More Fun With Magnavox: The 9300 Series
- Thread starter dcgillespie
- Start date
If mine were missing/bad and I didn't have spares I'd probably be more inclined to change it. It just happens to be that I'm in a situation where there isn't any need to do so. Generally I'm not much in favor of unobtanium parts, so a 12ax7 makes a lot more sense just because of availability.
Hummm.........A tease, with no idea how it's gonna turn out -- but it is in progress..........
Dave
Dave
Mike Stehr
Poverty Audio
Well, they have more than one secondary tap.
eickmewg
Well-Known Member
I would consider rewiring the 6EU7 sockets for 12AX7 tubes if it were a connection or two but it looks like a wholesale rewiring. I'll get socket adapters. I'd like to try the old Telefunken 12AX7's currently in my old Dynaco FM3 tuner in the 9302.
So I did an experiment with the 9302. I decided to try my Weber WZ68 SS rectifier in lieu of the JJ GZ34. I knew I would lose a little of the slow start of the indirectly heated GZ34 but the WZ68 does have a thermistor good for three or four seconds ramp-up. No real change with the amp, but here is the surprise, it is barely above room temperature to touch. Admittedly, I've ventilated it with a number of cooling holes, but its lack of heat was remarkable. Try that with the GZ34!
So I did an experiment with the 9302. I decided to try my Weber WZ68 SS rectifier in lieu of the JJ GZ34. I knew I would lose a little of the slow start of the indirectly heated GZ34 but the WZ68 does have a thermistor good for three or four seconds ramp-up. No real change with the amp, but here is the surprise, it is barely above room temperature to touch. Admittedly, I've ventilated it with a number of cooling holes, but its lack of heat was remarkable. Try that with the GZ34!
Mike Stehr
Poverty Audio
That's why I don't bother with them...drill out the rivets and toss. I understand some don't have replacements, but 9 pin sockets are usually cheap cash-wise.
Good ones!
Dave
Dave
Dave's up to something sneaky Folks.......
Last time I built an amp with two impedance taps- it was because I was using some of the "old school" Hammond output transformers (made before they went to the "easy wire" version). Some say those are sonically superior to the "easy wire" version.
Those older Hammonds had two secondary windings- one secondary winding with two taps (total of three connections) and one with two. You had to parallel the ground and 4 ohm tap of one secondary with the entire other to get 4 ohms, or use one whole first one for 8 ohms, or series the two end-to-end to get 16 ohms. I permanently paralleled the second secondary with the 4 ohm tap of the first, and just wired the output terminals for 4 and 8 ohms, not worrying about 16 ohms...
The other amp I have, which has two impedance taps, is one of my favorites- the HK Award A300. If Dave has found some of those OPTs, I'd be seriously interested in what he'd do with them!!
Regards,
Gordon.
Those older Hammonds had two secondary windings- one secondary winding with two taps (total of three connections) and one with two. You had to parallel the ground and 4 ohm tap of one secondary with the entire other to get 4 ohms, or use one whole first one for 8 ohms, or series the two end-to-end to get 16 ohms. I permanently paralleled the second secondary with the 4 ohm tap of the first, and just wired the output terminals for 4 and 8 ohms, not worrying about 16 ohms...
The other amp I have, which has two impedance taps, is one of my favorites- the HK Award A300. If Dave has found some of those OPTs, I'd be seriously interested in what he'd do with them!!
Regards,
Gordon.
Give the man a cookie!
Next Steps:
Any effort to improve the 9300 series from the existing modified form mandated a serious look at the output transformers. Early on, I had decided on trying out a Transcendar product for this part of the project, because they appeared to be of high quality, and I wanted to put that impression to the test. However, as many know, that company is not taking any orders at the present time, and gives no indication of doing so any time soon. Therefore, even if I got the last set to try out for performance capability measurements, it would have shot any ability to reproduce the circuit by others right out the window if I used them. But, I couldn't get any transformers from this company anyway, so that problem took care of itself.
In the realm of easily -- and currently available products then, next up was Edcor. I have only worked very little with Edcor products, and the work I did with them was of a completely different nature, which therefore doesn't give me much to work with regarding this project. I know many AKers have used Edcor output transformers over the years, and generally talk very highly of them. While I mean no disrespect to anyone at AK, virtually all of that praise is of a subjective nature derived primarily from listening tests, which doesn't provide the hard data I was looking for if the modified 9300 is to take significantly worthwhile steps forward in performance capability. More specifically, to do a full workup of upgrading the output transformer is a time consuming effort if done correctly, and I didn't want to make the expenditure only to find that the improvement was marginal, or less than that sought.
