1961 Zenith Stereo Amp Problem
 

Last weekend, I drove 4 hours to buy a console stereo. (Don't laugh, I know that sounds crazy). I've been looking for an early 60's Zenith tube-type for a while now, and finally found a nice top-of-the-line model with the extended stereo set-up and the optional reverberation.

I got it home and immediately replaced all the wax paper capacitors and the electrolytics (other than the filter caps, which seem to be fine). I also replaced all the old carbon resistors in the audio output circuits.

The amp uses two 6BQ5 output tubes in a push-pull circuit per channel and a 12AX7 as a pre-amp / phase inverter tube. This should really sound great, but it leaves a lot to be desired at the moment. At lower volume levels, it's pretty good but when I turn the volume control up to half or over, it's bringing in a lot of distortion.

Is anyone here familiar with the "current balance" adjustment pot on these amps? There is a potentiometer between pin 3 of the two 6BQ5 output tubes in each channel, with the center arms connected to each other and going through a 100 uf electrolytic and a 63 ohm, 4 watt resistor to ground. There is a 330k resistor coming off the plate of each output tube to a set of small terminals poking up through a hole in the chassis. The factory service manual says to hook up a meter between these terminals and adjust the "current balance" pot to get a zero-volt reading.

On mine (with both pots cranked all the way), one channel is down to .3 volts and the other channel is right at 2 volts.

Any ideas?

You could swap the tubes and see if the situation reverses. If so, your tubes are too different from one another. If not, it may be some more bad resistors.

I doubt that's the cause of the distortion. There is no substitute for an oscilloscope to see what's going on; otherwise it's like driving with a blindfold.

It probably sets the quiescent current in the PP output tubes to be the same. Doing so would reduce crossover distortion if it is biased towards the 'B' end of class a 'AB' PP configuration....As well as making tube aging more even.

The grid voltage on the diagram says it should be 0, but between the four output tubes, I have a range of between .5 and .9 volts.

Also, all 4 of the B+ voltage sources are about 32 volts higher than what the manual calls for. For instance, the B+ source going to the audio output transformers should be 375 volts, but is running right about 408 volts. The dropping resistors to bring down the main B+ for the other 3 sources are (surprisingly) accurate, so there's no problem there. I haven't replaced the 3 section filter cap, but since there is no audible "hum", I think they are OK.

Consider that the speakers could also be at fault

The high B+ could be a factor of the mains voltage you are plugging that set in to. When that set was new 115V AC was the household standard, but now a days it is not uncommon to see 127V AC at the wall outlets.

You guys really get 127 ? That's horrible,
Around here it's 115 - dead on 120, if I had 127 I would look into an autotransformer for the whole
House

Hello sometimes I get 130 volts output at the wall outlet I have replaced a lot of tubes and caps in my hifi units call your power company to address that voltage issue it may be a power transformer on the pole that could be bad..

That's pretty sloppy on the utility's behalf. I would try and file a complaint, not only is it wrecking equipment it is also making your bill higher than it needs to be.

The electric company just replaced the transformer on the pole in my back yard a few months ago. The voltage at my outlets is pretty steady at 124 volts. That is a bit high for the design of the amp, but I don't think that those extra 4 volts are enough to cause the B+ to be 30 volts off. The factory service manual says it's designed for 110 to 120 volts.

I know my speakers are good, because I've tried running another set and have the same outcome.

I'm wondering if the fault might be my output tubes. None of the tubes in this set have been replaced (with the exception of my substituting the 5U4 to see if there was any effect on setting the output tube bias). I never placed much stock in tube testers, and got rid of mine years ago, but I do wonder if these original output tubes have good emission levels now.

Check the cathode current of the output tubes. If
The tubes are really weak it would also explain the high b+ voltage

In the schematic you are getting the 'correct' B+ values out of do they list the line voltage they took that data at? If it was 115V, your line is running 9 volts higher, and the B+ winding of your power xformer/rectification scheme steps the line voltage up by a factor of 3 or more then that would be (3*9)=27 volts of extra B+ right there.

The schematic I have is a Photofact, and it says line voltage was maintained at 117 volts for the readings. Zenith rates the chassis at 110 to 120 volts. Thankfully, a generous videokarma member has offered to scan and email me a copy of the factory service manual:thmbsp: It'll be interesting to see what Zenith has to say.

I turned it on this afternoon and let her play for a couple hours. I noticed that after it had been on that long, the volume could be turned up much higher before distortion sets in than it does after only being turned on a shorter amount of time. To be honest, it just sounded better in every aspect after being on for a few hours. I know weak output tubes can sometimes perform better after being ran for a while, and weak output tubes could also be the reason for the bias being a little off. I guess it would also likely have an effect on adjusting the current balance pots for the grids, also.

If the tubes have seen a lot of hours and aged unevenly then it is possible they no longer match well enough for the current balance to function as designed.

In every one of my amplifiers, that uses them, the 6BQ5's are wasted.
I think I have two good 6BQ5's, to my name. :tears:

Thought someone might like a pic

It's fixed!!

After thinking about it for a while, I decided I really ought to replace those original 6BQ5 audio output tubes. I had read that the 6BQ5's are commonly used in guitar amps, so I made a phone call to a local guitar shop. They told me they had 4 of them in stock, for $7 each. I ran on over there to get them ($7 a piece is the best price I've seen on them). Three of the tubes are new production "Mesa" brand tubes and one is a NOS General Electric tube.

I knew when I powered it up the first time I made a wise decision. The volume control (which was set to almost the center position before) had to be cranked down. The sound is twice as loud and twice as clear as it was before. I had to turn the bass control down quite a bit to keep it at a comfortable level (it has been set wide open with the original tubes). With the set of 4 new output tubes, I had no problem at all adjusting the current balance control to exactly zero volts.

I flipped the chassis over, and lo and behold....the B+ was down 12 volts from what it had been before. This is much more in line with what I felt was a comfortable increase considering today's higher line voltages. I also noticed that after being turned on for an hour, the power transformer was still relatively cool to the touch (it was running pretty warm with the old tubes).

I am thrilled with how well this old girl is performing now!

Nice work, and a nice looking set.

I have to admit, this unit sounds exceptional. The bass is very punchy, without the "boominess" that my Magnavox solid state consoles have. The horn tweeters have rather short horns, and produce less mid-range sound than some exponential horns I have heard.

The tuner (like most top-end Zenith's of that era) is fantastic! With the built-in antenna, it brings in the weaker FM stations that most radios leave out. Even the AM bank is quiet and sensitive. As odd as it sounds, the FM receiver is not stereo....the record changer and tape inputs are stereo. This was Zeniths best in 1961, and in 1962 the replacement chassis for mine had FM stereo. I suppose the engineering department figured there weren't enough FM stations broadcasting in stereo in 1961 to make it worthwhile to add that extra circuitry. Unlike the Magnavox sets of that era, no provision was made for adding a stereo multiplex decoder chassis to the Zenith. The owners manual just mentions the option of adding an FM stereo receiver through the tape input jacks.

Now, I've just got to get the record changer back in order. The motor was froze, but I took it apart and cleaned it up and she's working again. When I turn it on, the record drops, the tone arm starts to move over, and then it comes back to the rest and turns itself off. The record changer is a VM (Voice of Music) built machine, and I'm really not that impressed with it. As I told someone else, I am tempted to find a later model Zenith 2G record changer for this stereo. I would hate, however, to loose the cool "Cobra-Matic" tone arm :)

With overly-high line voltage, I would want to watch the output tubes carefully in a darkened room to see that they're not red plating, specially on loud passages.