Pages: [1]   Go Down

Author Topic: Drake T-4XC PA Class of Operation  (Read 405 times)

KF0AEE

  • Posts: 128
    • HomeURL
Drake T-4XC PA Class of Operation
« on: September 14, 2021, 11:54:15 PM »

Howdy folks,

Which class of operation do the Drake T4 series PA run? I've read AB1, and the plate & grid voltages (650/-55) seem appropriate for this mode.

Trouble is, I'm seeing what looks like a single-ended class-A circuit on the schematic: the g1 are in parallel, and there's a single drive line running from driver tube to the grids. IOW, it looks like single-ended class-A.

I'm used to audio gear, so maybe I'm just missing something here.. but I sure don't see 6JB6 surviving 650V on the plate biased class-A!

Please clear this up, thanks
Logged

G3RZP

  • Member
  • Posts: 2254
Re: Drake T-4XC PA Class of Operation
« Reply #1 on: September 15, 2021, 05:41:17 AM »

Class AB1. With a tank circuit, it's very different to an audio amplifier where you would run two tubes in push-pull. In an RF application with a tank circuit, the tuned circuit 'fills in' for the half cycle when the tube is cut off i.e. the negative half cycle of grid volts.
Logged

KM1H

  • Member
  • Posts: 11155
Re: Drake T-4XC PA Class of Operation
« Reply #2 on: September 15, 2021, 10:45:14 AM »

When I was heavily contesting with a pair of C Lines I was using only Sylvania tubes and matching my own from a deep discount bulk buy that was shared with others.

Life was about 6 months at ~80W into the amp as built and a muffin fan on top of the RF cage extended that to 18-36 months depending upon how many contests I was in. The ARRL 10M contest worked them the hardest until I dropped output to 60W which still produced a good 800W out from a 1200W amp.

Carl
Logged

W1VT

  • Member
  • Posts: 6071
Re: Drake T-4XC PA Class of Operation
« Reply #3 on: September 15, 2021, 11:49:35 AM »

TV sweep tubes used to be plentiful and cheap so it was quite common to push them very hard.

Zak W1VT
Logged

KM1H

  • Member
  • Posts: 11155
Re: Drake T-4XC PA Class of Operation
« Reply #4 on: September 15, 2021, 02:02:14 PM »

But when they let loose during a contest or in a DX pileup it gets annoying. A strange thing is they never failed in the TR-6
Logged

G3RZP

  • Member
  • Posts: 2254
Re: Drake T-4XC PA Class of Operation
« Reply #5 on: September 15, 2021, 02:40:39 PM »

One of the many problems is that if you take two sweep tubes, for any given set of voltages there can be a very wide range of plate currents. If you are a manufacturer, the odds are that you'll have a lot of tubes from the same batch and the plate currents will match to some extent and probably enough for a short tube life. If they don't, you can quickly kill tubes. The 6146 family have a spec on this, and for the same electrode voltages, the plate currents can differ by a factor of 2:1. Which is why matched tubes are advisable....or a separate bias control for each tube....

However, the spread of plate currents isn't specified for sweep tubes - it wasn't needed for TV sets and controlling it by testing would cost - so it wasn't technically necessary for the intended application and wasn't done. Tough sh** for the ham radio manufacturers looking for PA tubes on the cheap...because this wasn't realised, separate bias controls for each PA tube weren't  provided and manufacturers relied on "batch matching" and ignore the 'later in life when tubes were rare' situation....
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Drake T-4XC PA Class of Operation
« Reply #6 on: September 15, 2021, 02:57:38 PM »

Dentron used to supply factory matched 6LQ6's in their sweep tube amplifiers. They had their own part number D-50A.

http://www.radiomanual.info/schemi/ACC_PA/Dentron_GLA%C2%B71000B_review_1980.pdf
Logged

KM1H

  • Member
  • Posts: 11155
Re: Drake T-4XC PA Class of Operation
« Reply #7 on: September 15, 2021, 05:52:09 PM »

Dentron used to supply factory matched 6LQ6's in their sweep tube amplifiers. They had their own part number D-50A.

http://www.radiomanual.info/schemi/ACC_PA/Dentron_GLA%C2%B71000B_review_1980.pdf

So did National for the NCX-500 rig that used a pair as did the TS-511C, TS-900 and Swan 500C. As did the Knight T-175 amp. The Swan Cygnet amp used 4. The Swan 260 and 270B had one.
Im sure I missed several for the magazine readers to find.

I built a 160 only amp with two using the C Line until I added it to the NCL-2000 along with WARC. In 86 I had a LK-500ZC with it all factory installed which is still my primary amp with the original Eimac 3-500Z's. ;D

The NCL is pretty much dedicated as an AM linear now for low power rigs.
Logged

KF0AEE

  • Posts: 128
    • HomeURL
Re: Drake T-4XC PA Class of Operation
« Reply #8 on: September 16, 2021, 09:42:26 AM »

This rather straightforward and focused question thread sure went OT and fast..

