Plexi 50 troubles

[uki]

Found a 100k 1/2W resistor for R36.

Replaced the balance resistors , 100k in place.

Bias range is now -34 to -26.

With the probes of the meter on each pin 5 of PA tubes, V4 read -5v and V5 read 6v .

Could increase balance resistors a bit , decrease R6?

[sluckey]

With the probes of the meter on each pin 5 of PA tubes, V4 read -5v and V5 read 6v .

I don't think you understand what I mean. Put the red probe on pin 5 of a tube. Put the black probe on pin 5 of the other tube. There will only be one reading.

Without moving any probes, turn the balance pot max CCW. What is that reading?

Next, without moving any probes, turn the balance pot max CW. What is that reading?
 

[uki]

I don't think you understand what I mean. Put the red probe on pin 5 of a tube. Put the black probe on pin 5 of the other tube. There will only be one reading.

Without moving any probes, turn the balance pot max CCW. What is that reading?

Next, without moving any probes, turn the balance pot max CW. What is that reading?

That is what I did , one probe(red) on V4 pin 5 and the other probe(black) on the V5 pin 5. Both probes at the same time on each tube.

With the balance pot in the center I got zero volts, rotating to one end the reading was -5 volts then to the other side it did increase until +6.

I'm guessing that isn't the readings you where expecting? By the way, no tubes on the amp, atm.

[sluckey]

With the balance pot in the center I got zero volts, rotating to one end the reading was -5 volts then to the other side it did increase until +6.

That's good. I misunderstood. What value are the two extra resistors? What value is R36? And with the balance pot set to center, what voltage do you measure from pin5 to chassis for each tube ?

[uki]

With the balance pot in the center I got zero volts, rotating to one end the reading was -5 volts then to the other side it did increase until +6.

That's good. I misunderstood. What value are the two extra resistors? What value is R36? And with the balance pot set to center, what voltage do you measure from pin5 to chassis for each tube ?

The two extra resistors for the balance are 100k, R36 is 120k(100k 10% nominal), the voltage at pin5 to ground about -30v ish bias pot was in the middle when I did the readings.

[sluckey]

the voltage at pin5 to ground about -30v ish bias pot was in the middle when I did the readings.

Need more negative voltage. For EL34s you probably need the bias adjust to swing between -30 and -45. How much swing do you have now?

[uki]

Need more negative voltage. For EL34s you probably need the bias adjust to swing between -30 and -45. How much swing do you have now?

Replaced R36 for 82k

Now I got -46 to -35 bias range

Bias balance range from both pin 5  -7 to +8
Balance resistors still 100k.

[sluckey]

Sounds like you're ready to put some tubes in it, bias and balance and close the book. 

[uki]

Sounds like you're ready to put some tubes in it, bias and balance and close the book. 

I was just thinking about that!   

I still need some beefier resistors, the one at R36 atm, is just 1 or 2 watts it is half of the size of the original one(220k 3W).
And the balance resisitors are only 1/4W.

[sluckey]

How's the bias mod working out?

[uki]

How's the bias mod working out?

The balance pot works great !

New Mullard tubes drain more voltage.

Did read all voltages, and I got readings from before the PA tubes replacement and bias mod.

Before:
V1                   V2              V3               V4               V5
P1 151            164            252              P3 464        464
P2 0                0                17                P4 460        459
P3 1                1                28                P5 -44         -44
P6 203            283            242              P6 -44         -44
P7 0                164            18
P8 2                165            28

Now:
V1                   V2              V3               V4               V5
P1 146            159            208              P3 459        459
P2 0                1                20                P4 453        454
P3 0                0                35                P5 -47         -43
P6 196            270            220              P6 -47         -43
P7 0                159            22
P8 1                160            35

cathode resistors:
V4 .021mA
V5 .021mA

Slowly turn bias from -53V toward -30V. Watch the milliVolts on the 1 Ohm resistors. For now, stop at 20mV (=20mA), which is on the cold side, but *safe*, and should play OK with slight hoarseness in note decays.

35mA may be a good final goal, but I would play it a long long time at lesser current to shake all the bugs out. Those vintage parts may resent being re-awaken after a decade of sleep.

Talking about bugs...amp still doing the volume change by itself, but it only happen after some amount of volume is open, start at the position where the OD begin, volume pots at about 9o'clock.   


