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Thunderbolt Mods?




 
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Author Topic: Thunderbolt Mods?  (Read 6945 times)
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WV9R
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« on: January 24, 2009, 06:37:17 PM »

Hi all, I'm back, thanks for all the help getting the Viking II going, as I said before I have a T-Bolt amp that I was going to look at next. Well I trusted someone on this and well I shouldnt have. Does anyone know of any mods for this amp that has you remove the 2 866 tubes? The tubes are gone and there is a resistor bank now mounted over the sockets. On the underside there is a board mounted with 6 47uf 450v caps. I can turn on the filament switch with no problems, but turn on the HV and the fuse goes right out.  Any suggestions?  It would make a nice doorstop, lots of weight hihi    but kind of expensive for that .. Any suggestons    Thanks in advance
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Ray
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« Reply #1 on: January 24, 2009, 06:47:56 PM »

Ray,
The problem with the Tbolt is the power suppy. The HV choke is one of the problems as is the HV plate transformer.
The rf deck is well built and very good for an early desk top linear BUT that pesky HV power supply sucks!
Check all the components in that area first.

Bill
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WQ9E
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« Reply #2 on: January 24, 2009, 06:51:47 PM »

Ray,

Well, somewhere it has to have replacements for the 866A tubes and I presume they are now on the resistor bank board mounted above the sockets.  At least this part of the amp is pretty straightforward so trace out how they were replaced.  There should be two strings of rectifiers, one for each 866A that was replaced.  At minimum there should be 8 1 KV rectifiers per string to allow a decent safety factor and 10 would be required to match the 866A PIV rating.  If there is any question about whether enough or the right ones were used (I am partial to 1N5408 which are 1,000 PIV 3 amp and dirt cheap) replace them.  If the HV filter caps haven't been replaced, then I would also replace them at the same time.

While you have the filter caps out (or disconnected) measure the resistance to ground for the HV and it should be slightly less than the bleeder resistance value (which did change somewhat over the life of the T-Bolt so check what is in yours).  

Also check the bias supply because a failure here will also create fuse blowing from excessive plate current and related collateral damage.

The T-bolt isn't the heaviest piece of gear I have (that would be the Desk KW or Gates rig) but it is without doubt the most awkward to work on given its weight and the difficulty of removing any iron to lessen the weight without also unsoldering a number of components.

Rodger WQ9E
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Rodger WQ9E
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« Reply #3 on: January 24, 2009, 07:42:06 PM »

Those things seem to be built as a CB amp. I rebuilt one a few years ago and found that it wants maybe 5 watts in for about 200 out carrier. That's it. The power supply is not very good for AM service. No real snot.
Keith
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John K5PRO
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« Reply #4 on: January 25, 2009, 03:18:00 AM »

Huh, that's not my experience. I got 400 watts or more from one at work, its class AB2.
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N3DRB The Derb
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« Reply #5 on: January 25, 2009, 08:44:57 AM »

main problem with teh Thunderjolt is they just don't run enough plate voltage. If Johnson had run 25~2700 plate voltage on the 4-400's, instead of the typical 2K under load,  they would have had something.
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« Reply #6 on: January 25, 2009, 08:55:16 AM »

yup, the hv is on the low side .... I had one as my first amp ... top output stock is about 800 W ... if your hv xfmr is ok and you stay with choke input (recommended unless you modify plate components) and you go solid state for rectumfinders then you need step start and an input transient snubber to ease up on on/off spikes .... I modded a later t-bolt to gg and vac cap for plate tuning and no hv choke ...it had about 3600V and made about 1400w and got hot as hell ... wound up with 3 fans on top pulling out heat ...an excellent shack warmer ....73 ....John

n.b.  in retrospect this amp would have been an excellent candidate for the Varney G2DAF circuit which works well at lower plate voltages ... consult RSGB handbook iffin you are interested ....beefus
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Beefus

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WQ9E
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« Reply #7 on: January 25, 2009, 09:08:16 AM »

The T-Bolt was really designed with the Johnson Pacemaker in mind and those two make a good pair.  It works fine on SSB and CW and does a good job of boosting the Pacemaker's low power AM up to a reasonable level.  But it was never meant to be the big gun on AM, that is the Desk KW's role.

I have a Pacemaker and Thunderbolt set up with an RME-6900 for receive and it makes a nice vintage sideband and CW station that can run AM.  With a good and stable exciter, the T-Bolt would have been a good RTTY amp back in the day since it could run in its high efficiency class C mode with FSK keeping the tubes and power supply from excess stress under continuous duty mode.

