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More Fun With Magnavox: The 9300 Series

Thanks Dave, that did the trick.

I've got to admit, my heart sank a little bit when you switched things up with the floating paraphase for the phase inverter. But I think that has more to do with where I'm at in my 9304-20 build than the efficacy of the switch.

I'm likely going to end up staying with the standard paraphase for the inverter as I've already purchased my coupling caps and the budget won't allow for a reboot at this time.

After doing your work on the 9300, do you feel that these amps will benefit from the modified pentode EFB while utilizing a BattRadioesque modified paraphase front end?
 
If the original power supply caps are replaced is it beneficial to change any of the values or should the schematic be followed exactly?
 
Kidmoe said:
If the original power supply caps are replaced is it beneficial to change any of the values or should the schematic be followed exactly?
Click to expand...

Kid -- You could use as much as 50 uF at 450 vdc at all four positions, while going any higher than this would return little if any significant results. You particularly would not want to go any higher than 50 uF at the rectifier cathode to protect the tube during any hot switching events.

Gordon -- The cathode configuration at the phase inverter is based on expediency. Understand that if the two triodes shared a common cathode resistor, it would need to be on the order of about a 1000 ohms. Now the NFB resistor used (1.8K) not only provides NFB, but also represents a DC path to help help bias both sections of the 6EU7 as well. But at 1.8K, it's too high to bias both triodes correctly. Therefore, the 2.7K resistor added to the bottom triode's cathode circuit then works in conjunction with the 1.8K NFB resistor to provide the correct bias for both sections. Bypassing the 2.7K resistor then maximizes the gain in both sections, while the small 100 ohm resistor between the cathodes of the two sections becomes the return element of the NFB voltage divider.

Dave
 
So Dave, you have kept the orig. OPTs with 4ohm Z out and just tested the 8ohm performance for our information so those of us with 8 ohm spkrs
can expect reasonable performance?
 
I was interested in the OPT upgrade too. I was also wondering if it would be any problem changing the 6EU7's to 12AX7's. I have a friend that has some nice NOS 12AX7's I might get a deal on.
 
Hi Dave,

Great thread and thanks for detailing all this information.

I have a question, will you be utilizing the balance feature/knob? if not why?.
 
The balance feature left with the old phase inverter design. Balance controls are not a normal item for basic power amplifiers as they are (normally) provided on the associated preamp they are used with in stand alone installations. If a balance control is required on the power amplifier chassis, it is indicative of the fact that something is wrong somewhere.........

To simply upgrade the original design by installing better OPTs would have been going about things backwards. If the circuit is holding things back -- and it surely was in this case in so many ways -- then simply dropping in better OPTS would do little to improve things. In improving any device, it is always best to start by improving from the weakest link, and moving up from there accordingly. In this case, that was the circuit, and therefore, where the order for improvement started.

With the weaknesses of the original design addressed in their entirety, once those modifications are made, the project is then at the right point to start addressing the OPT question. Since it is now the original OPTs that are the limiting factor, replacing them will allow the improvements made possible from the transformer change to really stand for what they are, and not have the circuit still hold even better transformers back.

New OPTs will require a new workup of the NFB and HF stability circuits, but everything else would remain the same. That's because the HF characteristics of every OPT are unique to a given transformer. Therefore, new transformers would require a new workup of those circuits with the new transformers in place.

As long as the socket wiring is changed accordingly, and the heater power is based on a 6.3 volt system, then the 6EU7 and 12AX7 can be used interchangeably.

Dave
 
The HF stability items are the 390pf + 1500 ohm resistor to ground on the voltage amp and the 47pf from plate to feedback loop, yes? Just making sure I didn't miss one. Mine already has different transformers, so my intention was to build mine based on these values without any HF stuff in it, and go from there.
 
Yep -- those are two of the main HF stability components. But the 47K input resistor figures significantly into the picture as well. Other components play a more minor roll.

Dave
 
Cool, thanks. Maybe this weekend I'll mess with mine. Mostly I'm curious what mine is doing now, since I've honestly never measured it.
 
Nice Job Dave as usual,

I remember a while back there was a bunch of us (you as well) involved in a similar amp/config where I'd mentioned I did not like the paraphrase splitter.
I ran a bunch of tweaks and trials to improve it, in the end the floating paraphrase was the change made, still not a fan of that one either, but it is better. :)

The driver circuits on most of these Maggies and the output sections needing attention along with there supply does leave a lot of room for improvement.
(but once all that is done they can be quite nice performers) :thumbsup:
 
Well, better late than never!! Sorry I've not been following this thread along since it's inception, but been too busy with DG-SE1s, and then got into this little Sylvania console amp....which is the subject of a couple of threads I have going here on AK as well.

