Thanks to Johnny Novice and Teh Slab Bacon for radio rescue.

[N3DRB The Derb]

went out last night to Slab's place with johnny, 2 Yaesu's, and a Gonset Amp named Jenny.lotsa stuff got fixed and progress made.
Slab also got delivered a HEAVY BOMB MAUL Thardarson  plate xfmr for the 4 X 1000 big rig.  He got the Gonset and will work on it as time allows, and Novice fixored the Yaesu which I managed to set on fire by forgetting the rig was powered up and spraying the bandswitch with contact cleaner. ) no , I'm not kidding) only damage was my pride and about 6 or 7 scorched wires, which he heat shrinked. Fixed it right up. We a stayed up late and ate a lot of chinese and worked on rigs. What could be better?

The Gonset has some stability issues and a parasitic, which is what blew up the bandswitch. If anyone has any input as to remove the parasitic and get it stabilized, we be all ears. It's a 40+ year old design. I'm sure it could be updated.

[K1JJ]

Horray to Mr. Slab and JN!!  Good job bros.


Yo Derb,

Don't worry, Vu Consultation Unlimited, will get you straight and on the righteous path.

Is that Gonset a grounded grid or grid driven amp and what tubes?

Does it use a tuned input ?

Describe the type of parasitics, stability issues and what you have to do to make it take off. Is it stable at all, on any band?  Any idea what freq the parasitic is on  -   and when it takes off, can you hear it on the band nearby or far removed?

Did you swap out the tubes?

Answer these questions and I'll have some suggestions for you to tame this twit.

T

[w3jn]

Twas a lot of fun.  Left Slab's about 2AM.  I set the alarm for 6 on the chance that I'd feel up to going to the York Fester....  I ended up going on about 3 hours sleep.

Scored a G-76 for a good price but other than that, wasn't a great fester.  They moved it AGAIN from where it's been for the past 3-4 years and it was a giant PITA to get to.

[N3DRB The Derb]

Tom,

We left the amp in the more than capable hands of the slab bacon. it's more for my education and re-training that I want to get teh am forum hive mind in on this.  I am insanely curious as to the reason why it's unstable. we didnt get a chance to change tubes or even put the cover on and test it, but at one point we got a 700 watt maul keydown out of it. It's got funky 811's in it, never did get to try it with the 572B's.

plates get moderate color and you can get a dip out of it with no drive applied. Slab will get it no doubt, but I'm curious as to what new methods are there to stop such things since that amp was designed in 1961. New better plate suppressors, how best to ground the grids for both dc and RF ( right now, they just go to ground through a .0047 cap)and such. I want to understand how to make it bettah and get some mo skillz.

Slab gotta work on his own maul, JN brought him a super bad Thor plate xmfr for the 4X1 rig. buzzardly and LARGE. He got mo monkey now.

Jhonny, you got more constitution than I do. We slept until 10 then went back for a 3 hour nap  sounds like the glory days of York are over for good, That was a great fest when it was at the fairgrounds.

update:  Slab found the trouble, and it was so obvious i should have hit us all in the face! the damn amp has no standby bias! I totally forgot the thing relied on the gonset TX for standby cutoff bias as it was made.

H'e 's gonna make sure those tubes are cut off. But I'm still interested in the subject of amp instability and how best to cure given problems. and the Gonset has a neutro cap. Yes , a neutro cap on a grounded grid amp. it's a metal bracket on a HV standoff that goes near or far from one of the tube plates to a coil wound in the opposing direction on one of the fil chokes terminated to ground. How do you adjust that lashup for where it needs to be?

[K1JJ]

OK Derb -

I didn't realize you left it with Slab - he'll get it running FB for sure. I thought maybe youse guys gave up on it for a bit.

No bias makes it idle very high and can make it more sensitive to taking off, but it should still be stable despite that.  Maybe the 811A's had a thermal runaway that spurned the instability too.. hard to say until the bias is applied correctly.

Sounds like your neutralization circuit should work as described.  According to W8JI and further posts by John/DEU, your 811A's can use neutralization. Some GG tube configs do not.

Naw, except for Richard Measures insisting on using nichrome wire for suppressors, there ain't nothing new in the whirl of basic tube amp design.  I was gonna suggest just the normal stuff to tame it down...  but let's let Slab get it going and it will be a non-issue...

Keep us posted and let me know if I can be of help if you get stuck.

T

[N3DRB The Derb]

how would you adjust the metal bracket Tom? Slab said maybe no HV on the amp, apply the drive and adjust the metal for minimum feedthru to the plate circuit.

I guess you could always use the old standby of checking he current on either side of your plate dip and adjust the thing until both your maximum current points is equal on both sides of the dip. You'd have to turn off and short the HV each time you adjusted it.

any method better than the other? I think the Heathkit APACHE used a metal plate.

Slab will do it right, but my skills have fallen off. I need to re fill my head with that knowledge again.

