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.
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.
Attachments
on my A5 VOTs . Stay tuned for more pics
![307adnimg_0002 [640x480].jpg](/community/data/attachments/97/97766-d68483f55f50d4a99d2b662c2d2e84ba.jpg)
![307dowimg_0001 [640x480].jpg](/community/data/attachments/97/97771-96fcf999731b4ea4974142ab11a03728.jpg)
Hello , yes , i'll scan the schematic in a few days : i still have a low hum (on 515 woofers ..) but would like to cancel it . I operate them with 300/0/300 PT / 5R4WG /2x2uF polypro /10Hys /32 uF . There is 325volts on the OT (3,3k primary ) and G2 is tied to A with 47ohms WW. G2 to the center of a 30ohms pot to 750ohms WW (+47uF 63volts) to GND. Heater is fed through 2 WW 0,39ohms resistors to the 6,3Vac . Grid leak res : 255k MF . The red block is a coupling silver mica capacitor 22nF /500v . The input tube : A 10k decoulping resistors goes to 32uF to feed a 6C6 with 80k MF in anode , g2 : 900K MF /0,22uF and Rk : 1,21k MF //47uF tantalum . The Ammeter in cathode goes to 45mA . Silver teflon wiring and iron plate 
Sound is astonishing for such a low power of 6 watts. Smoth and grainy . Bass is ... good , not so hard as with my 50W OTLs but they would perform for driving a 299/805 combo ! I've build a pair and will modify them for a 423C gas regulator in the 6C6 screen grid . As Tubelabe says , the 307As are worth the effort for the 15 Euros each i found them 
Sound is astonishing for such a low power of 6 watts. Smoth and grainy . Bass is ... good , not so hard as with my 50W OTLs but they would perform for driving a 299/805 combo ! I've build a pair and will modify them for a 423C gas regulator in the 6C6 screen grid . As Tubelabe says , the 307As are worth the effort for the 15 Euros each i found them
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.