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Old 20th February 2019, 04:26 PM   #171
Tubelab_com is offline Tubelab_com  United States
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While taking a break from shoveling the snow, ice and frozen mud that has taken over my driveway I decided to read back over some of this thread. Intrigued by SemerFi's circuit and its PNP current mirrors I remembered some of my experiments from many years ago.

At that time I was still an engineer working at a large Motorola plant and silicon from On Semiconductor (used to be Motorola) was FREE.....did I blow up silicon? Truth is, I have fried more sand than vacuum tubes in my 50+ years of electronics experiments.

From post #41 Gnobuddy said:

Quote:
what if we use a current mirror, arranged to force an output transistor to exactly duplicate the current in the output valve? Now the transistor should sound exactly like the valve.........we can modify the current mirror so that the second branch not only mirrors, but also multiplies, the current from the output valve? Then we could deliver triple, or quadruple, the current to the speaker
At the time I was making SET HiFi amps and I got the idea of making a boosted version of the 45 DHT tube. Armed with a box full of the biggest and baddest high voltage audio transistors that ON Semi makes, I went down this road.....only to wind up with a box full of toasted silicon.

Quote:
the transistor could be run off a much lower voltage B+ rail than the pentode. This would reduce problems with second-breakdown of the transistor.
YES, I had rediscovered secondary breakdown. running the transistor into the OPT, which was then connected to a lower voltage supply helped, and this allowed the use of a 600 ohm OPT. This is where my path took a different turn.

We have found a way to drive a chosen tube in a way that gives us a good sounding low powered amp. We know how how to mirror this current and multiply it and use it to drive say an OPT, which can present us with all sorts of inductive kick back voltages and currents which will kill the silicon, so....

Stick the mirrored side current into the cathode of a BIG fat TV sweep tube that has no problem eating whatever the OPT throws back at us. So, I tried mirroring the current of my little 45 tube, multiplied into the cathode of a big sweep tube, with both plates tied together feeding an OPT.....it works!

It is logical that a bunch of 6AS7, 6336 or 6528 triodes could be driven by the mirror and fed directly to the speaker.
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Old 20th February 2019, 04:26 PM   #172
efeemeka is offline efeemeka
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Quote:
¿Puedes hacer un amplificador OTL de clase A de 10 vatios por unos 100 euros (excluyendo el controlador del altavoz y el tanque de reverberación)?
Realmente lo dudo.
.

Is very ovius, yes...

This Amp you to coment, but of one only extreme, it is OTL, whit pcl86, or more way whit 1ecc83 an 2 EL86....yes 10 watts in OTl...of new...

An it can ser more less (too) price :-), by not put the output transformer, jiji

The Eminence wizzar whit 16 ohms it more way for you purposes....whit 102,8 db (103db), it way for my

Less cost
Less weigth,
Whit 800 ohms cone 6" more much less price , i pay for this cone 5$ :-).

The sound of guitar, is one part is in all chain of the sound since guitar to amp, and finality to speaker.

All chain in higth impedance in 800 or 600 ohms if this posible, and eliminate the block of output ( ladrille the output transformer)

I inclusive conduct the OTL of this type whit 16 ohms, of this form), and whit standars speakers whit others compensatións at one identical volume to compare whit 800 ohms in OTL.

Transparence not is one oblique expresión, it is not obstaculize, and not deteriorate the signal by the road.

This it is work in this moment and very less cost, in combo format , yes.

And include and more merit, this in higth and in low tensión,

in less voltaje less power ok but yes

Very big qualite, in some years you see one standar, in guitar this is only short time

Regards.

Last edited by efeemeka; 20th February 2019 at 04:48 PM.
 
Old 20th February 2019, 05:16 PM   #173
efeemeka is offline efeemeka
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I have in my saloon one venerable fender blackface of 1965, original not reissue, other fender deluxe tweed point to point,
One Champtone thirty teen, one fender champ ptp, and others...very big list for cite here all,

But i select for playing more good experience whit my guitar first the OTL of direct output, or capacitance output
Independent of the power of this.