[K1JJ]

how would you adjust the metal bracket Tom? Slab said maybe no HV on the amp, apply the drive and adjust the metal for minimum feedthru to the plate circuit.

Derb,

Yes, this is correct. I would do it that way.  It also helps to put the filaments on.... gives a stronger signal to work with sometimes, cuz once nulled out, the signal can get so small the stray pickup can mask the final tweaking.

I'll bet the adjustment is already pretty close to start, but touching it up will help.

The dip technique will work, but I'd rather use the feedthru null, cuz that's exactly what you are attempting to do and is probably more precise..

That amp will be stable as heck once Slabbed, GG and neutralized to boot... :-)

BTW, just got off the 75M DX window. Hadn't been on for at least 18 months. Had a blast tonight for about three hours, caw mawn.  I think the new tandem 2el quads are hearing a deeper layer of Euro/Russian pissweakers.

"OK boys....now listening for Russian stations that drive red '87 Yugos, running under 10 watts and have wives named Svetlana, only please."

Later -

T

[N3DRB The Derb]

h-ok TNX OM. Slab's gonna use it over at his place for a week or 2 with the yaesu ft 901 blowing it into his hi power DL and sock atomic yeallos into the mic while it sits there and pumps out whatever the recommended am input is.

I have the 2 yeasu's here an I am going to do some major cleaning and re org on the benches and fix Yaesu #2. Bad wx and thunderstorms here should make op condx bad for a few days. it's thunderstorming right now.

All it needs is a new board wired in which Ed sent me, then I'll outfit it with a am filter and all the am mods and I'll have my pair of operating radios. Then another KW Matchbox, hopefully messed up in some way cosmetically, and then another Gonset, mark 2 3 or 4, already set up for transceivers, an I'm done.

We'll go pick up the amp when I get the all clear from Frank. 10 more db in my cb.

[w3jn]

Tom Vu hon. Master of Amplifiers.  Johnny Novice not.

We gave up at 2 AM on acct of 1700V and 3 tired smelly hams not being FB for safety.

One other thing we noted, Tom, was that there was a plate current dip, and adjusting the loading seemed to do its thang, but doing so did NOT necessitate re-dipping the plate.  WTF??  First time I've ever seen that, but then I have limited experience w/ grounded grid ham-plifiers.  I suspect it goes back to the neutralization..?

[K1DEU]

Neutralizing in Grounded Amplifiers is commonplace and a minimum in design to me and here is Grandpa's (W8JI) article who designed many Ameritron Amps. AG6K also uses it in some of his 3K to 10K home brew Amps.

http://www.w8ji.com/neutralizing__amplifier.htm  This broadband stable method has been recently adapted by me for a grid driven (common cathode/filament) 4-1000 plate modulated by 833's amateur transmitter heard often on 75 and 160.

Tom, W8JI with his Globe Scout and 300 foot towers has quite the signal on 3,885 up here in Vermont.

73  Cu on 1885 or 3885. John, K1DEU

[W2VW]

Derb,

     Forget the 811s. Figure out which 572Bs make a good quad and try it with those. The Goonset was stable and worked OK with 572Bs. It was missing 40 meters IIRC due to the bandswitch being zorched. I would not count on that failure being from a parasitic. This amp came from a pile of amps where many had crapped out bandswitches. Again, you should be good to go as-is with Cetron 572Bs.
     

[N3DRB The Derb]

ok, Dave, will do. I'll take em down with me when I go back to Frank's. He made some mods such as getting rid of the wall wort and making everything internal to the keying circuit. it did 700 watts with the 811's. I've got 8 or so Cetrons for it.

Slab is better at such things than I am. I think we used to be about equal, but he's a much better tech than I am now. I've lost some ability due to the surgery and just being away from the game. amp will be good to go before it comes back here for sure and it will stay that way.

as you can see from the pic the bandswitch is now pretty much in new condition. 

[The Slab Bacon]

Well, here is the rub so far. After some carefull but thorough dissasembly and inspection, here is what I have come up with so far:

1. That amp as supplied by Gonshit has no built in blocking biass to cut the tubes off hard during standby. It relied on a -100v input from the exciter. I will be simply just using one unused set of contacts in the T/R relay to open up the cathode path to ground during standby. This will cut it off dead and positive.

2. Both the plate and filament chokes are quite small for the job at hand. Undersized would be a much better description. And to make matters worse, the HV feed wire that comes off of the bottom of the very small solenoid plate choke lays right on the filament chokes hidden in the subchassis. That small choke and 500 pF bypass cap is just not enough to keep the rf off of that wire. So I added a .01 / 2KV at the base of the plate choke.

3. I have fround that the overall lead dress and size of the existing Bypass caps to be rather inadequate for an amp of this power level. So I am reworking some of the lead dress and adding and enlarging the bypass caps where I feel necessary.

If all goes well around here I should be firing it off later this afternoon or evening. I 'll keep y'all posted to the resultz!!

