I've decided to abandon the idea of small resistors in the plate or cathode circuits, to measure the current. I've just finished calibrating my meters and there are accurate to 1.5%; that's more than accurate enough.
The 18 NOS 6J32P Svetlana tubes that I got from Russia have all been measured and graded now. I've been very lucky; I got seven matched pairs from the lot, and the real bonus is that those that matched for DC conditions also matched closely for GM. The lowest GM was 1.6 and the highest was 1.9, with most of them measuring at the top end. And they sound fine too. It was a very worthwhile purchase for $100. I think the guy on Ebay is located in Smolensk and he has lots more for sale.
that is good news....i have high regards for Russian tubes......
is your rig playing music now?
mine is still under construction, lots of other things to do, anyway the power sequencer and the psu istelf is tested and comfirmed okay.....
the amplifier board comes next...hopefully this weekend i can start testing the amp...😀
Here is the Ebay link for the seller in Smolensk that sold me the 6J32Ps. He has lots of other interesting tubes. LINK
Fan
There is no doubt that a fan is essential on this amplifier. I have tried several but the quietest that gives good air pressure and airflow is the 120mm Silent Wings 2 from Germany. I run it from one of the 12V heater windings and after full wave rectification, I drop the voltage down to 11V with a 22 Ohm resistor and smooth it with a 2200 uF electrolytic. The fan will not drive any reasonable amount of air unless it has a full 2 inches of space underneath, so I need to think about extending the feet or building some sort of frame to bring the amp up from the table surface.
There is no doubt that a fan is essential on this amplifier. I have tried several but the quietest that gives good air pressure and airflow is the 120mm Silent Wings 2 from Germany. I run it from one of the 12V heater windings and after full wave rectification, I drop the voltage down to 11V with a 22 Ohm resistor and smooth it with a 2200 uF electrolytic. The fan will not drive any reasonable amount of air unless it has a full 2 inches of space underneath, so I need to think about extending the feet or building some sort of frame to bring the amp up from the table surface.
Attachments
I keep the temperature in the listening room at about 19 Degrees C but the amp drives it higher when in use. Sometimes I need to open the windows even in winter when the outside air temperature is about 10 Degrees. In this part of Ireland we have cool summers and mild winters; we don't need air con.
Project Completed.
After 7 months, my Tim Mellows OTL amplifier is complete. I'm extremely happy with the sound of it. I'm putting a link here to the Google drive folder with all the photos from the construction. I hope more folks will be encouraged to start this superbly rewarding project. LINK
After 7 months, my Tim Mellows OTL amplifier is complete. I'm extremely happy with the sound of it. I'm putting a link here to the Google drive folder with all the photos from the construction. I hope more folks will be encouraged to start this superbly rewarding project. LINK
Good job God. I have all the parts for mine but it is third down on the list and I haven't had much time. I'll get to it someday.
Needless to say, there were a few problems along the way:
1) Very hard to get properly matched pairs of EF86s.
2) Getting high voltage resistors where specified was a problem.
3) Choosing the right fan gave me a lot of angst.
4) Waiting up to 6 weeks for parts from Russia and China.
5) Getting the correct spray paint for the chassis.
6) Making a suitable cover box for the big transformer took almost 3 weeks.
7) Getting rid of a nasty earth-loop hum meant running a separate earth strap to the tube pre-amp.
I'm listening to the amp right now playing Jazz and all I can say is that it is singing; best amp that I have ever heard, bar none, and I'm in the audio business for more than 50 years. My electricity bill will be much higher but at least the amp heats the listening room adequately on its own.
I'm very grateful for all the help and advice I got on this forum.
Last edited:
update, i settled for the NEC 12AX7, i seems to be better balanced than the JJ's..
output offset was 100mV ot there abouts....
with 200 watts for filaments alone, there is 350 watts total power draw,
so that each output tube dissipates around 37 watts each....
tommorow is the moment of truth, listening to it on real speakers....
output offset was 100mV ot there abouts....
with 200 watts for filaments alone, there is 350 watts total power draw,
so that each output tube dissipates around 37 watts each....
tommorow is the moment of truth, listening to it on real speakers....
Attachments
Yes, my JJs were not stable between sections. I found that Electro Harmonix were fine, although the JJs sound fine in my tube pre-amp so I did a swap. The EHs were pretty well burned in after a few years in the pre-amp. Good luck with connecting them to your speakers. I don't think you will be disappointed with the sound; it should sing.
I always rush to the speaker terminals with the digital meter to check the offsets when I switch on. I'm still nervous about blowing the drivers. I'll get over it after a while.
I've given my OTL a rest for the moment while I build a filter box under the fan to stop re-circulation of household dust which was affecting my asthma.
Very nice build!
I think you will find though that in due time, the insulation will degrade on the wiring to your filament connections of the output tubes. We use a bit of Teflon tubing that is placed over the wire in these locations- so if the insulation melts, its held in place by the Teflon tubing which melts at a much higher temperature. 1/2 inch of tubing should be sufficient for each connection.