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In Reply to: Hoping for a History Leason #1: Bogen / NTS-1125 posted by m8o on March 10, 2007 at 22:55:46:
I forgot to mention. I read many posts from folks over many years which went into be looking for a Bogen, and the 807 tube. I did read about 807 power tube based amps on this board and thank you eveyone over the years for all that was posted.
Follow Ups:
Hello there,
These have some beefy iron, don't they ? Some of the Bogen amps like these only have Line outputs, no Voice Coil taps. Worst case, you can use another step-down Line to VC tranny.
I have no experience with these, but their circuits were sort of common. There were J50, CH50, CH 60 and some more versions. But, I think this unit might have very high voltages to make 100-125 Watters. The 50-60 Watt versions ran over 600 Volts on the plates, 300+ on the screens of the 807s. I think the 100 Watters used closer to 700 Volts on the plates. Be careful with that HV. One nasty shock can stop your heart, i.e. kill. When probing underneath, use one hand only, keeping the other hand in your pocket, for safety. Attach the tester's negative lead securely to chassis ground, to assure one hand measuring. Also, remember to short the power supply caps, when actually working on these, as residual capacitance voltages do get stored in the cans.
Most of these used 6SN7 as cathode follower drivers, with CT chokes loading the output tube grids. While the R-C coupling devotees will like that, I prefer the versions with a 6V6 driver tube, splitting the phase, thru an interstage.
While I am here, thank you for the kind words on the Duke info. I do try to be helpful. It breeds goodwill and good karma. I never had a problem with sharing the knowledge.
If there is a 600-700V supply, augment that with some higher capacitance power supply caps in series. They didn't use much capacitance in the '50s. Energy storage is critical to bass response and speed and speaker damping/control.
While I was a legitimate, electronics service field engineer, I have become much more practical in these later decades. While theory still guides me, the practical engineering is more important to me. With sound equipment, it is the sound that counts.
If these amps have voice coil taps, they might be fine bass amps.
The short listening, in mono, I have done with some Bogen 807 amps, has me remembering them as sweet and punchy. 807s are very, very rugged tubes, much more rugged than any 6L6Gs that they resemble. Do not change to the 5933 industrial #, which does not sound as good.
Also, while it might be tempting to change output tube types, the 807s are consistently a sweet sound. I have used Tung-Sol 6550s at 600 Volts B+, but they do not sound sweet that way. 807s at 600 Volts can be very sweet. This amp's bias might have to be adjusted to a little higher negative number(more bias), for stability.
Good Luck with these High Voltage Brutes !
Hey, Thanx very much for your addition. I gathered already you're an interstage devotee; how did I guess. ;) Personally I'm a total neophite, but I'll be learning.I looked at some Bogen circuits but didn't find a 807 based one before. I did find one from those models discussed on DIYAudio with an 807, thanx. I'll look for more tomorrow.
I'm pretty sure I'll be ok with the OPT. The person I bought it from has been using it in his HT system. That's right, I just checked the eBay listing and it read "These NTS-1125 have two 16 ohm taps, 140v tap. both 16 ohm taps are strapped for 8 ohm speakers as wired now".
Thanx for the notes about the capacitance. I'll have to do that. I do think this amp will more likely be the one used driving the 4 ohm bass horn then the Dukane; though I will up-size the supply caps in those too eventually. I'm really going to need a 3rd of both this Bogen and the Dukane! ;) I'm definintely not planning on changing from the 807; it's what intrigued me in the 1st place.
And finally if you'll indulge me, I have a few questions that will illustrate just what a neophite I am with 'older' (ok, or 'real old') tube gear.
1) Is setting the bias higher as you mention making it operate furter in AB2 towards class B operation, or moving it more towards AB1?
2) If I don't have a service manual, is whatever pot I need to adjust to set bias 'intuitive' usually? And how do I know what value is 'right'?
3) When do I need to start old amps with a Variac? Only if they haven't been used for a long long time? Both these Bogens and Dukanes have been used (I think regularly) by the people I bought them from, so I think I don't need to start them with a Variac, but I'm wondering.BTW, feel free to ignore these questions if answering them would be too major a task; I bet it is. I undertstand. I should buy some old service oriented books. I purchased 3 books on tube gear written before the 50s a few months back, but they are more theory then service.
Just thought I'd mention, I figured out that the amp is either a HO-125 or GO-125.
Errr... I take that back. I sent the seller of a lot of photofacts on eBay a link to my pix of the amp, with the intention of buying the photofact for one it is, and he thinks it's neither of the two. He's going to see if he has a match (very nice of him to do for a $5 item).
No trouble at all...while I have the time...
Since these Bogens are being used, no Variac slow start-up is necessary. However, using a Variac is often a good thing. Your wall socket has about 121 Volts AC. Late nights, it might even yield 124V AC. Much of the gear we vintage nuts, I mean aficionados, use, was designed for 110-117VAC. I use gear from the 1920s and 1930s, often. I always use a Variac as a sort of isolation transformer, for the gear designed for 110-117VAC. Without the Variac 121VAC would overstress many parts.
When older gear has been stored for long periods, over 9 months, a slow start-up with a Variac is worthwhile. Items stored for years, must get slow increases of AC input voltages, or they will certainly blow parts. Let me describe how it can be done for the older, stored for a long time, amp variety.