Class AB1. With a tank circuit, it's very different to an audio amplifier where you would run two tubes in push-pull. In an RF application with a tank circuit, the tuned circuit 'fills in' for the half cycle when the tube is cut off i.e. the negative half cycle of grid volts.

Thanks. Yes, that does generally answer the question in that it fixes my confusion over the topology vs. CoO. But it does leave me wondering what prevents the use of Class C, being that we're already doing without ~180' of waveform as it is.
Logged

G3RZP

  • Member
  • Posts: 2254
Re: Drake T-4XC PA Class of Operation
« Reply #9 on: September 17, 2021, 03:26:52 AM »

Class C isn't linear, and you need linearity in the amplifier for SSB, i.e. the output power is directly proportional to signal level. In Class C, you need a certain level of signal at the grid before you get output. Class C normally has a ' duty cycle (or  'device conduction angle') of around 120 degrees. Higher efficiencies and more harmonic come from shorter duty cycles.  Some 58 years ago, I built an amplifier with an 807, running with 1700 volts on the plate, 500 on the screen and -300 on the control grid.  Driven hard with a 6L6G, on 40m it ran at about 220 watts input and 170 out, or about 77% efficiency. If you held the key down, for long, the plate glowed red! But 807s were cheap and readily available back then.....In the UK, they were much cheaper than comparatively rarer 1625, which was a 12 volt heater 807. That 807 lasted a long time and it wasn't bust when the amplifier got broken up....
Logged

KM1H

  • Member
  • Posts: 11155
Re: Drake T-4XC PA Class of Operation
« Reply #10 on: September 17, 2021, 10:57:14 AM »

Peter, back in the prehistoric years of 1957 I built a 4 x GG 1625 amp driven by another which was driven by a CE 10B.

There were certain manufacturers who brought the suppressor grid out thru the glass and tied it into the cathode pin. A little surgery on the phenolic base and some soldering iron heat to bring that lead out of Pin 6 and stuff it into the empty Pin 5 then allowed all 3 grid pins to be grounded. With about 1000V they made some good RF, I had no way to measure then, but used a Knight Kit SWR bridge as an output indicator.

Made many SSB contacts on 75, 40, and 20 and with help from others OTA made it sound decent. Hams helping others that way was the norm back then. The 10B used plug in coils and the amp used clip leads to change bands, no cabinets were used ;D

Can you imagine doing that today?

Never lost a 1625 as in sudden failure and I see them here every now and again as things get moved around (organized finally) as Id like to test them on the Hickok 752A
Logged

G3RZP

  • Member
  • Posts: 2254
Re: Drake T-4XC PA Class of Operation
« Reply #11 on: September 17, 2021, 04:22:23 PM »

Carl,

I have read that Central Electronics bought a load of that variety of 1625s and sold them for linear amplifiers after modifying them. Didn't they actually produce a four times 1625 amplifier using their modified tubes?  OK, that was back in the days when a ham licence meant you had some technical knowledge..

An old colleague, now in the US emailed me yesterday. His company had a problem which they thought was down a transformer on an RF IC output that they couldn't get the right answers from. None of them knew about leakage reactance on toroidal transformers or hpw to check an RF transformer - it hadn't occurred to them to put the correct resistive load on the secondary and measure the primary impedance with a 'notwork analyser'. They admitted they didn't know about RF..... I suggested that they started off by getting an ARRL Handbook and studying it.

I was asked by the boss man why an "amateur's handbook" would help. He went very quiet when i pointed out that amateur radio handbooks had been useful to the various armed forces since before WW2, and that the day before WW2 was declared (September 1939) the RSGB took delivery of 3000 copies of their Handbook and worried about how they would get rid of them. By November 1945, they had to have reprinted 185,000 copies to meet demand from services training schools...and it was on the "banned" books list for sending to POWs in Germany.  I believe the ARRL had the same problem in supplying enough copies of the ARRL Handbook..

I may well be considered to cynical and elitist when I deplore the lowering of technical standards amongst amateurs world wide......Tough !   i'm old enough that i won't be around that much longer to worry about it...


 
Logged

W9IQ

  • Member
  • Posts: 8866
Re: Drake T-4XC PA Class of Operation
« Reply #12 on: September 17, 2021, 05:25:06 PM »

You may want to check the title again. For  more than 15 years it has been The ARRL Handbook for Radio Communications.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

KM1H

  • Member
  • Posts: 11155
Re: Drake T-4XC PA Class of Operation
« Reply #13 on: September 17, 2021, 06:08:49 PM »

Quote
I have read that Central Electronics bought a load of that variety of 1625s and sold them for linear amplifiers after modifying them. Didn't they actually produce a four times 1625 amplifier using their modified tubes?  OK, that was back in the days when a ham licence meant you had some technical knowledge..

Peter, that was the P&H LA-400B and it could also be supplied with 837's, a seriously underrated and ignored tube designed around a military spec as were several WW2 era tubes.

Carl
« Last Edit: September 17, 2021, 06:16:00 PM by KM1H »
Logged
Pages: [1]   Go Up