Updating my 5 watt SE amp with Merlin's medium gain preamp

[pullshocks]

 So here is my old 5 watt single ended 6V6 amp, crying out for a new preamp and a few electromechanical improvements. 
The power amp is based on the AX 84 5W SE Building Block, but with tube rectifier, extra filter stage, and a choke.  It has a nice Heyboer OT. 
In the picture it is wired up with the AX84 “Blues” preamp which has a 7199 pentode front end, followed by a 12AU7 wired in parallel with LED bias.  I liked it at first, but as time went on I didn’t find the “squish” control that useful.  Also, for whatever reason, the designers did not include an input grid resistor and the amp tended to pick up radio signals.  And when my old Westinghouse 7199 crapped out I found replacements are hard to come by. 
So I was ready to try something new, and I have long wanted to give Merlin’s “Practical medium gain blues/rock preamp circuit” (From Designing Tube Preamps for Guitar and Bass, 2^nd edition) a try.   
Space is a little tight around the preamp tubes but through the wonders of DIY Layout creator I found that by rotating the tube sockets 90 degrees and adding a few more turrets I can fit  the Merlin preamp and others I might want to try.

[pullshocks]

 One other thing I needed to attend to was a deteriorated convenience outlet.  Yikes!
 

[pullshocks]

Much better

[pullshocks]

I really need to order some of the chassis mount single turrets for grid resistors, but here is my shop made substitute.  There is a spacer so no issues with chassis contact

[sluckey]

One other thing I needed to attend to was a deteriorated convenience outlet.  Yikes!

Easy to tell which is the hot side!   

[pullshocks]

Other than forgetting to reconnect the input jack ground, and master volume ground, start up went pretty well.


So far I am really pleased with it.  With Duncan '59 neck and Pearly Gates bridge pickups, it is pretty clean up to about 1 1/2, and then the overdrive starts ramping up as Merlin describes in the book.  Dialing the guitar volume knob down a couple tenths, cleans it up.


The "Bandmaster" tone stack is going to take a little getting used to.  I somehow ended up with the Bass control wired backwards


I put the bright cap on a switch, but with my hearing it is kind of hard to tell the difference/

[pullshocks]

One other thing I needed to attend to was a deteriorated convenience outlet.  Yikes!

Easy to tell which is the hot side!   

Yes that was an exciting moment when the 50 year old bakelite (or whatever other plastics they used back then) gave out.  I was plugging in a wall wart and it must have been tilted down slightly.

[davidwpack]

You're probably aware but they make adapters for the 7199 to accept 6U8 and 6GH8 tubes. I have them in two different Ampegs and like the 6GH8 a lot.

[pullshocks]

Finally getting back to this project.



Discovered I had used a 100K pot for treble instead of 500K called for in the schematic.  With the correct value, the tone stack works better, but still not really to my liking.   Just hard to find a tone, and when the bass pot is turned up past 7 or 8 bass tone kind of levels off and a weird more muffled tone comes on.


 May try replacing it with a TMB tonestack, or may move on to a completely different preamp circuit,

[shooter]

move on to a completely different preamp circuit,

 

This one I like, it's my "to upgrade version" the TS "in-circuit" is what I have, kinda cool, 3 settings; left side right side and max/min.  the one in the box is the upgrade one
fwiw
dave

[pullshocks]

Thanks Shooter  I found your original build thread on that amp.  Hats off for doing such a unique build.  Do you have any recordings with the amp?

[shooter]

have any recordings

Nope, don't play.  I get them test drove by my Gigging Son n his fellow guitar nerds.

It was designed to allow the player to play n not have to mess with knobs n such except at the guitar, the preamp can OD, the PA can be pushed into nice crunch.
pretty good range from easy jazz to 90's grunge.

works well with my 2X12 and single 15 but not so well with "underrated" speakers

[pullshocks]

That's a really cool father & son story.


Yeah, speakers can make a difference.  Today I took out the Eminence Ragin Cajun and swapped in a 12" baffle and an early  90's Celestion greenback, and kept the bass and treble controls half way up


I'm going to try a couple other speakers and some  lower gain tubes before I tear back into the circuit

[shooter]

lower gain tubes

typically use AU7s in the pre.  while they can "drop-in" an AX wired circuit, they really want the circuit re-adjusted to taste
 

a scope is handy so you can "set" clipping n distortion, then go back with a players ear for the fine stuff