Magnavox 1961 smaller tube console

[Motorola minion]

This is a thread related to 6EU7 subbed with a 12AX7. Because I have never seen one before, I was surprised to encounter three of these SE 6BQ5 models (!) last year. I paid $60 for the bottom one, assuming it had a 93-series amp- it didn't Another candidate for the (6EU7 conservation) conversion.

The other two pictured were brought by a customer. One blonde and one mahogany. I just finished the second one, so I figured a post was needed to identify these and the work needed for each. Ill just say that both had barely acceptable FM performance, even after alignment. But, even though there is no RF amp stage, the AM is on par with RCA - it seems to reject interference so well.

[electra225]

I would respectfully submit that the top two are 1960 models. "Concert Hall", model 1ST208F. I have not seen one in blonde before. The flat knobs and the Conquest changer are the giveaways. The bottom one is a 1960 Symphony with the AMP 175 amp. The 1961 models would have the S-600 changer and the 93 series amp.

Magnavox tube tuners without AFC, (middle of the 1961 model run) have variable FM performance. Some are fantastic, most are mediocre, and that is being charitable. They tend to not demodulate the FM stereo composite. Tuning to one end of the carrier or the other sometimes helps. A good FM oscillator, typically a 6EA8 is essential. Many times replacing the EA8 with a 6GH8A (note the suffix !) will help with FM drifting. A good antenna is essential as well. The definition of "good" varies depending on receiver location and station location. Tuner front end tube condition needs to be up to snuff. The AM side of the tuner, although basically an AA5 circuit with no RF amp, are typically good performers, loop antenna notwithstanding. You can add an external AM antenna to good advantage. I have three Magnavox stereos with AFC, all good FM performers. I have three Magnavox stereos without AFC. The CG is just okay. The Imperial is usually pretty good. The Symphony is not worth the trouble. I'm told tinkering with the discriminator improves performance in many cases, but such activity is above my pay grade. People who shelled out the big money for one of these things when they were new didn;t do so for radio performance. The record player was the big thing to Magnavox.

[Motorola minion]

Yes, that 1ST228 looks like my1960 Symphony 1ST208 and I mistook it for one. Those "twins" are 2ST259.

1ST208F on 2nd floor.jpg (60.76 KiB) Viewed 5928 times

[Motorola minion]

I have to agree the record player and furniture sold the average Magnavox, so I make sure those changers are carefully detailed and cabinets clean. The amp and speakers are rarely an issue either, but the FM sure is and it seems not worth my time. The more I tinker, the less hope I have that any tiny improvement will be noticed.

How can the same tuner (yes, German made die cast FM tuner) used in a Motorola, Zenith or RCA perform so much better and with selectivity that matters most a with a dial ever more crowded. FM at the low end (below 92) seems affected disproportionately, which is where all the good stuff is .

The 6DT8/12DT8 or any dual triode; 6BQ7, 6AQ8, 6EA8 are all good FM tubes but IF stages may be where Magnavox lags the others.

While I do not advocate tweaking IF transformers by ear, the discriminator transformer may be the only one I do recommend you try a 1/2 turn excursion both ways. My experience is similar with my 1947 Berkeley and1960 Symphony where tuning "through" the strong (spread out) stations change clarity of audio , while the weaker adjacent stations are "swallowed up". If you adjust the ratio detector slug and tuner simultaneously, a sweet spot will be found.

[electra225]

I have two or three big RCA table radios with AM-FM. They are pretty good performers and don't have the drift Magnavox tube tuners have. I have a 1946 Stromberg-Carlson 1121 with a fairly non-drifty FM tuner. I don't know why Magnavox tuners without AFC drift so badly. Once they are warmed up, the situation seems to be better. My issue with them is within the first 30 minutes or so of operation. The RCA's use a 6U8 as an FM oscillator, the SC uses a 6SB7 octal metal tube. The 6U8 is in the same family as the 6EA8 or 6DT8.

[electra225]

Dave your comment about tweaking the discriminator transformer interests me. I was taught that you don't even touch, look at, dust or have any association whatsoever with a discriminator unless you know what you are doing. Not saying you don't know what you are doing, but I sure don't. Will the discriminator have an effect on drifting? And, if so, how does it affect drifting? My CG is almost there, the Imperial is really not all that bad, the Symphony is a waste of time trying to listen to FM. The tuners are very similar as far as the front ends go. My question, then, would be why would three basically identical tuners behave so differently?

[Firedome]

No experience with these Mags or their FM issues, but I sure do like that blonde one, then again I so have a thing for blondes, my wife is one (well mostly gray now lol). Anyhow that cabinet design looks really good in light wood, I'd shed the DuMont in the LR for one of those if your customer wants to sell it... it'd be a great match for the RCA!

[Motorola minion]

I would not recommend doing anything unless you can't tune the FM station you want to hear. If it sounds good, it IS good.

Using a VTVM as per the SAMS alignment instructions is way better than tweaking by ear. If adjustments are done in the order listed and using an AM signal generator, following those instructions always results in improvement. The discriminator adjustment requires an FM generator, not easy to find. A "live" signal is needed also to adjust the MPX for maximum gain of the pilot carrier for stereo. I did this in an RCA and it made a huge difference.

The 55-series tuner below needed almost no adjustments and tunes the full range 88-108, even though the scale is compressed below 94 MC. This tuner uses a 6CB6 RF amp and 6U8 oscillator/mixer.

