|
Title: JTM45 "buzzing" and 350VDC at pin 6 Post by: lundergaard on May 03, 2010, 03:29:31 PM Hi there gurus...
I'm new to this forum, new to Ceriatone amps but a long-time amp builder and tinkerer (Matchless Spitfire, Chieftan and DC-30 clones. Marshall Plexi clone.). I went with a JTM45 board from Ceriatone because I simply don't have time to build complete amp circuits like in the old days. Ah kids... Anyway, i am using transformers made by Magnetic Components of Chicago (paper wound!) and built the whole thing in an old Fender M-80 stereo chorus combo chassis/cab. Things went fine. All voltages tested great on the pre-amp tubes and B+ with no power tubes installed.. When i put in the power tubes (Valve Art KT66s), connected the speaker out to my 8 ohm dummy load, i get a buzzing sound when i take it off standby. The freaky thing is that i can't tell if it's from the tubes, the choke or the OT. It is WEIRD. Voltages all look good EXCEPT... on Pin 6 of the KT66s i am only getting about 350VDC. Should be 400VDC. on the choke side of that 1K resistor feeding pins 6 I am getting the 400 VDC. So with that... my bias (in mV) is WAY off... like 80ma. ouch. The negative bias voltage, without tubes installed, is the expected amount (can't remember off top of my head). Was wondering if anyone had experienced this? I'm thinking of just ditching the 1K resistor and getting the proper 400VDC to pin 6 and then hopefully all will be well. The resistor DID check OK last test... but who knows now. Anyway thanks for any advice you got. If you need more voltage measurements i can get those tonight when i get home from work. cheers andy Title: Re: JTM45 "buzzing" and 350VDC at pin 6 Post by: lundergaard on May 04, 2010, 03:05:52 PM more data... OT is buzzing... not tubes. current is like 80ma though measured across the 1 ohm R still.
not sure if it matters but i'm connected to an 8 ohm dummy load. (series of 5 watt resistors in a little box.) the thing is warm after running this for a bit to get the voltages. a real head-scratcher! using the Torres book, i checked the resistance on the OT for primary and secondary. all within specs. thanks for looking andy Title: Re: JTM45 "buzzing" and 350VDC at pin 6 Post by: lundergaard on May 04, 2010, 03:31:29 PM uh oh... this could be as stupid as reversing pin 3 OT connections... oh brother.
will swap and report results in case someone else has this problem. andy Title: Re: JTM45 "buzzing" and 350VDC at pin 6 Post by: lundergaard on May 05, 2010, 04:28:08 PM duh. swapped OT wires on pin 3 power tubes and all good!
i sort of remember this from my plexi build from 10 years ago. memory loss... inevitable. anyway, if you are running your amp thru a dummy load and these wires are reversed, your OT sounds like a vibrating cell phone on the chassis. That's really the only audible indicator. Title: Re: JTM45 "buzzing" and 350VDC at pin 6 Post by: cmoore on May 07, 2010, 05:49:32 AM This is kind of funny, but also interesting. I know what this sounds like through a speaker, but I never use a dummy load, so I would not have known what to think. This is a good lesson to keep stored in the "memory banks".
Good Job Title: Re: JTM45 "buzzing" and 350VDC at pin 6 Post by: ampkits on July 07, 2010, 03:46:32 PM I am starting to use lots of dummy loads. Making em, to burn in amps. So thanks for the info! :) But we try it with speakers first to check tone, before burn in. I guess if let to run for a long time, that'd kill the tubes or OT for sure. Thanks nik |