Is more musical and tridimensional, and more sensitive at the pulsatión.

And less price, of course

Last edited by efeemeka; 20th February 2019 at 05:32 PM.
 
Old 20th February 2019, 05:58 PM   #174
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by 45 View Post
In my experience they are only transparent on the bench driving a resistor. Not so transparent when driving real loudspeakers. They are actually quite fussy when it comes down to mate them with loudspeakers. Proper solid state amps are better for me if no transformer.


Be assured that if the OTL made a better guitar amp it would be at least a common solution among top of the range custom amps. Instead it isn't.


Less expensive? Are you joking? I can do a 10W class A amplifier with 3 valves 1xECC83 and 2xPCL86, high quality capacitors and high quality transformers. The amp has two independent channels, one clean and one high gain with individual tone controls, presence switch. Only the first 1/2 ECC83 and output stage is in common. The clean channel outputs 10W super clean and 12W with some distortion (mostly 3rd and 2nd harmonics which results in a very light warm crunch when playing).

The output transformer is 16K plate-to-plate (a long story....) and weights less than 1Kg. Total parts cost is about 70 euros + 90 euros for the Eminence Legend 1028K + 30 euros for the reverb. If someone wants to buy it he would have to pay more because I have done the transformer myself however this would cost about 50 euros.

Can you make a 10W class A OTL amp for about 100 euros (excluding the loudspeaker driver and the reverb tank)?
I really doubt it.



I haven't tried for a guitar amp because I have tried for HiFi. Not good enough for me. Very fussy and thus anything but transparent in most cases.
For a guitar amp it should be easier however the sound of guitar amp is heavily dependent on the guitar, tone controls, effects of various types, speaker, gain stage that talking about the transparency of the output stage is like talking about everything and nothing at the same time.....

Hm. You say you've not tried it but you say 'in your experience' that the amp is only transparent driving a resistor. Can't have it both ways- I have to assume that the first statement is about a thing that didn't happen.


FWIW I get this sort of thing a lot, but I've been doing this successfully for a living for over 42 years so I know it works. If you don't want to try it, no skin off my back.



OTLs have an economy of scale- the bigger you make them the more efficient they are. But if you really wanted to make an OTL for under 100 Euros you could do it if you knew where to get the parts.
 
Old 20th February 2019, 06:17 PM   #175
45 is offline 45  Italy
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Quote:
Originally Posted by atmasphere View Post
Hm. You say you've not tried it but you say 'in your experience' that the amp is only transparent driving a resistor. Can't have it both ways- I have to assume that the first statement is about a thing that didn't happen.
hmmm instead I have more and more doubts that you know and understand what makes the sound in guitar amp.

Quote:
Originally Posted by atmasphere View Post
FWIW I get this sort of thing a lot, but I've been doing this successfully for a living for over 42 years so I know it works. If you don't want to try it, no skin off my back.
Successfully for you, I say. I have done my experience with OTLs in HiFi and they are inferior. In HiFi there are more precise rules on what an amplifier is supposed to do and they are inferior. Sorry to upset you but that is. For what other reason should they be better in guitar amp if the signature of the output transformer is diminished because of the narrower bandwidth?

Quote:
Originally Posted by atmasphere View Post
OTLs have an economy of scale- the bigger you make them the more efficient they are. But if you really wanted to make an OTL for under 100 Euros you could do it if you knew where to get the parts.
Again I doubt you are aware of what a guitar amp should look like and what should do. People don't want a big amp but a great amp which should be as small and light as possible. Big and heavy amps no one wants them anymore......
I don't need to buy the parts. I already have them. I have a couple of quads of 6AS7G's by RCA and some 6080's by GEC. Made in 60ies....but I prefer them with an output transformer.
 
Old 20th February 2019, 06:45 PM   #176
45 is offline 45  Italy
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Quote:
Originally Posted by efeemeka View Post
.

Is very ovius, yes...

