Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on February 17, 2018, 05:17:48 pm
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Hey guys !!
No it isn't about the band from 70s :icon_biggrin:
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
:laugh:
Question about the bias pot, where it have to be when doing the first power up ?
Check out:
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With no power tubes plugged in... adjust the bias pot for maximum negative voltage on pin 5 of the power tube sockets.
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Thanks Sluckey !!
Here some more pics !
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Good to see dry transfer letters again.
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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
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I use a Dremel tool with a cone shaped grinder bit to remove those center pins. They are rarely needed at audio frequencies.
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Hey sluckey, So those centre pins are used for something else??
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Typically used as a ground shield in RF circuits.
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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 :sad2:
It's best to have a spare socket that you can risk damaging.
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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!
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Power tubes heater wires have to be done same way as preamp ones? :w2:
(this is my second amp) :icon_biggrin:
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That looks like heater winding on my stock Bantam Bass. On smallish side.
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:bravo1: Uki, your wiring/lead dress on this amp looks much better than your 1st amp.
Very nice improvement! :icon_biggrin:
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:bravo1: Uki, your wiring/lead dress on this amp looks much better than your 1st amp.
Very nice improvement! :icon_biggrin:
Thanks a lot !! :smiley:
I got some more progress, check it out !
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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? :think1:
thanks
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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.
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Looks much better than my 2nd build! Awesome stuff!
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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
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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.
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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:
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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?
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Those voltage readings are mostly meaningless without tubes.
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Those voltage readings are mostly meaningless without tubes.
Is it safe to put the tubes on ?
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I got it !!!! :happy1:
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 ?!
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Much better. :thumbsup:
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Amp sounds good no hum, or any problem!
Is it possible to clean the tone in this amp ?
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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 :blob8: 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? :w2: :help:
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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 :blob8: 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? :w2: :help:
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.
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...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?
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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 (http://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697), 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 :icon_biggrin:
Question, is it safe to read the cathode current in the present state of things?
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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 :dontknow:
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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 :dontknow:
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?
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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
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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
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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?
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new meter readings :dontknow:
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
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new meter readings :dontknow:
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 :icon_biggrin:
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
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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.
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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
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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 !! :icon_biggrin: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?
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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?
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> 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.)
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What are the numbers now?
your old plates were ~~485 X .02A = 9.7w, you'll have to provide new plate and figure % :icon_biggrin:
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> 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 :sad2:
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 % :icon_biggrin:
How did you came up with that .02A ? :w2:
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 !
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20mV across a 1Ω resistor is 20mA. 20mA is .02A.
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how to tame the amp ?
that's a whole other thread :icon_biggrin:
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
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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.
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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
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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........... :icon_biggrin:
sometimes it is better to skip a cup of coffee and go with new, signal path caps are one of those coffee sacrifice places.
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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)
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sometimes it is better to skip a cup of coffee and go with new, signal path caps are one of those coffee sacrifice places.
I got some 22n new caps in hand just in case ! :icon_biggrin:
Need to replace volume pots, they are like granny of a granny :laugh: Did took it apart and cleaned it inside, was dirty man!!! It is working better now but feel like the volume goes to max before 10...
The amp sounds really good although it goes on overdrive really soon at volume 2 or so, knob at like 8-9 o clock.
Thanks @Tubeswell for the file good reading !!
I did the readings and the results are:
C11 cap have 24.2n
dc mV is jumping between .01 and 11.x every few seconds, 10-20 secs interval or so. The negative symbol keep showing up and disappearing while reading.
C12 cap have 24.4n
dc mV is also jumping just like C11
How much the dc voltage on a cap have to be to consider it a leak?
Thanks !
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How much the dc voltage on a cap have to be to consider it a leak?
I would not worry about it unless it is more than 500mV
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How much the dc voltage on a cap have to be to consider it a leak?
I would not worry about it unless it is more than 500mV
Thanks Sluckey, so those caps are good then! Where else to look at ?
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I did the readings and the results are:
C11 cap ... dc mV is jumping between .01 and 11.x every few seconds, 10-20 secs interval or so. The negative symbol keep showing up and disappearing while reading.
C12 cap ... dc mV is also jumping just like C11
How much the dc voltage on a cap have to be to consider it a leak?
