Tube Radio
Super Member
However a choke in the power supply can somewhat mitigate the power supply caps being limited by the rectifier tube given the choke has some DC resistance so long as the first filter cap is not over the max for the rectifier.
More Fun With Magnavox: The 9300 Series
- Thread starter dcgillespie
- Start date
Tube Radio
Super Member
A sub is definitely one way to go about doing it and with the lower bass not going through the amp and speakers, both will likely sound better.
Tube Radio
Super Member
A way to improve the filtering provided by the choke is to put a capacitor in parallel with it so that you form a LC tank that resonates at 120Hz.
I use this calculator
www.omnicalculator.com
I put in the choke value and the frequency and it tells me what cap value I need.
I did that in one amp and it took a 200mV ripple voltage down to 12mV.
So if the choke value in this Magnavox amp is 1H like I've seen in other Magnavox amps, the cap needed is 1.759uF. It's likely a 1.8uF cap would work just fine.
I use this calculator
Resonant Frequency Calculator
Use the resonant frequency calculator to check out the resonant frequency in an LC circuit.
www.omnicalculator.com
I put in the choke value and the frequency and it tells me what cap value I need.
I did that in one amp and it took a 200mV ripple voltage down to 12mV.
So if the choke value in this Magnavox amp is 1H like I've seen in other Magnavox amps, the cap needed is 1.759uF. It's likely a 1.8uF cap would work just fine.
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Problem there is the inductance changes somewhat with current load, and an AB amp swings a fair bit. Depending on the particulars if that filter is too sharp it might actually end up with considerably more ripple at full power than without the cap. Push pull design cancels most of it though so supply filtering isn't quite as critical.
Tube Radio
Super Member
Hadn't thought of that.
Tube Radio
Super Member
There are no dumb questions.
Would require a balanced signal being fed to the amp and an 8 ohm speaker being connected between both + speaker terminals and both - speaker terminals connected together.
The speaker + goes to the speaker+ terminal of the channel the input + connects to with the speaker - going to the speaker + terminal of the other channel where the input - connects. The input ground connects to the amp chassis.
The balanced signal can be generated by several methods:
1. Preamp with balanced output.
2. DRV-134 board which converts an unbalanced input to a balanced output.
3. Unbalanced to balanced transformer.
4. Split load phase splitter stage which can be made with a tube or a transistor.
Would require a balanced signal being fed to the amp and an 8 ohm speaker being connected between both + speaker terminals and both - speaker terminals connected together.
The speaker + goes to the speaker+ terminal of the channel the input + connects to with the speaker - going to the speaker + terminal of the other channel where the input - connects. The input ground connects to the amp chassis.
The balanced signal can be generated by several methods:
1. Preamp with balanced output.
2. DRV-134 board which converts an unbalanced input to a balanced output.
3. Unbalanced to balanced transformer.
4. Split load phase splitter stage which can be made with a tube or a transistor.
There is another way, but its not bridge mode, and it really depends on the output transformer if it makes any sense.
Parallel the inputs and parallel the outputs. The problem here is the stock 4 ohm transformers when run this way want to see a 2 ohm load. If you run it into an 8 ohm load the mismatch is probably enough that its not going to actually make any more power than it does run "normally" with an 8 ohm load on those 4 ohm transformers. Into a 2 ohm load though, it will make basically double the output power.
If you have other transformers, ideally ones with a 16 ohm output, it works more normally. Run an 8 ohm load on the paralleled 16 ohm jacks and you get 2x the wattage.
In bridge mode technically you get 4x the wattage since it doubles the voltage swing, but the impedance problem runs the other direction. 4 ohm to 4 ohm wants a 16 ohm load. Even with other transformers its not likely to have a less than 4 ohm tap. If you have a 4/8/16 secondary though, what you can do is reverse the primary and secondary. Ground the 16, and then you have 0/2.5/4/16 ohm outputs. 2.5 to 2.5 ohm in bridge mode wants an 8 ohm load. And yeah all that with the external phase inversion is needed.
so "yes" but with enough caveats as to make it less than practical.
Another thing you can do though is bi-amping. Active crossover upstream of the amp and split it into highs and lows. Probably vertical bi-amping makes the most sense here, so one amp per channel. One is for the bass and the other for the highs on each side. Need speakers that can accept this sort of input. Its not going to make any more power but it might improve dynamics a bit.
Parallel the inputs and parallel the outputs. The problem here is the stock 4 ohm transformers when run this way want to see a 2 ohm load. If you run it into an 8 ohm load the mismatch is probably enough that its not going to actually make any more power than it does run "normally" with an 8 ohm load on those 4 ohm transformers. Into a 2 ohm load though, it will make basically double the output power.
If you have other transformers, ideally ones with a 16 ohm output, it works more normally. Run an 8 ohm load on the paralleled 16 ohm jacks and you get 2x the wattage.
In bridge mode technically you get 4x the wattage since it doubles the voltage swing, but the impedance problem runs the other direction. 4 ohm to 4 ohm wants a 16 ohm load. Even with other transformers its not likely to have a less than 4 ohm tap. If you have a 4/8/16 secondary though, what you can do is reverse the primary and secondary. Ground the 16, and then you have 0/2.5/4/16 ohm outputs. 2.5 to 2.5 ohm in bridge mode wants an 8 ohm load. And yeah all that with the external phase inversion is needed.
so "yes" but with enough caveats as to make it less than practical.
Another thing you can do though is bi-amping. Active crossover upstream of the amp and split it into highs and lows. Probably vertical bi-amping makes the most sense here, so one amp per channel. One is for the bass and the other for the highs on each side. Need speakers that can accept this sort of input. Its not going to make any more power but it might improve dynamics a bit.
