JTM 45 Bias Bug*

[plexi50]

I built this JTM45 from a bunch of different spare parts laying around. I used Peavey transformers from a Heritage 100 watt chassis. I have used a full wave SS bridge recto and tapped onto one of the primary leads to supply the bias voltage. At the 180K bias feed resistor i have 200VDC. I think im reading this wrong as i should have my DVM set to AC before the rectification diode i believe. Anyway i CAN NOT get a bias grid voltage. I have tried many resistor combinations and have even removed the bias trimmer (which is good 25k) and tried several resistor combos to get me in any ball park of grid voltage. Have gone over my work and all looks correct. BUT! I do have a bad history of bias issues when using a SS bridge rectifier. Here are some pics . Dont know if they will help. Yeah i should have used a board but i tried another terminal strip build/

[sluckey]

When using a fullwave bridge you must pick up the AC for the bias circuit thru a .047µF cap. Copy the circuit on the bottom left of page 5 of this pdf...

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[plexi50]

That reminds me of that old 50's Hammond amp that you helped me with getting that bias working back in 2006.
I put a .047 cap from the rectifier primary lead to a (56K (one lead grounded) and 1N4007 diode bussed to cap-15K-cap 47K bias resistor to ground. Have tried 56k and 68K to no avail

Note! I am getting 200VDC on the bridge rectifier bias voltage tap without the .047 cap and 400 VDC with .047 on the bridge bias tap. No voltage on the bias circuit itself

[plexi50]

Got it running and sounding great. 475VDC plate with a -37.1 mv grid.  Thanks for the cap reminder Steve. Should have just ordered a JTM45 power transformer as it has the 5V rectifier taps. Probably will in a week or so. Just had to sink $800.00  for all new upper and lower control arm bushings and ball joints in my van. That was a great price considering all other shops wanted $1,800+

[sluckey]

so what fixed it? All I see is that now you have a 'real' orange cap and you moved to the other PT HT lead. 

[plexi50]

Im still trying to figure it out. I dont know for certain but i saw a small speck of solder on the - bias cap terminals strip that may have been touching the chassis. I used some blue solder wick to clean it and all other solder tab points. Next thing i knew is i flipped it on and watched my grid voltage appear at -49.9VDC. Then i installed the 25K trimmer pot and played with some r values before settling on 47K

This PT does have a CT but the voltage is 262VDC using it. The schematic for the Peavey Heritage shows 525VDC for the high power switch setting and a low power switch setting of 262VDC lifitng one of the HT leads

I choose to not use the CT. The preamp and EL34 power tubes current drawed the 525VDC down to 475VDC @-37.1 mv grid

I moved the bias feed to the other side of the rectifier because the schematic shows a .01 1KV cap from the CT to one of the HT leads. But i am not going to be using the lower voltage so i scrapped that idea and just left the bias voltage feed where it is now

[Willabe]

Next thing i knew is i flipped it on and watched my grid voltage appear at -49.9VDC.

Nice job, you've got a lot of stick to it in you.     

                            Brad     

[plexi50]

Thanks Willabe! If it wernt for Steve and many other geniusuz on this forum i believe i would be washing dishes at the White House. I just finished putting Orangedrops in a Deluxe reverb. I need a Bubba soda*