Original JTM45 re-cap & re-tube

[gui_tarzan]

I finally got to power this up today to check voltages and unloaded they don't look too bad. I am a little concerned about the bias cap voltages though, they are different

C1 - 514v - 32mfd (can)
C2 - 522v - 32mfd (can)
C3 - 479v - 32mfd (can)
C4 - 498v - 16mfd
C5 - -48v - 8mfd (bias)
C6 - -61v - 8mfd (bias)

Why would the bias caps be different voltages? All the resistors checked out within 10% tolerance of their values.

I think I just answered my own question, there is a 15K resistor between them. Coming off the 220K from the power tubes there is -48v so I think I'm good there.

[g1]

That's correct. Then after the second cap (the -48 one) there should be a resistor or trimpot (or both) going to ground. This is what sets the actual bias voltage (currently -48).

[gui_tarzan]

I've got 458v on the Tung Sol KT66s pins 3 and 453v on pins 4, Tung Sol's data sheet says not to go over 450v on the grid. I know it's only 3v but I like to be safe.

The bias is at -48v on pins 5, which seems high to me. Aren't they supposed to be in the high 30s at 450v on the plates?

[J M Fahey]

I've got 458v on the Tung Sol KT66s pins 3 and 453v on pins 4, Tung Sol's data sheet says not to go over 450v on the grid. I know it's only 3v but I like to be safe.

You must be kidding

The bias is at -48v on pins 5, which seems high to me. Aren't they supposed to be in the high 30s at 450v on the plates?

Maybe, maybe not, that´s not the main point.
*The* parameter, of which bias voltage is just a rough indication, is cathode (plate) current.

[gui_tarzan]

OMG does this thing sound great! Smooth, tight bass, great mids and sparkly highs. I think I'm in love. lol!

I'll hook it up to a couple of other speakers tomorrow, right now I'm playing it through a 30w Jensen out of a mid 60s organ and it is clean clean clean and sparkly. Might have a weak normal channel tube though, the volume is way down on it. It's loud on the second channel. These original Mullards really sound nice!

Yes, I'm kidding about the 3v. You do mean "anode", not "cathode" for the plate, right?

I'll also check the plate current tomorrow and see where it's at.

[Enzo]

No he meant cathode. Plate current alone determines plate dissipation, but we often monitor tube current at the cathode, with an adaptor or an installed 1 ohm resistor. In that case, though we are thinking plate current, we are actually measuring cathode current, which includes a small screen current as well as the plate current. So you are right too, just Juan didn;t mis-state it.

[gui_tarzan]

Oh, ok. I see what he meant now. Yes, I've got the adapter, will check tomorrow.

[gui_tarzan]

I changed the 56K bias resistor to 47K and I have 40ma (measuring using the adapter) now so at 438v plate voltage it's running just a tad over 70%.

[g1]

OMG does this thing sound great! Smooth, tight bass, great mids and sparkly highs. I think I'm in love. lol!

From this you increased your idle current with the only advantage being decreased tube life .
Or did it improve the tone even more?
From the best sounding, you should decrease idle current (increase 56K value) until it has a negative impact on the tone.
Then you will have best tube life possible for the tone you like.
The 70% magic number is pretty meaningless aside from a rough "do not exceed" max. value.

[gui_tarzan]

It sounds amazing. I tried a 27K and it sounded like crap.

[g1]

My point was that if it doesn't sound any better than the 56K, stick with the 56K as you will get better tube life.

[gui_tarzan]

Yeah, I got that. I'm going to put the 56K back in it before I take it back today. Thanks for the heads up.

[gui_tarzan]

Took the amp back to the shop that hired me to fix it. They're ecstatic and very happy with the results. As am I. The guy that played an LP through it said "listen to those harmonics - they're sparkly!" lol!



This one makes me feel good, it's not just another Fender (not that Fenders aren't any good!) but a timeless classic. I'll probably never see another one like it but who knows? The word is starting to get out that I can actually repair these. Of course some of the credit goes to you guys for steering me in the right direction on some of the theory end and I greatly appreciate it.