|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
60.234.110.135
Greetings 6as7g folk - I am just posting a finding today that was quite by accident . I was building my second monoblock whilst the first was running beside me ...Oh what a joy ! . Being in a workshop environment , I had a Fluke Temp probe & display handy ...and measured each tube . They were basically around the 110 - 120 degrees C ...but I did find one tube that was 140 + deg C ....which had me wondering why ~ So on its side she went , and found that the voltage drop across the 1 ohm resistors was much higher than the other tubes ...( and that was both sections ) ..so this tube was working quite alot harder than the others . Am I correct by this , in saying , that this tube has more gain ? ...I dont have a tube tester to compare results . BTW the bias current was set around 500 ma on the ammeter . Man , I just cant wait till the second is finished (:-)
Stu
Follow Ups:
That tube is running "hot", meaning it's mis-matched vis a vis your other tubes that are all drawing less current. It does not necessarily mean that the tube has any "more gain" than the others. You should probably discard that tube or put it aside as a last-ditch spare. You have discovered how you can use your amplifier to test and match its own output tubes. The amp should sound better and the output tubes last longer, if they are all in-synch.
Awesome DaveInVA & Lew , Thats fantastic advice , I have a spare tube , so I will exchange this Hot Puppy out . All the rest are much cooler indeed . I ordered 10 new Sovtek 6as7-g's for the second amp .. + while I was testing the first amp , I managed to flash over 1/2 of one tube in the first few minutes . Sadly this was caused because of a faulty tube in V3 or V4 - the arcing happened in the 6sn7 and then was followed by a huge flash in a 6as7g & a simultaneous bang in the speaker that made me nearly 20 years older / or younger ..im not sure which ! . Problems are all fixed now ...bias & balance is settled nicely . I pre cured the tubes as per recommendations . The damaged / stress placed on the output tubes was my mistake - I used some used ex radio spares 6sn7's ....but now have all new Electroharmonix . Overall - ive learnt alot already ...and building a left hand / right hand amp to my own physical design has been a real challenge . This week I hope to complete the 2nd amp ...then all I need is a site to host the pics. Cheers guys for your advice . Much appreciated . Stu
have 5 ohm 5 watt resistors in the cathode circuits. *Nuch* more effective than the 1 ohm resistors of earlier units.
I'm sure you meant "much" not "nuch".
Anyway, what does that do to the output impedance? I assume you take the output off the plate with the resistor on the cathode side.
No, we take the output off the cathodes. The cathode resistors are in series with that. But when you total up the entire lot of them, the result is a negligable increase in output impedance. However the tubes work better together so the impedance goes back down a little. The result is lower distortion- meaning better low level detail and smoother sound. The tubes last longer too.
nt.
Oh man - ive just finished wiring in my 1 ohm resistors too ..along with the 150 ohms tonight ....looks like back to the soldering iron for me before before I go much further on the 2nd amp . I new things were going too smoothly
Stu
The 1 ohm resistor will help somewhat to offset differences between the tubes but some will draw more current than others and if they are far enough off they may not last long. The newer Atma amps use resistors in the cathode leg instead of the plate circuit and should help spread out the tube differnces better.
| ||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: