3.5v on the heater???

[bristol]

Hi,
Just finished my build. Powered up OK, no sparks or blow fuse, lamp came on, but heater voltage reads 3.5v instead of 3.15. Voltage between pins 2 and 8 of the rectifier socket reads 5.12v. It this all right or did I mis-wire something?
Thanks

[novosibir]

Try a measurement between pin 2 & pin 7 on one output tube and verify to be between 5.8 & 6.9V

Larry

[bristol]

someone else is online! 7.11
thanks

[bristol]

My ac is reading 123 and so I would expect the heater to be a bit high, but not that high. I have traced the wiring 10 times and can't see any problems. Its a pretty simple circuit. I am wondering if the transformer is wound slightly wrong. I understand that a high heater voltage will cook tubes very quickly.

[bristol]

One other thing. I measured pins 4 and 5 of the rectifier and they are at 332 instead of 320. It seems everything is a little hot on the secondary side of the transformer.

[Big Mike]

that's still within about 5% ...is that an issue? Larry?

[novosibir]

Is this measurement with or w/o tubes in the amp?

If with tubes, then 7.11V on the heaters definitely is too much, though 332 instead of 320 on the recto tube is ok!

If measurements are done with tubes plugged in, voltage will drop down by the load.

Larry

[bristol]

Had no tubes on previous measurements.

Based on a private exchange with thousandshirts and your advice, I put in the rectifier tube (step D). I now measure 330vdc at the standby switch and -64vdc on pin 5 on V4 and V5 with the bias fully clockwise, -52vdc when counter clockwise. Directions say -45vdc clockwise. I do have 5.6k ohm resistors in series, right at the pin that a series resistor will prevent. They are one watt resistors. Should they be 2 watt? I would expect a small voltage drop. Am I good to go with adding the preamp tubes or is something wrong?

Also, not to get ahead of myself, assuming I am ok to go, I don't have my speakers yet. I crafted an 8 ohm, 80 watt load out of power resistors. Can I use it as a dummy load for further testing? It's purely resistive.
Thanks,
Charlie

[bristol]

The 5.6k resistors are to prevent KT66 ocillation.

[novosibir]

I now measure 330vdc at the standby switch and -64vdc on pin 5 on V4 and V5 with the bias fully clockwise, -52vdc when counter clockwise.

That's IMO the correct ballpark for KT66's! Will still drop a bit with tubes installed.

Directions say -45vdc clockwise.

That's for EL34 - CW -45 CCW -34

I do have 5.6k ohm resistors in series, right at the pin that a series resistor will prevent. They are one watt resistors. Should they be 2 watt?

No!

I would expect a small voltage drop.

No drop!

Am I good to go with adding the preamp tubes or is something wrong?

Add it!

I crafted an 8 ohm, 80 watt load out of power resistors. Can I use it as a dummy load for further testing?

By principle yes, but...

... if your OT leads are inverted by accident, you can't hear the squealing of your amp (because resistors almost always are quite like a grave), but this oscillating might kill your output tubes. You can't hear possibly hum, buzz, hiss... nothing!

Better wait for your cab!

Larry

[bristol]

Pardon me I I am slow, I want to be very careful.

So if I understand your reply correctly, the bias voltage range (-64 to -52) is good for KT66s and the build manual is wrong (-45vdc) ror KT66, correct for EL 34.

1 watt is ok for the 5.6k resistors.

And wait for the speakers.

One other thing:

I have noticed that the board is grounded only to the pot buswire (6 pots on the front panel), however, it's all floating above ground - no chassis connection. Is that correct or do I need to ground the bus wire to the chassis?
Thanks,
Charlie

[bristol]

Larry,
I do have a 15 watt speaker. If I keep the amp volume at 1, can I use it to test and set the bias, or will I blow it? Or could I wire the speaker in series with the resistive load to protect the speaker?
Thanks,
Charlie

[Flames1950]

I crafted an 8 ohm, 80 watt load out of power resistors. Can I use it as a dummy load for further testing?

By principle yes, but...

... if your OT leads are inverted by accident, you can't hear the squealing of your amp (because resistors almost always are quite like a grave), but this oscillating might kill your output tubes. You can't hear possibly hum, buzz, hiss... nothing!

Better wait for your cab!

Larry

If in your testing you show a good range of bias voltage as you turn the adjustment pot, but the bias current is skyrocketing when checked with the load, you could try swapping the output tranny leads to pin 3 of the power tubes to see if the high bias current goes away -- and then you'd know the leads had been swapped. (Shut it off fast if the current goes up of course!!) If the current adjusts nicely with the bias pot you're good. I've wasted time on that one before with a load when I first was getting started, simply because I couldn't hear the telltale howl with no speakers attached!

[bristol]

Thanks. I may try that unless I can use the low wattage speaker I have in series with the dummy load. See my previous post.

[bristol]

I discovered a ground problem. I have PECs and wired them using the bus bar across pin 3 approach rather than across all the pot backs - looks better. I had not, however, grounded the buss bar to the chassis. It led to some pretty strange voltage readings. Although with the grounding arrangement in the instructions, I never saw where the buss bar was supposed to hook to chassis ground unless the alpha pots do so on their own. I just wired in a ground and took the readings below. Hopefully, the ground will not create noise problems but if so, I'll play with it. I ran it to the earth ground that the AC cord hooks to since it was the closest ground lug.

V1 P1 218
V1 P3 1.8
V1 P4 3.5ac to chassis ground
V1 P6 220
V1 P8 1.8

V2 P1 155
V2 P3 1,29
V2 P4 3.5ac to chassis ground
V2 P6 326
V2 P7 183
V2 P8 183

V3 P1 243
V3 P3 44.6
V3 P4 3.5ac to chassis ground
V3 P6 237
V3 P8 44.6

My bias pot goes from -38 to -52vdc

My preamp tubes seem to run very cool. Warm, but no glow. Is that OK?

This all good??? Ready for power tubes? Can I put an 8 ohm 15 watt speaker in one speaker output and my 8 ohm 80 watt resistive load in the other, set the impedance to 4 and set the bias?? With no danger to the low wattage speaker?? And then actually plug in and rock??
Thanks
Charlie