Magnavox Imperial model 1ST233

[electra225]

I'm going to start a new topic on my old Imperial stereo. As you all know, ad nauseum, I am determined to find out why my Imperial sounds so horrible. I won't go into that anymore, just mentioning this for context.

I have been watching Uncle Doug videos on the more intricate processes of biasing amps. He does guitar amps, but the principles are the same for audio. How an amp works. He linked a CAD program that I will share if it stands me in good stead. One of his videos showed how to use the program to figure out how various values of cathode bypass caps would change the tone of an amp. He dealt particularly with cathode bypass caps in the pre-amp stage. (I didn't consider the pre-amp stages. I was looking at the power amp stage). Bill is satisfied with the performance of his Imperial, so I used the schematic for his and compared its component values with what was recommended by Uncle Doug's recommended program. According to that program, Bill's Imperial should be as poor a performer as mine is. His amp cuts bass response until about 1500hz. By changing the cathode bypass cap in the 12AT7 first audio stage in his bass amps, the crossover for the bass amp went from 29db gain to 56db gain and from 1500hz crossover to about 150hz. This adds credibility to my theory that the bass amps in Magnavox bi-amps don't work hard enough. Next, I'm going to compare the component values on the schematic for mine and see if they are different. If not, this is a good place to start. Merely changing one cap from .1uf to 1.0 uf, I would get an improvement in bass response as well as gain. And this is only the pre-amp stage. Changing the 12AT7 to a 12AX7 will actually cut bass gain and increase tremendously treble gain. This may get interesting. Stand by....

My investigation is very preliminary at this point. I do believe I'll find design issues in this stereo, plus items I overlooked the first time, due to pure ignorance. I'll bet right now I don't have the bias right in the output stages of the bass amps. There may even be issues in the treble amps I don't know enough to consider at this point. Enough people complain about weak bass that I know I'm not imagining it. The electronics in this stereo are robust enough that performance should be breathtaking. The program Uncle Doug linked does not consider a 6EU7.

Another consideration. This is a split-frequency amp setup. Where did Magnavox intend to run the midrange frequencies? If I get too aggressive in where the bass amp crossover is set, what will this do for the midrange? Do we want it in the treble amp? Maybe instead of changing the crossover frequency in the bass amp, perhaps it would be better to change it in the treble amp.....

[hermitcrab]

This is an interesting topic ... I often wondered about experimenting with different caps in the audio stages

[electra225]

I'm going to dig into the components of the stereo each by each. I'm going to measure the resistors in the phase inverter stage to make sure they are not drifted high and are close to the same value. I want to make sure both halves of the 12AX7 phase inverter are close to the same strength. I need to clean and test the balance control, hum control, timbre control. I believe the tone stack deals with couplets. Not the best to tinker with values, but I'll do what I can in the way of changing values for test. We need to make sure b oth halves of the 6EU7's are the same. Consider whether a 12AX7 conversion will increase or lose gain, and, if so, at what frequency. Make lots of notes and diagrams. Compare what I find this go-round to what I found last time. This amp has at least 5000 hours on it since I worked on it in 2014. I'll test all the tubes, and will consider a new quad and a half of 6V6 JJ"s. I want matched tubes in the treble amp as well as the bass amp. I didn't do that last time. I am going to sub the speakers with a set out of a solid state Stereo Theater just for test. This should be an interesting project we can all get involved in. We should know, by the time we get done, what, if anything, Magnavox designed into this stereo to attenuate bass response. And what specifically makes mine sound so horrible.

[William]

I guess my first question would be, Greg, did you compare components from my schematic to yours? I am very interested in knowing if they are the same, or with mine being one of the last Big Imperial built, if changes were made. If components systems were becoming popular around the time mine was produced could Magnavox changed things around to try and compete with the component worlds big bass?

Bill

[electra225]

I haven't seen any huge changes in electronic components yet. The two drawings are drawn somewhat differently, so what looks like changes at first blush have been the same components, drawn differently. The enclosed speakers are the biggest difference so far, but I'm just getting started. I don't want to commit one way or the other until I get deeper into it.

