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Post subject: 1977 Vibrolux Reverb doesn't match schematics
Posted: Wed Aug 01, 2012 3:49 pm
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I have a 1977 Vibrolux Reverb that doesn't match any of the schematics that I can find online even though all of the parts are original. The resistors in the dog house are 2.2k and 10k instead of the 10k 4.7k on all the schematics. The caps in the bias circuit are 80uf @ 100v instead of 50uf @ 70v. The 16uf @ 450v caps that feed the center tap to the o/p transformer are post standby switch instead of pre standby switch. These are some examples of the differences but not an exhaustive list. I need to find a correct schematic if one exists. The real problem is that the schematics reflect 450 volts on the B+ and I only have about 420 - after a cap job with Sprague Atoms (I also replaced the afore mentioned resisistors). This is before I flip the standby switch. This is when the B+ voltage loads down to about 300v. If I pull the brand new 6L6s the B+ returns to normal. I checked the 470 ohm 1 watt resistors on G2 but they are within spec. Have I got some bad caps? I've seen new parts be defective before. Could someone please enlighten me? P.S. I also have a new 5U4Gb in the rectifier...


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Wed Aug 01, 2012 4:39 pm
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pneumatica wrote:
Have I got some bad caps?


That's a distinct possibility. There was a recent post on this forum regarding new-production Spragues, with measurements literally all over the map when compared to the specified values. I regularly used the Atoms for all my clients' amps but have since switched to F&T's for consistency and peace of mind.

As for your amp not matching the schematic, this is not surprising given that FMIC was entering a transition period for many of the silverface amps. The B+ was pumped up on most of the models to wring a few more watts out of them. At least the Vibrolux Reverb was spared being morphed into an ultra-linear platform with that infernal pull-boost master volume control.

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Wed Aug 01, 2012 8:35 pm
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A few questions, OK ? How does the amp react with the old rectifier tube? What voltage readings did you get before the cap job? Did you bring the amp up slowly on a variac or at least a current limiter? Having the first filter reservoir after the standby switch isn't a bad set-up. IMO Art

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Thu Aug 02, 2012 6:39 am
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A friend asked me to take a look at this amp because it was "weak" sounding. He has had it since 1977 and it had all of the original parts in it - including the tubes. I found one 6L6 that had a crack in it and it had lost its vacuum. Also, there was a bad 7025 in it as well as a weak 12AT7 in the reverb section. I replaced the tubes with good ones and it sounded better, but not great. Since it was as old as it was, and no service had ever been done to it. I went ahead and replaced all of the electrolytic caps and the resisistors normally associated with a cap job. I did not check voltages before the cap job. The amp sounded much, much better at this time, but I wanted to check the bias settings and see if I could get it to optimal. That is when I discovered low voltage from the rectifier tube. I replaced it with a new one I had in my stock and got the voltage up to around 420v before turning standby on. The amp actually sounds pretty good now, but I suspect it could be a lot better with the proper power suppply. No I did not bring it up slowly as I do not own a current llimiter or a variac.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Thu Aug 02, 2012 7:02 am
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pneumatica wrote:
The amp actually sounds pretty good now, but I suspect it could be a lot better with the proper power suppply.


Have you tried connecting the amp to a better set of speakers? The original 10L5 Oxfords weren't the greatest-sounding speakers even when they were new and now being 35 years old I'm sure they're tired and ready for retirement. A vintage pair of Jensen C10Q's or CTS alnico's from a Super Reverb would likely do wonders for the tone.

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Thu Aug 02, 2012 7:41 am
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" No I did not bring it up slowly as I do not own a current llimiter or a variac. "

If you are going work on vintage amps --- I really suggest getting a good Variac (US made one, like a good used Superior Electric, Staco, or Powerstat). Or make the old "Light Bulb Limiter." These could really save you when first firing up an amps that's been sitting for awhile. Or after you've updating some components. Esp filter caps or trannies. (Good check of wiring issues before disaster strikes).

http://www.ebay.com/sch/i.html?_nkw=variac

http://antiqueradio.org/dimbulb.htm


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Thu Aug 02, 2012 11:15 am
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could just have a "weak" PT. I just did some work on a BF'd 74 VR and even after I matched/beefed up the power supply section to that of the older Fenders I still was only getting 440VDC with a bias @ 34mA. While 440VDC is ok its still a bit below specs. Tried a half dozen GZ34s. All pretty much the same. Amp sounds great even with the tired Oxfords. BUt new high quality speakers would make a big dif.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Thu Aug 02, 2012 8:07 pm
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Thanks for the advice and the links BMW2002Ti. I will look into these items. Ok, I've got possible bad caps and possible weak power transformer. Any more suggestions as to why the B+ drops from around 420v to 300v when the standby switch is closed?


