Plexi 50 (done)

[uki]

Hey guys !!

No it isn't about the band from 70s 

It is about the project I've started over a year ago, due to natural disasters it was in the shelf all this time but I'm back to it and did some nice progress !!  Oh it is the Plexi 50 !

related topics:
http://el34world.com/Forum/index.php?topic=20556.msg216668#msg216668
https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697


Question about the bias pot, where it have to be when doing the first power up ?

Check out:

[sluckey]

With no power tubes plugged in... adjust the bias pot for maximum negative voltage on pin 5 of the power tube sockets.

[uki]

Thanks Sluckey !!

Here some more pics !

[sluckey]

Good to see dry transfer letters again.

[uki]

It is been a slow pace work but it is moving on!

I got a little doubt now about the 9 pin sockets, about how to wire the heaters, because they have a tick and tall pin in the middle, should I use elevated heaters or the ones in the pic(will that pin get in the way)?

that pic is from: http://www.valvewizard.co.uk/heater.html

[sluckey]

I use a Dremel tool with a cone shaped grinder bit to remove those center pins. They are rarely needed at audio frequencies.

[TIMBO]

Hey sluckey, So those centre pins are used for something else??

[sluckey]

Typically used as a ground shield in RF circuits.

[pdf64]

With modern ceramic type sockets, that centre part holds the socket together, so if removing the protruding part, you may need to be careful not to weaken any such fastenings.
Don't ask how I know this 
It's best to have a spare socket that you can risk damaging.

[uki]

Got some more progress!

Thanks for the hints guys, Ill keep posting news.

Working on the wiring from pre-amp section to power section.
Check out the pic!

[uki]

Power tubes heater wires have to be done same way as preamp ones? 

(this is my second amp) 

[Bantam]

That looks like heater winding on my stock Bantam Bass. On smallish side.

[Willabe]

      Uki, your wiring/lead dress on this amp looks much better than your 1st amp.   

Very nice improvement! 

[uki]

     Uki, your wiring/lead dress on this amp looks much better than your 1st amp.   

Very nice improvement! 

Thanks a lot !! 

I got some more progress, check it out !

[uki]

Hey guys the amp is almost ready just few more wires to solder , but I'm in doubt about what path to take with the filter B and C, is it ok to cross the circuit if ok above or bellow the board, or should the wires go around the board? 

thanks

[sluckey]

I would route the wires under the board. When you reach the lower edge of the board (as seen in your pic) bring the ends up, wrap around the turret, and solder on the top side of the board. No problem if they cross each other or anything else.

[OrganicEffects]

Looks much better than my 2nd build! Awesome stuff!

[uki]

Hey guys !!!

Just have done the 1st power up !!! All seen well so far, just checked the for B+, with standby off it is reading 326 VDC, with standby on it is reading 503 VDC ! No tubes yet, what other readings do I have to check out before installing the tubes?

Thanks !!

I did also read the tube pins:
Does the readings check good?
V1
P1 - 425VDC
P2 - 0
P3 - 0
P6 - 429VDC
P7 - 0
P8 - 0

V2
P1 - 438VDC
P2 - 0
P3 - 0
P6 - 479VDC
P7 - 438VDC
P8 - 0

V3
P1  448VDC
P2  0
P3  0
P6  440VDC
P7  0
P8  0

V4
P3  496VDC
P4  500VDC
P5  -41
P6  -42

Bias pot range is -23 to -42

V5
P3  501VDC
P4  500VDC
P5  -41
P6  -42

[EKDENTON]

Wow Uki that looks nice. You did a great job on that one. I've been into Ham radio and away from tube amps for awhile. I built my first common mode current choke/impedence transformer today. Tested great, soon as I get it in a box I will give it a real test.

[jbefumo]

Nice looking build, Uki! Tidy workmanship isn't my forte ....


You got me beat by a year, but after 9 years of trying, this is what I've come up with:

[uki]

As far as I understand the preamp B+ is too high, is that correct? I have a guess it is the choke, maybe not the right one...

specs of the choke:  3Hy 120Ma 106Ohms  is this the right one?

[sluckey]

Those voltage readings are mostly meaningless without tubes.

[uki]

Those voltage readings are mostly meaningless without tubes.

Is it safe to put the tubes on ?

[uki]

I got it !!!!   

The amp is working , bit dirty for me but it does sound good with both volumes up about 25% does a nice tube overdrive!

Here is the voltages with the tubes!

