Hoi there,
Do not worry about the "low bank balance"; that's the same with most of us...
2 times 50 make's 100. Correct.
And why 2 x 50? That's the way these caps were made at that time.
It is probably a remaining of the tube rectifier era. 47uF is a big load for a tube rectifier.
Some manufacturers today do make a mistake by using a to big cap after the tube rectifier. I get these amps frequently on my bench...
Tube update? Why?
First finish one and use the tubes to test, repair and tune the second.
If all is well then, buy new tubes for the first and redo the bias etc...
Nice amps... I have to rebuild my Bass King T ...
Do not worry about the "low bank balance"; that's the same with most of us...
2 times 50 make's 100. Correct.
And why 2 x 50? That's the way these caps were made at that time.
It is probably a remaining of the tube rectifier era. 47uF is a big load for a tube rectifier.
Some manufacturers today do make a mistake by using a to big cap after the tube rectifier. I get these amps frequently on my bench...
Tube update? Why?
First finish one and use the tubes to test, repair and tune the second.
If all is well then, buy new tubes for the first and redo the bias etc...
Nice amps... I have to rebuild my Bass King T ...
The seller told me he had bought the amp from a church five years ago, and according to him and his knowledge concerning the amp´s history it still has the original power tubes in. Said they´re pretty noisy too so I´m guessing it would be better to throw in a new set of caps too? It´s like few generations of spiders had been living there. No one´s been in for a while so I wouldn´t be surprised if those tubes were in a need of a change.
And I was planning to replace these tubes with a pair of 6CA7's too and make it as much as a same toned as the one I have now. I´m currently trying to get a deal on some 15" Eminence speakers from the 70´s to get that nice vintage vibe on my other cab with this head.
You get that BassKing T up and running ya hear! I´d love to get one too someday. I now have an Eminent I at my practice space and those preamps sound killer growly on a bass. There´s like a zillion ECC83´s in there!
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Be sure the tubes can withstand the high voltage found in the Eminent.
Most new tubes will blow...
JJ is a good choice. I use them in all my Dynacord repairs and amps with high anode voltages.
They last...
It's not because a tube is old it has to be replaced.
Caps and resistors, they are the first source of noise in these old amps that have been stored for a long time.
As said; Fix nr one and then begin nr two.
The amp still looks decent.
I've seen more degraded ones...
Most new tubes will blow...
JJ is a good choice. I use them in all my Dynacord repairs and amps with high anode voltages.
They last...
It's not because a tube is old it has to be replaced.
Caps and resistors, they are the first source of noise in these old amps that have been stored for a long time.
As said; Fix nr one and then begin nr two.
The amp still looks decent.
I've seen more degraded ones...
First one, then the second. That´s my plan of course. I´ve had my share of torn apart projects that never got done.
I dug up the datasheet for the Electro Harmonix 6CA7EH. That should tell something. I have a good access to JJ 6CA7´s too and they are the same price too. 40€ for a matched pair.
I still haven´t got the hold of the second Eminent unit since it´s at my friends possession after he got it for me. My guitarist has that very same series unit and compared to my FIRST unit it has some differences. I shall post images of it later but I remember it had some fuses around the power tubes. I wonder wether it´s an update to the model that I have or the other way around.
PLUS! I must bring up the fact that the Eminent unit my guitar player has was taken to a store to have a general check up done after it had stood in storage a while and when we tested it out it sounded relly nice like it should, but after about half an hour of use it started cutting off the signal. Really bright flashes in the tubes when the signal got on and off. About the same that my unit did but it really started the fireworks around the caps output transformer.
I dug up the datasheet for the Electro Harmonix 6CA7EH. That should tell something. I have a good access to JJ 6CA7´s too and they are the same price too. 40€ for a matched pair.
I still haven´t got the hold of the second Eminent unit since it´s at my friends possession after he got it for me. My guitarist has that very same series unit and compared to my FIRST unit it has some differences. I shall post images of it later but I remember it had some fuses around the power tubes. I wonder wether it´s an update to the model that I have or the other way around.
