JTM45 smoked resistor

[stevewdewitt]

I have built my first JTM45 from a tubedepot layout. Voltages measure correctly on initial testing. When I plug in the rectifier, GZ34, The resistor between one of the Can Caps and the 470 Ohm and pin 4 of V4 and V5, 1K 3Watt, gets really hot and smokes. I have been using a current limiter and that seems to be saving me after replacing the resistor. But it still seems to be shorting somewhere.

Any advice would be great. I have used the links on this page http://music-electronics-forum.com/t26468/ to measure the voltages in step 1 of the Metropoulus directions. All the voltages are in line.

TIA
Steve

[big_teee]

Got any gut-shot pictures we could see.
Is the axact schematic and layout the tubdepot manual?
Output tubes, EL34s?
T
** Looked at several more modern JTM45 Layouts.
The 1k ohm resistor in question is always a 5 watt, not a 2 or 3 watt.
On regular 1987 circuit builds they use no 1k ohm, or 470 ohm resistors, but use 2 1k ohm 5watt resistors direct to the screens.
Also make sure you start with the bias voltage turned all the way up.
Until you bias the tubes.

[Bruce / Mission Amps]

Assuming it does this with the power tubes installed, try it without the power tubes OUT...uninstalled, to determine if the power tube's screens shorted to the cathode or "ground" internally, causing the excessive current draw.... and of course be sure there is no short on the actual tube socket's lugs 4 or a the darn filter cap!! LOL.
Also, make sure the power tube's screens have no B+ voltage applied if the plates do not... that can cause a big draw of current too.

[stevewdewitt]

Assuming it does this with the power tubes installed, try it without the power tubes OUT...uninstalled, to determine if the power tube's screens shorted to the cathode or "ground" internally, causing the excessive current draw.... and of course be sure there is no short on the actual tube socket's lugs 4 or a the darn filter cap!! LOL.
Also, make sure the power tube's screens have no B+ voltage applied if the plates do not... that can cause a big draw of current too.

Power tubes are not installed. They will be EL34s. I haven't gotten that far yet. This issue occurs when I put in the rectifier tube. I did have a SSR with a switch and the tube rectifier. But I removed that to rule it out as the issue.

I have used two different layouts. Both of the layouts had 1K 2watt or 3watt resistors in the offending position. The main layout I used is here https://d1sjrnpi226dnf.cloudfront.ne...pdf?1382391745 and the other I will attach. On the tubedepot layout it is the resistor on t47 and t51. I used ClassicTone trannys for JTM45. They have a tranny kit.

The voltages are as follows with no tubes and power on:
V1 pins 4 & 5 3.45AC, pin9 3.45AC
V2 pins 4 & 5 3.45AC, pin9 3.45AC
V3 pins 4 & 5 3.45AC, pin9 3.45AC
V4 pin2 3.45AC, pin7 3.45AC
V4 pin2 3.45AC, pin7 3.45AC
V5 across pin 2 & 8 5VAC

In image img_1661.jpg you can see the burnt resistor.

I also attached a number of photos in my profile album

[stevewdewitt]

Here is the other layout. I used this one because it has the same trannys that I have.

[stevewdewitt]

Resolved the issue. I pinched the screen/grid resistor wire from pin4 of V5 between the metal standoff and grounded it. Found it by doing continuity checks on everything. Couldn't see it from the top of the board.
Thanks

[big_teee]

Glad you found your problem.
One more question.
If you used the Transformer bundle kit for a JTM45?
It looks to come with a Classic Tone 40-18039 OT.
If that is the one you used it has a 8k primary impedance for KT66 tubes.
The normal Classic Tone OT for EL34s is 3.6k impedance part # 40-18025 .
So which did you get?
The Triodestore JTM45 layout is for KT66 Tubes.
T

[stevewdewitt]

Glad you found your problem.
One more question.
If you used the Transformer bundle kit for a JTM45?
It looks to come with a Classic Tone 40-18039 OT.
If that is the one you used it has a 8k primary impedance for KT66 tubes.
The normal Classic Tone OT for EL34s is 3.6k impedance part # 40-18025 .
So which did you get?
The Triodestore JTM45 layout is for KT66 Tubes.
T

I did get the one that uses the 8k primary. Any advice there? Should I use the KT66 tubes?

I have gotten it working and biased. It is a noisy thing. A
little squelching on the bright channel as well with no guitar connected it I turn it all the way up and the presence all the way up.

The light on the current limiter seems to be affected by the bias pot. I had it all the way down and the light was out. Adjusted correctly it is shining a bit.

[big_teee]

If it is a 4, 8, 16 ohms at 8k.
I think it would be 2, 4, 8 ohms at 4k.
Not sure what Ohm speaker cab you have?
T

[stevewdewitt]

If it is a 4, 8, 16 ohms at 8k.
I think it would be 2, 4, 8 ohms at 4k.
Not sure what Ohm speaker cab you have?
T

I have a 4 Ohm cab

[big_teee]

I have a 4 Ohm cab

You would use the 8 ohm switch position to match 4k primary output tubes to a 8K OT.

[stevewdewitt]

You would use the 8 ohm switch position to match 4k primary output tubes to a 8K OT.

My other options would be:
1. different OT
2. KT66 tubes

correct?

thanks so much. I may go with the other tubes. I nearly bought the KT66s to start with.

[big_teee]

My other options would be:
1. different OT
2. KT66 tubes

correct?

thanks so much. I may go with the other tubes. I nearly bought the KT66s to start with.

That would be the best solution.
If you want to run EL34s for now just use the 8 ohm tap, for your 4 ohm cab.
The layout with the 1k resistor with the 2 470 resistors is the KT66 way.
The modern way for EL34s is the two 1k 5w resistors like the 1987 circuit.
It is this one.
http://site.triodestore.com/50WPlexiLayout.pdf
They have done a lot of combining layouts.
You have it under control.
Good Luck
T
** Edit note
between the JTM45 and the 68 plexi (1987 Circuit) was the JTM50.
It used the JTM45 preamp with a tube rectifier and EL34s with the 3.6k OT.
It is this one. Notice it uses the 5w 1k screen resistors.
http://site.triodestore.com/JTM50layout.pdf

[stevewdewitt]

Glad you found your problem.
One more question.
If you used the Transformer bundle kit for a JTM45?
It looks to come with a Classic Tone 40-18039 OT.
If that is the one you used it has a 8k primary impedance for KT66 tubes.
The normal Classic Tone OT for EL34s is 3.6k impedance part # 40-18025 .
So which did you get?
The Triodestore JTM45 layout is for KT66 Tubes.
T

So I got some KT66s and sounds good. I took it out this evening and had it on standby for an hour or so then turn SB off and the fuse popped. Common OR problem? I have built 5 amps in the last year, none so ambitious as this one, and haven't had that happen yet under the same circumstances.

I know this could be more than just a fuse.

[stevewdewitt]

So I got some KT66s and sounds good. I took it out this evening and had it on standby for an hour or so then turn SB off and the fuse popped. Common OR problem? I have built 5 amps in the last year, none so ambitious as this one, and haven't had that happen yet under the same circumstances.

I know this could be more than just a fuse.

Ok, wrong size fuse. I put a 2A in the 3A position between pwr switch and PT.
Thanks