New amp build finally started. JTM45 with KT66.

[SG John]

That's great news. I wish I was there to hear it...

What tubes are in there right now?

It has the Gold Lion KT66s and some JJ 12ax7s in it. Once I get everything dialed in, working, and broken in, I’ll think about putting the NOS stuff in it.

 

[SG John]

And, so it begins.

 

[syscokid]

Have you met Mr Ac-switch-is-on yet?

 

[SG John]

Have you met Mr Ac-switch-is-on yet?

It was still unplugged for step one.

The Mrs. wanted my assistance, so I had to bugger off. Plate voltage is next. No camera tricks while I’m checking that.

 

[syscokid]

No camera tricks while I’m checking that.

Oh come on... You can do it!!

BTW, what are you measuring in that pic?

 

[SG John]

Oh come on... You can do it!!

BTW, what are you measuring in that pic?

Resistance between the center tap of the output transformer and pin 3 of V4 and V5.

 

[SG John]

I'm working on biasing the power tubes. After doing all the math, I get 1.753VDC at V4 and 1.774VDC at V5.

With the meter set on the OT center tap and pin 3 of V4, I get 2.17 VDC
With the meter set on the OT center tap and pin 3 of V5, I get 2.05 VDC

My trimpot is turn all the way clockwise to it's lowest value.

Is this too great of a difference, and is there a resistor I need to change?

Or, do I leave it alone and crank it up?

Thanks.

 

[ivan H]

Great to hear that it's up & running healthily SG John. It will sound a bit better when properly biased.
To aid the biasing proceeded, I recommend getting some test leads like this
Or better yet, like this
These (2nd pic) are called EZ Hook or IC test clips. They are a spring loaded syringe action & a small hook protrudes from the end when the back end is depressed. Allows hands to be kept out of there for the procedure. Cheers

 

[ivan H]

I'm working on biasing the power tubes. After doing all the math, I get 1.753VDC at V4 and 1.774VDC at V5.

With the meter set on the OT center tap and pin 3 of V4, I get 2.17 VDC
With the meter set on the OT center tap and pin 3 of V5, I get 2.05 VDC

My trimpot is turn all the way clockwise to it's lowest value.

Is this too great of a difference, and is there a resistor I need to change?

Or, do I leave it alone and crank it up?

Thanks.

Yes, you need to change a resistance value & there are two you can choose from to change.
The first is the AC dropping resistor, placed before the diode where the AC voltage is fed into the bias circuit. You would go to the closest lower value resistor in this case so as to increase you negative bias voltage a bit.
The second choice is the resistor that is in series with the trimpot. This is part of a voltage divider network. You would need to increase the value of this resistor (closest higher value to what you have in there) to increase your negative bias voltage.
If you can tell me each of the two values I'll give you their closest values. Cheers
Edit:
I'd change the resistor that is in series with the trimpot for the next higher value, but either way will work. Cheers

 

[SG John]

The resistor that is in series with the pot is supposed to be 56K, and this measures 57.5K.

The resistor before the diode is right on 220K.

I'm going to start looking to see what I have on hand.

 

[SG John]

I have more 56K Pihers, and some 47.5K Pihers in my stash. Nothing in the 220K range.

 

[ivan H]

I have more 56K Pihers, and some 47.5K Pihers in my stash. Nothing in the 220K range.

If you have a 62k or 68k for the resistor in series with the trimpot, it should get you into the necessary range.
A 200k or 180k for the AC dropper. Try 200 first if changing this resistor.
The power tubes are running a bit hot, check for red-plating if you power up again, but best to bias a bit cooler first.
This type bias supply resistance adjustment is pretty well normal when first powering up a build, or re-tubing. Cheers

 

[67plexi]

John, If it was me try a 180K not the 220K first that is what the schematic calls for.

 

[SG John]

If you have a 62k or 68k for the resistor in series with the trimpot, it should get you into the necessary range.
A 200k or 180k for the AC dropper. Try 200 first if changing this resistor.
The power tubes are running a bit hot, check for red-plating if you power up again, but best to bias a bit cooler first.
This type bias supply resistance adjustment is pretty well normal when first powering up a build, or re-tubing. Cheers

John, If it was me try a 180K not the 220K first that is what the schematic calls for.

Thanks guys.

I'll order some resistors in a bit. In the middle of cleaning my bench now.

I'll try the 180K first. It looks like the easiest.

 

[syscokid]

The resistor that is in series with the pot is supposed to be 56K, and this measures 57.5K.

Have any 68K's?

 

[ivan H]

John, If it was me try a 180K not the 220K first that is what the schematic calls for.

Thanks Plexi, I hadn't checked the schematic, & have not long woken up, just getting my second coffee. Cheers

 

[SG John]

Have any 68K's?

I have one, but it was cut real short to use elsewhere. It was accidentally placed where there should have been an 82K.

 

[syscokid]

I have one, but it was cut real short to use elsewhere. It was accidentally placed where there should have been an 82K.

Is it too short to add an extension to the leads? At least you can try the value while you order a new one.

 

[syscokid]

This is one of my favorite gadgets. And is pretty fun to use. $35-ish

 

[SG John]

Finally got my bags of resistors today. Switched the 220K voltage dropping resistor to a 180K as Plexi pointed out. Had to redo my math, as it actually brought my B+ voltage up to 440 volts. The Gold Lion KT66s are very, very close to my numbers. I also managed to finally get 7.67 volts difference between pins 1 and 6 of V3 after checking ten 12ax7s. Needs a bit more breaking in and tuning. I’m happy with the current sounds I’m hearing.