything wrong and it saves $100 worth of shopping time. It still saves a TON of money on big caps.

If I was still intending a pi filter consisting of cap/inductor/capwbypasscap what would be the advantages or disadvantages of using a cpaacitance multiplier on the first cap rung on the ladder versus the second cap rung on the ladder?

It looks like the kit uses just 4 22uf caps, but of course it is a capacitance multiplier LOL.
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Old 18th January 2013, 12:20 AM   #36
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You have to have a moderate cap directly after the rectifier or the cap multiplier can't do it's job. It will be trying to regulate pulsating DC.

You could do a Cap - CapMultiplier - Inductor - Cap and it would work. You will lose a lot of the energy storage capacity of the inductor though.
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Old 18th January 2013, 07:39 AM   #37
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Originally Posted by cyclecamper View Post
I am working on guitar amps, but these questions are really general tube amp questions and I'd like replies from a broad base of experience and knowledge, especially from the HI-FI arena.

I want to eliminate power amp tube hum or minimize it as much as possible. Then I'll start working on preamp hum & hiss.
Except for the very low level stages, DC heater power isn't necessary, unless you're using DH finals, and you'd like to keep DH finals out of a guitar amp. The delicate filaments don't stand up well to the abuse most guitar amps get at the hands of roadies. For most audio applications with indirectly heated cathodes, AC heater power works very well.

Most hum problems in audio amps come from poor lay-outs and/or poor grounding that lets the DC returns build up AC ripple voltages. For guitar amps, hum is also possible due to deliberately undersized power supplies -- the price you pay for compression and "sustain" caused by poor voltage regulation.

You can also get hum problems by means of magnetic coupling between a PTX and OPT, or a ripple choke and the OPT. A steel chassis can also encourage magnetic coupling. Usually, hum from ripple will have a frequency double the line frequency, and mag-coupled hum will be equal to the line frequency.

If you can o'scope, hum waveforms that look jagged and sawtooth-like are recharging pulses, and sinusoidal or near sinusoidal hum waveforms at the line frequency are mag-coupled.
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Old 18th January 2013, 07:45 AM   #38
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^agreed, in the last 6C33 SET i built the output tubes run on 12volts ac, even the preamp stage is ac, but dc lifted to 1/4B+....
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