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US Navy 803 tubes for home brew audio amp - opinions wanted

Discussion in 'Tube Audio' started by infullview, Nov 5, 2018.

  1. infullview

    infullview Active Member

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    I have several USN 803 tubes from Catoctin. They are circa WWII and were used as audio modulator tubes in high power AM transmitters. I was wondering if anyone has any experience with these, and if so, how they might work in a high fidelity tube amp application. Plates are capable of handling 200 watts so I could probably get 50 watts of audio in a single ended design without pushing them to hard.

    Any thoughts?
     

     

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  2. Palustris

    Palustris AK Subscriber Subscriber

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    If this is the same tube as a F128A then it will be very challenging to use. First of all, the filament requires 143W of power. You would need an 11V, 13A filament transformer. It looks like you could get a few watts out of it biased at -50V @ 3800V and 50mA. Now if you want to go class B push pull you could probably get 2.4kW with a 3000V plate supply although to get this amount of power you would need a 7500W plate transformer capable of delivering 2.5A at full power.
     
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  3. infullview

    infullview Active Member

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    Not the same tube. 50 Watts to light, max plate voltage 2K at 100 mA for 200 watts
     
  4. 604man

    604man Well-Known Member

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    USN Catoctin was a WW2 communications ship.
    my father was on that ship in WW2.
     
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  5. infullview

    infullview Active Member

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    134
    Cool. A bit of history to go with them. Further research tells me these were transmitting tubes not modulator tubes. I would imagine they would work for audio though. I did have one of these fired up for short time when I was in college as a colpitts oscillator on the AM band. I didn't have anywhere near the voltage and current on it to really produce, just wanted to see if I could get it to work.
     
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  6. 604man

    604man Well-Known Member

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  7. Retrovert

    Retrovert AK Subscriber Subscriber

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    Ages ago I was given some 3-500Z transmitting triodes and thought, wow, high-power SE amplifier. Then, after a bit of research, I realized the tubes were better off with hams, which is where they went. (I kept some smaller, pretty ones for my collection.) Here's the short version of why.

    As you noted, the plate voltage is typically about 2 kiloVolts. Yes, 2,000 volts. (Pausing to let that sink in.) Two. Thousand. Volts. DC, not AC, but still enough to kill you just as dead as William Kemmler if you're not careful.

    I have a pair of KLH Nines which need the power supplies rebuilt as the diodes are starting to fail. That's 5.7 kiloVolts DC, so it's not like I run away from HV power supplies. Just saying they're tricky to work on and I won't do it alone or without a home defib and my paramedic friend around just in case.

    The critical issue, however, is not electrocution but the requirement for custom-wound output transformers. Given the voltage and power requirements those would be expensive for Class AB at 50 Watts, but if you want to run Class A the transformer must not saturate from DC in the primary, and this requires a substantial core, which greatly increases the cost, at least if one wants linearity. Guitar amplifier is a lot simpler.

    A custom transformer, linear from 20 Hz to 20k Hz, matching 20k Ω to 16/8/4 Ω, and able to take 50 Watts is heavy and expensive. Maybe fifty pounds apiece, extrapolating from the 30 Watt Hammond SE (see below), figure a Cleveland per transformer since it's full custom and better specs. Plus the amplifier will exceed your floor loading.

    Pete Millet built a 30 Watt amplifier using a PL-177 transmitting tube running at 625 VDC with a 5k SE transformer (Hammond 1628SEA at $150 apiece, quite a baragin). See: http://www.pmillett.com/PL177.html
     
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  8. Retrovert

    Retrovert AK Subscriber Subscriber

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    I remembered where I saw one of the HV amplifiers in the ballpark of what you were asking about: http://tubelab.com/prototypes/833-se/

    But that's still "only" 1,500 Volts.

    This is the sort of amplifier to use in Fury Road:

    No gas needed for the flames, amplifier produces those from your speakers all on its own.
     
  9. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    If you are really serious about pursuing this type of an amp you need to read up on high voltage amp building. the ham radio guys are very familiar with the requirements of high voltage design. Needless to say high voltage amps don't come cheap. Check on the construction of single ended 813 amps which have many iterations. The 813 tube is a beam pentode and has the ability to power even the most demanding loud speakers like panel types in single ended mode.
     
