smaller AB761

[Beezerboy]

I built the Hoffman AB761 a few years ago and I love it. now I want a smaller version, around 15-18 watts... is the Hoffman Prinston Reverb what I want?

[tubeswell]

Yes

[Beezerboy]

and... I just came across Rob Rob's version with no trem or reverb. thinkin I might go that route since I rarely use trem and have a Flint anyway.

I see in his version that he increased the values of the 3 voltage dropping resistors to keep things in line due to the fact that we are losing 2 tubes. how about using a different transformer?   I have a spare Hammond 290PAZ which is 290 0 290... it makes 402 in the Hoffman Plexi (yeah, I know... SS rectifier). the A/B voltages in Robs are 420/410

just a thought...

[sluckey]

Take a look at this. It will fit a 12x6.5 chassis.

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

Your PT would be fine.

[tubeswell]

Take a look at this. It will fit a 12x6.5 chassis.

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

Your PT would be fine.

Missing Ground on the layout for preamp channel knock-down voltage divider?

[sluckey]

Yes it is. Hopefully the note on the schematic covers that.

[Beezerboy]

man I like the look of that!  I have the Princeton O/T on order since that was the main part I needed to make the other design... thinking about switching up. Hammond 1750H would be right for this one? tnx

[sluckey]

Hammond 1750H would be right for this one?

That's fine.

[Beezerboy]

so... I completed the RobRob 1164 version a while back and have been fooling with it a while. Built to the diagram except the power transformer was different & is about 10% low on initial voltage.  it's bedroom loud.... but not venue loud. also, all the voltages after B1 drop more than 10% of the spec values. I replaced some of the dropping resistors and now all the nodes are close to (minus)10% values so even steven.

everything below about 6-7 on the volume sounds nice (guitar at 6-7). if I get much over 7 on the amp, it starts sounding pretty distorted. its right on the edge of being able to use this amp in a small venue but it blurts out. it did all this even before I raised the B3/4 voltages

I put a Warehouse Veteran 10" in the cabinet, and used the Hammond OT they make for a Prince Reverb (1760 something) . I'm thinking one of these or both gotta go. I want to carry that sound I have at 6 out to about 9

I can see the Slucky AB Lite on the horizon. I almost dumped this project in March to go that way.... now I wish I would have

[tubenit]

if I get much over 7 on the amp, it starts sounding pretty distorted

If you can retain headroom and not have the amp distort on you, you may be able to have more useable volume?


Try changing the post phase invertor coupling caps from .1 to .047.  Try changing the cathode cap on V1a to 10uf and V1b to 5uf. Change V3b to 5uf.   Try a 5751 in V1.

Sound any better to you?  Anymore headroom? 


IF you still need more headroom, you could experiment with 47k & 47k (or a 47k with a 56k) for V3b plate resistor.


With respect, Tubenit

[Beezerboy]

thanks for the suggestions.  I'll try some of those things when I get some time. may have to order parts too

Edit: guess I should have said I built it MOSTLY to RR's diagram. I added a mid pot and a 470k grid stopper on the phase inverter..... both of those things he mentions for that amp. I also added a post PI master volume by replacing the 1M fixed resistor from the grid to tail with a 1M pot (it works fine)