I'm thinking of trying my hand a a guitar amplifier, and was wondering if a set of PL519s configured push-pull in an OTL arrangement might be suitable. This is not quite as mad as it sounds - because many guitar speakers are 32 ohms or more, it shouldn't be too difficult to get a decent amount of power out of these tubes, and the lack of an output transformer makes it an extremely affordable project.
Can anyone recommend a fairly simple OTL design using these tubes? It need not have the highest performance - this is for guitar use, after all.
Can anyone recommend a fairly simple OTL design using these tubes? It need not have the highest performance - this is for guitar use, after all.
OTLs work great for guitar.
Check these out:
YouTube - Tube OTL by Devildogamps
That's 6AS7 OTL with 6922s in the preamp.
Go for it!
Check these out:
YouTube - Tube OTL by Devildogamps
That's 6AS7 OTL with 6922s in the preamp.
Go for it!
Btw, an OTL is not minimalist, and no cheaper than a simple vintage type guitar amp.
The O.T. for guitar are not the item that makes or breaks a budget for a guitar amp. The cheapest Edcors are great for guitar.
But I agree that with the right tube, and high impedance speakers, the OTL is a very good option.
The O.T. for guitar are not the item that makes or breaks a budget for a guitar amp. The cheapest Edcors are great for guitar.
But I agree that with the right tube, and high impedance speakers, the OTL is a very good option.
I'm not so sure about that. The less desirable 519s are about $30-$40/pair on eBay, about the same as a pair of 6L6s. However, unlike the 6L6s, I don't need a $40 OPT...and, if I did my math correctly, I can get more power out of a pair of 519s into a 32-ohm load.
You might want to look here for a simple, reliable circuit:
http://www.diyaudio.com/forums/tubes-valves/167268-jbl-amplifier-story-new-post.html
I built a guitar amplifier based on nearly the same circuit, which looks like this. It runs 16 ohms with about 20 watts and about 50 into 32 ohms. Very nice tone and does bass really well. Instead of PL519s, it uses 6AS7Gs (which are cheaper and easier to find) but you could use PL519s in triode and it would work, though not with as much power. You would need a few more PL519s (six, I think) before they would make more power than the equivalent number of 6AS7Gs.
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I like the idea of your circuit, but that doesn't seem to point to anything OTL-related.
Oops! sorry about that- had the wrong text in my buffer. Here's the correct link. Most of the rest of the thread relates to that circuit.
http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp-3.html
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Any specifics on yours? I figure I may as well duplicate a known-good design.
Also, yours uses a single 6SN7 with two smaller tubes, which is very much different from the standard Atma-Sphere design.
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The 6SN7 is the driver tube, direct-coupled to the power tubes as you see in the schematic below, from the other thread. The smaller tubes are 12AT7s, arranged in a differential cascode as seen below, only without the CCS, and of course there are only 4 power tubes. I raised the power tube B+ to nearly 200V, and reduced the plate current to about 30 ma, to allow the amp to take advantage of a higher impedance load. It makes about 50-55 watts into 32 ohms.
I'm working on an OTL with an input transformer, the idea being that the front end of the amp is entirely single-ended, to keep the circuit as simple as possible, and allow for extra gain, volume controls and a tone stack. Its an idea I've had for a while, thinking that the 'pure' OTL would be lacking in the overdrive department (which turned out to not be the case at all- it has a fabulous overdrive sound!!). I was thinking I could get a little 'magnetic saturation' quality into the overdrive, without paying the price and weight of an OPT. If it works I'll post a schematic.
In the meantime, the 'pure' OTL approach works really well, and is also quite reliable, even in overdrive.
I'm working on an OTL with an input transformer, the idea being that the front end of the amp is entirely single-ended, to keep the circuit as simple as possible, and allow for extra gain, volume controls and a tone stack. Its an idea I've had for a while, thinking that the 'pure' OTL would be lacking in the overdrive department (which turned out to not be the case at all- it has a fabulous overdrive sound!!). I was thinking I could get a little 'magnetic saturation' quality into the overdrive, without paying the price and weight of an OPT. If it works I'll post a schematic.
In the meantime, the 'pure' OTL approach works really well, and is also quite reliable, even in overdrive.