This Amp you to coment, but of one only extreme, it is OTL, whit pcl86, or more way whit 1ecc83 an 2 EL86....yes 10 watts in OTl...of new...
.
And what speakers is going to drive with 2xEL86? 6" cheap cones? Haha. Then where is this superior performance???? If you don't spend in the amp to make it under 100 euros you will have to spend money on speakers and the size will not be compact. Useless in practice. Try to sell such an amp and see how many musicians would buy it. A guitar amp MUST be practical too even if you are not a professional because if you have to go around to play with friends after a while I am pretty confident you will leave at home and use a more practical one.

Last edited by 45; 20th February 2019 at 06:49 PM.
 
Old 20th February 2019, 08:55 PM   #177
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by 45 View Post
hmmm instead I have more and more doubts that you know and understand what makes the sound in guitar amp.


Successfully for you, I say. I have done my experience with OTLs in HiFi and they are inferior. In HiFi there are more precise rules on what an amplifier is supposed to do and they are inferior. Sorry to upset you but that is. For what other reason should they be better in guitar amp if the signature of the output transformer is diminished because of the narrower bandwidth?



Again I doubt you are aware of what a guitar amp should look like and what should do. People don't want a big amp but a great amp which should be as small and light as possible. Big and heavy amps no one wants them anymore......
I don't need to buy the parts. I already have them. I have a couple of quads of 6AS7G's by RCA and some 6080's by GEC. Made in 60ies....but I prefer them with an output transformer.

You can make an OTL guitar amp lighter than one that has an output transformer, unless the OTL is going to be inefficient by being too small. IME 20 watts is about as low power as you can get away with unless you really increase the loudspeaker load impedance.



I've won many awards and gotten plenty of nice reviews in the high end press so no worries there. I also play in a band; Music | Thunderbolt Pagoda
I've worked on many guitar amps over the decades. We used to build a Plexi knock off for an amp company back before they were driven out by the recession.



All of my employees play guitar. The only reason OTL guitar amps aren't more common is the simple fact that guitar players expect to see one of only a few power tubes types (KT88, 6L6, etc.) in an amp and if its something else they get freaked out.



I've run into a lot of mythology about OTLs in the last 40 years. The most common is actually that they are unreliable. That is only true if the circuit used is a totem-pole. Circlotron OTLs tend to be much more reliable. As far as speaker choice goes, SETs are as picky as OTLs are and SETs are doing quite well so no worries.
 
Old 20th February 2019, 09:45 PM   #178
45 is offline 45  Italy
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Quote:
Originally Posted by atmasphere View Post
You can make an OTL guitar amp lighter than one that has an output transformer, unless the OTL is going to be inefficient by being too small. IME 20 watts is about as low power as you can get away with unless you really increase the loudspeaker load impedance.
HOW??? Can you make a REAL example instead of going round in circles?

Then I have to remind you the TIPS I gave you to make it lighter in post 143.

I have made a real example of a 10W class A amplifier, including the actual Eminence driver it is using.

At 20W power level things become even worse for an OTL because for a 20W amplifier with output transformer I will only need 2x12AX7 and 2xEL84. Just one more tiny 12AX7 consuming little power. The PSU transformer will be a bit bigger of course but that's true for any amp.
I have made output transformer on 22x22 mm core size and it actually has HiFi performance! It could be even reduced to 20x20 accepting a bit more DC resistance but nothing really to worry about. Same as before for the 10W with PCL86's but 8K instead of 16K. I prefer the 22x22 because I can easily find the covers.....so not much weight added for 2x the output power.

Quote:
Originally Posted by atmasphere View Post
I've won many awards and gotten plenty of nice reviews in the high end press so no worries there. I also play in a band; Music | Thunderbolt Pagoda
I've worked on many guitar amps over the decades. We used to build a Plexi knock off for an amp company back before they were driven out by the recession.
So why do you keep going round in circles? Where is this lighter 20W amplifier?