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  10. infullview

    infullview Active Member

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    Thank you all for contributing to this thread. Here are my thoughts and ideas for discussion - and please feel free to tell me if any of these idea's are crazy.

    First, let's get past this high voltage paranoia. I've been shocked by the best of them and am very careful. I used to restore antique radios and many of these ran transformers that had voltages upwards of 1200 VAC.

    My design goal would be to shoot for around 60 watts of output.

    The nominal plate voltage/current for this tube is 1250VDC at around 120mA; it can be run a bit lower to accommodate a stock output transformer. Each leg of a center tapped 60w Hammond transformer can handle about 600VDC, so it should be able to take the full voltage across the primary in singled ended configuration.

    To save money I can probably use DC to DC converters to get the regulated B+ voltage I need. I have plenty of surplus, high current line transformers I can use to feed them with (I'm a bit of a pack rat).

    My biggest concern is where to source tube sockets for these beasts. Anyone have a source they can suggest?
     
  11. 6DZ7

    6DZ7 Super Member

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    That would be a push-pull transformer not made for SE input. Why not just find a conventional 50W amp circuit and plug that tube into it as an experiment. Try to determine the tube's usable primary impedance needs as close as you can first to get the best tranny for it.
     
    Last edited: Nov 6, 2018

     

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  12. infullview

    infullview Active Member

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    A push pull transformer just has a center tap. In the case of push pull, the center goes to B+; it can be also be used in a single ended configuration by wiring one side of the full primary to the plate side and the other end to the B+ side
     
  13. Retrovert

    Retrovert AK Subscriber Subscriber

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    That's not correct.

    The single-ended Class A (to distinguish from push-pull Class A) transformer must be specifically designed to not saturate from the standing DC in the primary, unlike a Class AB which has relatively little DC and thus requires only minimal core changes to accommodate. The design changes require air gapping (just like a swinging choke) and a more substantial core, plus winding changes if the low-end frequency response is not to be damaged by the gapping.

    Compare a conventional Class AB push-pull output transformer to a Class A single-ended output transformer and notice the substantial increase in size and weight. That's because half of the Class A core cannot be used.

    So one cannot simply use a Class AB push-pull transformer for single-ended just by ignoring the center tap. It simply doesn't work that way.
     
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  14. Retrovert

    Retrovert AK Subscriber Subscriber

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    Sigh. It isn't "paranoia". Nobody knows your level of expertise with lethal voltage, as you didn't state it, and as a general rule it is a good practice to caution about the dangers of high voltage because it can and will be lethal. What radio, BTW, runs with a supply of over a kiloVolt DC? Small signal tubes are typically used, and these typically run at three to four hundred volts DC.

    The electrostatic speaker world is filled with cautions about working on HV supplies. It isn't paranoia to warn those who may not have the required expertise, it is prudence and politness. The ham world is similarly filled with such cautions because these voltages will kill you dead, and your little dog, too.

    Again, one must use a single-ended output transformer designed to not saturate with Class A. The conventional output transformers will saturate and severely distort because the transformer stops working like a transformer.

    SMPS have noise issues, which is why linear supplies are generally used for audio. A high-voltage DC-to-DC converter is typically not rated for this type of current.

    Radio transmitter transformers will deliver the required voltage.

    Ham radio supply houses in the midwest. Maybe Fair Radio Sales or something like that. The auction place is another source.
     
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  15. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    You could do that , i mean , you will probably get sound out the speaker. but, as Retro says, when the core of the OPT becomes saturated, distortion will increase dramatically. It would be safe to say that the large output from the 803 tube will quickly overwhelm the PP OPT
     
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  16. Retrovert

    Retrovert AK Subscriber Subscriber

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    Overwhelm is spot on. The other bonus of DC through the primary is overheating and damage to the transformer.

    To the OP: if you don't believe us, I suggest contacting Hammond or any other output-transformer manufacturer tp inquire of technical support the consequences of using one of its push-pull Class AB transformers as Class A, either single-ended or push-pull. Engineers at the transformer companies make the big bucks because of an understanding of saturation issues and tradeoffs.
     
