More Fun With Magnavox: The 9300 Series
- Thread starter dcgillespie
- Start date
soundmotor
super modified
First time I've seen a stereo console Pilot amp. Damn, that one is pretty cool. They certainly did hold back on the PT.
nj pheonix
Lunatic Member
I don't know where i was , but i missed this one entirely. I just read cover to cover. I'll have to add this one to the notes. Thanks Dave (and everyone who contributed)!
ferninando
Lunatic Member
Reviving this old thread because I've been trying to wrap my head around the EFB circuit for the last day or so and I think I finally got it. It was puzzling me about the use of a negative regulator to produce a positive voltage to apply to the cathodes. And Dave's configuration of the LM337 is certainly counter to The Usual Way(TM) in all of the example LM317 / LM337 circuits I've ever seen. The light bulb went on when I read the following sentence in section 8.4.1 of the TI LM337 data sheet:
The reference voltage Vadjustment will typically be 1.25V higher than Vo.
...and then it all made sense. Want to produce 19V on the output pin at quiescent conditions? Just make a voltage divider from B+ that gives you 20.25V on the adjustment pin. Obviously, for an initial high voltage and adjustment capability you want the large-resistor->small-pot->medium-resistor as in Dave's schematic (you need to get it into the LM337's 35.8V operating range from wherever you're starting).
(...and now I'm pondering how one would use, presumably, the LM317 to get the same enhanced operation with negative voltage applied to the grid that tracked dynamic operating conditions in the same way as the cathode regulator EFB circuit does.)
(...yes, sometimes it pays to actually read the data sheet, rather than rely on canned circuit calculators...)
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Well then you use the "other" configuration for EFB, which I think Dave now calls Universal EFB. It does not use a 3 terminal regulator at all, rather it "follows" the screen/plate voltage by a sensing resistor that drives the base of a PNP transistor, which drives another PNP transistor wired as a buffer. The output of that buffer then provides the negative bias voltage for the output stage. The thread I boned up on before doing my own EFB-based amp was this Fisher 400 thread.
Look particularly at post #28 for schematic.
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Oh, yah, much different. Well, I'm in no great rush to wrap my head around that one just yet. For now, the cathode regulator is just the ticket for a couple of things waiting on the project shelf (a 9302 that will get the full DG treatment, as well as a Heathkit AA-151 that's destined to live in the master bedroom once my house remodel is finished). I'll probably also add it to my 8802 and the 8802 clone I built for my daughter.
I believe that particular configuration is intended to work with a negative DC bias supply, not off the cathode like the one used in the Magnavox. The version used in the Fisher 400 with the screen regulation component is what I intend to try out in my Sherwood 5500 when I get that together.
OK, so its 2016 and I have a meter that does not appear to be full of lies. I replaced my cheap 15 year old digital with a once expensive 30 year old digital. My bench now sports an HP 3456A, a meter once listed for $4000 in the mid 1980s, now available all day long for a C note.
Anyway, this is what it had to tell me:
I'll call them 8k transformers based on that data. These are Edcor CXPP-MS-8K transformers.
As you can see I measured them in slightly different ways just to see if it came out consistent, and it seems to have done that.
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This really is an amazing thread. A LOT of good info! I've got an EFB question and an OT comment:
Regarding EFB, I know the issue of having a well matched quad is important. Unfortunately, this isn't necessarily as easy as perhaps it should be. I'm honestly unsure of just how well 'matched' some of the matched quads I've purchased in the past have been. And, there's another issue here, which is; just how good are the 'new' tubes being manufactured. So, all this leads to the question of: Is it REALLY DIFFICULT to add an individual bias adjustment process to each tube while retaining the simplicity of the EFB itself?
Also, I've been using some Edcor and Transcendar OTs lately in some other builds.....and while I think the Edcors are a lot better than some of the 'console' stuff around, the quality of the Transcendar OTs is really top shelf. I know there are even better OTs out there.....and I've yet to actually own a set. I guess I too travel in the 'beer budget' circles. But, in spending $40 or $50 each for a set of OTs, I'd be incline to support that going an extra $10 or $20 for the Transcendar versions is probably well worth it if the amp is a 'keeper'.....and to be considered as 'representative' of one's best efforts. As a 'project to re-sell' or one that just passes time and uses up some oddball parts, then if you're looking to improve on the 'stock' console OTs, the Edcors seem to do a fine job in that respect. Just mho...... WC
Regarding EFB, I know the issue of having a well matched quad is important. Unfortunately, this isn't necessarily as easy as perhaps it should be. I'm honestly unsure of just how well 'matched' some of the matched quads I've purchased in the past have been. And, there's another issue here, which is; just how good are the 'new' tubes being manufactured. So, all this leads to the question of: Is it REALLY DIFFICULT to add an individual bias adjustment process to each tube while retaining the simplicity of the EFB itself?
