VT-25 in S.E.X. amp

[vetmed]

I tried without much success using a separate 7.5 volt transformer but its just too noisy. This is the SEX 2.0 with the transformer that is also used in the Crack amp. The heater winding is I am told  good for up to 3.5 amps @6.3 volts. So 2 questions: Can I power 2 DHT from a single heater winding? and will there be enough emission running the vt25 @6.3 volts? I've read some stuff by Steve Bench that says it should work with the added benefit of lower distortion, but at the risk of shortening tube life if the tube is run too hard. So I guess its really a question of if I can run 2 tubes using a single winding. Advice and comments welcome and appreciated.



Robert Lees

[Paul Joppa]

In two words, "no" and "no".

You need close to 7.5v DC to get a quiet VT25 that can run at full emission; the transformer cannot do this voltage.

The 10 even at full emission cannot deliver more than half the current that the SEX amp wants.

The plate impedance of the 10 is twice as large as the 6DN7, so the output transformer impedance is all wrong.

The gain of the 10 is half that of the 6DN7 so the bias and the driver gain is all wrong.

The 10 does not have second driver triode, so the chassis plate does not have enough holes.

In other words, it's a completely different amp.

[vetmed]

Guess I did not provide enough info. Check Steve Bench's site where he discusses" Starving" DHTs. Also the amp has been modded to run vt25s only. It has  Electraprint 10K to 8ohm parafeed transformers  and  Electraprint plate chokes which very nicely fit without any mods in the space where the original iron went. The intent right now is to use a separate outboard driver stage using a single 6SL7, but have had some noise problems with that, but I am pretty sure that I can cure that. The voltage doubler circuit rises to around 400 volts with the decreased load, necessitating a change in the last PS cap to something rated for higher voltage. The fly in the ointment is the 7.5 volt heater transformer which I cannot get to operate quietly, and which takes up a lot of space. So I thought trying to use the heater winding in the SEX amp that is already there.

[vetmed]

Tried hooking up the vt25s to the 6.3 volt winding. They light up but measure only 6.1 volts. If they had measured a bit higher I would have pursued it further. I posed a question about using switching supplies elsewhere, when they get here I'll report on if the "sound" is acceptable.


Robert Lees

[vetmed]

On the other hand I get 405 volts at the plate, 32 volts at the top of the 2000 ohm cathode resistor, and 7 or 8 mv of hum, the best I have managed so far. 32v/2000 ohms gives me 16 ma per tube which by the tube specs should be good for about a watt. I may even hook this up and see if it works! BTW I tried to include a photo but I am told that it exceeds the 512 KB limit, if someone can tell me how to get my image up I will be happy to post it or even a couple!



Robert Lees

[vetmed]

A couple of images, just in case you think I'm making this up...

[vetmed]

Just one more image, sounds great with either Sennheiser HD-497s or Beyerdynamic DT-880s. Getting about 7 mv hum in one channel and 8 in the other. This is noticeable when no music is playing but not when there is. I'm using it with my home brew transformer coupled preamp into a pair of UTC "ouncer" input trannies set for about 10:1 stepup into the VT-25s. Seems to work very well.