[Amps] Amps Digest, Vol 195, Issue 10--Replacing padder caps on an Alpha 77DX/SX

Fri Mar 22 11:04:32 EDT 2019

I have an Ameritron choke to go in my 77D, just need that bag of round 
tuits!

The Ameritron choke is not a panacea however. Used in the Alpha 78 there 
is a nasty fire starting resonance when operating on 30m. The close 
proximity of the metalwork within the cramped confines of the 78 chassis 
is the cause for sure. If you have a tightly packed amp it is worthwhile 
doing some 'in-situ' resonance checks.

My 77D is an early version and only has a single 160m position, I'm not 
even sure if there are additional bandswitch contacts so it can be 
modified.

If you work inside the amp compartment it is worth checking to make sure 
the nuts on the back of the ganged variable capacitors are firmly 
tightened down. I had a 'drift at power problem' caused by these coming 
loose and it was evident from the molten plastic of the load capacitor 
insulating brackets, since replaced by homemade fibreglass versions. The 
problem was of course on all bands not just 160m.

In addition to the changed padder caps between various versions of the 
77 they also changed the ganged variables. Early amps have a pair of 
500pF but the 1990 manual shows a 300pF load capacitor with an extra 
700pF ganged variable switched in at the 8-12MHz position and below.

Martin, HS0ZED

On 22/03/2019 01:20, Paul Christensen wrote:
>> "The 802 was a bit better and its been awhile since Ive been into those
> amps so dont know if the big Ameritron will fit. Carl"
>
> As I recall, the Ameritron choke and RF Parts RFC3 are the same.  I have
> replaced the B&W 800 series chokes with the RFC3 in Alpha 77Dx, 70V, and 70A
> amps.  There's ample room for the new choke although accessing the choke
> support screw is difficult.  On my 77Dx, I used a thin metal machinist's
> ruler, pressed against the threads to hold the screw into position long
> enough for the screw to grip the threaded ceramic post.  Then, it's just a
> matter of hand-tightening the choke to the point that the fiber washer
> between the ceramic and chassis is slightly compressed.  The RFC3 has a
> small wire gauge so not sure how well it would hold up in the 2-8877 77Sx if
> used well above 1500W.
>
> The new choke works great on all HF bands, including WARC.
>
> Paul, W9AC
>
> ----- Original Message -----
> From: "HP" <pfizenmayer at q.com>
> To: <amps at contesting.com>
> Sent: Monday, March 18, 2019 3:24 PM
> Subject: Re: [Amps] Amps Digest, Vol 195, Issue 10--Replacing padder caps on
> an Alpha 77DX/SX
>
>
>> Gary - if the drift is only on 160 - for hell of it -add another 2000
>> pfd or so plate bypass at cold end of the plate RFC - I had that
>> happen in another amp on 160 - allegedly an "unauthorized" factory
>> change had put a pair of 1000 pfd Z5U in instead of X7R. Those lousy
>> Z5U were actually dropping to about 500 pfd at 80 C .
>>
>> (yes I bought into the tale that that amp typically had bad padders
>> and I upgraded them all first - did not change a thing !!!! thats when
>> I started looking elsewhere -BINGO. )
>>
>> Hank K7HP
>>
>> Message: 3
>> Date: Sun, 17 Mar 2019 16:31:27 -0400
>> From: "Gary Smith" < Gary at ka1j.com >
>> To: amps at contesting.com
>> Subject: [Amps] Replacing padder caps on an Alpha 77DX/SX
>> Message-ID: < 5C8EAE9F.20487.19DF0C6D at Gary.ka1j.com >
>> Content-Type: text/plain; charset=US-ASCII
>>
>> Folks,
>>
>> I have been having drifting issues on 160 only, not 80M or above, with
>> my late model SX. I was thinking the 160 toroid transformers were
>> overheating but the tape is not brown and I've been assured the bit of
>> bubbling at the ends is from normal warming, not excessive heating.
>>
>> ----- On Mar 18, 2019, at 10:00 AM, <amps-request at contesting.com> wrote:
>>
>> | Today's Topics:
>>
>> | 1. Re: 3CX800A7 low grid current version? (Steve London) 2. Re: QRO
>> | 2500 MARK lll (John Bodine) 3. Replacing padder caps on an Alpha
>> | 77DX/SX (Gary Smith) 4. Re: Replacing padder caps on an Alpha
>> | 77DX/SX (Gary Smith) 5. Re: Replacing padder caps on an Alpha
>> | 77DX/SX (Jim) 6. Re: Question about grid current in 4CX or 3CX
>> | (Peter Voelpel)
>>
>> | --------------------------------------------------------------------
>> | --
>>
>> | Message: 1