That will be random tiny amounts of signal nose
What about C8 and C9? Did you measure those for leakage?
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What about C8 and C9? Did you measure those for leakage?
If I did this right, lift cap side that is connected to the pins 2 and 7 yes?
Got those readings and looks much better than the other two, it stays at .001 - .002 on both caps with the meter at 200mV.
The cap values read good as well !
Hmm where to look at next? :dontknow:
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where to look at next?
Is it a chicken or egg?, is your distortion at 2;
overdriven signal or
bad (wrong part, etc.)
a scope comes in handy here. short of that, put a 12AU7 in V1.
what are the 2 gain knobs on when vol @2 overdrives?
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where to look at next?
Is it a chicken or egg?, is your distortion at 2;
overdriven signal or
bad (wrong part, etc.)
a scope comes in handy here. short of that, put a 12AU7 in V1.
what are the 2 gain knobs on when vol @2 overdrives?
Amp is going on overdrive with volume at 2-3, I think those pots for volume are part of the problem, they are super old, and faulty, I got the impression the volume get to max before turning the pots to max. I did tried the bias adjust to fix the distortion that is when the red plate happened, I tried a lower gain tube(dunno which model it is, no label) but I only got one and its already in the other amp, not much difference thou.
Look the pictures of the knobs:
Is this amp supposed to overdriven like this ?
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sorry, I added a mv on mine, got confused :think1:
scratchy pot can also be dc through you caps so if you change a pot check the voltage where the couplers connect to the top of the pots
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What about C8 and C9? Did you measure those for leakage?
If I did this right, lift cap side that is connected to the pins 2 and 7 yes?
No, for those LTP grid caps, the grids at at a DC voltage (because of the current through the tail resistor).
You need to test the other end of those caps.
But before you do, What are the idle voltages on the LTP plates?
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What are the idle voltages on the LTP plates?
got 'em in reply 47, I'm not a PI guy, but they seem a smig low :dontknow:
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What about C8 and C9? Did you measure those for leakage?
If I did this right, lift cap side that is connected to the pins 2 and 7 yes?
No, for those LTP grid caps, the grids at at a DC voltage (because of the current through the tail resistor).
You need to test the other end of those caps.
But before you do, What are the idle voltages on the LTP plates?
I did the plate readings and today it is a bit higher than the other day
P1 217 (214)
P6 206 (203)
Is this the expected voltages? Shooter says it is a bit low.
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plate readings and today ... the other day
P1 217 (214)
P6 206 (203)
That amount of difference isn't unusual for a LTP, which suggests the grid voltages are probably okay.
What type of tube are you using for the LTP? a 12AX7? or a 12AT7?
And how big is the bias resistor? 470R? 820R? 1k? 1k2?
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What type of tube are you using for the LTP? a 12AX7? or a 12AT7?
And how big is the bias resistor? 470R? 820R? 1k? 1k2?
12ax7 JJ in the LTP
and the bias resistor is 15k I think, the amp is the Hoffman Plexi 50 (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf).
The readings for those caps are:
C8 22.1nF (200range)mV 0.0 - 0.1
C9 96.9nF (200range)mV 0.0 - 0.1
What next ?!
Thanks !
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I'm talking about the LTP bias resistor.
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I'm talking about the LTP bias resistor.
I've found this!
...You have a largish tail resistance (39k) because KF wanted to balance the plate resistors. And the HT voltage for the LTP stage is only 270V. Both factors reduce available plate-to-cathode voltage and bias voltage. Compare that to the LTP for (say) a Fender Brown Deluxe HT = 325, Tail = 6k8, much cleaner
Same situation yes? Just like in my amp!
Oh , the one connected to the cathode? 470 Ohms 1watt, and the tail resistor is 10k.
I did found out that R31 and R32 are a bit off of each other. R31 read 1215 ohms and R32 read 1025, gonna replace R31 with a closer one to the other. Could this be part of the red plate problem when changing bias?
