The AM Forum

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

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

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

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
WA1HZK

Huh, that's not my experience. I got 400 watts or more from one at work, its class AB2.

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.

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

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

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.

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.

In the resistive input position, a 350 ohm resistance is switched across the grid. This position requires no tuning of the grid circuit.

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.

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

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

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.

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

I know this is a pretty old thread, but maybe theres still some people out there that are familiar with the things mentioned in this thread.

I am whipping a modified Tbolt into action again and its using a Heathkit KS-1 modfied external PS, as the Tbolts plate xfmr was
removed, likely because as stated in this thread, it was marginal. The KS-1 has the diode tubes removed and solid state diodes with tube
form factors are in the original sockets. Yesterday I confirmed close to 3900Vdc from the KS-1, with it plugged into a 220V line service
so I believe thats all ready to go.

Im told my Tbolt was upgraded from AB1 to AB2, but am curious what to look for in the chassis to learn when it entails. All I can see thats non stock is a large square cap near the jack for the bias control  cable. Theres a photo in this folder where you can see it in my chassis.

https://app.mediafire.com/7plcwwj5onpvz

3900v out of a KS-1 is a little high.  3400v on today's line voltage (242 measured) is about right. This should drop to around 3100 under 600ma of load.  I wonder if the bleeder bank is open or the swinging choke is shorted.  Playing with a Chippewa/KS-1 right now so these numbers are fresh in my mind.


Steve

KB1VWC

Steve

Ok I remember you helped me on the Heathkit list back in Jan 2020 with the KS-1. Had to look that up as I remembered the name.
This ones got solid state diode tubes in place of the original 866 tubes so I think thats why the voltage is higher. When I shut it down,
the voltage diminished quite fast, so I believe the bleeder network is ok. Swinging choke, you mean the choke between the diodes and the
filter cap?

EDIT: My mistake onthe HV output, you are right it was 3486Vdc as per my notes.

  3486V sounds close enough to me..So I would guess no issues. If the choke were to be shorted you would have a lot higher voltage.  I would guess your line voltage is up around 248-250. That will get you the inflated numbers.

  Steve

Sorry, wrong thread...

Well I finished up making the T-R switch with a dow key relay and some other odds and ends I had laying around.
the DOW key had 2 sets of SPDT contacts basicly a DPDT, so I used those to perform the blocking and biasing
for the Thunderbolt cable. I also made a diagram here that shows the pinout and wire colors, see attached.



 

good work !

out of curiosity: how different is the rf section of the t-bolt
from the rf section of the desk kilowatt???

The plate tuning cap in the thunderbolt is rated either 2000 or 2500V.

Possible new problem - noticed the VR74 OA3 tube is no longer
lighting up when I power the amp with 115VAC.

At one point it was lighting up red like it had a filament glow as you can
see in this picture, its the tube in the upper left:

http://www.mediafire.com/view/kqz4t3o8sqc8znu/HPIM0694.JPG/file

But in this more recent pix its not lit:
https://www.mediafire.com/view/vrmf3prnj49fvww/hpim0831.jpg/file

In the schematic it seems to be handling something for standby bias perhaps?

http://www.mediafire.com/file/xekmcs4illyy72l/VIKING_TBOLT.pdf/file

I have the 2 pins tied together to form the "block" / standby condition as
the manual recommends. At first I thought
the older photo had the tube lighting because I didnt have these pins
shorted, but I just removed the bias jack for a fast test and that tube is still not
lighting. I suppose it could have gone out, as its old but also strange is that it was
lighting red like it had a filament, where I believe its suppose to glow
purple as the others. Could that tube only be operating for certain modes in the mode switch?

Regarding the tank capacitor voltage handling, I had looked into this a while back.
I looked up how much air gap would need to be for 4KV to arc over and it was something like 0.15mm which is less
than 1mm. I can tell the spacing there is greater than 1mm by eye, so unless I misplaced a decimal, all
should be well for the plate tuning cap to handle higher plate voltages.

http://www.mediafire.com/view/css4yxdpheqm7p2/HPIM0697.JPG/file

Ok I think I see what happened. That 4 pin plug is NOT KEYED - so you can put it in 4 different ways. I have pin 1 / gnd lined up to the ground pin in the chassis, so that ought to do it. I also marked the connector and socket so you can line up the marks and its now the correct orientation.

