Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on April 09, 2019, 05:27:43 pm
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Back to this amp.
After little use of this amp, maybe played it 4-6 hours tops total since it was done, something went wrong and amp is like failing, sound volume goes down and back up...
Then I decided to look at the power tubes while in operation, V4 is like flashing or sparking inside... :w2:
Probably related to the screen voltage 460 volts, that I couldn't find a way to reduce. So since the OT have UL taps and it have reduced voltage, it could be a solution for the problem. Haven't read the UL taps voltages yet.
Is it a good idea, does the UL taps need a resistor ?
Related topic:
https://el34world.com/Forum/index.php?topic=23039.msg246988#msg246988
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wait for a consensus, but I don't believe UL taps are meant to tame grid voltage. The primary reason I've found, they keep the tube in a happier place, more linear? in relation to the signal, not DC idle characteristics.
you should be able to add a tap for screens, but you might have to re-balance the downstream ones
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460V on EL34 screens in UL is not your problem. (When the tube conducts max, G2 dips to 300V.)
Bad tube. Bad joint. Failing cap or resistor.
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460V on EL34 screens in UL is not your problem. (When the tube conducts max, G2 dips to 300V.)
Bad tube. Bad joint. Failing cap or resistor.
My concern is the data sheet says 425 max, Its an EH tube.
Sorry sir I think I wasn't very precise there, the screens are in the B+ bus. I thought moving it to the UL could be a solution.
What caps or resistors should I look for? Its the Hoffman Plexi 50.
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UL taps need no caps or resistors. It's a direct connection UL tap > screen grid.
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My concern is the data sheet says 425 max
the UL tap is essentially another "choke" in-line with G2, chokes (inductors) are used to "stabilize" current, not really used for voltage dropping. you can see this by measuring volts DC on either side of your PS choke, compare it to the voltage drop across a PS resistor.
If you really want to drop the volts, as a test, disconnect the choke in the PS and replace with a 5k -10k, 5-10W R. NOTE: everything to the right will also drop and may need to be tweaked back up.
I agree with PRR, the G2 volts is probably not your dead tubes issue
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My concern is the data sheet says 425 max
the UL tap is essentially another "choke" in-line with G2, chokes (inductors) are used to "stabilize" current, not really used for voltage dropping. you can see this by measuring volts DC on either side of your PS choke, compare it to the voltage drop across a PS resistor.
If you really want to drop the volts, as a test, disconnect the choke in the PS and replace with a 5k -10k, 5-10W R. NOTE: everything to the right will also drop and may need to be tweaked back up.
I agree with PRR, the G2 volts is probably not your dead tubes issue
I see, if the voltage isn't the issue then:
1st thought, maybe I just got a bad new tube, the other PA tube looks fine. Swap the tubes for checking good or bad idea?
Took the amp out of the cab and examined the components for visual damage, all looks as before.
I did measured all the resistors and all have the expected reading.
Can't get a reading on the caps, is it because they are connected?
Read all voltages?
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Can't get a reading on the caps,
with amp off for long time, put a gator clip from ground to + side of cap, IT WILL SPARK, SOOO be quick, be confident :icon_biggrin:
now, while shorted, gator clip your meter, OHMS, black - ground, red - cap +
Now, disconnect the shorting gator, your meter should run up pretty fast to at least >100k ohms, hopefully infinity - open.
if so;
Read all voltages
preferably with tubes in, just grab BIAS, plate b+, and the last tap. compare to old readings then decide what's next
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with amp off for long time, put a gator clip from ground to + side of cap, IT WILL SPARK, SOOO be quick, be confident :icon_biggrin:
now, while shorted, gator clip your meter, OHMS, black - ground, red - cap +
Now, disconnect the shorting gator, your meter should run up pretty fast to at least >100k ohms, hopefully infinity - open.
if so;
Read all voltages
preferably with tubes in, just grab BIAS, plate b+, and the last tap. compare to old readings then decide what's next
The filter caps already had zero charge, zero voltage.
The caps across the circuit read open, except one in the preamp connected to the mid pot , the electrolitic caps does read some resistance, i did also read in farads for the filter caps, the caps still connected, 3 cap cans:
cap 1 - A 157.9 both positive pins connected
cap 2 - B 126.2 ???
cap 2 - C 55.5
cap 3 - D 60.7
cap 3 - E 55.9
Haven't read the voltages yet.