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I guess it is possible, but in my experience a simple SE design with a 20USD O.T. is both faster(easier) to build and less complex than a push-pull OTL. The little amp here : YouTube - OTL project
is as simple as I can make a guitar OTL, but I can make a SE amp with a 6L6 easier and cheaper if I want to. But I don't want to
I would love it if you do the OTL option, just saying that it's not the simplest route, that's all.
Hmmmm, perhaps u should concider something a little simpler than an OTL for a first build?
Get a kit!
CCS= Constant Current Source and is that bottom tube, V3, in the schematic. In my little amp it is replaced by a single 1 watt 110K resistor.
For the power tubes, I have two 2200uf 200V snap caps that I am using for each of the bridge supplies, so there are 4 total as you can see in the photo. I use a 1 ohm 5-watt resistor between the input and the output of each supply.
For the driver, I am running +/-300V with a power supply that is slightly trimmed down from the M-60. I have only 100uf on the plate of V4. That way if the amp is overdriven, there will be some modulation on the plate of the driver tube, which increases distortion, allowing the amp to cut 'through the mix' a bit more.
For the power tubes, I have two 2200uf 200V snap caps that I am using for each of the bridge supplies, so there are 4 total as you can see in the photo. I use a 1 ohm 5-watt resistor between the input and the output of each supply.
For the driver, I am running +/-300V with a power supply that is slightly trimmed down from the M-60. I have only 100uf on the plate of V4. That way if the amp is overdriven, there will be some modulation on the plate of the driver tube, which increases distortion, allowing the amp to cut 'through the mix' a bit more.
Are you sure? AFAIK finding a guitar speaker over 16 ohms will be hard work.
IMHO building an OTL guitar amp as your first DIY guitar amp is a bad idea, you are setting yourself up for disappointment. Do you even know what it will sound like overdriven?
There are 32-ohm 10" drivers made for the SVT cabinet. 16-ohm guitar speakers and not that rare either.
They overdrive great, and can sound like a Marshall or a Fender; they do metal overdrive really well.
My first DIY amplifier project was an OTL, back when I hardly had any idea what I was doing and long before the 'net. I had repaired many tube amps by that time so it did not seem like a stretch. The amp worked right away, FWIW. A small OTL is as worthy a project as any other tube amplifier for a first-timer. But I should point out that if you can't *fix* a tube amp that broke down, getting an amp that *has never run before* to a running condition is something else again!
He is very sure
Weber and Jensen have 32ohm speakers, and I bet some others do as well. Though I would rather have two 16ohm drivers in series. I really like two different 16ohm drivers in series b/c their slightly different impedance peaks spread the total out a little...or at least that's what I think. One 15" in series with one 12" is quite nice.If you check out my goofy vid (that I linked to previously, again below) of my one-tube OTL you'll hear the 6AS7 overdrive, and it's pretty cool. This simple amp was a test bed for my self, and features a simple gain circuit, but it's ok for pushing the OTL way into overdrive.
YouTube - MrSemperfidelis225's Channel
That would be a simple circuit, but it will not make power nearly as effectively as the Circlotron.
When you get rid of the output transformer, you are also rid of any reason for going single-ended; an OTL is not subject to crossover or notch distortion issues like push-pull transformer coupled amps can be. So the distortion character is more like single-ended, sans the 2nd and 4th harmonics (you get mostly 3rd, and not very much of it) if you keep the circuit simple as seen in the prior schematic.
But without a specialist guitar amp OPT, you'll never get that sound favoured by most guitarists of a saturating OPT. The actual OPT make a huge difference to the sound - try fitting a Fender OPT to a Marshall, or vice versa - you'll get a completely different sound.
Very hard to fake overloaded iron with an OTL.
Regards, Allen
Actually that's not what we found. The Marshall front end has a 'bug' in it that is responsible for much of the Marshall sound; the Fender front end has its own sound too. When you overload the amp, the amp has a nice saturation that every guitar player we have put it in front of has liked. If I had to guess, it comes off more like a Marshall and less like a Fender (I tuned it that way). You can tamper with that by working with the value of the bypass cap on the plate of the CF driver tube. With 200uf, the saturation is very soft, with a sort of compression before overload. With 50uf, the amp takes on a bluesy, wonkier sound like an older amp from the 50s.
A lot depends on the speaker load too- I recommend at least 16 ohms but your basic Marshall 412 cabinet is 16 so thats not that hard to come up with.
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