Buy Smart- 3-500Z Amplifiers with weak HV transformers

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, Nov 24, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    Note- TEN TEC Centurion Amps produce 1300 watts PEP due to more efficient transformer vs the Heathkit & Swan that should be rated for 1000 watts PEP. I should have mentioned that or place the amp in a mid-rated category.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The transformer is part of the formula, but not necessarily the most important part.

    Cooling/ventilation/air flow is very important -- not for the glass envelopes themselves (!) but for the pin seals and for power supply components. Whether the "transformer" is supplying everything (B+, filament power, accessory power) or just HV makes a difference. And transformer design makes a difference: As listed in #12 above, there are amplifiers with lightweight transformers that can provide sufficient power for 1500W RF output just fine -- depends on design. The Acom 1500W amp, and it really makes 1500W pretty easily (rated 1200W continuous key-down with no time limit) uses a <30 lb transformer. Unlike 3-500Z designs, that amp (and many others) use external anode tubes with pressurized cooling systems and blowers just for the tubes so the heat generated by the tube(s) isn't recirculated within the amp enclosure and doesn't make power supply components hot.

    3-500Z(G)s cool by radiation so even if the design uses chimneys and a blower, everything around them gets hot anyway.:p The chimneys and blower keep the pin seals cool, but don't prevent the radiation.

    "The entire design" requires analysis to draw any real conclusions.
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    [QUOTE="WB2WIK,The Acom 1500W amp, and it really makes 1500W pretty easily (rated 1200W continuous key-down with no time limit) uses a <30 lb transformer. [/QUOTE]
    Not all transformers are equal in weight vs performance. But the transformers I listed in the amplifiers are M-19 or M29 IE laminations or Hypersil C core. The transformer weights I listed are directly comparable. Further the power out of the transformers are listed on the Peter Dahl site and that is undeniable.
    The Acom may be M6 lamination so this specialty transformer is not used in the amplifiers I listed thus can be lighter for more power output. The Acom is a 3000VA rated transformer. Bottom line is my transformer weight vs performance is accurate and the Acom transformer weight does not apply.
  4. N2EY

    N2EY Premium Subscriber QRZ Page


    And there's more...

    Many were designed for unprocessed SSB and CW, not RTTY and similar.
    WA7PRC likes this.
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    The SB-220 is a great amplifier, just was designed for 1KW input with regard for the FCC rules at the time. 23lb transformer works well enough for 1KW PEP out.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't say "great," but it was easy to build and a bargain at the time.

    I built one and used it for years with no problems. But it's not a great design in several ways, one of which is that the input networks are not really adjustable without practically taking it apart. Another was the silly layout of the front panel controls, which made it just too easy to accidentally turn the bandswitch while "tuning up" and looking at meters.

    And the 23 lb transformer did turn out to be a weak link; lots of SB-220s have had transformers replaced, not to provide "more power" but because the original one failed (usually guys using RTTY or trying AM).

    For the price, it was a great deal, though. And a very easy kit! I built mine one Saturday afternoon and Saturday night it was on the air. Heath instructions were phenomenal, and it's actually a very simple kit with not many parts.
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    I believe the fair amount of SB-220 HV transformer failures noted by amateurs are related to using power output at or over 1200 watts PEP and long use such as in contesting or perhaps a constant 'long winded blabber mouth'. 1000 watts PEP out limit should keep the SB-220 HV transformer alive. The SB-220 HV transformer has a lower power rating than the Ameritron AL-80B transformer and that amp is rated at a maximum of 1000 watts PEP output.
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    Back when the SB-220 came out, Contesting and UR 5-9 had yet to be "The Cool Thing".

    The amp needs to be converted to a SB-220C , Contest version. :rolleyes:
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    Peter Dahl will convert to a contest version.
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- good luck finding a $750 to $1K legal output amplifier with two 3-500Z tubes. Two weeks and a few 1000 PEP out Heathkit SB-220s, one Drake L4B needing over 100 watts drive, two Henry consoles and one Ten Tec Centurion came up for sale. A few $2000 Ameritron AL-82 are/were for sale. And some were local pick up only including the Henry console amps that are too large to ship with standard shipping. Hey, there is always the Dayton ham swap as $750-$1K legal output desk amps apparently require careful planning to acquire.

    Apparently I did not know how lucky I was to find an out of state local pickup Amp Supply LK-500zb for $800 with new tubes and rebuilt power supply.
    Last edited: Dec 7, 2019

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