[Robert]

I have several tubes with loose plate caps and some of them have come off.
Is there a way to re glue them back on before resoldering them back on the tube?

[Ron Ramirez]

Yes, assuming the wire is still protruding from the top of the tube envelope.

Remove the old solder, remove the cap. A good choice for re-cementing the grid (not plate) cap is Sauerisen cement. It is in powder form, and must be mixed with a small amount of water. Careful - a small amount of powder is all you will need. Additionally, it will stand up to the heat produced by some tubes and is therefore a great choice for re-cementing tube bases to the glass. (However, most tubes with grid caps normally produce little heat.)

Once the cement is applied inside the grid cap, reattach it to the tube, making sure the wire protrudes through the tiny hole at the top of the grid cap. Let the cement dry overnight, then re-solder the wire to the cap.

[Texasrocker]

Ive had good luck for yrs using JB Weld for this purpose. Make sure all the old adhesive is removed from glass, clean with alcohol, then the JB Weld makes a very secure bond between the metal & glass after full drying time of 24 hrs. JB Weld can be found at Walmart ( automotive dept),some Hardware, & most Auto Supply Stores. Great stuff!!

[planigan]

Robert, from the replies I assume you get the message that they ain't plate caps, they are grid caps. PL

[Texasrocker]

Plate caps wont be found in vintage radio work, but can be found in Amplifiers indeed!
Take the expensive 8-pin octal type 6550, same as ( 5881) beam power amp tube, it can be replaced with a less expensive 6146 ( same socket) but some wiring mods are required at the sockets, and (add wire) to send plate connection up-top of chassis, to allow the 6146 plate connection thru the top plate cap connection of the tube. The 6146 type tubes are therefore a dangerous tube to work around, and require the heavy-duty type ceramic plate-cap connectors. The 6146 type tubes sound better than 6550s in the vintage ( all tube chassis) Ampeg SVT Bass Amps! Ive been playing thru these amps for yrs! The earlier 1970s model Ampeg SVTs used six 6146 tubes in push-pull,for 300 watts full clean pwr into 4 ohms. The later 70s model SVT amps used six 6550 tubes in push pull into 4 ohms. Both different amps chassis designs sound great, but the earlier 6146 design ruled for more overall power, tone, etc. Using matched sets ( sextets)are best for biasing these type amplifiers. The 6550s are getting more & more expensive! Therefore,tube conversions in these amps are worthwhile indeed.
To see a plate cap type tube basing diagram, see GE tube manual base # 7CK

The 6550 same as ( 5881) tube base # is: 7AC Same type socket connections as a regular ol 35L6, or 50L6 used in radio work.

[planigan]

Texasrocker, I stand corrected! You do learn something new every day. I was not aware that plate Caps were used on vacuum tubes. PL

[codefox1]

One of the reasons plate caps are found on high power audio and transmitting tubes is it allows higher voltage to be applied to the plates than would be possible with a connection to the tube base. BTW I like to use 807's instead of 6L6's some of my in Guitar and Stereo projects because they are cheap, plentiful, reliable, and super-retro looking.