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Another 813 Build




 
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w9jsw
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« on: January 19, 2019, 01:51:04 PM »

In this thread

http://amfone.net/Amforum/index.php?topic=44480.0

Tom, K1JJ made me an offer I could not refuse, in conjunction with Phil, AC0OB who offered the perfect iron for the build at flea market prices. I had decided to build a 8000 modulated by a pair of 811's mainly due to the cost of the 813 tubes and the lack of a suitable 2Kv power supply. Both of these issues were now resolved, so why not build the big rig!

As we start out I am going to outline my thoughts on how I will approach this build, subject to the vast knowledge of this board to help me "get it right". I have most of the parts in hand or on a truck somewhere.

The phases will be as follows:

Physical design combined with power supply build
RF Deck build
Modulator build

I plan to eventually have an existing Digital VFO (https://www.theladderline.com/node/37) driving a 20W SS amplifier/LPF (QRP-Kits) to drive the RF deck. I may use my Yaesu 757GXII in the interim if I decide to start the RF testing prior to assembling the SS components. For the audio, I will build the mosfet driver that Tom suggested. I have not decided if I will lay it out on a PCB or build it as he did. I plan to drive the audio chain with a combination of RPi 3B+ using some open source audio DAW tools. My work here is documented in another thread.

So that is the preamble...
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« Reply #1 on: January 19, 2019, 02:22:14 PM »

The physical layout will hopefully comprise a 6ft tall old buzzardly rack that began its life as a Motorola communications rack. It has a door and some nice silver louvers that will nicely dress it up. I just have to convince the plant guys at work that it will be better to let me have it rather than sending it to the crusher. This rack will allow me to put the transmitter components facing out and have the door on the back to avoid wayward cats/grandsons from being fried. I am not yet sure how I will place the supply components in the rack until I get some time to measure them all and see how to get them placed. I have not decided if I will paint it or leave it with its natural patina. It is that not so likable Motorola tan color.

My objective is to build this power supply in a way that I can disconnect the supply from this rig and directly plug it into my Drake L4B Linear that was purchased sans supply. I have the same power cable that Drake used that will allow me to build it to be electrically compatible (except hardwired for 220V only). That way I can just reach over and unplug the PS and plug it into the Drake using the Drake switches to turn it on. I know I could make it fancy and switchable but I really don't see the need and this is just so simple to do it this way. I have all of the Cinch Jones connectors. I am going to make one change - no Millen HV connectors. I am going to use RP-BNC high voltage connectors.

The power supply iron comes from a Gates BC-1T. I will have the transformer, the 8uf oil cap and the choke. I bought 2 more 32uf 4500V oil caps so will have a total of 72uf of filter capacitors. Gonna need a substantial soft start for that much capacitance. I have not decided on a design yet. I have the supply shown in the attached schematics, but will be re-drawing the supply so show the partitioning and component groupings. Going to also as a variac into one leg of the 240V to allow me to control the overall voltage.

Trying to decide if I should make a small control chassis to house the HV meter, switching and screen/bias supplies or if I extend the size of the RF Deck facepate and put the primary supply components there. Taking a look at the size of the screen power trans and the filament transformer, I don't think the RF deck has enough space and they have to be on top. The bias supply is small enough to be under the chassis. Even using my planned 17x17x4 chassis, there is just not enough space I fear. Have to think about that.

Tom, I need to see a pic or get a description of the diode block. I plan to use 15 of the 6A10 diodes per leg. I would like to see how you laid these out and the use of the fuse wires that have protected the strings from disaster as you wrote in some of your threads. I assume it is on plexiglass on ceramic standoffs but the specifics are not firm in my mind.

John

* 813 Transmitter.pdf (576.25 KB - downloaded 141 times.)
* L4B-Schematic.pdf (3927.72 KB - downloaded 103 times.)
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« Reply #2 on: January 19, 2019, 06:52:52 PM »

No sense in cramming things in making it hard to work on.
A big transmitter should be big, just as happy chair is happy.


* happy-chair-is-happy_9278.jpg (73.42 KB, 489x366 - viewed 184 times.)
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« Reply #3 on: January 19, 2019, 09:59:53 PM »


Tom, I need to see a pic or get a description of the diode block. I plan to use 15 of the 6A10 diodes per leg. I would like to see how you laid these out and the use of the fuse wires that have protected the strings from disaster as you wrote in some of your threads. I assume it is on plexiglass on ceramic standoffs but the specifics are not firm in my mind.

John


John,

Sounds like a nice plan!  Take a lot of pictures and post them.

