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[AE0Q]

AFAIK, on some of the older ETO-alpha amps, the hi-low B+ function was done via a tap on the secondary of the plate xfmr....via an (optional) $80.00  spdt vac relay.  Most folks would order the amp without the optional vac relay..and leave it hardwired for the higher tap.

It's interesting to see the evolution of the ETO engineering decisions.

In the early Alpha amps, the big 77/77Dx/77Sx, they used a big open-frame, HV-rated double-contact relay, contacts rated for 1A @ 1,000 VDC (also spec'd at 200mA @ 5,000 VDC), to select the HV secondary AC tap from LO to HI HV.  Very expensive relays, like $300+ each...

Then in the 76/76A/374A/78 series, they used a big, open-frame, monster (3" x 2.5") black phenolic-base DPDT relay, contacts rated 25A @ 277 VAC, to select the HV secondary AC tap going from LO to HI HV.  Cost $50...

Then in the 86 and 87A they changed to a tiny (1" x 1" x 0.5") SPDT relay with contacts rated for 8A @ 400 VAC, also on the HV secondary AC wiring.  Cost $10...  I've only heard of one failing out of 1800 amps built with those.

I don't get it.  Why would anybody use a 277/400 v rated relay to switch HV taps ??   A SPDT  RJ1A / GH-1 would make more sense.  3.5 kv @ 60 hz and 25 amps  CCS.  Way back then, a RJ1A woulda been dirt cheap. Then u have the option of either tap, when coil de-energized.  They come in 12vdc, 26.5 vdc and also 115 vdc coil voltages.

The Jennings must have been more expensive at the time?  The 77/78 amps had a Jennings RJ1A for the OUTPUT T/R relay (it's not a latching relay), but a fairly large Magnecraft reed relay for the INPUT T/R.

Had to be a cost thing...  For sure that little thing on the 87A/86 was/is dirt cheap compared to a vacuum relay!  And it never fails...

[KA4WJA]

Glenn, Jim, et al,
Glenn, thanks for the clarification on why the Alpha 86 has a "low voltage" position....Japan (and some EU) certification...I never thought of that.  Cool.

The HI/LO HV switch allowed the 86 and 87A to be approved in Japan and some European countries that have lower power limits.  The later 89 does not have it, the CW/SSB switch changes the tube bias voltage.

And, fyi....here's a pic of the big Magnecraft relay for the 77dx/sx...(somewhere I think I have a spare....and, think NOS they sell for ~ $150?)....and also a pic of the smaller relay (MPJA) for the Hi/Lo positions of the 78 (and it sells, new, for $7.95 now...but, I don't have a spare.)

In the early Alpha amps, the big 77/77Dx/77Sx, they used a big open-frame, HV-rated double-contact relay, contacts rated for 1A @ 1,000 VDC (also spec'd at 200mA @ 5,000 VDC), to select the HV secondary AC tap from LO to HI HV.  Very expensive relays, like $300+ each...

Then in the 76/76A/374A/78 series, they used a big, open-frame, monster (3" x 2.5") black phenolic-base DPDT relay, contacts rated 25A @ 277 VAC, to select the HV secondary AC tap going from LO to HI HV.  Cost $50.

And, Jim...you are so right about the plate / tank load z, on paper, being the same when changing the B+ and plate current (by changing the drive from the exciter)....and, while I actually haven't done it in decades, if memory serves, with my SB-221 it did work...but, fact is that was > 40 years ago, and I think I only did it once (as part of initial testing, tuning, etc.) after building the thing.   After that, I don't think I ever used the "CW" (low-voltage) position, and always tuned-up and operated in the "SSB" (hi-voltage) position. 

Notice on all those older amps with a high or low  B+ position, that the higher B+ position is always 1.414 X higher.   The idea here is when tuned to 1 kw dc input on the lower B+..... then switch to the higher B+... then INCREASE the drive, such that the plate current increases by 41%,  on paper, the plate load Z did not change at all, and the amp should be tuned up correctly.  ( I would just pulse tune the amp on the  higher B+ position, using a pep wattmeter, and be done with it).

In some ham applications, max PO is not required, or not allowed.  We are only allowed 750 watts out on CW / FM/ data modes here in Canada.  If only the higher B+ was available, and drive reduced, and amp retuned for lower PO, then the plate load Z would DOUBLE.   The tank loaded Q would also DOUBLE. Tank coil taps are all in the wrong place. uh on each band would have to be doubled.

FYI, for anyone curious, here are quotes from the Heathkit SB-220/221 manual, and Alpha 78 manual, regarding how-to tune-up using the OLD RULES....again this was because of the old FCC rules, requiring a maximum 1000 watts DC input CW...(2000 watts DC input PEP)...{fact is you ARE tuning for max output / best linearity, with a drive level that keeps the amp, and tubes, within their specs!....it's just that with the OLD RULES, we also had to keep within the spec of 1000 watts DC input CW (or 2000 watts peak PEP, SSB)... And, also take note that there is no mention of "output" power, other than the Heathkit manual referencing the "relative power output" metering (REL PWR)....as "output" power wasn't in the rules, nor in many / most spec sheets nor manuals....although Alpha does mention the output power is some of their legacy manuals/ads....you are tuning for minimum GRID current with some specific drive level (best linearity), all-the-while keeping within the "1000 watt DC input" CW (2000 watt peak PEP, in SSB)...kinda makes me wish new hams learned from the old rules? so that they'd understand linearity, grid current, etc... }

SSB PROCEDURE
( ) Tune up the exciter and Amplifier as for CW
operation. NOTE: In the absence of the recommended
oscilloscope monitor, either the
PLATE METER or the REL. PWR. indication can
be used to monitor SSB transmission. The
PLATE METER indications are easier to follow.