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preferably with tubes in, just grab BIAS, plate b+, and the last tap. compare to old readings then decide what's next
V4
p3 464v
p4 459v
p5 -44v
p6 -44v
cathode resistor 20.8 mA
V5
p1 464v
p2 460v
p3 -43
p4 -43
cathode resistor 18.3 mA
It is about the same as last readings
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Brrrr. That's biased very cold.
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Brrrr. That's biased very cold.
I did try while ago to bias it hotter but its when one of the PA tubes start to red plate, and come to this state. Its biased at 20mA as PRR suggested. (https://el34world.com/Forum/index.php?topic=23039.msg253715#msg253715)
What I got is a bad tube yes? Is the circuit ok ?
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What I got is a bad tube yes? Is the circuit ok ?
without changing bias for now;
does it play without a tube arcing?
if you can play for an hour, bump up the bias a tad, if a tube gets ugly swap it, does the problem "follow" the tube?
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What I got is a bad tube yes? Is the circuit ok ?
without changing bias for now;
does it play without a tube arcing?
if you can play for an hour, bump up the bias a tad, if a tube gets ugly swap it, does the problem "follow" the tube?
The tube that red plated is in V3 position now, it was in V4 position before, when it did red plate.
Yesterday the amp did play while it doing the arcing as you said, but the volume sound goes down and up.
About 40-50% less volume when it drop.
Just about 10 mins ago: ...
I'm puzzled now, I did as you said, "play for an hour" , about that, the problem didn't happened :w2:
Man this thing plays loud rock'n'roll sound like !!
By the way the OT gets somewhat warm, it is possible to keep touching it. Temp is about 40C or over.
So bad solder joint?
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Man this thing plays loud rock'n'roll sound like !!
that's what we LIKE :icon_biggrin:
did you verify that the 1.5k grid R's were ok?
If so, you're probably ok
I'd have a spare tube or 2 just in case it is a bad one
a quick check if it's on the bench, pull the 2 PA tubes, unplug speaker, ohm the outside 2 primary OT wires to the middle wire, they should be reasonably close, wiggle the wires while the meters hooked up just to "see" if something is loose.
I like to put one probe in the socket where the tube goes, that way you catch all the connections, then clip the other probe at the choke that feeds the CT OT
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did you verify that the 1.5k grid R's were ok?
If so, you're probably ok
I'd have a spare tube or 2 just in case it is a bad one
a quick check if it's on the bench, pull the 2 PA tubes, unplug speaker, ohm the outside 2 primary OT wires to the middle wire, they should be reasonably close, wiggle the wires while the meters hooked up just to "see" if something is loose.
I like to put one probe in the socket where the tube goes, that way you catch all the connections, then clip the other probe at the choke that feeds the CT OT
Did play another 5-10 mins all good.
The resistors read good, they are 2.2k 5W, you mean screen resistors yes?
Been thinking about a couple spare tubes... that money thou is for the KT88 amp.
The amp is in the bench still, did the readings , no PA tubes, no speaker.
Choke read 106.8 ohms
OT does read 101.6 ohms(V4) in one side and 131.9 ohms (V5)in the other.
Reading across the choke, 207 ohms in one side and 237 ohms in the other side
30ohms off, that may be the cause of so much distortion? Also the cause of red plate?
... In a perfect OT each half of the primary winding would be exactly the same. But because of the way OTs are usually wound, one side will have a little more resistance than the other side. 150Ω and 172Ω are close enough, ...
I did use a cheap meter at that time.
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if you have an arcing tube, throw it away before you damage the OT. The resistance readings are not exactly what you'd want, but i've measured OT primaries that worked fine with a similar difference. That said, I have also had a couple of amps with similar differences of the secondary DCR incite red plating of the tube connected to the lower R side of the winding with matched tubes. if the red plating follows the tube, no matter the socket, then the tube may be gassy (bad) or grossly miss-matched.
--pete
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if you have an arcing tube, throw it away before you damage the OT. The resistance readings are not exactly what you'd want, but i've measured OT primaries that worked fine with a similar difference. That said, I have also had a couple of amps with similar differences of the secondary DCR incite red plating of the tube connected to the lower R side of the winding with matched tubes. if the red plating follows the tube, no matter the socket, then the tube may be gassy (bad) or grossly miss-matched.
--pete
Is there any way to balance the OT ? Bias balance does help with it ?
Amp is on, did play a bit, no bad behavior so far after about 40 minutes.
Got a clip for you guys !
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nice sound!