To my knowledge then, and for the performance level I was seeking, that left the Triode Z-565 48. I have worked extensively with most of the orignal Dyanco transformers, and also Triode's reissue of the fabled A-431. In every example of this reissued transformer I worked with, I found it to act -- and react -- exactly as the original device -- and I do mean exactly. Most importantly, the sub and supersonic characteristics mimic the original product as close as any manufacturing effort can produce of such a product -- which is so important in a NFB amplifier, while power handling, power bandwidth, and distortion all remain identical as well. These transformers cost more, but my tests confirm Triode's claim that they are manufactured exactly like the original product, except for the use of improved insulation -- and of course with the Z-565 48, that the secondary is now 8 Ohms tapped at 4, instead of 16 Ohms tapped at 8. My experience with the quality of the reissue A-431, and the fact that all of these transformers were designed by David Hafler, was really all I needed in making a decision to go forward. I knew that the reissued Z-565 would fill the bill of upgrading the output transformer perfectly. A new set of these transformers will set those who are seriously interested in pursuing a higher level of performance from their 9300 back about two C notes. As the upgrade unfolds, each of you will have to decide if the upgrade is right for you with respect to return on investment. As a preliminary comment however, I am quite confident that there is no readily available (currently produced) transformer that can come close to the performance capability that these transformers have to offer, as will be shown as the details unfold.
What the upgrade is NOT:
The Z-565 is of course the central piece of Dynaco's ST-35 basic amplifier, and their SCA-35 integrated amplifier. The designs of those units are well known, well discussed, well praised (or criticized as the case may be), but universally given high marks for the output transformers they employ. Both of these designs employ an AF amplifier stage (triode in the ST, pentode in the SCA) which is direct coupled to a triode cathodyne (or split load) phase inverter, which drives a UL connected output stage. Because these designs are sooo well known and thoroughly dissected into more individual efforts of analysis than I can count, I therefore have NO desire to simply build an SCA/ST-35 circuit into the Magnavox chassis just because the all important piece to do that is now mounted in place. The Dynaco circuits are well proven high performance circuits. But this project has at its roots a Magnavox 9300 series amplifier, with its simple as dirt paraphase style phase inverter, pentode connected output stage, and only a moderate amount of NFB to enhance its humble existence.........and people love the darn thing. And, its easy to see why. It is so easy to listen to with virtually zero listener fatigue, producing a sound (with good speakers) that belies its simple design.
For those who have followed along to this point (or actually modified your amplifiers to the design offered in this thread), you will know that to the basic design, we have:
1. Converted the output stage to operate under the control of EFB(tm), boosting power, lowering distortion, enhancing tube life, and making for cooler overall operation.
2. Converted the phase inverter from a straight paraphase design, to a floating paraphase design. This makes for a forced balanced drive to the output stage independent of the specific driver tube used, and importantly, does so over the whole of the audio bandwidth, with a minimum of phase distortion in the process. It is a significantly improved design over that of the original offering.
3. Modified the power supply with detail improvements that (a) use the extra filament winding to buck the AC line for more accurate power transformer secondary voltages, (b) provide for a fixed balanced AC heater circuit, and (c) change out the rectifier tube to a GZ34 to provide for delayed warmup, higher overall B+, and less drop under heavy dynamic loads.
4. Modified the NFB and stability networks to achieve best possible stability, consistent with best possible performance from the other performance attributes (distortion, frequency response, etc.) of which the original output transformers are capable, while keeping the original overall level of NFB in tact.
These changes made a significant improvement over the performance of the original design -- improvements that are both clearly audible, and measurable. If another upgrade is to be offered then on top of what has already been done, it too had to be significant, while still using the same basic Magnavox formula (topology) presented above. Therefore, on top of the modified design previously presented, the upgrade focuses on two additional areas of opportunity -- the output transformer as discussed in this post, and another area which will be discussed in the next post. Understand that in performing the upgrade to your amplifier, it assumes you have already modified it as discussed above. With that then, here's a little something to wet your whistle (now go easy; remember, this unit is still considered to be a development mule.......):
Dave
Next Steps:
Any effort to improve the 9300 series from the existing modified form mandated a serious look at the output transformers. Early on, I had decided on trying out a Transcendar product for this part of the project, because they appeared to be of high quality, and I wanted to put that impression to the test. However, as many know, that company is not taking any orders at the present time, and gives no indication of doing so any time soon. Therefore, even if I got the last set to try out for performance capability measurements, it would have shot any ability to reproduce the circuit by others right out the window if I used them. But, I couldn't get any transformers from this company anyway, so that problem took care of itself.
In the realm of easily -- and currently available products then, next up was Edcor. I have only worked very little with Edcor products, and the work I did with them was of a completely different nature, which therefore doesn't give me much to work with regarding this project. I know many AKers have used Edcor output transformers over the years, and generally talk very highly of them. While I mean no disrespect to anyone at AK, virtually all of that praise is of a subjective nature derived primarily from listening tests, which doesn't provide the hard data I was looking for if the modified 9300 is to take significantly worthwhile steps forward in performance capability. More specifically, to do a full workup of upgrading the output transformer is a time consuming effort if done correctly, and I didn't want to make the expenditure only to find that the improvement was marginal, or less than that sought.
