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Tube Guitar Amp - Running Output Tubes in Spec?
Tube Guitar Amp - Running Output Tubes in Spec?
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Old 6th February 2019, 03:08 PM   #11
nauta is offline nauta  Canada
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Hi

The OP began this thread with a mistaken notion that needs to be removed from public consciousness; allegedly "correct" because it is repeated often, and thus forgivable that people still believe the mistruth.

Data sheets for tubes used to list specs that accommodated variation of the tube manufacturing process AND for anticipated variation of the end-user's application. The tube manufacturers later changed the data to reflect only their own variation.

Some companies today still list out-of-date data out of simple lethargy and because they are not the ones that designed the tube in the first place- they have just copied the physical design. Others list data that does not represent the limits of the tube because their in-house test equipment falls short of those capabilities. An example of the latter is Svetlana USA, where there test rigs were 500V max, so apps for many tubes did not list voltages higher than 500V. The big tubes followed existing data provided by the original manufacturers as these products were simply straight re-issues.

As far as guitar and bass amps go, all the high-powered stuff (usually fixed-biased) uses the tubes within their specs. Cathode-biased amps fail sooner because of poor heat management. If you see these running "out of spec" it is a combination of two factors: original mains voltage was lower than today, so the PT puts out more voltage today. In new designs, uninformed "designers" and hobbyists add together the plate and screen power ratings to set the idle condition - not the way to do it - mistakenly thinking this will achieve higher audio output.

In upside-down tube amps, heat management is not great by design, and the issues are made worse since these are portable amplifiers that get banged around and transported in a less than ideal manner.

When reading the tube data, remember that it is all continuous values, so plate power is like a light-bulb power rating - run it at max all day as long as you can remove the heat. Hiwatt biased their amps this way. Tubes handle voltage stress much better than current or heat stress, and retain this ability their entire life. Tubes will easily last fifty years or more if the demand on cathode emission is conservative AND they are not mechanically upset.

If by "AB" you mean fixed-bias, this will extend tube life if you do not run the tubes as hot as they can go. Most cathode-biased amps are actually AB to meet their maximum audio output. For example, an AC30 produces about 20W at its class-A limit, then switches to B for the last 10W to reach 30W out. Do the math or simply scope the amp.

Also keep in mind that you can use a tube at far less than its capability. Just because you want a low output power does not mean you have to use small tubes - those will give you their small-tube tone. if you use octal-based outputs, then you have the option of plugging in a dozen different types with their own character. This does not change the maximum 15W output, say, as that is determined by the PT and OT; instead, you just have more range of tones provided the heater can support the heater currents needed.

For preamp tubes, the usual "300V" you see is based on the use of an inductive load with its attendant flyback voltage. For 12AX7 and 12AT7, the open-circuit voltage rating is conservatively about 540V - 600V for the T, but a 12AU7 is over 1,200V. Figured I should mention this since no doubt you believe that preamp tube specs are being exceeded, too?
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Old 6th February 2019, 04:34 PM   #12
BJosephs is offline BJosephs  United States
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nauta,


Again, I did not assert that there was anything wrong with the way classical designs run their output stages. I’ve built several myself and they worked fine until it ripped them up to build something else. If you think there is a fundamental misconception regarding just how much a tube can handle then go start a thread to work that issue. All I asked was if anyone had built an amp using the recommended conditions (as published) to see if folks like the sound or if there is some sort of tone magic that comes for pushing beyond those limits. If you took exception me characterizing this as “torture” or “abusive” then I regret that added color in my wording. My intention was not to perpetuate a myth but to have a fun discussion. That doesn’t seem to be in the cards since so much of the focus is staying on what the limits are or ought to be.


Brian
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Old 6th February 2019, 05:10 PM   #13
nauta is offline nauta  Canada
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Hi Brian

For the concept of "building at a given voltage versus what does it sound like", there are some common observations.

The first is that higher-voltage tends towards cleaner, more linear operation. This has a benefit for good sustain for clean or distorted sounds that do not require output tube distortion. In a preamp, this holds true as well. Higher voltage equals crisper cleans and distortion with good attack.

Another notion is that headroom is reduced when you use lower voltage. This is pretty obvious.

Another is that for a given OT primary impedance, reducing voltage reduces power, again totally obvious.

If you take a specific amp circuit and dial its voltage down, at first it will get rounder and mellower but it may not in fact get more distorted. The gain of the tubes goes down when you go really low, but this actually works out okay as the signal swing is proportional to the supply.

You can look at the output stage in isolation and try to figure out the tonal differences for Va=300V and Va=500V, say. At either voltage you could get the same amount of power with a few tubes depending on the amount you want to get - if the power is below the tube's capability then this works out okay. Say you can get 15W at 300V or at 500V with a given tube. At 300V, you might bias it hotter if you doggedly follow the 70%-rule than you might were you to set it by ear but within a metered safety value (see below).

At 300V, maybe you've set the 6V6 to 30mA, which is only 9W of plate heat. At 500V the same 9W would be only 18mA. The heat is the same so the thermal impact on the sound is the same. However, the transconductance is different for these situations and that changes the "apparent gain" of the tube. The transfer curve of the tube is different and the signal hits a different part of that curve in each voltage environment. Part of the reason for this is that the idle currents have been adjusted to constant power for the different conditions; were you to scale all the voltages and let the current scale too, then the tone would be same for both voltage levels (which is why Power Scaling works so well).

Tube biasing: The limiting factor to idle power is the plate power rating which should not be exceeded on a DC-basis. A 6V6 rated at 14W can be biased up to that 14W at 100Va or 500Va. You simply follow the power equation P=VI and you have different values of I as V is changed. This makes it clear that any tube chart that shows single "magic" idle current for each tube type is wrong, or at least incomplete.

