Beer of the Bass Posted December 18, 2013 Share Posted December 18, 2013 (edited) I got my bias probe from Tube Town in Germany, as it was the cheapest I could find one at the time. It's just an adaptor which puts a 1 ohm resistor in line with the cathode for easy measurement, with attached leads for your meter. [url="https://www.tube-town.net/ttstore/product_info.php/info/p471_Bias-Probe-Octal--KIT.html"]https://www.tube-tow...Octal--KIT.html[/url] Edited December 18, 2013 by Beer of the Bass Quote Link to comment Share on other sites More sharing options...
3below Posted December 18, 2013 Share Posted December 18, 2013 Good help from many above. Once you get into the swing of the circuits and understand what they are doing it is not difficult. Very good advice about discharging filter capacitors. I gave myself an unforgettable experience with my Hiwatt 200 in my youth. Those caps hold a lot if charge, a breathtaking amount in fact. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 19, 2013 Author Share Posted December 19, 2013 Thanks guys, some brilliant advice here. I'm an all-valve noob, and a bit scared of them with all the current that's in them (probably a good thing!) so it's really good to know exactly what I'm doing before I get the soldering iron out. I'm thinking I'll attached the bias mod with wires to the PCB going to the chassis so it can be tweaked from the outside, along with some circuit tester jack sockets. Then I can keep an eye on it periodically without having to open her up Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted January 2, 2014 Author Share Posted January 2, 2014 Well I did it! Adding the Bias mod was actually easier than making the bias probe actually. I wasn't expecting it to arrive in kit form. I first checked the bias with the old valves in before I did the mod. It was running at about 20mA - which according to the Weber Bias calculator was yielding about 8 watts per valve. So I was getting just under 50 watts out of my 160W amp! I installed the new JJ 6L6s (matched sextet) which were getting about 18mA. once I'd done the bias mod I checked the plate voltage, worked out that 44mA would get me the 70% max recommended power and adjusted the bias up to 40mA to be safe. this got me about 18 - 20 watts from each valve. So 100 - 120w total, more than double what it was running at! Sounds good too The only odd thing was that I could not get a bias reading off all the valves (old or new), but as they all lit up and its making a noise I figured this may be an issue with the probe rather than the amp. As it's a matched set the global bias adjustment should suffice, and I've left 10% margin. Thanks everyone for your help with this, I wouldn't have even attempted it without your wise words! Now to gig it (I'll be taking a backup amp just in case) Quote Link to comment Share on other sites More sharing options...
6v6 Posted January 3, 2014 Share Posted January 3, 2014 (edited) [quote name='brensabre79' timestamp='1388674318' post='2324640'] Well I did it! Adding the Bias mod was actually easier than making the bias probe actually. I wasn't expecting it to arrive in kit form. I first checked the bias with the old valves in before I did the mod. It was running at about 20mA - which according to the Weber Bias calculator was yielding about 8 watts per valve. So I was getting just under 50 watts out of my 160W amp! [/quote] Great news, well done! Note, idle plate dissipation is not the same as power output - so if each tube is dissipating 8 watts, this just means they are running very cool (which would definitely affect the tone and amount of available headroom due to crossover distortion and premature saturation of one half of the waveform), not that you are only "getting" 8w per tube. You probably were still getting more than 50w output, but as you discovered, the waveform would distort early (the only way to prove the output power is to run it into a dummy load with a sine-wave going in, then measure the output voltage with an oscilloscope) [quote name='brensabre79' timestamp='1388674318' post='2324640'] I installed the new JJ 6L6s (matched sextet) which were getting about 18mA. once I'd done the bias mod I checked the plate voltage, worked out that 44mA would get me the 70% max recommended power and adjusted the bias up to 40mA to be safe. this got me about 18 - 20 watts from each valve. So 100 - 120w total, more than double what it was running at! Sounds good too [/quote] Great, sounds like you've got it bang on - I bet you'll be amazed how much better it sounds cranked now it's biassed properly with new tubes [quote name='brensabre79' timestamp='1388674318' post='2324640'] The only odd thing was that I could not get a bias reading off all the valves (old or new), but as they all lit up and its making a noise I figured this may be an issue with the probe rather than the amp. As it's a matched set the global bias adjustment should suffice, and I've left 10% margin. Thanks everyone for your help with this, I wouldn't have even attempted it without your wise words! Now to gig it (I'll be taking a backup amp just in case) [/quote] The not getting a bias reading sounds suspect. If the bias probe worked on one tube, it should work on all of them, so this IMO could point to some remaining issue inside the amp. "they all lit up and its making a noise" just means all the tubes have filament current (not necessarily that they are all working and making a noise) It could be that the tubes you got no reading from have a fault which needs rectifying, if so you are still not getting full output from the amp and some of your shiny new valves are just expensive lightbulbs I'd check the following: - Unplug the amp and drain the filter caps - (Edit) Unplug all the power tubes to ensure