JM Signature Build

[martin manning]

Finished the grounds and the heaters only to find that I'm out of 1ohm resistors for the bias check.................the "mouser wait" ensues.

I'd be improvising here, like using the DC resistance of the OT's primary to set the bias.

That's because you're a lot smarter than me :)

Nah, this is easy. Measure the DC resistance of both sides of the OT, CT to each plate lead (and they'll be slightly different), then use that to calculate bias current by measuring the voltage across the same points, I = V/R. It's easiest to calculate a target voltage for one side, clip your probes on, and bring the bias voltage up until you hit it.

[dreric]

I got the amp finished up! didn't blow up! but also no sound

My pre-amp voltages are WAY off, 380-420 volts, I think enough to may the amp soundless. Plates on the 6V6 are around 450 volts, high?

Does anyone have a reference on that my pre-amp voltages should be? I've looked in the threads I can fine but no one has shared a voltgae chart.

Thanks

[M Fowler]

No voltage chart here but it seems as though your tubes are not pulling down the voltage.

[dreric]

No voltage chart here but it seems as though your tubes are not pulling down the voltage.

Agreed, I haven't had this trouble before, where would you suggest I look?

[dorrisant]

Read the preamp cathode voltages...

[martin manning]

- Identify the schematic you built to.
- Verify that all filaments are glowing, and that the filament voltage is correct.
- If you can set the bias on the power tubes do that.
- Measure voltages at all power supply nodes, and at all tube pins, and post them.

[dreric]

- Identify the schematic you built to.
- Verify that all filaments are glowing, and that the filament voltage is correct.
- If you can set the bias on the power tubes do that.
- Measure voltages at all power supply nodes, and at all tube pins, and post them.

Martin,

All the filaments are glowing. Filament voltage is 7.25 volts

There seems to be a problem with the bias.

The bias voltage on the board is -57.5 volts.

With the bias pot rotated fully clock wise (lowest reading) On the tubes:
V6: 27 ma
V7: 8.2 ma
V8: 6.0 ma
V9: 4.2 ma
If I turn the bias pot counter clockwise voltages go up but the imbalance remains.

I've included some picture of the bias circuit, not 100 sure I did the right.......

I build the amp to the schematic at on page one. This schematic shows the PS with the two rock power reduction system which requires two taps. This confused me so I built the PS based on the earlier schematic included in this post.

The node voltages are:
1. 528 VDC
2. 527 VDC
3. 505 VDC
3a. 498 VDC
4. 442 VDC
5. 442 VDC

For clarity I an using 50 watt iron and 4 6V6.

Seems like I need to get the bias supply problem figured out first.............if I did that wouldn't the voltages drop down?

Thanks for your help.

[martin manning]

All the filaments are glowing. Filament voltage is 7.25 volts

OK, good but that is a bit high. You may want to address that later.

There seems to be a problem with the bias.

The bias voltage on the board is -57.5 volts.

With the bias pot rotated fully clock wise (lowest reading) On the tubes:
V6: 27 ma
V7: 8.2 ma
V8: 6.0 ma
V9: 4.2 ma
If I turn the bias pot counter clockwise voltages go up but the imbalance remains.

Yes, get this sorted out first. Sounds like your raw bias voltage is good, but check it through the bias pot and over to the junction of the two 220k grid resistors on the main board. Check the voltages on 6V6 pins 3, 4, and 5 and post them.

[dreric]

All the filaments are glowing. Filament voltage is 7.25 volts

OK, good but that is a bit high. You may want to address that later.

There seems to be a problem with the bias.

The bias voltage on the board is -57.5 volts.

With the bias pot rotated fully clock wise (lowest reading) On the tubes:
V6: 27 ma
V7: 8.2 ma
V8: 6.0 ma
V9: 4.2 ma
If I turn the bias pot counter clockwise voltages go up but the imbalance remains.