Found this info:

1.) When the drive power, using a steady carrier, is slowly increased the grid current (either screen or control grid) will at some drive level suddenly rapidly increase. The sudden rapid grid current increase will be disproportionate to the plate current or drive power increase! DO NOT go past the point where grid current starts to rapidly increase with small changes in drive power level.

Could this be the problem?

[PRR]

> Found this info:

That's a radio transmitter. A very different thing.

Sudden jumps at a certain combination of gain and treble suggests oscillation.

[uki]

Sudden jumps at a certain combination of gain and treble suggests oscillation.

How to find out if that is the case? Can transformer miss match cause oscillation ?

[shooter]

a scope is your best tool for chasing oscillations.
when you say;

amp still doing the volume change by itself

does it change "both" ways?  increase and decrease, or only decrease?

[uki]

No scope, don't even know how to use it   

Before replace the PA tubes it was going down and back up, now it is going up and back down. I though maybe the OT is miss match    Gotta fix this, PA tube does a little light inside when I hit chords. Dunno if it is normal.

The other day you said to read resistance from caps, if infinity it is good, if there is resistance it means it is leaking? replace ?

[shooter]

depending on cap, meter, circuit, you might get infinite, or high (>100k).  the best way to check leaky caps in signal path, lift the "right" (upstream) side and check for VDC, should be really low, like mV or less.
PS filter caps should "hold" a charge during amp idle,  They shouldn't lose volts much if any
keep taking your bias down a smidge, the last amp I set bias on landed very cool, as I hammered the strings there was a "perceived" volume increase. I considered that normal since the tubes were now screaming at the large "jump" in drive signal 

[shooter]

don't even know how to use it

here's some reading when your fingers get to sore to play 

https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/all

[uki]

Can't afford an Scope at this time,

How bad it can be if the OT/speaker isn't working on the expected impedances/miss match in relation with primary impedances, or load resistance?

I think it maybe related to the whole problem with the amp, not just that
Here is why, I got the calculations for OT impedance ratio. Going to do it all over to make sure.

32 * 32 = 1024    1024 * 4Ω = 4096Ω
25 * 25 = 625     625  * 4Ω = 2500Ω
19 * 19 = 361     361  * 4Ω = 1444Ω

32 * 32 = 1024    1024 * 8Ω = 8192Ω
25 * 25 = 625     625  * 8Ω = 5000Ω
19 * 19 = 361     361  * 8Ω = 2888Ω

32 * 32 = 1024    1024 * 16Ω = 16384Ω
25 * 25 = 625     625  * 16Ω = 10000Ω
19 * 19 = 361     361  * 16Ω = 5776Ω

The amp have connected to it in the 8ohm output(tap) a 8ohm speaker.
25 * 25 = 625     625  * 8Ω = 5000Ω

Looking at the tube data sheet from Mullard, tubes working at 450 , load resistance is 7k.
B+ atm 459

So right  now the amp is working with round  5000Ω

But following the chart and math(if I did it right)

7k / 625 = 11.2k

25 * 25 = 625     625  * 16Ω = 10000Ω   so I should be using a 16Ω speaker(s) in the 8Ω tap for better match , is that right?

Im trying to eliminate possible problems to get the amp fixed.

[shooter]

IF I'm reading you correctly;
Ideal based on the datasheet I looked at confirms your 7K @450
you're at 5K.  What OT are you using?  datasheet?
going to 10k via 16ohm tap appears to me to put you ~~ the same distance from 7k, only on the other end of the bell curve.
shouldn't be any worse/better than 5K, so try it, WATCH it, especially the current.  I like watching the current while playing, then back off (mo neg) idle bias accordingly

[PRR]

Bring it to the threshold of acting funny. Get a plastic chop-stick. Move wires around. Does it get better or worse?

Monitor grid voltages. When a tube oscillates its grid will usually go strongly negative.

Post pictures. Maybe someone will see that the pink wire is too close to a preamp socket.

[uki]

What OT are you using?  datasheet?

  unknown   got it from an old hi-fi amp
No markings on it.

I have the feeling the OT isn't for EL34   

  so I should be using a 16Ω speaker(s) in the 8Ω tap for better match , is that right?

Oops I got that backwards:   8Ωspeaker in the 16Ω tap, how about that, closer to 7k yes? Or bad idea ?

32 * 32 = 1024    1024 * 8Ω = 8192Ω

WATCH it, especially the current.  I like watching the current while playing, then back off (mo neg) idle bias accordingly

How do I do it? across the cathode resistors? PA tubes go brighter inside while playing is that normal ? I never really paid attention to it before.

