Just for kicks I decided to build the ECC82 headphone amp circuit pictured.
My question is, R20 & R26 call for an 8.06k resistor. Would it cause any problems
if I just used an 8.2k resistor? I really don't want to order some special resistor.
My question is, R20 & R26 call for an 8.06k resistor. Would it cause any problems
if I just used an 8.2k resistor? I really don't want to order some special resistor.
I don't see why not. As it is part of the overall feedback, it will causes slight gain increase in the gain. More important might be matching, but you will probably not even notice a small gain difference. Go for it.
You could always parallel 8k2's with 470k... Odd that 8.06 is the only E96 value, the rest are E12 apart from a couple of 91k's
One more issue I hope you can help with. The filaments are fed by the emitter of T4 (BD139),
The tube is very slow to show filaments lighting up and I only get 9V across the filaments.
Wouldn't it be best just to short the E/C connection of T4 to deliver the 12v to the filaments?
The tube is very slow to show filaments lighting up and I only get 9V across the filaments.
Wouldn't it be best just to short the E/C connection of T4 to deliver the 12v to the filaments?
I see no reason to include anything north of the positive rail. Presumably it’s meant for some kind of supply filtering and anti reversal protection. If you have a decent supply it’s not needed. There are likely better options if supply filtering of a dirty supply is needed. I see 4 diode drops on the way to the filament or 2.4 volts. The tube still functions with the reduced filament as not much is asked from it except distortion for the tube sound.
It's kinda bizarre, it's got a reduced filament voltage (apparently a design "feature" of the circuit) AND a very reduced plate voltage (usually 150 to 250V). I don't think the circuit is to filter a dirty supply, it appears to do a "soft start" with C5 and R7. The time constant (to 63% of full voltage) is 39 seconds (!). "Real" tube circuitry warms up faster than that.
I've seen reduced plate voltage for a "tube mic pre" so it gives more "tube sound" than at the usual higher plate voltage, but I've not seen a lower filament voltage like that. Is there some discussion of this from the source of the schematic? I wonder if the purpose is to extend tube life or reduce cathode emission or what.
Also, it's a cathode follower configuration (and it's a good bit of transistor circuitry for a "tube amp"), which doesn't give gain, and surely gives a different transfer curve than the usual common cathode circuit.
I've seen reduced plate voltage for a "tube mic pre" so it gives more "tube sound" than at the usual higher plate voltage, but I've not seen a lower filament voltage like that. Is there some discussion of this from the source of the schematic? I wonder if the purpose is to extend tube life or reduce cathode emission or what.
Also, it's a cathode follower configuration (and it's a good bit of transistor circuitry for a "tube amp"), which doesn't give gain, and surely gives a different transfer curve than the usual common cathode circuit.
Agree. I don't see any positive function of a start up delay in this circuit, much less a 39 second time constant. The circuit seems more suited to a guitar effects pedal and not a headphone amp. Still if the result pleases someone, then who am I to say its wrong.
Looking at the circuit diagram I see (left channel) 1 amplifier stage followed by an emittor follower, a cathode follower and again an emittor follower (darlington pair) in the signal path (and a feedback configuration). What is producing the 'tube sound" in this amplifier? That ECC82?
I understand if you wanted to use your junkbox parts, and it sounds if you are up and running with the 8.2K resistors. However, 8.06K is a standard 1% resistor. ~50 cents at Mouser with plenty of stock.
https://www.mouser.com/ProductDetai...=sGAEpiMZZMtlubZbdhIBINbvJI4P3EasQL63kwWizNE=
Reiterating post #3, why is that a 1% value when those around it that also determine gain and feedback, R16, R17, R18, R21, are 5% values? if the others don't need to be 1%, then it seems some sort of waste or overkill (or whatever the word might be) for R20 to be. One percent values don't cost much more than 5% thesedays, but this looks like an engineering inconsistency.
I need to be careful here, but the whole design seems to me to be an engineering inconsistency. Its not clear what the intent was. Its something however.
Tubes behave awkwardly at starved plate voltage, and messing the heater voltage is a viable way to manipulate with the characteristics. The designer may have tweaked it down to 9V deliberately. Feel free to play around with it though, it won't explode.
"Tubes behave awkwardly at starved plate voltage, and messing the heater voltage is a viable way to manipulate with the characteristics. The designer may have tweaked it down to 9V deliberately. Feel free to play around with it though, it won't explode."
So, why not using a car radio tube type in this amplifier?
So, why not using a car radio tube type in this amplifier?