A comparison of 6BQ5 console amp designs, Mag and Fisher

[Firedome]

Ran across this on AK, an interesting comparison of a 1960-ish Fisher 6BQ5 Coronet or Electra console amp vs a Magnavox 6BQ5 console amp (not clear what model, can anyone venture a guess based on the description? it seems to be one with ceramic cartridge input if that tells us anything), but it's all somewhat above my pay grade to evaluate, anyone care to make a comment on the validity of this?

"Now the driver stage design and OPT used are both very decidedly well above that of Magnavox EL84 designs, but the penalty of Fisher's driver stage design is the rather low sensitivity it produces (2.0 vac per channel required for full power output in the Model 1000). Magnavox, squeezing out every last oz of functional capability from its designs, used a paraphase style phase inverter that has much more gain (typically requiring less than 1 volt for power) than Fisher's cathodyne design, but also has much poorer AC push-pull balance across the audio spectrum (and therefore higher distortion). However, since Fisher was using this amplifier with a significantly better control preamp than Magnavox ever used, the high input level requirement of the Model 1000 was not a concern in the overall design of the Executive. Magnavox -- often using no more than passive tone controls between a high output crystal phono cartridge and their power amplifiers, needed as much sensitivity as they could squeeze out of their amplifiers. Ergo, performance was sacrificed for sensitivity in their designs".

What say ye? BS or legit commentary.

[electra225]

Everyone is entitled to their opinion.

The only P-P stereo amp with 6BQ5's that Magnavox ever built, as far as I know, was the 93 series. My affection for the 93 series is well-documented.

[Firedome]

So all the other Maggie PP amps were 6V6gt based. Never had a tube Mag, only a '72 SS R231, which I find a very impressive performer, so still learning about them. The big amps in the CG etc certainly seem impressive, with their large transformers.

Don't know who wrote the comparison above or their credibility regarding paraphase vs cathodyne, which seems to be the heart of their argument, nor the actual difference in OPT transformers. I know Fisher bought in theirs, I forget who from, will have to consult my files. Did Magnavox make their own or buy them, and if so from whom. Scott was one of the few who wound their own in house, but someone as big as Magnavox may well have, which allows for tighter quality control.

[electra225]

A guess as to the supplier of Magnavox transformers would be Mallory. All the Magnavox stuff I have is equipped with Mallory filter caps.

Many of the "comparison" reviews take pot shots at Magnavox. Why not Motorola, or RCA, or even Admiral who all made amplifiers quite similar to the 93 series. I guess they always pick on who's on top. No fun picking on second best.

[Firedome]

I don't believe Mallory made transformers, #s on them might give a clue, I'd be interested to know. The output transformers in particular, play a very large role in the ultimate quality of the sound of an amplifier.

Experience and opinions may vary, and this is just my own opinion based strictly on my own experience, as it must be, but as far as consoles go IMO TOL models from Fisher, Scott, Pilot, Stromberg-Carlson, to name some of them, were the top tier. I have owned all of these, including 5 Fishers (the Scott was SS, the rest tube). Unless one has experienced the whole variety it's difficult to make a valid judgement. Hey I like them all, from the most humble to models that cost $2500 in 1960 (about $25,000 today), I'm not putting anything down, but there is a pecking order, as in all things, that should be acknowledged. One must drive a Cadillac in order to make a valid comparison to a Chevy, but all should be appreciated for what they are and preserved.

[electra225]

Mallory was in Indianapolis. My aunt worked there for like a billion years. They built lots of electronic components besides capacitors. Magnavox wasn't even building their own speakers by the time the stereos came out. I can't imagine Magnavox winding their own transformers. I believe I have an old Mallory catalog around here someplace that lists IF transformers. When I get the back off a Magnavox, I'll see if I can find codes on the PT.

Your comment about "best to least' is exactly why I have tried to collect a Magnavox from the smallest model with P-P amp to the Concert Grand. All sitting side by side. It is amazing how good the smaller ones sound in relation to the CG with all its exotic electronics. About the only difference I see is the size of the cabinet.

[hermitcrab]

I also like them all, last thing I think about is there performance ... if the amp is run by a hamster in a running wheel I could care less , as long as it looks good and sounds good... it passes my inspection...

[electra225]

[Firedome]

Interesting concept to have the entire line up, from least to top!

I know of a guy in Maine who's into vintage outboards, yet another of my crazy hobbies, who particularly likes the 1958 Johnson line, one of their flashiest ever, especially the '58 Super SeaHorse 50 V-4. He decided to find the entire line up from 3 hp to 50, restored them to like new, found original Johnson showroom stands, and set up a display in a room that duplicates a Johnson showroom in 1958, just like in this pic (that kid would be me in 1958, crazy about outboard motors even then).

How cool would it be to assemble something like a Magnavox Dealer showroom circa say 1962 with the entire line-up of Stereos, TV, and radios, complete with banners, quite an undertaking, but that would be amazing! Doug H. in Chicago could almost do that with his extensive collection of Zenith stuff I'd bet.

Re; transformers I'd bet Mag bought them from one of the many makers, Triad, UTC, Chicago Standard, Magnetic Windings, Thordardson, Haldorson, Peerless... there were loads of them that did just that and it was typically far easier to spec them and buy from one of the OEMs. If anyone can give me #s off them I might be able to ID it.

[electra225]

I only have one hole in my "least to best' lineup. A little Imperial. I thought the Stereo Theater would be the same, but that's not true. The 300 series Stereo Theaters used the little Imperial cabinet. The 400 series Stereo Theater used the little Imperial electronics. Usually, not always. Not in the case of my 418. I need a little Imperial to make my lineup complete, seven instruments in all. On my bucket list.....

[Motorola minion]

The plate and bias voltages applied to the 6BQ5 were lower in the amps made for reliability. If you see more than 9 volts on the cathode, there is more than 300 volts on the plate. Fisher, Motorola and Zenith used Amperex or Mullard EL84 instead, often labeled 6BQ5 and made in Holland or Great Britain.

Most amps RCA and Magnavox designed for reliable operation using standard parts. Most 6BQ5 made by RCA, Sylvania, Tung Sol, etc was fine for these and the TV's that used a 6BQ5 for a vertical or audio output tube. These standard replacement tubes, when installed in an amp that subjects them to higher voltages, will wear them out prematurely.

I proved this fact using a Zenith SE 6BQ5 amp that was originally equipped with Amperex EL84s, that were still OK. Measuring the cathode voltage gives a pretty good picture of the tube's condition. As the tubes get weaker, the cathode voltage drops along with the emission. There was a noticeable loss of fidelity from a borderline weak RCA tube, where bias* was -6 volts. When the OEM Amperex or new Russian JJ or Sovtek EL84 were installed, bias returned to 9 volts and fidelity restored.

Doing the same with an RCA SE 6BQ5 amp, specified bias was -7.2, resulted in MUCH less difference in sound. In one of those more powerful amps, dropping cathode voltage from 9 to about 6 or 7 volts will introduce distortion and clipping at high signal input. Tube is LESS biased, that is to say the tubes bias is LESS *negative in respect to cathode and resulting operation is more prone to distortion because the tubes operating (quiescent) characteristics have changed from its design.

[electra225]

At 120+ volts line voltage, I see the plate voltage on the 6BQ5's in my 93 series Magnavox amps tickle the 350 volt range. IIIRC, they want 11 volts on the cathode. There are those who downtalk the 6BQ5 due to how hard they are typically driven. I have run one 93 for almost ten years like that without apparent harm. I may measure cathode voltage again one of these days to test out your theory of checking tube strength measuring cathode voltage. I appreciate that tip. IIRC the factory spec on the plates of the 6BQ5's in the 93 series amps is 325 volts, at 117 volts line voltage. Magnavox runs their 6V6's at 295 volts on the plate. My experience has been that running a 6V6 at 350 volts on the plates might not work very well. I don't know what running 400 volts on the plate of a 6BQ5 would do to the tube.

[Firedome]

I have noticed that the more "deluxe" brands like Fisher, Scott &c do tend to have specified Amperex, Telefunken, or Mullard EL84 output tubes as standard, whereas in my Dumont, GE, and most other console amps that I've had GE, RCA, Sylvania 6BQ5 tubes were called for in the original spec. I've also noticed that the premium brands almost always came with Telefunken Ecc83s not US 12ax7s or even 7025s. I'm postulating that the reason for this may have been that a more demanding circuit called for tubes that would take extra abuse or were lower noise in preamp, and the higher priced brands could justify the extra expense. Or was it just that the Amperex salesman gave them a better deal? lol