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470R is on the hot side for a 12AX7 LTP. You want more like 820R to 1k. If the LTP bias is too hot, if can muck up the LTP duty cycle, resulting in red-plating on one side of the output stage under large signal conditions
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470R is on the hot side for a 12AX7 LTP. You want more like 820R to 1k. If the LTP bias is too hot, if can muck up the LTP duty cycle, resulting in red-plating on one side of the output stage under large signal conditions
The amp still going on OD at low volume, although it sounds really loud!! The tone is nice and the amp is very responsive, but I would like some more clean tone at low volume, and get that nice OD when volume is about 7-8.
The PA tubes are swapped to see if the difference in mV get fixed, it still about the same. Does that means the tubes are slightly unbalanced?
I did replace the LTP bias resistor with a 997Ohms, I did notice a voltage change across several points in the amp after replacing the LTP bias resistor, and I got readings for everything I could think of:
B+ nodes
A 461
B 460
C 326
D 279
E 262
V1 P1 150
P2 0
P3 1
P6 201
P7 0
P8 1
V2 P1 161
P2 0
P3 1
P6 279
P7 161
P8 162
V3 P1 245
P2 17
P3 28
P6 242
P7 18
P8 28
V4 P1 across 1ohm resistor 17.5 mV
P3 458
P4 456
P5 -43
P6 -43
V5 P1 across 1ohm resistor 20.4 mV
P3 458
P4 456
P5 -43
P6 -43
Thanks !!
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what happens when you 0 one pot, does the other have more range, try that with both
tack a 1Meg R to one pot, outer pins, leave the other pot 0 what happens
anyone you know, work with have a scope
troubleshooting the signal path without knowing if it's a gain thing or a circuit flaw is a big crap-shoot that usually leads to self installed problems
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V4 P1 across 1ohm resistor 17.5 mV
P3 458
P4 456
P5 -43
P6 -43
V5 P1 across 1ohm resistor 20.4 mV
P3 458
P4 456
P5 -43
P6 -43
If you have 2 well-calibrated V-meters for measuring the tube current simultaneously, you'd soon be able to see whether the difference in tube current between V4 and V5 is due to slight fluctuations in the supply at different measurement times, or because the output tubes aren't absolutely evenly matched. (I suspect the latter, but having said that, they're not that badly mis-matched.)
If you're concerned about losing bias voltage, make sure that:
1) The bias supply circuit is functioning properly by checking (and re-flowing any bad) solder connections:
2) The bias supply filter caps are functioning properly (which you would usually notice sound-wise if they aren't);
3) The output tube grid pin clamps are sufficiently tensioned (you can re-tension them with a small jeweller's flat-head screwdriver - with the amp off and unplugged, and the caps discharged)
Always check you have the requisite negative DC voltage on the output tube grid pins before plugging output tubes in.
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what happens when you 0 one pot, does the other have more range, try that with both
tack a 1Meg R to one pot, outer pins, leave the other pot 0 what happens
anyone you know, work with have a scope
troubleshooting the signal path without knowing if it's a gain thing or a circuit flaw is a big crap-shoot that usually leads to self installed problems
Well I don't know anyone that have a scope.
I did the 1M resistor in the first volume pot but it didn't change the behavior, those pots are very old and I will replace then when I get some new ones.
If you have 2 well-calibrated V-meters for measuring the tube current simultaneously, you'd soon be able to see whether the difference in tube current between V4 and V5 is due to slight fluctuations in the supply at different measurement times, or because the output tubes aren't absolutely evenly matched. (I suspect the latter, but having said that, they're not that badly mis-matched.)
If you're concerned about losing bias voltage, make sure that:
1) The bias supply circuit is functioning properly by checking (and re-flowing any bad) solder connections:
2) The bias supply filter caps are functioning properly (which you would usually notice sound-wise if they aren't);
3) The output tube grid pin clamps are sufficiently tensioned (you can re-tension them with a small jeweller's flat-head screwdriver - with the amp off and unplugged, and the caps discharged)
Always check you have the requisite negative DC voltage on the output tube grid pins before plugging output tubes in.
Very good advices that I'm taking notes and will keep in mind. Thank you for that!
Only one meter atm that can measure mV, this is what I got: https://media.dooca.com.br/raisa.com.br/produto/zoom/multimetro-10a-linha-viva-md-1500-icel-14946192255304.jpg
Would be a good thing to add a bias balance pot to the amp, besides the bias adjust one? That way the current could be adjusted even yes?
