Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on June 18, 2014, 05:00:44 pm

Title: Hole-y 7025/12AX7A?
Post by: Voxbox on June 18, 2014, 05:00:44 pm

I've got these two RCA valves from a 1960 Vibrasonic.


One is clearly etched 7025, the other has no remaining markings other than the red/orange RCA logo, which is the same on both valves.
However, the second valve has some round holes in the plates through which can be seen a spiral heater element.


Does anyone know what type the hole-y valve is?




Cheers, VB

Title: Re: Hole-y 7025/12AX7A?
Post by: HotBluePlates on June 18, 2014, 07:19:59 pm

The tube on the right is a Philips-brand tube, maybe a Mullard. The seam at the top of the envelope was not found on American tubes that I've seen.


Look for a faint etched marking on the side of the envelope, down near the pins. It's 2 lines of info; I'd look for a right-triangle or a "B" on the second line.

Title: Re: Hole-y 7025/12AX7A?
Post by: Voxbox on June 19, 2014, 01:51:57 am

Thanks HBP,
Both are marked RCA, so it's possibly an RCA badged Philips?
Another thing is that the spacers are circular on the one with the holes, but on the one etched with 7025, they cut off parallel to the plates.
There are no markings like you describe though.


What I was really wondering is if it is a 7025/12ax7 or some other type?
Cheers,VB

Title: Re: Hole-y 7025/12AX7A?
Post by: HotBluePlates on June 19, 2014, 04:06:20 pm

Quote from: Voxbox on June 19, 2014, 01:51:57 am

Both are marked RCA, so it's possibly an RCA badged Philips?

It's probably a European tube, we just don't know which brand yet.

Do you have a high-res picture of the plates looking straight on? The first picture has them canted and is too small to see any detail, the second is a side-view. I might be able to make a better guess by seeing the plates from the front.

Quote from: Voxbox on June 19, 2014, 01:51:57 am

What I was really wondering is if it is a 7025/12ax7 or some other type?

Put it in a 12AX7 socket. If it has full-gain, it's a 12AX7.

And after the 50's, anything marked "7025" was probably nothing more than a selected 12AX7 (in my opinion). The modest improvements to create a 7025 would have made more economic sense to apply to all 12AX7's after a period of time. The manufacturer could also charge more for the 7025.

Title: Re: Hole-y 7025/12AX7A?
Post by: Voxbox on June 19, 2014, 04:42:31 pm

Thanks HBP


from straight on they look the same!
I showed them best I could to show the holes as I thought they were the distinguishing feature. The gain is the same as each other and other 12ax7/7025 types.
I'll have look in a few days and do a high-res shot - I'm away for the weekend with the missus and the dog, leaving the laddie in charge.....


Cheers, VB