To my knowledge then, and for the performance level I was seeking, that left the Triode Z-565 48. I have worked extensively with most of the orignal Dyanco transformers, and also Triode's reissue of the fabled A-431. In every example of this reissued transformer I worked with, I found it to act -- and react -- exactly as the original device -- and I do mean exactly. Most importantly, the sub and supersonic characteristics mimic the original product as close as any manufacturing effort can produce of such a product -- which is so important in a NFB amplifier, while power handling, power bandwidth, and distortion all remain identical as well. These transformers cost more, but my tests confirm Triode's claim that they are manufactured exactly like the original product, except for the use of improved insulation -- and of course with the Z-565 48, that the secondary is now 8 Ohms tapped at 4, instead of 16 Ohms tapped at 8. My experience with the quality of the reissue A-431, and the fact that all of these transformers were designed by David Hafler, was really all I needed in making a decision to go forward. I knew that the reissued Z-565 would fill the bill of upgrading the output transformer perfectly. A new set of these transformers will set those who are seriously interested in pursuing a higher level of performance from their 9300 back about two C notes. As the upgrade unfolds, each of you will have to decide if the upgrade is right for you with respect to return on investment. As a preliminary comment however, I am quite confident that there is no readily available (currently produced) transformer that can come close to the performance capability that these transformers have to offer, as will be shown as the details unfold.
What the upgrade is NOT:
The Z-565 is of course the central piece of Dynaco's ST-35 basic amplifier, and their SCA-35 integrated amplifier. The designs of those units are well known, well discussed, well praised (or criticized as the case may be), but universally given high marks for the output transformers they employ. Both of these designs employ an AF amplifier stage (triode in the ST, pentode in the SCA) which is direct coupled to a triode cathodyne (or split load) phase inverter, which drives a UL connected output stage. Because these designs are sooo well known and thoroughly dissected into more individual efforts of analysis than I can count, I therefore have NO desire to simply build an SCA/ST-35 circuit into the Magnavox chassis just because the all important piece to do that is now mounted in place. The Dynaco circuits are well proven high performance circuits. But this project has at its roots a Magnavox 9300 series amplifier, with its simple as dirt paraphase style phase inverter, pentode connected output stage, and only a moderate amount of NFB to enhance its humble existence.........and people love the darn thing. And, its easy to see why. It is so easy to listen to with virtually zero listener fatigue, producing a sound (with good speakers) that belies its simple design.
For those who have followed along to this point (or actually modified your amplifiers to the design offered in this thread), you will know that to the basic design, we have:
1. Converted the output stage to operate under the control of EFB(tm), boosting power, lowering distortion, enhancing tube life, and making for cooler overall operation.
2. Converted the phase inverter from a straight paraphase design, to a floating paraphase design. This makes for a forced balanced drive to the output stage independent of the specific driver tube used, and importantly, does so over the whole of the audio bandwidth, with a minimum of phase distortion in the process. It is a significantly improved design over that of the original offering.
3. Modified the power supply with detail improvements that (a) use the extra filament winding to buck the AC line for more accurate power transformer secondary voltages, (b) provide for a fixed balanced AC heater circuit, and (c) change out the rectifier tube to a GZ34 to provide for delayed warmup, higher overall B+, and less drop under heavy dynamic loads.
4. Modified the NFB and stability networks to achieve best possible stability, consistent with best possible performance from the other performance attributes (distortion, frequency response, etc.) of which the original output transformers are capable, while keeping the original overall level of NFB in tact.
These changes made a significant improvement over the performance of the original design -- improvements that are both clearly audible, and measurable. If another upgrade is to be offered then on top of what has already been done, it too had to be significant, while still using the same basic Magnavox formula (topology) presented above. Therefore, on top of the modified design previously presented, the upgrade focuses on two additional areas of opportunity -- the output transformer as discussed in this post, and another area which will be discussed in the next post. Understand that in performing the upgrade to your amplifier, it assumes you have already modified it as discussed above. With that then, here's a little something to wet your whistle (now go easy; remember, this unit is still considered to be a development mule.......):
Dave
Last edited:
Thanks Gadget! Three things would be most helpful:
1. Full power bandwidth.
2. Frequency response.
3. 10 kHz square wave presentation.
Also, is your unit the modified design but simply using the Edcor transformers -- or where other modifications made? If so, what are they, and what are the specs of the transformers you used? Finally, is it still using pentode based operation?
Thanks!
Dave
1. Full power bandwidth.
2. Frequency response.
3. 10 kHz square wave presentation.
Also, is your unit the modified design but simply using the Edcor transformers -- or where other modifications made? If so, what are they, and what are the specs of the transformers you used? Finally, is it still using pentode based operation?
Thanks!
Dave
Son-of-Vere
Super Member
Aha! The long awaited transformer transformation is finally here! I will be following this with great interest, as I'd like to rework my Baldwin frankenamp into something better.
I would certainly think so. Dynakit Parts is a quality outfit. If their Z-565 offering weren't the same as the original, then it would not meet the original performance specifications for the ST-35 kit they offer. I would not hesitate to use the Dynakit Parts transformer -- and I did not mean to exclude their product as a viable option. I consider the transformers from either of these two sources to be of superb quality -- I just happened to have the ones from Triode on hand to develop the upgrade with.
Dave
Last edited:
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