• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Has anyone used an opamp to split phase for a tube amp?

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no such hangup when it comes to silicon in the signal path of their chosen source.....even a turntable.

I stuck a couple of opamps inside my old Technics. Best phono stage I have made. Why? Because it's dead quiet. You can turn the volume up until you hit the threshold of acoustical feedback, and there is zero hum. It used a couple of those TI opamps that you can't get any more LMEsomething. The circuit is in the opamp data sheet.

Phase splitter.....stick an LND150 right into the socket holes of the 12AX7 in many guitar amp circuits......sounds good, just don't tell the metal mechanic at the guitar that you snuck a couple pieces of sand inside the amp.......
 
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here's an experiment for someone with 2 sets of identical speakers on their tube amp.
play tube amp into 1 pair of speakers, then use a 10k / 220ohm spitter to feed the signal from the speakers into a SS amp that then drives the other pair of speakers. match the volumes of the speakers.
I am guessing the sound would be identical.
 
I don't know for sure. I selected the LND150 for one reason. At the time I was designing a ultra low budget guitar amp. The LND150 was the cheapest high voltage mosfet in the Mouser catalog. I have since used it in split load and LTP splitters with good results, but I would imagine that any low capacitance fet would work very well.

For applications that need more current, requiring a higher dissipation fet I have used the On Semi NDF02N60Z, but it has been placed on the endangered species list.
 
Shannon Parks released an Arduino/tube pre-amp design over at diytube

I have collected all the $$$ parts for a big tube amp. Something in the 400 to 500 WPC range. Once I finish building my new lab, and knock out a few half finished projects, I'm going to build it.

At those power (and cost) levels things can go very wrong very quickly. Something like an Arduino is needed to keep track of things like tube current, temperature, output tube condition, and peak output voltage. It needs to be able to shut things down quickly in case of imminent disaster, but allow for things like a blonde with a guitar preamp set on KILL!

See note below.

Once these things are realized it becomes possible for all sorts of cool stuff like "in amp" tube testing and matching (needed for proper operation of toroidal OPT's).

Note:

There is always talk of building the "big one" but few really big tube amps are actually built. I have seen 525 watts flow from one of Pete Millett's big red boards, but that was a one off experiment. Building a totally reliable amp that can take a beating is a different story. Nothing can kill the fun like watching some expensive parts like OPT's burst into flames.

I have made an OPT go from working properly to 6 inch flames in a few mS. All it takes is a speaker load going open at full power. In my case I was cranking about 150 watts through an 80 watt OPT when the load resistor blew (open circuit) the 700 volt B+ just toasted the OPT instantly.

If you have time to kill, read this guy's story. He builds a big amp only to have the fire gods descend on it. The thread ends with no conclusion, and the builder disappears from this forum......almost 4 years ago. My opinion is poor OPT design, but nobody really knows. The OPT fries in post #231.

http://www.diyaudio.com/forums/tube...voltage-driver-ab2-operation-gu81m-tubes.html

Do you think a blown speaker is a possibility with 500 watts of power? How do you protect against disaster?......experiments with "sacrificial" OPT's will take place before building the big one.
 
Something like an Arduino is needed to keep track of things like tube current, temperature, output tube condition, and peak output voltage.

I have a cheap Atmel 8051-based chip in my amp with Plitron transformers monitoring cathode currents of all output tubes, in addition to the OT fuses. It shuts things down in the event of over-current anywhere in the output stage. It has gone off when there were problems.

I have an autobias circuit to maintain DC balance and had a bad solder joint in the power tube B+ supply that was causing the supply to intermittently collapse and come back up. Well, the autobias circuit would try to compensate for the reduction in current in the tubes when the HV supply cut out. When the supply came back up, the tubes would be drawing a lot of current and the amp would automatically shut down. Took me forever to find the problem but at least I didn't damage anything expensive in the process.

When you spend $1000 on transformers and $250 on output tubes, I think a cheap monitoring circuit is money well spent.
 
