Pseudo Kalamazoo Model 2 Build Questions

[TurboGuitarMelton]

Hi


I built a sorta Kalamazoo model 2 with a 12ax7 preamp, a 6av6 oscillator, and a 6v6 output. My preamp plate voltages aren't what I would expect. I'm used to seeing Fender preamp plate voltages that are  170v or so and these are all the way up to 250v! I am 90% sure that the reason they are so high is because the 4.7k Cathode resistors  aren't drawing the current that a 1.5k would. However, when I initially built the amp I noticed that the tremolo was much weaker with the 1.5k resistors in there so I put in the 4.7k resistors the original KM2 schematic called for instead.


If I want the lower preamp voltages and the stronger tremolo should I just bump up the power resistor going to the preamp node or up the 100k plate resistors? Or is there a tweak I can make to the tremolo so it will work better with 1.5k resistors? Lower the 150k oscillator tube plate resistor so that the tube can swing more voltage with 1.5k cathode resistors?


Here is my schematic with voltages


Thanks!

[sluckey]

If I want the lower preamp voltages and the stronger tremolo should I just bump up the power resistor going to the preamp node

Yes. You show 22K but the orig. schematic by Miles shows 100K. Change that 22K to 100K and your V1 voltages will be more in line with the original. Your other voltages all look fine to me and I would not do anything with those. However, Miles used a 250Ω "sag" resistor. If you don't have a sag resistor you can lower the other voltages by using one.

I would not mess with any cathode resistors or plate load resistors. Everything you want to do should be done in the power supply. Here's the original Miles schematic in case you have not seen it. The schematic you posted is an edited copy of Miles' schematic.

     http://sluckeyamps.com/misc/K2.pdf

[j_bruce]

Interesting build.

I have a Kalamazoo Model 2 that I use for harp and I have been wondering if I could beef up the power by running two 6bq5 output tubes in parallel with the proper output transformer and get a similar tone. The real debate is if the tone comes more from the speaker than the circuit? If so I would not be able to get the same tone.

I don't use the tremolo so that could go away if I built a new version.

Good luck with it!

[DummyLoad]

Interesting build.

I have a Kalamazoo Model 2 that I use for harp and I have been wondering if I could beef up the power by running two 6bq5 output tubes in parallel with the proper output transformer and get a similar tone. The real debate is if the tone comes more from the speaker than the circuit? If so I would not be able to get the same tone.

yes. 1/2 the reflected load and double the OT & PT power ratings. i'd run individual cathode resistors for each 6BQ5.

the speaker is what you hear, but the shape of the signal sent to the speaker is all the amp.


--pete

[PRR]

> beef up the power by running two 6bq5 output tubes in parallel with the proper output transformer

You need a bigger power transformer too. (Maybe not: at this scale a 1*6V6 amp often uses a 2*6V6 PT, because the smaller part is not enuff cheaper to justify.)

2X power is NOT a big difference. Far less than "twice". For the specific case of a very steady drummer and a 1-tube amp being 66% of drum volume, 2-tube may get you to 100% of drum volume and a fair fight. However the drummer can play louder, and most will.

A better bigger speaker will make a small amp big. 4x10" cab is hard on the back but blows-down any single-speaker cab.

[shooter]

I would not be able to get the same tone.

I like to think of tone unique to an amp
speakers being a big part;
would highly encourage what PRR suggested, plug your II into a Marshall 412

If you want to go future, spec Iron for a KalamazooII pre, SE 6L6 and a 212 cab, the neighbors WILL know