Plexi 50 troubles

[uki]

I got the cellphone playing through the PI       the volume is low thou. Presence pot works with it !

Oh and I forgot to mention the cap cans are 50uF/50uF/500v

For you last post, I'm a bit lost at:

> NO signal Tap A VDC, BOTH PA tube currents 

Read VDC, but how do I get the current ? Cathode resistor ?

> 5mA

Is it .005 ?

[Tony Bones]

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

I'd go back and make sure that the contacts on all of those are clean, including the ohms switch. Deoxit helps if you can get it easily. The odds of this fixing the problem are not great, but not tiny either.

this thead.

Yeah, I read that thread three years ago.  Things change ...

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

That blue glow is totally not a problem.

[sluckey]

Can we get this amp back to a stock Plexi schematic?

1. Are you using the OT screen taps? If so, do you know for a fact that you have them wired correctly? A screen tap must be used with its corresponding plate tap. If you are using the screen taps I highly recommend that you NOT use them. Just tape them off and wire the tube screens just like a Plexi schematic.

2. I'm also suspicious of your high resistance bias balance circuit. I suggest you go back to the simple Plexi bias circuit until this weird problem is sorted. You can always fancy it up after the amp is working properly.

Once you do these two things we'll be in a better position to try to solve your issues.

[Colas LeGrippa]

Feed the amp section with the plate of the first ax7, disconnecting everything else in the path.
It ll give you a good clue.

[shooter]

yes .005

even with low input, did the volume change up/down?

[uki]

Can we get this amp back to a stock Plexi schematic?

Yes ! Doing it already !  Balance circuit removed !

1. Are you using the OT screen taps? ...

Never connected them.

yes .005

even with low input, did the volume change up/down?

No volume changes while playing through the PI , did it run for about 10 mins.

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.

Ok let me see if I got this:

read voltage DC in tap A,  460vdc ish

read current for PA tubes(across cathode resistor)  0.021mA

0.021 + 0.021 + 0.005 = 0.047

460 / 0.047 = 9787.23404255

that is 10k resistor.

I'm not sure about the power, how to get there?

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

By last tap you mean tap E ?

[shooter]

22W *2, so 50W  P=E * I.  You can get by for a real quick test with ~~30W
Attach to TAP B, quick measure, look for HOT or smoke.  What this does is "simulate" the tube load the PS "see's".  I do this before I EVER plug in a tube.  I'm about 90% sure your PS is ok, unless you used unknown, old caps.  So, keep doing the other stuff 1st.
I'm also about 90% sure it's from PI to speaker

here's a handy ohm chart

[uki]

22W *2, so 50W  P=E * I.  You can get by for a real quick test with ~~30W
Attach to TAP B, quick measure, look for HOT or smoke.  What this does is "simulate" the tube load the PS "see's".  I do this before I EVER plug in a tube.  I'm about 90% sure your PS is ok, unless you used unknown, old caps.  So, keep doing the other stuff 1st.
I'm also about 90% sure it's from PI to speaker

here's a handy ohm chart

> P=E * I

What is E ?

I don't have 10k 50w resistor,  I got 12k 5w , 10k 10w, 15k 10w,  330r 25w

PS filter caps are all new, this

Now the PI caps are old , really old. the little brown caps look in the pics.

[shooter]

really old

Now those are some oldie moldy!!

I would unsolder the "right" side and see if they are leaking DC volts (C8, C11, C12)
measure VDC from the right side (unsoldered) to ground, anything more than ~~ 20mV DC I'd replace.  with new ones 

E is volts, same as V.  depending on age we either learned it as E or V or HT, or...

[uki]

I did read again the voltages for PA after removing the bias balance mod,

I did notice this since the first time the amp was working, while reading V4 pin 3 it does a big noise, meter goes crazy for a second then the voltage show up. Did lost one cheap meter reading this pin, while ago.

V4 P3  477 vdc
         4  472 vdc
         5   -53 vdc
         6   -53 vdc
         cathode resistor   0.002mV

V5  P3  477 vdc
          4  475 vdc
          5   -53 vdc
          6   -53 vdc
          cathode resistor  0.011mV

I did notice a lot of downgrade in tone with the rollback.

I would unsolder the "right" side and see if they are leaking DC volts (C8, C11, C12)
measure VDC from the right side (unsoldered) to ground, anything more than ~~ 20mV DC I'd replace.  with new ones 

We have done this before here and here, they did ok back then, could they drift in this time? Gonna get it again anyway.

[shooter]

drift in this time?

should be good, just next order, add a couple caps

[sluckey]

-53v is way too high for el34s. You need to be between -40 and -45.