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  17. infullview

    infullview Active Member

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    OK I see your point on the output transformer. So bottom line is the cost is probably going to be a lot higher for single ended OPT. Do you think I could go push pull and degrade the plate voltage and current to be within spec for an off the shelf OPT? Please understand that I'm trying to be creative with cost. BTW I believe it was a Kolster cathedral radio that had the 1200 volts. Each leg of the CT transformer went to an 80 tube for full wave rectification at about 600VDC. Standard practice back then was to use several chokes (one of them being the speaker's electromagnet) and small (4uF) caps to filter the B+. This also had the effect of dropping the voltage to the plate to about 450VDC. Oh ya, and it burned a hole in the end of my finger that took forever to heal. Lesson learned.
     
  18. Retrovert

    Retrovert AK Subscriber Subscriber

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    Cost for Class A iron is always significantly greater than for Class AB. Crossover distortion in Class AB is not always objectionable, let alone noticeable. Reasonably balanced tubes tend to have most of the overlap cancel in the transformer. Most, but not all. (Bias shifts over the life of a tube.) The promotion of Class A is not always solely based upon measurable science and engineering; sometimes bragging rights come into play. 'Nuff said on that.

    Using the field coil of a speaker as a choke, BTW, is a great way to couple 60 Hz and 120 Hz mains noise into the speaker generating some impressive IM distortion. That technique was done for cost savings, not sonic quality.

    The hole in your finger is exactly why I suggested that high voltage can be dangerous! You got a good lesson which didn't kill you. I had mine with a hot chassis when I was fifteen, right across my chest. My dad had his with a hot chassis (he had warned me, and I sort of understood, but didn't understood about death caps) and a CRT anode which had been discharged. (Nothing like dielectric absorption to knock one on one's ass.) I've learned to listen to stories about the hazards of dangerous things. You're lucky, BTW, that it wasn't an RF burn. Those can do a lot of damage and take even longer to heal and leave nasty scars.

    Anyway, even assuming you are willing to move to high-voltage Class AB, you still face the difficult problem of obtaining a transformer to match a very high output impedance (as high as 10k to 20k) to your speakers. A custom-made 50 WPC Class AB to do the required matching is, of necessity, very expensive. I don't know exactly how expensive, but the costs for similar custom-made transformers suggest it could be around a thousand dollars. You can send an email to any of the custom houses and ask. I know that five hundred doesn't get one very far in the full-custom world when it comes to wide-band response at high power.

    I'm not going to tell you how to spend your money—you should naturally take your own counsel on that—but here is a suggestion to be taken with as much salt as you like. Many Class AB amplifier can be built without much difficulty to deliver 30 WPC. If you built one of those I expect you'd be very happy. You could purchase a nice pair of output transformers for what a single 30 WPC Class A transformers will cost you. In fact, for the same money as your Class A project I would expect you could build a four-channel setup, which would allow you to bi-amp the speakers. That should give you all the clean power you need.

    I'm not really into power, since I listen at very low volumes. This is why I can still hear to 16k Hz, at least that's where my computer speakers start to roll off. I'm only interested in the power to the extent it provides good transient response without clipping. Headroom may really be what you're seeking; I don't know what volume levels you require. All of this is something to think about.
     
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  19. infullview

    infullview Active Member

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    So when you're taking impedance I assume you are referring to the primary OPT impedance required for the tube. It sounds like this project is more of a pain in the butt than it's worth. I just look at those giant tubes and think WOW I'd bet they'd make a great amp! BTW I was 12 when I put together my first cigar box AM transmitter (about 12watts) and the finger burn was around the age of 17. Even when you know it's dangerous you can still make a mistake. In my case, it was a wire that had not been secured and hit the end of my finger - don't remember why it was powered on though. My brother was into electronics way before me and used to make 40 watt AM transmitters with 6L6 tubes - he used to peak them by lighting a 40 watt florescent tube into the air.

    I take it that your opinion is abandon this idea and go with something more conventional? It just seems like anything to do with tubes is so damned expensive these days. You can't even find a decent rig to rebuild for under $700. I know I could cobble something together on my own using conventional parts and these old 803 tubes, but as you said it probably wouldn't sound very clean. I even looked the possibility of buying a kit from Korea, but these too are insanely expensive and I there are no US distributors (probably due to liability involved). Any suggestions? Back in the day, I would have gone to Lynn, MA to a place called John Meshna surplus and supply. I could get bags of parts, tubes, and chassis for almost no money. Those days are sure gone.
     
  20. kward

    kward AK Subscriber Subscriber

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    Is this 803 tube a triode or pentode? I can't find any data sheets for it.
     

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