Center to center pin measurement between pins 2 and 3 is greater then center to center pin measurement between pins 1 and 4.
That is the key.

Yep, what Pete said.   Its not obvious, but a little time spent looking, and you'll see it.

Different gas regulator tubes light up as they regulate their specific rated voltage.

Each gas ionizes at a different voltage, so therefore tubes for different voltags have different gasses, and therefore the colorful combination. Orange is usually a neon gas color.

here is a chart from the RCA RC-25 tube manual.

Update on progress.

Had success on 80m loading up with RF from the rigg into the Windom antenna, made sure to limit at 300W because of the balun limitation. Seemed to to not bad, 2 rounds on a net, kind of medium length transmissions both times, good reports - was happy.

Then I tried loading up on 160m with the Dentron MT-3000A tuner, and could not get any dip. The tubes were complaining with orange and red on the plate elements, so stopped that experiment abruptly.

Went back to 80m again with the same settings. Got the dip, about the same plate current and up to 300W carrier again, but the tubes were glowing red after about 5 seconds, and I dont believe I saw that when I did my net the prior night.

After moving the mode switch from TUNE to LINEAR, saw one of the 150V regulator tubes was glowing purple all on one side of the glass - definately not like the others. As per the advice of KB1VWC suspected bias issues, so opened the chassis up and inspected the VR tubes. One of the 150 and also a 105V regulator has white film on the getter area and the other has it on a seperate section away from the getter area, and I am told these are hosed.

I'm suspecting they have seen their day as they seem to be date coded 59-43 and 59-48 on the tube base. 1959 week 43 and 48 respectively as date of build by RCA, so they have earned their keep for sure as they are likely the original tubes in the linear.

I was going to brainstorm  SS jiggery for the entire string, but found some NOS online very cheap so ordered some.

The 2 bad ones are also kind of loose when you move the glass by hand - the socket is for sure not gripping them tight like the othe ones.


 

The frequency range of the stock Thunderbolt is from 3.5 to 30 MHz. Why would you try to load it up on 160 meters?

I heard additional inductance and capacitance would be all it needs , in line after the amp to the tuner, (with the Tbolts settings wide open) so I thought my Dentron Tuner would be able to compensate, but I guess not.

Apparently a bigger plate choke is needed than whats in the Thudbolt , for starters.

If you really wanted to do this, you might start by reading this article in January 1991 Electric Radio:
"Modifying the Johnson Desk Kilowatt for 160 Meters"
It's for the desk KW but there are lots of similarities between it and the Thunderbolt.

Ok alot of new progress, some good, some not so much.

The NOS tubes are in and lighting nicely.

Noticed when the amp was in LINEAR mode, one of the new ss diode I put in for the
negative bias supply went out. I took them both out, they were 600 piv rated which ought to have been fine.
I put in 2 1N4007's which are 1kv piv. Ran it again in LINEAR at idle, and checked them, not getting warm so thought ok. Later on everything died, and the 5a fuse for the filament xfmrs blew. Originally there was an 8a type in there with a sagging metal strip behind the glass, meaning it was close to blowing.

After studying the schematic, when in LINEAR mode the switch places the full screen supply voltage right to the grid via the bias cable pin 4. In standby, we already have the -180v from the other supply on the grid. The result is naturally a high drain of current from the negative supply, which explains the bown fuse, blown diodes and removing the tube for diodes.

Johnson placed a 270k /1Wresistor on the mode switch between terminal 6 and 5 to remedy this condition I believe. I have one that measures 280k / 1w and placed it in, and so far after longer operation times, no more fuse or diode blowing.

So I believe that glitch is resolved.

I had taken note that during the vreg tube flakeyness, one of the 4-400A's was glowing red during the 300W pep operations. I though likely bias related, but even with the issue stabilized, one tube stil glows red. If I remember right my first good test on 80m with 300w yielded no red glow from either tube, which made sense as this is
not alot of work for the tube.

I do have a spare set and will swapping the current tubes to each others socket and see if the issue follows, as a clue to whats going on.

