Hello,
I´m new on the forum. And pretty newbie with these tubeamps too.
I own a Dynacord Eminent II amp. 80W Solid state preamp and 2xEL34 in the output. One day at practice while playing my stack went dead quiet. I checked the two secondary fuses in the backpanel and those were blown. So I set a pair of new ones and it was good to go for about an hour or so after which it died again. Then after another round of fuses the head worked fine.
Then one day I get loud crackling and fireworks like it was a Rammstein show from the output section. The top lid is always open so I could clearly see where the sparks were flying. Now the cathode resistor (10ohm 1W) on one of the EL34´s was blown wide open. So I put a new one in and actually matched a pair and changed to the other cathode resistor too (R418 and R419)
So I went on and set the tubes back in and set the amp in on-position. the second I flicked the standby switch I get a nice loud pop from the speaker. Still got sound. so I just started playing some chords. And once I strummed out a nice loud chord, that same cathode resistor plew open.
Should I suspect there is a short in that very EL34? The tubes were oriented the same way they were before. And the tubes have had some time on them so I wouldn´t be surprised.
Here´s the schematic for the amp.
http://www.freeinfosociety.com/electronics/schemview.php?id=361
So, my plan was this: Take out the old EL34´s out and put a matched pair of 6CA7EH´s in. These should go hand in hand with EL34 amps.
I´ve been looking into biasing a lot for the last week or so and been going through the schematics a lot as well as the innards of the amp. First I assumed that I had a cathode biased amp in my hands but now I´m not that sure. AND THIS IS THE ONE IMPORTANT THING I´M HERE TO FIND OUT!
As I understand both EL34´s should have grid reference to ground. As the schematics tell there is this very 10ohm resistor between cathode and ground and the grid has it´s connection to ground through this resistor. And then and there I was confused. There are the two 25K trimmers (R412 and R413) that someone said was used for biasing the tubes. And this was something I´m hoping to be the answer.
I´m not sure if I was very clear but I hope someone can make some sense of this and I´ll be updating the info here if any questions rise up.
Thanks in advance!
K
I´m new on the forum. And pretty newbie with these tubeamps too.
I own a Dynacord Eminent II amp. 80W Solid state preamp and 2xEL34 in the output. One day at practice while playing my stack went dead quiet. I checked the two secondary fuses in the backpanel and those were blown. So I set a pair of new ones and it was good to go for about an hour or so after which it died again. Then after another round of fuses the head worked fine.
Then one day I get loud crackling and fireworks like it was a Rammstein show from the output section. The top lid is always open so I could clearly see where the sparks were flying. Now the cathode resistor (10ohm 1W) on one of the EL34´s was blown wide open. So I put a new one in and actually matched a pair and changed to the other cathode resistor too (R418 and R419)
So I went on and set the tubes back in and set the amp in on-position. the second I flicked the standby switch I get a nice loud pop from the speaker. Still got sound. so I just started playing some chords. And once I strummed out a nice loud chord, that same cathode resistor plew open.
Should I suspect there is a short in that very EL34? The tubes were oriented the same way they were before. And the tubes have had some time on them so I wouldn´t be surprised.
Here´s the schematic for the amp.
http://www.freeinfosociety.com/electronics/schemview.php?id=361
So, my plan was this: Take out the old EL34´s out and put a matched pair of 6CA7EH´s in. These should go hand in hand with EL34 amps.
I´ve been looking into biasing a lot for the last week or so and been going through the schematics a lot as well as the innards of the amp. First I assumed that I had a cathode biased amp in my hands but now I´m not that sure. AND THIS IS THE ONE IMPORTANT THING I´M HERE TO FIND OUT!
As I understand both EL34´s should have grid reference to ground. As the schematics tell there is this very 10ohm resistor between cathode and ground and the grid has it´s connection to ground through this resistor. And then and there I was confused. There are the two 25K trimmers (R412 and R413) that someone said was used for biasing the tubes. And this was something I´m hoping to be the answer.
I´m not sure if I was very clear but I hope someone can make some sense of this and I´ll be updating the info here if any questions rise up.
Thanks in advance!
K
Hello and welcome to the forum.
First I would pull the rectifier and power tubes and measure what you get for voltage at pin 5 of power tubes sockets.
edit: I was looking at the wrong schematic and see there is no rectifier tube. You could do the same thing sans pulling rectifier tube (because it doesn't have one) but I am unsure of the voltage handling capability of the PS caps. Looks like a voltage doubler, and if the caps are rated for 500v you should have up to 1k voltage rating for the OT center tap which looks to be 750v loaded (it will rise unloaded).
Be careful around these high voltages!
First I would pull the rectifier and power tubes and measure what you get for voltage at pin 5 of power tubes sockets.
edit: I was looking at the wrong schematic and see there is no rectifier tube. You could do the same thing sans pulling rectifier tube (because it doesn't have one) but I am unsure of the voltage handling capability of the PS caps. Looks like a voltage doubler, and if the caps are rated for 500v you should have up to 1k voltage rating for the OT center tap which looks to be 750v loaded (it will rise unloaded).
