CJ: That schematic callout is so helpful! Thanks. Now I at least have something to go on to discuss it with my tech. Bias trems generally sound better to me, but this particular opto circuit has such a unique, wonderful sound. I'm definitely leaving it. That trem (with the extra click for "hyper") and reverb are a big part of why I like these amps so much. No need to mess with success.
Speaking of Ampegs, Gemini II inbound!
- Thread starter mad dog
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If you get the chance, could you take a few gut shots of the chassis; top and bottom? If I get the pics, I'll stop ragging you about home-brew recordings ..... 8) Thank you. J
Hi TH; I got to thinking about the zener application illustrated here (page 82, Electric Guitar Amplifier Handbook, by Jack Darr):teleharmonium said:
In addition to some good text, the book contains several hundred schematics that I went looking through to see if I could find zeners used in high-voltage tube amps on either or both primary B+ or screen supplies and didn't find any. Here's an example from the schematic of the Fender 75:
The zener is tied into the channel-switching opti-coupler and its footswitch; I think it generates a reference voltage but it isn't regulating B+ or screen supply. If the plate voltage is supposed to vary with the amplitude of the signal, and if there were a zener on the primary and if reaching the zener's breakdown voltage, it shunted the B+ to ground, I think the output waveform would be clipped; that is, unless the power supply is ready to burn up and the zener's function was safety, I don't think a zener on the B+ would be such a good idea. They might find a better use in screen regulation or in regulated power supplies but without 'chaining', any zener beefy enough to handle several hundred screen volts might be a little on the expensive side for run-of-the-mill commercial amps.
They are frequently used in bias circuits for both tubes and transistors; Gibson GS 100 for example and others. John
FNG
Enlightened Member
I really wish I knew what the hell you guys were talking about. 
8)
8)Hi Pat; I feel the same way when the discussion turns to chord structure and scales. I didn't get much past Vol 2 of Sams Basic Electricity and Electronics ... but it was farther than I got trying to learn to sight read music or mess w/ Tab. Those who can, play; those who can't, well ... they spend their time putzing with bench projects :wink: John
teleharmonium
Member
Hey capn,
the zener diode usage that I am (very casually) familiar with is as an after the fact modification for amps that have excessive plate current, or in homebrews with the same problem. I would not expect a schematic from a manufacturer to use it in that particular application as they would simply design it with an appropriate PT and power supply circuitry to avoid the issue and save the expense. My amp tech added it to a Bedrock combo I used to own which had crazy high voltage, it was burning up tubes like a Music Man and would go into high frequency oscillation. I've heard of others putting them into Music Man amps. I don't think I have the circuit handy but I've seen it and I do remember it is pretty simple.
the zener diode usage that I am (very casually) familiar with is as an after the fact modification for amps that have excessive plate current, or in homebrews with the same problem. I would not expect a schematic from a manufacturer to use it in that particular application as they would simply design it with an appropriate PT and power supply circuitry to avoid the issue and save the expense. My amp tech added it to a Bedrock combo I used to own which had crazy high voltage, it was burning up tubes like a Music Man and would go into high frequency oscillation. I've heard of others putting them into Music Man amps. I don't think I have the circuit handy but I've seen it and I do remember it is pretty simple.
Hi TH; can't argue w/ success. I was mostly curious why their use isn't more widespread and was trying to think up a good reason why not. I once built a regulated preamp supply based on an Audio Amateur Magazine design; highest voltage in that chassis was 240v. Built around a TO-3 transistor and a string of 3 zeners; it worked fine. Your tech must have had a good idea where to set the breakdown voltage; no funny business in the signal? I know that oscillation / squelley thing....not pleasant :evil: Cheers! cj
FNG
Enlightened Member
capnjuan said:
Dang, I can't play either!! :?
Sorry for the veer.... :mrgreen:
S'ok; veer all ya want. IMO, some of the funniest moments and exchanges on this BB occur in connection w/ veer; 'Consistency' - the hobgoblin of little minds ... a little non-linearity now and then is a good thing! JohnFNG said:Dang, I can't play either!! :?
fronobulax
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capnjuan said:
Actually, the complete quote begins with "A foolish consistency" so there is some wiggle room for those of us who like to be consistent once in a while
http://www.bartleby.com/59/3/foolishconsi.html
And might suggest that the definition of 'A foolish consistency' might be consistency for no sake other than its own .... usual identifiable syntax: "for the sake of consistency ... " which is strongly recommended when non-linearity isn't.fronobulax said:Actually, the complete quote begins with "A foolish consistency" so there is some wiggle room for those of us who like to be consistent once in a whilecapnjuan said:
Hi Teleharmonium; your tech was/is ahead of the commerical curve. From Ted Weber's site: Weber VST Weber is marketing high-wattage, power-dumping zener diodes: application photos from Weber's site:teleharmonium said:
These are 'break-down' diodes; one end connected to a DC supply leg; the other to chassis ground and when the voltage detected exceeds the diode's rating, it shunts or re-directs the DC to ground until the detected voltage falls below the diode's breakdown voltage effectively regulating the DC voltage as the the specified breakdown voltage.
Most vintage gear was built when AC line voltages were 110V; today they are closer to 118-120V. As a result, tube rectifiers see higher secondary AC voltage and everything is operating at higher B+ voltage. Although these might not help a tube rectifier, zeners would damp B+ voltages downstream.
