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Messages - Dimi Pana

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The Newcomer's Forum / Re: Amp repair?!
« on: February 13, 2019, 12:11:30 AM »
Excellent points, now I have lots of reading to do. Hmmm, I did not think about the zip tie getting loose because of the resistor heat, it's not much but it does warm up. I wanted a quick solution so I can put the amp back together and play. I will revisit this hopefully in a few months, I intend to do a few mods to this amp, the double block connectors is a solid solution I will do that. My friends EVJ is indeed a newer production (judging by the serial number) but still the exact same version 3 (identical PCB, components, etc.). So I am not sure what's going on but for sure this amp was designed by mistake/accident who knows, with heaters voltage higher than what it should be. Anyway, I am so glad mine now works and (to my ears) it works well. Thanks again, cheers!

The Newcomer's Forum / Re: Amp repair?!
« on: February 12, 2019, 12:36:53 PM »
Thank you both for the feedback, I do appreciate the free tutorials, and as a formerly clueless end-user, I can now confidently assert that hey, when it comes to expert opinion there is no substitute, you have definitely opened up my eyes to matters that I was completely unaware of.  :duh


#1. The amp is fixed and packaged back to its original state and it works REALLY well. I was able to find and ..."borrow" a few low Ω resistors from work, and after some mucking around, guess what? A .27Ω resistor (5W @10%) brings the heater voltage to its ideal range. Actually, at start up it shoots up to about 6.38V then goes down to around 6.28V and finally stabilizes exactly at 6.3V. Sorry for being pedantic, I was just playing with different values and just got lucky! I used some spade terminals soldered to the resistors leads dressed the connections nicely with heat shrink tubing, found a nice spot away from the mains and used some existing holes in the chassis (the ones where the heater wires enter from the PT) to thread a zip tie that fastened the resistor against the chassis with enough space for air to circulate around it. Like already said, even after of plenty of time of playing the amp, the resistor is comfortable to the touch, warm but not hot, actually while still in the open (e.g. chassis not back in the cabinet) a laser thermometer showed about 78F and after I put it in the enclosure that spot (touched from the outside) never felt hot, actually it was barely warm, perhaps because the chassis now dissipates the heat better(?!?). So it is done and THANK YOU SO VERY MUCH both/all!   :dbtu:

#2. I installed a new JJ EL84 and instead of a 12AX7 I opted for a new 5751 also JJ. The sound is sweet, it distorts much later and although it is probably the psychological aspect of it, I think it sounds better than before. So, looks like my $75 investment at a recent garage sale paid off. Even better, now I know a lot more about how this thing works (my first tube amp, btw) I did not spent a dime for the repair (other than time and readily available parts) and guess what, all this reading made me realize the EJV is probably one of the most modded low-power amps out there, to the point that I now feel ..."dangerous" enough to attempt some of this mods. I know what you're thinking, we'll never get rid of this guy, he "will be back"!!!   :grr

#3. I was also able to borrow that other EVJ from my friend and guess WHAT?!? As far as voltages, it is EXACTLY THE SAME !!! The PT provides 7.6VAC for the heaters and with my house's 125 VAC mains (btw, is that normal, or a bit high?), and tubes installed, when measured at the pins, I read a ..."nice" 6.93VDC. My friend said he had this amp for about 5 years now, he does not use it for gigging, just home practice/jamming, and he has never changed tubes at all!?! Again it is probably my ears, but my amp is quieter at any volume level and actually if cranked mine sounds more defined, the other is getting "muddier" earlier. Anyway, ... So you were of course right all along (not that I doubted any of you) still I am not sure why mine had the problem I experienced which led to the discovery of the "elevated" heater voltage, but honestly, I am not going to ...overthink this any more/longer.   :-[