Note also that you can get lots of power at low voltage but it means running the tube to its maximum current more of the time, which wears out the cathode faster. Going high-voltage means one can use less of the cathode capability of the tube and the tube lasts longer. However, this brings in the new wrinkle of OT primary impedance and how that affects sound? Higher impedances means higher turns ratios and a bit harder to get the same performance as a low-turns-ratio part. For guitar, this matters less than for hifi and the subjective difference is either liked or not.

Also, you have to remember that whatever the output stage does, it is only a part of the whole amp and the whole guitar system. You have to tweak the preamp and other things to get to the sound you really want and you can do this regardless of the output stage voltage - mostly.
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Old 6th February 2019, 05:57 PM   #14
Printer2 is offline Printer2  Canada
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Quote:
Originally Posted by nauta View Post
Note also that you can get lots of power at low voltage but it means running the tube to its maximum current more of the time, which wears out the cathode faster.

I thought the cathode boils off electrons at a constant rate. Seems I was wrong. You would think Leo got it right running higher voltages drawing less current from the cathodes.
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Old 6th February 2019, 06:43 PM   #15
aut0m4tic is offline aut0m4tic  United States
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Brian
I've wondered the same thing. I have to imagine that the published data provided the tube manufacturer's a margin for safety and reliability. I also have to imagine that amp manufacturer's made decisions based on cost just as much as sound.

I'm in the process of building a test bed amp that could do just what you asked. (I am interested in the high power testing that tubelab has done) Ive stripped out a hammond ao-29 organ amp, and bought an adjustable negative voltage module (-36v), a smps to provide a stable screen voltage, and meters to keep everything sane

I am metering: b+ current, cathode current, and screen current. I would just need to get 6.3v and 5v filament transformers, so I can use my variac to get the b+ exactly to the datasheet value.

I enjoy the learning process as much as I enjoy actually using the amps!

I need to get 4 more volt meters, so I can monitor: heater, b+, screen, and bias constantly. It's a pain to move a single voltmeter around after each and every parameter change.

One thing I have noticed, with my VERY limited testing, is that its possible to use many values of screen and bias voltage to get the same plate dissipation. So far, I've stayed within the tung-sol published plate dissipation levels, so screen dissipation has not been a problem. No preference to tung-sol, it's just what came up in a search engine.

The picture shows just how crude the setup is, and its definitely a work in progress!! Sometimes education doesn't look as pretty as a fender champ

20190206_105631.jpeg
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Old 6th February 2019, 06:54 PM   #16
PKI is offline PKI  United States
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Tube Guitar Amp - Running Output Tubes in Spec?
Doing some variac experiments I found that I much prefer sound of EL84 in my guitar amp running lower voltage that would be closer to the specs.
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Old 6th February 2019, 06:59 PM   #17
VictoriaGuy is offline VictoriaGuy  Canada
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Tube Guitar Amp - Running Output Tubes in Spec?
Did you have independent filament (heater) supply, or was the variac controlling the input AC power to the whole amp?

A VVR is an excellent tool for exploring tone vs B+ supply voltages.
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Old 6th February 2019, 07:02 PM   #18
PKI is offline PKI  United States
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Tube Guitar Amp - Running Output Tubes in Spec?
Default Tube Guitar Amp - Running Output Tubes in Spec?

Unfortunately heaters were affected and I could only go so low before started hearing negative effect of low heat. I really would like to find a PT with lower voltage for the plates. No luck yet.

Vvr?
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Old 6th February 2019, 07:09 PM   #19
VictoriaGuy is offline VictoriaGuy  Canada
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Tube Guitar Amp - Running Output Tubes in Spec?
Quote:
Originally Posted by PKI View Post
Vvr?
Try this for an overview before searching for 'VVR tube guitar amp'....

https://dalmura.com.au/static/VVR%20info.pdf
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Old 6th February 2019, 08:01 PM   #20
nauta is offline nauta  Canada
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Hi

There is a certain amount of electron emission caused purely by the heater, but the load on the tube substantially increases that emission rate. If you had a tube idling all the time at extremely low plate current, the heater would burn out before the cathode was deteriorated. Similarly, if the tube was idled at heavy load the cathode will wear out before the heater. Either way, it usually takes quite a while.

Note that Power Scaling came before VVR, with the latter being a misinterpretation of the Power Scale circuit despite the fact that Dana could see the entire implementation. Kevin describes this in detail on his forum theultimatetone.com.

The article linked above is interesting but has a lot of misinformation and simply wrong information. For example, for the mosfet regulator with current limiting a simple approach is shown where a resistor is tied to the source and the gate zener goes to the other end of this resistor. In a simulation all the parts are perfect but in real life the gate threshold voltage can vary by a few volts. That means that one mosfet might limit current somewhere near where you wanted but none of the rest do until current is amperes! It's a "clever" way to do it in the worst sense of the word.

Current limiting is a neccesity the way vvr works, but you should use the tried-and-true predictable transistor with current-sense resistor. Vbe is way more predictable than Vgs so the current limiting will be accurate.

Using SMPS modules to support linear circuits is a bit tricky as the SMPS makes a boat-load of noise. Grounding has to be really well laid out and you need chokes and wierd ceramic caps that are 100nF at DC but 22uF at 200kHz - RF near-voodoo stuff - hehe. I don't see why you don't simply use a standard choke and cap or RC off the plate supply to make the screen voltage?

For the bias supply, it is easy to make a linear negative rail for a fixed negative voltage, then use the conventional bias-set network to provide controlled grid voltage. Bias-set network = pot in series with resistor to ground, with wiper tied to the grid leak resistor. What can be easier? need two pots? wire them in parallel.
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