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PostPosted: 09 Nov 2010, 10:20 
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Location: US Pacific Northwest
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I've occasionally wondered why we (the US) settled on 120VAC@60Hz and a major part of the rest of the globe 240V@50Hz. Are we weird, even though we started this whole electric grid thing, or is everyone else weird? I know why we settles on AC vs DC, I'm just curious on the particular voltage and frequency. If I learned this in electronics class, I don't remember it. That's been too many electrons over the jumper since.

If you're interested, read this: http://electrical-science.blogspot.com/2009/12/history-of-power-frequency.html

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PostPosted: 09 Nov 2010, 13:50 
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Suncalc wrote:
Quote:
I've occasionally wondered why we (the US) settled on 120VAC@60Hz and a major part of the rest of the globe 240V@50Hz. Are we weird, even though we started this whole electric grid thing, or is everyone else weird? I know why we settles on AC vs DC, I'm just curious on the particular voltage and frequency. If I learned this in electronics class, I don't remember it. That's been too many electrons over the jumper since.

If you're interested, read this: http://electrical-science.blogspot.com/2009/12/history-of-power-frequency.html

The history is quite convoluted!

One nice thing about 60Hz instead of 50Hz is that those in the U.S. on a 60Hz system can use power transformers designed for 50Hz systems (but not the other way around). Try to use a tranny designed for 60Hz at 50Hz and you'll probably burn it up (with no load!).

I still have to wonder WHY IN THE WORLD the sound and performance of a tube with electrons emitted from a cathode wire (directly heated) that is changing polarity 60 times a second would be superior to the sound from a tube where the electrons are emitted from a cathode that is "stable", with the only AC in the cathode being the signal. If 60Hz is better than DC for filaments, then surely 100Hz or perhaps 40Hz would outperform 60Hz, or at the very least SOME frequency other than 60Hz would be better. Or is someone out there saying that 60Hz AC is THE optimal heater supply frequency that surpasses the performance of both DC and any other AC frequency and it's TOTAL COINCIDENCE that the mains frequency just happens to be the same? It sounds kind of naive to think that 60Hz (or 50Hz) AC heating for filaments is the best there is when the only other alternatives I've seen to DC are some elaborate current source DC supplies designed to present a high impedance path to ground on both sides of the heater (not a bad idea really).

As far as high frequency audio signals are concerned, consider that the voltage gradient on the filament will change very little during several cycles of a high frequency signal. At 6 kHz it will take 100 cycles of audio signal for the heater voltage to make just one period. Looking at a window of 1/600th of a second near the peak of the 60Hz waveform will reveal that the 6kHz signal essentially sees almost unchanging DC on the cathode within the window of about 1.6ms during which the heater voltage looks like DC when you "zoom in" on the 10 cycles of 6kHz "riding" the crest of a much longer wave. The implication here is that high frequencies are not affected by AC heating since the rate of change of the 60Hz waveform is so slow as to appear non-existent to a high frequency signal, at least near the peak of the 60Hz heater voltage waveform.

All I'm doing is pointing out some technical reasons that AC heating should sound no better than DC. HOWEVER, I didn't say it does or doesn't. I'm just giving reasons why it should and shouldn't be different. Perhaps someone out there sees something I don't.

Conversely, consider a 20Hz audio signal occurring during the zero crossing of the 60Hz heater voltage as the voltage change/time is at its max for the heater supply. During the waveform cycle of the 10Hz signal that occurs during the zero crossing of the 60Hz heater, the voltage on the cathode is changing significantly with respect to time while the signal voltage isn't, or at least the instantaneous rate of change is much lower. In order for this to work without modulating the signal with the 60Hz heater voltage, the AC in the filament must be perfectly balanced so that the current density in the tube changes from side to side 60 times a second in absolute perfect symmetry so that the sum total of induced 60Hz current is zero and no PS hum becomes audible.

