Need advice troubleshooting 1971 Univox amp (tremolo schematic included)

[LucifersTrip]

Thought I'd ask for a little advice, since I do very little with speakers...

I picked this up recently and have been having a blast testing it out with various fuzzes. It's from 1971 and is one of the earlier solid state models, at least for Univox.  You can tell...Univox bragged that it had 10 transistors (looks like a bunch are germanium).



All the electronics work well (volume, bass, treble, tremolo, reverb, 3 inputs), but the bottom speaker, (which was replaced by a previous owner) is silent (or near silent).

The original is a 4 ohm Univox - stamped speaker and looks like (it's faded) 30 Watt max. The replacement (from the 90's?) is a 4 ohm  Lanzar OA10 that's 150 Watts (OA = open air).

The two speakers are in parallel with the main outputs going directly into the Lanzar. Leads then run in parallel to the Univox.
Again, with both speakers connected, the Lanzar seems silent.

I've only done a couple quick things so far:

Measured the continuity between the parallel speaker connections. OK
Measured the resistance across the Lanzar. 4 ohms
Disconnected the Univox speaker. The Lanzar motorboats.
Disconnected the Lanzar speaker. The Univox is silent.

Does there need to be 4 ohms across the speakers to functions properly or is there something else...
If you search amp speaker motorboating, the common advice is to look for bad caps, but shouldn't that
effect both speakers?

Just thought I'd ask for suggestions since I'm a speaker novice, but it just seems blown.

Here's the schematic (or at least a very close one for the U-150RX. I don't know that the "X" stands for.

On a related note, someone may want to build the trem into a pedal, if it hasn't been done already.  Same
transistors as the Shin-Ei Companion fuzz. (Does it need 35 volts?)



edit:


thanx

[Gus]

do you have a small amp you can test the speakers on?  This might be a good way to tell if the speakers are good.

Maybe both are replacements and have different sensitivitys

The schematic has 4 ohms marked by the speakers and shows the speakers wired in parallel
So should the speakers be 8 ohms for a 4 ohm load?
or
4 Ohms for a 2 ohm load?
It does not appear to be clear to me what the load should be

Are the speaker wired in the proper phase?  You want them moving the same way so they don't cancel


Interesting schematic
The preamp gain stages look to be bootstrapped from the emitters 33uf, 100uf and the first stage of the reverb 100uf
Output looks something like what R.G. has at GEOFEX about the Vox amps

When I worked on amp circuits that I did not know the history of I test all the caps and parts that I can before I power the circuit up
The 1000uf 35V electrolytic cap that couples the amp to the speaker can get beat up from AC current besides ageing and drying out
I would pull and check all the electrolytic with a meter for at least the uf values
Check the bootstrap caps
the 2nd stages 100uf from the emitter of the 2SC536F has feedback from the output going to that node

[KazooMan]

I agree with Gus on the output impedance issue.  It is strange that the schematic has 4 ohms printed on it and then you have two 4 ohm speakers in parallel. 

I suggest that you post your question over at:

www.music-electronics-forum.com

It's a forum with many service techs who are always happy to help out, much like the experts on this forum.

Tell 'em I sent you.  I post over their with the same username.

[R.G.]

It's not clear what the speakers ought to be from the schematic.

As to motorboating, it's a common issue with amplifiers with interstage transformers. The transformers don't (usually...) have enough low frequency response to keep the feedback loop stable in the face of odd loads. Worse, the output capacitor makes the phase shift in the interstage transformer worse. it is possible that you could get things to work better by increasing that 50K feedback resistor, but there is the risk that it will go unstable on you.

That doesn't mean that the Lanzar isn't a problem, just that it's possible that it's not. The right advice is to test it with another amp.

I'd say the first step is to look at the caps. Are they more than 10-20 years old, or worse, original? If they are, first go do a cap job and replace all the electros. In particular, the output capacitor can directly mess with the amplifier's stability since it's inside the feedback loop, and so is the power supply filter cap.

[PRR]

> a small amp you can test the speakers on?

