JTM45 Sounds beautiful then Main Fuse blows!

Discussion in 'Amp Tech Center' started by koochems, Feb 19, 2018.

1. petebFriend of Leo's

Apr 25, 2003
That's right, Clint points out that you have measured the screen current directly, even though a little bit indirectly and it does check out, 2 mA is alright.

Better numbers will help.

Because right now the cathode current does not equal the screen current added to the plate current.

Until better numbers are found, it looks like more current or electrons are entering v5 at the cathode than electrons are leaving v5 at the screen and plate combined.

And that would make problems for the fuse.

Last edited: Mar 11, 2018
2. SnfoilhatTele-Holic

Age:
37
Apr 8, 2016
Oakland, CA

AC power supply stuff

5VAC winding out of 2A total:
1.9A GZ34

6.3VAC winding out of 5A total:
2.6A 2@KT66
0.9A 3@12AX7
____________
3.5A

HT power supply stuff
B+ estimate 345V x 1.4 = 483V. 483V - 20V (rectifier) = 463V @ 150 mA

Cathode current estimate = 28 mA + 34 mA + 2 mA + 2 mA + 2 mA= 68mA

I don't understand how the people at Metroamp came up with target plate voltages of 400VDC if this is the power transformer they recommend. So setting that number aside, I think all of your voltages are too low.

Your amp is drawing 95% of the 5VAC supply, and only 70% of the 6.3VAC, and only about 50% of the HT. So your measured voltages should be even higher than the nominal 5VAC, 6.3VAC, and 463VDC.

To my understanding, that implies a problem bigger than just getting it biased correctly.

This PT has international primaries. Is there any chance you got the USA (120V) and Japan (100V) mixed up? Do you have a safety bleeder resistor installed that should be 220K but you misread the color bands and installed 2200? A shorted tube? Bad OT (I think someone mentioned this earlier)? Some crazy high wattage pilot light?

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3. koochemsTDPRI Member

Jan 19, 2016
denver
Well I've uncovered something a bit distressing... I have 2 16ohm greenbacks wired in what I remember parallel but it was so long ago they are actually wired in series! This increases the load to 32ohms when the amp impedance selector set to 8ohms! Sheesh, not sure if this is the root cause but will rewire the speaker cab in parallel to match the amps ohm setting. Hopefully this hasn't damaged the OT or speakers by now. Also, the magnetic components 40-18039 45 Watt OT is installed.

Voltage drop readings are as follows:
V4 = 0.780 VDC
V5 = 1.124 VDC

Measured across pins 4 and 6. These values provide lower plate current in turn higher screen current. How to get the screen current under control, hmmm?...

I have not tried swapping tubes to see if the spark followed. Is it still worth trying knowing my load is wired incorrectly and my screen current is higher than expected for each power tube?

4. clintjFriend of Leo's

Apr 4, 2015
Idaho
Get that load fixed first and foremost. Once that's done, see where you sit as far as tube numbers. The tube flashing over may have helped saved the OT windings from damage by limiting voltage, however the tubes may be damaged now.

5. petebFriend of Leo's

Apr 25, 2003

I was wrong, the screen current is fine.

I should have used your 1 or 2 volt drop from your voltage tables.

Because the screen resistor is 1k, the voltage is the milliamps.

1 volt is one mA, two is two, .78 is .78 and 1.124 is 1.124.

If the current thru the tube is 30 milliamps, one or two mA thru the screen is only 3 or 6 percent which neither are too high.

I agree get the load right.

6. koochemsTDPRI Member

Jan 19, 2016
denver
I rewired the speakers in parallel (load = 8 Ohms now) and took more readings. Most everything is the same! Still seeing 2.8AC in heaters... It sounds like this may be an issue? Shall I replace tubes with new and risk damaging in attempt to replicate the issue?

Only noticeable differences:

Cathode Current (measuring pin 8 with mV DMM setting:
V4 = 24.0mA
V5 = 30.8mA

Voltage drop across pin 4 and 6 with 8 Ohm load
V4 = 0.616 VDC
V5 = 0.992 VDC

OT resistance is a bit lower @113.1 and 104.0 Ohms. I Really appreciate the help all!!

7. clintjFriend of Leo's

Apr 4, 2015
Idaho
Are you running direct from wall power, or are you on a limiter right now? If it's wall power, pull all the tubes and see what unloaded heater voltage is. I'm hoping there's no transformer damage from the tubes arcing like they were.

8. petebFriend of Leo's

Apr 25, 2003
The numbers look fine, for idle, DC conditions.

Maybe the Ac is the problem??