#4. Regarding the wattage measurements, I used the 7.8VAC coming from the PT, and the 1A nominal current the two tubes draw per their data sheets.  But then I measured with my DMM the actual current and found it to be 1.8A, thus the widely varying figures I stated in an earlier post. I now understand how it really works, thanks Enzo and Phil. I am not sure why though, currently with the fix that draw has gone down to about 1.35A so there is still something on top of the filaments that draws the .35A current. Not worrying about it, after all the schematic does not show the heater circuit so I do not know if there is something else in there other than the heater's filaments.   8|

#5. This is not a question but I would be ungrateful if I did not mention again how appreciative I am for your help. If you guys ever happen to visit the "Windy City" PLEASE drop me a line, I'd like to return the favor.


Cheers to all and take care!       

The Newcomer's Forum / Re: Amp repair?!
« on: February 11, 2019, 03:17:37 AM »
You are correct, overthinking is a novice's natural reaction while treading unknown territory for the first time. IMO, I'd rather overthink than underthink, with high voltage amps it is also safer, I think.

Now, you have given me a lot of good advice, i.e. indeed the probes amount to about 0.2 Ω so the actual value I measured is about 0.6 Ω which is indeed within the theoretical 0.58 Ω. Ι looked up online, cost will not be an issue, any such resistor (.5 to .75 Ω @ 5 or 10 W or ...more) is going to cost way less than actually shipping it first class mail. Not worth ...overthinking that.

Can you also please help me with the following: Without inserting the voltage dropping resistor, there are two (orange color) wires coming from the PT providing 7.8 VAC for the heaters circuit and the tubes according to their data sheet should draw about 1A combined (in practice I measured 1.8 A but what do I know?). Anyway, according to Ohm's law the power should then be (that's easy P=VxI) a theoretical 7.8 W to an actual 14.04 W. Provided that my thinking is correct, a 5W resistor is insufficient in both cases. It is my understanding that the more power a resistor is rated at, the better it dissipates the heat. So what is the recommended value in my case?

Also, what is best: One resistor for the entire heaters circuit in series to one of these orange wires, or should I split the total recommended Ω value in half and add one resistor per each wire?

Regarding the PT voltage (and with my limited knowledge about amps) I doubt the designer screwed up so bad I mean the schematic I got says the PT provides 6.3 VDC but I am reading close to 8 VDC in mine, which after rectification you well know is going to be over 10, so as Phil said, someone screwed up big time or something is iffy with this particular PT. I still have not been able to borrow a friends identical EVJ to compare with mine, it will be interesting to see what's going on.

Anyway, thanks also for the info about running cold or hot, since I still have to get the proper resistor, I will try to tweak it just a tad higher, just to be OK according to the tube's data sheet. Btw, a few weeks ago when I started having problems with this amp I had no clue about heaters voltage etc. Now I understand that tube manufacturers say 6.3 V is ideal, a +/- 5% is acceptable and the maximum tolerance is up to +/- 10%. So, I hope around 6 V is OK and I heard somewhere that less heater voltage might help the amp distort a little later and the EVJ being as a simplistic design as it is, it does suffer from that also.

Again all this is just an excuse to play and learn with an amp that I can afford to ...destroy (not intentionally of course) so thank you all, and thank you Εnzο for the input.

The Newcomer's Forum / Re: Amp repair?!
« on: February 11, 2019, 01:04:15 AM »
Success!  :dbtu:

Well, it is is not finalized yet but I managed to bring the heaters voltage down to 5.89 VDC, so then I was able to play with the EVJ for about an hour, clean, overdriven, pedals, straight into it, no funny sounds, high or low volume, no crackling, amp is not overheating, and most importantly it did not blow any tubes.

So the method Phil suggested has been tried and it works, I had to use a combination of four (!) resistors, 1, 2.2, 5 and 15 Ω (that's what I had available at home), in parallel, to achieve the lowest possible resistance value, which in theory is .58Ω but in practice my cheap DMM showed 0.8 Ω (well these were wirewounds at 10%).

