JTM45 Build - Bias Circuit Error - No Voltage at Bias Diode - HELP...

[cluster]

Hi All,

I just slapped together a JTM45 and started checking voltages. All looks good except the bias circuit. I get voltage on either end of the 220K bias range resistor just before the diode. I also get voltage (to ground) on the diode end attached to the 220k. But NO voltage on the other end of the diode?? Very, very little and it's NOT negative. I thought the bias diode was just suppose to reverse the voltage coming from the 220k but in my case I don't get that. I tried a few diodes and still no dice. What am I doing wrong? No negative voltage after the diode means I can't bias the power tubes. Thx

[The Dude]

Voltage before the diode will be AC. Voltage after the diode will be DC. Are you measuring correctly? The diode doesn't "reverse" the voltage. It rectifies the AC. Also, the polarity of DC after the diode depends on how the diode is installed- either anode or cathode towards the resistor. Do you have the diode installed correctly?

[drewl]

Is the ac for bias circuit before or after standby switch?
some amps it is after.
Pull output tubes and and see if there's bias voltage with standby switched.

[cluster]

Voltage before the diode will be AC. Voltage after the diode will be DC. Are you measuring correctly? The diode doesn't "reverse" the voltage. It rectifies the AC. Also, the polarity of DC after the diode depends on how the diode is installed- either anode or cathode towards the resistor. Do you have the diode installed correctly?

Ok... makes sense. I think where i messed up is I installed a solid state / tube rectifier switch near the rectifier socket. It's possible I inadvertently rectified the bias supply voltage before it even reached the 220K (face palm). See attached. I attached the bias supply wire pin 6 (after diode). Instead I need to attach it to pin 5 instead? Right? (ie: before the diode to retain the AC voltage going to the 220k). First time installing this kind of switch. The diagram is for a Fender style with Cathode bias. I have a fixed bias which needs a supply. Thx

[The Dude]

Your bias supply should come off of the HV winding before the diodes for the B+ rectifier circuit. It is a separate supply altogether. The resistor knocks down the AC voltage to a proper AC supply for the bias circuit. So yes, the bias supply should come from pin 5 of the rectifier socket.

[Helmholtz]

I attached the bias supply wire pin 6 (after diode)

That explains your problem. At pin 6 you have of positive voltage which will be blocked by the bias diode.

Nevertheless it would be good to see a drawing of your bias circuit.

[cluster]

Is the ac for bias circuit before or after standby switch?
some amps it is after.
Pull output tubes and and see if there's bias voltage with standby switched.

I'll check tomorrow. Thx

[The Dude]

Yes, it would be good to see exactly what you built. That said, here's a schematic of a JTM45 for reference.

http://marstran.com/66JTM45Schematic.gif

[cluster]

Yes, it would be good to see exactly what you built. That said, here's a schematic of a JTM45 for reference.

http://marstran.com/66JTM45Schematic.gif

Thanks so much. I solved the bias issue as per your recommendations. But now i have a new issue with high voltage at the Preamp and PI.

KT66 and 12AX7 tubes IN, I get the following...

V1 pin1 = 322VDC should be (~180)
V1 pin6 = 322VDC should be (~180)
V2 pin1 = 192VDC should be (~155)
V2 pin6 = 324VDC should be (~280)
V3 pin1 = 257VDC should be (~205)
V3 pin6 = 227VDC should be (~195)
V4 plate = 400VDC looks good
V5 plate = 400VDC looks good
bias = -46VDC
Wall = 115AC

Dropping Resistor for preamp =10K 3watt
Dropping Resistor to PI = 8.3k 3watt


Not sure why this is since I used the recommended resistor sizes. Any Ideas?

See attached...

Please excuse the sloppy wiring. I know it looks like a mess. All I had was 22 gauge silicon wire which was a pain to wrap for hooking up the heaters. Need to get some proper solid core PVC wire for next time. Also this cheap Chinese chassis was a PIA to work on. Nothing fit and I had to re-drill ever hole to make my components fit.

Should I be upping the dropping resistors to bring the voltages down? I should have measure the screens to see. I'll do that when I get back.

[Helmholtz]

Are all tubes heating, can you see the filaments glow?

[cluster]

Are all tubes heating, can you see the filaments glow?

Fixed it. Bad tube at V1... also had the ByPass cap at V1a reversed (facepalm). Now my treble knob isn't responding. Need to look at the wiring there. Thx

[The Dude]

......Please excuse the sloppy wiring. I know it looks like a mess......

Oh, I've seen much worse than that!

[cluster]

Ok... found out why my treble pot wasn't working. A really tiny strand of wire was jumper'd between terminals. I couldn't even see it. Just by chance I ran my probe between terminals looking for issues and caught it.

So on to the next issue: Why is my screen voltage not lower than my plate voltage?? Plate =412VDC Screen = 412VDC (pin4). Pin 6 ~412.5. I'm using 470 ohm 5 watt screen resistors.

My configuration is:
V4-pin6 connected V5-pin6 connected to one side of the 50/50 mains cap.
V4-pin6 connected to 10k dropping resistor on the tube side of the board.
470ohm screen resistors between pin6 and pin4 on V4 and V5
The opposite side of the 10k dropping resistor is then connect to 1) PI plates and 2) one side of a 32/32uf cap.

Any ideas??

[Helmholtz]

We need to see your actual schematic.

But screen voltage being equal as or even a couple of volts higher than plate voltage is not uncommon - and no problem.

Without the shared 1k screen resistor and assuming that the choke has a DCR of 100R, screen voltage will only be about 1V lower than B+.

Plate voltage at idle depends on OT primary DCR and somewhat changes with idle current.