This may be very important, in the EL34EH data sheet it says: no more than 425v in the screen, but there is 456v, could this be the cause of the excessive distortion?
Check out those pots :icon_biggrin:
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Screens should be kept around 420V on EL34s IMHO, especially modern production ones. You should use decent Rg2 (1k to 2k2, rated at 5-7W) on the screen pins. The 'design problem' with EL34s is the signal grid is wound close to the cathode, in order to achieve maximum gain. But this makes the signal grid prone to collecting emissive coating material from the cathode over time, which then means the grid starts charging up, and losing its bias voltage. If the grid leak resistance are too high, this can further contribute to loss of bias, leading to thermal runaway and red-plating.
Also there are opinions out there that modern production EL34s should be nominally de-rated to 20W (instead of the advertised 25W) due to reliability problems.
(see the 'Lord Valve' recommendations at the bottom of this page: http://www.duncanamps.com/technical/lvbias.html (http://www.duncanamps.com/technical/lvbias.html)
This means setting them up to idle at 30mA max if you're running the plates at 460. This often mean modding the bias supply circuit to get the grid voltage more negative to the point where it will reliably stay put under normal operation.
Bias balance shouldn't be necessary if you have the tubes reasonably closely matched.
Without getting a scope onto the signal path, its a guess as to where 'distortion' is originating from.
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I got two new CTS pots for the volumes and 2.2k 5W resistors for the PA screens.
It did take the amp to a new level :icon_biggrin: , now does have some clean sound, and it is much more pleasant to hear, but it is still breaking up somewhat too earlier, about volume 4-5.
The resistor didn't drop much the voltage for the screen of the PA tubes, from 456 it went down just about 2-3 volts , there is a little improvement of the mV in the tube that was higher now is a bit lower, dropped about 0.05mV.
Going higher with the screen resistors would do the trick ?
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I'm not a player sooo....
have you played through a Marshall plexi style amp, it seems to me the players I know like it for it's "crunch"
it's "good" distortion, I don't recall anyone wanting clean :dontknow:
I know the default for mine was 2 dark channel 4-6 clean side, BUT they each had "their own" sound, bad, imho. what made the amp is the mix. Not saying to quit tweaking, just maybe know what the amp was designed for, otherwise you might wind up tweaking it into a fender :icon_biggrin:
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I'm not a player sooo....
have you played through a Marshall plexi style amp, it seems to me the players I know like it for it's "crunch"
it's "good" distortion, I don't recall anyone wanting clean :dontknow:
I know the default for mine was 2 dark channel 4-6 clean side, BUT they each had "their own" sound, bad, imho. what made the amp is the mix. Not saying to quit tweaking, just maybe know what the amp was designed for, otherwise you might wind up tweaking it into a fender :icon_biggrin:
On a Plexi nope, but I did play on a Marshall full tube once long time ago, it was a JCM900 it had a clean tone, I didn't fiddle with it enough thou to find out what it could do. I do play on a hybrid model quite often with tube preamp (https://www.instagram.com/p/Bjae_lfHJyl/)(dunno the model) but it isn't the same thing as a full tube amp...
Man the amp sounds much better now, I do like the crunch !! At 50% volume it rocks !!! And I do like the volume mix thing!
The thing is: in the data sheet for EH EL34 it says 425volts max for the screen, the screen of the tube is at the moment at 454, it will stress out the tube yes, how to drop the screen voltage down to the specification levels?
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Its probably not going to be easy to drop screen voltage without installing a complicated regulated screen voltage supply.
It would be easier to instead increase the -ve voltage range on the bias supply circuit, in order to ensure you can get the cathode current down to about 20mA. This may mean having to going up 'one notch' swapping out the bias range resistor. Being able to bias the output tubes on the cold side like this will also help increase the headroom all round, even if it means the bias ends up deep towards Class B.
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it will stress out the tube yes,
yes, but you're within 10%, and it rocks, and it's loud, and it makes you smile when playing, budget for a new set next year :icon_biggrin:
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It would be easier to instead increase the -ve voltage range on the bias supply circuit, in order to ensure you can get the cathode current down to about 20mA. This may mean having to going up 'one notch' swapping out the bias range resistor. Being able to bias the output tubes on the cold side like this will also help increase the headroom all round, even if it means the bias ends up deep towards Class B.