It will be fun to watch the rig come together. I am excited that you will be trying to duplicate the RF  layout of Chuck's rig.  Your rig will have evolved by 2-3 builds. IE, it would have taken you several homebrew rigs before you discovered and employed these quality parts and advanced homebrew AM techniques on your own.  You have covered everything from the MOSFET audio driver, computer audio processing, digital RF drive, stiff HV supply, excellent RF parts and layout, commercial iron and big modulators - big audio to RF ratio... nice frickin rig!

I especially like that you listen to advice from others here and actually go out and do it. A more stubborn ham (like me) will do some things his way and regret it later...  Grin


The HV diode stack:

It's just a rectangular piece of Plexiglas mounted on four ceramic pillars.   Some guys use green G-10? fiberglass which looks very professional.  The diodes are mounted in two 15 diode lines. Drill 30 holes  for 15 diodes  and another 30 holes for the other 15 diodes.  Connect them together and solder the leads.   You can lay them out any way that looks pretty as long as the two legs are spaced apart by a couple of inches to avoid dust arcing, etc.    I like the look when the individual diodes are laid side by side instead of in a line.  

The HV fuses:  Big commercial vacuum HV  fuses are expensive... so I mounted  two ceramic pillars spaced 2"-3" apart, and at least 2" high.    Strip a piece of RG-213 shield wire (the outer braid) or equivalent. This is very fine wire. Place 1 to 2 strands in parallel to bridge the gap of the two ceramic pillars.  Use one fuse in series with the stack junction -  at the fullwave diode output just before the filter CHOKE.   You are using choke input so this will give some surge reduction. This assumes you decided to put the choke in the + lead, not the xfmr CT as mentioned earlier in a post.

Bear in mind when a fuse blows, the wire will probably flame an arc a few inches around it and below it... and above it. Mount the fuse in the clear and away from the diodes!  

You will know how many strands to use by experience. Start with ONE strand and if it blows from normal operation, then add another strand.   Bear in mind that these fuse wires can fatigue after a lot of HARD turn-on surges, so if one blows during normal operation after a few months, this probably means to add another strand.   You COULD use the wire/current tables to estimate wire size needed, but trial and error works just as well.  Your step-start will be important for your planned 72 uF of capacitance. A very slow step-start may mean no fuse fatigue problems at all.

The HV fuse definately works. I've had direct shorts for various reasons in the several rigs it feeds and never lost a diode stack in at least 25 years now.  But when the HV fuse does blow, it will leave a bit of soot on the pillars, depending on the intensity. Generally, the 240 breaker will pop at the same time as the HV fuses blows. Sometimes the fuse blows and the 240V breaker  does not.  This is what you want to see.  Coupled with the 25 ohm HV series resistor, we have an overall failsafe system.

BTW, my 4X1 rig HV supply uses three strands of RG-213 braid and has remained intact for a few years now. (140 uF @10 KV, 30 A choke)   Probably cuz I haven't been on the air...  Wink


T
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« Reply #4 on: January 20, 2019, 08:37:40 AM »

Thanks for the diode stack info. Got it laid out on paper. Will get a piece of plexi this week and get it built.

I have decided build a power control center. Here is how it will work.

240v is supplied to the HV unit and up thru the control cable to the cinch-jones connector. It goes into the L4B amp or control head then back out to the supply as switched 240v. I plan to put a relay in the HV unit to bypass this current path so that the voltage going back is low current. This is a common update for L4B supplies to allow for reduced current on the switches. This will also insure that I am not carrying full current for the trans up and down the cable. The L4B transformer is 700VA. This Gates one is at least 1500VA. The 240 will have circuit breakers and soft start in the HV unit.

Should I have the control head get a separate 120V or should I send a neutral up the cable and split the filament and supplies across the load and run it all off of 1 240v feed? The schematic shows sending ground on pin 7 but no neutral. Pin 8 is not connected in the L4B supply but it is grounded on the L4B amp chassis. I could lift that ground and pass neutral on pin 8. My house is build according to the 4 wire code so my neutral and grounds are separate except at the transformer pole where they are bonded together.  

I think this will be safe. My reasoning is that I am the one that will be using my HV with my amp. There would be concern if someone took my HV supply and plugged it into another amp. The concern being that the neutral and grounds would be bonded at the amp. My amp being plugged into another L4B supply would be no concern at all, I believe.

Thoughts?

