Partially-bypassed means the value of the bypass capacitor is not large enough to completely bypass the cathode at all frequencies. The solution to your triode mode problem is more filtering in the power supply. I’m leaning towards single-ended 6V6GT or 12AB5 but will hold off making that decision until the power system is up and running. In this case, the price you pay is in power dissipation. They also feature cathode followers but not DC-coupled ones, if I remember correctly. A turret board provides better mechanical reliability and ease of maintenance or modification. The “driver” tube, or phase inverter as it is more appropriately known, is under no more stress than any of the other preamp tubes.

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Still needs a ton of tweaks. There is a world of difference in the tone of an improper PCB layout and a proper one, just as there is between a proper PTP layout and an improper one. Aug 8, In parallel with Facebook post on your site Yes, I want more technical info, a lot cathodde, as do many others I know. Go here for more info and a handy web calculator followdr capacitor lifespan: While the amplifier is biased at idle to a point not exceeding the plate dissipation, which seems okay on the surface, this is only half the story.

Vibro-Champ with a different tone stack is near the top of my mind.

Which is the cathode follower?

Then install the power tubes. I don’t use eyelet boards, because they don’t allow a good mechanical connection; instead, they depend solely on the solder for mechanical strength at the connection.

So it’s hard followerr know what tube is right for what application. For example, if you have a standard Bax stack with a couple of 1Meg pots, you can assume the impedance plot of the network looks like a maximum of around 1Meg at some frequencies. That’s what it’s job is – to shut the amp off to protect it in the event of a short circuit.


An amplifier typically has large filter capacitors that can draw quite a bit of inrush current when the amp is first turned on and they are fully discharged, and this can occasionally cause a fuse to blow, particularly if the fuse is old. The presence of a phase inverter tube does not automatically mean the amplifier is class AB, just as the presence of a cathode resistor doesn’t automatically mean the amplifier is class A.

In addition, since the output stage and phase inverter don’t have a gain control stuck in the middle, the presence and depth controls have the full cathore gain of the output stage to work with, so you can have wide ranging high and low end boosts from these controls.

Which is the cathode follower? | The Gear Page

You can vary either the feedback resistor, the shunt resistor, or both. Power handled by the Rk is 11V x 0. Since fol,ower tube’s control grid is a very high impedance, much higher than the value of the resistor, the resistor value also sets the input impedance of the amp.

That had more tubes than a twin reverb! This value is not large enough to completely bypass the cathode at all frequencies, so you end up with more gain at midrange and high frequencies.

Amps that get their distortion exclusively from the preamp section will function much better with an effects loop, because the output stage is designed to run clean, so the effects can be used to better advantage. Is it the loop itself, or what you put in the loop? All single-ended output stage guitar amps operate in class A mode. You can use a load-line superimposed on the characteristics of the tube to determine the max plate voltage, idle current, and load impedance that will allow you to bias the tube to the optimum class A operating point.


In fact, unless you know the plate voltage, plate bias current, and output transformer reflected impedance, you can’t tell the class of an amplifier just by looking at the schematic. Luckily I had a bunch squirreled away from an old amp I parted out. Assuming you were trying to drive into a K load, you’d want to scale your Bax stack to an impedance of 10K or lower.

Can’t wait to begin firing it up!

The AC coupling capacitor coupling between the phase inverter and the output tubes precludes any flow of grid current anyway.

May 28, 5. So instead there’s clever use of the multimeter, breadboarding, experimentation, and sinking extra time in, since I’m resisting doing things the right way.

In push-pull class A, one tube amplifies the sine wave, the other amplifies an upside-down version of it degrees out of phaseand the two complete signals are added together in the output transformer to produce a new sine wave of higher power than you could get with a single tube in the single-ended configuration. The best way to get rid of the hum is to generate a DC filament supply, but this isn’t always practical, because of the high dissipation caused by the large current draw of the filaments.

Never built a fixed-bias amp before and so never gotten to play with power tube bias-vary tremolo. The output of an amplifier, at 4, 8, or even 16 ohms is, for all practical purposes as would relate to a tone stack, an zero ohm source, so you have that part covered.