First, you must have a load on the amp, that is a speaker, or heavy duty 8-16 Ohm resistor, connected to the proper output taps. Set the Variac to 50VAC. Remove the output tubes before turning on. Turn on the Variac with turned on amp connected, yes, without the output tubes. Over long time periods, hours, you slowly increase the Variac AC voltage, a few volts at a time.
The reason I remove the output tubes, is very practical, learned from many years of fooling around with tube gear. The output tubes draw the nmost current. They are the likliest, to ruin the power supply caps, the fastest. Without the output tubes, we can reform many of the original power cap cans, very easily, as proper voltages are reached.
So, we can monitor the AC leakage on the power supply caps. Use a meter with the neg. lead secured to chassis ground, and use only ONE HAND to move the positive meter lead, carefully. As the leakage gets very low, less than .3 Volts AC, we can slowly increase the AC Voltage from the Variac. If an output stage electrolytic gets hot, back off on the Variac, and wait, hoping it will cool and possibly reform. When cool, turn up the Variac, a little at a time. Check the AC leakages as you go. This can take one hour to many hours, even more than a day, depending on the encountered problems.
If the AC leakages stay low, keep increasing AC, SLOWLY. Amps that are good, will reform quickly. Monitor the DC voltages at the power supply caps, also. As we increase the AC in, the DC comes up quickly.
You can get rated DC Voltages on the power caps, long before 110VAC !
That is important. As we reach rated DC voltages, back off on the Variac, back to about 60V AC. Then, insert the pairs of output tubes, and start the process over again, monitoring AC leakages.
As the AC leakages get low, step up the AC SLOWLY. Keep on keeping on, and soon, or later, the amps can work with 110-120VAC.
Bias is not all that tricky. First, try to realize that bias is really measured from the cathode to the grid. While fixed bias amps can measure negative bias from ground, any tube's bias is really the difference between its cathode voltage and its grid voltage. With tube manual or diagram at your side, you can see which socket pin is grid and which is cathode and which is plate, screen, etc.
Without a schematic, tube manuals can give insight to certain bias voltage points at certain plate voltages. Decreasing bias, that is less negative, decreases plate voltage, increasing current, closer to Class A or AB1. Increasing bias, increasing negative bias value, also increases plate voltage, but decreases idling current, closer to Class AB2. Don't sweat the details about Class here. Bias must be measured with no signal into the amp. Remember to have the amp connected to a load. Tube manuals are a guide. My amps often have different bias values than the manuals, but I like to run my triodes hard, in Class A or AB1, often written as A Prime in the 1930s. By the way, books from the '20s and '30s are much more informative.
So, in the case of the brutish Bogens, let's say their bias is about -30V, with 600 Plus Volts on the plate, 300V on the screens. Those 807s will run very hot, even yielding red spots on the plates. Raising the negative bias value to -35 or possibly -40, will reduce the idling current, lessen the red, add stability and life to your amp. If there is no adjustable pot/knob, you might have to change reistances, if needed. There are resistors, in the bias circuit, that are in series with the bias voltage, while others are in parallel, connected toward ground, the bleeder resistors. Juggling these resistor values is what changes bias.
If this is your first amp setup or "repair," DO NOT change anything on the Bogens, until you understand what you are doing. There are very dangerous voltages under that amp ! It would be safer, and better for you, to use a much smaller amp in your first experiments.
Absorb this all, and gather some more questions. Read and Reread.
The more you read, the more sinks in the brain. Some of the theory needs to simply be remembered, rather than understood. The best theory books are the early radio books and magazines, before WWII. I learned the most from 1920s and 1930s texts.
Gotta Go...Later...
What can I say, but 'WOW'. ;) Thank you doesn't do it justice. That was very edifying. I keep reaeding it over to make sure I commit it all to memory. That's years of hard won experience there laid out in such easily digestible fashion. Thanx again....and, I'm buying a variac, if only to have. (actually, I may need it for one of the three amps I'll be using as 'tweeter amps'.) Wish I had it already because the Bogen arrived today, and I'm picking the Dukanes up Sunday. The 3rd bass horn arrived today too; and I have all the mid and tweeter horns and drivers... Not sure I can wait to turn it all on.
Hey, Be patient, now. Good things come to those who wait, patiently.
With the Very High Voltages underneath the Bogen, your calm, confident movements are needed !
When I was studying electronics, rereading stuff, over and over, while monotonous, was necessary. There is so much theory, that must be respected, and remembered, even if not fully understood. The best example of that, is the theories of how current flows. When working with tube stuff, or electricity, whether current flows from positive to negative, or vice versa, is not relevant. What is most relevant, and hardly ever mentioned in any theory textbook, is Never get in the way of that current flow !
For safety, don't be the conductor of that current. That is why the practice of one handed testing is imperative. Using two hands, and stumbling or fumbling, getting in the way of the current, there is a high likelihood that you could get the shock across your chest/heart. That could be fatal...
So, in the future, when really checking cathode bias, from cathode to grid, first secure the test probe to the cathode connection, using only one hand. Then, probe the grid with the other test probe, using only one hand. I might sound repetitious, but our health is important...Even one handed shocks are nasty. For those less confident, use leather, or thick rubber, or real high voltage gloves.
By the way, m80, thank you for the kind words. Best of Luck and Happy Listening...
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