Just a couple of comments on prior posts, one as related to the use of 'new' 5AR4s vs some of the current production units. Dave, PLEASE jump in on this.....as in reading this thread I don't believe I saw a note anywhere on the total current drawn from the HV in the PS of this unit. But, my thoughts on 5AR4 rectifiers are that the new ones are fine in amps where you're not approaching the current limit stated for the tube...which if I recall, is about 240 or 250 ma. I don't think this amp is even over 200ma.....or perhaps even significantly lower....particularly with EFB, but I'm thinking that in most any amp with under 200ma of current draw, the 'new' units....even those Chinese units from Tube Amp Doctor or Ruby...they would work fine! However....just my opinion here...but in amps like the Dynaco ST-70, the MkII/III, or even some of the 'integrated' units like the Scott 299C/D or 233 series...those amps really deserve a 'vintage' rectifier just to assure that the current levels are as 'available' as possible. Not saying that the new Sovteks, JJ, or others 'won't' do the job....but I'd far rather have a nice Amperex, Mullard, Telefunkin, or even an old US version in a 'big' amp. Dave, maybe you could comment as to the total current draws w/ or w/o EFB in this amp?

Also, as to the EFB, this Sylvania amp I'm currently working on is very close in many ways to this Maggie. PI stage is pretty similar, but the AF stage is Pentode driven....and the amp uses a 6BL8 vs the 6EU7/12AX7. Also the Sylvania came with it's output section already 'split' into two cathode resistors....with NO bypass caps. So, there are some other issues at play in that amp as well....particularly as related to the power supply, screen voltages, and supply voltages at the plates of the driver tubes. BUT....one BIG advantage that I discovered is that the 7.6K primary impedance on the OTs is at 8 ohm....not 4. So, this amp is getting a good bit of attention from me...which brings me to the point: Dave, do you think I could pretty much 'copy' your EFB as you have it in this Maggie circuit and 'adopt' it into the Sylvania, providing I massage the other voltages to the OTs and 6BQ5s to reflect the same basic values as those in the Maggie?

Needless to say...... more 'incredible' work from the 'master'!! Like a few others, I too have one of these 9300s on the shelf....and just may have to take the jump with it as well. OH.....I wanted to ask if you think these basic changes would also apply to the 6V6 version?

Thanks Dave!! Tom D.
 
Tom, for whatever it may or may not be worth, my Fisher TA-600 is running a cheap as chips Sovtek 5AR4 and its holding up OK. Output stage on that runs somewhat hotter than the Magnavox would. My Bogen DS-225 runs a pair of them, and they also seem to be holding up OK.
 
Thain, I'll PM you about my 'rectifier' experience.....but my comments were more about the Chinese versions than others. However, in general, I just have a personal affinity toward the Euro or US version as I have had some bad experiences with the others. For what it's worth, I now do that 'diode' mod in some cases as well, and in particular where the use of a Chinese 5AR4 might be employed. I'm just 'skittish' about them....but, the price is right! I think the Ruby versions were selling for under $10 for a while....... not bad when a Mullard was going for $100+. I wouldn't worry at all about using most any 5AR4 in this Maggie amp though....... but I would definitely think twice about a Ruby in an amp like your Fisher. Just my humble opinion...... WC
 
Tom,

If you are going to use a new production 5AR4 use the Sovtek as I have personally had the least amount of issues with them. Then its just a 15 buck or so difference from the Chinese Bottom feeders.

Frannie
 
"...Chinese Bottom Feeders..".......lol I like that! Yea, I think you're right Frannie. That's pretty much my thoughts as well. I've got a few amps where I'm not too concerned about my 'carp' rectifiers (sorry.....if you're not a fisherman, the 'carp' reference may not make any sense....but it's known as the apex 'bottom feeder' in most aquatic habitats...lol). But, the Sovtek, and perhaps even the JJ....they might be OK in most circumstances. I still lean toward the Mullard and Amperex as being my favorites.......when I can afford them! They're like 'worth their weight in gold' kinds of tubes, so I simply can't afford to put them in everything. But, if I could....... I would!!
 
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