AGAIN, I'm not saying DC heating is better. I'm just asking a lot of theoretical questions about AC heating and wondering out loud how all the problems above are solved or contribute to superior sound. I have yet to read any theories or technical explanations about WHY AC should sound better than DC for heaters, but I've heard the opposite argument many times.

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PostPosted: 09 Nov 2010, 22:35 
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Interesting information.... Thanks Matt. Didn't realize how convoluted early power was. Guess if I ever travel to Japan, I'll need to keep in mind they have split system - yeah, like THAT's ever going to happen. :P
_

That 6C33C puts out alot of heat! I think I read it was 63W equiv. Get 2 or 4 of them together and....That would make a heck of a "Betty-Crocker Micro-wave Oven"! I'm thinking "Chestnuts roasting over open 6C33Cs..." :sing:

Thinking aloud some more... A Hammond 272JX should be able to push a 2 - 6C33C OTL. It has 6.3V @ 8A filament windings. It also has 300-0-300V @ 250mA HV and 5V @ 4A. ...and I have one! 'Course would have to be monoblock. Hmmmmm :idea: Great, now I've got another project idea rolling around up there. It's getting kinda crowded! :lildevil:

So, here are the projects I am now considering (if your interested);
A 300B SET (monoblock)
A 12AX7 PP (based on the FireFly amp design that used 12AU7s) (monoblock)
A 6C33C OTL Monoblock
A "complete" stereo amplifier w/phono centered around 6T9 PP and 6C10 triple-Triode compactron (original project)

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We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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PostPosted: 17 Jun 2013, 00:17 
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sampleaccurate wrote:
Last night I finally finished my first DIY amp, a single ended triode 300B stereo amp.

I used SMPSs for the heaters, and a direct coupled 6SN7 driver for the 300B tubes with Hammond audio output transformers..

Brought it up slowly this morning on a variac, no smoke, voltages looked about right. Finally plugged in some speakers and a CD player and it worked - first time!

I was missing some resistors and had to make a modification that is limiting the power right now, but at low volume it sounds sweet. Very clear, nice highs. I LIKE!!! Next up is a KT88 stereo oddwatt. And today from Ukraine my GM70s arrived in the mail for a future high power SET amp.

Image
Image
Image

No pictures of the inside yet. I confess - it's messy. I need to clean it up.


Hi, I am a new member and I am from Viet Nam, I would like to DYI one like that and could you pls which component (Tube, resistor...) do I need to make this amplifier. May be I have to buy it from Amazon. Thanks for your help in advance and waiting your feedback soonest


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PostPosted: 17 Jun 2013, 11:47 
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Typhoon, all the information lies within the schematic shown in the link:
--> Direct coupled 6SN7 driver for the 300B tubes
The difference is that Stephen uses an SMPS for filament power and a solid state HVPS design rather than the shown tube based HVPS.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


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PostPosted: 10 Oct 2013, 02:42 
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Joined: 28 Feb 2013, 05:45
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Hi
First of all: Very nice project.

I have a question about the schematic: I don't understand the resistor (or is it a pot?) between pin1 an 4 of the 300 tube. Could anybody help ?

Best regards
Christoph


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PostPosted: 10 Oct 2013, 09:13 
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Joined: 27 Jun 2011, 10:13
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Location: PA
Its a hum ballance for the 300B. Since ac is used. It can be elimated if you use a center taped transformer.


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PostPosted: 10 Oct 2013, 09:42 
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Got it .Thanks wolfdog.

Regards
Christoph


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PostPosted: 10 Oct 2013, 15:09 
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Centre tap tranni is probably the best way to go. What are your impressions of the sound. I have a 300B SET and love it. Nothing compares.

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PostPosted: 27 Dec 2013, 23:54 
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Joined: 06 May 2009, 21:53
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Hi, is there an advantage to using large OP transformers on an amp that only outputs less then 10watts?
Has anyone used or recommend Edcor iron for this amp, I find them a lot less money then Hammond, or is Hammond better quality then Edcor?


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