>> | Date: Sun, 17 Mar 2019 14:18:38 -0600
>> | From: Steve London <n2icarrl at gmail.com>
>> | To: amps at contesting.com
>> | Subject: Re: [Amps] 3CX800A7 low grid current version?
>> | Message-ID: <121ccba3-78d1-e23b-6122-24af452cd1bf at arrl.net>
>> | Content-Type: text/plain; charset=utf-8; format=flowed
>>
>> | This sounds like the same tube that Alpha was selling a few years ago.
>> It was
>> | also touted as "low grid current". The specs are exactly the same as
>> | the standard tube, as far as I could tell. Alpha specifically did
>> | not
>> recommend
>> | using it in the Alpha 87.
>>
>> | The designation on the Alpha tube is:
>> | VTU-3CX800A7LG "Alpha Tubes" 1450
>>
>> | BTW, in my converted 76PA, I do NOT find the pair of 3CX800A7LG's
>> particularly
>> | easy to drive with the Alpha broadband input transformer. It
>> | typically
>> takes
>> | 60-70 watts for 1500 watts output, depending on the band.
>>
>> | 73,
>> | Steve, N2IC
>>
>> | On 03/16/2019 02:33 PM, Floyd Sense wrote:
>> | > I see that RF Parts carries a "low grid current version" of the
>> 3CX800A7 tube,
>> | > but haven't been able to locate any technical info on the tube.? I
>> assume that
>> | > this version of the tube might result in lower grid current than
>> | > the
>> standard
>> | > version, at a given power level with all other things being equal.?
>> Can someone
>> | > verify that and are there any other considerations when using such
>> tubes in my
>> | > Alpha 89 amp?
>>
>> | > 73, K8AC
>>
>> | > _______________________________________________
>> | > Amps mailing list
>> | > Amps at contesting.com
>> | > http://lists.contesting.com/mailman/listinfo/amps
>>
>> | ------------------------------
>>
>> | Message: 2
>> | Date: Sun, 17 Mar 2019 16:21:31 -0400
>> | From: John Bodine <john.bodine at gmail.com>
>> | To: Amps Amps <amps at contesting.com>
>> | Subject: Re: [Amps] QRO 2500 MARK lll
>> | Message-ID:
>> | <CAFJ4jdbWEUkr2u=QYjsjZrCuYkY=UkNpaaaSY6biKCxVz4dR-A at mail.gmail.com>
>> | Content-Type: text/plain; charset="UTF-8"
>>
>> | Try one tube at a time. Probably just one tube went.
>>
>> | On Sun, Mar 17, 2019, 9:29 AM gudguyham--- via Amps
>> <amps at contesting.com>
>> | wrote:
>>
>> | > Sounds like a bad tube
>>
>> | > Sent from AOL Mobile Mail
>> | > Get the new AOL app: mail.mobile.aol.com On Saturday, March 16,
>> | > 2019, Bob Gibson via Amps <amps at contesting.com
>> | > > wrote:
>>
>> | > I have a problem that just came up with my QRO amp..First I blew a
>> 1/2a
>> | > fuse on the bias board..I replaced it turned it back on then a
>> | > 1/2a
>> fuse
>> | > blew on the LV and bias board again but not the same fuse..Screen
>> voltage
>> | > is OK and HV is OK. When I hit with 10 watts the fault light comes
>> | > on
>> and
>> | > kicks the amp off and I see the grid current meter go all the way
>> over..I
>> | > see no burnt parts or smell parts...could this be a grid short in
>> | > one
>> of
>> | > the tubes? I have pulled the tubes and nothing looks bad below..I
>> think I
>> | > will send the tubes down and have them check just to make sure it
>> | > is
>> not a
>> | > problem with one of them..Bob W5RG
>> | > _______________________________________________
>> | > Amps mailing list
>> | > Amps at contesting.com
>> | > http://lists.contesting.com/mailman/listinfo/amps
>>
>> | > _______________________________________________
>> | > Amps mailing list
>> | > Amps at contesting.com
>> | > http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>> | ------------------------------
>>
>> | Message: 3
>> | Date: Sun, 17 Mar 2019 16:31:27 -0400
>> | From: "Gary Smith" <Gary at ka1j.com>
>> | To: amps at contesting.com
>> | Subject: [Amps] Replacing padder caps on an Alpha 77DX/SX
>> | Message-ID: <5C8EAE9F.20487.19DF0C6D at Gary.ka1j.com>
>> | Content-Type: text/plain; charset=US-ASCII
>>
>> | Folks,
>>
>> | I have been having drifting issues on 160 only, not 80M or above,
>> | with my late model SX. I was thinking the 160 toroid transformers
>> | were overheating but the tape is not brown and I've been assured the
>> | bit of bubbling at the ends is from normal warming, not excessive
>> | heating.
>>
>> | 160M & 80M transformers L7 & L5
>>
>> | http://doctorgary.net/toroid-1.jpg
>>
>> | http://doctorgary.net/toroid-2.jpg
>>
>> | Upper band transformer L6