I been thinking that for a small imbalance you might??? be able to use a 10ohm under 1 tube, and a 1 ohm under the other kinda thing, but I'm not good enough to actually know before hand :icon_biggrin:
I'd take DL's suggestion, new tube, but I would label and keep the suspect for experimenting only.
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I'd take DL's suggestion, new tube, but I would label and keep the suspect for experimenting only.
I did let the amp on yesterday for about 6 hours played about 3, no problems happened...
nice sound!
Thanks !! :icon_biggrin:
I been thinking that for a small imbalance you might??? be able to use a 10ohm under 1 tube, and a 1 ohm under the other kinda thing,
Lets say that is possible, how to do that, resistor in series with OT tap to tube pin ? or resistor to ground?
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Lets say that is possible
paraphrased; "ALL things are possible, but not ALL things are beneficial" :icon_biggrin:
I have done it to self biased PP, never fixed bias, that said, IF I was to try it would be with the 1 ohm R's used to monitor current, and I probably would chicken out anything > 20 ohms.
the "better" way would be bias the "hot" tube to your happy place and play
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I want to add a balance bias pot to the amp for better control over the PA tubes, would this work ?
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no
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no
Is it totally wrong or is it missing something?
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Totally wrong. It will do nothing. But you have fixed bias amps. Try it and see for yourself. Meanwhile, think about it and I bet you will figure out why. :think1:
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Totally wrong. It will do nothing.
Argh :BangHead: :laugh:
But you have fixed bias amps. Try it and see for yourself. Meanwhile, think about it and I bet you will figure out why. :think1:
There is no ground, that is one thing. I don't fully understand it.
Bias adjust + bias balance is the goal, I it found at Robinette page.
It is going to be trick to adapt it.
How about the UL taps ?
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Your circuit simply changes the resistance value of the two 220K grid resistors. For example, with a 100K pot you could adjust the value of one resistor to be 320K and the other would be 220K. You could even make those two resistors have exactly the same value (approx. 270K each). But you cannot change the bias voltage that appears on the grid of the output tubes because there is no current flow through your pot or those 220Ks and with no current flow you do not drop/change any voltage. Bias remains the same.
Rob's circuit works but requires that special pot with a tap. The simplest solution I've ever seen is to simply have two separate Fender style bias adjust pots. For example see page 2...
http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
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Your circuit simply changes the resistance value of the two 220K grid resistors. For example, with a 100K pot you could adjust the value of one resistor to be 320K and the other would be 220K. You could even make those two resistors have exactly the same value (approx. 270K each). But you cannot change the bias voltage that appears on the grid of the output tubes because there is no current flow through your pot or those 220Ks and with no current flow you do not drop/change any voltage. Bias remains the same.
I see, it is just another resistor in combination with the 220k ones.
Rob's circuit works but requires that special pot with a tap. The simplest solution I've ever seen is to simply have two separate Fender style bias adjust pots. For example see page 2...
http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
Ooops that was the wrong picture.
How about this one ?
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Got schematic for the above.
Have to figure out how to implement in the circuit, not much space there, maybe a small board just bellow the two 220k resistors.
Would this work ?
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Imho, you're overthinking/overdoing the balanced bias thing. A slight mismatch, caused by either tubes or output transformer won't (necessarily) hurt your tone. Bias the hotter tube correctly, as someone already said, and leave the colder one, well uh, cold. Better still, with your transformer, to use a slightly mismatched pair: colder tube on the hotter side and vice versa. This way you don't have to get finnicky with the bias balance trimpot nor pay for platinum premium tube matching, ha!
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Imho, you're overthinking/overdoing the balanced bias thing. A slight mismatch, caused by either tubes or output transformer won't (necessarily) hurt your tone. Bias the hotter tube correctly, as someone already said, and leave the colder one, well uh, cold. Better still, with your transformer, to use a slightly mismatched pair: colder tube on the hotter side and vice versa. This way you don't have to get finnicky with the bias balance trimpot nor pay for platinum premium tube matching, ha!
That is a good idea !!
Played yesterday with pedals and one or maybe both PA tubes are like flashing inside, and there a behavior like something is falling, the volume goes down and up ... a day before did play with no pedals, just the guitar in the amp, for about 3 hours no problem, the amp stayed on for about 6 hours under observation and the problem didn't happen.
I'm not worried about the tone it does sound good, althought break up too soon for my style of playing,
but the amp isn't working properly. There is definitely, something wrong with the amp.
When biasing the hot tube hotter, it does red plate.
I think balancing looks to be one thing that will help to stabilize the amp? I did stop writing and decided to look at the amp again...
