New amp build finally started. JTM45 with KT66.

SG John said:
According to my past notes from other builds and repairs with comments by IvN and Plexi, that is perfect. It’s the stuff in the yellow frame of my last photo and the filter cap that are off.

One of the lugs on the filter cap has wires coming from the choke, B+ center tap of output transformer and standby switch(my notes are downstairs, so I may be wrong).

That is too high. I’ll ring stuff out again tomorrow and look under the turret board.
Everything else seems to be ok. I’ll run it for brief periods to get some numbers after I check the turret board.
Click to expand...
I'm sure you'll get it figured out. Seems like you're surrounded locally by enough knowledgeable friends that want to make sure that you have a successful build.
 
syscokid said:
I'm sure you'll get it figured out. Seems like you're surrounded locally by enough knowledgeable friends that want to make sure that you have a successful build.
Click to expand...

It was funny today. Scott was saying because I did such an impeccable job of wiring and soldering, it is more difficult to troubleshoot because everything looks perfect.
 
Hi John. I apologise for not being here of late, I've been recovering from an accident.

Almost 700V at the main filter & 560V at the pre-amp filter is awfully high.
Can I ask what your heater supply (AC) voltage is?
You can take these readings either directly at the PT or any valve socket.
Reading from either "side" of the supply to ground (or Center Tap), it should be around 3.15 VAC "per side", or, reading "across" the whole supply (from 6.3 to 6.3 on the PT, where the indicator lamp is connected) it should read 6.3VAC.
Both these figures are for "tubes fitted" (with no tubes fitted the readings will be a little higher).
This will let us know if you are using the correct primary tap (voltage selector tap) on the PT.
Chris's PT's usually have a 100VAC tap on them. Putting 120VAC into that 100VAC tap would result in high voltages like that what you are seeing. It may not be the problem, but best being sure.
It does look very good, a great job for a first build. Hopefully we can get it all sorted, it would be a shame to damage any of those fine components. Cheers
Edit:
No "old glass" fitted during the de-bugging period please. Cheers
 
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ivan H said:
Hi John. I apologise for not being here of late, I've been recovering from an accident.

Almost 700V at the main filter & 560V at the pre-amp filter is awfully high.
Can I ask what your heater supply (AC) voltage is?
You can take these readings either directly at the PT or any valve socket.
Reading from either "side" of the supply to ground (or Center Tap), it should be around 3.15 VAC "per side", or, reading "across" the whole supply (from 6.3 to 6.3 on the PT, where the indicator lamp is connected) it should read 6.3VAC.
Both these figures are for "tubes fitted" (with no tubes fitted the readings will be a little higher).
This will let us know if you are using the correct primary tap (voltage selector tap) on the PT.
Chris's PT's usually have a 100VAC tap on them. Putting 120VAC into that 100VAC tap would result in high voltages like that what you are seeing. It may not be the problem, but best being sure.
It does look very good, a great job for a first build. Hopefully we can get it all sorted, it would be a shame to damage any of those fine components. Cheers
Edit:
No "old glass" fitted during the de-bugging period please. Cheers
Click to expand...


Hey Ivan. I hope you're ok, and not hurt bad.

As for the amp, what I got at the transformer was:

6.3 (1) = 3.01
6.3 (2) = 3.01
Together, they were 6.03VAC

My high voltage windings (pins 4&6 GZ34) were:
329 VAC
328 VAC
Together 660 VAC

5V = 343 VDC
5V = 343 VDC

According to the drawing that Chris sent me, the BLACK wire is 115 VAC. White is 100VAC.

Don't worry, only the JJ 12ax7s and Gold Lion KT66s were used.
 
I found that the 100K resistor between pins 1, 6, and 7 of V2 was loose at pin 1. It must have dropped when I soldered the wire from the turret board there. Now that I've fixed that, all my voltages are within 1-45 volts depending on what I'm checking. There is noise coming from pin 2 of V2 when I touch it with a probe. Everything under the board is secure, just checked it.

It works, but not as loud as it could be.

Calling it a day, and will look at it further tomorrow.
 
SG John said:
There is noise coming from pin 2 of V2 when I touch it with a probe
Click to expand...
Is it quiet when you don't touch it? Cuz, if it is quiet, just don't touch it anymore. But seriously...
Pin 2 (&7) are the very sensitive grids of the preamp stages. Some noise is normal when making contact with the grid pins. Unless you're describing a totally different type of noise... :hmmm:
 
SG John said:
Hey Ivan. I hope you're ok, and not hurt bad.

As for the amp, what I got at the transformer was:

6.3 (1) = 3.01
6.3 (2) = 3.01
Together, they were 6.03VAC

My high voltage windings (pins 4&6 GZ34) were:
329 VAC
328 VAC
Together 660 VAC

5V = 343 VDC
5V = 343 VDC

According to the drawing that Chris sent me, the BLACK wire is 115 VAC. White is 100VAC.

Don't worry, only the JJ 12ax7s and Gold Lion KT66s were used.
Click to expand...
Thanks for the concern SG John. No major injuries, just knocked around & quite sore for a period.

