4x 6AG7 Amplifier

Discussion in '"Boat Anchor" & Classic Equipment' started by NW2K, Feb 11, 2019.

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  1. N2EY

    N2EY Ham Member QRZ Page

    Interesting! Ol' Wayne.....classic con-man. I encountered him in an online forum some years back, when he gave a very distorted history of US Amateur Radio in the 1960s. It was almost fun, proving him to be dead wrong on so many things.

    "He never let the truth, facts or logic get in the way of a story - particularly if that story put $$ in his pocket."

    Him and Don Miller.....except for Don it wasn't about money.

    73 de Jim, N2EY
  2. KM1H

    KM1H Ham Member QRZ Page

    My own 4 x 6AG7 GG amp came from CQ and no photo was published. This was in 56 as I had the General and a 10A by then. I have no idea of the output with only ~ 350V but I got my feet wet in SSB 75-40-20M only .Instead of the output network shown I used a pi network with a clip lead bandswitch

    Next came the 4 x 1625 in GG at about 900V that was driven by the 4 x 6AG7's. The amp was reduced to parts but I still have the modified tubes. IMD...what was that? It was all buried in with the AM on either side anyway:eek::rolleyes:. SSB was only in a narrow window then and it was a constant war with AMers so no attempt was made to clean up signals that I can remember. That had a ~$1 bandswitch from a BC-375 TU and covered 80-15.

    I also built the W2EWL exciter which was used mobile on 20 and replaced the TBS-50

    After a stint in the USN with the early 60's Collins URC-32 I was allowed to use on ham bands for phone patches and some ragchews I didnt go back to AM on HF for several decades. The next amp was a NCL-2000 prototype (I worked for National 1963-69 and it cost me $20) which I still use with 160 and WARC added with low power AM rigs.
    Another $20 prototype has been on 6M since 1964.

    The newest amp here is a 1986 LK-500ZC with the original Eimacs still putting out 1200W

    W1BR likes this.
  3. K9STH

    K9STH Ham Member QRZ Page

    My first "linear" amplifier was a WRL Globe Chief 90A. I built the 75-meter DSB transmitter that was in the June 1962 issue of Popular Electronics. I plugged the output of the DSB transmitter into the crystal jack on the transmitter. The transmitter had been modified per the manual to use with a VFO.

    I really did not know that a Class "C" amplifier was not suitable for SSB / DSB operation. However, the darned thing worked and I never got any reports of distortion, etc.! I had a habit of running the transmitter at around 120-watts input even though the specifications were for 90-watts input. With the pair of 807 tubes in the output, at 75-meters I was probably getting around 80-watts output.

    Although the transmitter was designed to use crystal control, I ran a VFO into the crystal socket to be able to move around in frequency. Since the carrier was attenuated, none of the SSB operators ever noticed that I had a USB signal as well as the normal LSB signal.

    Glen, K9STH
  4. NW2K

    NW2K Ham Member QRZ Page

    But...why not? These tubes seem happy. While 6AG7 datasheets exist for common uses, I cannot find any hard engineering data for the 6AG7 used exactly in this manner. :)

    Why? For the same reason I do any of this. This amp represents the highest, best use of four of my gaggle of 6AG7s. I'll report on lifetime if I see some changes.

    A few more notes on the amp. I replaced the output link with a pi network and the output increased a bit. I flipped the cap-input switch on the HV supply and I can now see 1000v on the plates (key down, 1260v key up) and the cathode current reads 260ma. Sine wave output into 50 ohms is 85w with 8w of drive from the AT-1. The pi network is about 250pf, 7.5uH, 1750pf, at 3.55 MHz. The plate impedance is likely closer to 2000 ohms.