Quote:
Originally Posted by atmasphere View Post
All of my employees play guitar. The only reason OTL guitar amps aren't more common is the simple fact that guitar players expect to see one of only a few power tubes types (KT88, 6L6, etc.) in an amp and if its something else they get freaked out.
Apart from people beliefs and expectations the true reason why OTL amplifiers are not popular, never were and never will is that the best speakers have low impedance. A 32-150-600-800R driver will never be as good as a 4-8R driver. That's physics. The first thing that can be noticed is that all things being equal a lower impedance driver will be more efficient, if done right.

Quote:
Originally Posted by atmasphere View Post
I've run into a lot of mythology about OTLs in the last 40 years. The most common is actually that they are unreliable. That is only true if the circuit used is a totem-pole. Circlotron OTLs tend to be much more reliable. As far as speaker choice goes, SETs are as picky as OTLs are and SETs are doing quite well so no worries.
That's not mythology it was true at beginning. It's rather bad reputation that never went away. The mythology is that the absence of the output transformer makes them better. Better for what?
SE or PP amps can be both with or without transformer. Surely a SE OTL is the worst amp one can think of...

Last edited by 45; 20th February 2019 at 09:56 PM.
 
Old 21st February 2019, 02:39 AM   #179
Tubelab_com is offline Tubelab_com  United States
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This thread is also starting off in much the same way the HBAC did too.

Note:
HBAC = Hundred Buck Amp Challenge thread

The Hundred-Buck Amp Challenge

Quote:
I have made a real example of a 10W class A amplifier
I started making guitar amps in the 60's as a kid with no money. My only parts store was the county trash dump where hundreds of old TV's, radios, and HiFi consoles went to die. The prices were good though, free, just bring some tools and a shopping bag. I started making Champ clones, many with TV tubes.

I have had a guitar in my hands off and on for nearly 60 years, and built hundreds of amps. Some were sold, some given away, and others just used for a while, then robbed for parts. Most used tubes, but I did go through a solid state phase in the 70's and early 80's since silicon was free. In the 90's I did make soemguitar amps for sale. They were either "Turbo Champs" a SE design with anything from a 6V6 to a KT88 making from 5 to 15 watts, or push pull amps of 35 to 100 watts. I started Tubelab, making HiFi amps in the early 2000's. Every one of my HiFi designs had to eat my guitar playing with at least 10 DB of overdrive as part of its initial "torture testing." Ever play guitar through 300B's? I have, in both SE and push pull.

The amp I currently use is a 4 watt push pull job I built for about $45 cabinet and chassis included, for the HBAC. It was deemed that the current cost of 12AX7 / ECC83 was too much for most builders to use in their amps. It would be interesting to see how your amp was built for 100 euros when 20 of them went for tubes.

I have however NEVER played guitar through an OTL, and even though the engineer in me says I would prefer a transformer driven speaker, I will reserve judgment until I actually TRY it.

I am currently in the midst of a redesign of my TSE product, as after 14 years of sales some of the parts have gone extinct. Once that is done, I may tinker with some sort of OTL.

As has been stated 10 watts is kind of an odd spot for an OTL with conventional speakers. The 600 ohm speaker stuff is far too oddball, and hard to find. 2 X 16 ohm speakers, or 4 X 8 ohm speakers is far more conventional. I could rewire the speakers that I currently use, and know to make a 32 ohm load.

Getting 10 watts across that 32 ohm load needs about 18 volts RMS and 600 mA of idle current in class A. The cheapest power transformer is an isolation transformer with 2 X 120 volt windings......that's a lot of wasted power to make 10 watts. I have a big box full of used 6AS7's some 6336's and 6528's, but to make the 100 euro mark some cheap tubes are needed. I dug through my collection and found some TV sweep tubes that currently list for $3 and have a peak current rating of over 1 amp each......

Quote:
Surely a SE OTL is the worst amp one can think of...
Maybe I'll start there. It's easy, and I only need about 50 volts peak to peak swing across the speaker.

Ralph, I followed the link to the Bandcamp page and listened while I typed this. I was intrigued to see a real "beast full of tape strips" kind of Mellotron.
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Old 21st February 2019, 07:09 AM   #180
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Hey George, you take it as a challenge and that 's fine.

Someone else is certain but cannot make an example. That's not fine.
 

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