they don't interfere with the readings, we're measuring the amp wiring not the tubes - Locate pin3 on each of the power tube bases (it should be in the same place on all of them) - Use the continuity/ohms (buzzer) setting on your meter and prove the plate connection (pin3) of each bank of three valves is connected and there's no broken wires (ie between v5, v6, v7, then again between v8, v9 and v10) - Test the continuity (buzzer again) between the cathode (pin 8) of each bank of three as above - Use the ohms (low range, say 200ohms-1k) setting to check the resistance between pin4 of each pair of tubes (e.g between v5 and v6, then again between v6 and v7, then repeat for the other half of the poweramp). This should be around 200ohms (because there are two 100ohm screen grid resistors inbetween). If any are open circuit, you could have a burnt out screen grid resistor (can happen when a tube goes bad) If none of the above show up any problems, I'd inspect all of the tube sockets carefully for damage, and if all is OK fire up the amp and carefully take some measurements on the highest volts range of your meter (primarily check the plate and screen grid voltages all look roughly the same). Be very careful if you do this and make very sure you meter is on volts not current (don't ask me how I know this ) if you've also been measuring bias. Edited January 3, 2014 by 6v6 Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted January 3, 2014 Author Share Posted January 3, 2014 Wow, thanks 6v6. I'd better check it out then, as I only got a bias reading on 1 out of 3 tubes! Would the amp even work if only one or two of the tubes were actually giving any output then? Maybe this explains why it seemed so underpowered...? Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted January 3, 2014 Share Posted January 3, 2014 [quote name='brensabre79' timestamp='1388762702' post='2325761'] Wow, thanks 6v6. I'd better check it out then, as I only got a bias reading on 1 out of 3 tubes! Would the amp even work if only one or two of the tubes were actually giving any output then? Maybe this explains why it seemed so underpowered...? [/quote] Yes, the amp will work and not sound unusual so long as there are at least one pair of valves working. Hence the guitarists' practice of taking two out of your 100 watt Marshall or Fender twin to make it less loud. 6V6s troubleshooting procedure is sound advice, so I'd try that first. Quote Link to comment Share on other sites More sharing options...
6v6 Posted January 3, 2014 Share Posted January 3, 2014 [quote name='brensabre79' timestamp='1388762702' post='2325761'] Wow, thanks 6v6. I'd better check it out then, as I only got a bias reading on 1 out of 3 tubes! Would the amp even work if only one or two of the tubes were actually giving any output then? Maybe this explains why it seemed so underpowered...? [/quote] If two of the six are working (one on each side of the output transformer primaries) then the amp would work, but not very well. And yes, if for some reason one or more tubes is not working, the amp would be very underpowered. Not necessarily the case here, but worth checking out It wouldn't normally be necessary to test the bias on every tube, but if you suspect a fault, it's probably an easy way to gain confidence each tube is working, as it proves the plate, grid and cathode connections are all OK. I'd also measure the screen-grid current via each of those 100ohm resistors (but don't do this unless you're confident measuring with the amp on and disassembled), as if any of those are burnt out (or otherwise failed open-circuit, e.g dry solder joint) the tube it's connected to won't work but the amp will still make sound, albeit at reduced power. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted January 3, 2014 Author Share Posted January 3, 2014 OK thanks 6v6, I'll check it out tomorrow. It's a bit of a pain to get into as the sockets are on the PCB (I know, not the best) so you have to lift the whole PCB out to get underneath. Is it OK to switch on the amp with no tubes in then? (Rubber gloves on ) Quote Link to comment Share on other sites More sharing options...
6v6 Posted January 3, 2014 Share Posted January 3, 2014 [quote name='brensabre79' timestamp='1388768488' post='2325884'] OK thanks 6v6, I'll check it out tomorrow. It's a bit of a pain to get into as the sockets are on the PCB (I know, not the best) so you have to lift the whole PCB out to get underneath. Is it OK to switch on the amp with no tubes in then? (Rubber gloves on ) [/quote] Personally I'd try to avoid powering it up with the PCB lifted, it's very easy to accidentally move something and either hurt yourself or the amp You could do all the continuity tests by sticking the meter probes into the power tube sockets with the tubes unplugged. The other alternative is measuring components on top of the PCB - e.g all those screen grid resistors are probably sat on top of the PCB where you can probe the leads? It is generally OK to power up the amp with no tubes, but I'd suggest doing the powered off continuity/ohms tests first as you can derive almost as much info from those measurements and it's much safer. Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted January 4, 2014 Author Share Posted January 4, 2014 OK Thanks! Well I did the power off tests and all checked out OK. Pin 3 continuity across all 6 sockets Same for pin 8 200 ohms between the three on each side for pin 4. I'm thinking that as the bias probe is quite tricky to get in there because of the bear claws and it may be that these were pushing it out again. Its possible I just didn't have it in the socket properly. Quote Link to comment Share on other sites More sharing options...
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