Bring it to the threshold of acting funny. Get a plastic chop-stick. Move wires around. Does it get better or worse?

Monitor grid voltages. When a tube oscillates its grid will usually go strongly negative.

Post pictures. Maybe someone will see that the pink wire is too close to a preamp socket.

Will do sir , I got pictures right now, can't do tests right now, I'll get it.

[pdf64]

...Looking at the tube data sheet from Mullard, tubes working at 450 , load resistance is 7k.
B+ atm 459...

Do you mean this info https://bms.isjtr.ro/sheets/129/e/EL34.pdf
If so, which set of operating conditions are you looking at?
It's just that the 450V 7k table at the top of p2 is for cathode biased UL
Note that I think that the Vb refers to the HT supply at max (ie unclipped) output; hence a 'non class A' idle HT / plate voltage would be higher.
A plentiful degree of HT sag being what keep Marshals from killing their EL34 in short order 
5k is a pretty good load for any of the big octals, should get 40W without squeezing the tubes as hard as the ~3k5 normally used for EL34 in fixed bias.

[shooter]

How do I do it? across the cathode resistors?

yup, Fixed bias, hopefully you have 1ohmers.  I use 2 meters, I measure plate volts, NO signal, I get the no signal current, do the math, call it 9W/tube.  then start playing a "loop" (drives the wife nuts:) while watching the meters, I get a "feel" for average current. take that #, do the math, IF I'm going above max plate most of the time, lower bias a smidge, repeat until that "average playing current" falls closer to < max dissipation.

[uki]

Bring it to the threshold of acting funny. Get a plastic chop-stick. Move wires around. Does it get better or worse?

  Nothing changes while fiddling with the wires or components, I did poke all over the place 

Monitor grid voltages. When a tube oscillates its grid will usually go strongly negative.

Pins 5,6 voltages are dropping about 100v  from 450 ish to 350 ish.

Bias voltage drop to around -55 while playing.

reading across cathore resistors, it does some big peaks,
V4 from .021mV to .170mV
V5 from .021mV to .125mV   

when the sound volume goes up by itself, the current drop immediately
from those big peaks to about between .080mV and .090mV.

This all happen when strumming all strings really hard, amp volume at 9 o'clock.

Bias adjust pot is at maximum negative, which is -46

[PRR]

.

[st]

PRR's observation is pertinent--check and doublecheck whether that's the source of your problem.

If it isn't: when this problem happens, is the chassis in or on top of a speaker cabinet?

[uki]

The chassis was on the bench alway from speaker cab, while doing the tests. The problem happens where ever  the chassis is.

Oh by the way, I got some speakers on bench for the computer, the other day when I was reading voltages, while touching the filter caps positive pin with the meter probe, a scratch sound like was happening in the speakers for the computer, every time I did touch the caps, some huge magnetic field happened at that moment. The speakers were about 12 inches far from the chassis.

Here another two pictures from different angle.  Are the wires far enough from pots?

[st]

So it is not speaker-vibration induced.

Chasing oscillation without an oscilloscope is very hard. At the end of the day, it is more likely something that's too close to something else. PRR suggested tonestack (preamp) to output transformer, which seems like a good possibility. The only ways to find out  are by moving things around (wire/transformers ) or inserting temporary shields (piece of aluminum plate for instance, grounded to chassis with a clip lead, put between tone pots and output transformer wires, then pray it doesn't fall around and cause a fire ).

In the meantime, i keep thinking of other things you might want to eliminate. Here's one more:  Are the laminations of your output transformer packed tightly together? Or are they somewhat loose in one of more corners?

[shooter]

using Hoffman's schematic

some hacks to try, can be done mostly with gator clips

[sluckey]

It's been nearly 3 years in the making. Four threads and hundreds of posts. Using a donor OT that was never verified. I believe this amp is cursed!   

[uki]

It's been nearly 3 years in the making. Four threads and hundreds of posts. Using a donor OT that was never verified. I believe this amp is cursed!   

    


The OT is unkown and as you figured out while ago, or it isn't the original OT from the old gear or was rewinded.

It have the MOJO thou !! The amp sound really good !!

      Working on it right now !

[sluckey]

The amp sound really good !!

So why are you working on it!!!

[uki]

The amp sound really good !!