>>
>> | http://doctorgary.net/toroid-3.jpg
>>
>> | I tested the plate caps with my LCR and all tested as WNL. I cannot
>> | do a leak down test with this LCR meter.
>>
>> | As there's no sign of overheating I can find anywhere and I haven't
>> | been beating on the amp, I'm assuming the drift is from the 160M
>> | padder caps breaking down over the years and they should be
>> | replaced.
>>
>> | Alpha could have made them easier to access, they're right
>> | underneath the band switch, screwed to the front RF panel and the
>> | front panel of the RF deck is not removable (this would have made
>> | access a cinch). The only access to the front area of the deck is
>> | from above and from the left side.
>>
>> | I did not remove the side panel at this time, I'll do that when I
>> | order & get the padders. Here's the view of the padders from above.
>>
>> | 100pF band position A
>> | http://doctorgary.net/100pF.jpg
>>
>> | 1000pF band position B
>> | http://doctorgary.net/1000pF.jpg
>>
>> | I understand the early models had one 500 pF padder and later models
>> | like this one have two 1000 pF padders in parallel.
>>
>> | The two paralleled 1000pF caps for
>> | band-switch position B are easily
>> | accessible from the side, the two
>> | paralleled 100 padders for Band-switch A are located deep where I
>> | can't get a screwdriver in to undo them.
>>
>> | For anyone who has replaced these on one of these amps, have you any
>> | suggestions of how to best get to the 100 pF caps and remove/replace
>> | them with a minimum of effort?
>>
>> | Also, I'll probably get the HT58 padders from RF Parts, any other
>> | worthy places I might also check into?
>>
>> | Thanks & 73,
>>
>> | Gary
>> | KA1J
>>
>> | ------------------------------
>>
>> | Message: 4
>> | Date: Sun, 17 Mar 2019 19:02:58 -0400
>> | From: "Gary Smith" <Gary at ka1j.com>
>> | To: Amps at contesting.com
>> | Subject: Re: [Amps] Replacing padder caps on an Alpha 77DX/SX
>> | Message-ID: <5C8ED222.12990.2936F6 at Gary.ka1j.com>
>> | Content-Type: text/plain; charset=US-ASCII
>>
>> | Folks,
>>
>> | > Alpha could have made them easier to access, they're right
>> | > underneath the band switch, screwed to the front RF panel and the
>> | > front panel of the RF deck is not removable (this would have made
>> | > access a cinch). The only access to the front area of the deck is
>> | > from above and from the left side.
>>
>> | I should have said ETO instead of Alpha, even though we all know the
>> | current Alpha group did not make these old amps, I should have been
>> | more clear.
>>
>> | 73,
>>
>> | Gary
>> | KA1J
>>
>> | ------------------------------
>>
>> | Message: 5
>> | Date: Sun, 17 Mar 2019 21:12:14 -0500
>> | From: Jim <jimw7ry at gmail.com>
>> | To: amps at contesting.com
>> | Subject: Re: [Amps] Replacing padder caps on an Alpha 77DX/SX
>> | Message-ID: <50f6cf1f-9d74-361a-1f51-edec27577337 at gmail.com>
>> | Content-Type: text/plain; charset=utf-8; format=flowed
>>
>> | I just rebuilt a Ten-Tec Titan 435 that had horrible drift issues on
>> | 160 meters. The RF current in the load cap on 160 is very high!
>>
>> | I used strings of 100 pF 3KV CERAMIC disk caps. *No more drift.*
>> | Mouser part number is: 80-c330c101JHG. Replace a 500 pF doorknob
>> | with 5 of these in parallel.
>>
>> | Don't waste your time with doorknobs, they cant handle the current.
>> | I just ripped the original doorknobs out of the Titan 425 and
>> | replaced them with strings of these. 20 second key-downs now no
>> | issue on the Titan 425 on 160 meters.
>>
>> | I would stay below 1.5:1 SWR to keep the voltage down. With high
>> | SWR, the voltage *can be* quite high on the loading caps.
>>
>> | Don't use mica caps either. I tried them. They just blew up.
>>
>> | Some info in the archives of the amps reflector about this subject.
>> | Do a search from contesting.com amps archives. There also some info
>> | on W8JI's site, www.w8ji.com
>>
>> | Email me privately and I can send you some pictures of what I did in
>> | the Titan. I'll try to post the pictures on my photo site later this
>> | week and share the link.
>>
>> | Thanks
>> | 73
>> | Jim W7RY
>>
>> | On 3/17/2019 3:31 PM, Gary Smith wrote:
>> | > Folks,
>>
>> | > I have been having drifting issues on 160 only, not 80M or above,