++1. When you don't know if the amp is sick, the speaker(s) is sick, or both, you can not reliably test without a 3rd-party reference (another amp or another speaker).

> shouldn't that affect both speakers?

This is strange. Another reason to get additional cross-checks.

> It does not appear to be clear to me what the load should be

It does say "4 Ohms" on the line common to both speakers.

I did a long derivation but not worth posting.

It won't make an honest 20W in 8 Ohms. It can touch 20W in 4 Ohms.

Guitar-amp ratings are often dishonest.

Considering its age, I would *only* load it at 8 Ohms. Maybe it was sold with 4 Ohm load, but the difference in power-out is small, while the difference in transistor stress is large.

I wonder if the "Univox 4 Ohm" speaker is even original. It might have been wired two 4 Ohm in series, but the (untrustworthy) plan shows parallel. IMHO there's no way it was two 4r in parallel.

This "Lanzar" appears to be a car sub-woofer?? If so, it is ALL wrong for this application. The Univox g-speaker is tuned for max output above 80Hz, strong midrange. A sub-woofer is tuned for acceptable output *below* 100Hz in a small box, which means very weak midrange (and bass output you can't get from guitar even if it were properly boxed).

Get two same-model 16 Ohm *guitar* speakers and parallel them. They only need to be 10-Watt guitar rating, so you don't have to pay for mega-magnets.

Considering its age (43 years!!), and original cheapness, it is no shock that it is marginally motorboaty. You do have to replace electrolytics occasionally. Start with these:

[bhill]

And if after the cap job you still have problems, give the folks at http://www.ssguitar.com/ a shout. There are several very knowledgable techs with decades of experience in SS amps. As I recall teemuk does occasionally post on this board, but you will get a better response there. After all, Teemu Kytalla wrote a very very in depth book on solid state guitar amplifiers that does a very good job taking you through everything you could need on the subject. That book is available as a pdf on the ssguitar site, so something to fill your bedtime reading hours.

[LucifersTrip]

I just wanted to thank everyone for the info & suggestions...and especially PRR for the detail.

It seems the consensus is to test the Lanzar first...Is the Ruby good enough for a simple test? (*edit: yes)

Regarding the ambiguity of the "4 Ω" on the schematic, you guys are questioning whether it's a load rating or the actual spec of the speakers?
I guess it's possible that the Univox speaker is also a replacement, but it would seem less likely (I've seen pics of another that had the exact
same Univox logo stamped...did someone remove an 8 ohm Univox and replace it with another early 70's Univox 4 ohm?).

Is it really a good probably that a cap could be bad when everything else works and sounds great (with one speaker, of course)? The amp is super quiet...not even an inkling of hum.

I'm using this as a home practice amp to screw around with fuzzes, so don't need mega volume. I haven't had the need to go past 50-60%,
which is loud enough give some nice ear-ringing after 15 min or so...All the controls work as they should, with no crackling and a good smooth sweep for all (there's no bunching). I have enough treble at full but not enough for harshness or chime. If I had to nit pick, I'd say the only thing I might occasionally need is a bit more bass, but the amount I'm hearing is consistent with what I've heard in other smaller, cheaper solid state amps from that time period.

I'd really like to avoid replacing both speakers, especially since the Univox is vintage and sounds cool. If the Lanzar is a dud (*edit: it's a dud for incompatibility, not for lack of working), can someone suggest another...in parallel or series? (PRR, I know you suggested to load at 8 ohm).

>>>it is no shock that it is marginally motorboaty

I think "marginally" is an understatement. The Lanzar is silent (*edit: or seems so) until the Univox is disconnected, then it motorboats loudly. There's no gray area.

Thanx again for the suggestions. I'll look for something I can try the Lanzar with and search for more info on the amp.
=================

*edit: tested with Ruby and both actually work.

The Univox sounds very good with a full range of bass & treble and can hit a reasonable volume.
The Lanzar (as PRR suggested) is bass-y & muddy and is at about 50-60% of the volume of the Univox.

