Now that I finally (hopefully) have the oscillation under wraps I've gone back to playing with this circuit.
By varying the screen voltage on the first stage I can shift the operating point and thus the distortion level.
changing the G2 voltage also changes the gain of the stage over an Av= 21-92 range. To compensate I've switched to a gang pot (not in simulation) with one section acting as a volume control which is wired inversely to the G2 bias pot to compensate for gain change. It tracks within a couple of dB and I think I can get it down to 1dB or less.
Right now it varies the anode voltage from mid bias to almost cutoff. I thought about increasing the adjustment range to allow operation anywhere from saturation to cutoff, but the gain swing is non-linear and I can't compensate easily with a gang pot.
By varying the screen voltage on the first stage I can shift the operating point and thus the distortion level.
changing the G2 voltage also changes the gain of the stage over an Av= 21-92 range. To compensate I've switched to a gang pot (not in simulation) with one section acting as a volume control which is wired inversely to the G2 bias pot to compensate for gain change. It tracks within a couple of dB and I think I can get it down to 1dB or less.
Right now it varies the anode voltage from mid bias to almost cutoff. I thought about increasing the adjustment range to allow operation anywhere from saturation to cutoff, but the gain swing is non-linear and I can't compensate easily with a gang pot.
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You can try it with 6V6, but I will keep 6P30B-R in mine. Sub-miniature tubes are dirt cheap, don't require sockets, and are very reliable, being manufactured for space and military equipment.
I used to (oh, those younger and happier days) play with tube circuits completely stoned, usually making some mistakes but then I'll get shocked and regain my concentration. There's nothing like a good electric shock to regain consciousness. Much better than coffee.
At work in a perfect world? Don't know what perfect is but working doesn't look that great to me.
Preliminary thoughts on GU50 SE
EF80 with variable gain -> ECC81 -> bass&treble -> ECC81 -> master volume -> GU50 -> 5K transformer, with around 500 volts on the plate and a resistor feeding G2.
Will the gain be enough, or I need an EF184 and an ECC83? Or maybe 2 stages in front, with an ECF80? All valves are available already, so what do you guys think?
EF80 with variable gain -> ECC81 -> bass&treble -> ECC81 -> master volume -> GU50 -> 5K transformer, with around 500 volts on the plate and a resistor feeding G2.
Will the gain be enough, or I need an EF184 and an ECC83? Or maybe 2 stages in front, with an ECF80? All valves are available already, so what do you guys think?
If I had to pick one output tube for my amp it would be the 6V6. This is not to say that I do not like other ones but if I had to choose one this would be it.
costis_n, if you figure that you get half the tube gain in the circuit you still have 1200 before the tone stack, and 30 after it to drive the GU-50.
80mA and 500V puts you at -27.5 V bias, so you need 55Vp-p drive to the grid.
Figure your gain as EF80 - 40, ECC81 - 30, Tonetstack - 1/5, ECC81, 30...
I get a 5X loss thoerugh my tonestack (80% loss) and it can vary a great deal depending on the tonestack, but using this value you get 55Vp-p at the GU50 with 15mV input, so I think you will have plenty of gain, and probably enough gain for overdrive.
I'm running 40, 1/5 (tonestack), 25, 25, 2, but I only need 4Vp-p drive.
You may want to move the tonestack after the EF80 and before the first ECC81, but use a high Z tonestack if you do as it will heavily load the pentode output (which may be good for distortion if you want that).
80mA and 500V puts you at -27.5 V bias, so you need 55Vp-p drive to the grid.
Figure your gain as EF80 - 40, ECC81 - 30, Tonetstack - 1/5, ECC81, 30...
I get a 5X loss thoerugh my tonestack (80% loss) and it can vary a great deal depending on the tonestack, but using this value you get 55Vp-p at the GU50 with 15mV input, so I think you will have plenty of gain, and probably enough gain for overdrive.
I'm running 40, 1/5 (tonestack), 25, 25, 2, but I only need 4Vp-p drive.
You may want to move the tonestack after the EF80 and before the first ECC81, but use a high Z tonestack if you do as it will heavily load the pentode output (which may be good for distortion if you want that).
Doesn't have to be a car speaker:
Weber Ceramic Signature 6x9" Speaker
ALNICO:
http://taweber.powweb.com/weber/asig69.htm
Weber Ceramic Signature 6x9" Speaker
ALNICO:
http://taweber.powweb.com/weber/asig69.htm
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I wish I had some. I made all of my guitar amps long before Tubelab existed. I don't have pictures of any of them. In fact I have very few pictures of any of my creations despite the fact that I was into photography and had a darkroom.
My parents gave me a used electric guitar for a birthday present when I was 6 or 7. I didn't have an amp and I think they liked it that way. My parents had an old Magnavox mono HiFi that they gave me when they upgraded to stereo. I cut a guitar cord in half and twisted the wires onto the tone arm where the phono cartridge was and I was hooked.
It was the 1960's and old radios, HiFi's and TV sets were available everywhere for free. I started making my own guitar amps from their parts. By the time I graduated from a technical high school (they tought vacuum tube electronics) I had built a 200 watt tube guitar amp (8 metal 6L6's) and a 1200 watt solid state (24 X 2N3773's) PA amp for a local rock band (big power for 1970).
I made guitar and stereo amps on and off over the years until about the mid 80's. Wife, kids, job.....I had a Carver stereo and a POS guitar amp. No time to tinker, no time to play guitar, didn't even play the stereo.
When my daughter was in high school she played in a band and was in the high school marching band. Band kids were here all the time. I fixed their amps, and tweaked a few. I started making amps again. There were push pull amps and "turbo champs" no two amps were the same and all were sold or even given away. I made a turbo champ with junk parts and two JBL 5X7 inch car speakers that I pulled out of my car when I traded it in. All was built into a pine box. That little amp just screamed. I made another with two Audiovox 6X9's that I bought cheap when the local K-mart closed down. I made it with MDF. I thought the sound was far too clean but the kid that got it played an ES-335 and liked the sound. I made one using some Audiovox 5 1/4 inch round car speakers but I blew the speakers with a 10 watt amp! When my daughter graduated high school most of her friends left south Florida. I have no idea where the amps went.
So fast forward about 12 years and we are here. Yes it has been probably 14 years since I completed a guitar amp build. I started one more than once, but never finished it. So I have some catching up to do. There are plenty of people here building Champ like amps, so I am trying something that I have never done before. The first attempt sounded terrible so I cranked up the power supply until it fried. I started a second amp design on perf board bud didn't get very far before I had to leave town.
Over a two week period I threw several new ideas and a few old ones into a schematic and laid out a PC board. I haven't finished it yet since I am now behind at work. I plan to build and test it to see what happens. I don't expect everything to work, but I have to start somewhere. As it stands now there are 5 tubes, 8 mosfets, 7 pots and it all adds up to $98.
I still have the wood shop class one night a week. I have made small cabinet for an 8 inch speaker out of pine. More cabinets will me made in the coming weeks.
]
There were a few amps made in the late 60's and early 70's that came with 6X9 inch speakers. I remember an Acoustic (brand name) solid state PA amp that had two speaker columns using several 6X9's in a line array horn arangement. Quite loud and clean for its size and weight.
Ideally the voice coil moves the cone back and forth. It also imparts vibrations across the paper cone. The cone will flex as it moves back and forth. Usually a guitar speaker is made out of stiff paper and may have ribs in its surface to break up the secondary resonances and cone flex. There will be "standing waves" across the cone surface, some may be reflected from the cone edge back toward the voice coil. As the power level increases the cone can not be completely controlled and the cone mevement is said to "break up" imparting some unique frequency and power dependent distortions to the sound. A 6X9 is not symmectrical so there will be more and different distortions created by the cone movement itself than in a round speaker.
A HiFi speaker usually has a well damped cone to absorb the standing waves. This comes at a cost in efficiency. A vintage guitar speaker is made to be loud. Modern guitar speakers with high power handling capability may have some extra damping to prevent cone destruction. The same goes for car speakers. The new stuff that handles 20000000 watts doesn't "break up". The old 70's vintage Jensen car speakers would rock, but the blue foam surrounds didn't like me at all. I fried a bunch of them when I worked in the stereo store. 50 watt Marshall Super Lead + Jensen car speaker = blue foam pieces all over the floor.
Sometimes the stsnding waves and break up modes can be seen by sprinkling a small amound of baby powder on the cone and driving the speaker with an amplified audio oscillator. Vary the frequency and power level and you can see places where the powder just dances on the cone. Don't do this on a good speaker. If the powder finds its way into the voice coil it will make the speaker sound really bad.
I doubt that cone break up will come into play at the 1 or 2 watt level though.
It seems that there is a point where the cone stops acting mostly
piston like and begins to add its own tone beaking up like a bell
and adding significant harmonics. Some call it cone cry since the
breakup distortion can be at farily high frequency and imparting a
crying like quality to upper pitch tones.
I'm not an expert with guitar speakers but the Weber people are
and they talk about differences involving ribs in the cone and
doping that can be applied.
Beranek has some nice pictures of cone breakup modes in his classic
text on Acoustics - not sure if they are available online.
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Has anyone ran across a readily available 10k primary pushpull transformer in the 15watt range? I have been searching for one but I have not come across anything. I may end up just buying an XPP model from Edcor and play the waiting game.
I thought about trying a power toroid. If I could find one with a 240v CT primary and a 7 volt secondary wouldn't that get me in the ballpark? Roughly 9,400R primary into an 8R load?
I thought about trying a power toroid. If I could find one with a 240v CT primary and a 7 volt secondary wouldn't that get me in the ballpark? Roughly 9,400R primary into an 8R load?
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8K ohms 15W PP for $25 at musical power supply dot com.P-P OUTPUT XFMRS - Musical Power Supplies Inc.- Quality Tube Amplifier Parts You CAN Afford
It's a one man shop and he says right on his web site it's a hobby business and he says he'll do custom work if it fits on an existing frame so maybe you can get 10K?
I'm looking at his 5W transformer for a mini-amp that might even be battery powered. It would be cool to have a battery powered tube amp
220V inverters on epay are cheap. You can get 300V from it, and use it's case for tube chassis. I was considering such an option.
No need for rectification. Take 308 DC from before 50 Hz chopper. Or remove 50 Hz chopper (power MOSFETs)
I used 750VA 120V for one double-powered hybrid project.
Nice!
How much noise was there?
I did not hear any. Except hick-up on peaks of power (400W x 2 class AB)
But 100VA inverter for a battery guitar amp would be more than plenty I guess.