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 2:16 am
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Yeah Pneuma, think you have to observe the voltge at the PT before and after the rectifier, at standby switch"on". I would also check the 6.3 filament voltage. Have you checked all of the ground connections? Not forming the power caps slowly, can easily cost 20% horsepower. But I don't see this as the basic discrepancy. Especially if the condition was happening prior to the cap job. I certainly wouldn't expect a 120 volt drop with the 5U4GB. I could live with half that drop. Art

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 3:35 pm
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Before the standby switch is on I have about 345 volts on each of the plates on the 5U4(700v total from plate to plate) - perfect. Out from the rectifier before standby is switched there is 420v - a little low from 450 but not too bad. Then when standby is switched, it drops to 300v and the voltage on each of the plates drops to 330v (660 total from plate to plate). If I remove the 6L6s, the B+ voltage returns to 420. I tried removing pre amp tubes, but they only accounted for a few volts of drop. I know the power tubes are good because they are brand new plus I tested them on a very good Hiccock tester. I have checked the 6.3 filament voltage and it is good. The negative bias voltage will only adjust to about -36v whereas the schematic shows a nominal -48 volts. A visual inspection indicates that the grounding is in order. What say ye?

As I was reading my own post, I noticed what I had posted about the negative bias voltage and thought that maybe one of the resistors in the bias circuit could have drifted WAY out of spec. I think this would allow me to have a relatively normal voltage until I flipped the standby switch. Then, when the current is flowing through the whole circuit - including the bias circuitry- the loading could take place. I will check that stuff now. Does this make any sense to anyone else or am I chasing a shadow?

Nope, that wasn't it. All of those resistors are dead-on. Hmmm.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 5:57 pm
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What is the Plate current or Cathode current on the output tubes?

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 7:02 pm
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Thank you for the warning. I have a bias probe, and I am quite familiar with how to check the plate current without it as well. I was an electronic tech in the Navy 25 years ago and have maintained a career in industrial electrical, electronic, and instrumentation every since, but this hobby with vintage tube amps only cropped up about a year ago. I am still learning the finer points of these little animals and this is the first time I have seen this problem. I am trying to talk myself out of making the guy spring for a power transformer if I can, but I am leaning more and more towards that end. (that wasn't supposed to sound crappy, but I guess it did. sorry). I really appreciate anyone's input and I am all ears. I want to learn as much as I can about this interesting subject - really.
I can balance the tubes at around 15 mA current. There is a hum balance circuit rather than the traditional bias circuit. I can unbalance the tubes and get around 16 mA out of one, but balanced it is around 15 mA. Do you think the tap on the power transformer that supplies the negative bias voltage could have a shorted winding and that is why the negative bias won't come up to around -45 or -50 volts and then when the circuit is completed with the standby switch it manifests by loading the B+ voltage? Just grabbing at straws.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 8:11 pm
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I am having a hard time with your Plate current readings for the output tubes, especially considering that you say that the bias voltage is way low, and installing the output tubes drags the B+ down from 420 VDC to 300 VDC.

The power transformer is a possibility, but not very likely. Monitor the AC voltage going to the rectifier tube (compare with standby on or off), see if it drops also. Also check the DC current out of the rectifier (compare with standby on or off). If the AC voltage feeding the rectifier drops proportionally to the drop in DC voltage (ie, 420 to 300), and you have low current out of the rectifier, then that points to the PT. But if you have a very high current out of the rectifier, that points to something loading the rectifier too much. Even though it's new, you can't rule out the rectifier as the possible problem.

I would also have a close look at all of the things that you've changed, especially the filter cap polarities. Maybe you've lost (or created an extra) ground connection somewhere along the way. If not a PT problem, odds are very good that it's something that you have changed that is causing the problem.

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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 9:40 pm
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shimmilou; I am probably not making things clear. Can you describe what it is that you are having a hard time with on the plate currents, etc. I could try to give a better explanation of my readings.
The first thing I did when I discovered the low voltage was to go back and inspect my work to be sure I didn't leave a solder splash or some other kind of shoddy workmanship. It all looked good to me. I have looked at the rectifier voltage before and after the standby is switched and have 700 vac accross the plates of the rectifier before switching the standby and 660 afterwards - about 6%. The drop is obviously not proportional. I will double check my work to see if I got something backwards. Thanks for the tips.


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Post subject: Re: 1977 Vibrolux Reverb doesn't match schematics
Posted: Fri Aug 03, 2012 10:21 pm
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Boy, don't I feel like a dufus now! Especially after that "I've been an electronic technician for 20 some-odd years" rant. I put one of the caps for the bias circuit in backwards. When I put it right "lo and behold" everything got right. Thanks for the reality check.

shimmilou, you nailed it. "I would also have a close look at all of the things that you've changed, especially the filter cap polarities." I looked this thing over several times while trying to figure the problem out and overlooked that cap every time.

I am very thankful to everyone that gave advice in this thread. It is great to have folks that will share. Great to know about the current limiter, speakers, variac and so on. I hope that some day I will be able to pass on something to someone else on this site that will help out.


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