Yes, get this sorted out first. Sounds like your raw bias voltage is good, but check it through the bias pot and over to the junction of the two 220k grid resistors on the main board. Check the voltages on 6V6 pins 3, 4, and 5 and post them.

As I woke refreshed and invigorated for the holiday weekend I realized that I had put pulls into the power tubes rather that tubes "known" to be good. So brand new tube went in and the bias problem is gone.

Off supply -60 vdc
At the pot -60 vdc
Wiper -60 to -30 vdc depending on position

V6-9
pin 3: 528 vdc
pin 4: 528 vdc
pin 5: -56 vdc (and sound comes from the speaker when the probe touches the pin)

Bias reads about 32 ma

Still no signal, onto the preamp tube voltages

[martin manning]

528V and 32mA is almost 17W! The idle current needs to be down around 18mA with that much voltage on the plates. Seems like your plate voltage is too high even with the 4x 32mA. What do you have for mains voltage?

[67plexi]

Eric, I think for 6V6 275-0-275 would be the perfect B+ secondary voltage
for your build you may wish to find an other power transformer.
My guess under load should be around 340VAC. Your RCA Black plates will
love you at that voltage.

Best, Steve.

[dreric]

528V and 32mA is almost 17W! The idle current needs to be down around 18mA with that much voltage on the plates. Seems like your plate voltage is too high even with the 4x 32mA. What do you have for mains voltage?

I'm past the Jim Kelly zone!

From the rectifier: 508 vdc
node 2: 508 vdc
node 3: 486 vdc
node 3a: 483 vdc
node 4: 428 vdc
node 5: 529 vdc

V1
pin 1: 0
pin 2: 0
pin 3: 430 vdc
pin 6: 0
pin 7: 0
pin 8: 430 vdc

V2
pin 1: 0
pin 2: 0
pin 3: 496 vdc
pin 6: 0
pin 7: 0
pin 8: 507 vdc

V3
pin 1: 493 vdc
pin 2: 0
pin 3: 6 vdc
pin 6: 489 vdc
pin 7: 0
pin 8: 6 vdc

V4
pin 1: 306 vdc
pin 2: 0
pin 3: 0
pin 6: 0
pin 7: 0
pin 8: 2.27 vdc

V5
pin 1: 304
pin 2: 0
pin 3: 0
pin 6: 0
pin 7: 0
pin 8: 2.27 vdc

V6
pin 1: 499 vdc
pin 2: 26 vdc
pin 3: 102 vdc
pin 6: 501 vdc
pin 7: 78 vdc
pin 8: 103 vdc

[dreric]

Eric, I think for 6V6 275-0-275 would be the perfect B+ secondary voltage
for your build you may wish to find an other power transformer.
My guess under load should be around 340VAC. Your RCA Black plates will
love you at that voltage.

Best, Steve.

Steve

here's the specs PT the PT 330-0-330

[martin manning]

Looks like there is some error there, as I don't know how node 5 could be 100V higher than node 4.

On V1 and V2, pins 3 and 8 are cathodes, and should have only a volt or two on them. V4 and V5, pins 3 and 6 show no voltage, where they should be similar to pins 8 and 1 respectively. V5 cathodes (pins 3 and 8) should be much higher.

Take a step back. Make sure that your meter is working properly, and that the battery is fresh. Remove all of the tubes, and check voltages through the entire power supply, making sure that the correct node voltages appear at the plate load resistors, the reverb transformer, and at the correct socket pins. I suspect there are wiring errors in the preamp (V1, V2, V4, and V5), so recheck them.

[67plexi]

What I see is your 100uf @ 450v is wired as a voltage doubler
You an flip #2 100uf @ 450v around that would give you 50uf @ 900v and use a 270k bleeder resistor from + to + and use #1 100uf to standby to choke

I cant see under your board to see how you wired it.


If it was my amp I would go 220uf @ 450v x 2 both positive on the front
270k 2-3 watt resistor and ground the back negative on the capacitors
110uf @ 900v and use #1 capacitor positive as B+1 filter.

Steve.