    David, thank you very much for the suggestions! I will take a look at the 6JD5. 73, Dean
  5. NW2K

    NW2K Ham Member QRZ Page

    Update: Tubes are still fine after significant use. I've repackaged the amp and it now uses five 6AG7's. Here are some pictures. For the archives, I have also included two pictures of an amp, which uses an ARC-5 coil, built and used regularly by Colin K7FM. I went back to link coupling for simplicity and the amp gives 10db (80w out for 8w in). Coils can be changed and this coil is for 40m. 73 Dean NW2K

    5x 6AG7 Amplifier 3.JPG 5x 6AG7 Amplifier 4.JPG 5x 6AG7 Amplifier 6.jpg K7FM 4x 6AG7 Amp 1 Small.jpg K7FM 4x 6AG7 Amp 3 Small.jpg
    K9MOV likes this.
  6. W1BR

    W1BR Ham Member QRZ Page

    Pair of 1625 tubes in a converted SCN-274N junker will do an easy 100 watts when converted to an amp. Sadly, there are too many examples available for those conversions.
    N2EY likes this.
  7. KM1H

    KM1H Ham Member QRZ Page

    Sadly?? Why?? There are probably hundreds or more of untouched ones still available than the handful of ARC-5/SCR274N collectors that whine non stop. As an amp there are even more very common/available choices to use to convert. Then for any efficiency the correct 1625's have to be found and modified for GG. The 6AG7 and its 9 pin 6CL6 replacement are far from scarce and a few hams using them wont deplete stock either....only the audiophools can do that. A couple of ham amps and a bunch of CB versions depleted 6V filament sweep tube stock in about 20 years and the higher filament voltage ones are rapidly following along.

    I wont say what Ive been given for NIB 6JS6C, 6LQ6, 6JB6, and others recently. Stashed away years ago they are far better than a 401K. I still have several of the audiophool tubes they drool over from the 20-30's NIB as I had the foresight to get most for free or close to it in the 50-70's from radio/TV repair shops that were closing.

    The 807, other 1625's, 6L6, 6V6, and other beam tubes are not adaptable and are stuck at grid driven AB1 at low efficiency.

    Last edited: Apr 6, 2020
  8. KM1H

    KM1H Ham Member QRZ Page

    There is no surprise there! Unless the 6AG7 has a lot of wasted space inside the metal jacket it is a far more rugged tube than most realize but are still prone to blabber away. I never replaced the ones in my amp nor in the 10A, 10B, 20A 's I used to drive transverters into the 90's.
  9. W1BR

    W1BR Ham Member QRZ Page

    View attachment 647145
    Whoa... original condition goes back to them being usable today. They required very little to be used as designed--the designers were not idiots. Most of the
    issues go back to poorly regulated power supplies and other hammy hambone mistakes. I have a good collection of fairly decent SCR-274N and ARC-5
    radios. Finding an original example that covers a ham band is getting a lot harder, and prices are ridiculous. I was looking for a T-17 for 160 meters
    and found out they are going for several hundred bucks... my intent is to put them on the air with minimal modifications, and that I can hopefully
    use the correct racks so they are interchangeable. I don't like hacked vintage ham gear, and I have the same feelings for some of those military radios.

    My better examples are shown below, several others are in cue
    for cleanup and returning back to what they should be.

    Regarding "linear" or AB1, that was not my intent in commenting. But, regarding linearity, isn't the beam forming electrode accessible by opening
    the socket on most, and removing from the cathode to permit GG linear service?


    ARC-5.jpg ARC-5.jpg
    Last edited: Apr 6, 2020
  10. KM1H

    KM1H Ham Member QRZ Page

    Pete, this was your comment I was relying to:

    Your reply has nothing to do with turning a junker TX into just an amp.

    When you said amp I was "assuming" you meant linear since I had been talking about 4 6AG7's in GG and driving 4 1625's in GG as linears.

    I dont remember all the details but I had a hard time finding 4 to modify as it appears RCA was the most common by far. It and a few other brands connected the beam electrode to the cathode inside the envelope and were useless.

    These days I have no interest in those TX , the W2EWL conversion was the last, but do use a few BC-453's as a Q5er plus a few QF-1's. I do have a BC-454 and 455 for nostalgia as that is what I used between the regen I started with and the used HQ-129X I soon used on Novice 15M about mid 1956. Of course they were modified to bandspread only by removing tuning cap plates:eek: The ones here now have been strictly shelf queens. I scrapped the HB 6AG7 6L6 80 40 rig for a used Viking Adventurer, a great little rig that Ive added to my current collection along with the matching screen modulator. I did most of my shopping in Radio Row via bus and subway from LI until I was allowed to drive and picked up a 49 Ford for those Saturday excursions.


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