This leads me to think that it's possible (when both hooked up in the Univox amp) that the Lanzar was actually making some small sound, but was completely overshadowed by the Univox. I did put my ear right up to the Lanzar and couldn't hear anything, but maybe not a good volume test.

[PRR]

If you have zero budget: put the Lazar in your car, shove a dinnerplate in its hole, and use just the one Univox speaker.

[LucifersTrip]

If you have zero budget: put the Lazar in your car, shove a dinnerplate in its hole, and use just the one Univox speaker.

Budget is no concern within reason (the amp is worth ~ $200-300 and less with no original speakers)...just want to drop a second one in there that's compatible. Am I correct thinking that the Lanzar is "silent" because the circuit can't drive a 150 watt speaker? The Univox is 10",  30 watt, 4 ohm

I did a little more research and a couple people noted the Univox's were Jensen. One person said theirs has a couple 8 ohms in there.

Would I be looking for something like these?  I will consider pulling both and buying two.
http://tinyurl.com/kdh65gf (only 8 ohm)
http://tinyurl.com/mpmbtjs (only 8 ohm)
http://tinyurl.com/l6u3mxa

[bluebunny]

. . . the Lanzar is "silent" because the circuit can't drive a 150 watt speaker?

I would say not.  Plenty of reports around these parts of people driving e.g. Marshall 4x12s with sub-watt amps (e.g. Murder One) to great effect.

[R.G.]

Well, if the amp is loud enough with just the univox speaker, take a pair of cutters and clip the speaker wires to the Lanzar. All done. 

It makes a reasonable looking hole filler.

As noted, even mini amplifiers drive huge speakers OK.

[wavley]

The Lanzer's quietness is most likely because guitar speakers often in the 100 dB sensitivity range where car/home audio speakers are often in the 80's for sensitivity, that's a pretty big difference.

[PRR]

Here's a plot of a good typical guitar speaker (Eminence Legend 1275) and an especially good subwoofer (Adire Shiva). Both are Twelves because there's more data around; Tens will be similar but a few dB less both ways.



In the guitar zone, the G-speaker is 8-10db (more than twice) louder over the fundamentals and a stunning 20db louder in the harmonics. Also note that the ear typically judges location by the higher tones, they are more directional and usually a better guide to/from food/danger.

So I believe the sub "is" working. But two such speakers adjacent, you will be unable to hear any guitar from the over-mellow sub with all the racket from the light-cone screamer.

[LucifersTrip]

"This leads me to think that it's possible (when both hooked up in the Univox amp) that the Lanzar was actually making some small sound, but was completely overshadowed by the Univox."

Thanks for confirming with the facts!

Still, the odd thing is that the Lanzar cannot be run by itself or it motorboats...but of course, when in parallel it appears silent.
...and the Lanzar does work with the Ruby.

...but the point's moot since the Lanzar will be gone shortly.

[teemuk]

Does there need to be 4 ohms across the speakers to functions properly or is there something else...

Ohm's law.... you know.

You know the output voltage swing and output power rating, right? What load do you need to produce that power. It's a matter of simple calculations taking up a few seconds.

Take the schematic amp (U-150R) for example: Rated output power is 20W average, the output theoretically swings from 0V to 35V minus voltage drops across the output devices and power supply sag. A good estimation is about +/- 17 Vpeak swings.

If P = U^2 / R then R = U^2 / P, and then we just fill in the blanks
(note that because I use peak voltage in the equation I also use the peak power which is 20W*2)
R = 17^2 V / 40W = 289V / 40W = 7.2 ohms.

So that's close enough to 8 ohms, IMO.

[PRR]

> the Lanzar cannot be run by itself or it motorboats...

I didn't want to get into this.....

The power amp has NFB around it and at least three high-pass poles near the bottom of the audio band. Same as a phase-shift oscillator. It wants to oscillate.

Clearly with the factory-fresh capacitors and original speakers, it didn't.

Not only have the caps faded with time, the sub-woofer has a low resonance frequency. The guitar speakers resonate around 90Hz, the sub maybe 40Hz. There is an impedance peak at resonance. This is a fourth filter pole. Evidently it interacts with the "1000uFd" output cap, the added pole now closer to the cap and transformer poles reduces stability to the point that it oscillates.