V1 P1  142 VDC
     P2      0
     P3      1
     P6  185
     P7      0
     P8      2

V2 P1  156 VDC
     P2      0
     P3      1
     P6  279
     P7  156
     P8  162

V3 P1  204 VDC
     P2    11
     P3    40
     P6  208
     P7    12
     P8    40

V4 P3  489 VDC
     P4  487
     P5  -42   
     P6  -43
     
V4 P3  489 VDC
     P4  487
     P5  -42   
     P6  -43

How's that ?!

[sluckey]

Much better. 

[uki]

Amp sounds good no hum, or any problem!

Is it possible to clean the tone in this amp ?

[uki]

Something isn't right!!  One of the power tubes V4 is red plating, I did notice it but only after tunning a bit the bias, I'm not sure if it did happen before the change, but I did watch for it in the very beggin of the first times testing the amp it didnt happen, it was adjusted to -36 from previous -43.

I was reading the voltages just after turning standby on, so I did read V5 which was ok and then when reading V4 pin 3 it did burn    my meter(a cheap one)... amp still working thou, bias is back to maximum negative bias, haven't turn the amp on yet. what to do?   

[purpletele]

Something isn't right!!  One of the power tubes V4 is red plating, I did notice it but only after tunning a bit the bias, I'm not sure if it did happen before the change, but I did watch for it in the very beggin of the first times testing the amp it didnt happen, it was adjusted to -36 from previous -43.

I was reading the voltages just after turning standby on, so I did read V5 which was ok and then when reading V4 pin 3 it did burn    my meter(a cheap one)... amp still working thou, bias is back to maximum negative bias, haven't turn the amp on yet. what to do?   

Are you plugged into a Light Current Limiter?  I leaned a valuable lesson recently, that is, stay plugged into a light limiter until you are certain that everything is stable.

That's a bad feeling when you discover you fried a transformer.

[pdf64]

...One of the power tubes V4 is red plating, I did notice it but only after tunning a bit the bias ... was adjusted to -36 from previous -43...

The purpose of the bias voltage is to keep the plate / cathode current in the correct range.
Hence the benefit of 1 ohm cathode resistors, as they allow a simple, direct, uncomplicated, accurate, low hazard measurement of cathode current.
So, what is the cathode current at -43V, and at -36V?

[uki]

Guys thanks for the reply!

Well I have probably used a OT that is underated for this amp, it was take from an old radio/pickup amp, I couldn't find schematics for it and it did came without power tubes, so not much clue about what it was.
Also it had e rectifier tube, which drops the voltage right?! So lets say if it was 6v6 tubes in there, with recto tube, then, maybe the OT couldn't be able to handle el34s with diode rectifier, could that be the case?! I think this info is important.


About reading the

...One of the power tubes V4 is red plating, I did notice it but only after tunning a bit the bias ... was adjusted to -36 from previous -43...

The purpose of the bias voltage is to keep the plate / cathode current in the correct range.
Hence the benefit of 1 ohm cathode resistors, as they allow a simple, direct, uncomplicated, accurate, low hazard measurement of cathode current.
So, what is the cathode current at -43V, and at -36V?

About that... well that is what I was going to do when my meter went bye bye, 1st thing I did was to take back the bias pot to max negative voltage, then I was reading V5 then V4 meter fried, I keep watching and about same time tube was red plating, just few seconds after.

I gotta buy a new cheap meter, not risking the good one I have, I still learning about all of it 
Question, is it safe to read the cathode current in the present state of things?

[shooter]

is it safe

I would leave the PA tubes out for now.

set the pot to max -, AND verify with a known GOOD meter
I would also verify with said meter the OT's ohms readings; output and primary
I would at least tack in a 1ohm cathode R for troubleshooting

the size in your other post looks to be a pretty beefy OT, looks like Hammond

[uki]

Are you plugged into a Light Current Limiter?  I leaned a valuable lesson recently, that is, stay plugged into a light limiter until you are certain that everything is stable.

I don't have one, but it is now in the to do list! Thank you very much for the advise! And I'll keep that in mind for now on.

is it safe

I would leave the PA tubes out for now.

set the pot to max -, AND verify with a known GOOD meter
I would also verify with said meter the OT's ohms readings; output and primary
I would at least tack in a 1ohm cathode R for troubleshooting

the size in your other post looks to be a pretty beefy OT, looks like Hammond

There is 1Ohms resistors in the cathode of the power tubes.
Yeah indeed the OT looks beef !! I did had the same impression, not sure if you remember it is the UL OT we discussed some time ago.
I'll do the readings and post, as soon I get new meter.

Oh and by the way, the OT was getting a little warm in the secondary side, not too much just a little, but the primary was cold. Does this tell anything?

[uki]

Got some time today to check the amp!