Still 5.89 VDC is on the low side (not sure if that's good or bad) but sound/tone wise it had no perceivable change. I think an actual value between 1/2 and 3/4 of an Ohm should land me in the sweet spot of around 6.3 VDC, and actually I'd rather run a little on the "colder side" for tube longevity, am I right about that?

So a few questions:

1. As expected, the resistors got a little warm, still was able to comfortably touch them I mean after an hour of operation, they did not burn hot at all, so that's good. I could be wrong but the lower the resistance the warmer they appeared to be, the 15Ω was almost not warm at all. They were all 5W at 10% so what do you think for the final fix, should I get one at a higher wattage (thus bigger) or it is better to split the load (heat) across many of equal value. And I do not really need a heat sink, just bolt them against the amp chassis, right?  Also can you suggest where I can buy such resistors?

2. This is more of a philosophical question but it is funny how the PT the designers decided to use is putting out more voltage than what it is healthy for the tubes. Or you think the PT is developing a problem? From previous posts I understand that if the PT is on its way out, most likely it would have provided less voltage not more, am I right?

Thank you all and especially Phil for sticking with me along the way, so far this has exceeded my expectations, I really appreciate your help!


The Newcomer's Forum / Re: Amp repair?!
« on: February 10, 2019, 01:11:41 AM »
OK, sorry for the late reply, got busy at work this past week. However, I was able to do some more "digging" and this is what I found. Also, keep in mind that all following measurements were taken with standard 125VAC grid voltage (e.g. without using the step down transformer) and with the tubes inserted, speaker connected, no guitar connected and volume all the way down. I did not let the amp stay on more than 1-2 minutes, I took the measurements and powered-OFF. Nothing "bad" happened, tubes were tested and found OK, no ...smoke or funny smells, you know what I mean eh?!  ;)

So, here's what I found:

#1. The PT outputs 7.8 VAC which feeds the tube heaters circuit. As correctly pointed out by Phil, the heaters of a 12ax7 and a EL84, combined, should draw about 1A. However, an actual measurement showed that at power-ON the current "rushes" to a little below 3A but it quickly starts falling and settles around 1.8A after about 30 seconds or so (I am assuming this is when the heaters have reached full operating temperature). Now, if I measure the heaters' voltage at that point, I get a consistent 8.5VDC at both of them.

#2. Through trial and error I found that the amount of resistance needed to bring the heaters voltage (measured at the pins with tubes inserted) down to its optimal range (e.g. +/- 5% of 6.3 VDC) should be between 0.5 and 1Ω. How so you may ask? Well, I had a few 5W wirewound resistors (5, 2.2 and 1 Ω) and tried them all. I started with a 5Ω but it brought the voltage too low (about 1.85VDC) the tubes did not even glow, then I tried the 2.2Ω, voltage went up but still low so tubes were barely glowing, then I tried a 1Ω and the voltage went up to about 5.38VDC, the tubes did glow but I ...chickened and did not try to play through the amp, I was afraid I might do some damage.

So, now I have two questions:

A. How do I (accurately and confidently) calculate the proper resistance value in order to achieve smack 6.3VDC at the pins of the heaters?

B. Assuming this is accomplished, can I consider this a solid/final solution or is it a necessary workaround/hack that works but might cause problems down the road. If it is not a, then what do I have to do to bring the voltage down without using the brute force solution (in series resistor). Someone I asked mentioned something about building a simple zener diode voltage regulator. I googled that and from the little I read it sounds like a viable solution. Can anyone please weigh in on this?

And as a ..."bonus question" can anyone explain why instead of about 1A, I measured 1.8A drawn by the heaters circuit? Also, Phil, by "block connectors" you mean that in the pic right?

The Newcomer's Forum / Re: Amp repair?!
« on: February 01, 2019, 12:57:08 PM »
Τhanks for the info, it prompts me to study more, and that's a good thing. Tube amps seem simple from a first look but it can get complicated once you understand how each component feeds into the other from one stage to another. I am really excited about getting my hands to a working EVJ and taking some measurements comparing to mine. I should be able to report back by tomorrow afternoon. In the mean time can anyone elaborate (but not much and you can always provide a ink) on what is the effect voltage variance has on the heater filaments. And by "effect" I mean both electronically as well as from an audio (tone) perspective. Thanks!   

The Newcomer's Forum / Re: Amp repair?!
« on: January 31, 2019, 06:42:22 PM »
Phil hi -

Yes, it makes sense, basically follow the heater circuit starting from the two orange cables coming from the PT, through the rectification then to the cap. The 4700μF cap is obviously the one from the heater wires to ground. The only problem with doing all this AND taking measurements is removing the PCB from the case, turning it upside down, re-attaching all wiring, etc etc. The way the PCB is, its solder pads side is hidden under the visible side. Anyway, I will definitely get the identical EVJ this Saturday and will be able to assess what's going on after I take measurements and compare. I can certainly put in there a better (higher quality) bridge with the desired DC output OR like you said the easiest MOST effective remedy is to calculate the appropriate resistor value. Thanks and I will be back this weekend with more!

Edit: Btw, how do I calculate the value of the resistor? I know I have to use Ohm's Law (R=U/i) I know the target voltage and I need to find the Resistance. But how do I know what the Amperage is? Are you saying I should measure the amperage without any modifications and go from there? What about the tubes' filaments, that's resistance too, right? Shouldn't that be taken into account also? ???

The Newcomer's Forum / Re: Amp repair?!
« on: January 29, 2019, 01:08:58 PM »
Thank you all for the suggestions. I am providing you with some extra feedback I was able to collect from a friend of a friend who has an identical EVJ and also need confirmation about some ideas of my own.

1. By this week's end I should be able to have that identical EVJ borrowed and I will do some measurements and compare with mine. At that point we will be able to determine which part or the "equation" is ...faulty. The PT, the bridge, something else, who knows?

2. I am trying to confirm that the bridge that rectifies the AC to DC for the heaters is that black square component with the four legs. You are probably not able to see it well in the pics I uploaded, it sits between the two spade connectors where the two orange wires are going in from the PT and the (big) 4700μF capacitor. However there is also an array of four black diodes forming -clearly- a rectifying network, what I do not know for sure is which goes to the heaters, and which to the rest of the amp. Just by their location I am guessing the diodes are for the main amp circuit and the bridge for the heaters.

3. Can you provide either a quick (if possible) guide how to test all possible components or a link where I can read up? I mean I understand an ideal scenario is to remove them from the PCB and test independently however this is not possible at this time, I will wait until I had the chance to compare with the other working EVJ and draw some -hopefully final- conclusions. Still, how do I test a bridge, or a diode array, or that big capacitor? I am also a bit concerned about high voltages. Can you please advice.

4. Someone pointed me to the possibility that all this tube swapping I did (remove, insert, remove, etc.) may have loosened the pins in the socket. So the theory is that a pin that makes no contact at all or worse one that makes intermittent contact may be the culprit. Who knows, maybe I was hasty and damaged it, but again bear in mind that it is ONLY the power tube and not the preamp tube that's acting up. Still the heater voltage is the same at both the power and preamp socket. Do you see where I am driving at?

Anyway, thank you again, looking forward to your replies.

PS: Not sure where you guys are but here in the NW suburbs of Chicago we are bracing for one of the coldest 48 hours in this area's history, we expect temps of about -13 and probably double that with the windchill. All I can say is: Good luck!

The Newcomer's Forum / Re: Amp repair?!
« on: January 28, 2019, 06:54:32 PM »
WOW galaxiex !!! Where on earth did you find this?! I love building little projects like that. Essentially I am making a step down transformer that is more portable than what I already have now and although I was looking for a more direct approach this workaround is certainly an option plus a bucking transformer is always a useful thingy to have on my bench. Thank you so much, where did this come from, do you have more projects you can share like that? I'd be very interested. Thank you!