>> | > with my late model SX. I was thinking the 160 toroid transformers
>> | > were overheating but the tape is not brown and I've been assured
>> | > the bit of bubbling at the ends is from normal warming, not
>> | > excessive heating.
>>
>> | > 160M & 80M transformers L7 & L5
>>
>> | > http://doctorgary.net/toroid-1.jpg
>>
>> | > http://doctorgary.net/toroid-2.jpg
>>
>> | > Upper band transformer L6
>>
>> | > http://doctorgary.net/toroid-3.jpg
>>
>>
>> | > I tested the plate caps with my LCR and all tested as WNL. I
>> | > cannot do a leak down test with this LCR meter.
>>
>> | > As there's no sign of overheating I can find anywhere and I
>> | > haven't been beating on the amp, I'm assuming the drift is from
>> | > the 160M padder caps breaking down over the years and they should
>> | > be replaced.
>>
>> | > Alpha could have made them easier to access, they're right
>> | > underneath the band switch, screwed to the front RF panel and the
>> | > front panel of the RF deck is not removable (this would have made
>> | > access a cinch). The only access to the front area of the deck is
>> | > from above and from the left side.
>>
>> | > I did not remove the side panel at this time, I'll do that when I
>> | > order & get the padders. Here's the view of the padders from
>> | > above.
>>
>> | > 100pF band position A
>> | > http://doctorgary.net/100pF.jpg
>>
>> | > 1000pF band position B
>> | > http://doctorgary.net/1000pF.jpg
>>
>> | > I understand the early models had one 500 pF padder and later
>> | > models like this one have two 1000 pF padders in parallel.
>>
>> | > The two paralleled 1000pF caps for band-switch position B are
>> | > easily accessible from the side, the two paralleled 100 padders
>> | > for Band-switch A are located deep where I can't get a screwdriver
>> | > in to undo them.
>>
>> | > For anyone who has replaced these on one of these amps, have you
>> | > any suggestions of how to best get to the 100 pF caps and
>> | > remove/replace them with a minimum of effort?
>>
>> | > Also, I'll probably get the HT58 padders from RF Parts, any other
>> | > worthy places I might also check into?
>>
>> | > Thanks & 73,
>>
>> | > Gary
>> | > KA1J
>> | > _______________________________________________
>> | > Amps mailing list
>> | > Amps at contesting.com
>> | > http://lists.contesting.com/mailman/listinfo/amps
>>
>> | ------------------------------
>>
>> | Message: 6
>> | Date: Mon, 18 Mar 2019 09:19:36 +0100
>> | From: "Peter Voelpel" <dj7ww at t-online.de>
>> | To: <donroden at hiwaay.net>, <amps at contesting.com>
>> | Subject: Re: [Amps] Question about grid current in 4CX or 3CX
>> | Message-ID: <B4151E78081946A0BA36EE4E8A8C1CE1 at SHACK>
>> | Content-Type: text/plain; charset="iso-8859-1"
>>
>> | Hi Don,
>>
>> | The grid dissipation of the 4CX10000D is 75W.
>> | It takes 150W of drive for 250mA of grid current with that tube in
>> class-c
>> | with anode and screen voltage supplied.
>>
>> | Are you thinking of a different tube?
>>
>> | 73
>> | Peter
>>
>> | -----Original Message-----
>> | From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of
>> | donroden at hiwaay.net
>> | Sent: Freitag, 15. M?rz 2019 16:38
>> | To: amps at contesting.com
>> | Subject: [Amps] Question about grid current in 4CX or 3CX
>>
>> | Question :
>>
>> | For the larger tubes with handles ( commercial service )
>>
>> | Does grid current increase or decrease as plate voltage and screen
>> | voltage is removed. ?
>>
>> | If it takes 250 watts to produce 100ma of grid current in a 4CX10,000.
>> | is the grid just a big dummy load and is it doing the tube harm to
>> | dump 250 watts into it when both plate and screen voltages are removed ?
>>
>> | Don W4DNR
>>
>> | _______________________________________________
>> | Amps mailing list
>> | Amps at contesting.com
>> | http://lists.contesting.com/mailman/listinfo/amps
>>
>> | ------------------------------
>>
>> | Subject: Digest Footer
>>
>> | _______________________________________________
>> | Amps mailing list
>> | Amps at contesting.com
>> | http://lists.contesting.com/mailman/listinfo/amps
>>
>> | ------------------------------
>>
>> | End of Amps Digest, Vol 195, Issue 10
>> | *************************************
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
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