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Full plate voltage 12ax7 preamp to Class D?
Full plate voltage 12ax7 preamp to Class D?
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Old 6th February 2019, 01:14 PM   #1
Lefty78 is offline Lefty78  United States
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Default Full plate voltage 12ax7 preamp to Class D?

So I have seen this trend a few places and don’t know how to design a power supply that works.
Henriksen’s new Forte guitar amp has a 12ax7 running at “full plate voltage” into the preamp and then into a class AB power amp I suspect.
Milkman’s Them Amp pedal has a 12ax7 preamp running at “high voltage” then into a class D board.
I don’t think these are just 12-24v buffers.

What kind of power supply could run a “high voltage” 12ax7 preamp and a class D board?
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Old 6th February 2019, 02:25 PM   #2
Palustris is offline Palustris  United States
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Since the operating points you reference e.g., “full plate voltage” and “high voltage” are meaningless terms, the first place to start is to understand the operating points for a 12AX7. The tube manual has all the information: 12AX7 data.
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Old 6th February 2019, 02:50 PM   #3
DF96 is offline DF96  England
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This thread should probably be in Instruments and Amps.

High voltage for a 12AX7 probably means 200-300V supply rail i.e. the voltage you need if you want low distortion but don't want to use a CCS anode load. Used properly, a 12AX7 will simply amplify the signal - just like a properly-used opamp! Hence for a guitar amp no point in doing it except for marketing reasons?
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Old 6th February 2019, 06:31 PM   #4
jazbo8 is offline jazbo8
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Full plate voltage 12ax7 preamp to Class D?
Moved to I&A forum.
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Old 6th February 2019, 06:50 PM   #5
Printer2 is offline Printer2  Canada
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Probably minimum voltage for our guitars is about 150V. I have a design I want to use into a Class D amp but it is waiting its turn on the old project runway.
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Old 6th February 2019, 07:38 PM   #6
PRR is offline PRR  United States
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> What kind of power supply could run a “high voltage” 12ax7 preamp and a class D board?

Write down your estimated power needs.

One 12AX7 is likely 240 Volts 0.002 Amps.

One transistor POWER amplifier is likely nearer 24V 2A, 48V 1A, much variation.

Note the vast difference in V and A on each supply.

It makes no sense to build 240V 2.002A, then waste-off 90% of the voltage (over 400 Watts of heat!) to get 24V 2A.

There are ways to kick-up 24V to 240V. Until recently this was non-trivial. Today you can buy pre-engineered up-converters, and that may be one path.

The other path is, clearly, TWO supplies. With incredible luck you might find the power transformer from one of the several hybrid tube/transistor amplifiers. More likely you buy two power transformers, and go on from there.

Note that a LOT of commercial boxes run 12AX7 or 12AU7 as low as 24V to the plate, and people buy them.
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Old 6th February 2019, 08:16 PM   #7
nigelwright7557 is offline nigelwright7557  United Kingdom
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I designed a hybrid 12au7/class ab amplifier.
I used the +/- rails from the class ab amplifier as the power supply for the valve and it worked fine. I think the class ab amp was +/- 45 volts.
I even dropped the 45 volt rail to 12.6 volts for the 12au7 heater using power resistors.
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Old 6th February 2019, 09:20 PM   #8
nauta is offline nauta  Canada
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Hi

"High-voltage" is relative. Higher than what?

Recom has a DC-to-DC converter that produces around 200V out from various possible input voltages depending on the specific module (R15-150B). You can stack two to have up to 400V. These are about $50-60 so not likely to be what is used in any of the products mentioned above.

Considering that a 12AX7 only needs 2mA or less for both sections combined, which even at 300V is only 600mW of power, it is simple to use a voltage multiplier (caps and diodes) driven by a CMOS oscillator. Rolls does this for phantom power for microphones. The alternative is to use a coil and mosfet to pulse it, driven by a controller chip - all standard stuff for hobby switch-mode projects.

When a tube is used for actual gain and not as a buffer, it imparts a bit of character to the sound. So, tubes are added for more than just marketing.

All the digital mixing and recording guys have software plug-ins that emulate vintage tube gear and other analog stuff to help warm up the sound. Digital sound has been a big boom for analog and especially tubes. At first it was thought that everything before digital was obsolete, now you can't get enough of it back - haha!

Class-D amps have lots of problems. A friend of mine was telling me about these great class-D amps he got from Europe. The distortion was listed as kinda low but at wierd frequencies. It was suggested to me that these freqs were possible null points of the supply and the amp where distortion bottomed out into a resistive load. There's some marketing!
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Old 6th February 2019, 09:47 PM   #9
Printer2 is offline Printer2  Canada
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Under $10 for a boost module that can boost 12V-24V to 400V. Enough current for a tube output section from the specs. I need to wire up mine.
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Old 7th February 2019, 05:13 AM   #10
dotneck335 is offline dotneck335  United States
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Quote:
Originally Posted by DF96 View Post
Used properly, a 12AX7 will simply amplify the signal - just like a properly-used opamp! Hence for a guitar amp no point in doing it except for marketing reasons?
I think a LOT of people will totally disagree with that. There is nothing better than a tube for a guitar preamp; the 12AX7 being one of the best---all the modeling amps in the world are just trying to get THAT sound!
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