10 mins later:
I just read again the OT primary, there is something wrong with it, at first one side wasn't giving any readings, :w2:
then I shake the wires , fiddled, wiggled ... got reading
now it read
131 ohms in one side(V4) and 126 ohms in the other side(V5), loose wire inside the OT? bad joint ?
Then after re-soldering the OT primaries
112 ohms in one side(V4) and 127 ohms in the other side(V5)
Keep changing?
:help:
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There is definitely, something wrong with the amp.
I think balancing looks to be one thing that will help to stabilize the amp?
I seriously doubt bias balance has anything to do with your problem.
Bad connections could be the source of the problem. Especially check anything in the bias supply up until the grids if the power tubes.
I also wouldn't discard some kind of oscillation as a possibility.
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+1
Then after re-soldering the OT primaries
the arc'd tube should be replaced now that your wiring is fixed, then just run it and watch.
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+1
Then after re-soldering the OT primaries
the arc'd tube should be replaced now that your wiring is fixed, then just run it and watch.
First thing in the morning, did read again the OT
124.8 ohms V3 126.4 ohms v5 why does it keep changing ?
Re-soldering all the bias joints as well.
The feedback loop resistor read 4.7k while connected to the OT , disconnected read 47k, is it expected?
One of the LTPI resistor is one 1k, but 470 ohms in the schematic, replace it ?
Going to get new tubes today(unmatched pair), And the parts for the bias balance, why? Cos the matched tubes are too expensive (https://www.multcomercial.com.br/audio-parts/valvulas/el34.html).
Is that bias balance circuit correct?
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> 124.8 ohms V3 126.4 ohms v5 why does it keep changing ?
Not much of a change. Could just be the way the probes contact the leads. Make your probes needle sharp. Now always try to pierce the solder joint and you will get more consistant readings.
> The feedback loop resistor read 4.7k while connected to the OT , disconnected read 47k, is it expected?
yes
> One of the LTPI resistor is one 1k, but 470 ohms in the schematic, replace it ?
yes. Why was it ever 1K???
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Copper has a pretty large temperature coefficient. 130 ohms while hot and 125 when cold is completely reasonable. It would only require about 20F increase.
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> 124.8 ohms V3 126.4 ohms v5 why does it keep changing ?
Not much of a change. Could just be the way the probes contact the leads. Make your probes needle sharp. Now always try to pierce the solder joint and you will get more consistant readings.
Today it read V3 103.3 , V4 126.4
V3 tap (blue tap of the OT) have a initial peak reading of about 130ish then rapidly drop to 100ish.
> One of the LTPI resistor is one 1k, but 470 ohms in the schematic, replace it ?
yes. Why was it ever 1K???
I dunno !! :laugh:
Why Fender uses in that part of the circuit 6.8k and Marshall 10k ?
Got two new Mullard tubes(unmatched), gonna do the bias/balance circuit .
Is the bias/balance schematic I draw correct? Remove the 47k (R35) resistor from original bias circuit?
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Is the bias/balance schematic I draw correct? Remove the 47k (R35) resistor from original bias circuit?
Sometimes you just have to build it and see if it does what you want.
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Is the bias/balance schematic I draw correct? Remove the 47k (R35) resistor from original bias circuit?
Sometimes you just have to build it and see if it does what you want.
The arrangement for the bias on the following schematic looks about the same yes? A bias adjustment pot and bias balance pot!
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Your circuit simply changes the resistance value of the two 220K grid resistors. For example, with a 100K pot you could adjust the value of one resistor to be 320K and the other would be 220K. You could even make those two resistors have exactly the same value (approx. 270K each). But you cannot change the bias voltage that appears on the grid of the output tubes because there is no current flow through your pot or those 220Ks and with no current flow you do not drop/change any voltage. Bias remains the same.
Based on the circuit above got another idea, check it out!
Does it work ? :think1: :help:
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It will only take a few minutes to temporarily wire that mod to the board. You don't need any tubes plugged in to test it. No animals will be harmed during this test. Just do it and tell us what you think.
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It will only take a few minutes to temporarily wire that mod to the board. You don't need any tubes plugged in to test it. No animals will be harmed during this test. Just do it and tell us what you think.
What about the trimpot and resistors value? I got 20k trimpot, what value for the resistors?
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I don't know. Just experiment.
I don't like a bias adjust and bias balance system because the two pots are interactive. Tweak one and you'll need to re-tweak the other. May have to do this several times. I've already shown you the simplest circuit to do this. And a bonus is the pots are not interactive. Set one to the bias point you want. Then set the other to the bias point you want. Done.