PLUS! I must bring up the fact that the Eminent unit my guitar player has was taken to a store to have a general check up done after it had stood in storage a while and when we tested it out it sounded relly nice like it should, but after about half an hour of use it started cutting off the signal. Really bright flashes in the tubes when the signal got on and off. About the same that my unit did but it really started the fireworks around the caps output transformer.
How is the project going?
The Eminent 2T has a lot more electrolytic caps to be changed: look at all the orange parts in the preamp section. It also uses stabilzed voltages in the power supply - again full of electrolytic caps.
But first and beforemost the tube section must be restored. Electrolytic caps as mentioned (btw: Jan Wüsten's 100µ+100µ works well in the position of the 100µ+50µ and is cheaper (larger capacity is better unless it is too large)).
One of the caps is isolated. That isolation must be retained: the cap is on a base potential of about 400 V, and that's the voltage of its outer surface against ground!!
The small electrolytic cap C401 at the ECC81 also needs to be changed.
Use "upright" Piher PT15 trim pots to replace the flakey original trim pots. A small defect my destroy not only the EL34s but also the output transformer. And that is a part of 100 € (Welter electronics has them). If there is a fuse of .125 mA installed in the bias circuitry - that one needs to be removed and bridged. A loss of contact/break of the fuse will have the same effect as both bias pots breaking at the same moment!
Biasing: the 25 mA (0.3 V at the cathode resistors of the EL34) is a bit too hot for EL34s at those voltages. I bias these amps at 0.21 V, and they still sound good. If there is flashing in the tubes of an otherwise restored amplifier clean the sockets (after unloading the electrolytic caps through a 47 kOHm/11W resistor)
The Eminent 2T has a lot more electrolytic caps to be changed: look at all the orange parts in the preamp section. It also uses stabilzed voltages in the power supply - again full of electrolytic caps.
But first and beforemost the tube section must be restored. Electrolytic caps as mentioned (btw: Jan Wüsten's 100µ+100µ works well in the position of the 100µ+50µ and is cheaper (larger capacity is better unless it is too large)).
One of the caps is isolated. That isolation must be retained: the cap is on a base potential of about 400 V, and that's the voltage of its outer surface against ground!!
The small electrolytic cap C401 at the ECC81 also needs to be changed.
Use "upright" Piher PT15 trim pots to replace the flakey original trim pots. A small defect my destroy not only the EL34s but also the output transformer. And that is a part of 100 € (Welter electronics has them). If there is a fuse of .125 mA installed in the bias circuitry - that one needs to be removed and bridged. A loss of contact/break of the fuse will have the same effect as both bias pots breaking at the same moment!
Biasing: the 25 mA (0.3 V at the cathode resistors of the EL34) is a bit too hot for EL34s at those voltages. I bias these amps at 0.21 V, and they still sound good. If there is flashing in the tubes of an otherwise restored amplifier clean the sockets (after unloading the electrolytic caps through a 47 kOHm/11W resistor)
Hello,
I actually just ordered a pair of JJ 6CA7´s and a pile of Piher upright trimmers about 10 minutes ago. I´ve been dong a lot of other stuff aside and been broke as usual so haven´t had the opportunity to restore the amp(s).
I got a gift certificate to a music electronic store so I went on and bought the tubes I desired to use, since they didn´t have the appropriate caps for the output/tube section.
Or as least I thought so before I was reading your comments, BEA. 100µf+100µf caps seem to be pretty easy to obtain after all. It´s good to know they can work in the position of the 100µf+50µf cap. I still feel like I would be more comfortable replacing the caps with about the same orientation and wiring that the original has. And with the screw type installation.
I also put it on my list earlier to change the C401, C406 and C406. Those are easily done at the same time as I take the whole tube board out and check everything.
And I must say those orange caps really creep me out. They look like some sort of tumors.
Right now I´m still in the middle of moving house so I shall take this task upon me when I got all the space I need. Then I will take sharper photos of both of the units since they have bit of a different arrangement on the tube section. There is at least that one fuse in the bias circuitry that you, BEA, mentioned.
