designing a new amp, from pieces of an old amp

[wsaraceni]

what types of things do you normally need to look at when designing a new amp, while using pieces of an old amp.

for example. let's say you want to take a simple preamp from one amp, ef86 followed by a tone stack with treble and bass and a volume knob. and instead of feeding a phase inverter and a pair of el84s, you'd like to make it single ended into a single el84. i know you can probably just take the output of the volume knob, and put it on the input of the power section of the el84, but who knows if it'll sound any good, work well, be too much overdrive, not enough. etc.

here is an example.
what if i wanted to take this preamp http://www.prowessamplifiers.com/schema ... matic.html

into this poweramp
http://audiohi-fi.narod.ru/cxems/Ac4.gif

while making it solid state rectified and no tremolo.

this amp will have an ef86 feeding an el84 and thats it.

now, the preamps on these two amps are very similar, so in theory, you should be able to make it to work. but what would you be missing?

[tubeswell]

Your PT:

- Assuming a 6.3VAC-0 or 3.15-0-3.15VAC winding, your heater current rating should be at least 960mA (plus the lamp draw, if you're going to run the lamp off the heater winding)

- For a SE EL84 I'd use 190-0-190 to 220-0-220VAC with SS rectification. Anything with a current rating from around 60mA minimum ought to be adequate on the high tension winding

Your OT: 7k - 8k reflected load. 5-10W gapped iron, or bigger if you want more bass.

And use a CLC whole-of-supply filter (with a minimum 50mA HT choke) in the power supply if you want a quiet amp.

[wsaraceni]

thanks. the spare transformers i have now are the 269EX (71ma) which should work but may be a little low on the voltage side. and the OT i have is the 125DSE. I can replace either or both of these if it made sense. but would prefer to use them since i already have them. for the choke. how do you determine the value? and placement. you run the PT off of the first cap and replace the 5w resistor with a choke? i have read that the single ended amps benefit greatly from a choke, but it is normally left out for cost reasons.

[tubeswell]

A whole-of-supply filter goes between the rectifier and the plate supply node. Look at the tweed princeton 5F2 schematic for location of the type of filter I am talking about.

[wsaraceni]

simple enough.

but now the bigger question. where can you learn to really design amps. to me, copying a dr z mini z would get me what i'd like. a cool single ended amp for home. but i really wouldnt learn anything from it and would probably end up better off just buying one. instead, it would be much cooler to do something slightly different as I dont see a reason to "clone" something currently in production.


assume you could just put the two together. it would look something like this. i could build this, but where do you go from there. appologies for the crummy ms paint mashup

[Luthierwnc]

You're going to need more filtration for a solid-state rectifier. Maybe look at 32 and 32 for the first two nodes. Also, your schematic shows 250v on both legs of that FWBR. That's like 650 volts DC! A 269 series transformer is more like 380VCT which puts you in the low 300's after full-wave (not bridge) rectification. A little low but it should work.

Go here for more power ideas:

http://duncanamps.com/psud2/index.html

Skip

[wsaraceni]

thanks. i happen to have a 32/32 can so i can use that. i just copied and pasted so there are is a lot that isnt accurate. i'll update it when i get a better program, or re-draw it by hand. i also assume the 22k resistor before the third cap should be 2w minimum

[wsaraceni]

how would this tone stack feed into the power amp. the ac4 has the resistor, but the 66 has the phase invertor. i'd assume i need to figure out the correct signal level and adjust that resistor accordingly. mathmatically, how would you go about doing that. at first, i was just trying to do this as an "exercise" in learning about amps. but if you think it might work well enough, i may just go ahead and build it.