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Old 19th July 2010, 05:37 AM   #1
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Default Solid state rectifier for AC30 clone

Hi,

I am thinking of building a guitar amplifier based on the AC30 schematic using parts that I have stripped from an old stereo amplifier. The power transformer that I have is rated 350-0-350 and only has a single 6.3VAC secondary (no 5V for valve rectifier or another 6.3V). The power supply in the original schematic uses a 5AR4 and the B+ is approx 320V. Would this solid state alternative be ok to use? Any advice would be appreciated as I'm pretty new to tubes.

The schematic is here
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Old 19th July 2010, 06:49 AM   #2
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I don't know if that 900 ohm resistor will drop enough voltage (did you calculate it?) but if it does it will have to be rated for about 100 watts dissipation because you are going to be dropping over 100 volts through it. A 350-0-350 transformer puts out too much voltage for the VOX circuit, but you can use it if you don't mind wasting about half of the transformer's power through R1.
Using solid state rectification will alter the sound of the amp; you need to keep a tube rectifier in your amp to keep that "VOX" sound. You can use a couple of 6AU4 or 6AX4 damper tubes for the rectification and still be able to ground the 6.3 volt winding.
Daniel
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Old 19th July 2010, 07:53 AM   #3
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I use Copper Caps from Weber Amps in my Fender Deluxe for years without problems. The advantage is that in any moment you want, you can change it for a real tube rectifier because they use standard tube sockets. And not is expensive.

https://taweber.powweb.com/store/ccapord.htm
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Old 19th July 2010, 09:31 AM   #4
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Hmm yeah that power supply works in PSUD2 but real life may be a different story with 100W dissipation across a single resistor. Those copper caps sound like a great idea. Wouldn't actually be able to swap out for a tube rectifier though with no filament connections present but that's not a worry if they're as good as people say they are. I'll do some research and see if they could work.
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Old 19th July 2010, 09:58 AM   #5
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OK it seems that using a copper cap rectifier and a very small first filter cap could work but only if the output transformer B+ connection were connected after the choke. Would this work? Also, would having a small first filter cap increase power supply ripple? My supply now looks like this:
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Old 19th July 2010, 10:03 AM   #6
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Hi Hayden - I don't think your model is realistic at all.

Have you put all the specs for the transformer into the model? If not, you won't get the right result. The 31ohm secondary resistance spec shown in your diagram is the stock setting in PSUDII, so I figure you haven't changed the specs...

Rather than having a fixed resistive load, put a current tap (or two...). Set the current for the idle current of your amp.

The VOX amp has two voltage taps off the B+ - one for the preamp section and the other for the outputs. You need to model this too.

Now you are nearer the mark.
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Old 19th July 2010, 11:41 AM   #7
Ian444 is offline Ian444  Australia
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PSUD will show the ripple and the voltages at each point in the circuit. Have a look to find the resultant ripple. Sometimes you have to magnify the resultant waveform to see it. Read the instructions on how to zoom in.

A 350-0-350 tranny with diode rectifiers will give around (Ian runs to the valvewizard site to check) 420V under load. The total load would probably be close to 200mA. You need to lose around 100V, or maybe just 70V if you use JJ EL84 and run a 350V B+. The amp is cathode biased so you'll lose another 10 or 15V there. The 60 ohm choke will drop 60 x 0.2 = 12V. So you need to lose around 45V or 75V for 350V or 320V across the tubes respectively. For 45V drop at 200mA you need 225 ohms and 75V you need 375 ohms. The power dissipated will be 9W and 15W respectively. An aluminium-housed power resistor of twice the rating heatsinked to the side of the chassis with heatsink grease is how I would do it.

The 5K resistor in your PSUD sim will not load your supply correctly. Use Ohm's law. For say 350V and 200mA, you want 1750 ohms load. The valve wizard has good info on power supplies. I wouldn't get real fussed about whether its valve rectified or solid state.
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Old 19th July 2010, 12:20 PM   #8
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All the dropping resistor calculations so far have assumed a constant load. In actual use, the DC current will vary, so the voltage across a dropping resistor will also vary. This will introduce power supply "sag" - not necessarily a bad thing in a guitar amplifier, but how close it will then be to the performance of an AC30 is debatable. A zener diode string, or "power scaling" could be used for a fixed voltage drop.

A valve rectifier such as the 5AR4 could be used with a separate heater transformer. Jaycar Electronics in New Zealand sell 2 amp multitap transformers. These have a 6-volt tap which could be used, with a suitable dropping resistor, for a 5 volt heater supply.
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Old 20th July 2010, 12:04 AM   #9
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Seems to me that a choke-input filter could be the way to go here.
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Old 20th July 2010, 01:09 AM   #10
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yep - but we are all guessing until we know the power capability of the PTX...
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