Be careful around these high voltages!
Last edited:
I took a couple of pics for the jury. I always thought it had a rectifier tube since the preamp section should be solid state. I took the photos around the ouput area and the "rectifier tube" is TELEFUNKEN, ....7W.. I would have to guess 12AT7W or such. There you can also see the 25k trimmers.
See if you can make any more sense to the situation, especially concerning the rectifier.
There is also another schematic to the amp. But that one is for the model with preamp tubes. And the schematic looks pretty much the same concerning the OT section.
See if you can make any more sense to the situation, especially concerning the rectifier.
There is also another schematic to the amp. But that one is for the model with preamp tubes. And the schematic looks pretty much the same concerning the OT section.
Thanks for the feedback! I´ve also read about the 6CA7 that their plate dissipation would be even a bit higher (30+W) than the usual spec of 25W for EL34. Could I bias the tubes a bit hotter? Or should I stay with the 25ma? Or is that something the tubes do it by themselves with the cathode resistor? Just so I understand correctly?
And about the burnt resistor(s), do you think a short in the EL34 would be the most obvious reason for that to happen? And could you identify the "rectifier" tube if I was to update that as well.
Thanks again!
And about the burnt resistor(s), do you think a short in the EL34 would be the most obvious reason for that to happen? And could you identify the "rectifier" tube if I was to update that as well.
Thanks again!
Considering the age of the amp do a few checks first, now that it is open.
Be carefull! There is a 700 and more dc on many places...
Especially be carefull when you flip the amp.
The transformer connection points are visible and can easely be touched...
Without the EL34 inserted, measure the voltages.
Are they reasonably close to the noted ones on the schematic?
Check the electrolytic caps and change those that show signs of "end of life".
There must be plenty of them.
Then the bias trimmers.
They must be allmost black at the back and the wiper is very fragile.
I see that they are the originals. Change them.
Then... Make sure the new tube's you will use (be it EL34 or 6CA7) that they can handle the 700 plus volts found in the Dynacord amps!!!
If not; they will blow very soon after installation or even instantly.
JJ tubes will handle that voltage.
Check the resistors in the power amp part for correct value.
If that all is done and found good, insert a set of known good tubes that can handle 700+ volts, connect a load or speaker.
Use a 50W bulb in series with the amp (AC 220V side) and switch the amp on.
The bulb will lit initialy and then dim a bit.
Press the standby switch so that the amp is in the working state.
Take a few measurements around the power tubes.
They should be close to the values as seen on the schematic.
No hum or any other weird sounds?
Switch everything off and remove the bulb.
Connect the amp direct to the ac and measure again.
Then take the amp out of standby and measure again.
The last measurements should follow thos on the schematic.
Adjust the bias and your amp is "as new".
Good luck.
Be carefull! There is a 700 and more dc on many places...
Especially be carefull when you flip the amp.
The transformer connection points are visible and can easely be touched...
Without the EL34 inserted, measure the voltages.
Are they reasonably close to the noted ones on the schematic?
Check the electrolytic caps and change those that show signs of "end of life".
There must be plenty of them.
Then the bias trimmers.
They must be allmost black at the back and the wiper is very fragile.
I see that they are the originals. Change them.
Then... Make sure the new tube's you will use (be it EL34 or 6CA7) that they can handle the 700 plus volts found in the Dynacord amps!!!
If not; they will blow very soon after installation or even instantly.
JJ tubes will handle that voltage.
Check the resistors in the power amp part for correct value.
If that all is done and found good, insert a set of known good tubes that can handle 700+ volts, connect a load or speaker.
Use a 50W bulb in series with the amp (AC 220V side) and switch the amp on.
The bulb will lit initialy and then dim a bit.
Press the standby switch so that the amp is in the working state.
Take a few measurements around the power tubes.
They should be close to the values as seen on the schematic.
No hum or any other weird sounds?
Switch everything off and remove the bulb.
Connect the amp direct to the ac and measure again.
Then take the amp out of standby and measure again.
The last measurements should follow thos on the schematic.
Adjust the bias and your amp is "as new".
Good luck.
Thanks for a very detailed and informative instructions! The only thing I´m always concerned about is when the caps are safe to measure/touch. I´d like to replace the big cans too. I was thinking of replacing the trimmers with new ones anyway. I assume I should take some readings off them first so I can set the new ones about the same for a sake of not destroying everything.
I shall look into this. I´m not really sure what do you mean by running the lightbulb in series with the amp, "THE AC 220V SIDE".
It´s really a great amp and does any job you give it, so I really wouldn´t want to leave it hanging. Thanks for everyone´s input and more is always welcome!
K
I shall look into this. I´m not really sure what do you mean by running the lightbulb in series with the amp, "THE AC 220V SIDE".
It´s really a great amp and does any job you give it, so I really wouldn´t want to leave it hanging. Thanks for everyone´s input and more is always welcome!
