Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 06, 2015, 02:38:11 pm

Title: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: kagliostro on June 06, 2015, 02:38:11 pm

This last days I received some tubes and among them there are some metal tubes

so a question arrived to my mind

Assuming you have the choice to get a tube in the metal version or in the glass version

which one is to be preferred ?

Thanks

K

Title: Re: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: VMS on June 06, 2015, 02:54:10 pm

Glass.   :smiley:

(https://dmillarblog.files.wordpress.com/2013/07/mackintos.jpg)

Title: Re: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: kagliostro on June 06, 2015, 03:49:00 pm

Yes, I can understand  :grin:

---

Other factors besides the design factor ?

Franco

Title: Re: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: darryl on June 06, 2015, 07:04:22 pm

Metal 6L6 and 6V6 output valves have the external metal can attached to pin 1, so it can be grounded to act as an electrostatic shield. Many newer ( post 1950's ) amplifiers assume that glass bottles are being used, and pin 1 on the valve socket is either left open circuit, or used as a convenient tie point for a grid or screen stopper resistor.

If you plan to use metal valves, make sure the valve sockets are wired correctly for them. For example, having the screen voltage accessible on the external metal can could be a mite unpleasant if you inadvertently made contact with it!

Title: Re: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: PRR on June 06, 2015, 09:32:44 pm

There's true all-metal tubes, and glass tubes in metal shells. I do not know how to tell (without a saw).

Metal tubes will most-all be "vintage", made mid-1930s to mid-1960s (at latest).

The first year of metal tubes, there were production problems.

A particular problem with metal: you can't tell if they light-up. I went crazy trying to diagnose a chassis where a 6SJ7 worked for only a few seconds once in a while. It was a broken heater (the chassis had been thrown into a large trash dump, where I found it) which sometimes touched enough to work. If it had been glass I would have noticed the no-glow days sooner. (It did run a little warm, not cool enough to seem wrong, but this was all conducted heat from the rest of the chassis.)

Generally, I do not think there is ANY must-have difference between metal and glass. Tubes of both constructions of similar vintage seem to work just the same.

Title: Re: Metal Tube Vs Glass Tube - In general which is to be preferred ?
Post by: kagliostro on June 06, 2015, 09:38:59 pm

Thanks Darryl & PRR

those are the info for which I was looking for

Franco