Revisiting The GU81M: Probably Not The best Choice For A Linear Amp!

A closer look at the GU81 tube and also Mike's GU81 linear amplifier.

[@m0dad]

A beast of a valve and the power supply. Very interesting video thanks for sharing Justin...David M0DAD

[@ericrawson2909]

I love this old tech. I have a 5C/450A which is similar but even larger. I haven't tested it, I must one day. I also have a beautiful 4212E triode, but sadly when I tested it, it had very low emission.

[@bob_mosavo]

Thanks 😁

[@markward5943]

Beautiful. Such a shame to tear down.

[@MM0IMC]

Another reason to stick with grounded grid, especially for a newbie amp builder...

[@yuehhsu1786]

We have built an amplifier what using a pair GU-81M, it works great even on 10 mters, it can output 2200W+ on 10 meters, the output capatance it is not problem, you can use a roller inductor ,

[@MM0IMC]

It's a shame to have to decommission them. 😮 I note that Perian Marcel YO6PMX, used a 4:1 balun for the input impedance to bring the tube impedance down from 2k Ohm to 500 Ohm.

[@buttyboy100]

I've had a QY5-3000A valve on the shelf for years. I collected all the transformers required for the PSU then ran out of interest time and space. It does require fan cooling, which would be a nuisance in my little 8' x 12' shack. It would have to be a two box design as the PSU HT transformer alone weighs 80 pounds. Heavy had a hernia repaired already I don't fancy heaving that thing about😂

[@The_3_Triangle]

i have an amplifier with a pair of gu81m the amp was amazing (cost me only 400€) yeah it's has some disadvantages like high heater current...also need relatively complex in/output π network for work....but still worth it....provide 3100W PeP on 160meter and 1900W in 10 meter ...and the biggest advantage's is about this capable to run like 24/7 in full power....i using a 10KVA 3 phase HV trafo....no multiplier or else straight 2 way rectifier and a huge cap on anode side.... and all voltage has an individual linear regulator on them....works well but few issues the amp is big currently living in the rack cabinet.... luckily the amp is relatively quite.... currently i building an HF amp with 2 GU78B i interested for the result....thx the video anyway

[@itsonlyme9938]

I built a HF amp using 2 x 813's they needed 10 volts @ 10amps and difficult to get them to work on 10 meters. about 600 watts. My latest project was reverse engineering a Heathkit SB200which was using 572B tubes to Russian pulse radar ceramic UHF tubes, GI6B x2 or I could have used GI7B's . the heater is 12 volts @ 2 amps each and with 2.4kv and about 20 watts of drive will give 800 watts of carrier on 10 meters where as 813 x2 and 100 watts only gave 600 watts. the GI6B's are very cheap just make a hole in the chassis 36mm and clamp with a cap clip. They are triodes

[@bob4analog]

Did you make your GI6B amp grounded grid?

[@yuehhsu1786]

use roller inductor replace the fixed inductor and replace variable capacitor (plate tune) use fixed capacitor on higher band(21MHz or higher) no extra fixed capacitor. I'm sure you will get same power in 10m as low band

[@PE1JAS]

I wanna build one with the only one GU i have: but indeed all points to consider....... and what a nice Delta power supply that is, from our Dutch hams in Zeeland! 73 pe1jas

[@moviestudioland]

He should sell you that amp before it is pulled apart.

[@MrTalaue]

I would not use vacuum relays for band switching. The lead length within the Pi circuitry of that amplifier is far to long for eficient use on the high bands,

[@thewtf]

Yes, some of them are a bit long

[@moviestudioland]

ebay sell the project to free up space

[@moviestudioland]

Sell the GU81 amplifier your friend built. People would want it. Don't destroy it. No sense in that mate.

[@thewtf]

Dont worry I did not scrap it the end! Kept it and got it working nicely! Works very well

[@docvolt5214]

​@@thewtfwould love to see the schematic of it, obviously if possible, I've got a pair of these beasts a friend of mine got when escaping from Russia and I would love to see how much winding I would need to do to get an output transformer for these ahah

[@erwe1054]

А почему ? Линейность оставляет желать лучшего ? Вы просто не умеете их готовить