A modified Magnavox 8802 in the Gillespie style

[dallen9494]

Thanks!

 

[mdye]

Resurfacing an older thread here.

I'm considering a build with spare parts that I have laying around here based on the final schematic posted in this thread. I've got a gutted Maggie 8802 chassis with original power transformer, and a pair of output transformers from a Harman Kardon TA224 receiver that used 6V6/7408 in PP. According to the Photofact for the TA224, these transformers are 6.2K primary with 8 and 16 ohm secondaries.

First off, I'm wondering if these OTs would be suitable to this design given that the original used an 8K primary transformer. Any thoughts?

Secondly, assuming the this could work with the tweaking of the feedback network, can anyone help me determine the correct resistor values that I should be using?

Thanks!

 

[dcgillespie]

I would actually measure your transformers to see what the primary impedance actually is. Sams was famous for getting the primary impedance wrong on a number of smaller amplifiers. For example, I believe that they show the 8800's original transformers as 6500Ω (or there abouts) units as well. The HK's transformers just might be higher as well.

The FB resistor simply sets the amount of feedback, but the response of the transformer determines how the response of the circuit is tailored -- and that takes a scope and square wave generator to properly adjust. I would suggest simply building the power amplifier section of the TA224, since those are the transformers you have. That way, the feedback and stability work is already done for you.

Dave

 

[gadget73]

^^ very much this. I've done some transformer swaps and more often than not, what you end up with for a circuit far more closely resembles the circuit the transformers came from than what you're starting with. There isn't anything special about the 8800 circuit to begin with, so its not exactly a tragic loss to swap it to something else.

 

[mdye]

Thank you both for your feedback. I was considering just using the TA224 circuit, and I think that maybe the A300 used the same transformers so maybe that's another schematic to look at. Would one just start the input at the last AF amp stage before the phase inverter?

 

[gadget73]

If thats whats inside the feedback loop, yes. Usually it'll be whatever is after the the volume or balance pot, whichever control is the last in the circuit. Not specifically familar with that amp, so I don't know what exactly its got.

 

[fxer1982]

I finally found some time to finish the amp I built for my son for Christmas. Thanks thorpej and dgillespie for all of your help! I just finished listening to it for an hour or two, I might have to give him some socks instead!

which output transformers do you have running this....if ya dont mind where ya get that rocker switch too??? ive been looking for the same switch...