The cathode current is already at 20mA on V5 and V4 at 17mA.
Leave as is now ?
Well the overall sound is very good, no noise or hum, I did play on it with a different guitar that have a more clean tone and I was able to get some clarity on the notes, also plugged in a 4ohms speaker cab(2x12 150w each) but the tone is somewhat dry and metallic, on the 8ohms cab(1x12 80w) it does sound much better, more round and warm. In the EL34 data sheet says plate resistance 15k ohms so I guess a 16 ohms cab would have a better range for the PA tubes.
I still need to finish the cabinet, only the skeleton is done...
Thank you !
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plate resistance 15k
my sheet show Ra-a @450vdc ~7k
the tone is somewhat dry and metallic
2X12 will make her happy, but you need to roll a bunch of speakers to find the sweet spot, maybe check out the cabs n speakers section of the forum for ideas
I run a modern eminence re-issue with an '80s creamback, AND it sounds best with the cream-back on the "right" looking at it :dontknow:
I get sweat-clean from the left and the right takes care of the rest
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Those PA tubes are from Electro-Harmoniks, here is their data sheet. (https://el34world.com/Forum/index.php?action=dlattach;topic=23039.0;attach=72445)
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I believe plate resistance is NOT the same as plate load. Need checking but plate resistance stays basically the same while plate load is dynamic
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Oh the load resistance in the data sheet says 'nominal'.
my sheet show Ra-a @450vdc ~7k
You might be right about that, then if that is the case then putting the 8ohms cab in the 16ohms set in the amp will do the trick? Or is it a bad idea?
I have done those calculations while ago for the OT, I think this will help now to get the best out of the amp.
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
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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 shaking the bugs out, weird behavior happening with the volume controls, the sound doesn't come up right the way as it is turned up, the sound is coming up when the volumes are at about 3, and suddenly it just appear like there was a fast spinning of the pot, to the actual amount set with the pot. What to do ?
Thanks !!
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What to do ?
both ch or just 1
sketchy input jack, sloooow plate voltage, bad cable, solder connection...
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What to do ?
both ch or just 1
sketchy input jack, sloooow plate voltage, bad cable, solder connection...
Both are doing same thing. Could this behavior being caused by the screen resistor that was swapped previous to it?
Would it cause slow plate voltage, in which tubes(if that is the case)?
I did find the problem, the new pots are defective, the old vintage pots are back in place and both volumes are working as expected !! :icon_biggrin:
Now wait for the cabinet !!
Thank you !
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slow plate voltage
put your meter on the "D" node before you power up, jack in some canned music into the input jack before power up, set volumes ~~ 4 before power up.
power up, watching the meter, does it "react" with the sound, like, start at 200vdc the just to 300vdc when the music kicks in? (bs numbers for example only :icon_biggrin: )
the schematic shows 1k screen R, what did you go to?
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slow plate voltage
put your meter on the "D" node before you power up, jack in some canned music into the input jack before power up, set volumes ~~ 4 before power up.
power up, watching the meter, does it "react" with the sound, like, start at 200vdc the just to 300vdc when the music kicks in? (bs numbers for example only :icon_biggrin: )
the schematic shows 1k screen R, what did you go to?
2.2k 5w I did the trick to fix the excessive distortion, it still does distort at high volume :icon_biggrin:
Thanks to Tubeswell, his idea !!
Guess what?! I did find the problem, the new pots are defective, the old vintage pots are back in place and both volumes are working as expected !! :icon_biggrin:
Now wait for the cabinet !!
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Almost done now with the cab, only missing the back panel !
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Done !!
Thanks to all of you that came to my aid !!
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Looks good! Well done! :thumbsup:
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Great work! I see you have one input instead of the standard 4 inputs on a Plexi, did you configure it so both channels are jumped?
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Great work! I see you have one input instead of the standard 4 inputs on a Plexi, did you configure it so both channels are jumped?
Thanks !!
Here is how it is in the circuit, I did it just like in the Hoffman 6V6 version, there is one volume for each side of V1, so it is possible to mix the bright with the dark channels. My amp is the Plexi 50 with EL34s.
https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf