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Hi All, I am new to the Group, a friend of mine suggested to ask my problem here as there are many capable Hams on this group. I have a GS35 on 2m and has been working for years, I have changed the tube during it's life and blown a bias transistor here and there. The problem I am having now is one I have never seen. When I apply HT to the Amp usually the Amp is cut-off with the cut-off resistor and no current flows. Now I get Negative Grid current and Anode current starts to flow, and the more I increase the Anode say from 1000v to 1500v the current increases. The things I have changed in the Amp: Tube - New tested good tube installed. Bias Board - tested adjustable from 0 to 55 v Filament Transformer HT & Heater feed-thru cap. Usually when the Grid Bias transistor goes - I am using a BDX66B from DL4MEA design ( https://www.qsl.net/dl4mea/2g35.htm ) the amp does not do anything so not sure that Bias is the problem. I anyone might have come across this negative grid current problem please let me know. 73 Philip 9H1PA

I am doing some updating to my Alpha 76PA 3-tube amplifier. I have switched the T/R relay to a pair of vacuum relays and changed the "relay" (AKA Send or PTT) request to send from 24 Volts to be a compatible 12 Volts for modern transceivers. I am about to replace the original Alpha Plate Choke with a re-wound Ameritron style taller choke. While I'm at it I have a, new-but old, Cera-Mite 5000 PF, 15 KV door-knob capacitor. I'm planning to replace the original plate blocking capacitors with this door knob cap. The original plate blocking cap uses dual original 2000 PF, 6 KV ceramic disk capacitors. I've has this door knob cap a long time and I'm no longer planning on building a tube amplifier from scratch. As you can see I've fabricated two copper straps to place the cap in the 76pa amp. I fabricated these straps to put to door knob cap into a previous Alpha 76PA I had. And when fastened in place it is quite sturdy and has adequate clearance from its surroundings. This amplifier will be kept in my station as a back-up for my Solid State PA. I've found SSPAs to be less tolerant of operator errors than tube amps. But I still like the instant on and 6 meter capability in my current SSPA (403A/Flex PGXL). What are your thoughts on this replacement other than it is overkill in this situation? 73, Dave, w6de

I’m working on a used AL-811 for a friend. The 811 tubes were bad, so I replaced them with some Penta Labs 572b tubes (A whole another adventure). I also replaced the tube sockets, and installed GDTs on the tube sockets and added some bias diodes on the center tap of the transformer. I made sure there were not MOVs in the input board. The issue I’m having is high SWR (over 2:1) on 80 meters and I can’t get full drive from what appears as the exciter is folding back. All the other bands are fine. I’ve attempted to adjust the 80-meter input coil with no effect on SWR/PWR in. I’ve checked the input switch for continuity and from the input through the 80-meter input coil with no problems. Switch alignment looks good. I remove and tested C4 (for 80 Meters) a 500pF cap and it tested good. I’ve removed the 200-ohm padding resistor just to see what effect it had on SWR and nothing. Obviously, I get more output for the same amount of drive. If I’m at the high end of 75 meter with the tuning slug all the way at the end of the coil form, the SWR just starts to drop, and I see an improvement in drive level. With the padding resistor in circuit, I’m showing about 50 watts for drive and 60mA of grid current. With the amp in bypass the radio will do 120W no SWR. I’m wondering about values of C4 and if anyone else has experienced this behavior after swapping to 572b tubes? Don’t know how the amp performed prior as the tube replacement. Thanks for any productive comments in advance. Jim – AA7CL

These tubes were literally dug up last weekend at a former "VHF Telegraphy" commercial site in the UK. Does anyone recognise what they are ? Ideally I'd like to know what equipment they were used in, date is likely 1960 - 1980, all the printing is lost to the rain/soil. Tim

Hello I've already presented the refurbishment of this L7 amplifier here. I implemented a cathode bias and connected the grids directly to ground. I noticed that, regardless of the bias, I can't output more than 400 W. At 7.100 MHz · The gain is 13 dB. · The plate voltage under load is 2450 V. · The plate current is 310 mA. · How can I increase the output power? · What do you notice on an amplifier equipped with 2 3-500Z tubes in parallel? -- F1AMM Fran?ois

Justin, I have a couple of HF-2500 amplifiers I've used for many years and experienced the same failure of the meter lamps. The earlier HF-2500 (single meter) has two bulbs and the later (two meter) has four. I put up with no lamps for sometime and then just gave in to fixing the problem when another issue cropped up. I noticed that the plate voltage indicated on the front panel meter of the HF-2500 (dual meter) seemed quite low, and did not match the voltage read with a HV probe on the anodes of the PA tubes by over 500VDC. I decided to fix it, and replace the three meter lamps that were not lit. At the time I did not realize what a tall-order fixing the HV meter would be. Lamps first, I replaced the four incandescent lamps with 3800K Bright-Warm-White LEDs from a pinball supply company. These are in a #47 lamp form factor and are commonly used to replace lamps in boatanchor gear with 6.3VAC on the lamp. The current draw was something like 25mA each and I wired each meter pair in series. I also strapped a .001uf disc across each lamp to keep any RF out of the LED. There is also a published mod to the metering board where you replace the 7812 three terminal 12VDC 1A regulator with a 78S12 2A regulator. It solves the marginal current supply issue. The real pain was fixing the HV meter. As you may already know, the HV PS board is located under the RF deck. Nearly every part in the tank circuit must be removed to access the HV power supply board, as well as the front panel and more. It requires all this removal of "stuff" to take out a plate under the RF deck components that the HV board is mounted on. When I finally got the HV board out I found that the series resistors for the meter had gone way up in value. This probably was due to the dissipation in the three marginal wattage resistors, and the heat from all the bleeder resistors in the confined space. The air flow from the fan has about half of the air going to the right-hand tube closest to the fan, and the second half of the air passes through the HV compartment before it get to the left tube. So the left tube runs at a higher temperature. Go figure. While I was in the HV compartment I replaced all the filter capacitors with 105C types and replaced all the bleeder resistors and metering resistors with higher dissipation parts. I left the diodes as-is. Once reassembled, it worked great and the HV meter read correctly. After all this work I saw an ad from Palstar who apparently bought the Commander line and did a lot of upgrades on the already great amplifier. One significant change they made was to install a "door-plate" on the bottom of the RF deck to make easy access to the PS circuit board. If I ever have to do this again, I will cut a door in the bottom, hi! 73 Bill N4LG At 03:24 AM 4/11/2025, KG2RG via groups.io wrote: >[] >[] >[] >[] >[] >[] >[] >[] >[] >[] >[] > >Hello, >I have a Commander HF 2500. Have not had it to long. When I got >it, only 1 of the 4 meter bulbs was working. After about 3 month, >the bulb went dark. >I believe there is a 12V 1A regulator that >supplies the meter bulbs and the relay. I read the pulls are >Festoon type and need to be no more the 0.08A each so to not >exceed max current and cause the relay to no key. I finally got in >there and they are not the fuse looking type bulbs. They are >little bulbs with with short leads soldered to the meter board. > >i finally got in there because I had another Issue with the >standby switch. I think the standby switch was intermittent and a >relay was not keying sometimes. I started noticing the problem >when I would first turn on the amplifier after the short warm-up., >I placed the standby switch into operate and the amp would not >key. At first, I thought this issue may have been the regulator or >something else, but I started to notice the symptoms would often >happen after using the standby switch. I originally planned to >bypass the switch and see if the problem went away and then I >could replace the standby rocker switch. But when I got in there, >I noticed there was another set of poles th