Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: terminalgs on September 26, 2011, 10:29:26 am
-
The EMI REDD 47 preamp uses two OA2 voltaqge regulator tubes:
schematic: http://i.imgur.com/WLkoS.png (http://i.imgur.com/WLkoS.png)
OA2 datasheet pg.1: http://i.imgur.com/cnpk3.png (http://i.imgur.com/cnpk3.png)
OA2 datasheet pg.2: http://i.imgur.com/d6O6S.png (http://i.imgur.com/d6O6S.png)
Since the OA2 is a 150v regulator, does using them in series as
shown in the REDD47 schematic produces 300v ? how is the
value of the 220k resistor in parallel with the top OA2 determined?
pg.2 of the OA2 datasheet says "sufficient resistance must always
be used in series with the OA2 to limit current through the tube."
In the case of the OA2, what resistors provide this?
the minimum supply voltage for the OA2 is 185, but it doesn't seem
like the minimum of two in series in this fashion doesn't mean 370v,
because of the 220k R. is that true?
-
Since the OA2 is a 150v regulator, does using them in series as shown in the REDD47 schematic produces 300v ?
Yes, think of the 0A2 as a 150v zener.
how is the value of the 220k resistor in parallel with the top OA2 determined?
I'm not sure that resistor is even needed, unless it's there to help insure the bottom tube fires. The exact value is probably not critical, but needs to be big.
datasheet says "sufficient resistance must always be used in series with the OA2 to limit current through the tube." In the case of the OA2, what resistors provide this?
The three 1K 2W wirewound resistors.
the minimum supply voltage for the OA2 is 185, but it doesn't seem like the minimum of two in series in this fashion doesn't mean 370v, because of the 220k R. is that true?
I don't think the 220K is a factor. That minimum supply voltage is just a recommendation to insure the tube will fire for a long long time (think tube life). The 0A2 has a parameter called striking voltage (also called ignition or firing voltage. That's the voltage required to cause the gas to glow which in turn causes the tube to regulate the voltage across it. The striking voltage is usually a few volts greater than the regulated voltage value. A new 0A2 strikes at 155V. So, really, you would need a minimum of 155V for a new 0A2 to fire. But as the tube ages, the striking voltage increases. Using a minimum of 185V insures that the tube will have a long useful life.
Since this circuit uses two 0A2s in series to produce a regulated 300V, you'll need to have a larger unregulated B+ input. 350 to 400 should be good enough, depending on the current load for the regulated 300V.
-
The mil-spec sheet says ionization is 165V, or 330V for two in series.
The REDD plan cited calls for 370V supply, 366V at the cold 0A2s. So it "should" fire both. However with the 220K, the bottom 0A2 feels the full 366V and ionizes for-sure.
When bottom 0A2 ionizes, assuming it breaks near 150V, the voltage across the 220K and the top 0A2 is about 200V.
Once you start to ionize, you want to bring the current up. Action below 5mA is undefined and wonky. Ideally we'd come up to 5mA, suggesting a 40K resistor. However after top 0A2 fires this becomes a 4mA leak around the top 0A2. Since we only have a 5mA-30mA range, now the string must pass 9mA-30mA. That does not leave much margin for E88CC variations or "370V" variation. 1mA initioal ionization is pretty solid, not to meet 144V-153V spec but enough to ensure "about 150V" drop so the upper 0A2 is sure to get enough voltage.
I don't think it HAS to be this way. You could lose the 220K and 99+% of units would fire; if not, you should replace the oA2s because they are maybe-marginal.
I don't like gas-tube hiss fed direct to amplifier stages, I don't think the circuit "needs" regulation. However it is a Holy Grail circuit so I guess you can't tamper.
-
PPR & SLUCKEY,,, thanks for the replies! makes sense now!