Low Power SSB
( ) For 1000 watts P.E.P. operation, switch the exciter
only to the SSB mode. Leave the amplifier
MODE switch at CW /TUNE. [low-voltage position]
( ) Adjust the exciter drive control so the PLATE
METER will indicate between .12 and .2 ampere
with average speech. Hard voice peaks should
not exceed .250 ampere.

High Power SSB
( ) For 2000 watts P.E.P. operation, switch both the
exciter and the amplifier MODE switch to SSB. [high-voltage position]
( ) Advance the exciter drive level until the PLATE
METER reads from .2 to .3 ampere with average
speech and no higher than .33 ampere on hard
voice peaks. A higher drive level will cause
"flat topping."

And, from the Alpha 78 Manual:

Final Adjustment for 2 kW PEP SSB Input.  For high power (2+ kW PEP) SSB operation of the ALPHA 78, first complete the adjustments for l kW tune-up as in section 3 - 6 .2 above . Then return the exciter to STANDBY or RECEIVE. Press the amplifier SSB button and confirm with the MULTIMETER (in HV position) that the HV is 2300 + 100 vdc.
Switch the exciter to the SSB mode and slowly increase its output level while speaking normally into the microphone. Correct operating adjustment is achieved when either (a) the highest swings of the PLATE CURRENT METER reach approximately 0.45 A., or (b) the highest swings of the MULTIMETER (monitoring GRID current) reach approximately 50 mA. At this operating level - - FCC maximum legal power of approximately 1 kW average dc plate input and 2+ kW PEP input - -  no LOAD-TUNE adjustments are required, other than those already performed during the initial tune-up a 1 kW input. [assuming you did increase your exciter / drive level accordingly] (Refer back To section 3-3 for discussion of proper operating levels) 

Fair winds to all.

73,
John,  KA4WJA

[W9AC]

I don't get it.  Why would anybody use a 277/400 v rated relay to switch HV taps ??   A SPDT  RJ1A / GH-1 would make more sense.  3.5 kv @ 60 hz and 25 amps  CCS.  Way back then, a RJ1A woulda been dirt cheap. Then u have the option of either tap, when coil de-energized.  They come in 12vdc, 26.5 vdc and also 115 vdc coil voltages.

With a 3.5KV (5KV peak) spec, the GH-1, HC-1, or RJ1A relays are probably marginal due to close contact spacing, even in a vacuum. 

In the case of the Alpha 70/77 series amps, a wide, open-frame Magnecraft W158-HVX relay is used on the HV secondary; it's designed for HV switching.  As Glenn pointed out, they're very expensive when found today, and probably expensive relative to most other 70/77 amp components back in the 1970s. 

Paul, W9AC

[VE7RF]

I don't get it.  Why would anybody use a 277/400 v rated relay to switch HV taps ??   A SPDT  RJ1A / GH-1 would make more sense.  3.5 kv @ 60 hz and 25 amps  CCS.  Way back then, a RJ1A woulda been dirt cheap. Then u have the option of either tap, when coil de-energized.  They come in 12vdc, 26.5 vdc and also 115 vdc coil voltages.

With a 3.5KV (5KV peak) spec, the GH-1, HC-1, or RJ1A relays are probably marginal due to close contact spacing, even in a vacuum. 

In the case of the Alpha 70/77 series amps, a wide, open-frame Magnecraft W158-HVX relay is used on the HV secondary; it's designed for HV switching.  As Glenn pointed out, they're very expensive when found today, and probably expensive relative to most other 70/77 amp components back in the 1970s. 

Paul, W9AC

You may be right.  It would make for a good experiment.   A GH-1 will typ hi pot test to just over 7.5 kv.

[W9AC]

A GH-1 will typ hi pot test to just over 7.5 kv.

About 25 years ago, I purchased several Magnecaft spares from Newark Electronics.  But not having those replacements, I would look at the Jennings RJ2B for the Alpha 70/77 series.  Still expensive but more plentiful today than the Magnecraft.  Not sure if it could be shoehorned into an Alpha 86, 87, or 89 model.

Paul, W9AC

[VE7RF]

A GH-1 will typ hi pot test to just over 7.5 kv.

About 25 years ago, I purchased several Magnecaft spares from Newark Electronics.  But not having those replacements, I would look at the Jennings RJ2B for the Alpha 70/77 series.  Still expensive but more plentiful today than the Magnecraft.  Not sure if it could be shoehorned into an Alpha 86, 87, or 89 model.


Paul, W9AC

Jennings has gone out of business...toast.  The Gigavac equivalent of a Jennings RJ2B is called a G2.  The damned G2's have gone UP in price from $200.00  to a whopping $303.00  from MGS.   I just bought 3 x G2WF's a few months ago. The G2WF version has a flange base...and threaded inserts on all 3 x contacts..which makes attaching copper strap, or ring terminals  a snap.  You can also get em in a threaded base, and also 12/26.5 / 115 vdc coil, and also with just hollow cylinders for the 3 x contacts.  They too, hi-pot a lot higher than spec.

We used paralleled  G2's / RJ2B's  on several amplifier projects..(for TR relays).   But they have to be paralleled correctly, in order for the current to split 50-50.  Local engineer ham buddy and myself tried to speed em up with an over voltage, since he is a hardcore QSK op.  We managed 2.4 msecs on relay #1....and 2.6 msecs on relay #2.  (tested individually before paralleling contacts on his YC-156 amp).   Good enough for QSK, and he's happy.

Back when gigavac had it's ..'ham program' running, the 'G2ham' was only $112.00    I kick myself for not buying 2 dozen of em. Program ended, and price went up to $200.00  Now it's  $303.00   I use 8 of the damned things on my 40m yagi, to switch segments. ( 4 x segments each for REF + DIR...and 7 for the DE).