[uki]

Can we get this amp back to a stock Plexi schematic?

1. Are you using the OT screen taps? If so, do you know for a fact that you have them wired correctly? A screen tap must be used with its corresponding plate tap. If you are using the screen taps I highly recommend that you NOT use them. Just tape them off and wire the tube screens just like a Plexi schematic.

2. I'm also suspicious of your high resistance bias balance circuit. I suggest you go back to the simple Plexi bias circuit until this weird problem is sorted. You can always fancy it up after the amp is working properly.

Once you do these two things we'll be in a better position to try to solve your issues.

I got those items in place !

-53v is way too high for el34s. You need to be between -40 and -45.

Now it is  at -46 V4 current is .030v already, while V5 is only .017v     
here is about the point where the previous tubes started to red plate,
this is why the bias balance control.  This pair of tubes are unmatched.

Nodes A and B at 464v

[shooter]

swap the tubes around, does the current follow the tube or stay with the socket?

[uki]

swap the tubes around, does the current follow the tube or stay with the socket?

Does not stay with the socket, same readings as before.

From this point Sluckey want to find out the cause of that 70v drop while playing.

[shooter]

Does not stay with the socket, same readings as before.

This tells me you are way off on match, Follow Sluckey, when you get time try n get 2 tubes +/- say 5mA instead of the 13mA you got now.

my thought, when you hit that HOT tube hard, it's cooking at 3X max plate

[uki]

This tells me you are way off on match, Follow Sluckey, when you get time try n get 2 tubes +/- say 5mA instead of the 13mA you got now.
my thought, when you hit that HOT tube hard, it's cooking at 3X max plate

I only have those previous tubes, the EH matched pair, but I dunno if they are good. The amp was playing ok with them. But the Mullards sound way better !

[shooter]

V5 was .017mA, keep that one.  Then take your "suspect" others and put them in 1 at a time.  use the one that get closest to .017.

what sounds good won't count til you are happy the amp is working 

[uki]

V5 was .017mA, keep that one.  Then take your "suspect" others and put them in 1 at a time.  use the one that get closest to .017.

what sounds good won't count til you are happy the amp is working 

I got the tubes matched(almost) EH on V5 with .018v and Mullard on V4 with .019v !

[shooter]

SWEET, now go fix your amp 

where do the #'s fall, idle no signal

[uki]

SWEET, now go fix your amp 

where do the #'s fall, idle no signal

I'll get the big resistor next week!

What you mean with where do the #'s fall, idle no signal  ?

[shooter]

TAP A vdc
TAP A * .017  = tube dissipation at idle
tap A *.018

so I take it it plays ?
IF so, ONLY play CLEAN, 1 channel at a time, play long enough to be comfortable it does or doesn't do the volume up/down thing. 

[uki]

Little variation after powering off on.

A 458vdc  *  .018 = 8.2w
    458        *  .017 = 7.7w

It does not change volume while playing it clean, both channels checked, one at a time.
before this the volume behavior was happening when volume was at the point where the amp start to OD, about volume 3.
In the last video the volume change did not happen.

Also I did watch the tap A while playing at lvl 2(clean), it does drop almost 10v. PA tubes go might hot, can't keep touching.

[sluckey]

Adjust the bias to increase tube current to 36mA (36mV across the cathode 1Ω resistors). How does it behave and sound?

[uki]

Adjust the bias to increase tube current to 36mA (36mV across the cathode 1Ω resistors). How does it behave and sound?

Current set at 36mA in the hotter tube(Mullard) V4, Tap A is reading 450 vdc  the voltage drop is now smaller but with peaks of 50v
the current goes up about double while playing. No red plate in about 10-15 mins.
Now about the tone, not very pleasant , probably because of the different brand of tubes, the EH tubes doesn't sound good in comparison with the Mullard. EH tubes sounds more harsh and dry, Mullards more round and warm.

The volume random change is happening with both channels open and at about 3, where the OD start, bellow that doesn't happen.
The volume went up/down twice just as the volume was increase to about 3 and strumming the strings hard. So I did turn down the volume immediately. With the volume bellow the amp does a clean sound.   

[sluckey]

Temporarily disconnect the NFB. Sound any better?

[uki]

Temporarily disconnect the NFB. Sound any better?

W/o the NFB the amp does sound better, the volume change did not happened, so it is something in that part of the circuit, a cap maybe?
The amp goes a bit microphonic thou.

[sluckey]

Reconnect the NFB. Then swap the OT primary plate leads. What happens?

[uki]

Reconnect the NFB. Then swap the OT primary plate leads. What happens?

Amp does whistle right the moment the standby is engaged. I did turn off right the way.