I did notice the tube not glowing too red had a clip on the base that was not touching the metal base, so I fixed that.
I also took the metal straps off the anodes, and shined them up nice as they were a little corroded. After that the tube seemed to glow red at a slower time than before. I am also seeing on long SSB transmissions the reflected power climb up gradually and at some point the forward power will go past 300w as the tube stays red longer. Once it kools down, I can repeat this cycle over and over where my fwd and reflected power are back to normal till the tube stays red for too long. I was told its kinda normal for some redness in these tubes at their optimum settings but I think this is maybe a weak tube.

I can work 300W in the tune mode like this and the tune glows much less red if even at all. I will have t check again
but I think eventually the problem cycles up again after a longer time even withth e greatly rediced bias on the tubes in that mode.



 

One of the set screws in a 4-400A heat sink was loose, so that explains why one tube was more eager to glow red
under low power levels - starving for plate current mose likely.

Some more pitfalls encountered when trying to tune on 40m - likely should not have with that loose anode, but before I discovered it , was too late. This NP 600V / 0.5u F cap leaked oil -

https://www.mediafire.com/view/olwnysxze66bw2j/oil_cap.jpg/file

Apparently a mod, was hooked from ground effectively in parallel with C116. Measures a bit high ESR, so tossed it.
Looks like its only in effect when the LINEAR mode is selected. Someone speculated its an anti-humm mod on the screen supply.

I dug into the wiring more and updated the schematic with my findings, and
cleaned it up a little:

https://www.mediafire.com/file/c59nbazluuk0tqk/TBOLT_KA2PTE_.pdf/file


So they have lifted the cathode of the final tubes from ground, and are
running the plate meter through
that, which explains the current reading at idle. Though almost 75mA through
the screen seems kinda alot.

I have included my ss diodes in place of the tube, they are 1N4007's and I
put small resistors in line
with them to hopefully act as fuses. I also replaced C104 which looked like
it was original with a 150uF
as per the schematic. This is 5x higher than the original but tests so far
seem to show its ok. Theres about -188V on that cap now which is the standby bias.
Im told the extra 30 or so volts ought not matter much from the manuals -150V
and the tube sheet says the max is -500v.

The (2) 1K resistors are now shown after tracing them out. Apparently making
a higher voltage drop than R116
was when in TUNE or LINEAR mode. I believe this is the AB2 to AB1 mod but
cant fully understand how
limiting the current to that 75V regulator tube does the trick.


Right now with no plate volts and the 4-400A tubes removed entirely my
screen volts is 367 (from terminal 1 on the mode sw) with no plate current
at all in TUNE mode.

In CW mode the 75V neon tube goes off, which I believe is normal and screen
volts jumps to 519. Move to LINEAR position and the screen is
519V. No grid or screen current at all , which I believe is expected as
there are no tubes. These voltage readings are a little over the ones
in the manual, so I guess they are acceptable.

When I leave things as they are and put one or more tubes, this is where
there are issues now that werent there before.
They seem not to change with the bias cable in or not, but for safety I left
it in so there is negative bias on the tubes.

So now with still no plate volts and one 4-400A tube _IN_ a socket the screen as
measured on terminal 1,  is
67V in TUNE, and 204v in LINEAR or CW. In CW there is 100mA
of current on the plate meter, screen current is about 25mA with no grid
current. In LINEAR its 50mA of plate current with about the same screen
current. In TUNE there is perhaps about 2x what was normal plate current,
which is the "needles width" reference I was told is normal (by another thudbolt owner)  
- I guestimated to be  10mA so its now maybe 20mA or so.

Maybe the plate current meter was sticking, but when I worked the amp with decent results,
never saw idle current on the plate. When I put in a decent RF signal, then keyed into TX,
then I got plate current.


UPDATE: I concluded 1 or more the NOS tubes I bought may have reached the end of their life. So have some more on the way.

Well lots of progress since last post. Thanks also to the people who are helping via the PM's, greatly appreciated.

Found some connections on the VR tube termnals were breaking down from the heat causing a short to other wires running near them. Fixing that has stabilized the tube output. Still had too much current draw in LINEAR or CW, and concluded the 4-400A's were biased like a diode because the grid bias was not making it there, due to an open L8.