9. clintjFriend of Leo's

Apr 4, 2015
Idaho
That impedance mismatch quadrupled the impedance seen by the power tubes, which does one very nasty thing. It drastically raises the AC voltage at the plates under load. At the low side, it could drop plate voltage to near zero easily. At the high side, plate would skyrocket to insane values and cause that light show the OP was seeing.

10. koochemsTDPRI Member

Jan 19, 2016
denver
The amp is plugged directly into wall power.

I removed all tubes and tested VAC with power ON and standby OFF:

3.0VAC supplied across heaters (should be 3.15VAC)
4.9VAC across pins 2 and 8 from the rectifier (should be > 5VAC)

Ugh... does this mean my power transformer sustained some damage since these voltages are lower than expected?

11. clintjFriend of Leo's

Apr 4, 2015
Idaho
Never saw an answer to snfoilhat. You do have the PT primary configured for your wall voltage, right? Blue as common, and the appropriate lead chosen for your wall voltage? (And what is your measured wall voltage?)

12. koochemsTDPRI Member

Jan 19, 2016
denver
I have the GRY, ORN, WHT, and BRN heatshrunk together on top the PT, with the BLK 120V routed to the mains switch, earlier build pic below. I do NOT believe I have a safety bleeder installed unless it was on the original schematic. Is this for amp safety, draining caps? Fairly confident the power tube isn't 100% as there were multiple arc flashes V5.

I still haven't tested with new tubes in, but to clintj's point I tested the unloaded heater voltages and they are lower than expected so it seems isolated to the circuit / transformers?

13. petebFriend of Leo's

Apr 25, 2003
I think that you have found that the amp works, even with the 32 ohm load.

Now that the speakers are rewired, you should have fixed the root of the problems.

have you tested and played the amp with the rewired speakers?

If the heater voltage is a little low, the amp should still work.

didn't the amp sound beautiful with the filament heaters at 5.6V and the wrong load?

it can only get better.

14. koochemsTDPRI Member

Jan 19, 2016
denver
LOL. It did! I was so happy with the sound. I'll try and make her sing today after work. I'm sure the headroom, tone has to be drastically different now... I'll find out tonight!

15. koochemsTDPRI Member

Jan 19, 2016
denver
Replaced the old tubes with the new tubes and let them heat up. Took readings, and most all was the same except B+ is now 475Vdc (+10Vdc difference) and the cathode current is relative similar between the two power tubes - which make sense as the previous set were most likely damaged. I'm really happy with the sound! The tube pulsing is almost completely gone, I played the amp in a dark room to find any variation in the screen grid and there is the slightest bit when the amp is pushed but nothing concerning or out of the norm. Appears the load may been the biggest contributing factor. I'll for sure keep in eye on the amps performance going forward. Really appreciate all the feedback from this thread, I would not have thought to check my cab without your expertise. Learned alot! Thanks again!

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16. nasdakTele-Meister

Mar 22, 2013
france
i don't think the amp would work at all if heater and OT primary where really connected ?????

17. koochemsTDPRI Member

Jan 19, 2016
denver
Hi All, this amp has gone down once again. I actually gigged with it..... which is quite scary, cuz it made it through. I realized the normal channel was no longer working (no sound), so took a look and the power tubes were redplating. Something went wrong and I'm not sure, hopefully it is salvageable. I pulled out all tubes and started taking readings - I didn't even get to "standby on" before finding an issue.

Settings:
Amp Power ON, standby OFF, Rectifier Tube in, no other tubes installed,

Heaters V1 - V5 = 3.0AC
voltage across 2 and 8 for rectifier = 5.0AC
One side of Standby switch = 334VDC
Control grid V4&V5 pin 5 = 0.0VDC!

So clearly something is wrong with the control grid (measurement should be -45vdc) and I'm not sure what to do. I've looked at the board, resoldered some cold joints, made sure no shorts present... Any advice?

18. petebFriend of Leo's

Apr 25, 2003
Take the power tubes out until you track it down.

There must be a place where the bias voltage disappears.

19. dan40Tele-AfflictedSilver Supporter

Aug 19, 2015
Richmond Va
Start at the point where the ac voltage enters the bias circuit. After the diode, the voltage becomes dc so set your meter accordingly. Take readings along the entire bias circuit until you reach the control grids. You will likely find a bad capacitor or diode that is causing your voltage loss.

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20. wanderin kindTele-Meister

Age:
64
Apr 24, 2018
Olympia WA
check the polarity of the caps in the bias circuit, they need to be installed "backwards" that is, positive to ground instead of negative. if your tubes arced out again, they may have taken out the diode, so check that also. measure DCR from bias voltage node to ground. what do you read?

remove feedback wire from output secondary. you might have your output transformer leads swapped. Classic Tone is notorious for bogus color codes and phase swapping. this is probably not the case as you would have heard a deafening squeal if was backwards, but at this point, you want to run down any suspects.

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