So why are you working on it!!!

The volume does up and down by it self...  I did what Shooter suggested , jumped the tone stack and cut off the feedback loop, presence at zero, the problem happen now more easily.

As I play harder the volume goes up, like a jump, like a boost switch, almost double the volume of the amp, and then goes back down, as I play softly, all by itself.

[sluckey]

As I play harder the volume goes up, like a jump, like a boost switch, almost double the volume of the amp, and then goes back down, as I play softly, all by itself.

That's a feature than many pickers would pay for. 

[shooter]

It have the MOJO thou

you know once you fix it, the MOJO will be over 

does it matter which input?

[uki]

As I play harder the volume goes up, like a jump, like a boost switch, almost double the volume of the amp, and then goes back down, as I play softly, all by itself.

That's a feature than many pickers would pay for. 

    There is a Roland or Boss hack with similar feature, clean setup with soft picking and distortion setup with hard picking.

I was just thinking, the volume of the amp was louder before it started happening, then it did start "switching" down/up, it is reversed now, up /down now. Like loosing power, dunno. Cutting a piece of aluminum to shield the OT wires

The OT and PT are very tight bolted. Position of Choke maybe suspicious, right bellow the bass pot.

does it matter which input?

Only one input jack.

[shooter]

Like loosing power, dunno

I like playing in the deep weeds;
monitor Tap A vdc while playing, get a ballpark range
then, switch to VAC, repeat

[uki]

Like loosing power, dunno

I like playing in the deep weeds;
monitor Tap A vdc while playing, get a ballpark range
then, switch to VAC, repeat

Here is what happen:
I did read it right in the cap can pin.

Node A idle 456vdc
While playing drop to 386vdc

Reading in AC
idle 000
playing highest peak 007  just as show in the meter.

I did replace 2 of the preamp tubes, V1 and V2, no big changes but the tone.

The PA tubes go might HOT V4 is the hotter one, after playing for about 5mins, can't keep finger on it for more than a second or two.
No red plate, but there is the little bright light inside, it does go off when sending signal, or it seen like it, dunno.

The volume was at about 5 or 50% open.

The volume problem seen to happen when the volume pots are at about 9 o'clock, about where the amp does go on OD,
not sure about it thou,

[Tony Bones]

Concerning the volume jumping up/down, I tend to agree with PRR that parasitic oscillation is likely. OTOH, I've also seen this happen because of a marginal connection, usually one with oxidation. Tube sockets and input / output jacks are likely, but pots can do it too. Are any of yours taken from the old amp? If so, clean them well along with tube pins.

I think Shooter asked this, but I don't remember your answer: does the problem happen with either volume control when the other is set to minimum?

[Tony Bones]

No red plate, but there is the little bright light inside, it does go off when sending signal, or it seen like it, dunno.

Can you make a video of this with all of the room lights turned off?

[shooter]

007

7 what? mV, Volts, Kv 

about where the amp does go on OD

I'm not good enough on blocking distortion, does the volume ALWAYS go down 1st, then up?

[sluckey]

Node A idle 456vdc
While playing drop to 386vdc

That's a 70V drop! I'd say you have a power amp or power supply problem. Troubleshoot this symptom and stay out of the preamp.

We know the OT is questionable. Tell us about the PT.

[shooter]

betting the math works out for the V drop because of his I boost idle to full roar

V4 from .021mV to .170mV

Uki, start a go fund me page n I'll send a $1 for a scope 

[uki]

Concerning the volume jumping up/down, I tend to agree with PRR that parasitic oscillation is likely. OTOH, I've also seen this happen because of a marginal connection, usually one with oxidation. Tube sockets and input / output jacks are likely, but pots can do it too. Are any of yours taken from the old amp? If so, clean them well along with tube pins.

I think Shooter asked this, but I don't remember your answer: does the problem happen with either volume control when the other is set to minimum?

Most resistors, 6 caps, volume pots, all sockets, PT, OT, input jack, ohms switch, all from the old amp, this thead.

I did disassembly the old pots to clean the resistive trail when I removed it from the old amp, was really dirty, volume was cutting off in certain spot in one of them.

I just unsolder its wires and did check the resistance and the working of it, pots are working ok, wires now back in place.

Haven't tried one volume at a time    

Ok I did that , after resoldering, the problem didn't happen, (but it doesn't mean its gone) because the other day same thing happened, it stopped and came back next day....

