Dual 50 Plexi/JCM 800

[purpletele]

Put all tubes back in, disconnect the NFB wire, and recheck voltage on pin 3 of each power tube.

With NFB disconnected I get 409 V and 407 V

I put it on standby to calculate the bias, set up the meter and now I get an indication of a short from the light limiter.

[sluckey]

How does the amp sound with NFB disconnected?

[purpletele]

How does the amp sound with NFB disconnected?

I didn't get there, I wanted to set the bias and then play it.  I switched to Standby and clipped my lead onto my 1 ohm resistor and the light limiter started glowing a lot brighter than it had been.

I switched everything off to contemplate

[sluckey]

Which OT do you have? Be specific. Post a link.

[purpletele]

Which OT do you have? Be specific. Post a link.

I flipped the switch and let the light limiter absorb the rush, it settled down to a normal low glow but it startled me, Shocked would be the wrong word.

As the amp was warming up it jumped in and out of a solid signal and settled to a strong quiet idle.

The amp sounds and plays like it is within the right range of power and touch.

The JCM side sounds real Ice Pickey, the JMP side sounds good, I will try with a single coil and I can tell if the tone is right or thin

I posted the Classictone 40-18025 below.  I don't think i'm shorting out, but I spooked for a second.

[sluckey]

You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.

[purpletele]

You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.

The black wire is connected to a speaker ground only, I will extend the ground from the jack to the power ground.

I dropped some voltage where everything is at 395 V now.  I described the sequence in the note above.

I'll reconnect the NFB and see what happens.

[sluckey]

I described the sequence in the note above.

Huh? What? Where?

[purpletele]

You must have the black wire on the secondary connected to chassis ground. I suggest using the power ground point. Reconnect the NFB wire. If the amp screams then reverse the OT red and blue plate leads.

The grounding on the power ground helped with the surge.

The amp is back at 1 volt or less at the plates

**Correction, plate voltage is jumping in and out.

397 V at the fuse OT connection 397 V at the plates. 

[sluckey]

Plug the amp straight into the wall. You don't need the lamp limiter now. Did you swap the OT plate leads? And?

[purpletele]

Plug the amp straight into the wall. You don't need the lamp limiter now. Did you swap the OT plate leads? And?

Oh man did it yell at me!!

[sluckey]

Swap the plate leads again.

[purpletele]

Swap the plate leads again.

I must have a bad connection that I cannot find.

After I reset the Plate Leads I was getting 427 V.  I played it for a while and then it dropped out while I was on the JCM side.  It went to 390 V.  The JMP side stayed strong and deep but the JCM side was weak and thin. 

After all of that I was checking voltages and my Klein meter kept jumping to AC after I would get a low reading on the Plate. 

Funny stuff going on.  I get 1.5 volts on V6 and it goes right to AC mode when I hit pin 3 V4

This amp is winning tonight!  I am beginning to think the OT is failing.  I didn't have any event that would make me think I fried it, but it sure seems suspicious.

[sluckey]

Klein meter kept jumping to AC

I'm kinda beginning to suspect that meter. I've never had a meter to jump from dc mode to ac mode. every meter I've ever used required you to turn a knob to go from ac to dc. Maybe your Klein wants a fresh battery.

Tell me more about this Klein. Exact model number so I can look up the specs and manual.

EDIT... Never mind. I think your meter has a SEL button to toggle between AC and DC mode. This just sets some logic state in the meter brain. This logic state is being upset when you try to measure the plate voltage on this amp for some reason. If fresh batteries don't fix it, there is likely nothing you can do about that. Try another meter if you have one.

[Slimtim]

yep never had a meter go to ac from dc either.do you have a spare or borrow one from someone for a few quick tests?

[purpletele]

Klein meter kept jumping to AC

I'm kinda beginning to suspect that meter. I've never had a meter to jump from dc mode to ac mode. every meter I've ever used required you to turn a knob to go from ac to dc. Maybe your Klein wants a fresh battery.

Tell me more about this Klein. Exact model number so I can look up the specs and manual.

EDIT... Never mind. I think your meter has a SEL button to toggle between AC and DC mode. This just sets some logic state in the meter brain. This logic state is being upset when you try to measure the plate voltage on this amp for some reason. If fresh batteries don't fix it, there is likely nothing you can do about that. Try another meter if you have one.

I changed the batteries in my Klein meter.

Fired the amp up, here is the sequence:

1. Once it is warmed up I get 465 v on the plate.
2. Pull off and measure V5 Plate and the voltage is on a sequence voltage drop, but it goes from 465 to 1.25 Volts and then drops at a decimal at a time until it is less than a volt.

Then I get disgusted and turn it off.

I think I fried the transformer with the bias trim pot being backward.

[sluckey]

You gotta ask why that Klein only misbehaves when you have the NFB wire connected? Seriously, try another meter before you order an OT.

If I had that amp and also had a super duper 50W Plexi (probably with the same OT), I'd temporarily swap OTs.

[purpletele]

You gotta ask why that Klein only misbehaves when you have the NFB wire connected? Seriously, try another meter before you order an OT.