The OT secondary read:

red+brown= 124.6Ohms
red+blue= 112.9Ohms  (this side is in V4 which is red plating)
blue+brown= 236Ohms

In the power tubes there are some numbers 1412 and 1409(V4) does it have anything todo with matching?

The secondary read:

black+red= 1.1Ohms
black+brown= 0.9Ohms
black=blue= 0.6Ohms

Does the OT looks ok? If the problem isn't the OT what next?

Thanks

[shooter]

so you got a good meter you trust?

without the PA tubes in is this still the case;

Bias pot range is -23 to -42

[uki]

so you got a good meter you trust?

without the PA tubes in is this still the case;

Bias pot range is -23 to -42

Yes I got a good meter, reliable one.

Bias pot is now doing -28 to -53 is that expected w/o PA tubes?

[shooter]

new meter readings 

ok, set it to -53 without tubes
power down and wait til discharged
put in power tubes
set your meter up for Volts DC.  (IF it's auto range leave it, IF you have to select a voltage range, pick 0-10vdc)
clip + lead to the cathode - 1ohm junction for V4
clip -  lead to the  ground - 1ohm junction for v4
power up, take it outta standby (IF it has one)
get your reading accurate but quick, power down
now do the same for V5 and post what you got

[uki]

new meter readings 

ok, set it to -53 without tubes
power down and wait til discharged
put in power tubes
set your meter up for Volts DC.  (IF it's auto range leave it, IF you have to select a voltage range, pick 0-10vdc)
clip + lead to the cathode - 1ohm junction for V4
clip -  lead to the  ground - 1ohm junction for v4
power up, take it outta standby (IF it has one)
get your reading accurate but quick, power down
now do the same for V5 and post what you got

Extra info: some components like: caps, resistors, sockets, transformers, used in the amp are really old, like 50 years old or so 

Bias set to -53 max negative voltage.
Caps discharged,PA tubes installed.
Meter set to 0-20vdc, there is no 0-10, there is 2vdc and 200m
Reading across 1Ohm cathode resistor
V4 reading is giving 0, so 0-20 is too high
So I did set to 200m and got the following:

V4  2.3mV
V5  1.4mV

Ok something is odd there yes? What next?

Thanks

[PRR]

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.

[shooter]

Watch the milliVolts on the 1 Ohm resistors

since you only have 1 meter and V4 "might" be a problem child, monitor it's 1ohm, your initial readings indicate it's pulling lots more current that it's brother

[uki]

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.

Thank you sir!

I did work the mV towards 20mV slowly and constantly watching the tubes, did play a bit on the start point and went up in small paces, until 20mV no red plate so far, the amp is been on for over 2 hours and under observations, did play at 20mV for about 30 minutes. Looks to be stable, there is one detail thou, I did notice it now, the amp is upside down, no problem, but when it was red plating even with the -53 bias volt set, it was replating when turned into the position it will be in the cabinet, haven't turn the amp yet to verify if it is really the case. Gonna play a little bit more !! 

Watch the milliVolts on the 1 Ohm resistors

since you only have 1 meter and V4 "might" be a problem child, monitor it's 1ohm, your initial readings indicate it's pulling lots more current that it's brother

Thank you Sir !

Yes for sure it is unbalanced , with 20mV on V4 and 18mV on V5, thinking about add a bias balance pot, and balance the mV on tubes. Feel like V4 warm up more than V5. Here is one interesting video about it:
 

How to implement bias balance pot in this amp?

[sluckey]

Yes for sure it is unbalanced , with 20mV on V4 and 18mV on V5, thinking about add a bias balance pot

That's perfectly balanced. Crank it on up to your desired operating point. What are the numbers now?

[PRR]

> it was replating when turned into the position it will be in the cabinet, haven't turn the amp yet to verify if it is really the case.

Either your electrons are falling-down, piling-up, getting over-excited, etc.

Or you have a loose connection, good (kind of) one way, bad the other way.

(Yes, air convection cooling changes with orientation but not enough to cause red-heat. The glass will sit a little hotter if less air convects over it, but the plate is much-much hotter than the glass and little 150C-170C changes hardly affect the 700 degrees C on a red-hot plate.)

[shooter]

What are the numbers now?

your old plates were ~~485 X .02A = 9.7w, you'll have to provide new plate and figure % 

[uki]

> it was replating when turned into the position it will be in the cabinet, haven't turn the amp yet to verify if it is really the case.

Either your electrons are falling-down, piling-up, getting over-excited, etc.

Or you have a loose connection, good (kind of) one way, bad the other way.

(Yes, air convection cooling changes with orientation but not enough to cause red-heat. The glass will sit a little hotter if less air convects over it, but the plate is much-much hotter than the glass and little 150C-170C changes hardly affect the 700 degrees C on a red-hot plate.)

