Kalamazoo Hamfest find P&H LA-400 Ham CB 1625 Tube type linear Strange amplifier

Scott & I attended the Kalamazoo Mich, hamfest today. He purchased this cool linear amplifier for a very low price. It runs 4-1625 output tubes. Very nice cosmetic condition. The style reminds me of Central electronics. Made in Lafayette, IN. We inspected the amp, found a missing tube. Then we found that the tubes had been modified! Base configuration does not match the tube data. It appears that the transformers are OK. But the amp needs some maintenance before we can attempt transmitting. I will be posting a follow after we resolve the issues. Fun stuff!

[@mrradio2187]

Definitely got that 'Central Electronics' look...

[@ordinaryaverageguy76]

10:00 The P&H amp is a grounded grid type amp. The 1625 tubes required modification to make them suitable for GG operation. Normal 1625 has the beam-forming grid internally connected to the cathode. That connection has to be separated for GG operation. Certain brands of 1625 had the grid & cathode leads connected in the base instead of inside the glass envelope. Those could be modified for GG work. It was often done by cutting a window into the phenolic base & working the wire leads that way. It looks like P&H modified tubes in their factory & you were expected to buy replacements from them (hence the different pin-out of the base, to make it sort of "proprietary").

[@chrishoffman6537]

Cause that’s what’s up!

[@PlanetaryThoughts9861]

OK, that makes sense. Is there a short version of why a GG type amp is better than any other type? Does it solve a biasing issue?

[@ordinaryaverageguy76]

@@PlanetaryThoughts9861 GG amps do not need neutralization at MF & HF frequencies. Many tubes (so-called zero-bias tubes) can be run in GG with grids directly connected to ground. So, no need for grid & screen bias supply circuits. Very simple construction. This LA-400 has a biasing terminal on the chassis rear but the manual says it's an optional use. I suspect it's related to how some RF amps when idling (e.g., during receiving time) can generate RF noise, & this may be an option for a large negative bias to silence the amp completely.

[@johnparichuk8367]

Take a look at the June 1955 issue of QST. There's an article about this amplifier and modifying the 1625 tubes by removing the bases and replacing them with 6-pin bases.

[@d-labelectronics]

Thanks, I will take a look

[@Tom-W7TMD]

Neat Looking little amp! I hope we get to see it fired up!

[@n8iws484]

Such a clean chassis. Good luck on the search.

[@electrolytics]

Very cool. I'm glad it was the fuse and not the transformers. Very odd with the tube pins. Thanks alot. Hope to see the follow up.

[@danrussell9357]

Great Video, Looking forward to Part 2!

[@renegade44040]

I have an experience to share with you. I was friends with Layton Kor and I can tell you personally he would be very happy to see the younger climbing generation.

[@peterpetropoulos9607]

the roller coil looks like it’s from the ARC5 series transmitters, the right top air cap looks like it’s out of the BC375 tuning units, and U won’t be surprised if all the interior parts are from older military rigs.

[@ordinaryaverageguy76]

The roller & window are definitely robbed from ARC-5 or SC-274N series transmitter.

[@deancovey8193]

I was there could have got a perfectly good running Clipperton-L I was selling cheap lol maybe see you at Marshal in the spring

[@R50_J0]

You seemed thinner at K-Zoo yesterday than last year at Adrian. You’re doing OK?

[@randyab9go188]

That amp used "modified" 1625 tubes.

[@jims2222]

I agree. Likely DIY modified.

[@ordinaryaverageguy76]

@@jims2222 Personally I think it was a P&H factory modded tube set. The chassis has matching 6-pin sockets, after all. So, I think P&H was modding tubes that they sold to their linear amp customers. Most of the 1950s-ish grounded-grid mods to those tubes did not change out the base pins, they just moved a wire to an unused original base pin (pin 2).

[@cheycasters]

🎸🎸🎸🎸

[@peterpetropoulos9607]

time to pull out the 807 stash

[@d-labelectronics]

807's are 6vac filament power 1625's are 12

[@ordinaryaverageguy76]

Besides the different filament voltages, the 807 also has the beam forming grid connected internally to the cathode, just as the 1625 does. You'd have to perform the same modification to any 807 as they did for the 1625 tubes to use in this amplifier. I do _not_ know that there are 807 brands that brought the beam forming grid lead outside the glass envelope (as did some 1625 makers).

[@kaiwenwu1947]

Hello sir. Maybe you have covered this but I can't find it. I have a Marshall valvstate 8080 combo. Iv been restoring, it's great. All works well. BUT. On the clean channel the bass control does nothing I can hear at all. ???? 😮 mid and top are fine. ? And bass mid and top are fine on the valve channel, please help. Is it a capacitor maybe I can't find faulty , just hoped you had seen this before. I would be forever in your debt. Great channel very well explained, and safely done. Mr Kaiwen Wu " in the UK. Cheers