K
To clarify the function of the smaller tube: Have a look here http://www.peel.dk/Dynacord/pdf/Eminent II T (unknown revision) (2).pdf
Its a ECC81 (12AT7) working as preamp (left part) and phase inverter (right part) in front of the power tubes.
The tube uses only a small number of mA, from my experience you can let it in, it will not change the result significantly.
Its a ECC81 (12AT7) working as preamp (left part) and phase inverter (right part) in front of the power tubes.
The tube uses only a small number of mA, from my experience you can let it in, it will not change the result significantly.
Moved to Instruments & Amplifiers forum. This is where the instrument amp guys hang out.
This was my logic too. And since all the preamps (4 channels) were doing just fine and the ouput section doesn´t contain that many caps I decided to replace all of them and then move to the tube part of the issue. Just to narrow the hazzards down a bit.
As I also replaced the power tube cathode resistors I took notice that the hadn´t drifted at all. They were still in the 5% tolerance given.
About replacing the caps. There´s two 40uf electrolytic caps on the PCB. The two yellow/gold ones visible in the photos too side by side. Looks like pretty hard to come by around here and also pretty pricey. 20+ euros each. And then another observation, The other big capacitor in schematics is 100uf + 50uf. Now, the one in my unit is 100uf + 47uf. How precise these values are? I´m going to take the old big caps off and install two small electrolytics in the big cans for the look so either way it´s not a problem. Just curious.
Hi,
"I suppose this was meant with the light bulb in series with the amp. "
Yes, that's an easy and cheap way to protect the amp for further disasters.
Very usefull...
Then the caps:
Check the site of "Ask Jan First"
Ask Jan First ® ; electron tubes and more
KON96070 Dreifachelko 50+50+50 uF bei 550/600V - 20,22 Euro.
KONEK47 47 uF 500 V - 5,83 Euro.
The 3 x 50uf can be wired as 1 x 50uF and the remaining 2 of 50uF in paralel to create a 100uF.
Make sure the voltage is as specified or higher...
It's a bit of money, but the amp is worth it.
Dynacord had a good reputation.
I'm a big fan of Dynacord and I understand they do no longer build guitar amps since the mid eighties due to the massive import of cheap and of disputable quality of amps from the far east.
Great amps and build like a tank.
ps: the values at that time had a value +/- 20% as noted.
Let us know.
"I suppose this was meant with the light bulb in series with the amp. "
Yes, that's an easy and cheap way to protect the amp for further disasters.
Very usefull...
Then the caps:
Check the site of "Ask Jan First"
Ask Jan First ® ; electron tubes and more
KON96070 Dreifachelko 50+50+50 uF bei 550/600V - 20,22 Euro.
KONEK47 47 uF 500 V - 5,83 Euro.
The 3 x 50uf can be wired as 1 x 50uF and the remaining 2 of 50uF in paralel to create a 100uF.
Make sure the voltage is as specified or higher...
It's a bit of money, but the amp is worth it.
Dynacord had a good reputation.
I'm a big fan of Dynacord and I understand they do no longer build guitar amps since the mid eighties due to the massive import of cheap and of disputable quality of amps from the far east.
Great amps and build like a tank.
ps: the values at that time had a value +/- 20% as noted.
Let us know.
Thank you Tarzan! Now, that´s one thing I thought I´d never say.
That is one great site/store! I think I´m gonna get my cans there. I discharged the caps yesterday through a 2K2 ohm 15W resistor. I´ll return with more when I have some money to buy my replacement parts. And I also do appreciate this workhorse to the point where I don´t really mind paying a lot to get it running smooth. I actually play bass through it and it really delivers.
K
That is one great site/store! I think I´m gonna get my cans there. I discharged the caps yesterday through a 2K2 ohm 15W resistor. I´ll return with more when I have some money to buy my replacement parts. And I also do appreciate this workhorse to the point where I don´t really mind paying a lot to get it running smooth. I actually play bass through it and it really delivers.
K
The project is still on hold due to low bank balance but the preparations are being made. One question and one happy announcement:
The C2 in the schematics is the big can 50uf+50uf. It´s connected parallel and then it´s 100uf total right? Then why is it a 50uf+50uf can? is it because of the voltage rating?
Then the happy news. I first bought this amp for 250€ and at that time I knew it was pretty cheap for being in such good condition. Now I got a new one thanks to one good friend lending me the money and it only cost 100€. I suppose I´ll do the same capacitor and tube update on that unit too but at least now I have a good back up, which I thought I would never have!
K
The C2 in the schematics is the big can 50uf+50uf. It´s connected parallel and then it´s 100uf total right? Then why is it a 50uf+50uf can? is it because of the voltage rating?
Then the happy news. I first bought this amp for 250€ and at that time I knew it was pretty cheap for being in such good condition. Now I got a new one thanks to one good friend lending me the money and it only cost 100€. I suppose I´ll do the same capacitor and tube update on that unit too but at least now I have a good back up, which I thought I would never have!
K
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