The control head will have fused switching for filament 120V. I plan to use those nice computer power connectors to feed the 120 voltage out of the control head up to each chassis (mod and rf) to the filament transformers. I will have 2 RP-BNC connectors to loop the HV from the HV unit and feed it out to the mod deck. Then 2 more connectors to loop it from the mod deck to the RF deck. I can buy pre-made HV cables with nice insulated ends. I will also have an HV meter on the control head so there will be a resistor string there to drop the voltage to a measurable value. Variac on one leg of the HV input feed to control overall HV output.

The fused screen and bias supplies will be built entirely on the control unit chassis, including the VR-75 tube and small variac. See schematic for locations where the split occurs (the split at VR75 is not correct on the schematic). The meters for these supplies will be located on the RF front panel.

I plan to have a BIG RED SWITCH on the primary 240V feed mounted prominently somewhere that can shut the whole mess down. I don't want to have to be reaching to unplug something if the main relay decides to fuse closed or some other major failure.

Well, I think that is the plan so far. I will revise my schematics to reflect this partitioning plan. I like my schematics to be a good roadmap for the build and a good reference if I have to go back 5-10 years later to repair something.

John


 
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« Reply #5 on: January 20, 2019, 01:56:18 PM »

I always use the HV fuse of magnetron ovens, they are ok upto approx 3 kV, small and enclosed in an isolating cartridge. And when they blow, i do the same as you did, use thin wire to repair the fuse (if you don't have spares or a shop closeby)
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« Reply #6 on: January 20, 2019, 04:19:41 PM »

Quote
I had decided to build a 8000 modulated by a pair of 811's

Is this going to be a push pull circuit? I built an amp using 8000's way back around 1978 and it worked great in push pull. It had more output than 810's in the same circuit. 8000's are quite rare so if you run out of 8000's, 810's are not hard to find and will most likely other than bias adjustment, will be a drop in replacement.

Is your power supply going to be cap input? That's a lot of capacitance and could result in an electrical explosion if something were to short out.  The link below has fuses and holders.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Microwave+Oven+High+Voltage+Fuses&_sacat=0

There are other fuses listed at higher ratings in amperes by looking around eBay but most are rated at 5 KV.

I cannot express forcefully enough to make every effort to make sure that your capacitors are discharged completely and that a ground shunt or short be in place at many points where lethal voltage may be present! Just shorting something out with a screw driver is not enough! The shorting mechanism need to be in place throughout while high voltage repairs are being made!
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« Reply #7 on: January 20, 2019, 05:21:39 PM »

Points noted on the danger. Power supply is choke input as shown in the attached 813 schematic.

Do you have an example of a shorting mechanism that is safe when in operation?

Here is what I am building - http://www.amwindow.org/tech/htm/813/813.htm

I _was_ building an 8000 rig...

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« Reply #8 on: January 20, 2019, 07:01:42 PM »

The Collins KWS1 uses a leave spring that shorts the HV as soon as the top plate is removed. Very simple and safe system. If someone has a KWS1, he can make a pic of it.
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« Reply #9 on: January 20, 2019, 07:52:08 PM »

Quote
Do you have an example of a shorting mechanism that is safe when in operation?


Commercial systems use an interlock system. I'll get back to you with an idea or two sometime tomorrow. Too tired tonight to strain the brain  Grin Grin

I guess I need to read more closely to understand what you are doing? All I had to do is read Cheesy Cheesy
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« Reply #10 on: January 20, 2019, 09:29:38 PM »

I use perf board from....  RadioShack.com for my diode stacks.

Less drilling 😇

--Shane
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« Reply #11 on: January 21, 2019, 01:10:41 AM »

I suggest a 4 wire 240V cable to the power supply, then run 120V, neutral, and GND to the controller from a jones connector etc. on the power supply. You can be sure it's all un-powered whenever you disconnect the single 240V plug, and since the control has a special plug, it can't be plugged into a 120V outlet by error.

The reason I suggest this is because I used to run a separate 120V power cord to the control box and got a very nasty surprise while working on the unplugged Tucker transmitter, as the station control was still plugged in and supplying 120V to the transmitter in a roundabout way. The worst part was not the shock, but the large cut on my arm as I jerked it out of the bowels of the transmitter.
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« Reply #12 on: January 21, 2019, 02:22:41 AM »

First thing you know John is that next you'll be feeding the output of the 813 into a setof parallel 833s or 4-400s. Grin

Phil - AC0OB
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« Reply #13 on: January 21, 2019, 06:37:36 AM »

Is there no end to this madnesss?  Grin

Great suggestion, Patrick. I will incorporate it. That way no mods to the L4B.

