| Author: | Torquil Srensen (registered user: 92 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 17:29 ( . ) |
Hi! I was thinking about cathode followers, and got the idea of connecting the speaker without using an output transformer and that this could save some weight. This is an old idea of course, and I found an article about it from 1951 by Fletcher and Cooke, in "Electronics" magazine: [link] After doing some searches I think these are referred to as OTL tube amps? But I have not found any audio recordings of OTL guitar amps. I found some OTL hifi-amps on youtube, but not guitar amps. If OTL sounds good for guitar, isn't this a good way to reduce weight? So there must be some cons, otherwise they would be used more, right? Anybody know of pros and cons for OTL guitar tube amplification? Links to some recordings? Thanks Torquil Sørensen |
| Author: | Torquil Srensen (registered user: 92 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 18:33 ( . ) |
Ok, I found some clips near the middle of this page: [link] They offer some kind of OTL amp. The clips sound OK, but not exactly my kind of tone in most of the clips. The clean tone in the first clip was quite nice. Maybe they should have specified if the distorted tones were preamp or poweramp distortion. It could be interesting to make a low powered OTL amp if it is not too complicated. I would like to keep things simple, not much more complicated than the standard designs offered on AX84.com. - Torquil |
| Author: | Stephen Keller (registered user: 3502 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 19:35 ( . ) |
On 11/29/2009 @ 18:33, Torquil Srensen wrote :It could be interesting to make a low powered OTL amp if it is not too complicated. I would like to keep things simple, not much more complicated than the standard designs offered on AX84.com.
Simplicity and OTL designs are somewhat mutually exclusive ideas. Generally the way to get low enough output impedance to drive a loudspeaker directly without the impedance matching transformer requires running many low-output-impedance tubes in parallel. If you look at the design in that article you cited, it shows four 6AS7Gs on each side of the push-pull arrangement. That's eight triodes in parallel per side. That particular design delivers a relatively low 7 watts into its 16 ohm load, but it requires a 400-watt power supply to run it. Stph |
| Author: | Torquil Srensen (registered user: 92 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 20:17 ( . ) |
On 11/29/2009 @ 19:35, Stephen Keller wrote :
: Simplicity and OTL designs are somewhat mutually exclusive ideas. Generally the way to get low enough output impedance to drive a loudspeaker directly without the impedance matching transformer requires running many low-output-impedance tubes in parallel. If you look at the design in that article you cited, it shows four 6AS7Gs on each side of the push-pull arrangement. That's eight triodes in parallel per side. That particular design delivers a relatively low 7 watts into its 16 ohm load, but it requires a 400-watt power supply to run it. : : Stph -- Thanks, Stephen. Yes, I was kind of hoping that a more efficient design had emerged since that one. Btw, one commercial amp of this type is claimed to weigh 2.5kg and produce 50W of power, and uses only four ordinary power tubes. I dislike those web sites that are one huge page full of praise for a product, but it was this one: [link] It apparently uses "no heavy iron transformers anywhere", so it has an non-traditional PSU. I think it uses some complicated electronics for the impedance matching (some MOSFET & high frequency stuff going on), so that would probably not be interesting for me then. But it does suggest that a quartet of EL34 cathode followers should be able to produce 50W in this type of circuit. If connected to a 4x12'' 16 ohm speakers in series, the impedance mismatch would possibly be smaller. Or maybe 8x12 is neccesary... I won't bother to do any calculations, I'm pretty sure I'll be using an OT in my next amp :-) |
| Author: | Celeste hall (registered user: 1313 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 20:40 ( . ) |
Voltage gain is all tobu, but current buffering is switching SS "The impedance conversion is done with dc-dc switching converters operating at fixed frequencies well above audio frequencies." Personal experience with switching anything tells me it has a really solid botton end , but harsh highs, likely starting around 1000hz. PS is switch mode as well, so it is going to contribute to the high frequency noise, hash and ailiasing down. I would bet it sounds great at a show or in the store, but grates on you if you try to live with it. |
