The Amp Garage

Hi All, For the past year or so, I've been trying to reverse engineer the Tubeworks RT2100 Mosvalve amplifier from it's schematic, cause I wouldn't mind either A) Building a Low-power version, or B) Condensing it into a lil pedalboard PA or line driver. The problem is that I cannot for the life of m...

The 560v B+ is from the mfr of the PT (Mercury). The others are simply guesses/targets. From previous threads about the choke-input screen supply I should expect 60%-65% of B+ there. I'm probably being optimistic by targeting 350v there. The preamp and bias voltage are easily adjusted by resistor s...

Every chance I'm asking a stupid question, but how did you get those numbers for the DC voltages? The 560v B+ is from the mfr of the PT (Mercury). The others are simply guesses/targets. From previous threads about the choke-input screen supply I should expect 60%-65% of B+ there. I'm probably being...

Every chance I'm asking a stupid question, but how did you get those numbers for the DC voltages?

With a safety resistor that is tens of times larger than the nominal speaker load the output is greatly reduced. You don't need wattage anywhere near the amp's rated output power. So purely for a safety resistor, I could get away w/ a pair of 960Ohm 25W Wirewounds in parallel? 30 * 16Ohm = 480 at W...

As I see it, the point is that to avoid the protecting resistor's resistance overly affecting the speaker's frequency-impedance characteristics, its resistance should be about 3x the speaker's peak impedance. So about 30x the nominal speaker impedance. Ideally, the resistor's power rating should be...

trobbins wrote: ↑Fri Aug 14, 2026 4:29 amYup

Ah brilliant. I used your Eminar 70W bass doc as a guide to reverse engineer (And now Re-design) my own for years. I believe ours have the same OT.

With a square-wave-like output the tubes are essentially switching between saturation and cutoff. Without a load rapid switching into cutoff can generate high (kV) flyback spikes on the OT primary due to the energy stored in the (e.g. leakage) inductance. A base load resistor across the output help...

As I see it, the point is that to avoid the protecting resistor's resistance overly affecting the speaker's frequency-impedance characteristics, its resistance should be about 3x the speaker's peak impedance. So about 30x the nominal speaker impedance. Ideally, the resistor's power rating should be...

Under the same premise, would a parallel pair of 50W 330Ohm resistors be an effective failsafe for the 16Ohm tap on a 75-ish Watt amp? Like in the event I don't have access to a reactive load, I could still do some live measurements? Or in this case would it be better to do a pair of 33Ohms for appr...

This Reco may be for a bit further down the line, but when you have somewhat of a handle on general electronics, there is no book better than the Art of Electronics by Horowitz and Hill. Just endless circuits tidbits. That is a solid read. In that same vein I'd recommend the Radiotron Designers Han...

This Reco may be for a bit further down the line, but when you have somewhat of a handle on general electronics, there is no book better than the Art of Electronics by Horowitz and Hill. Just endless circuits tidbits.

1/8W gonna be a fair bit hissy ya reckon? No discernible difference in hiss. Hiss is thermal noise , a product of the resistance value and the temperature around the resistor. The size and material are not a factor, only the resistance and the ambient operating temperature. Current noise is related...

There's not supposed to be any DC on those pots, power is a non issue. The reason we tend to like bigger higher power pots is they are typically lower noise 1/8W gonna be a fair bit hissy ya reckon? Doesn't sound like you're building something high gain, I doubt it will be an issue. Even then, I've...

Snicksound wrote: ↑Fri Jun 19, 2026 7:40 pm There's not supposed to be any DC on those pots, power is a non issue.

The reason we tend to like bigger higher power pots is they are typically lower noise

1/8W gonna be a fair bit hissy ya reckon?