Tube Radio
Super Member
It will likely improve the frequencies above the crossover point as the amp channel no longer has to reproduce the bass.
That said a simple 12 dB/octave high pass and low pass crossover like what Motorola did with their three channel consoles would likely do the job.
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Tube Radio
Super Member
True
bean_counter
Active Member
Wow, great thread! Finally made it through, tho I admit to just skimming some of it.
Been out of the game for a while; are the Z-565 clones unavailable? I picked up a really nice console with a 9302 amp; I’ll rebuild with the Z-565 if I can, if not I’ll use the Edcors with gadget’s circuit. But I would really prefer the Z-565.
edit - corrected typo, amp is 9302, not 9202
Been out of the game for a while; are the Z-565 clones unavailable? I picked up a really nice console with a 9302 amp; I’ll rebuild with the Z-565 if I can, if not I’ll use the Edcors with gadget’s circuit. But I would really prefer the Z-565.
edit - corrected typo, amp is 9302, not 9202
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Honestly I'd give a go to using the stock transformers first. They may surprise you with how much they don't suck.
I think Musical Power Supplies is making a Z-565 clone that is supposed to be true to the original but someone else may know more on that. I'd consider MPS either way, though if I'm wrong about them making a Z-565 you'd have to do some fine-tuning to make sure it all plays well. I have a set of MPS iron on a 6L6 amp that I'm pretty happy with though.
I think Musical Power Supplies is making a Z-565 clone that is supposed to be true to the original but someone else may know more on that. I'd consider MPS either way, though if I'm wrong about them making a Z-565 you'd have to do some fine-tuning to make sure it all plays well. I have a set of MPS iron on a 6L6 amp that I'm pretty happy with though.
bean_counter
Active Member
Thanks Gadget! Good to see someone is making them. Not too bad a price, either.
I usually like to get it all done from the start and not go through iterations, other than tweaks, but maybe I’ll give the stock transformers a whirl.
I usually like to get it all done from the start and not go through iterations, other than tweaks, but maybe I’ll give the stock transformers a whirl.
Lavane
Lunatic Member
I agree, try using the original output transformers first. I wound up leaving them in my 9303. They're probably the best OPT's Magnavox had made.
They sound good. Of course, the secondary winding is 3.7 ohms, so 4 ohm speakers are best to get full output of the amp. I've been using 6 ohm and 8 ohm speakers with mine and the amp sounds good and gets plenty loud.
Mine has been heavily modified, and I do have it biased a little hotter than most I think.
In the end, I just didn't see a reason to use any other output transformers since the factory ones sound so good.
You have more thread reading fortitude than I- when you mention "using the Edcors with Gadget's circuit, can you tell me what post # that is in or grab the link? What is different- the feedback tuning?
I'll pile on- Built my 9300 as the Gillespie "mark one" build using the stock transformers, typically run 6-8 ohm speakers, and have never wanted more from it. Plenty of power for small-mid size room. The amazing thing about Dave's EFB circuit and operating point is that the amp has a lot of swing headroom, so the power level for 8 ohms is not that much lower than for 4 ohms, so it is a great performer either way. Dave's schematic has the feedback and stability fine tuned for the stock iron, allowing you to focus on a successful build, get a handle on the EFB circuit and fine tuning bias, see what it can do, then go from there. If you use different OPTs, you will need to tweak the feedback and stability, which requires a more advanced skill level.
Since you bought the complete console, save that tuner- it has the exact same cadmium plating on the chassis and shield boxes as the amp chassis, so makes great raw material for hole patching.
bean_counter
Active Member
Hi wparks,
Gadget’s separate thread with the schematic for the Edcor transformer is:
and yeah, I never throw anything away until my wife makes me lol. I’m thinking of building a power supply for the 7002-00 multiplexer, I’ve got an old Pilot mono FM tuner with a MPX out connection that I picked up years and years ago.
Gadget’s separate thread with the schematic for the Edcor transformer is:
Magnavox 9300 with Edcor CXPP25-MS-8K iron
This is my own little side thread to document my version of Dave Gillespie's modified Magnavox 9300 using Edcor transformers. Original thread here: More Fun With Magnavox: The 9300 Series This specifically exists to answer these questions: Thanks Gadget! Three things would be most helpful: 1...
audiokarma.org
and yeah, I never throw anything away until my wife makes me lol. I’m thinking of building a power supply for the 7002-00 multiplexer, I’ve got an old Pilot mono FM tuner with a MPX out connection that I picked up years and years ago.
I should mention it does better power with tubes that aren't worn out. I'd have to go back and measure it with the set in there now to get an exact number but its somewhere around 13 watts with a good set of Valvo vs a little under 10 with the tired old RCA tubes.
I actually just replaced the wafer sockets in it last weekend. One had broken so I did both, and the random top mount socket I had used for the cathode follower always annoyed me visually so now its got a proper set of old US manufacture bottom mount sockets that look like they belong there.
I actually just replaced the wafer sockets in it last weekend. One had broken so I did both, and the random top mount socket I had used for the cathode follower always annoyed me visually so now its got a proper set of old US manufacture bottom mount sockets that look like they belong there.
bean_counter
Active Member
Thanks for the tip on fresh outputs. It still has the original Magnavox stencil ‘West Germany’ EL84s; I haven’t tested them, but I imagine they’re pretty tired. The console is actually working - a rare find for me - but it does sound tired.
I’m going to try the original transformers. I’ve ordered a quad of JJs and a Sovtek 5AR4. The 6EU7s I did test, and they’re fine. I’m going to tear it down completely and replace sockets, but I’ll keep the 6EU7 vs rewiring for 12AX7, they’re good and I have a few good pulls in my stash.
I have a lot of the parts I need in my stash, this should be a fun, relatively cheap project.