| Author: | tedmich (registered user: 378 posts ) |
| Date: | Mon, Nov 30th, 2009 @ 13:52 ( . ) |
On 11/29/2009 @ 20:17, Torquil Srensen wrote :
On 11/29/2009 @ 19:35, Stephen Keller wrote :
: : Simplicity and OTL designs are somewhat mutually exclusive ideas. Generally the way to get low enough output impedance to drive a loudspeaker directly without the impedance matching transformer requires running many low-output-impedance tubes in parallel. If you look at the design in that article you cited, it shows four 6AS7Gs on each side of the push-pull arrangement. That's eight triodes in parallel per side. That particular design delivers a relatively low 7 watts into its 16 ohm load, but it requires a 400-watt power supply to run it. : : : : Stph : -- : : : Thanks, Stephen. Yes, I was kind of hoping that a more efficient design had emerged since that one. : : Btw, one commercial amp of this type is claimed to weigh 2.5kg and produce 50W of power, and uses only four ordinary power tubes. I dislike those web sites that are one huge page full of praise for a product, but it was this one: : [link] : : It apparently uses "no heavy iron transformers anywhere", so it has an non-traditional PSU. I think it uses some complicated electronics for the impedance matching (some MOSFET & high frequency stuff going on), so that would probably not be interesting for me then. : : But it does suggest that a quartet of EL34 cathode followers should be able to produce 50W in this type of circuit. If connected to a 4x12'' 16 ohm speakers in series, the impedance mismatch would possibly be smaller. Or maybe 8x12 is neccesary... I won't bother to do any calculations, I'm pretty sure I'll be using an OT in my next amp :-) -- the link you posted is for the Milbert GAGA amp, which uses the designs of David Berning to run multiple tubes, auto everything and a SS OTL output. Apparently having an output section similar to a switching power supply run backwards to turn the high voltage tube AC into high current speaker AC. Berning's HIFI amps are very well reviewed and cost $10k+, while the guitar version willl be leass than 1/4 that. They have sound clips posted [link] NOT a gain monster but very responsive sounding. All the most knowledgeable amp designs that have commented on this design have only one criticism I have heard "I didn't invent it!" |
| Author: | tedmich (registered user: 378 posts ) |
| Date: | Mon, Nov 30th, 2009 @ 13:56 ( . ) |
On 11/29/2009 @ 18:33, Torquil Srensen wrote :
Ok, I found some clips near the middle of this page: : : [link] : : They offer some kind of OTL amp. The clips sound OK, but not exactly my kind of tone in most of the clips. The clean tone in the first clip was quite nice. Maybe they should have specified if the distorted tones were preamp or poweramp distortion. : : It could be interesting to make a low powered OTL amp if it is not too complicated. I would like to keep things simple, not much more complicated than the standard designs offered on AX84.com. : : - Torquil : -- oops! see you already got the clips linked! The patent is here: [link] you can get the full text pdf. unlike the US patent office...build one? |
| Author: | Celeste hall (registered user: 1313 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 20:46 ( . ) |
The only truly all tube guitar OLT amp I know of is the Rendition One. It solves part of the impedance issue by using a 32onm speaker array. That alone likely adds 75% to the output power. If you were to use a 4xSomething cab with all 16 ohm drivers wired in series you would make the amp even happier. |
| Author: | Stephen Keller (registered user: 3502 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 21:23 ( . ) |
There are also more efficient designs for OTL stages, but nothing comes to mind that is inherently simpler. Stph |
| Author: | Celeste hall (registered user: 1313 posts ) |
| Date: | Sun, Nov 29th, 2009 @ 21:26 ( . ) |
I think the inverted totompole is aabout as effiecient as they get, but not really any simpler. OTL is not going to loose you much seight because of how inefficeient it is. |
| Author: | Merlin (registered user: 4140 posts ) |
| Date: | Mon, Nov 30th, 2009 @ 16:48 ( . ) |
Probably the cheapest way to play with OTL is to build a headphone amp. An ECC82/12AU7, ECC81/12AT7, ECC88/6DJ8 can all be used to drive headphones in an SRPP or White cathode follower arrangement. Not quite as rock-and-roll though I guess! With a couple of 16ohm speakers in series you could make a sub-1W OTL practice amp the same way, *fairly* cheaply, with a couple of 6V6s or something, but you'll always get serious power wastage. |