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Thread: Taming high B+

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    Taming high B+

    I'm getting to some upgrades on my Dynaco ST70 that I've been thinking about for a while. I've got the Triode Electronics improved power transformer, which makes a substantial difference, but I've still got the single 5AR4 bottleneck. After resisting it for a long time, I've experimented with SS rectification, and I have to admit that the difference it makes is huge. Missing bass magically appears, and dynamic transients are tight and controlled. I love the idea of tube rectifiers and their soft-start feature, but I'm tired of the sag in an amp that I'd like to be more accurate and articulate and less tubey. I recently rotated a powerful SS amp through my system and got spoiled.

    My ST70's power supply is a mix of polypropylene and series electrolytics, so the power supply is good up to 630V or higher. JJ KT77s were used for these tests, and they have a datasheet max on plates *and* screens, like the original KT77s, of 800V. (Whether this is wishful thinking or not, nothing blew up.) In all the cases below, I could tweak bias to get appropriate plate dissipation.

    Even with a 5AR4 and the standard tube complement, I get ~460V on the plates (410V on the schematic) with 122VAC input. Using a plug-in diode module, I get 490V on the plates. While that's technically acceptable for KT77s, the datasheet limit on EL34s and 6CA7s is 425V on the screens. (The JJ KT77s actually seem to sound really good running at these high voltages with SS rectification.) I had been running EH 6CA7s till one bit the dust after five years.

    One suggestion I've received is lowering the B+ by making the first stage capacitor small, splitting the difference between capacitor and choke input to tune in the desired voltage. This suggestion includes using a larger choke after the first stage and piling on lots of capacitance after the choke. Inrush current and instant B+ would be handled in one of a number of ways either via thermistors, time delay relays, or the good old-fashioned standby switch.

    Question: Is lowering the B+ this way a better idea than the one I've read about on this forum of putting a resistor in series with the ground return of the power transformer, possibly with a Zener in parallel to keep voltage drop stable? Pros and cons?

    ***

    I've noticed with many upgrade/replacement transformers for vintage equipment that the secondary voltages tend to run very high. I understand that this is due to the upwards creep in wall voltages, but since we all know about that, it's frustrating to me that the people who design and manufacture these transformers don't correct for it rather than making them as though we're all still living in 1960.

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    Some of the new Hammond stuff has 125V taps, the odd Weber PT also. Nice to have proper heater voltage, and sane B+ levels for filter caps that may be running too close to rated voltage.

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    Reducing the B+ by using smaller capacitors before the choke works perfectly. I use motor capacitors in series for this job - a recent build had 1.5uf before the choke (no increase in choke value) and then I treated the output of the choke as though it was just the correct rectified voltage for subsequent R-C nodes. You may need to add an additional node - as thought the choke and preceding cap doesn't exist.

    The downside is that you have caps and a choke (plus balancing resistors if you're using series caps to get to a safe working voltage) as pre-conditioning components. A real downside is if the amp doesn't already have a choke. The upside is the supply ripple into the rest of the circuit is lower than if it was straight off a rectifier.

    If no choke is fitted I always use a Mosfet dropper as per R.G's Mosfet Follies.

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    Mick, would that technique work where you have a field coil? Or would it mess with the speaker action?

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    I'd need to take a look in PSUD to see what happens. Reducing the input cap moves the circuit closer to the characteristics of an LC filter and I'd expect that what the field coil 'sees' would be different to a more efficiently implemented CLC filter as per the stock arrangement.

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