Posted on Jun 23 2012 11:54 PM
"it appears to use a 'small' 9-pin driver tube. As such, it can't produce the 'drip' of a real 6G15 Fender reverb unit."
Not true. In fact, this statement is libelous. Good thing for your I'm a nice guy, and I think your comment ignorant rather than malicious.
I don't wish to start an argument with BJB, but the statement appears to be grounded in the mistaken notion that unless a tube has an octal base, it cannot be a power pentode. You would all be well advised to get an RCA tube manual and look at it a little more closely than he apparently ever has. Take a look, for example, at the 50C5, which is a miniature 7-pin version of a 50L6, which is a 50-volt filament version of a 6L6, which is substantially more powerful than a 6K6.
My circuit uses the power pentode half of a dual triode-pentode to drive the tank and is more than up to producing a dripping wet output. If anyone wants the mp3s that show this they can contact me at tubekit101 at hotmail. There are many big files. Multiple megabytes.
Another mistaken idea that seems rampant in these forums from those whose knowledge is only superficial is that a transformer is needed to drive a reverb tank. It depends on what the reverb tank is designed to be driven by. In this case, the tank has a high impedance input coil (high being very relative here; actually it is only 1450 ohms), not the 8-ohm impedance of the Fender unit. An 8-ohm tank is like a speaker and needs a transformer (which limits its frequency response). A 1450-ohm tank is a suitable load for the plate circuit of a pentode directly, without a transformer (using a capacitor to block DC). Moreover, this direct coupling of the driver output to the tank allows a wider frequency response than sending the signal through a transformer. See the Reverberocket circuit for an example of a transformerless design. Then listen to that amp and tell me a transformerless design can't sound dripping wet, if you still think so.
The Fender design uses five gain stages: Two dual triodes and a pentode driver. One of the Fender gain stages is an "extra" 12AX7 triode gain stage which is used simply to invert the signal. Look at the circuit. The first stage output is thrown away in a resistor divider that feeds only a fraction of the signal to the next stage. All it does is invert the signal to match the polarity at the mix point.
Our design uses only four gain stages, eliminating the redundant inversion only stage by simply reversing the leads at the tank input. (Fender could have done the same thing by reversing the leads of the transformer primary, but they had an "extra" gain stage and used that instead.)
The two tubes are a 6EB8 and a 6KE8. The 6EB8 contains a high gain triode (similar specs to a 12AX7, AF=100) and a power pentode which can feed up to 25 mA into the tank--more than enough to drive a 1450 ohm coil. In fact, you can set the drive conrol so high that you are overdriving the coil so much that it is beginning to distort in the tank, if you want that effect. We haven't tested to see how long you can do that before burning out the tank, but you have the freedom to do that if you wish with our design. If you choose, you can operate it within its rated input specs and get as much dripping wet as you can stand because the mix can be set where there is virtually no dry signal at all.
The 6KE8 is another dual triode-pentode, but in this case, the tank recovery amp pentode is a very sensitive high gain pentode (high voltage gain, not high current gain, like the other) and produces a gain of twice what you can get from a 12AX7. It's partner triode is a medium-mu stage that functions very nicely as the cathode follower to buffer the dry input from the guitar and lower the output impedance.
If you've never heard the device, shame on you for dismissing it as unworthy of your attention with a load of ignorant falsehoods.