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More Fun With Magnavox: The 9300 Series

Dave G. please reply.

Long ago soon after referbing my 9303 I bought these hammond OPTs
https://www.tubesandmore.com/products/P-T1608
10w 8Kohms CT 4,8,16 ohms PP to replace the stock OPTs.
With idea of getting more bass output.
Do you think these would be good to use in your 9300 redo without messing
anything up that you carefully worked out for these amps?
Or should I just keep the orig. OPTs and not be concerned about it.?

as always, thanks for your expert advice Dave.

fern
 
The transformer you selected has the correct primary impedance (don't use the UL taps), and just over a 50% increase in core size over the that of the original transformer. It will also certainly allow for a proper impedance match to the load as well. No doubt it will allow for improved LF power output capabilities as well as extending the basic LF response as well. As to how much improvement there will be to the power bandwidth on the high end will depend greatly on how much interleaving is used in the coil form. It is my experience (from a couple of decades ago) that Hammond transformers perform pretty well on the low end of the spectrum, but are challenged on the high end -- although their specifications imply that this has improved with later models.

All you can do is try them. They will likely work in the modified circuit just fine, although just as likely will not operate with optimum HF transient and frequency response, as the modified design is adjusted to make the most of those performance parameters with the original transformers.

For perspective, and purely from a performance standpoint, the transformer you have selected has a core size that is about 80% of that used in the Dynaco Z565 transformer, but is still less than 50% of the core used in Fisher's SA-100 transformers -- two pieces of well respected 6BQ5 iron. While there is room on the chassis for either of these transformers, the installation of transformers of this size would require very good planning, and particularly so with the latter pieces. It would also clearly cease any relationship the unit retains with its original Magnavox heritage, other than the chassis and power supply! The units you have selected should provide a nice performance boost, without completely trashing the Magnavox image. If you try them, let us know how it turns out!

Dave
 
Interesting question ferninando, I also have a new pair of those T1608 Hammond transformers that I got
for another project and never used them.

If you do decide to try those T1608 Hammond transformers on your 9303 Magnavox I also would like to
know what the outcome is and how it sounds with the Hammond OPT, if it works out I will likely do the
same to my 9303, I do like the idea of having 4,8 and 16 ohm taps at the output.

Robert
 
I also have a set of OPT's I was hoping to connect to my 9302 or other 6BQ5 amp. They are from a Stromberg-Carlson ASR-333 or ASR-433 and quite heavy.
I hadn't mentioned it here earlier because I thought is was somewhat off topic. But it would be nice to make use of them.
 
After I attend to some family matters during the first quarter of the new year, I plan to revisit this project with the more aggressive modification plans that changing output transformers would require. The good news is, that with the modified circuit as presented, it is fully ready as is for that event, so that installing better OPTs will allow them -- and the modified design -- to operate to the best of their capabilities.

Dave
 
Dave, if it would be of any use to you, I can ship you down my 9304 with the Edcor transformers if you'd like a broader sample of non-stock iron to experiment with. With some luck I'll have actually figured out what the primary impedance is for sure on those by 1Q 2016. Its on my list of things to get back to as soon as UPS gets my meter to me.
 
I call shotgun, I'm heading to Orlando on vacation over the winter. Dave, if you'll PM me your location I'll drop my 9304-20 off on the way through. You're not exactly enroute, but my little guy loves the Atlanta Aquarium.


;) just kidding.
 
I will be doing Dave's basic mods as they now stand with the orig. opts and hold off with new opts til after Dave is able to revisit the OPT
issue.
I think we will all be better off by doing that.
 
Dave, Thanks for all you have done. Enjoy the holidays with your family. I can wait until next year when you get some free time for further improvements.

-Morris
 
yeah Dave. you need a break from all this.
At least from all of us who constantly ask you questions.
But like a tru audio enthusiast/EE engineer you give the answers we need.
Thanks for what yu do for us.
 
dcgillespie said:
As a basic rule, the power supply caps on any piece of equipment that is 50+ years old should get replaced if the unit is expected to deliver long term dependability with regular use. Also, while they often can fail in a rather benign way (that is, just slowly disappearing from the circuit), they can also fail in such a way as to take out other components with them -- even a power transformer in extreme cases. The bottom line is that you have to define the service you expect a given piece to return. A piece that is going to see regular significant duty, and is possibly mounted up in a case or where it's not easy to get to makes for a no brainer -- replace the cap now even if it's currently good. For the development mule however, where the only time (currently) that it's on is when it's right in front of me on the work bench, replacing a currently good cap is a much lower priority relative to the goals for the project.

