Rx For the Magnavox 8800 Series

Looks good now! A 450 volt can cap will be fine as that voltage rating is still well above the actual turn on surge voltage. Assuming you are using the same power transformer, it will work with an 8600 series amplifier, but the actual B+ voltage levels produced with the above configuration in the 8800 amplifier of this project are 310V, 305V, and 275 volts -- not the 255, 250, and 240 volts shown above. Therefore, for use with an 8600 amplifier, you'd likely want to keep the rectifier tube for the added drop, or add an appropriate resistor between the output of the diodes and the first filter cap, or possibly both. If a resistor is needed, it will need to be determined experimentally.

Dave
 
Dave - Big thanks for the thumbs up, I'm a visual logic schematic guy and a 65 year old newbie to tube amps, ears still intact except for the top end. And, as regards the 8600, your "Flea" series will give me all I need to know per AK postings. I thought replacing rectifier tubes with SS diodes was dangerous given surge voltage danger upstream. Thanks again.
 
coz; I recently experimented with the build-out resister you referenced previously. In my case I calculated ~39k of impedance was needed on the inverter output, resulting in pretty symmetrical clipping, without negative measurable impact to the output measurements. I don’t have the test equipment to measure THD, so don’t know if this method negatively affects THD as Dave mentions. Comparison of 1KHz before and after.

F614B976-1EF1-4055-BD43-C9C739192F11.jpegF9BBFF4A-3F2D-44A6-9329-E2EE75AC0CE3.jpeg
Bill


While waiting around in the airport lounge, I found some relevant information on the floating paraphase inverter in Morgan Jones' book, where he discussed the need for a build-out resistor to equalize the output impedance of the inverter, it might be worth trying when I get back from the trip to see how it behaves in the UL output stage. Here is the link to Google books.
 
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Dave,

I very much appreciate your "Flea Power 8600" guidance with simple schematics for visual folks, the 8800 is harder for me to see. You said:

If desired, the Bias, DC Balance, and AC Balance controls can be eliminated from this project as follows:

1. The resistors making up the Bias and DC Balance circuits can all be eliminated, and a single 220Ω 2W resistor be connected between the paralleled output tube cathodes and ground (along with the cathode bypass cap).

2. The AC Balance control can be eliminated by replacing the 430K resistor with one of two matched 470K resistors, and the existing 470K resistor with the other matched 470K resistor, that also has a 10Meg resistor connected in parallel with it. Tie the free end of the two matched 470K resistors together, and where they connect, also connect the free lead of the .047uF cap that goes to the bottom grid of the drivertube.


Any chance you (or anyone who has done this) can post a scribbled up schematic for the 8800 without the controls? I am a "paint by numbers" guy.

Thanks!
 
5DB6DF7D-AA55-40A3-B3BF-99F9229CAFFD.jpeg
Here is the front end from my edited working copy, l included the DC balance circuit in mine so it’s already drawn in. (Disregard my values.) I don’t have access to my PDF editing software anymore to change it for you. It is well described by Dave, draw it out how you think and we can take a look at it before you install.

Bill

Dave,

I very much appreciate your "Flea Power 8600" guidance with simple schematics for visual folks, the 8800 is harder for me to see. You said:

If desired, the Bias, DC Balance, and AC Balance controls can be eliminated from this project as follows:

1. The resistors making up the Bias and DC Balance circuits can all be eliminated, and a single 220Ω 2W resistor be connected between the paralleled output tube cathodes and ground (along with the cathode bypass cap).

2. The AC Balance control can be eliminated by replacing the 430K resistor with one of two matched 470K resistors, and the existing 470K resistor with the other matched 470K resistor, that also has a 10Meg resistor connected in parallel with it. Tie the free end of the two matched 470K resistors together, and where they connect, also connect the free lead of the .047uF cap that goes to the bottom grid of the drivertube.


Any chance you (or anyone who has done this) can post a scribbled up schematic for the 8800 without the controls? I am a "paint by numbers" guy.

Thanks!
 
San --

1. The two cathode terminals (pin8) should be connected together.

2. The 220Ω resistor (ideally 200Ω) should be connected in parallel with the 100 uF cap -- so that.......

3. The 220Ω resistor and 100 uF cap connect between the two cathodes, and ground.

I hope that helps!

Dave
 
I actually just today ordered two 50k pots and will finally add the AC balance into mine. Made some room under the chassis for it by removing the volume control and extra source switching I had installed awhile back.
 
Finally getting around to converting an 8802 to Dave's circuit and thought I would do a simplified version since I already have a quad of closely matched output tubes. Thinking of just installing bias test points and pots to keep the two channels' outputs the same but also keep the chassis drilling to a minimum. I had a couple of questions.

After installing the pots for bias, Am I shooting for 80ma total cathode current per channel?

Also, using the simplified schematic below, are there supposed to be 470k grid leak resistors off the outputs or did those go away when getting rid of the output balance circuit?

 
Mr. Guess's revision of my original schematic is (obviously) in error:

1. There should in fact be a 470K from pin 5 of each output tube to ground.

2. The 200Ω cathode bias resistor shown is a good average value to start with, but ultimately, optimum performance is produced when the quiescent cathode current is set for 40 mA per tube.

Dave
 
Late to the party, but I just picked up a 8802 last week. Still absorbing all this info.

Thanks to Dave for putting in a lot of work!
 
Ok, this is an opinion related question i realize but I finished reading the first 3rd of this thread. I just rebuilt an 8802 and a 175-67 at the same time. I'm trying to decide which one to keep. The 175 can def drive my speakers better with the extra 4 watts per channel but that 8802 is such a small footprint and sounds damn near a good but slightly less push...but after reading this thread, it sounds like the 8802 was an after thought by Maganavox. Has anyone done Dave's B+ upgrades to their 8802 and did it make a diff? Are these little guys worth tweaking more or should i just keep this 175 and move on with my life.....;-)
 
Regarding the 8802 with a simple refresh it is a respectable 6v6 amplifier with a pleasant sound, which Dave provides a baseline set of measurements for reference. His mods done in entirety result in a more powerful amp with excellent (HiFi) measured specifications all documented in this thread. The mods can be done partially which Dave explains, providing some improvements in the measurements. Mine measures about 90% as well as Dave’s final design, as I used less optimal output transformers that I already had on hand. I cannot speak to your 175 amp being a better choice or not. Because Dave took the time to document both baseline and final measurements for the 8802 that gives you something to compare the 175 against.

Bill
 

Hey now Mr. G I'd like to do this to my 8803 but leave the unit in my console and connected via the molex umbilical to the pre amp. Is this feasible, or do the necessary changes in the power supply make this impossible. Is there a different mod scheme that is better suited to my program? I'm looking to basically resto-mod a handsome little unit I picked up over the weekend.
 
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