FLEX 15 Background
I designed the Flex 15 after receiving many requests for an amp that sounded like the Rock Block but with more power, 2 channels, and an effects loop.  It started out being a 15 watt amp (thus the name), but ended up being 25 watts.
What Rock Block owners seemed to like most about it was the overdriven tone and the pick dynamics.  I had already determined that the key to acheiving these attributes was to use a singled-ended output stage and no negative feedback.  The singled-ended output provides more even harmonics when overdriven compared to a push-pull output stage.  Also, triode operation provides more gradual clipping compared to pentode operation.  Negative feedback, used in most guitar amplifiers above 5 watts, results in a harsher overdriven distortion and reduces the dynamics.  No negative feedback is used in the Flex 15.
My own experience with amps rated at 15 watts and more was that the clean tone was OK, but when the output tubes were overdriven the distortion was too harsh and "ice-picky" for my tastes.  My goal was to design an amp that had both great clean and overdriven tones.
I decided that this new amp should provide about 25 watts clean and 50 watts distorted with a switchable 5 watt mode.  This power level would be usable for most gigs and practicing and would be ideal for recording.  I couldn't find any examples of 25 watt singled-ended triode guitar amps so I had to design the Flex 15 from scratch.  I knew it was crazy to design a 25 watt single-ended triode amp, but I decided to try.
I looked at triode curves for several types of power tubes before determining that EL34 and 6L6 tubes would be the best choices.  3 power tubes in parallel would be needed to acheive the desired power level.  There is also one 12AX7 and two 12AT7s.  One 12AT7 is used to drive the output tubes and one is used to buffer the effects loop.   I found that very large transformers are required for this configuration.  The transformers used are as large (and expensive) as those in 100 watt push-pull amps.  The output transformer weighs 11 pounds.
It took me about one year to get the prototypes working as well as I had hoped.  Most of that time was spent eliminating hum (more difficult with single-ended designs), eliminating oscillations (harder with no negative feedback), and making the channel and effects loop switching as quiet as possible.
The final results have far exceeded my expectations, and I am very excited about this amp.  I'm confident that those of you who give it a try will find that it is the best amp you have ever heard.
Kim Hardee
Surprise Sound Lab
Rock Block
Flex 15
and Demos
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