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I don't know. Just experiment.
I don't like a bias adjust and bias balance system because the two pots are interactive. Tweak one and you'll need to re-tweak the other. May have to do this several times. I've already shown you the simplest circuit to do this. And a bonus is the pots are not interactive. Set one to the bias point you want. Then set the other to the bias point you want. Done.
Oh I see ! I thought it would be easier to fit in the board, but as I found out it is harder to tweak ! Now the way it is in the Sunn Spectre is much easier to tweak, but probably harder to fit in the board. Gonna study that !
It does work :icon_biggrin: !! But the voltage increased a lot, to less than -10v :blob8:
I did put a jumper in the 47k resistor connected to the 50k trimpot, no changes :w2: hopping the voltage to go down... :think1:
How to get the voltage down back to the range for bias ?
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Try 100Ks for those two resistors.
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Try 100Ks for those two resistors.
100k resistors did drop some voltage but just about -10v.
Then with 330k resistors, the bias adjust trimpot does -35 to -21
and the balance pot -36 to -34.
With 430k resistors bias voltage got close to -40 to -24, with the balance pot in one end, with the balance in the middle, bias voltage is just bellow -40, about -39ish.
Balance pot gives about 2 volts range,
Looks like still need about -10 volts to get in the desired range
How's that sounds like so far?
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Looks like you're heading down a deep rabbit hole. Don't talk to the smoking caterpillar. :icon_biggrin:
You're way too high with the resistor values. Go back to 100K and play with R36 value. Decrease value to increase voltage.
But before you do anything else, put the balance pot at center position. Set the bias voltage to some convenient level on pin 5 of one of the tubes. Not looking for any specific voltage. Now connect one meter probe to pin 5 of one tube and connect the other meter probe to pin 5 of the other tube. Ideally you want zero volts on the meter with the balance pot set to center. Now rotate the pot to one end and record that voltage. Then rotate the balance pot to the opposite end and record the voltage. One end should be a positive voltage and the other end should be a negative voltage. The wider this range of voltage, the more likely you will be able to balance tubes. But if you only swing from +1v to -1v, you will only be able to balance tubes that are already very closely matched.
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play with R36 value. Decrease value to increase voltage.
I got to buy some resistors, none to replace R36(220k 3W) at the moment with 3W or with smaller values like 200k 180k or so, I got tiny ones with 1/4W and 1/8W, maybe some with 1/2W gotta dig the stash.
Looks like you're heading down a deep rabbit hole. Don't talk to the smoking caterpillar. :icon_biggrin:
You're way too high with the resistor values. Go back to 100K...
:laugh:
430k in the last test
What happen to the circuit with those high value resistors ?
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I think your balance voltage swing will just get smaller and smaller, making the balance pot less and less useful.
Do the test I suggested. What have you?
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I think your balance voltage swing will just get smaller and smaller, making the balance pot less and less useful.
Do the test I suggested. What have you?
I notice decreasing the balance resistors also decrease the bias range.
I'm still digging resistors...
In the Williamson, balance pot is 10k and bias 25k. 20k and 50k in place atm.
I thought the pot balance could be smaller as well, to increase negative voltage?
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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?
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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?
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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.
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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 ?
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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.
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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?
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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.
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Sounds like you're ready to put some tubes in it, bias and balance and close the book. :icon_biggrin:
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Sounds like you're ready to put some tubes in it, bias and balance and close the book. :icon_biggrin:
I was just thinking about that! :icon_biggrin:
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.
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How's the bias mod working out?
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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. :dontknow: :help:
Found this info (https://www.w8ji.com/loading_amplifier.htm#Improper_Loading_of_Amplifier):
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?
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> Found this info:
That's a radio transmitter. A very different thing.
Sudden jumps at a certain combination of gain and treble suggests oscillation.
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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 ?
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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?
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No scope, don't even know how to use it :laugh:
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 :dontknow: 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 ?
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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
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don't even know how to use it
here's some reading when your fingers get to sore to play :icon_biggrin:
https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/all
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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.
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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
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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.
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What OT are you using? datasheet?
:dontknow: unknown got it from an old hi-fi amp (https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697)
No markings on it.
I have the feeling the OT isn't for EL34 :laugh:
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.
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...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 :wink:
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 :icon_biggrin:
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.
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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.
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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 :icon_biggrin:
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
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.
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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?
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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?
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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 :w2:) 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 :icon_biggrin:).
