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Old 27th August 2005, 08:38 PM   #1
Klimon is offline Klimon  Belgium
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Default Stupid question?

Hello,

I'm building a preamp with one gain stage, using an old power trafo. The schematic says I need 330volts AC from the trafo, mine has only 300. I would like to build the power supply as specified on the schematic. If I'ld pick a lower value for the anode resistor, too achieve the proper working voltage, would there be any drawbacks (except maybe the ps being designed for a voltage a little higher)?

Thanks!
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Old 27th August 2005, 09:20 PM   #2
DUG is offline DUG  Canada
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If your AC is 10% low then your DC will also be abot 10% low

If your plate swing is not very much (100Vp-p) then you will not notice it much. (IMHO)

get the graph of the tube you are using, plot the load line under both conditions and see if there is a difference.

Then decide.

I usually design around what I have.

As others would say..build it and listen to it.
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Old 27th August 2005, 09:56 PM   #3
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If your preamp is a single gain stage, I wouldn't worry too much about a slightly lower B+ voltage.

Can you post a schematic?
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Old 27th August 2005, 10:42 PM   #4
Klimon is offline Klimon  Belgium
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Yes, loadlines... I'm more of a tweak and listen kind of guy, but indeed the time has come to learn to read them...

I'm building the 12b4 preamp schematic + ps as seen on diyparadise.com
Schematic: http://www.diyparadise.com/12B4pre.html
Please note that Yeo (the guy from the site) uses a 12k anode resistor, and not 8.5k as indicated on the schematic

Power supply: http://www.diyparadise.com/simplepreamp.html
(it's the one at the bottom of the page. Yeo uses it with both the pictured 5687 and the 12b4 preamp)

More info on the 12b4 pre can be found on the mentioned site.


Now we're at it...Maybe I could use a rectifier tube with less voltage drop (6x4 isn't too 'efficient'), e.g. GZ34? Should I look at the loadlines of the rectifier, or can one swap rectifier tubes without too much fuss - respecting offcourse the max. Voltage, ampere and capacity limits and realising B+ will change?

Something I couldn't get an answer to yet:
Is a power supply a carefully designed circuit that's married with the signal circuitry, or does one take some general rules into account and improvise? For example if you take Duncan amps ps simulator (which is above my head for now), it seems nearly impossible to get a good 'plot' (that rises fast to the high voltage) when inserting large capacities (e.g. 470F). Why is it then that some people (who design kits so are expected to know their stuff) design a ps with a 470 cap? Do they know how to make it work after long and meticulous designing, or don't they measure/simulate at all and just choose some capacity at random - and listen?

Why do some very experienced designers always put a smaller value cap (say 10) right after the rectifier, then followed by larger capacities. And why do other very experienced designers always put a smaller value cap always on the B+ side of the ps?

Life's mysteries

Thanks for your replies
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Old 28th August 2005, 12:15 AM   #5
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Rectifier tubes have a maximum capacitor value for a capacitor input filter. It is usually specified in the tube manual. Use a capacitor that is too big, and you will shorten the tube's life. This is because the tube is trying to charge this capacitor as it is warming up (before it is really ready to supply full current). In extreme cases you will see sparks in the rectifier upon power up. Using a big capacitor after the choke of resistor is usually ok since the resistance limits the current. This is less of a problem with lower voltages, or small transformers. The use of a standby switch, and a soft start circuit can allow bigger capacitors. Some tubes are more prone to sparking out than others. Other than this restraint (which is often ignored) the power supply is like the rest of the amp, a matter of personal preference.

I can't speak for other designers. but I will usually design the amp first using laborotory test power supplies. Then I will design the power supply and check to make sure I get the same performance. On a single ended amp (which has poor isolation from power supply noise) the power supply is critical. Push pull amps are more tolerant of power supplies.

In your case I would build your preamp, using the transformer that you have. Tweek it. Try different plate and cathode resistor values. You should be able to find a sweet spot for your particulat amp that works with the slightly lower voltage.

The 6X4 is a lossy tube but it works with your existing 6.3 volt filament winding. A 5AR4 will require a dedicated 5 volt winding that is isolated, since it will be connected to B+. If you have this winding then try it. I have tested several flavors of cheap 5AR4's and get the most voltage out of Sovteks. I have not tried any of the expensive tubes. I am using two tubes in a voltage doubler configuration to generate 1100 volts for 845's. I find about 35 volts difference between the Sovteks and some Westinghouse branded Chinese tubes. The other ones fell in between.
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Old 28th August 2005, 01:40 AM   #6
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Some possibilities for lower forward drop rectifiers are the 6BY5 double damper diode and the 7Y4. Damper diodes have a reputation for a very small forward voltage drop.

Another technique for raising the B+ rail voltage is to use a CLC filter instead of a CRC filter. Ripple is lower and, given the lowish DCR of the choke, voltage drop is lower too. BTW, if you go the CLC route, 15 muF. in the 1st position is PLENTY. Locate the bulk of the energy storage after the choke.
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Old 28th August 2005, 11:47 AM   #7
Klimon is offline Klimon  Belgium
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Hello,

Thanks for the info!

I took another look at my power trafo, and it's actually 305V and has a 14v winding for filaments. I'm sure I can reach the required B+ by using a GZ34 and can always put the 14v winding in series with the primary to make B+ even higher. I'll be using a separate (also retrieved from an old amp) trafo for the 12B4 and GZ34 filaments.

The ps filter uses two chokes and is choke-input, so no problems with capacity! The many Hs will give me the opportunity to work with low capacity, which means I can use motor-run capacitors (paper-in-oil: they give an immense spaciousness to the sound!!!) **** electrolytics
(Funny you can't say **** on this forum. It's like the peeping sound whenever someone says the word **** on American television...Damn!)

Another good thing about using a GZ34: I found a Siemens branded Mullard in an old amp, testing as new. It's the version with seven notches in the plates; the Mullards are acclaimed to sound very good and last for several decades. Lucky me
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