While in there decided to check C10, which was completely invisible - amazing the amp worked at all with it like that. Then found nearly all the coupling caps on the Sw2 array were shot as well. With max coupling there was perhaps 300pF, which explains when I tried to load on 40m - the amp self destructed.

I have updated the schematic quite a bit in the previous download link. They have made the plate curent meter the cathode current meter and placed (2) 0.01 caps from the cathode to ground....I guess trying to bypass the cathode since the ammeter is now sort of a resistive load - not 100% sure the idea of that.

Also T103 had been updated with the voltage connections. Im told I ought to wire it to 120 from 110, and was curious if anyone knows which 2 wires are for the 120v connesitons? Theres one black wire, and (2) white / black striped ones.

Set this aside a while back as I learned I have a parasitic happening from tuned grid to plate when in tuned grid mode with the selector. The 2nd new RF choke had smoked again, arousing suspicion....and there was a popping noise from one of the tubes that preceded it.

Before I risk another RF choke, I took KB1VWC's advice and am setting about replacing all the RF bypass caps in the chassis which are 0.005uF, rated at a voltage unknown as the manual does not give a voltage. Here is a picture of one of them:

https://www.mediafire.com/view/vhyal4a0pwxbcdz/HPIM0855.jpg/file

I have a load of Xicon (p/n: 140-202p9-472k) class 1 ceramic disc caps I bought yrs ago which are 2kv rated at 0.0047uF and started installing , but wanted to double check in case these ought to be RF rated? I believe class 3 are RF rated, but not sure if that matters much.

Been a while since I fired up the Tbolt. Gonna use if for my special event station in June, so I opened it up and modified the filament switch so its now a DPDT center off. The down posiiton disconnects the screens of the 400A's
for neutralizing mode and I drilled a hole in the top cover above the neutralizing cap for easier access.

I placed in a set of JAN type 400A's I picked up last year in nearly factory condition that have thorium coated
filaments and reconfigured the regulator tubes so there are (3) 150 and (1) 100 for a screen of 550V which
seemed to be the best result I saw so far in past tests.

I noticed the filament to the new tubes is a higher voltage, so they glow a low white color now, and the
voltage on them is about 5.3V and I want to get it down to 4.9 somehow. I calculated a 4.5 ohm 18W resistor
fed to the primary ought to bring the 120V there down to about 111V so I may try that method.

I set about neutralizing last night, but since there is 3400V on the adjuster from the plate supply, I had to
turn the power off, then move it up or down and see if it changed the reading on the wattmeter. It was too
awkward doing it that way, so I loaded up into a dummy load for a max of 600W. I noticed the corner tube
was the only one with the plate turning red, so apparently it was doing all the work, hence the 600W max
power out.

I killed plate power, put the cap at max, then backed off 1 turn and now the other tube was red like the other
and I could now redip with more drive and saw about 1100W on the wattmeter to the dummy load.

When I unkeyed the 80m test signal, I noticed the plate current stayed where it was and the tubes continued
to glow red? I killed plate voltage, and they stopped drawing power. I guess they were self-oscillating perhaps
due to the neutralize cap still needing tweaking, so I re-tested after backing off another 2-3 turns from the current
setting, and when I unkeyed the drive signal, the tubes no longer stay active.

I had thought perhaps my T-R relay stuck, but I keyed and unkeyed and could hear it operating while the
tubes stayed running so I guess they were self oscillating.

Tue procedure in the manual says to look for grid current as the way to neutralize and to be on 20m.
I was on 80m, so if I operate on other bands, will there be a conflict?

In my junk box, I have a solid state "Semtech SA 7477 7823" which replaces the 866.
Alas, I Googled this and found nothing?

I did find this:
https://www.ebay.com/itm/374321329464

Found this info on SOLID STATE RECTIFIER REPLACEMENTS:
https://www.kevinchant.com/uploads/7/1/0/8/7108231/international_rectifier_ss_replacements.pdf

I've had on of those amps before and never had neutralization issues. But mine ran 2000-2500V. The semtech should be an 866 replacement.