Where are the 307A / VT225 amps?

[corbato]

Googling throws up little

On paper the specs look nice for a little PP stage. Some details here

Has any one any experience? DHP looks tempting.

[tubelab.com]

I have seen little info on these tubes, except for a link a couple of years ago that claimed they "sounded better that a 300B". I have a box full of them, in the same millitary packaging as the ones in your link. I will try them sooner or later, but I was thinking SE.

[Zen Mod]

I have dozen ,too ;one day mebbe I'll have time for little playing
there is tride data ,collected somewhere on the net

[coresta]

Hi ! I have some of these too ... nice DHPentode As a driver for an A2 100TH amp ?

[Zen Mod]

Originally posted by tubelab.com
I have seen little info on these tubes, except for a link a couple of years ago that claimed they "sounded better that a 300B". I have a box full of them, in the same millitary packaging as the ones in your link. I will try them sooner or later, but I was thinking SE.

Lab man- did you tried this nice toob in a meantime?

[tubelab.com]

No, unfortunately I haven't got to them yet. They are still on the shelf waiting for their chance to make sound. There were 3 events last year that slowed my tube time down a lot. I hope to catch up this year, but for now I am 1200 miles from home using a borrowed computer. I finally get to meet my grandson, and see my daughter for the first time in nearly 3 years.

[tubelab.com]

Lab man- did you tried this nice toob in a meantime?

This kind of slipped off of my radar screen until I was over at the warehouse preparing a lot of tubes to be sold off. I found the large box of VT225's. I had written a note on the box to check them out, but the rated dissipation was low at 15 watts. I decided to wire a pair into a TubelabSE and crank them up. The worst that I could do was blow some stuff up! Is 15 watts really 15 watts? The plates look like 30 or 40 watts.

NO, 15 watts must be some ultra conservative military or telephone service rating. I wired the tubes in triode mode with the screen connected to the plate via a 100 ohm resistor. The suppressor is grounded. B+ is 350 volts (screen rating is 300) with about 335 volts across the tube. I cranked the tube current up a little at a time stopping at 10 mA increments to check for glow in a dark room. I got to 80 mA with no visible glow. I didn't go any further. 80 mA is 27 watts.

I applied a sine wave and made some measurements. The power at clip was 8.5 watts in one channel and 8.9 in the other. I am using 5K ohm OPT's and these are about the same power levels that I get with 300B's in the same amp. I backed the drive off to get 8 watts and set the bias for minimum distortion. Both channels liked 65 mA for 4.1% and 2.9% distortion. Distortion at 1 watt was 0.8 % in both channels. This is a dissipation of 22 watts which should not fry the tubes too quickly.

I hooked up my speakers and played a few songs. These tubes sound at least as good as my Chinese 300B tubes, maybe better! I was planning an amplifier build using the big Edcor transformers and 300B tubes, but I may use these tubes instead.

An interesting note... I tried UL mode. The gain went up but maximum power output did not increase. Don't quite understand this behavior yet. These tubes will not go into the lot to be sold.

[pmillett]

Grrr... one of my favorite tubes. I was hoping nobody would "discover" them. Guess I better buy a bunch now before the price goes up.

Tubelab, if you'd like to dispose of any of yours, shoot me an email...

Pete

[coresta]

Hey Tubelab! great ! i'm building a pair of SE"T" with them at this moment ... a very nice toob indeed .. i ' m sure it is the effort worth with paralleled electrodes like a 300B . A 6C6 in front and it would do the WE91' job !

[Zen Mod]

tnx lab man

as always - useful and interesting info