Any amp that can safely run AM linear at near the old legal limit would have to be capable of far beyond the old 1KW CW / 2KW SSB rating; a Henry 4K ultra would probably make a fine AM linear.   I have run my TX-1 and Chippewa a bit on AM and it will come pretty close to running the current legal limit on AM; although Heathkit advised that it could be run at 1 KW carrier input it does not have the capability for clean amplification of fully modulated AM under those conditions.   Otherwise controlled carrier works pretty well with the linear amps and I have run my Drake 4 line gear with the L-4B this way; it does give the AGC on the other end a workout.

Rodger WQ9E
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« Reply #8 on: January 25, 2009, 09:31:33 AM »

main problem with teh Thunderjolt is they just don't run enough plate voltage. If Johnson had run 25~2700 plate voltage on the 4-400's, instead of the typical 2K under load,  they would have had something.

Beefing a T-Bolt would be next to impossible, even with an external power supply.

The plate tuning variable cap spacing is way too close to run more plate voltage. The 4-400s have marginal cooling, no forced air system sockets or chimneys. They'd burn the paint off the cabinet running any more power.

The amp *is* perfect for use with an AM exciter like a Johnson Ranger, but you need to use a 6 to 10 db pad in between. Instructions for building a correct pad are in the T-Bolt manual.

Ray, pull the 866 rectifiers from their sockets and see if the fuse still blows. That'll tell you if the plate tranny is OK or not. If the fuses don't blow, then your problem is either bad 866s or a short downstream from them.

Note that if the 866s have been shook or moved around from their normal vertical position, you *have* to let them cook with only filaments on for maybe a half-hour before applying HV, or they'll flash back and blow fuses. A good substitute for the 866s is a pair of 3B28 rectifiers, if you don't put solid state diodes in. The 3B28s are much more reliable and stable and available surplus.

Be CAREFUL! Short everything out with a Jesus stick before working inside the beast.
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« Reply #9 on: January 25, 2009, 12:16:59 PM »

The high voltage transformers burn out in those. The one I had before already had a surplus outboard transformer. It did well for 100W AM carrier and 400W PEP but the tubes got very warm doing that. The most I ever got out of it was with 2000V/500mA input and about 550W out. The comment about using QRP transmitters is right! 10W drove that AB1 amp mightily. One option is to disconnect the grid tank and put a 50 ohm non indiuctive resistor there as termination. Then drive with somewhat normal levels, no grid tuning necessary.
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Pete, WA2CWA
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« Reply #10 on: January 25, 2009, 02:29:38 PM »

One option is to disconnect the grid tank and put a 50 ohm non indiuctive resistor there as termination. Then drive with somewhat normal levels, no grid tuning necessary.

In the resistive input position, a 350 ohm resistance is switched across the grid. This position requires no tuning of the grid circuit.
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WV9R
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« Reply #11 on: January 25, 2009, 05:47:42 PM »

Hi All,  Well I started at the HV t-former and disconnected the secondary, and fired it up, fuses held, so I worked my way through the amp in this manner. Got all the way to the 4-400's, with both of the anode caps removed all is good , if I hook 1 of the 2 tubes up it will hold, switch them and poof. So my question is  how can I test the 4-400's, is this making any sense at all? As everyone can tell I'm not that well versed in this area....I appreciate all of the help offered here.
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Ray
WV9R
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« Reply #12 on: January 25, 2009, 06:42:21 PM »

Hi all,  does anyone know of any resistance checks I can do on these tubes to check them out?   Also does anybody have any 4-400a tubes in their spare parts box?  Thanks Ray
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Ray
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« Reply #13 on: January 25, 2009, 07:38:26 PM »

I would not activate a 4-400 with screen and grid voltages but the anode disconnected ... this is a pretty good way to destroy the screen ...  I would test them 1 at a time with lowered screen voltage (the tune position as I recall) ... also, you need to be carefull of high fil volt'g ... will very likely need to add resistance in the fil xfmr primary to bring back to spec ... there are lotsa 4-400 available even on e-pay ...73 ...John
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Beefus

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to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
Steve - WB3HUZ
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« Reply #14 on: January 25, 2009, 07:52:00 PM »

Mods? Change the screen voltage by using different VR tubes in the screen supply. Tron did this on a T-bolt and improved the peak output capability. I can't remember which way he went, up or down on the voltage. But it worked. I operated the amp and it would put out some pretty good peaks on AM, even at 200 watts output.
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« Reply #15 on: January 26, 2009, 10:00:34 AM »

yes Steve, the stock screen VR is 750 - 800 V which is from the Eimac spec.  Bill Orr reported Collins research from the late 50's with screen V reduced to 300 V and grid bias reduced to 65 V or so ... went gg and got impressive performance ....73 ...John
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Beefus

O would some power the gift give us
to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
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