Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More!

First post. Had to do it to thank Mr Gillespie. Thank you!! I had an chassis and power transformer that ended up working out very well. Voltages are almost spot on. Used 100 ohm resistor and 100 ohm hum balance pots for the bias setting. Hooked up a cd player and some Heresy speakers and was really quite blown away. Dead quite and the sound is fantastic! Once again thank you for the wonderful amp!
 

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Hi Tube -- Welcome to the AK Tube forum! And thanks for reporting on the results of your build! Gotta love it when you can re-purpose existing components into a new device -- some of the best projects start out that way.

Glad you're enjoying the amp through your Heresys -- I play mine through Cornwalls, and it amazes me every time I use it.

Thanks for giving the circuit a try, and the AK Tube Forum as well!

Dave
 
I guess Klipsch is the way to go. I'm using KG 4's and the little amp sounds fantastic!

I bought one of Wharfcreeks amps in April.

After a few weeks of listening I'm still amazed how good it sounds. It's been going for a couple hours a day and it's been perfect.

Thanks Tom and Dave!
 
I just wanted to post an update. I got some time to finish up my rebuild and I can say I am very impressed. Thanks to everyone for inspiring my to take on this project. I learned a lot and should have gotten to know tube amps better a ling time ago.

Right now my bias current is running at 48.7 uA, is that too high?

Here is a pic.
 
Sphinx -- Congrats on getting your amp up and running! With the voltages posted on the schematic, the ideal output stage cathode current draw is 49 mA (i.e. milliamps, which I assume you meant rather than uA, which stands for microamps). If that is is the current draw your tubes are producing, then that is perfect!

Time to enjoy some good music and your hard work!

Dave
 
20180523_122507.jpg I have been reading Dave's posts in the thread on More fun with Magnavox 93xx about adding a input level control (I got so excited about the results on 8200 that I picked up a 9302 to work on). In posts 388 and 413 mentions that adding a volume pot to the input drops off some of the HF response. I added a 100k pot that I keep in 60% to 75% range to the input of my 8200. Did I limit the HF by doing it without a preamp of some sort? Here is how I hooked the pot. Sorry for the bad drawing skills.
 
Sphinx -- in this case, you should be fine, because the value of your resistors are so low. If you're driving the amplifier straight from a CD player or other SS source, then the drive impedance is such that you could even use a 50K pot, or 10K and 39K fixed resistors if that is the way you're going. In either case, if the new pot or fixed resistors are a permanent addition to the amplifier, you can remove the original 470K resistor or leave it in as you wish.

Dave
 
I think that the capacitance of the interconnects will have the major effect on hf response. Lowering the resistance at the input should negate that somewhat, my controller switch point pot is 10k by itself, any other parallel resistance in the circuit lowers the resistance load the driving source encounters further. A robust line drive capability of the source is desirable for both interconnect capacitance and loading resistance.
 
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Hi To All. I am new here and owns a 8601. I have a blown output recently and gotten a pair of replacement output from Antique sound. Model: PT31 as shown below.
It works but it has a loud siren background noise.
Can anyone teach me how to resolved this loud siren ring coming thru the speaker? Any schematic to show what to remove or change?
My set is still stock except wired to play as stereo....was running very fine then.

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Blitz -- Very likely you new transformer is wired with the phase reversed from that of the original transformer. If you currently have the transformer wired with the Blue lead to the 6BQ5, the Red lead to the power supply voltage, the Black lead to ground, and the Green lead to the speaker, then it is most surely reversed. Correct the problem by disconnecting the Green and Black leads and reconnect them in reverse of how they were connected. That is, ground the Green lead, and connect the Black lead to the speaker connections.

I hope this helps!

Dave
 
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Thank you Dave
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Hi all. Well my wife and I have enjoyed our 8601-20 with Dave's mod for quite awhile now. An old friend gave me a set, (don't know what console he took these out of)
8601-20 amp, micromatic turntable, and a 7911-00 tuner/preamp. All are in mint condition, both cosmetically, and electrically. I will change the output impedance to 8 ohm like I did my last one. I will build a cabinet for these. My question is this. Other than new 8 ohm opts, what more can I do to this 8601-20 as per Dave's mod, and still operate it as original, and still use the tuner/preamp tone controls?
 