                                                                        The Slab Bacon

[K1JJ]

1. That amp as supplied by Gonshit has no built in blocking biass to cut the tubes off hard during standby. It relied on a -100v input from the exciter. I will be simply just using one unused set of contacts in the T/R relay to open up the cathode path to ground during standby. This will cut it off dead and positive.

Good idea, Slab.  That's a good way to key the amp.

Maybe you already planned to, but be sure to put a  ~50K resistor across the relay contacts as well as a .01 1KV disc cap.  Opening the cathode to infinity opens the door for other problems.


BTW, funny about your "targeted" CQ's on FD... 

[WA1GFZ]

Check the filament choke with a grid dip meter for a high q VHF resonance that can make it go wild. I always bypass my input choke with about 2 K two watt in series with a .001.
4X3 I used a lower C and R. I seem to remember a 100 pf dork knob in series with 100 ohm. This kills the high Q resonance.
 

[The Slab Bacon]

I had the amp fired up last night on the bench: It now cuts off dead in standby.
When keyed with no drive I now see not even the thickness of the meter pointer difference in the static plate current as i try to make it oscilate with full plate voltage and no drive. I only see this slight difference with the plate tuning cap fully open (unmeshed). It is much much better than it was. I now want to see what happens when I adjust the neutralizing tab and put it back into the case.

                                                                    the Slab Bacon

[K1JJ]

When keyed with no drive I now see not even the thickness of the meter pointer difference in the static plate current as i try to make it oscilate with full plate voltage and no drive.
                                                                    the Slab Bacon

Slab,

Looks like you're almost there.

The pictures of the rig look pretty decent - not a bad looking linear, esp when 572B's are added.

It might be a challenge, but too bad they don't blow the air from the bottom  to top - up around the tube seals and let the heat rise naturally. 

I have a "Smegma" commercial pair of 3-500Z's linear where they used a fan off to the side and no chimneys - as with the Gonset. This produced no filament pin cooling which resulted in repeated solder dripping out the the pins and dead fils. In addition, the glass envelopes would get hot on one side and cooler on the side facing the fan. What a JS setup for 3-500Z tubes that require forced air cooling!

It had no "sub-chassis box" to focus the air. I built a 7" X 8" aluminum box and remounted the tube sockets with a new squirrel cage directly to the back of the sub-chassis box with a variac to slow it down.  I also added chimneys. What a difference!  The fil pins stay cool, the glass cools evenly and even the plates glow less at a given power level. Now there's less noise and more tube air due to less turbulance.


With the Derb amp: Even mounting a muffin fan UNDER the 811A's/572B to blow air upwards would be an improvement - but will require some thought and work. It may require putting legs on the amp so that a muffin fan(s) can be mounted underneath.  I usually drill a circle of 3/8" holes around each of the tube sockets to let more air pass than the standard socket allows.  With a chimney, this works even more FB.  Even with the Gonset layout, it could be done. (w/o chimneys)   

But this is a job for the Derb later on...

T

[WA1GFZ]

could also put a fan over the tubes and suck up.
I did that with my old SB401 with 2-56 screws to attach the fan so didn't even have to drill any holes.

[N3DRB The Derb]

did someone say fans?

one of the free items from the barc minifest. new 115 vac fan with air duct and guard.

Slab: it would be ultra kewl to be able to plug this in that socket on teh back of amp and have the relay contacts key the voltage to the fan. trouble is, where ya gonna find a socket for it? I think thats a job for me later too, but thats the plan. for right now, I can just plug the fan in a socket.

PS: thats the yaesu #2  EE getting ready to get fixed with the board Ed sent me. I did hook #1 back up last night and look around but condx were so bad I just shut it off. 

PS #2: thx to JN for fixing #1 so fast you could not see his hands move. ( I set it on fire.) no really, I set it on fire. 

I'm a lucky guy. This is gonna be a great year full of radio.

[K1JJ]

could also put a fan over the tubes and suck up.

Yeah, after looking over the pic of Derb's amp again, a muffin fan(s) on top sucking air up would be easiest.

Though more efficient for the seal and pins, access to the bottom looks like quite the project the way it's laid out and wud require some metal work.

 Maybe 115V  muffin fans on top in parallel that are throttled back with a Variac wud be efficient and quiet.

Derb: I'll bet the inside fan running all the time wud be great for standby and the two muffins keyed-on for TX wud work FB, as you invision. 

You might even run the inside fan off the same Variac to quiet it down too.   Just a small change in voltage can sometime make a big difference in noise while keeping a good flow.  With three fans going, you can distribute the flow and keep them quieter.

Just some "cool"  ideas to get the Derb creative juices flowing, caw mawn,

T

[W3RSW]

The fan's probably synchronous. Put a 2uf/440v or AC rated cap. in series with one of the leads to cut it down to quieter speeds.

[N3DRB The Derb]

picture....

[AF9J]

Awww!  He looks a lot like my Sarah!  I bet he's a real sweetheart too!

73,
Ellen - AF9J   

[Ed KB1HVS]

picture....

 Happy Birthday Derb.. (nice pussy ya got there )