I have lucked into two different pairs of 400 watt Plitrons. Under normal circumstances they would be well beyond my budget. I need to protect them because a blown OPT would mean junking the amp.

The usual failure mechanism for a tube OPT is a flashover arc due to extremely high voltage. If the current in an OPT (or ANY inductor) is ever abruptly shut off, without any path for current to flow, the voltage can theoretically increase to infinity until a path for current to flow IS FOUND! This is the principle behind Tesla coils and automotive ignition coils. A tube amp usually has a rather stiff power supply connected to the OPT.... BIG FREAKING SPARK HAPPENS!!!! Once an arc strikes, it is fed by the B+ voltage until the fuse blows, or all paths for current to flow are GONE!

I watched an Ampeg SVT (400 watt bass amp with 6 X 6550's) catch fire when the speakers blew in the early 70's. Later I got to autopsy it. Both the OPT and power transformer were toast, along with several other parts.

This is where the "don't run a tube amp without a speaker" rule comes from.

That rule unfortunately is not so cut and dry. Under some circumstances you CAN run a tube amp without a speaker and have no ill effects. I do it all the time. This is the case if at least one of the output tubes remains in conduction. Under normal circumstances, an SE amp runs class A and can be operated without a load. If the amp is driven to clipping without the load, it is no longer in class A and the tube current will reach cutoff, and bad things can (and likely will) happen. In a P-P amp things again are OK as long as the amp is not driven to clipping, Keep in mind that the gain goes up a lot, with a near infinite load on the tubes, and all bets are off if there is a feedback loop, because it may go unstable.

So does this mean that everything is OK as long as a speaker is connected. NO, it does not. A HiFi amp is probably safe with most speaker loads since we don't generally listen to extreme clipping. A guitar amp, is not. Guitar amps are often driven well into clipping. Most guitar speakers exhibit resonance within the frequency range of the guitar. The speaker can be resonant with an impedance in the 20 to 40 ohm range right about where the OPT runs short on inductance and begins to saturate driving the tube current up. Now run those tubes into saturation (one tube is cut off, while the plate of the conducting tube goes negative) and the OPT appears unloaded while the core was magnetically saturated. Inductive kick back possible???? Well I have measured over 2KV in a guitar amp that runs about 425 volts of B+, yes possible, and probable.

The common guitar amp cure is a diode clamp, but this sounds nasty. My GDT experiments were inconclusive....but will resume when time permits.
 
Ok, so here is what I have had rolling around in the back of my mind for a while. I build a couple of O2 headphone amplifiers and have been truly impressed by them. I have even used them as a preamp and although I prefer my Decware CSP and Aleph 1.7P, it was very neutral. It also has been extensively documented and measured and has amazing specs. So why don't we modify the front end of that circuit for use as a phase splitter/front end for a similar tube amp? I understand that for the most part an opamp circuit is not to much different than the next but we have very detailed measurements of the performance of this circuit.

The real question I have with doing such a thing is; should it be wired with one regular channel and one inverting or as a quasi-paraphase circuit to achieve the best performance. AGDR has developed an inverted O2 amplifier design with some measurements, which we could use as a basis if we went that way. Granted it has a slightly higher level of noise but I am not sure it would really affect anything in the real world.

Heck we could even power it with rechargeable batteries for really quiet operation. We would just need higher voltage batteries or to double up on the 9Vs.

Thoughts?
 
Thank you for that link.

What I try to find - being low at electronics skills - is what final schematic was used on post #32 for listening and testing !? complete amplifier: opamp gain and phase splitter and push pull tube output ...

I am interested in such a simple design with finals KT88 push pull, but I do not have the knowledge to design myself a complete and best working circuit. So I try to find from pieces from others experiments ... :(
 
Sorry but I dont understand! Thank you.

The reason for opamp splitter was to avoid phase shift and disbalance on high frequencies in a tube phase splitter with split load, so called Concertina? If to shunt resistors in anode and cathode by capacitors this problem is solved. Far the more, if to combine Concertina with LTP the result is even better.
 
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