About flipping the amp(that was just a guess), no changes, all still the same, now voltages, when reading pin 3 there is a spike in the meter with over 500 volts before it get the right number specially in V4, maybe that is what killed my cheap meter 

What are the numbers now?

Voltages on PA tubes are:
V4  P3 460
      P4 463
      p5 -43
      p6 -43

V5  P3 460
      P4 463
      p5 -43
      p6 -43

What are the numbers now?

your old plates were ~~485 X .02A = 9.7w, you'll have to provide new plate and figure % 

How did you came up with that .02A ? 

The amp goes on overdrive very easy with volume at just about  2(distortion machine)!!! I would like to get some clean tone with more volume, how to tame the amp ? 

Thanks !

[sluckey]

20mV across a 1Ω resistor is 20mA. 20mA is .02A.

[shooter]

how to tame the amp ? 

that's a whole other thread 

reading pin 3 there is a spike in the meter with over 500 volts before it get the right number specially in V4,

just idling, no signal that shouldn't happen. 
swap tubes and see if it stays or follows the tube. 
thump the tube
thump the wires
Is it "consistent" like every 2seconds

[tubeswell]

If it distorts really easily, you may have an out-of-balance LTP (i.e. from one (or more?) of the coupling caps leaking DC). This could be either the LTP 'master' triode coupling cap, or the LTP 'slave' triode decoupling cap, or one of the coupling caps feeding the output tubes. Whichever, a leaky coupling cap will throw off bias voltage. If it interferes with the LTP duty cycle, it will cause a distortion problem when you push a signal through the amp, and possibly even cause red-plating in one output tube when you crank the signal.


Check the LTP plate voltages at idle and see if they look uneven.  Then test the coupling caps around the LTP for DC-leakage.

[uki]

reading pin 3 there is a spike in the meter with over 500 volts before it get the right number specially in V4,

just idling, no signal that shouldn't happen. 
swap tubes and see if it stays or follows the tube. 
thump the tube
thump the wires
Is it "consistent" like every 2seconds

I did change the way I been reading those high voltages, before, while the high voltages going I was going with the probes to were to read. Now I do 1st plug the probes and then turn on the HV. No peaks this ways, and the voltages stay the same all the time at idle.

If it distorts really easily, you may have an out-of-balance LTP (i.e. from one (or more?) of the coupling caps leaking DC). This could be either the LTP 'master' triode coupling cap, or the LTP 'slave' triode decoupling cap, or one of the coupling caps feeding the output tubes. Whichever, a leaky coupling cap will throw off bias voltage. If it interferes with the LTP duty cycle, it will cause a distortion problem when you push a signal through the amp, and possibly even cause red-plating in one output tube when you crank the signal.


Check the LTP plate voltages at idle and see if they look uneven.  Then test the coupling caps around the LTP for DC-leakage.

Thanks sir!!

This is a great information and it is indeed what is happening, one of the tubes(V4) red plating if bias is around -36(working now at -43 no red plate), I was trying to fix the distortion by tunning the bias pot.

V3 plate voltages:

P1 214vdc
P6 203vdc

So one of those caps are leaking! Those caps are really old. If I got this right the caps you mentioned probably are: C8, C9, C11 and C12; C10 as well? , the amp is the Hoffman Plexi 50. Now how can those caps be tested for leakage ?

Thank you

[shooter]

Now how can those caps be tested for leakage ?

for c8,11,12, unsolder the "right" side, (as viewed in the schematic)
measure the "floating" lead, it should be 0 to small mV, BUT, as long as you have it half removed...........

sometimes it is better to skip a cup of coffee and go with new, signal path caps are one of those coffee sacrifice places.

[tubeswell]

As shooter said, you use your VDC meter to measure for DC on the 'AC-side' of each cap, by (unsoldering and) lifting the AC side from the circuit and clipping your DC meter between this (now) 'floating' cap lead and the ground return, and switching the amp on and waiting for the (start-up) voltages to stabilise. A good cap will show some (change in) DC voltage as the amp powers up but this will drop away completely as the voltages stabilise to full power. A leaky cap will still show DC (even after the power supply voltages have stabilised).

See the diagram in Fig.4-4 on page 57 of the attached chapter from the Jack Darr book.


Note: the 'side' you need to measure for DC on the LTP slave triode decoupling cap, and on the master triode coupling cap, are both on the opposite side to the grid-side (because the grid sides of both of these caps both should have DC voltage on them under normal operating conditions - because these are connected directly to the tail resistor/bias resistor junction by each respective grid leak cap)