My friend has that very same era unit and it has that fuse too. It went through restoration in a shop but I remember that same fuse was still there. And that amp went all over the place: flashing bulbs, no sound, SOUND! very freaky. So that time we left it be.
So bottom line. I now have the output tubes covered. I still need to get some money together to get the proper caps. But slowly getting there. Right now my mind isn´t actually right there so I will just get my shopping list together and just go through the schematics over and over.
Thanks again! And all the input is welcome!
I actually just ordered a pair of JJ 6CA7´s and a pile of Piher upright trimmers about 10 minutes ago. I´ve been dong a lot of other stuff aside and been broke as usual so haven´t had the opportunity to restore the amp(s).
I got a gift certificate to a music electronic store so I went on and bought the tubes I desired to use, since they didn´t have the appropriate caps for the output/tube section.
Or as least I thought so before I was reading your comments, BEA. 100µf+100µf caps seem to be pretty easy to obtain after all. It´s good to know they can work in the position of the 100µf+50µf cap. I still feel like I would be more comfortable replacing the caps with about the same orientation and wiring that the original has. And with the screw type installation.
I also put it on my list earlier to change the C401, C406 and C406. Those are easily done at the same time as I take the whole tube board out and check everything.
And I must say those orange caps really creep me out. They look like some sort of tumors.
Right now I´m still in the middle of moving house so I shall take this task upon me when I got all the space I need. Then I will take sharper photos of both of the units since they have bit of a different arrangement on the tube section. There is at least that one fuse in the bias circuitry that you, BEA, mentioned.
My friend has that very same era unit and it has that fuse too. It went through restoration in a shop but I remember that same fuse was still there. And that amp went all over the place: flashing bulbs, no sound, SOUND! very freaky. So that time we left it be.
So bottom line. I now have the output tubes covered. I still need to get some money together to get the proper caps. But slowly getting there. Right now my mind isn´t actually right there so I will just get my shopping list together and just go through the schematics over and over.
Thanks again! And all the input is welcome!
Done and done, man! Well actually just done, since I still have the other model to go through. And that one is somewhat different from the inside.
In this 1967 model I set up brand spanking new set of JJ 6CA7´s. Pulled out the electrolytic caps in the output and ECC81/power tube department and replaced them with F&T caps with proper voltage ratings. Also changed the bias trimpots and the cathode resistors to pair of new matched ones. In the circuit they measure 12.6 Ohms and 13.6 Ohms. So I did my math and got the bias to about 17.4mA per tube. I went by the book on that one and set it pretty close to the 70% of max plate dissipation.
Also I got pretty close readings in reference to the schematics when I measured around the OT and the tube sockets and after consulting a guy that knows about these amps got the information that the big filter caps weren´t actually original and leakage measure ok, so I didn´t have to change them just now to get the amp running again. But I will change them when I get the money.
Heading over to rehearsal space in a couple of days to properly test the sound, since at home I had just a poor 4ohm/15W speaker acting primary as just a load when powering up the amp.
Thanks for everyone for their direction and feedback. And when the time comes to start dealing with the other unit I shall use this same thread for that.
Thanks again!
In this 1967 model I set up brand spanking new set of JJ 6CA7´s. Pulled out the electrolytic caps in the output and ECC81/power tube department and replaced them with F&T caps with proper voltage ratings. Also changed the bias trimpots and the cathode resistors to pair of new matched ones. In the circuit they measure 12.6 Ohms and 13.6 Ohms. So I did my math and got the bias to about 17.4mA per tube. I went by the book on that one and set it pretty close to the 70% of max plate dissipation.
Also I got pretty close readings in reference to the schematics when I measured around the OT and the tube sockets and after consulting a guy that knows about these amps got the information that the big filter caps weren´t actually original and leakage measure ok, so I didn´t have to change them just now to get the amp running again. But I will change them when I get the money.
Heading over to rehearsal space in a couple of days to properly test the sound, since at home I had just a poor 4ohm/15W speaker acting primary as just a load when powering up the amp.
Thanks for everyone for their direction and feedback. And when the time comes to start dealing with the other unit I shall use this same thread for that.
Thanks again!
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