If I had that amp and also had a super duper 50W Plexi (probably with the same OT), I'd temporarily swap OTs.

I'll try another meter tomorrow. 

Said What??  My super duper Plexi is working great, don't want to disturb the mojo.

Thanks for the help

[purpletele]

Back In The Game

I had tested the plate voltage with another meter, but it was a cheapy.  It had just kind of redlined and didn't know how to react. 

I had to let it go for a few days, but I did order a spare.

I performed a test bench retrofit of a new OT, (I didn't mount it).  The amp warmed up and was in quiet idle at 400 V on V6 and 315 on V5.  That didn't look promising.

I disconnected the NFB and I have a steady 410 V at each plate and the bias is steady at 44 mV.

Summary:  I am not sure that the OT is fried, but the new one that is hooked up temporarily is at least holding a voltage while the NFB is connected.

NFB: My first thought is do I have the bias system hooked up correctly.

Bias: At 410 V I set the bias at 43 mV but I am bottoming out on the low end adjustability on the bias system.

Tone: The amp plays well on both channels.  The tone for each channel is reflective of its design.  I didn't have a chance to turn up the MV, but at lower levels the amp sounded distinctly like a JCM 800 and a 1987x and it sounded nice.

The amp feels like it is operating at 92%.  The NFB is obviously an issue.  I have posted a new photobelow.

[DummyLoad]

short? see area circled in red in attachment.


--pete

[MFowler]

I don't like the looks of many of the solder joints on the turrets looks bare of solder to me.


Mark

[sluckey]

Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.

[purpletele]

short? see area circled in red in attachment.


--pete

Thanks Pete,

Those turrets are close, but I can slide a straight edge between the gap, so nothing is touching.

[purpletele]

Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.

Thanks, I'll try that this afternoon.

The amp is sounding nice, it's almost there.

BV

BV

[purpletele]

Your 1Ω bias resistors mounted on the sockets don't appear wired correctly. The correct way to wire them is to jumper pins 1 and 8 together. Then connect the 1Ω resistor between pin 8 and chassis ground (preferably use the power ground point). Do this for each tube socket.

Thanks, I'll make that change now.

The amp is sounding nice, it's almost there.

BV

BV

[purpletele]

Afternoon Tests

I changed the 1 Ohm resistor for V5 and V6, I used a solder post to tie the ground and the 1 ohm resistors together.

Things are still wonky:

1. With the NFB disconnected the voltage starts at high 300's and bleeds voltage like a metronome.

2. If you play it when it first heats up is sounds good, after a few minutes it is real tinny and you can tell the voltage has dropped.

3. When I measure the voltage after playing it is really low and continues to drop decimals of voltage while powered up.

4. If I start up with the NFB connected, then the voltage is low to start, mid 300's and it drips away like a faucet.

Photos of the grounding on the power tubes.

Thanks for any ideas.  It took me a while to dream up this issue.

[shooter]

with the NFB

the dripping, and metronome are with your ears, or meter?
without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB

[purpletele]

with the NFB

the dripping, and metronome are with your ears, or meter?

I have a meter on it and visually watching the voltage drop.

without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB

I'll test it.

[purpletele]

with the NFB

the dripping, and metronome are with your ears, or meter?
without any tubes in, monitoring your "last" tap VDC, what happens in 15min With NFB, and 15min Without NFB

Shooter I get the following:

514 V on the plates with the NFB connected

517 V on the plates with the NFB dis-connected

[shooter]

so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm.  did you fix/replace your sketchy meter?

[purpletele]

so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm.  did you fix/replace your sketchy meter?

Busy day at the Jungle today.

I'll do the steps as suggested.  I changed the batteries in the Klein. 

I was looking at the following as a second meter:

https://www.amazon.com/SSEYL-FLUKE-Digital-Multimeter-F17B/dp/B00SF3SQ50/ref=sr_1_fkmrnull_1?crid=3LFK3V3IKEV6E&keywords=fluke+17b%2B&qid=1552692307&s=gateway&sprefix=fluke+17b%2Caps%2C205&sr=8-1-fkmrnull

These meters look like they are built in Asia somewhere and built for the Amazon market.  Any other recommendations?

[purpletele]

so put all the pre tubes in and repeat
if it's steady, add the PA tubes, repeat, somebody, or some thing is sucking current and it should be getting mighty warm.  did you fix/replace your sketchy meter?

493 V NFB Connected Pre-Amp Tubes installed   Voltage is steady and not dropping

495 V NFB Dis-Connected Pre-Amp Tubes Installed. Voltage is steady and not dropping.

475 V NFB Connected Power Tubes installed.  Voltage drops like a Zeppelin

475 V NFB Connected Power Tubes Installed. Voltage drops like a Zeppelin.   

BV

[shooter]

It's time to beg, borrow or buy another meter to be sure, same check.

If it still drops without NFB, I'd be inclined to swap out the tubes, then the filter cap feeding the PA tap.