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« Reply #14 on: January 25, 2019, 06:37:14 AM »

385 pounds of iron arrives today  Cool
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« Reply #15 on: January 25, 2019, 05:07:44 PM »

385 pounds of iron arrives today  Cool


And six pounds of 813s, caps and sockets are on the way to you today now that the ice storm is over..

The postal worker wanted to know if I wanted to pay an extra $10 for a "fragile" sticker.  I said no and  figgered it was packed well enough with styro and bubble pack so that the tubes would be FB.    813s are possibly the most rugged big transmitting tube ever made.

So what's the latest on the project over the last week?

The most difficult part may be finding a name for your rig. I've had rig naming contests here in the past.  Just so you know..."Fabio", "Dr. Love",   "Dual Quads" and "Rico Suave"  are already taken. Or you could do like most hams and just call it "The 813 AM Rig."     Grin


T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #16 on: January 26, 2019, 06:41:47 AM »

Thank you Tom for keeping your promise on this deal and your help along the way. I would not have taken this leap without support.

Going to do the physical layout of the power supply today, and build the diode array. I assume I should be using the center conductor from some poly RG58 for all voltage interconnect on the supply?

Have electrical connectors showing up to do the interconnects to and from the control panel. Also going to lay out the screen/bias transformers and power supply circuits on the chassis. I have a free-standing supply for the 24v relay power.

Want to thank Phil for delivering the iron. As we were talking in the parking lot, for a fail-safe Phil suggested a large relay that is energized from the line power that holds off a large resistor when on. When line power is removed it is placed across the caps to quickly drain them and keep them shorted. He suggested a 200W one with sufficient resistance to bleed it off quickly. What can I do to reduce the arcing on the relay - some decoupling caps across the relay will not be enough. Going to do a little googling to see what others have done. Guess I could us one of those nice russian spst vacuum caps here as well?

Thoughts?

Also will spend time this morning doing the final schematic for the PS.

Names? Gort is taken also. Was thinking "Lord please don't let this PS kill me" but that is too long.  Huh

John

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« Reply #17 on: January 26, 2019, 12:45:22 PM »

Here are the latest schematics. Reworked the power so it shows the HV unit vs. the power controller. Still have to design a soft start. Suggestions for a KISS approach?

20A or 30A breakers? 30A or 40A main breaker on the panel? Guess I should check the Gates schematic to see what they did.

On edit - the Gates fused at 30A providing 3500W of power at 85% modulation running a pair of 833s driven by a pair of 833s. That works out at ~15A steady state. This transmitter should pull about the same or somewhat less, I would guess. I think I will have breakers at 25A on the HV unit and a 30A electrical circuit. Soft start will help here on the surge current.

Phil gave me the Gates filament transformer also. Looking to see if I should use it. It has multiple 10V filament feeds. What is the rule of thumb on how far the filament transformer should be away from the tubes themselves?

Added the antenna switching and the proposed sequencer order. Please help me here on the initial sequencing plan.

Is it necessary to have the 33K/200W additional R on the supply? It will drain the power at 80ma after the mains are shut off. That is 200W of power at 2.5kV.

Wondering if I should drop the 8uf oil cap. That was the original Gates cap. I have added 64uf of additional capacitance. Why bother with the 8uf can...

After this design session, I still have parts to buy. Thought I was about done... oh well..

John

* 813 Transmitter.pdf (1273.99 KB - downloaded 103 times.)
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« Reply #18 on: January 26, 2019, 01:10:20 PM »

Forgot the HV fuse...
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« Reply #19 on: January 26, 2019, 05:17:32 PM »

Quote
He suggested a 200W one with sufficient resistance to bleed it off quickly. What can I do to reduce the arcing on the relay

I think this may be a non issue since you are bleeding off to a resistor and not an inductor.

What value of resistor are you planing on using? Bleeder resistor typically rage from 50 to 100 K depending on how stiff you want the regulation. I always suggest using resistors in parallel to give you a safety margin in the event that one resistor goes open. For example two 100k in parallel to arrive at 50k. 
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« Reply #20 on: January 26, 2019, 06:26:05 PM »

Quote
...Bleeder resistor typically rage from 50 to 100 K depending on how stiff you want the regulation. I always suggest using resistors in parallel to give you a safety margin in the event that one resistor goes open. For example two 100k in parallel to arrive at 50k. 

I bought a bunch of 200k 2w 5% metal film resistors for use as bleeders. With 200k across a 400v capacitor 0.8w are dissipated, worst case. That's a 2.5:1 safety factor. I parallel units to get the amount of bleed I want. A one-size-fits-all solution.

