12at7 cathode follower

Great fretboard skills, the best guitar and amp your budget allows, a cathode follower… Wait a second! A cathode follower? And how does it make me sound better? Good questions, all of them. But whatever descriptive adjectives you want to use, TubeMeister Deluxe definitely serves up a refreshingly different flavor of sound.

And one of the main reasons why it ladles out such tasty tone is a much-loved and often talked about amplifier circuit called a cathode follower. Ask vintage amp enthusiasts about the cathode follower at your own peril. What is this legendary cathode follower, and — most importantly — how does it make our guitar tone better?

To put it almost simply, a cathode follower is an impedance conversion stage within an amplifier circuit that transforms a high-impedance signal into a low-impedance signal. The latter is less susceptible to interference and can have a beneficial effect on tone. It takes a fairly firm grasp of electrical engineering to understand the intricacies of the theory behind the cathode follower. For our purposes here today, suffice it to say that not every preamp circuit under the sun stands to benefit from an added cathode follower.

The upper half of the wave represents distortion, while the lower part stays half clean. This mixture of clean and distorted sounds represents the typical flavor — pure attack and dynamic response mixed with a delicious creaminess — of a cathode follower-endowed amp. Other amps have it both ways, with one channel that works with a cathode follower and another that does without. Cathode follower circuits are said to lend the tone a special flavor, as a number of classic guitar amps would go to show.

A lot of this is down to the harmonics, generated by the cathode follower stage, that enrich the output signal. In the TubeMeister Deluxe 20 and 40, the cathode follower stage is largely responsible for shaping the brown sound, as this most popular flavor of tone has come to be known.

They certainly pack a bigger punch than the added two and four watts, respectively, would account for. And this response is very musical, with just enough natural compression to sound and feel good.

Again, these amps come across as bigger, beefier and bolder. Cathode follower? The Deluxe amps mix the best of traditional tube tones with state-of-the-art technical features, making your life as a guitar player more fun, easier — and better sounding — than ever before! Take a bow, designers! Classic tube tone, cooked up with the finest ingredients and the tastiest traditional recipes, all ready to go right out of the box: 21 st century amps like these truly are a crossroads where tradition and modern-day innovation converge; where the cathode follower and FRFR coexist in the happiest of unions.

Are you bothered about whether your amp has a cathode follower? Does the technology behind guitar tone matter to you? Just click on this beautiful picture below to read our little TubeMeister 18 to TubeMeister Deluxe 20 case study:. They also feature cathode followers but not DC-coupled ones, if I remember correctly. Glad you enjoyed the post Jimi!The cathode follower is normally used as a buffer between a one bit of circuit and another, so the second one does not cause loss of gain or increased distortion by loading down the first one.

The cathode follower has very high input resistance, and low input capacitance because it does not suffer from the Miller effect. It also has low output resistance, so wide bandwidth can be maintained when driving heavy capacitive loads like long cable runs. However, whereas the DC-Coupled cathode follower can produce a unique sort of overdrive, the AC-coupled version is not so useful for this since the input couling cap prevents the flow of quiescent grid current.

It is normally used only as a tonally transparent buffer stage, such as for driving the output of an effects loop, for example. The output may be taken directly from the cathode 'Out1' in the diagramor in the case of the self-biased circuit from the junction of the bias resistor Rb and load resistor Rk 'Out2'. I prefer the latter because this allows the bias resistor to do double-duty by isolating the valve from load capacitance somewhat, which tends to improve stability.

Yes, a cathode follower can oscillate at very high frequencies if you're not careful, particularly with capacitive loads like cables.

In some designs you may see Rb bypassed with a capacitor, which maximises input impedance. However, this defeats any stabilising effect of Rb and is frankly a waste of time.

The price we pay for this is gain, which is slightly less than unity. Because the AC cathode follower is normally used only as a transparent buffer for driving difficult loads, it makes sense to use a medium- to high -current valve that can provide plenty of drive.

12at7 cathode follower

Designing an AC Cathode Follower For the following example we will design a cathode follower to act as a buffer for an effects loop. The second triode in the ECC82 could of course be used to amplify the returning effects signal. Choosing the Load A cathode follower is simply an ordiary gain stage that has been turned on its head. Although the load is in the cathode circuit rather than the anode, a load line can still be drawn in the usual way see the basic triode gain stage.

Generally we would like the load resistor Rk to be smaller than the exturnal load impedance that the cathode follower will be expected to drive. If it isn't, then the AC load line will rotate so much that output signal swing headroom will be much less than anticipated.

It is therefore desirable to use as low a value for Rl as possible, as low as we dare without taxing the power supply too much.

Operating at the highest anode current we can afford will maximise bandwidth and drive into capacitance loads. In this case an 18k load resistor has been chosen, which is low enough to maintain a reasonable swing into loads as heavy as 10k.

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In this example the HT is V. Biasing Since we are only desining a clean buffer we probably want to maximise headroom and minimise output impedance by choosng a fairly warm bias point.