Drifting is another issue altogether. Automatic Frequency Control alone improved the stable operation of Magnavox solid state tuners way beyond the tube designs preceding it. Like the 59 series tuner in the twins, I have two RCA tuners of this vintage with equally annoying tuner issues (12DT8), but they do have AFC.

[Firedome]

It seems the SS Mags improved FM performance and stability greatly, I was very impressed with the sensitivity, spurious signal rejection and stereo audio quality from WSKG Jazz on my late Uncle's Maggie P231 recv'r when it was on his basement workbench, using just a simple flat-ribbon antenna. It was about a '71/72 mid-line model.

[electra225]

The later Magnavox tube tuners with AFC are good performers with no FM drift. I have three, 2 five knobbers and a six knobber. The front ends are quite similar in both types. I have wondered if the eye tube affected FM drift, but the Symphony, the worst drifter, doesn't have one.
.

[Motorola minion]

The eye tubes were mainly monitoring the AVC voltage, which is the DC equivalent of the carrier signal strength.

The frequency drift of an FM oscillator is a combination of thermal changes and parts tolerances. Mica and ceramic capacitors come in several flavors to compensate for changes in a resistor, etc. AFC somehow used a diode to sample frequency-voltage and varied FM oscillator frequency in proportion. It was a pretty tight loop and did not involve IF-AVC, which is independent of frequency.

Inside those "pull string tuned" German tuners used by RCA and Magnavox, is a sloppy-soldered mini PC board and variable capacitors to align it. In most cases, these adjustments are at the end of their range, pointing to other issues.

[electra225]

The conversation I referred to was on the "big forum" a few years back. The point was made that if the eye tube or its circuitry were wonky, the AVC voltage could be upset, thereby lowering the sensitivity of the tuner and lowering the volume. Shorts, gas in the eye tube plus drifted resistors in the circuit, could upset AVC voltage. Models without the eye tube this would not apply to. It might be well to check this circuit just because to eliminate this as a potential problem area.

[Firedome]

Why were those tuners made in Germany in the first place? It's not like RCA in particular didn't have the engineering & technology to make one of their own. And the US TV makers made their own or bought them in from US makers like Sarkes-
Tarzian &c, didn't they? Seems odd.

[electra225]

Sarkie's plant in Bloomington and the RCA plant were in close proximity to each other. I'm not sure what their business relationship was. I wasn't aware that RCA and Zenith used foreign-built tuners.

[Motorola minion]

German-made FM tuner sub-chassis for 1961 is 59-series and with few changes, AFC was added to 79-series tuners (the 1962 version). Abandon all hope for that first picture, the second picture shows a 79 series exterior view, with AFC line to a feed thru cap on top. I do not recommend removal of this circuit board, unless a bad connection exists within that shell. I had to do that with a '61 RCA, same troublesome tuner

Comparing schematics, just the AFC diode, cap and resistor was added then a few surrounding cap values went down to compensate, twin-triode tube 6DT8. The diode allows a dc voltage sample from the discriminator* to "tweak" the oscillator. I just had to look these up, unsure of what I had posted earlier.

Tuning eye tubes do use AVC. So, gassy - drawing excess current form that high-impedance AVC line would be a problem. None of my 60s Mags have an EYE tube, just the 47 Berkeley and 48 Windsor do.

(* zero when tuned correctly)

[Motorola minion]

Regarding the better tubed 200 series tuners 55,57,77-series, I have to agree the 6CY5 RF amp and 6EA8/6U8/6GH8A osc-mixer was wayyy better than this experiment in global sourcing.

How come Zenith and Motorola with very similar German-sourced tuners worked so much better

[electra225]

My guess would be that Zenith was heavy into FM early. Their FM tuners of almost any description were top notch. Motorola made commercial radio equipment, so it may have been in their DNA to make quality FM equipment.

[TC Chris]

My guess would be that Zenith was heavy into FM early. Their FM tuners of almost any description were top notch. Motorola made commercial radio equipment, so it may have been in their DNA to make quality FM equipment.

Remember that Zenith tuned FM with variable inductors, while everybody else was using variable capacitors. Many AM car radios also used variable inductors, slugs moving in coils. Maybe for durablity in the car setting? I always wondered why Zenith chose that system for their FM front ends when almost nobody else did.

Chris Campbell

[Motorola minion]

My guess would be that Zenith was heavy into FM early. Their FM tuners of almost any description were top notch. Motorola made commercial radio equipment, so it may have been in their DNA to make quality FM equipment.

Remember that Zenith tuned FM with variable inductors, while everybody else was using variable capacitors. Many AM car radios also used variable inductors, slugs moving in coils. Maybe for durablity in the car setting? I always wondered why Zenith chose that system for their FM front ends when almost nobody else did.

Chris Campbell

Possibly due to a good relationship between Edwin Armstrong and Zenith.
I have some early AM/FM table radios by Motorola, Westinghouse and Philco.
The FM performance is awful on those and CU reports on table radios bear this out.

[electra225]

CU also panned RCA and Zenith televisions in the late 1950's, early 1960's, in favor of lesser brands. We all know that RCA and Zenith were at their prime during that time.

The problem with using a 6GH8A as an FM oscillator in a Magnavox tube tuner is that they are known for being wonky themselves. You can introduce new and evasive problems using one. You have to make double-sure, if that is possible, that the GH8A you are using is good. The models without the "A" suffix don't work at all in my experience. When the 6GH8A is good, my experience using them has been positive.