MrAqua, your question is really a good one. As I see it, in changing the speakers to an 8 ohm load, the need to replace the output transformers is really important. I believe that Dave pointed out rather early in this thread that the 'impedance mis-match' between the 4 ohm output transformers used on the 8600 series amps, and the 8 ohm speakers used by most folks in their home audio system....this rather 'robbed' the amp of a significant amount of power alone.......like about 50% or so. So, if you're switching to 8 ohm speakers, making the change to the output transformers is really a good plan. Because of the change in secondary output impedance, the changes made on Dave's design for the 'Feedback' section should probably be included as well. So, you'll probably want to find the old feedback circuit, find what is probably just a single resistor, and replace that with the 10K / 300pf network that Dave designated. But, beyond this, here's the rub: Dave did this project with the intent of redesigning this amp as a 'stand-alone' power amp. He did some calculating on current capacity, and in LOSING all the peripheral sections for radio/pre-amp...... the power transformer 'could' supply the needed 100 ma of current when built as per his design. But, you're keeping all that stuff, right? So, if you attempt to re-work the output section to the way Dave designed the bias, you'll be significantly increasing the current draw from that section, as well as retaining that from the other sections. This may prove too much for the power transformer itself. So, it might be best if you simply left that section of the amp alone. The addition of the Zobel network might also be considered.....that being the 47 ohm resistor and the .1 / 100V capacitor coming off the output side of the output transformer (seen in the circuit as connected to the Feedback line). This may prove beneficial in reducing some potential distortion. But, beyond these three changes (Output transformers, Feedback network, Zobel network), I don't know that you'll accomplish much by doing anything else. You could play around with the bias section.....and 'maybe' increase the bias load some, but you'd risk putting too much current draw on that transformer. My humble opinion is that beyond those 3 things listed any further changes will likely not provide any real 'audible' improvement. If you get really ambitious, you might consider re-wiring the 6EU7 socket to accommodate a 12AX7 tube. 12AX7s are a LOT less expensive than 6EU7s!! But, that's a bit of a challenge in that little amp! I've done it twice, once because I had to....the old socket was shot! I might add that if you DO decide to do this, replacing that tube socket with a better one is also a good idea!!

OK..........there's my $0.02 worth! Hope it helps give you something to think about. Other's may feel differently or have 'additional' suggestions. So.... good luck with your final determinations.

Tom D.
 
Thanks Tom. Good to hear from you. We are in consensus with this. Should I install the choke as I did in my other 8601? I know that it will not have the same audio quality as my other 8601 mod, but the tuner/preamp, and turntable are in such great condition that I hate to separate them.
John.
 
John, on a 'stock' 8600 chassis I believe the position that Dave used for the choke was over the top of one of the chassis cut-outs that was used for a molex connector coupling to the other units. As such, I don't think you'll be able to put the choke in the same location. If you can find room for it....'Great'!! But, I'd have to again caution you that the Choke Dave selected was part of his overall design....which included the 50 ohms of resistance within the choke itself. I think it would be prudent to go back and look at the 'original' design, and see how Magnavox had their filter section wired up. Honestly, like I said, I don't recall how the original was set up......so there may be attachment points that aren't part of Dave's schematic. I think I'd go 'sans' choke at first, and see how it all works out. If you hear significant power supply hum, then maybe finding a way to mount a choke wouldn't be a bad 'addition' to the unit. But, maybe a 4th stage to the filter itself might also be a 'fix' for that kind of problem. I'll just mention that in the builds of this amp that I do most often, I use a Hammond power transformer that produces just a bit too much B+ for use 'as-is'. So, to dial things down, and in consultation with Mr. Gillespie, I use a 4th stage.....and basically plug it in directly after my rectifier. And, I use a relatively small value cap at this point, like a 8uf / 450V. This provides minimal charging demands on the rectifier itself, plus it doesn't 'build' voltage into the unit that then has to be curtailed with a dropping resistor. As it is, I still have to use the dropping resistor.....but when I was using 33 uf caps, I built even higher voltage levels thus requiring bigger resistors. Anyway, I now run the 8uf cap right off the rectifier socket, followed by my main dropping resistor, then on to the second stage of the filter section. At that point I connect the choke. The output of the choke is where I target my 265 volts....just as shown on Dave's schematic. Then the 3rd stage filter, the 820 ohm dropping resistor, and the final filter stage where I have 255 volts going to the output tube screens and the driver tube plate supply. I believe this would be designated as a C-R-C-L-C-R-C setup. I've NEVER had any hum problems!!

Anyway............ if you can find a place for the choke right from the get-go.....then as long as it also fits 'electrically' (meaning the 50 ohms of resistance is OK in that position), then go for it. I don't see it hurting anything. I think the Hammond 156 Choke Dave specifies in his schematic is rated for 200ma......so, that should be sufficient for any of the 'other' unit's needs as well as that of the amp.

TSD
 
Thanks Tom. I will make the changes, and listen for any hum. If there is, I will address it with the Hammond 156 Choke. There seems to be room to mount it on top between the 6EU7, and the 6BQ5, all the way forward. Also, I read that Dave recommended to omit the NFB mod if I left the stock circuit intact, as it was already addressed in the original circuit. Did I misread it? John.
 
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I'm not sure what you're referencing, but it doesn't sound right to me. Could be.....but I'm just not sure about it. From my perspective I would have believed that the 'original' feedback circuit would have been designed around the output voltages produced by the 4 ohm output transformer with an appropriate 4 ohm load. It may be that an 8 ohm output transformer with an 8 ohm load will put out about the same voltages....comparatively speaking. That being true, then I guess the feedback circuit would be roughly the same as well. But, as I understand it, there IS a difference in voltages produced...which is the necessary factor to drive the increased load. So, with an increase in output voltage coming from the 8 ohm output transformer.......it's my understanding that some modification in feedback may be beneficial. But, I'll beg off here and see if someone else, or perhaps Dave may have some comments. Lord knows I do NOT have any kind of definitive answer. I think I've pretty well exhausted my commentary here.........so hope some of it has helped.....and I'll just wish you luck from here out!!

Tom
 
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