Is the balance control on yours on a hollow shaft on the bass control, or is it on the chassis?

The Concert Grand uses a cathode follower. I discovered that the Imperial does as well. Instead of the 12AX7 used by the CG, the Imperial uses the ubiquitous 6EU7. Goody, I just LOVE that tube. (Not). Apparently, the rather complicated tone stack with the timbre control and all that nonsense has enough signal loss that the Maggie engineers decided they needed a cathode follower to bolster the signal. The tone stack is in the tuner, but is late enough in the amp that you typically don't find a cathode follower. There is only one stage of amplification after the tone stack. The treble amp uses a 12AX7 first audio, the bass amp uses a 12AT7. Interesting....

[William]

I don't remember that one, I will check and get back to you.

Bill

[electra225]

One drawing shows the balance control optionally located on the chassis. I'm not sure what difference that would make on tone.

[electra225]

Let's start at the top. The schematic for my 1960 Imperial is Sams 563, folder 10. The schematic for Bill's 1962 Imperial is Sams 626 folder 9. The model number for my tuner is 64-01-00, for Bill's 64-03-00. The main difference in the tuners I have seen (quite preliminarily) is that mine has a 6E5 tuning eye tube, his has a tuning meter. My tuner cathode follower supplies 250 volts to the amp chassis, his spec is 270. There are four tone stacks in the two tuners, both use the same component values.

The amp is a different kettle of fish altogether. There are several differences in component values and element operating voltages. The plates in the output tubes on mine run at 320 volts, in Bill's they run at 350 volts. That is a lot of difference. Cathode and screen voltages are also higher in Bill's than in mine. The tone choke, the component that essentially serves at the crossover network between the bass amp and the treble amp is rated at 420 ohms on mine, 480 ohms on Bill's. There is a 4700 ohm resistor ahead of the treble amp control on Bill's, that value on mine is 2200 ohms. So they have made considerable changes in tone compensation, exactly the animal I am chasing. I'm not sure I understand what difference this all makes, but now I know they made changes in the following two years of production.

I believe at this point I'll find out where they get the extra voltage to the amp. I think this confirms my suspicion that Magnavox did not drive the bass amps hard enough in my stereo. They do drive it harder in Bill's. Then I'll go component by component in mine to make sure I don't have a failure of some kind. Then test all the tubes. My suspicion I will find one or more out of spec. After I'm sure I have a good basis to start, I'll change components on by one then note any difference. This is going to be a longer project than I envisioned. I think I'd best put the CD changer on the Symphony so we can have music. This is going to be a fun project. Maybe we'll get some answers to the anemic bass complaint. IF Bill will put up with me asking about his, we can make comparisons between mine and his that works like it should. We are fortunate to have an example of how an Imperial should sound....

[Conelrad]

Next time I stop over, I'll bring an audio generator and a good mic/amp/VU meter.

We'll see just that amp does with the audio.

DG

[electra225]

I found how Bill's bass amps get
more B+ than is in mine. The power transformers are different, although they carry the same part number, 300172. His has an 840 volt secondary, mine is 580 volts. I'm not sure what difference that would make in tone, but Magnavox obviously decided to drive the amps harder. Trouble with that thinking in the way of bass response, not only do they drive the bass amps harder, they also drive the treble amps harder. Methinks the tone quality difference lies elsewhere.

Dennis, anything you would care to contribute to this project would certainly be welcome. I'm in over my head by a good measure....

[William]

Ask away, Greg, and I will do what I can to help you through this.

Bill

[electra225]

Thanks, Bill. I think I have a plan of attack. We'll take this a step at a time. I hope to learn a lot in this project. I can't be much dumber about this stuff than I am now.

I went thru my notes on when I did the Imperial the first time, in 2015. I had posted a question on ARF about the frequency crossover on the 81-series dual-frequency amplifiers. A man local to me was kind enough to respond. IIRC, he is a retired electronics engineer and was kind enough to give me his suggestions. He did a mock-up of some kind, likely mathematically, on the amp and came to the conclusion that were was a "peak" in the crossover frequency that would give more strength to midrange frequencies and "would have the tendency to attenuate bass response". This is basically what I found using the site Uncle Doug recommended, using the resistance of the plate resistor, the value of the cathode bypass cap in the first audio amp.