The Newcomer's Forum / Re: Amp repair?!
« on: January 28, 2019, 06:12:30 PM »
ΟΚ, so here in the NW 'burbs of Chicago, I've been spending most of my free time removing snow, maintaining the snow blower and preparing for the next snow, so amp repair was put in the back burner. But here's something interesting, I was able to use the amp with no problem and no tubes burning up by powering it through the step down transformer. It sounds good too! So what's the remedy for my ...blues?! A new power transformer? Or calculating the values and adding some resistors in-series to bring down the heater voltage? Or perhaps a amp? xP Chime in please with your experts' advice, my boss is hounding me to bring the step down transformer back to the shop but unless I find a permanent solution this ain't gonna happen soon!

The Newcomer's Forum / Re: Amp repair?!
« on: January 25, 2019, 10:10:54 PM »
I was about to test the heater voltages with the tubes on when I had the idea to test the transformer output BEFORE it goes into the bridge rectifier. Well, it is 7.8 VAC guys, so I think we found the problem. For some reason the transformer voltage is too high. Not sure what cause it, but that's what I think is going on. Am I right/wrong? Chime in please! So after it exits the bridge and gets rectified, it will be even more hence the 9+ voltages I was measuring yesterday. Still have not put in the tubes, but does it make any difference at this point? Is it normal that all of a sudden the PT will output more of its nominal voltage. If you think I should still measure the heater voltages with tubes installed let me know - Thank you!

EDIT: While I was thinking what could have caused the PT to do that, I remembered I have a mains step-down transformer at work. I am a PC technician and sometimes we need to bring down the mains AC if it is too high so it won't fry PC equipment. Anyway, I plugged it all in and guess what. With the AC stepped down the heater voltage is a healthy 6.5 Volts. At this point I plugged the tubes in and played my guitar through the amp, no problem at all. I am posting some pictures, so what do you think? I am so lost about WHAT could have caused the PT to go ...crazy!?

The Newcomer's Forum / Re: Amp repair?!
« on: January 25, 2019, 03:45:44 PM »
OK, I can certainly do that (test heater voltages with tubes installed) BUT I am afraid I will burn the power tube again. If I just turn the EVJ ON but with no guitar plugged in and volume all the way down, am I still stressing the tubes, especially the power (e.g. if there's nothing to output) or I am going to be OK for at least a minute or so I need to take the measurement?

On a different note, to test the bridge rectifier I need to remove it from the PCB correct?


So just to clarify because the schematic does not help, there are two components that can be failing between 120 VAC input and the tubes heater filaments: The power transformer and the bridge rectifier. I see a capacitor but that is not in series it is going directly to ground. Shouldn't there be some sort of resistor in there too? I am no expert but my limited knowledge tells me there should be at least one controlling the voltage to the heaters. YES/NO?!

Thank you all for the input!

The Newcomer's Forum / Re: Amp repair?!
« on: January 25, 2019, 01:21:27 PM »
Phil, hi and thank you again for the insight.

The weird thing is this amp was working OK for about a month after I purchased it, early last Nov 2018. Well, I did feel that the front face-plate was (probably) running a little bit on the hot side, but had no other to compare, the amp was working, so I ignored that. Then in mid-December while I was playing it started the crackling and hissing noises, and the amp stopped making sound. I took it to a friend who has the same amp, he removed the tubes, and found the power tube had burned out.

I felt that for a used amp off of eBay that was expected so I embarked on replacing both tubes with new so I can have a fresh start and a baseline. After some research and because I felt the amp distorted too soon, I decided to go with a pair of JJ tubes only instead of a 12ax7, I put a 5751 in the preamp which has 30% less gain and it did help a bit with what I was looking to accomplish.

