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[Amps] Fwd: Re: SB-220 Bias Issue, Too High of Plate Voltage?

Subject: [Amps] Fwd: Re: SB-220 Bias Issue, Too High of Plate Voltage?
From: Jim <>
Date: Mon, 20 May 2019 08:05:42 -0500
List-post: <>

MOST of the Harbach stuff is safe, but costs at least twice what it should. Stay away from the Nichrome parasitic suppressors (too much L, that makes 'em run WAY hot on 28 MHz). Rebuild your using Globar (WAY expensive) or NOS carbon composition resistors. Ebay seller 'bigsmythe74' has 56/10%/2W for $1/each plus $4/order. See #400880759592 <>.

In the HV meter multiplier string, Heathkit used three 4M7/1W carbon composition resistors to TRY to make 'em last (it didn't work). CC resistors absorb moisture ('cuz they're not sealed), and then soar in value. It's more noticeable in high nominal value resistors. Later aftermarket SB-22x rectifier/meter boards substituted fourteen 1M0 Metal Film resistors. The easiest/cheapest thing to do is upgrade the OE PCB. I replaced R6/R7/R9 with 4M7/1W/5% "Metal Glaze" resistors (Vishay pn VR68000004704JAC00). The datasheet (*link* <>) sez they're OK to 10KV. Digi-Key stocks 'em for $0.78 each. The rectifiers are kinda wimpy and should be upgraded to 1N5408 (3A/1KV). Order an extra rectifier to add between -HV and chassis for meter protection. Tom W8JI shows how here (*link* <>).

The electrolytic capacitors in the HV supply can be changed to CDE 381LR-series. If you stay below about 300uF, a step-start circuit (Harbach "Soft Start") isn't needed. Everyone sells 'em for about $6/each (to fit the OE plastic holder, you want 35mm OD). Replace the 30Ω/7W wirewound resistors w/ 100KΩ/3W Metal Oxide types. Your HV filter capacitors will like less heat.

I like Jim W7RY's "QSK" circuit. It drives FAST vacuum/reed relays that eliminate hot-switching when using VOX (fone) or semi/full break-in (Morse). You also get a relay driver (Harbach "Soft Key"), adjustable operating bias, relay contacts that will never oxidize, and dead quiet operation.

The cooling fan from Grainger is Dayton pn 4M070 (*link* <>)... for over twice the cost from Harbach ($50). For the extra cost, Harbach supplies a fan blade.

vy 73,
Bryan WA7PRC

On Saturday, May 18, 2019, 5:54:30 PM PDT, Jim <> wrote:

Sorry for not changing the subject.

If that is the true voltage, and you had to add that many diodes, then you have another problem.

An oscillation or parasitic of some sort.  Better measure the voltage with an accurate meter or check your multiplier resistors in your high voltage metering circuit.

Thanks 73 Jim W7RY

On 5/18/2019 6:56 PM, Joe wrote:
Only relying on the Meter on the amp, do not have a volt meter that goes that high.

But 3500+ a smidgen, maybe 3600

On 5/18/2019 6:51 PM, Jim wrote:
What is your MEASURED plate voltage?
Thanks 73 Jim W7RY
On 5/18/2019 6:11 PM, Joe wrote:
This AMP had it before and ran sooo cool it was wonderful.

Now with the new transformer I literally had to double the amount of diodes to ger the idle low enough so ever with ZERO WAYYS Drive I mean no watts at all put it in transmit mode and the plates 5 seconds later would be the color of an orange!

Finally when I got the idle current down to 120 or so at least with no power the plates stay gray. be even a medium length CQ, like say CQ CQ CQ CQ CQ CQ DE WB9SBD WB9SBD K

We are again bright orange.


On 5/18/2019 5:45 PM, Jim wrote:
Why do you think you need such a thing Joe?
Thanks 73 Jim W7RY
On 5/16/2019 11:23 PM, Joe wrote:
Has anyone installed one of these into their SB-220?


On 5/10/2019 7:55 PM, Jim wrote:
I tried to post a schematic and the moderator wont allow it for some unknown reason.

Schematic is attached.

Jim W7RY

On 5/10/2019 9:52 AM, Lou Everett, Sr. wrote:
Where is it?


On Fri, May 10, 2019 at 7:41 AM Jim < <> <> <>> wrote:

    Here is the modification (simple schematic) to the cut-off bias in
    Kenwood TL-922 amplifier.

    --     Thanks
    Jim W7RY
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