In this case a bias of about The AC load line corresponds to an extrernal load of 10k in parallel with the existing 18k resistor, which is about the heaviest any normal piece of audio equipment is likely to present.

From this line we can see that the maximum output swing will be about 32Vpeak which is more than enough. The first is cathode bias or self bias, just like with an ordinary gain stage. A close standard is ohms and will do fine. Note that if the anode current is close to 8mA then the load resistor Rk will dissipate 0. It should therefore be rated for 2 watts at least. The grid-leak resistor Rg can be the usual value of 1Meg, although it can be made somewhat smaller while still maintaining a high input impedance.

This is because it appears bootstrappedi.

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This happens because the AC signal at the cathode is identical in phase, and only fractionally smaller in amplitude than the input signal. Obviously, the same signal also then appears at the junction of Rb and Rk, albeit very slightly attenuated. What this means is that at the bottom of Rg there is an AC signal almost identical to the signal at the top of Rg.

The difference in voltage between the top and bottom of Rg is hence very small, meaning very little AC current flows in it. Since resistance is voltage divided by current, the apparent resistance to AC signals appears much larger than the actual resistor value. Input capacitance Because the anode remains at constant potential the Miller effect is eliminated, and due to internal feedback the grid-to-cathode capacitance Cgk is also bootstrapped, i.

This is the same as saying the capacitance appears dividedand it was already very small to begin with! The total input capacitance is therefore less than 10pF and can be completely ignored.There are lots of things it could be doing… a second input, a line input, another gain stage….

Or take a page from the venerable Fender Bassman, and use it as a cathode follower. The cathode follower is a non-gain stage that separates the preamplifier from the tone stack.

Why would you want to do that? The tone stack, with multiple paths to ground through capacitors and tone control potentiometers, places a rather heavy load on the previous preamp stage. The preamp stage is high impedance and does not drive a load all that well.

The cathode follower has very low input capacitance and very high input impedance, so it is a very light load for the preamp and a better match impedance-to-impedance than preamp-into-tone stack. On the other side, it has very low output impedance and is a powerful current source, so it can drive the tone stack effortlessly.

The theoretical bark is worse than the actual bite, and despite all the impedance mismatches, lots of signal gets through to the tone stack. On the other hand, nobody ever accused a Bassman or a Marshall which copied the circuit of sounding bad.

Mark Huss did this mod on the green board Blues Junior years ago.

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He has instructions and some sound clips on his site:. C and D cream board. Photos are at the end of this page.

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I cut-and-pasted another tube section into the Blues Junior schematic. The half-tube connected by the colored lines is the unused half of V2, designated V2A.

The only additional component, other than the wires, is the cathode resistor. The voltage drop across the cathode resistor raises the cathode to a high voltage that gets modulated by whatever signal appears on the grid.

Unlike amplification stages, the cathode follower does not change the phase of the signal. What goes in is what comes out. V2B, by the way, is the tone stack recovery amplifier. This is the stock circuit board. The first step is to clip both ends of this jumper, close to the board.

Trying to pry or wiggle the entire part loose while keeping the solder molten is an excellent way to tear the solder pad from the circuit board. I used a 60 drill in a Dremel tool fitted with an inexpensive accessory chuck. The chucks are readily available by mail order and in home centers and hardware stores.So, instead, let's look into a much-misunderstood tube-circuit topic: cathode-follower-based power amplifiers.

I didn't know just how misunderstood this application of the cathode follower was until I came out with the TCJ Push-Pull Calculator programwhich includes the transformer-coupled, push-pull, cathode-follower-based power amplifier in its library of eight tube power amplifier topologies. Cathode-follower power amplifiers Cathode-follower-based output stages are rare—surprisingly rare. A conservative guess would be that Yet the cathode follower produces far less distortion and a much lower output impedance than the comparable grounded-cathode amplifier does when working into the same load.

How the cathode resistor cleans up vacuum tube amplifier performance

Furthermore, the cathode follower is not straddled with the Miller-effect capacitance that the grounded-cathode amplifier is burdened by. Before answering this question, let's review the assumptions. First of all, how much less distortion does a cathode follower produce than a grounded-cathode amplifier? Using a 6SN7, a cathode load of 20k, and an input signal of 1V into a cathode follower yields the following graph:. Note the absence of distortion harmonics beyond the third harmonic and the low distortion less than 0.

Not nearly as good. The total distortion is closer to 0. Is this possible? Still, it is fun to compare simulation to reality. Well, at least the 0-grid voltage line is close. But how good is the tube manual's graph? My guess is that most graphs in tube manuals are not born of a curve tracer's output, but only the result of a handful of spot point data lists and a French curve and a steady hand.

Consider this: most tube manual graphs existed before Tektronix came out with their famous curve tracer mid s ; what was used in its stead other than a high-voltage power supply, an ammeter, pad and pencil? No so good. Notice the bottom of the curve tracer plots, where it bends severely.

To be fair, I should put my own tube math model True Curves tm to the same test.

12at7 cathode follower

Live Curves model vs Sofia tube curve tracer plots. Not as bent on the bottom as the real thing, but quite accurate above 2mA for this sample 6SN7, other 6SN7s will, of course, trace a different set of plate curves.

One of the reasons is the bit floats that SPICE is limited to, opposed to the bit floats I have access to; another reason is the large exponents overwhelming the SPICE engine without the benefit of programmatic error handling and exponent limiting. Beyond the limitations of the SPICE model, a fundamental error in comparing these two circuits is at hand.Forums New posts Search forums. What's new Latest activity New posts.

Latest activity. Log in Register. Search titles only. New posts. Search forums. Log in. For a better experience, please enable JavaScript in your browser before proceeding. Thread starter Robert Herndon Start date Aug 15, Robert Herndon Ambassador of Tone. It works in that position, but really drops the volume level Anything else happens besides the volume drop?

This gives a very nice "chug" on muted F 's with complete clarity and it's very loud yet still controllable at stage volumes. Amp settings taken just now:. The 12AU7 in V1 reduced the gain too much for my liking, but tonally it was a very great, classic rock sound. In V2, with a standard ECC83 in V1, it has a really nice classic rock tone, but it loses its punch - you must run an overdrive pedal to get it cooking I have not tried 12AU7 in V3.

In V4, it reduced the amp's volume level about 2 dial position markers. Good clarity, but too much volume loss. The 12AU7 has great potential as a classic rock tone tool, but if you like higher gain levels, it's not the right tube, IMHO. Cheers Mitch. V3 is the cathode follower position, and the tube used here needs to be tough.

Spiral filament 12AX7 tubes have problems in cathode follower circuits due to the fact that most cathode follower circuits in guitar amplifiers exceed the maximum cathode to heater rating of volts. It's also said that you should avoid long-plate tubes in this position in the DSL's because it can cause unwanted feedback.Discussion in ' Tube Audio ' started by goldearOct 17, Log in or Sign up. What types of tubes are best for use as cathode followers? Messages: 8, Location: Seattle.

This is a question which I have had for a long time.

12AU7 / ECC82 Cathode Follower Tube Preamp Schematic

What I'm not clear about are the trade-offs involved in choosing a high-mu, high-gain tube like a 12AX7 over a medium-mu low-gain tube such as a 12AU7, or something more in-between such as a 12AT7. It seems to me like having a lower-impedance output would be and advantage.

But I'm not clear as to what makes one tube have a lower impedance than another. Is having higher-gain and degenerating all of it via the feedback in the cathode follower with a tube like a 12AX7 advantageous, over having lower gain, and therefore not degenerating as much of the gain in a tube like a 12AU7? I know that back in the 60s that using 12AX7s as cathode followers was common. But in the 80s and 90s it became more popular to used 12AT7s and 12AU7s. But I have no idea why that was, other than to assume that it had been learned that the 12AX7 didn't make for as good of a cathode follower.

Any insights that can be given here would be appreciated. Messages: 44, Location: Southern NJ. Usually it has to do with plate resistance. Lower plate resistance means it can produce more current, aka lower impedance.

12at7 cathode follower

If you think of a tube as being a resistor between the plate and cathode connections, thats what the "plate resistance" figure actually describes. Ohm's law tells us that less resistance at a given voltage means more current flows. Messages: 9, Location: Macedon NY.

Transconductance is important, as the higher it is, the lower the plate resistance. Though in some cases, 12AX7 will be good enough, depending on what it has to drive.Discussion in ' The Cellar ' started by surface54Sep 20, Log in or Sign up. Marshall Amp Forum. Joined: Nov 6, Messages: 1, Likes Received: Joined: Apr 13, Messages: 4, Likes Received: 1, I really like the Tung Sol Surface! It packs a nice thump. Marshall MannSep 21, I really dig it!

If this is a DSL amp, just remember V3 has two triodes. MartyStrat54Sep 21, AT7 sounds alot smoother, LPS is bigger.

12at7 cathode follower

If you want a lower-gain cathode follower tube, you'd be much better off using a Electrically it's a much better match. Personally I don't like using anything less than a 12AX7 in a Marshall circuit.

At best you end up with an amp that sounds like a Fender trying to sound like a Marshall. RiverRattSep 22, I've never seen that on any Marshall document about the DSL.

Do you have a link, or are you just looking at a schematic? MartyStrat54Oct 2, Lane SparberOct 2, Thanks Lane. I wish someone would have piped in about this over two years ago. I have been listing V1 and V2 as the only gain stages based on info I got off of a respected tube site. Thanks for the heads up, Big Dooley. A 12at7 in most cathode follower circuits is a bad idea. But why not try a 12az7. It has the same characteristics of the 12at7 and can handle the heater-cathode difference.

Just to add, a is great too. IMO, I think that you will get more out of the 12ax7 cathode follower pushing a tone stack unless you can make up for it with another preamp stage.

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You roll the tubes and be the judge though. You must log in or sign up to reply here. Show Ignored Content. Share This Page Tweet.


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