[electra225]

I know you guys like pictures. These are just general pictures of the rat's nest that is Magnavox wiring. They put the Multiplex adapter in the hottest spot in the cabinet, right over the output tubes. Not a lot of dust for not anything being done in almost ten years. The rectifiers look baked. The speakers are coded 232, which would make them genuine Magnavox speakers. One has a bell cover, one doesn't. The Channel 1 speaker doesn't have threads in the magnet, so I don't think it's ever had a cover. Notice the big holes in the speaker enclosures.

[electra225]

I have the electronics out and on the bench. Of course, the changer had to come out in order to get the tuner out. I'll go thru the changer in this operation, I think. I have no idea what it needs other than a cartridge. I see a receipt from VM Enthusiasts when I went thru this stereo in 2015 for motor mounts and a drive tire. The bridge in the Euphonics U-1 cartridge had turned to goo, if I remember right.

And, Bill, I may have told you wrong. This amp does not have JJ output tubes. It has RCA's. The rectifiers are Raytheon. I have a receipt from AES for a quad of matched NOS tubes....

[William]

That looks pretty much like mine except I do not have the MPX to remove. I did remove the changer to get the tuner out. The speakers look the same too, but I'm not sure what my EIA codes are.

Bill

[electra225]

Is one of your speakers missing its bell cover? Does your speaker enclosures have holes like that, only filled? Do you reckon using thin wood or even cardboard would seal the holes in the speaker enclosures adequately? These speakers have large magnets and voice coils, lots bigger than the ones in the Symphony. The cones seem awfully stiff to me, but what do I know?

[William]

Yes, one has a cover, and one does not, and they both have big magnets with big voice coils. They look very heavy duty. I do have the same square holes but mine are screwed and glued and are about the thickness of the bottom of the cabinet. It was done heavy duty so nothing will rattle when it's shaking with bass.

Bill

[TC Chris]

Nice pics. Did the speaker enclosures have backs on them? Those holes in the bottom look very on-purpose, as though they were placed to affect the sound in some desired way.

Chris Campbell

[electra225]

The speaker enclosures do have backs. Plywood, stained both sides, rough finished one side. I have some lauan I'll staple over those holes for test. If I get the bass back due to them being plugged off, I'll do something more heavy duty. If they rattle when the stereo is "shaking with bass" that will be the sweetest nasty noise I ever heard!

I have found, with the exception of some of the tubes in the IF strip and in the MPX, every tube in this poor, neglected old Imperial is bad. Not just a little bad, not just failing the life test, (which they all do). One of the output tubes in a bass channel tests about 20%. On the life test, it falls to zero. The 12AT7 phase inverter tube is really weak. It has an Amperex 12AX7 all the audiophools go gaga over. It is junk. One half of one rectifier is really weak, both fail the life test. There is considerable what Shango calls filament boiloff in the rectifiers and output tubes. I did indeed install a quad of matched NOS output tubes in the amp in 2015. Never again. I have old tubes already, why pay good money for more? I believe I saved a pair of NOS Sylvania 5U4's. IF I can find those, I'll use them. I put the tubes back in their original locations. My next test is to run some voltage checks. I want to see what B+ is doing. Then get a baseline of performance. Then start checking component by component. I need a 6E5 eye tube, I should have one. I wonder what else I'll find....

BTW, Bill, I know where there is a spare Multiplex adapter if you'd like one.....

AS I had feared, all three 6EU7's in the tuner test weak. One is still the original. I see some of the tubes coded the 46th week of 1960. I thought this had an earlier build date than that. I need to research this. If you look at the second picture above, the amp is coded the 24th week of 1960. And I can report that PB Blaster has kept the cad dust at bay on the amp and tuner chassis. There is some dust, but no cad dust.

[TC Chris]

If there are backs, then those holes were there on purpose, perhaps a sort of bass reflex design. I'll bet covering them makes things worse.

Chris Campbell