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Old 18th July 2007, 06:59 PM   #21
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Any chance we could return to the original subject of this post? Specifically, how on earth I can build a cheap OTL amplifier for a 64-ohm load?
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Old 18th July 2007, 08:51 PM   #22
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Hey Spas

Did you check out this thread ?
Simplest OTL?


The circuit from PRR seems easy to build
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Old 18th July 2007, 10:04 PM   #23
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OTL amps are not really projects for people who haven't worked with tube circuits before. If one tube decides to short you better have fast blow fuses because your speakers won't last very long.

But here is a circuit that is simple and does not need a split power supply.
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Old 18th July 2007, 10:28 PM   #24
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by astouffer
............. here is a circuit that is simple and does not need a split power supply.
Hi.

As I have posted in other OTL threads:-

although this circuit does not have a transformer, it is choke loaded and therefore is not really 'OTL', as the performance depends on the choke.

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Old 18th July 2007, 11:07 PM   #25
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Quote:
Originally posted by quinnling
Hey Spas

Did you check out this thread ?
Simplest OTL?


The circuit from PRR seems easy to build
That DOES seem easy to build. 3W distorted with one tube into an 8-ohm load should mean that I'll get at least a good 30W with four tubes into a 64-ohm load. I wonder if I could use the other half of the 12AX7 to get a bit of gain?

Quote:
Originally posted by astouffer
OTL amps are not really projects for people who haven't worked with tube circuits before. If one tube decides to short you better have fast blow fuses because your speakers won't last very long.

But here is a circuit that is simple and does not need a split power supply.
That's a single-ended amp. It's not going to be nearly powerful enough for my purposes.

On the subject of speaker protection, I'm not stupid - the speakers will be wired in series with an 80UF polypropylene capacitor, just in case. (The fundamental frequency of a guitar is usually 100hz, so if the rolloff is below that, it suits me just fine.) Furthermore, I intend to add some sort of emergency shutoff (likely using solid-state components - maybe a comparator?) that will trip a relay, removing voltage from everything from the heaters.

As an added bonus...these are $7 tubes.
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Old 21st January 2009, 02:30 PM   #26
puginfo is offline puginfo  Malaysia
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from where i am, we get 240Vac from the wall socket. if the main power trans is to be omitted for simplicity, the 240Vac fed directly into a silicon bridge which would hold 1000Vac then on smoothered by say LCLC or even RCRC, we should have like 240*1.35= about 300Vdc. if something lower like 200Vdc is needed, then Vdc can be further regulated via a pass mosfet/bjt cct.

someone on here hilited that rectification this way would be dead dangerous and im trying to understand why. could some one throw some light pls? thks
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Old 21st January 2009, 04:06 PM   #27
Jeb-D. is offline Jeb-D.  United States
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Quote:
if the main power trans is to be omitted for simplicity
That's not safe

Also, it will require a floating power supply if you want to direct couple your OTL to the load. There is no way to float mains rectified power, as it is already referenced to ground. Circlotron and Futterman variants, both require a type of floating supply for direct coupling. Otherwise the speaker can get damaged from DC offset.

Use a 1:1 isolation transformer at least. They are cheap and plentiful.
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Old 21st January 2009, 04:37 PM   #28
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by puginfo
from where i am, we get 240Vac from the wall socket. if the main power trans is to be omitted for simplicity,
NEVER try this !!!!!!

Quote:
someone on here hilited that rectification this way would be dead dangerous and im trying to understand why. could some one throw some light pls? thks

DEAD
DANGEROUS



Discussion of non-isolated power supplies is prohibited on this forum and will result in posts being removed or the thread being closed.


Andy
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Old 21st January 2009, 04:53 PM   #29
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You should have a look at Berning's OTL patents as well.
I forget the number, but you should be able to Google it.

Not truly transformerless, but an oddity abusing smaller
modern switch type transformer with an analog chopper.
Output step down transformer doubles as high voltage
step up for the plate.

Assuming you will use some inexpensive off the shelf DC
power supply of lower voltage...
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Old 21st January 2009, 10:13 PM   #30
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Quote:
someone on here hilited that rectification this way would be dead dangerous and im trying to understand why.
In most of the world one side of the mains or the center of the mains is connected to an earth ground. That makes one or both sides of the mains HOT (120 volts +) with respect to ground. Omit the isolation transformer and you DIRECTLY CONNECT this hot circuit to the ground circuit of your amp. Yeah there might be a rectifier, choke or other component in series, but it doesn't matter. Now the ground circuit of your amp is DIRECTLY CONNECTED to your guitar, turntable or CD player. This is electrically equivalent to taking your guitar cord and plugging it into one terminal of your wall (mains, line or whatever) socket. Would you do this?

Lets say you still don't understand, lets say that the microphone is grounded, and your guitar is hot with 120 or 240 volts, you are all sweaty because you are jumping all over the stage, when you touch the mic stand with one hand while your other hand is still on the guitar. YOU ARE TOAST! Now Google "Stone the Crows" and you will see that this is exactly how the lead guitar player was killed! Don't even think about it.

Another more reasonable request. If you design a guitar amp that is made to operate with a non standard speaker impedance, use a non standard speaker connector. Otherwise someone WILL plug an 8 or 4 ohm cabinet into it and zap, tubes will fry, and the speakers might.

Quote:
My experience of 6336 is very bad, they are not built very well and can't withstand almost any over current for any time however short
I have been abusing 6336A's quite a bit over the past year. I agree that they are not going to work in an OTL due to the peak currents involved. The little piece of ribbon that hooks the cathode to the base pins blows like a fuse. They will however work well at currents up to 1/2 per section. At that level they are virtually bullet proof.

Quote:
OTL amps are not really projects for people who haven't worked with tube circuits before.
I would tend to agree with this statement. OTL's are tempermental beasts. Even a well designed one will blow up occasionally. Add the typical abuse that a guitar amp sees and you will eventually get very frustrated with it. After blowing up some tubes and maybe speakers, the cost will be higher than a conventional amp too.

Now how do you build a super cheap guitar amp? Well, in the 60's old TV's were free, and I could build guitar amps for free too. That is a bit harder today. You are going to need a power transformer! It will be a lot cheaper if it doesn't say "tube" or isn't associated with the word "tube". Hoe do you get HV? Well look on Ebay for something called an industrial control transformer. it is usually possible to find one that was made to step 480V down to 240 or 120 volts. Turn it around and you can make 120 or 240 volts into 480 volts, add a solid state rectifier, and you have a bunch of power for cheap. a 120 or 240 to 240 volt version can be useful too. 6 volt transformers are reasonable for the filaments. If not 12 volt transformers from an old battery charger, or even a 24 volt control transformer can be found cheap, wire the filaments of identical tubes in series.

Well, what about the output transformer? Well, again since we are not looking for HiFi specs, look for anything that has the right ratio (about 25 to 1 voltage ratio) with a center tapped winding. A 480 (2 240 volt windings) to 12 or 24 volt industrial control transformer works reasonably well. Some toroidal power transformers will work. The idea is to try anything close that can be obtained cheap or free. It may even be possible to use a tube OPT that is under rated since we are not looking for frequency response down to 20 Hz. Fender was famous for using under sized output transformers. A 20 watt "HiFi" OPT can do 50 watts in guitar amp duty if it was reasonably well built.

Yeah maybe it will distort, maybe it will prefer 4 or 16 ohms because the ratio isn't right. Experiment. It will be a lot easier to make a reliable amp this way than an OTL.
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