Weber's page also includes a built-in calculator to ID the diode matching the buyer's volts-to-be-dropped and amplifier wattage. I don't see why these couldn't be used in the primary B+, as a screen regulator, or regulation of preamp supply voltage. Would require knowing the amp's actual power supply out DC voltages v. design and picking a target not-to-exceed voltage for each tap.
Considerably easier than scratch- or kit-building a regulator, requires punching/drilling holes in the chassis, some consideration for location viz heat generated, an ever-so-simple and elegant solution to regulated supplies .... not to mention extending the life of tubes! Cheers, cj
teleharmonium
Member
capnjuan said:Hi Teleharmonium; your tech was/is ahead of the commerical curve. From Ted Weber's site: Weber VST Weber is marketing high-wattage, power-dumping zener diodes: application photos from Weber's site:
These are 'break-down' diodes; one end connected to a DC supply leg; the other to chassis ground and when the voltage detected exceeds the diode's rating, it shunts or re-directs the DC to ground until the detected voltage falls below the diode's breakdown voltage effectively regulating the DC voltage as the the specified breakdown voltage.
Most vintage gear was built when AC line voltages were 110V; today they are closer to 118-120V. As a result, tube rectifiers see higher secondary AC voltage and everything is operating at higher B+ voltage. Although these might not help a tube rectifier, zeners would damp B+ voltages downstream.
Weber's page also includes a built-in calculator to ID the diode matching the buyer's volts-to-be-dropped and amplifier wattage. I don't see why these couldn't be used in the primary B+, as a screen regulator, or regulation of preamp supply voltage. Would require knowing the amp's actual power supply out DC voltages v. design and picking a target not-to-exceed voltage for each tap.
Considerably easier than scratch- or kit-building a regulator, requires punching/drilling holes in the chassis, some consideration for location viz heat generated, an ever-so-simple and elegant solution to regulated supplies .... not to mention extending the life of tubes! Cheers, cj
Yeah, my guy is real good, but working on amps is a sideline and he's a busy guy so I have learned to be very patient.
The zener diode he installed back then had a heat sink over the top, it kind of looked like he had installed a PC CPU in the amp ! It brought the plate current down to the safe zone, but it was still on the high side, and the amp just didn't have the sound I was looking for. It was good for harder distortion but the cleans and semi cleans weren't happening compared to the amps I picked up after that one, so it was pretty easy to let it go. I normally wouldn't have considered a mod that required a new chassis hole, but the amp was unstable and ate tubes like a Music Man.
I've got about a 5 year backlog of amp projects. Boo hoo for me. But I've tried a bunch of techs and every other one has disappointed me, or worse, and I don't want to ship my stuff. The one time I tried that, with a well known amp repair guy, he had an amp for a while and never could figure it out, even though it's a make he is familiar with and talks about online. So I kind of came to the conclusion that some of the amp techs that are legendary on the internet, aren't all that, and that it is best to stick with the guy I trust, no matter how long it takes.
Hi TH; however done, the good news is that you got your amp stabilized. I thought these zeners were pretty interesting; have my nose face down in a Guild Thunderbass now thinking how simple it would be to used them .... There's a pretty good commerical amp shop down in West Palm Beach that I use. If I get myself all messed up, I can go there if necessary. If I have something I can't fix, I can just take it there. Cheers. cj
And speaking of ampegs, how is your Gem II coming along capnjuan??
Hi Michael; tried to pull the trigger but, based on a recent eBay / Gem II / BIN transaction at $450 for a complete, spotless Gem II, they wouldn't wiggle on price. Discussion was compounded by $100 in shipping - in fairness to them, 1,500 miles and maybe 75-90 pounds. I got them to send me pics showing fairly extensive face-plate pitting; the kind you see from extended periods in unheated basements or garages.mad dog said:
I've had some success dealing with the pitting but the prize was the 15" Jensen and the same humidity that pits the face-plate can turn the speaker cone to the consistency of a paper towel ... am waiting to hear from the Guiness Book of Records for the most flaccid speaker cone; I say proudly that I own the fartiest 12" Magnavox speaker ... it farts just looking at it ...
Anyway, the Gem II was a consignment item with - guessing - 30% going to the shop leaving seller-in-fact w/ say $350; I wanted them to throw me a $25 bone based on the comparable hoping they'd split the hit; they said no deal. FWIW, it's still sitting there.
Based on your/matsickma's/Gilded's/John K's respect for the 7591, I bought a poor man's 7591 Ampeg/Thunderbird, a Univox U305R:
2 X 7591, 6CA4 rectifier, 15" Jensen/Muter speaker, trem/reverb, 2-channel, found original Univox dual footswitch on eBay, semi-closed back, 32" high like the Guild Brett/Bart Mavericks to help bypass the tilt-back. It's in VG cosmetic condition and electronically only needs the usual intervention; caps, cord, one pot for sure, and power switches. The speaker cone is intact, dry, and tight. PCB construction w/ a reasonable build standard. Not expecting miracles but I've got the schematic and will get it to baseline and see what happens.
It's also in the house at 1/3 the cost of the Gem II and there's no reason to think the Gem II wouldn't have needed as much or more intervention. Did you see the 2 Sano's I posted? I thought the first one was especially interesting, like yours?
Regards, John