[shooter]

Keep working with Sluckey
this is a fwiw;
this amp
http://el34world.com/Forum/index.php?topic=20104.msg210740#msg210740
I never did get it "fixed"
at clean, played fine, guitar or music, the minute you hit OD things when sideways.
I changed enough pieces, spent enough scope time to be reasonable convinced my OT is to blame, core saturation, weird impedance stuff that needs spooky math 
 

[sluckey]

Well, put the OT primary plate leads like they originally were.

At this point I would buy a real Marshall 50W OT. Hoffman still has them.

[uki]

Well, put the OT primary plate leads like they originally were.

At this point I would buy a real Marshall 50W OT. Hoffman still has them.

Can't buy over sea with the current currency rates,   1 USD to BRL 3.93947

Anyway, so it is the OT that doesn't go well with the rest of the amp ?  Bad idea to keep playing with it?

[shooter]

Bad idea to keep playing with it?

how much are tubes , and possibly PS worth to you

You're now a highly qualified troubleshooter, fix some amps and get some money 

fwiw, the amp I mentioned, it's still out being used, just not overdriven

[uki]

You're now a highly qualified troubleshooter, fix some amps and get some money 

Not a bad idea !!   

The OT was probably for a pair of 6V6 and it is probably rated for 20-30W tops, that is my own conclusion, because the volume past 4-5 doesn't increase volume, the amount of distortion tells something is out of place. The original amp had a tube rectifier so more voltage drop, that is what makes me think this. Here another thing, while running at 21mA it does sound much better, does it add up clues for you?

Ok new OT for this amp. Gonna put it to rest for now !

Thanks a lot guys , your help is very much appreciated !! 

 

[sluckey]

Anyway, so it is the OT that doesn't go well with the rest of the amp ?

That's not what I'm saying. I said, "At this point I would buy a real Marshall 50W OT." Parts substitution has always been a valid troubleshooting technique. Doesn't always fix the problem though.

If you suspect that OT may have been for a pair of 6V6s, then you must also suspect the PT was for a pair of 6V6s. You had the whole amp. Don't you know what tubes were supposed to be in it? If the iron really was meant for 6V6s then just put 6V6s in it and call it a Plexi6V6. Mine sounded pretty good! 

[uki]

I dunno if it was running a pair of 6v6    The old Fidelius came without, the power tubes and rectifier, It had on one 6SL7 one 6SN7 one EF86 and one 12ax7.

About the transformer, here is a pic of print.

And a schematic I draw for the power section, maybe wrong.

[Tony Bones]

That iron certainly looks big enough to support 6L6/KT88. The vintage amps I've found that were set up for 6V6 usually had much smaller OT.

[shooter]

the 500 cathode R is kinda common on 6V6, my notes say 500-750
I got 270 - 330 for EL34's.  BUT those are ballpark based on plate V
and the Iron like tony said is BIG

you could scrap the fixed bias and try self biased.  you will change "volume" some, and tone will change, with self bias you usually set to 80-100% dissipation since the harder you play the less likely you smoke important parts

[shooter]

went re-reading...some

32 * 32 = 1024    1024 * 8Ω = 8192Ω
8k is kinda 6V6 value, 34's is kind of 3.5k, pointed out by PDF64 for fixed bias
so I would be inclined to put a 4ohm speaker on the OT and play clean, if all's good, move into OD and see if it sounds better.

I doctored up this datasheet for  "options" til you get a new OT
the UL data 20% 43% will be a guess, all the modern UL's I've bought are 40%, but that's NOT scientific 

[sluckey]

If pin 1 was not connected to anything then it ain't an EL34.

[uki]

If pin 1 was not connected to anything then it ain't an EL34.

Pin 1 was connected with pin 8.

[pdf64]

Bear in mind that generic 'off the shelf' OTs might have been marketed for various tube sets, eg the Radiospares Deluxe used by Marshall for the early JTM45 listed EL34, KT66, 6V6, 6L6, EL84 as suitable tube types (though that has multiple primary and secondary winding taps / arrangements available)  https://imgur.com/a/zLmbj

[uki]

Is this model 40-18025 a suitable OT for the amp?

Thanks

[sluckey]

I don't know.            Are you building a 50 watt Marshall amp? 

[uki]

I don't know.            Are you building a 50 watt Marshall amp? 

 
It is built already, the plexi50

 

... I said, "At this point I would buy a real Marshall 50W OT." Parts substitution has always been a valid troubleshooting technique. Doesn't always fix the problem though.

That is what I been thinking. To try an OT designed for this amp.

[shooter]

I think the dollar might weaken soon, get a good exchange rate, buy it, play it