One volume at a time, Dark channel seen to be more stable, both did work ok.

The amp volume only seen to go up until the knob reach about 50% after that not much increasing, not noticeable. It does a big step at between about 20% to 30%.

About the light inside the tubes. I did replace those tubes few days go, the previous ones were lighting up, some of the guys said they were arcing.

The video with lights off, both volume knobs at 50% LOUD.

&feature=youtu.be

betting the math works out for the V drop because of his I boost idle to full roar

V4 from .021mV to .170mV

Uki, start a go fund me page n I'll send a $1 for a scope 

   I was looking at this one !  Or it or the transformers for the KT88 amp.

Node A idle 456vdc
While playing drop to 386vdc

That's a 70V drop! I'd say you have a power amp or power supply problem. Troubleshoot this symptom and stay out of the preamp.
We know the OT is questionable. Tell us about the PT.

The PT is really old, I bet 50 or more years old. got the PT diagram.

007

7 what? mV, Volts, Kv 

about where the amp does go on OD

I'm not good enough on blocking distortion, does the volume ALWAYS go down 1st, then up?

I think is 7 volts, meter was at 700, the screen showed  007 peak. AC mode.

The amp goes on OD with knobs open past 20%, you can see it in the video few posts ago.

It seen to be random the volume behavior, sometimes go up/down,  then down/up,  I think Sluckey is right might be something with the power amp or power supply.

[Tbone55]

At this point a colonoscopy might be the only option. 

Seriously, I don't have anywhere enough experience to try and provide any advice on this. I've read through all of the posts as it helps me understand amp circuitry better. Based on my observations it seems that the OT is very suspect. Also, the volume pot makes me a suspicious. I've had volume pots and tone pots on my guitar cut in and out without touching them. Spray the pots with some cleaner and spray some as well around the connections. How about heating up the solder joints and reflowing some solder? Check continuity between each connection point of the components and the wires. Measure resistance and capacitance values. Isolate the different sections of the circuit in order to narrow down the problem. Start from the input and go through each section. You need to do this by process of elimination. A lot of great help has been provided and since  you haven't been able to find the problem maybe a simpler approach might be in order. You're working with unknown components that may be suspect. Do you have any other components you could borrow from another amp that you know are working properly that you could try? I'm just throwing this out there as an option as I know how frustrating it can get. Hope you don't mind me chiming in. Hope you find the problem soon.

Cheers!

[uki]

I appreciate very much all the help, been learning a lot from all you guys!!

 

About the DC voltage drop, where to look ? Filter caps?

[shooter]

the PT values look good, that's a fine old tranny!
the volts is the current is the volts.........

kt88 tranny 

try this;
find C8 on the schematic I posted, take the wire off the wiper of the treble pot, stick it into an MP3 player headphone jack, play, watch, listen.  the player should have a volume control.
so...MP3 to wire to C8 to PI, (I usually have an 1/8 plug that I tack onto the wire)

[uki]

the PT values look good, that's a fine old tranny!
the volts is the current is the volts.........

kt88 tranny 

I was thinking about the same thing !   

try this;
find C8 on the schematic I posted, take the wire off the wiper of the treble pot, stick it into an MP3 player headphone jack, play, watch, listen.  the player should have a volume control.
so...MP3 to wire to C8 to PI, (I usually have an 1/8 plug that I tack onto the wire)

Can use a cellphone? So what I need is a jack to plug in that point of the circuit, bypassing the preamps, and going to the PI directly , need to ground the jack too yes?

[shooter]

is it under warranty 

pull V1 so there is no tube connected to the input, run a gatorclip from wire to "tip" of your input jack, 1/4 to 1/8 

[shooter]

forgot to add this 

I’m no longer capable of  operating heavy equipment so here’s some homework

NO signal Tap A VDC, BOTH PA tube currents

ADD the 2 current values together, then ADD 5mA

OHM a resistor value R = V/I
OHM a Power value for your R, then multiply * 2 (close works:)
Find your Resistor, make it.


Now that you got your Resistor, clip 1 side to the last PS Tap, clip the other to ground.
PULL ALL TUBES
Clip meter across your resistor, set for VDC
Power up, voltage should be close to full stable within 30 seconds.

IF it’s about the same as you measured for tap A AND there isn’t any smoke, let it COOK for 5min, walla, 1/3 of amp tested!  Kinda
the music to PI should decide another 1/3rd