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?
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using Hoffman's schematic
some hacks to try, can be done mostly with gator clips
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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! :icon_biggrin:
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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! :icon_biggrin:
:laugh: :l2: :l4:
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 !
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The amp sound really good !!
So why are you working on it!!!
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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.
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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. :laugh:
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It have the MOJO thou
you know once you fix it, the MOJO will be over :icon_biggrin:
does it matter which input?
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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. :laugh:
:l2: 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.
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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
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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,
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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?
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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?
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007
7 what? mV, Volts, Kv :dontknow:
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?
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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.
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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 :icon_biggrin:
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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 (https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697).
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 :BangHead:
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 :icon_biggrin:
:laugh: I was looking at this one (https://sp.olx.com.br/sao-paulo-e-regiao/audio-tv-video-e-fotografia/osciloscopio-mesco-ao-1221-604233478?rec=l) ! 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 :dontknow:
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.
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At this point a colonoscopy might be the only option. :icon_biggrin:
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!
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I appreciate very much all the help, been learning a lot from all you guys!!
:happy2:
About the DC voltage drop, where to look ? Filter caps?
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the PT values look good, that's a fine old tranny!
the volts is the current is the volts.........
kt88 tranny :l2:
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)
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the PT values look good, that's a fine old tranny!
the volts is the current is the volts.........
kt88 tranny :l2:
I was thinking about the same thing ! :icon_biggrin:
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?
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is it under warranty :icon_biggrin:
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
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forgot to add this :l2: :think1:
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
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I got the cellphone playing through the PI :icon_biggrin: 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 ?
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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 (https://el34world.com/Forum/index.php?topic=20476.msg215697#msg215697).
Yeah, I read that thread three years ago. :laugh: 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.
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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.
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Feed the amp section with the plate of the first ax7, disconnecting everything else in the path.
It ll give you a good clue.
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yes .005
even with low input, did the volume change up/down?
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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 ?
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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
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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 (https://www.multcomercial.com.br/capacitor-eletrolitico-radial-50uf50uf500v-fet.html)
Now the PI caps are old , really old. the little brown caps look in the pics.
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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 :icon_biggrin:
E is volts, same as V. depending on age we either learned it as E or V or HT, or... :think1:
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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 (https://el34world.com/Forum/index.php?topic=23039.msg253248#msg253248).
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 :icon_biggrin:
We have done this before here (https://el34world.com/Forum/index.php?topic=23039.msg253830#msg253830) and here (https://el34world.com/Forum/index.php?topic=23039.msg253975#msg253975), they did ok back then, could they drift in this time? Gonna get it again anyway.
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drift in this time?
should be good, just next order, add a couple caps
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-53v is way too high for el34s. You need to be between -40 and -45.
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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
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swap the tubes around, does the current follow the tube or stay with the socket?
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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.
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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
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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 !
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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 :icon_biggrin:
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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 :icon_biggrin:
I got the tubes matched(almost) EH on V5 with .018v and Mullard on V4 with .019v !
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SWEET, now go fix your amp :icon_biggrin:
where do the #'s fall, idle no signal
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SWEET, now go fix your amp :icon_biggrin:
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 ?
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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.
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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.
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Adjust the bias to increase tube current to 36mA (36mV across the cathode 1Ω resistors). How does it behave and sound?
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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.
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Temporarily disconnect the NFB. Sound any better?
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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.
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Reconnect the NFB. Then swap the OT primary plate leads. What happens?
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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.
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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 :dontknow:
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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.
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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?
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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 :icon_biggrin:
fwiw, the amp I mentioned, it's still out being used, just not overdriven
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You're now a highly qualified troubleshooter, fix some amps and get some money :icon_biggrin:
Not a bad idea !! :icon_biggrin:
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 !!
:happy2:
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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! :icon_biggrin:
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I dunno if it was running a pair of 6v6 :dontknow: 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.
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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.
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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
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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 :icon_biggrin:
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If pin 1 was not connected to anything then it ain't an EL34.
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If pin 1 was not connected to anything then it ain't an EL34.
Pin 1 was connected with pin 8.
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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
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Is this model 40-18025 a suitable OT for the amp?
Thanks
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I don't know. :dontknow: :think1: Are you building a 50 watt Marshall amp? :wink:
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I don't know. :dontknow: :think1: Are you building a 50 watt Marshall amp? :wink:
:w2:
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.
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I think the dollar might weaken soon, get a good exchange rate, buy it, play it :laugh: