Crossover distortion is a very real issue in valve amps. Pick any push pull amp with adjustable bias, set the bias for too low of an idle current, run a low level (under 1 watt) sine wave and look at the scope.
😕 I beg your pardon?
They do, happily, all day long, that's why they must be forward biased into some or a lot idle current to begin with.
Guess what that "kink" in the middle of the waveform is :
An externally hosted image should be here but it was not working when we last tested it.
I have seen crossover distortion at high power levels on the output of a 400 watt toroidal output transformer as the frequency was lowered towards 20Hz that did not have the same distortion at lower power levels. I believe the core was not large enough and was saturating but I could be mistaken. I also saw blocking distortion at lower frequencies in the same amp on the inputs of the output tubes because the driver tube did not have enough current capability to drive the capacitor coupled input. Changes to the driver circuit fixed that problem and a good portion of the output transformer problem. I could drive the output tubes into positive grid current without distortion(at the kt88 input) but it still had some crossover distortion on the transformer output at 20Hz driving full power.
Hi Guys
"The 400" chapter of TUT6 goes through several designs using standard tubes to achieve 400W+, along the way demonstrating how much power can be attained from those common types. You don't see MI amps pushing the tubes anywhere near their limits, except in cathode biased amps, where the tubes are brutalised by heat.
The Fender PS-400 produced 435Wrms with three pairs of 6550s and could do so forever (provided you don't drop it). The OT is the exact same size as the Hammond 1650W, same impedance primary just different secondaries. In this amp, a voltage doubler supply produces just under 800Vnl (700Vfl) for the plates, and 390V for screens and everything else. The grids are driven through a transformer, allowing positive grid drive with low distortion. All of this is within tube safety rules.
As others have pointed out, you have to build amps like this as monoblocks. They are just too heavy to move carefully otherwise.
As TUT3 shows, the 1650T and 278CX can allow four big-bottle tubes to produce 160Wrms with just a 30 lbs weight. Just a few db quieter than 500W and easier to move. TUT3 also shows an SVT build using standard transformers and a much simpler PA that allows modern tones or vintage ones.
None of the examples above require exotic or expensive transformers or tubes.
Have fun
Kevin O'Connor
"The 400" chapter of TUT6 goes through several designs using standard tubes to achieve 400W+, along the way demonstrating how much power can be attained from those common types. You don't see MI amps pushing the tubes anywhere near their limits, except in cathode biased amps, where the tubes are brutalised by heat.
The Fender PS-400 produced 435Wrms with three pairs of 6550s and could do so forever (provided you don't drop it). The OT is the exact same size as the Hammond 1650W, same impedance primary just different secondaries. In this amp, a voltage doubler supply produces just under 800Vnl (700Vfl) for the plates, and 390V for screens and everything else. The grids are driven through a transformer, allowing positive grid drive with low distortion. All of this is within tube safety rules.
As others have pointed out, you have to build amps like this as monoblocks. They are just too heavy to move carefully otherwise.
As TUT3 shows, the 1650T and 278CX can allow four big-bottle tubes to produce 160Wrms with just a 30 lbs weight. Just a few db quieter than 500W and easier to move. TUT3 also shows an SVT build using standard transformers and a much simpler PA that allows modern tones or vintage ones.
None of the examples above require exotic or expensive transformers or tubes.
Have fun
Kevin O'Connor
I think they all show some distortion. Isn't the center picture showing a clipped signal.
The distortion in the last photo might even sound good for a bass amp. I'd guess it is about 10% THD which sounds "clean" for guitar but not "sterile" I think even people playing smooth jazz need about 10% THD.
Totally depends on what 'sound' you're looking for in an amp. There's not a single right amount of THD (or frequency response, etc.) that will satisfy every player. I like mine a bit more clean 😎
Without getting into a dispute - Tube amps can produce crossover distortion under the right conditions, especially if the tube bias is insufficient at low drive levels on the output tubes ( at low volume ). Since most tube amps are run in class A or AB1 it's a rare phenomina, as the control grid does not normally draw current. On the other hand Class-B tube stages will display such if poorly biased.
To all the subsequent posters - yes you can MAKE a push-pull valve amp give crossover distortion (at very low levels just like a transistor amp), but you have to specifically design/adjust the amp to do so, no vaguely correct design would approach it.
The first signal is unclipped showing over 500 watts clean output. The second is fully clipped, showing the drive circuit is capable of overdriving the output tubes and at that point it is delivering 630 watts. The last is demonstrating full output at 20Hz, 492 watts which is where it counts for me for playing bass. I would like to see specs on the fender ps-400 or ampeg svt for output at this frequency but I've never been able to find them.
ok
i'm not reading all this but...
this will cost thousands and can be the biggest pain in your *** ever... o and will take a long time
and why the hell do you want a big tube amp???
solid state is the way to go for BIG systems or a hybrid amp with tube drivers solid outputs
or make a high and mid amp and a solid for lows
i'm just sayin this is a big task
i'm not reading all this but...
this will cost thousands and can be the biggest pain in your *** ever... o and will take a long time
and why the hell do you want a big tube amp???
solid state is the way to go for BIG systems or a hybrid amp with tube drivers solid outputs
or make a high and mid amp and a solid for lows
i'm just sayin this is a big task
It's already done, go big or go home! This is a hobby of mine and I love tube amps and wanted one that does not exist so I built one. Is it heavy and was it expensive? Yes and yes and I knew that going into it.
Because he can I guess. The better question is; why wouldn't you want to build a big tube amp!
I don't know about you guys, but it's these fantastic extreme builds that put a smile on my face and inspire me. It's out of the box and it's awesome.
I am not easily impressed but that amp of yours hits the button!
Nice work. Being a bass player, all round musician and electronics engineer of many years you have got the best of all worlds. Lots of grunt at the bottom end, (I would test with a pair of Nexo 18's) and what looks a nice balance.
Well done.
Nice work. Being a bass player, all round musician and electronics engineer of many years you have got the best of all worlds. Lots of grunt at the bottom end, (I would test with a pair of Nexo 18's) and what looks a nice balance.
Well done.
My dad used to build big tube amps too, and he doesn't get it any more since he got into transistors. But why build what you can buy? The point of building is to try, design, experiment, customize, dream, enjoy, inspire...so these extreme amps are very interesting.
Hi Guys
Listen to any Mesa amp if you want crossover distortion and a dead lifeless tone. The accountants design the amps and this means CHEAP tubes.
Biasing cold means more of the tubes they buy can be fitted into production amps.
Tubes will last for decades if they are not mechanically upset. Musicians have been force-fed a lot of garbage about changing tubes every six months. Total waste of money!
Tubes have a break in period, just like strings. They lose some gain but then are on their "tone plateau" where the tone changes very little over decades of use.
As an alternative to the sea of tubes, a single pair of 4CX250s will easily produce 500W all day and night while being bounced around on the road. These are metal-ceramic tubes, about the size of a short 6L6 but looking like a jet engine. They need forced air cooling and the sockets and chimney typically used with them are available from all the ham suppliers. The sound is neutral, like a 6L6 or similar - no edge, no glassiness. 4CX250s were used by the military in portable transmitters.
Used to build 700W bass amps using these in pairs. The total weight of iron was about 40-lbs.
Another alternative that uses less expensive tubes would be 811As: one pair will do 340W and two 680W. 500W is no problem and the tubes cost no more than receiving types.
The Svetlana SV572-160 amp referred to earlier in this thread did not need eight output tubes to produce the power it did - four would do that. Eight was chosen for marketing. A pair will produce about 250W and has a very transparent sound. Heater current is high, and the tube itself is quite large, so not a first choice for portable equipment.
Note that toroids should never be overdriven at low frequencies. They function perfectly, so fail perfectly, producing an audible 'clink'.
As far as moving those big amps, at the risk of making the package even larger and heavier, the pro way is use double flight cases with heavy padding in between. The padding allows soft rubber wheels to be added and the vibration going over hard surfaces is much reduced.
Have fun
Kevin O'Connor
Listen to any Mesa amp if you want crossover distortion and a dead lifeless tone. The accountants design the amps and this means CHEAP tubes.
Biasing cold means more of the tubes they buy can be fitted into production amps.
Tubes will last for decades if they are not mechanically upset. Musicians have been force-fed a lot of garbage about changing tubes every six months. Total waste of money!
Tubes have a break in period, just like strings. They lose some gain but then are on their "tone plateau" where the tone changes very little over decades of use.
As an alternative to the sea of tubes, a single pair of 4CX250s will easily produce 500W all day and night while being bounced around on the road. These are metal-ceramic tubes, about the size of a short 6L6 but looking like a jet engine. They need forced air cooling and the sockets and chimney typically used with them are available from all the ham suppliers. The sound is neutral, like a 6L6 or similar - no edge, no glassiness. 4CX250s were used by the military in portable transmitters.
Used to build 700W bass amps using these in pairs. The total weight of iron was about 40-lbs.
Another alternative that uses less expensive tubes would be 811As: one pair will do 340W and two 680W. 500W is no problem and the tubes cost no more than receiving types.
The Svetlana SV572-160 amp referred to earlier in this thread did not need eight output tubes to produce the power it did - four would do that. Eight was chosen for marketing. A pair will produce about 250W and has a very transparent sound. Heater current is high, and the tube itself is quite large, so not a first choice for portable equipment.
Note that toroids should never be overdriven at low frequencies. They function perfectly, so fail perfectly, producing an audible 'clink'.
As far as moving those big amps, at the risk of making the package even larger and heavier, the pro way is use double flight cases with heavy padding in between. The padding allows soft rubber wheels to be added and the vibration going over hard surfaces is much reduced.
Have fun
Kevin O'Connor
in 80´s you had to diy, because in east europe hifi electronics were scarce.
today you have to diy because what can you buy is ..meh
There is absolutely nothing wrong with the Mesa Amp, apart from them being too loud on stage!. What is the objection from moving the output valves from class AB1 to class B2 If one wants non linearity set the bias with a selector switch -40 for EL34 or -45 for 6L6. The valves will sound like a Marshall JMP100. A very sought after sound.
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