Dave
Click to expand...
I can vouch for this. I picked up a 9304 that had a bad can cap in. Someone ran it and it took out three of the output tubes and destroyed the PT in the amp. There was melted electrolytic all over the top and underside of amp and blobs of it in the console. It was a big bad ugly mess. Out of amp testing of the transformer just causes it to be an incredibly unsafe smoke bomb, so away it goes.

All that being said, is there a recommended new replacement for this power transformer? I'm a little unsure of the specs, but I'd sure like to resurrect this amp by getting a good part and follow some of Dave's recommendations.
 
gfxmla said:
All that being said, is there a recommended new replacement for this power transformer? I'm a little unsure of the specs, but I'd sure like to resurrect this amp by getting a good part and follow some of Dave's recommendations.
Click to expand...

The Photofact for the 9302 says: 600VCT @ 170mA, 5V @ 3A, 6.3V @ 300mA, 6.3V @ 4A.

If it were me resurrecting an amp that exploded like this, I'd just switch to SS rectification (which in turn allows you to use larger filter caps) to save on the 5V winding.

That said, the Edcor XPWR008 seems perfect for this amp: 600VCT @ 200mA, 5V @ 3A, 6.3V @ 4A. You don't get an extra 6.3V winding for the bucking mod, but the primary is rated 120V rather than 117V on the original, so maybe not necessary anyway. You'd probably have to fiddle to get it to lay-down mount, or you could just punt and vertically mount it.


$65. Hard to beat that.
 
thorpej said:
The Photofact for the 9302 says: 600VCT @ 170mA, 5V @ 3A, 6.3V @ 300mA, 6.3V @ 4A.

If it were me resurrecting an amp that exploded like this, I'd just switch to SS rectification (which in turn allows you to use larger filter caps) to save on the 5V winding.

That said, the Edcor XPWR008 seems perfect for this amp: 600VCT @ 200mA, 5V @ 3A, 6.3V @ 4A. You don't get an extra 6.3V winding for the bucking mod, but the primary is rated 120V rather than 117V on the original, so maybe not necessary anyway. You'd probably have to fiddle to get it to lay-down mount, or you could just punt and vertically mount it.


$65. Hard to beat that.
Click to expand...
Thanks for the specs and info thorpej. I don't know how I feel about a 5 week lead time from Edcor though.

I can order this thru Mouser and get it hella quicker.
http://www.mouser.com/ProductDetail/Hammond-Manufacturing/372FX/?qs=iSWJLKjVRH6l6v6REZIMcw==

Nearly twice as much as Edcor, but I can start working on it this weekend if I desired.

...decisions, decisions...
 
However in the power transformer space, Edcor makes really top notch stuff. They run really quiet and for the price they are nigh impossible to beat...if you can afford the 4 - 6 week wait time.
 
kward said:
However in the power transformer space, Edcor makes really top notch stuff. They run really quiet and for the price they are nigh impossible to beat...if you can afford the 4 - 6 week wait time.
Click to expand...

Yes, I really like the Edcor transformers. That 4-6 week wait time isn't hard and fast, though; I once received an order in 3 weeks. Granted, it's not a "THIS WEEKEND!!!!" kind of thing, but I patience is a virtue, right? :-)
 
thorpej said:
Yes, I really like the Edcor transformers. That 4-6 week wait time isn't hard and fast, though; I once received an order in 3 weeks. Granted, it's not a "THIS WEEKEND!!!!" kind of thing, but I patience is a virtue, right? :)
Click to expand...
Yes, it is a virtue, but I am not a virtuous kinda guy...;)
 
Im putting together a parts list for this and the 8600 mods. I have some 5k 1/2 watt pots for the bias. Will this work or should I use a higher rating?
 
That is more than ample. The power dissipated by the pot is miniscule at best.

Good luck with your projects!

Dave
 
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