[sluckey]

I'm suspecting some HF oscillation when NFB is connected is confusing his meter. There's just no way for voltage to really drop that much without seeing some smoke somewhere. Probably need a scope to prove/disprove my suspicion.

[shooter]

without seeing some smoke somewhere

that's why I put in the 15min disclaimer 
I suspect you're correct since he didn't mention any smoke

[purpletele]

without seeing some smoke somewhere

that's why I put in the 15min disclaimer 
I suspect you're correct since he didn't mention any smoke

No smoke, and I let it run for at least 15 minutes.

How did I get a case of High Frequency Oscillation and what should I do to correct that?

[shooter]

High Frequency Oscillation

you know the gremlin with the spiked hair smoking a cig, that's the guy you're after 

typical, positive feedback, induced signals, bad DC filtering, harmonic tubes.  I've chased him a few times, he's a B ch to kill 

[DummyLoad]

without seeing some smoke somewhere

that's why I put in the 15min disclaimer 
I suspect you're correct since he didn't mention any smoke

No smoke, and I let it run for at least 15 minutes.

How did I get a case of High Frequency Oscillation and what should I do to correct that?

a shot in the dark: try removing the plate to plate (47pF SM) cap from the LTPI - it's the cap connected from pin 1 to pin 6 on the LTPI tube.

you may have a high(er) level preamp signal in proximity to a lower level preamp signal and they are relatively in phase.

an oscilloscope really helps to troubleshoot these kinds of issues.

pull the the LTPI tube and power up - is B+ stable? if yes, then LTPI or something before the LTPI is the issue. if B+ is not stable, then the issue is with the output stage. 

--pete

[purpletele]

a shot in the dark: try removing the plate to plate (47pF SM) cap from the LTPI - it's the cap connected from pin 1 to pin 6 on the LTPI tube.

you may have a high(er) level preamp signal in proximity to a lower level preamp signal and they are relatively in phase.

an oscilloscope really helps to troubleshoot these kinds of issues.

pull the the LTPI tube and power up - is B+ stable? if yes, then LTPI or something before the LTPI is the issue. if B+ is not stable, then the issue is with the output stage. 

--pete

Pete,

I get the 475 V and then it drops immediately as before.

Thanks

BV

[DummyLoad]

what does it drop to - e.g. bottoms out at?


--pete

[purpletele]

what does it drop to - e.g. bottoms out at?


--pete

The voltage dropped quickly to 335 V.

It has stayed at 335 V for about 4-5 balls in the water at 17.  (15 -20 minutes)

BV

[purpletele]

I'm suspecting some HF oscillation when NFB is connected is confusing his meter. There's just no way for voltage to really drop that much without seeing some smoke somewhere. Probably need a scope to prove/disprove my suspicion.

How do you resolve such a matter and what would create a HFO?

[sluckey]

I don't know that it's even an issue. It may only occur when you put your meter probe on the plate socket pin. I've had amps that would emit a screeching/arcing sound when my meter was connected to the plate of one tube but would behave quite well when connecting to the other tube plate. Never resolved that. Just said to self, don't do that.

So, other than this plate voltage issue, how does the amp behave otherwise? Sound good, bad, or ugly? Any red plates?

[shooter]

Just said to self, don't do that.

yup, same!  My 100X probe ALWAYED messed up plate readings.  I just made my I readings via Rk and accepted the math, and audio measurements at speaker.

[purpletele]

I don't know that it's even an issue. It may only occur when you put your meter probe on the plate socket pin. I've had amps that would emit a screeching/arcing sound when my meter was connected to the plate of one tube but would behave quite well when connecting to the other tube plate. Never resolved that. Just said to self, don't do that.

So, other than this plate voltage issue, how does the amp behave otherwise? Sound good, bad, or ugly? Any red plates?

The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.

[shooter]

then it drops voltage

are you wired like the layout on page 1, reply 8?  specifically the "A" cap?
If so can you clip your meter + on cap, - on chassis (Not a stud, or ground point, just gator-clipped to chassis metal), set for VAC and watch it from power on, for ~~~~5min or so

[Willabe]

The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.

Is this happening with your meter hooked up?

Or is it getting thin sounding and your thinking it's because the voltage has dropped?
 

[purpletele]

The amp sounds decent for maybe a minute, then it drops voltage and gets real thin and tinny.

Is this happening with your meter hooked up? Or is it getting thin sounding and your thinking it's because the voltage has dropped?
 

It does happen when the meter is hooked up.


I can also not be on the meter and just play it after it warms up, it sounds full then it shifts gears and feels real thin and tinny, that timing is consistent with the meter readings of the voltage.


Thanks for thinking about it.

[purpletele]

then it drops voltage

are you wired like the layout on page 1, reply 8?  specifically the "A" cap?
If so can you clip your meter + on cap, - on chassis (Not a stud, or ground point, just gator-clipped to chassis metal), set for VAC and watch it from power on, for ~~~~5min or so

I thought that would be the next suspect.  I'll run the test and get back.

Thanks for the input.

[shooter]

It does happen when the meter is hooked up.

IS this what doesn't happen;

feels real thin and tinny

in other words; it works fine without meter?????