Also, I've been using some Edcor and Transcendar OTs lately in some other builds.....and while I think the Edcors are a lot better than some of the 'console' stuff around, the quality of the Transcendar OTs is really top shelf. I know there are even better OTs out there.....and I've yet to actually own a set. I guess I too travel in the 'beer budget' circles. But, in spending $40 or $50 each for a set of OTs, I'd be incline to support that going an extra $10 or $20 for the Transcendar versions is probably well worth it if the amp is a 'keeper'.....and to be considered as 'representative' of one's best efforts. As a 'project to re-sell' or one that just passes time and uses up some oddball parts, then if you're looking to improve on the 'stock' console OTs, the Edcors seem to do a fine job in that respect. Just mho...... WC
Shadowdog
Super Member
Tom I believe there are schematics on the Tronola Electronics site for individual output tube pot setups.
http://www.tronola.com/
http://www.tronola.com/
Thain, I may give Jim a shot. I'm still pokin' around.....and I may be able to come up with bunch of 'good' 6V6 tubes from which I just might be able to match up something fairly closely here on my own. But, if not, I see what Jim has to offer and at what price. AES is selling some 6V6 tubes in matched quads.....but the Chinese versions start around $50 and they go up to nearly twice that by the time you hit the 'Tung Sol' re-issues. I recall finding boxes and boxes of 6V6 tubes just a decade or two ago...... it was like no one wanted 'em any more! Now.... I guess they've become like 7199s or even 12AX7s to some degree. Anyway...... if I do the individual bias adjust circuit with the EFB, I may just have the problem overcome with that additional effort.
Randy, thanks for the tip back to Tronola. I've read most of what's in there....... and in reality I believe I've read all of it. I remember reading Dave's 'New Look......' article shortly after it appeared in Tronola. Then those other articles by Arthur Grannell....who other than his documents in Tronola, I'm completely unfamiliar with. However, he sure seems to know what he's doing. Hopefully his 'individual' set-up with work with my 6V6 tubes. I'm still way ahead of myself here, as I've not even started the 'build' process yet. But, looks like we're going to get a pretty serious snow storm this weekend....... like upwards of 2 feet??? UGH!! But, if so, I hope to be hunkered down with soldering iron fired up and getting a few things accomplished. Tomorrow it's off to the grocery store so I can 'weather the storm'.....so to speak. I spent all day today moving my brother's stuff from the home he just sold in Rehoboth, Delaware to his other home in Washington DC....thanks to some help from my girlfriend's son. We started at 9 am this morning and didn't finish until 9 this evening....with about 300 mile of drive time taboot! So, I'm beat....and the ONLY thing I have on the agenda tomorrow is a trip to the grocery store. But, from there.......hopefully I can get back into my 175 build and make some progress.
Thain and Randy....thanks again!! Tom D.
Randy, thanks for the tip back to Tronola. I've read most of what's in there....... and in reality I believe I've read all of it. I remember reading Dave's 'New Look......' article shortly after it appeared in Tronola. Then those other articles by Arthur Grannell....who other than his documents in Tronola, I'm completely unfamiliar with. However, he sure seems to know what he's doing. Hopefully his 'individual' set-up with work with my 6V6 tubes. I'm still way ahead of myself here, as I've not even started the 'build' process yet. But, looks like we're going to get a pretty serious snow storm this weekend....... like upwards of 2 feet??? UGH!! But, if so, I hope to be hunkered down with soldering iron fired up and getting a few things accomplished. Tomorrow it's off to the grocery store so I can 'weather the storm'.....so to speak. I spent all day today moving my brother's stuff from the home he just sold in Rehoboth, Delaware to his other home in Washington DC....thanks to some help from my girlfriend's son. We started at 9 am this morning and didn't finish until 9 this evening....with about 300 mile of drive time taboot! So, I'm beat....and the ONLY thing I have on the agenda tomorrow is a trip to the grocery store. But, from there.......hopefully I can get back into my 175 build and make some progress.
Thain and Randy....thanks again!! Tom D.
OK, another question: I just completed the build of this 'driver' circuit into my 175 overhaul. After a few little 'glitches' which I've now corrected, I've got it dialed in pretty well. However, I'm not sure but what my build application might benefit from the 'input reduction' part of the circuit Dave posted.....as it drives the 6V6s pretty well without too much twist on my pre-amps VC. I think if I reduced the sensitivity it might 'react' a bit more tamely. However, I'm having a bit of trouble trying to figure out just want Dave has got in mind with this part of the circuit. I see the 330K resistor and the 56pf cap across it....but I don't see where Dave has intended this to be installed. Can I get an 'assist' with this? And, FWIW...the driver circuit is awesome! After I completed the amp's basic build today (still using cathode bias).... I listened to a Bruce Hornsby CD straight through for about 45 minutes....very dynamic music, lots of percussion, GREAT piano licks, good guitar work.......and a GREAT test for the new amp! Simply put: Stunning!
jazbo8
Well-Known Member
"For those who wish to maintain the original sensitivity level of 1Vac, a compensated reduction network is shown on the schematic that should be inserted between the input jack, and the junction of the 47k/470k resistors."
Jazbo, many thanks! I read through all 14 pages last night.....and I missed it! I'll get it installed yet this morning and see how it functions. Thanks again! Tom D.
Gadget and Larry, I did order a matched quad of tubes from JM. A good price on a quad of the new Tung Sol reissue versions.....and now that my 175 is pretty well done I'll install them and see how they sound. Again, thanks for the tip. TSD