It is at this point that I tried about 4-5 different brands just to see if my ears could tell the difference. I decided I could not tell any (significant) difference so I settled with the JJs. However, at this point the problem started getting serious. During my tube swapping I did not blow any tubes probably because I never played the amp too much or too hard.

So now that I settled on two JJ preamp + power tubes I cannot play more than 2-3 minutes and the amp sound crackles, hisses and fizzles and the power tube (ONLY the power tube is gone!?)

So what could cause the higher voltage to the heaters? Btw, this version 3 of the EVJ was specifically made with DC heater voltage to eliminate noise which was a problem with earlier versions. So there is no purpose going back to AC.

But what's causing the excess voltage going to the heaters? Could it be the transformer? Or the next logical think would be the rectifier. I look at the schematic and do not see any resistors after the bridge rectifier so I am assuming the path is:

120 VAC ---> transformer ---> 6.3 VAC ---> bridge rect. ---> ~8 VDC ---> tube heaters.

Am I correct?

Also, if for some reason the DC to the filaments is so high how come the preamp tube does not burn out. Plus when I did the first measurement I found over 9 volts, so am I right thinking that the bridge rectifier may have gone bad?

Now that I am getting more comfortable with all this, I will go back and do some more careful and invasive measurements. In the mean time, how can I "advertise" in this forum what my problem is in the hopes that someone particularly expert with this amp can help me out?

Thanks again!     

The Newcomer's Forum / Re: Amp repair?!
« on: January 24, 2019, 01:53:31 PM »
OK, sorry for delaying to do what you asked me, I am digressing, but we expect snow and subzero temps in Chicago as of later today so I decided to stay home and prepare the "fort" (salt-check, snow blower-check, electric generator-check, etc, you get my point).

But I also managed to find the time and took the measurements, so here it is.

I first checked R14 (did that BEFORE even powering up) and it read 217 Ω, which I think is good (so I did not mess with removing C5) .

Then, with 125 VAC coming from the grid here's what I get:

Preamp tube

Probing DC pin# to ground:

Pin 1 ---> 345 V

Pin 2, 3, 7, 8, 9 ---> 0 V

Pin 4, 5 ---> 9.30 V (!?)

Pin 6 ---> 345

  *Probing between pin 4 and pin 5 ---> 9.45 V (?!? but read comment below about the heater pins)

Power tube

Probing DC pin# to ground:

Pin 1, 2, 3 , 4, 6 and 8 all show 0 V

Pin 5 ---> 9.43 V

Pin 7 ---> 385 V

Pin 9 ---> 384 V

 *Probing between pin 4 and pin 5 ---> 9.61 V (hmmm ?!?)

BUT wait !!!

With time (as circuit warmed up more?) the heater pins voltage also kept going up in both tubes!?! I watched it go all the up past 9.85 V and eventually powered-off at 10.05 V fearing something is (likely) wrong and it was still rising!? Think we found the culprit?

Thanks again for all the advice, waiting for your diagnosis and possible remedies. I hope this is an easy (and not so expensive) repair!  ;)

- Dimi.

The Newcomer's Forum / Re: Amp repair?!
« on: January 23, 2019, 09:38:04 AM »
Phil -

Thanks for your quick and informative answer. I have version 3 of the Epiphone valve Junior which luckily has the DC heater circuit. Thanks for confirming that there's something strange with the schematic as you can see it's lifted from a venerable source but you never know right? I'll try to find a better schematic I'm almost certain there's one floating online for all different versions of the EVJ. I am at work now I will do the measurements you asked me when I get back home. Just to clarify you are asking me to do that with both tubes, not only the power tube that's giving me the problem. Also you want me to measure between 4 and ground and then 5 and ground or between 4 and 5 only? Finally I might be able to obtain an identical working amp from a friend-of-a-friend etc LOL which could be useful for comparison purposes. Much obliged for the help!

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