The Amp Garage

Is this a head? or a combo? (How close is the pan's output transducer to the PT?)

My amplifier seems to have a funny tone stack where there is only a treble and mid cap... but no bass cap. I think the 'mid' cap actually works interactively with the bass pot value and affects its frequency range also. If you have an iPhone or iPad, there's a great app called Tonestack Kit. https:...

^What pdf64 said^

And use your R-meter to confirm you have DC continuity between each filter cap ground lead and the main ground return point.

Yeah that will also work. But in the SVT, the ‘fixed voltage’-end of the CF load resistor is referenced to a really negative voltage, so that the 12AU7 cathodes can be DC coupled directly to the output tube grids (to eliminate blocking distortion). The CF load resistance doubles as the output tubes’...

Check that your preamp filter caps are actually grounded. (If not, the preamp gain stages will interact/couple with each other through the power supply rail, because the dropper resistance ends up acting like another load resistance in series with the plate resistors, causing a fizzy mushy sound)

For the PT, I am specifying either 400VAC or 410VAC at 450-500mA secondary, a 55VAC bias tap, and two 6.3VAC taps; one for heaters (9A) and the other for accessory stuff (3-4A). For my bar napkin math using a 400VAC secondary... With a 400VAC secondary, 400 * 1.3 = 520VDC rectified under load 520VD...

Protection mode can trip in for a number of reasons, including that the protection mode control circuit is powered by a secondary winding from the filament PT, and the filament winding from that same PT is connected to the output tube board with push-on spade terminal connectors that arc when the co...

R.G. wrote: ↑Mon Aug 12, 2019 2:07 pm I... I thought about recommending a full wave rectifier bridge to replace the single diode, but at the low voltages in a heater supply this might not give enough DC to properly run the fan, as a bridge has two diode drops instead of one. ...

Voltage doubler?

2.1mm switching jacks

Bad cat with the insanely high EL84 voltages? Maybe failing EL84s?

g2 voltage affects gm. Higher Vg2 = higher gm and vice versa. So if your Vg2 supply is lowered, it will take a bigger g1 input signal for the given output signal at the plate. This isn’t ‘compression’ as such (although the difference is moot to some). Rather it is decreased input sensitivity. But an...

It won’t do any harm. If the resistor is appropriately power-rated, it won’t get hot enough to bother the filter cap.

V1a is supplied by R76 V1b is supplied by R81 Which ‘V1’ do you want to change? Whichever one it is, going from 216V to 200V won’t make any difference in the way the amp sounds. 16v is not a significant enough change to make a noticeable difference. However, if you wish to find this out for yourself...

There are various ways. You could install a voltage regulator in the HT rail after the reservoir cap, with the output voltage set at the desired ‘B+’. RG Keen has some MOSFET designs. Merlin Blencowe also covers this topic in his Designing Power Supplies book. If you use a big-ass enough tube, you c...