I took one of my Simple P-P boards that was assembled according to the instructions. Yesterdays experiments were done simply by wiring an octal socket in place of the 9 pin EL84 socket. THe screen grids were connected to a seperate power supply since sweep tubes and the 26E6 can't handle the full plate voltage on G2. A second variable supply was used to run the board.
Todays experiments were done by converting to fixed bias for flexibility. The cathode resistors were jumpered and the 470K grid resistors were connected to a negative power supply instead of ground.......Now that I think about it 470K is too big for sweep tubes probably helping the first 6BQ6 to runaway.
The Simple SE schematic can be found here. Operation at 500 volts was never intended, but I don't mind blowing stuff up.
http://www.diyaudio.com/forums/tubelab/167250-sp-p-final-connections.html
Look at the red board thread. I have squuezed 250 WPC out of that one and have been annoying the neighbors with a new build at 125 WPC using $4 output tubes.
http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design.html
Thanks George. I am ordering a quantity of 12BQ6GA's that I'm planning to blow up.
Scott
Hey George, since I have quite a few of these (thanks again) I got a naughty idea of what to do after I finish my grand daughter's mini-console project. (have the PA section done on that one now BTW).
I wonder how they might work triode strapped in an Inverted Futterman configuration (is that circuit patented BTW?). Thought it might be fun to try an OTL for the woofer amp in my main system. Could be fun, no?
I wonder how they might work triode strapped in an Inverted Futterman configuration (is that circuit patented BTW?). Thought it might be fun to try an OTL for the woofer amp in my main system. Could be fun, no?
Don't know much about the inverted Futterman configuration. OTL especially for a woofer will involve rather large current swings unless you have high impedance speakers. Just a few watts into an 8 ohm speaker will involve peak currents over an amp. This usually requires multiple sweep tubes or multiple voltage regulator triodes wired in parallel. I don't think the little 26E6WG's are up to the task. Look for tubes with big wide cathodes.
My limited OTL experiments have worked best with 6336A's or 6LW6's. Both have gotten rather expensive lately. There are several sweep tubes with peak current ratings in the 1 amp+ range that are still cheap. I look for them at hamfests in the $1 range, and got over 100 in Dayton this year. The 6AS7 works good too, but you need a few in parallel and they need to be reasonably matched. Matching is the hard part since they are all over the place. I have several hundred used ones in fair to poor condition. Someday I will rig up a test circuit to find some sets.
mmmm.... yes. I don't see any max current ratings in the data sheet. The data sheet for the 6LU8 for example gives maximum DC cathode current (75mA) and peak cathode current (260mA). The 26E6WG data sheet just gives values for a typical design (60-66mA). So guess I will probably be using an OPT with these. Possible exception is a linae array idea I have rolling around in the back of my noggin. 
Horizontal deflection types will work OK as OTL finals. Of course, you'll need to parallel up in order to get the necessary currents to drive the speeks. I considered a design like that based on 6BQ6s. To get the required current, they needed to be driven a bit into Class AB2, but that's NBD if you use a source follower grid driver. That does require more 6BQ6s, and the need for some serious heater power since the heater requirement for the 6BQ6 is: 6.3V @ 1.2A: 7.56W per tube. I have a bunch of 6BQ6s, but would rather hold them in reserve for a project I already designed and built. Still, getting ~40W from a single PP pair and more conservative operating conditions ain't half bad. (Condo living doesn't exactly play well with mega-powered VT amps, so I don't really need much more than 40W anyway.)
If/when I get around to doing an inverted Futterman design, I decided that something like the 6AS7 or the 6C33C would do better, in that these types can give the current while staying in Class A*1. The 6AS7 is especially attractive since you get two triodes in one bottle, and so need just half the number of holes in the chassis. (Even better would be hollow state up front, and source follower in back, but no glowey bottle coolness either.)
As for patents, the Futterman patent ran out a long time ago.
Of all the tubes I tested the 6LW6 wired in triode could pass an amp with the lowest voltage drop. I don't have my numbers handy but it was in the 50 volt range. The 6336A with both sections paralleled was almost as good. The 6AS7's were in the 100 volt range. I don't have any 6528's. Grid current was a given in all cases.
If you are choosing a sweep tube for any low impedance design like an OTL or a cathode follower, look at the peak cathode current spec. Go for at least an amp.
Thanks Miles. My woofers are currently running fine on 35WPC of sand so I don't need a lot of power either.
I figured that the Futterman patents were probably long expired but I didn't know whether the inverted version, being a later development, might be patented by someone else.
36KD6 is intriguing with reasonable price and up to 1.4A peak cathode current. Wonder what the mu and rp are for triode connection.
I figured that the Futterman patents were probably long expired but I didn't know whether the inverted version, being a later development, might be patented by someone else.
36KD6 is intriguing with reasonable price and up to 1.4A peak cathode current. Wonder what the mu and rp are for triode connection.
Most big sweep tubes have Mu in the single digit range (4 to 8) and rp in the hundred ohm range at the current we are considering.
I have been lucky at finding 6DQ5's (1.1 amp), and 6KN6, 42KN6 (1.5 amp!) at hamfests for $1 each. I also got 45 NOS 35LR6's for $3 each (1.3 amp). Anything that was EVER used in a ham or CB rf amplifier or audio amp is stupid expensive.
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