Starting my next build (finally decided)

[hesamadman]

So I have posted a lot of random things over the last few months. What it has finally come to is to use my Hammond 290ax PT and my recently acquired hammond 1750E OT, and build based off of the Fender Princeton Reverb B1270 power amp.
http://el34world.com/charts/Schematics/files/fender/Fender_princetonreverb_b1270.gif


I chose this for multiple reasons. One being that I had PT already and one of the factors was just determined when talking about my cathode biased 30 watt. I knew that cathode biased amps ran hot but I didnt really think much of it. I've been searching for a 15 to 20 watt cathode biased power amp to base my build off for a few months but after the cathode bias discussion I figured "why not just use a fixed bias schematic". I am not including reverb or tremolo on the amp.


I am a little confused by the bias circuit of this amp. The way that it is affiliated with the vibrato intensity pot is throwing me for a loop. Since I want to eliminate the vibrato all together, how will I incorporate a bias pot?

[sluckey]

Just like this...

     http://sluckeyamps.com/tdr/tdr.pdf

Except R38 will be a 100K/1W and will connect to the 330VAC lead from the PT.

[hesamadman]

No more questions your honor






For now 

[sluckey]

You may be interested in this Deluxe AB763 Lite amp. The only difference is the PI...

     http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf

Hoffman may have a version of this also.

[uki]

You may be interested in this Deluxe AB763 Lite amp. The only difference is the PI...

     http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf

Hoffman may have a version of this also.

How many watts this one does ?!

[sluckey]

About 20.

[hesamadman]

I like the lite version. Thanks for showing me these. They will definitely be good references. I had two questions.

1. Would vibrato be easy to implement into this circuit should i decide to use it? I've used a gretsch 6151 tremolo before but I'm not very experienced with tremolo

2. I saw you mentioned this was roughly 20 watts. I'm not very familiar with how the wattage works in an OT. Can I still use the 15 watt OT I have?

[sluckey]

Tremolo is easy.

I suspect a 15W OT will handle all the power that little PT can deliver.

[hesamadman]

According to hammonds website the PT is a replacement for a princeton as well as the OT

[hesamadman]

You may be interested in this Deluxe AB763 Lite amp. The only difference is the PI...

     http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf

I changed my mind about the PT. Im going to just go ahead and order a Hammond 290BX rather than the AX I already have.
Sorry to edit this message so much. I figured I would go ahead and ask a few questions at once as I am getting my parts list together.





-I have a handful of 22uf 500v caps. I always forget what Ive read about using bigger caps on the power supply filtering. I know they call for 20. Would my 22 be sufficient?


-I have never done a tube rectifier before.What is a good rectifier tube to use in this circuit?


-I dont understand the way tube rectifiers are are setup. Why does the 5v tap have a 220k drop down resistor and dual 20uf filters?


- Noticed the bias tap on your drawing is 38volts. According to the hammond PT specs, my winding will be 50v. Will I need to make any changes?

[sluckey]

Im looking at this and was curious how I could get my bias voltage using a http://www.hammondmfg.com/pdf/EDB290AX.pdf

See reply #1.

[Willabe]

-I have a handful of 22uf 500v caps. I always forget what Ive read about using bigger caps on the power supply filtering. I know they call for 20. Would my 22 be sufficient?

Yes that'll be fine, it's 2uF more, you'll never hear any difference.

-I dont understand the way tube rectifiers are are setup. Why does the 5v tap have a 220k drop down resistor and dual 20uf filters?

It has nothing to do with the 5acv heater, it's a bleed resistor to bleed of the dcv from the filter caps when you turn off the amp. It only kinda looks like that because of the way it's drawn. Remember the 5acv heater has the whole dcv (rectified) floating on it.

The 2 @ 20uF are in parallel so it's 40uF total. You could just use a single 40uF cap there if you want.

[sluckey]

-I have a handful of 22uf 500v caps. I always forget what Ive read about using bigger caps on the power supply filtering. I know they call for 20. Would my 22 be sufficient?

yes

-I have never done a tube rectifier before.What is a good rectifier tube to use in this circuit?

5AR4/GZ34

-I dont understand the way tube rectifiers are are setup. Why does the 5v tap have a 220k drop down resistor and dual 20uf filters?

The resistor and filter caps are not part of the 5V filament circuit. They are the B+ filter caps and bleeder resistor. The 5V winding is used only to heat the rectifier tube filaments (pins 2 and 8). The B+ is taken from the tube cathode and sent to the filter caps. Notice that the cathode is connected to pin 8 also. IOW, the cathode and filament SHARE pin 8 and there will be 5VAC as well as B+ voltage on pin 8. To correctly measure the 5VAC, connect one probe to pin 2 and the other probe to pin 8. To correctly measure the B+ voltage, connect one probe to pin 8 and the other to chassis ground.

- Noticed the bias tap on your drawing is 38volts. According to the hammond PT specs, my winding will be 50v. Will I need to make any changes?

Maybe, maybe not. The only change you may need to make is the value of R34 (sometimes called the bias range resistor). If your negative bias voltage is too high, increase the value of R34.

[hesamadman]

Thanks guys. That answered my questions. I did notice that my 22uf caps are 450volt. I know on your drawing Sluckey, it shows 500v. Fender used 450 in the schematics that I have. Is there an issue if I use these 450? Also, im guessing a rating of 50v is ok for the 50uf caps in the bias supply? Actually I have 47uf/50v here.