1 unit, 200k bleed.
2 units, 100k bleed.
3 units, 67k bleed.
4 units, 50k bleed.
5 units, 40k bleed.

Don
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« Reply #21 on: January 27, 2019, 08:53:39 AM »

Working on the step start part of the circuit.

My transformer is a big Gates transformer. Inrush current is going to be big given that I have 72uf of caps on the secondary. For safety reasons I decided that I will have good old fashion switches to cut off the power. They then feed to a relay where I can switch the HV from either the control head or the L4B Linyaaar amp. This relay is controlled by a 120v feed. After the relay I will have a variac on one or both legs of the feed depending on what I can find. Next in the line is the step start. I have read about them from various uses, mostly on amps with considerably smaller transformers. I think this one needs to be substantial.

I am going to build the step start along these lines. Maybe Phil can help me with estimating the specs on the transformer.

http://amfone.net/Amforum/index.php?topic=41787.msg303932#msg303932

I am going to use the 40H estimate until I can measure the trans. Not sure my little LC tester can go that high.
Caps - 72uf
Sec current - 1.5A
100K Bleeder

TC - R*C =7.2s (a long time)

R = L/TC = 5.55 ohms

Fuse = 1.5*2.5=3.75A

So looks like a 6ohm 200W R in series with a 4A SB fuse?

That TC is really long. If I go to switching HV during a QSO I think I will need a better solution.

Going to drop the 33K extra bleeder. I think I will use the existing 100K bleeder with a mechanical interlock to short out the filter caps if the case is opened.

New schematic. Going to start the PS build this afternoon if I don't get a hernia carrying the iron to the shack in the basement.

John

* 813 Transmitter.pdf (1262.23 KB - downloaded 78 times.)
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« Reply #22 on: January 27, 2019, 10:39:03 AM »

Was thinking "Lord please don't let this PS kill me" but that is too long.  Huh

John

I've got that power supply in my shop.  Knocked me with 6kv.


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« Reply #23 on: January 27, 2019, 11:16:57 AM »

I read the post - scary. That is why I am going with RP-BNC as suggested in a reply to that post. No Millen HV for me.

Glad you are ok.

John
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« Reply #24 on: January 27, 2019, 11:56:02 AM »

Hi John,

Without going into formulas, I'll give you my practical experience on step-starts...

There are many ways to approach this. I personally like keying the HV for transmit/ receive. The goal is to reduce the initial major surge, that capacitor-only supplies can reach   10, 20, 30+++ amps, depending on the components and voltage level.   The diodes can handle this as well as most capacitors, but it does raise havoc on longevity if pushed to an extreme.

In your favor you are using a big choke input, which will oppose the flow of current as the mag field initially builds. This is a good thing.

Most of the initial charging occurs very quickly, in under a second.  A step start is the best way to add more "cushion" to the surge.  My suggestion is a 5-10 ohm 200 watt resistor that stays in the 240 VAC primary for 1/4 to 1/2 second and then gets bypassed - is about right for your supply.  That 1/4 to 1/2 second can get "buried" with the other functions of sequencer antenna/ driver switching, amplifier key up, etc.  The resistor contactor (and any other associated relays in the 240VAC primary) should be a robust 240VAC type, 24VDC coils, about the size of a lemon with heavy contacts, open frame... you know the type.  No RS 12 V relays.

You will see your AM carrier come up before the step start drops out and then there will a few DB power peak afterwards. You may hear the second click of the relay unless you use some kind of audio delay as I do, to remove all the shack noises on key up. IE, the audio comes on last.

If you make your step start timer variable, you will be able to find a compromise between key-up speed and reasonable current surge. I run this same setup and component values on my own 140 uF, choke input 4KV HV supply.  My total delay is about 3/4 second and the HV supply is barely stressed on key up.  The HV  B+ line fuses also provide protection and a good guide to how hard the system is being stressed.  If the HV fuses fatigue after only a few short days of use, then you need to either increase the fuse rating or decrease the surge startup level.
 
BTW, I will try to get to your other questions later... For now, your question about switching in the additional bleeder after shut down... you already have fixed bleeders on the filter caps, right? Then the extras are a redundant system for safety and is always a good thing if you want to go thru the extra building effort. Most hams don't bother, but if you are extra nervous, then do it.


No, the small 8 uF will just mostly take up space... stick with the two 32 uF beasts. Use it as a spare.


T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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