Glad you got the voltages sorted.
The voltages you quoted are all within spec.
The rectifier tubes heater winding 5V = 343VDC is about correct for when the amp is on standby.
When the amp is switched off standby this should rise to about 5V = 450VDC (because the main filter cap is now hooked up to it). This voltage can be read at the rectifier (pin 8), the main filter cap or the standby switch.

Touching your meter probe to many points in the circuit, or on valves socket pins while the amp is on will result in all sorts of popping & buzzing noises, & can be especially loud, as Sysco noted if you touch pins 2 or 7 of any pre-amp tubes, also pins 5 of the power tubes as these are control grid pins.

I'm not sure how low the output volume is, but I'm imagining that you have not yet set the power tube bias & that the negative bias voltage is set to the maximum available negative voltage (tubes biased cold)??
The output volume will increase some as the output tubes are biased hotter.

Waiting until you are rested & clear headed before getting back to it is a good idea. Cheers
 
Hey Ivan, feel better!

Very nice, neat build SGJohn!
I know you have sound but the probing noises brings up:

Using the sound / no sound while chopsticking around grid or plate pins is also a valuable "divide and conquer" method if you have no throughput - your input jack signal is not reaching the speaker.

Starting at the output tubes and moving upstream through the circuit you will come to a point where you have probe sound, then you dont.
This helps isolate where the problem likely is.
 
jtcnj said:
Hey Ivan, feel better!


I know you have sound but the probing noises brings up:

Using the sound / no sound while chopsticking around grid or plate pins is also a valuable "divide and conquer" method if you have no throughput - your input jack signal is not reaching the speaker.

Starting at the output tubes and moving upstream through the circuit you will come to a point where you have probe sound, then you dont.
This helps isolate where the problem likely is.
Click to expand...
Thanks JTCNJ.

I have used this simple method using the multimeter probe (a "pop" test) to locate the faulty stage in amps that aren't passing signal for many, many years. Dead simple & a sure way of locating the problem area of the circuit.

One thing that I didn't think of yesterday SG John, that "may" be causing low output volume. V2 "may" have been damaged, depending on exactly which socket pin that the 100k resistor wasn't connected to. That's not to say that it is damaged though. Cheers
 
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ivan H said:
Thanks for the concern SG John. No major injuries, just knocked around & quite sore for a period.

Glad you got the voltages sorted.
The voltages you quoted are all within spec.
The rectifier tubes heater winding 5V = 343VDC is about correct for when the amp is on standby.
When the amp is switched off standby this should rise to about 5V = 450VDC (because the main filter cap is now hooked up to it). This voltage can be read at the rectifier (pin 8), the main filter cap or the standby switch.

The output volume will increase some as the output tubes are biased hotter.

Waiting until you are rested & clear headed before getting back to it is a good idea. Cheers
Click to expand...

Thanks for the info. I've been at it a couple of hours now. One other thing I noticed last night was that the "Z" with the wiring at the input jacks was also incorrect. Got that sorted. Next, I went through all nine 12ax7 tubes that I have. Got something quieter in V1 and V2, and the best one I could come up with for V3 was an 8.66 Volt difference.

When I go through the voltages, my 5V tabs are now 384VDC in standby, then drops to 358VDC when I turn on the amp out of standby mode. I also get that at pins two and eight of the rectifier, standby switch, and the 32/32uF filter cap. Where I had so many voltage issues, could I have fried the filter cap?

My plate voltage is 347VDC, which seems too low to me. When you do the math for plate dissipation, you end up with a bias value of about 48.07 mA. Shouldn't it be down in he 30-40mA range?

I've tried both Mullard/RCA GZ34 rectifier tubes that I have, and get all the same numbers.



ivan H said:
One thing that I didn't think of yesterday SG John, that "may" be causing low output volume. V2 "may" have been damaged, depending on exactly which socket pin that the 100k resistor wasn't connected to. That's not to say that it is damaged though. Cheers
Click to expand...


That tube is now in the "bin", along with another. I'm thinking of grabbing a few more new ones just to compare values. I have eight assorted JJ, Sylvania, Baldwin, and GE 12ax7s around. Also have some GT and Sovteks, but the glass seems slightly larger and is tough to get them in the sockets.
 
67plexi said:
John, Pin 8 V-2 I hope that's 1.46 Volts not 146 Volt's
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No, it's 146.

The Mojo drawing says it's supposed to be 209V feeding pin 8. It's coming off the turret with the 100K resistor and 250pF capacitor with the 56K resistor bridged over from the .022uF caps.
 
syscokid said:
Aren't those Merren PT's for the JTM45 suppose to be putting out 450vdc for the B+...?
Click to expand...


It looks like I smoked the output tubes. Both KT66s have black circles on the sides of the bottle, and my B+ is now 470 volts.

Time to order some more tubes. I don't care if I kill Gold Lions, I just don't want to kill the 1950s GEC KT66s that I have.
 
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