With the other speaker parallel, its sub-resonance impedance is low enough to mask the high impedance of the sub, and it is stable (probably *just barely*).

See the ghostly line in my graph? That's the g-speaker impedance. The sub is similar but bumped near 40Hz. With both in parallel, we have 4 Ohms at 20hz, 9 Ohms at 40hz, 6 at 60Hz, 9 at 90Hz, 4 Ohms at 300Hz. With just the sub we have maybe 70 Ohms at 40Hz, and that very-steep rise from 20hz to 40Hz. Rising is inductive. We have that big 1000uFd cap out of the amp. Induct plus capac makes resonance, damped by resistance, but there isn't much resistance in this circuit.

Your units are 4 instead of 8 Ohms so all impedance numbers are half; same trends.

While the sub alone violates the Designer's assumptions, power amps "usually" are not so fussy. This suggests that values have drifted (caps are going sour).

[PRR]

Also the output impedance of the power stage, before feedback, is high, around 50 Ohms.

A hasty sim-run shows such a source, 1000uFd, and a typical 40Hz subwoofer, shows large phase-shift at the feedback point. It crosses zero degrees at 23hz (lower than nominal resonance because of 1000uFd against speaker mass), but rises fast to 90 deg at 11hz, 135 deg (a conventional safety limit) at 3hz, and 170 deg at 0.5Hz, asymtotic to 180 degrees to zero Hz. Amplitude peaks at 22hz and is down 20dB at 5hz (121 deg).

On top of that there is bass phase shift in the cap under Q2, and the transformer from Q3 to Q4/Q5. 180-121= 59 degrees away from oscillation. If we have just 30 degrees phase shift each in these two nets, at 5Hz, we make the magic 180 degrees and maximum oscillation. They have to be designed below 3.5hz, and preferably well below.

That's a loose-rocks analysis. The numbers aint right. But something like that. It needs caps at least as large as it had when new. (The transformer *probably* hasn't drifted; they usually work or fail outright.)

------------------

If you must run the Laskar alone, a 10 Ohm 10 W resistor across it will probably control the motoboating until the caps decay a few years more. If you want a very low max output very mellow amp, this will do it. You are throwing 99.44% of your precious 20 Watts away as heat (a good guitar speaker only throws-out 99% of electric signal as heat). All the cone top-resonances which put the guitar ahead of the band have been brutally damped-out in the subwoofer. I have a kitchen-radio which might play guitar louder.

I agree that an amp worth $300 with genuine speakers, which can't be re-speakered for $300 (unless you are impossibly lucky), is a quandry. Dozen-watt den-amps are under $100 in any pawn shop.

[Gus]

I did post about checking the the electrolytics and did mention the 1000uf and 100uf caps
PRR went into detail

I also posted it looks like a the output some VOX amps had.  
If you go the R.G.s site look at the stability section in "Thomas VOX SS Amp Tips".  
The schematic in this thread does not have a Zobel network and it has feedback from the cap to speakers node

[GFR]

I had a smaller brother of this amp, a U-65G (branded as Giannini instead of Univox). The circuit seems virtually identical (I didn't compare the schematics side by side, just from memory - even the power supply voltage is the same). Well, I had the reverbless version. The U-65G had a 12" speaker (I think it was 8R).

Mine started to hum a lot, it was a dry cap.

The input stage is not very guitar friendly, with a too low impedance. It helps to use a buffered  output pedal in front of it.

The power stage and that inerstage transformer are just great. It's the bluesiest you can get without real tubes My amp is no longer among us, but I still keep the transformers in a box. I hope to have time to build a new incarnation of it someday.

Curiously, there was a higher wattage amplifier from the same line with no interstage transformer. It didn't sound good at all

[GFR]

PS> A nice trick with the U-65 was to set the line voltage selector on the wrong position. We have 127 VAC here but if you set it to 220V, all internal voltages were dropped and it would have a nice crunch sound at quiet bedroom volume levels