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Hi,
I just gutted my Fender Hot Rod Deluxe to convert it into a Vox AC15. Here is the schematic and layout I used and some pictures of the build :
Schematic :
http://www.blueguitar.org/new/schem/..._handwired.pdf
Layout :
https://dl.dropbox.com/u/28610725/Tu...Board%20v2.pdf
Pictures :
http://imageshack.us/a/img641/8828/dscf0997d.jpg
http://imageshack.us/a/img198/7179/dscf0996q.jpg
http://imageshack.us/a/img811/5323/dscf0995l.jpg
http://imageshack.us/a/img411/8844/dscf0992p.jpg
http://imageshack.us/a/img22/6493/dscf0998i.jpg
There are some differences between my build and the schematic :
- I used silicon diodes for the rectifier. I kept the Fender design for it and added a series resistor to get some voltage sag out of it.
- As the Fender trafo is 325-325 instead of the 300-0-300 expected in the Vox, I dropped some voltage through two 1N5364B zener diodes in parallel (33V). I've seen this trick in the TAD Tonebone 6L6GC to EL84 adapter. The EL84 are over voltaged, but that's the way the AC15 is designed (about 370V on the plate at idle...)
- The filter caps in the PSU are MKP polypropylene. Those weren't too expensive so I replaced the electrolytics by those.
- The output tubes are not supplied by the choke because the Fender one is only rated for 50mA, that's not enough for the 100mA of the EL84 in the Vox circuit.
- I replaced the EF86 pentode circuit by the first half of th 12AX7 normally used in the Brilliant circuit, as it's done in some AC30s. I decided to do it after reading Merlin' Blencowe's book and to avoid microphony problems.
- I added a master volume pot, as implemented in the latest AC15 designs.
- I removed the "low" input jack, as I do not plan tu use it and as I don't want to drill holes in the enclosure.
Unfortunately, it doesn't work fine. It seems that there is a problem with the output tubes but I'm not sure, that's why I'm looking for some help : I don't want to buy a pair of EL84 and fry them in a few seconds.
Here are my experimentations until now :
When I first switched it on, I noticed that the EL84's plate became red hot, and after I measured the idle current I found out I had an idle current of 113mA for both tube + the screen grid current. I also noticed that I had a non negligeable voltage at the control grid : about 15V ! This value seemed to increase with time, as well as the grid/cathode voltage decreases, and thus idle current (deduced by measuring cathode voltage) increases. I didn't let the tube run like this during a long time, hoping I didn't let them long enough to fry them...
Taking a look at the PSU, I measured 391V right after the full wave rectifier (instead of the 440V without load). I'm a bit surprised that the trafo is already on its knees as it is rated for 120mA (from Hammond's equivalent datasheet, http://www.hammondmfg.com/pdf/EDB290UX.pdf ). I measured 374V after the series resistor (B+), which is about the voltage Vox runs its EL84 in the AC15, so that shouldn't fry the tubes.
Another thing that surprised me is that the zener diodes I added in series (1N5364B, 33V) only drops about 10V instead of the 33V expected.
As the control grid is the only place where some current can flow for a voltage to take place, I deduced that I had some control grid current (15v/220k so about 68uA). Does that come from faulty tubes ?
Then I tried to increase the cathode resistor. I had a 470R/5W so I put it in place of the 150R. Now the tubes run a lot cooler but I still have about 15V at the control grids, so those are still sinking current. I tried to plug it in a speaker (for now I ran the amp through a 7,5R/50W dummy load). The amp isn't as loud as a 15W tube amp should be, and I can hear hum. The normal channel seems quite fine, but the brilliant channel starts oscillating as I increase the gain. The frequency of the oscillations is about 20kHz. That occurs with the dummy load as well as with the speaker, so that doesn't seems to be related with microphony
I then disconnected the output tubes to check if the oscillations take place already in the preamp. It doesn't : everything seems just fine without the EL84, frim the input stage to the output of the PI.
Another thing : when I run a test signal into the amp with the EL84 in place and the dummy load, I can slightly hear it as I increase the output level ! Is that normal ?
Do you see any mistake in my wiring ? I triple checked the layout against the schematic, and the build against the layout but I might have made some fundamental stupid error in the design process... I'm always looking for learning and understanding what's happening so any explanation on those phenomena would be appreciated !
Best regards.
Eric
I just gutted my Fender Hot Rod Deluxe to convert it into a Vox AC15. Here is the schematic and layout I used and some pictures of the build :
Schematic :
http://www.blueguitar.org/new/schem/..._handwired.pdf
Layout :
https://dl.dropbox.com/u/28610725/Tu...Board%20v2.pdf
Pictures :
http://imageshack.us/a/img641/8828/dscf0997d.jpg
http://imageshack.us/a/img198/7179/dscf0996q.jpg
http://imageshack.us/a/img811/5323/dscf0995l.jpg
http://imageshack.us/a/img411/8844/dscf0992p.jpg
http://imageshack.us/a/img22/6493/dscf0998i.jpg
There are some differences between my build and the schematic :
- I used silicon diodes for the rectifier. I kept the Fender design for it and added a series resistor to get some voltage sag out of it.
- As the Fender trafo is 325-325 instead of the 300-0-300 expected in the Vox, I dropped some voltage through two 1N5364B zener diodes in parallel (33V). I've seen this trick in the TAD Tonebone 6L6GC to EL84 adapter. The EL84 are over voltaged, but that's the way the AC15 is designed (about 370V on the plate at idle...)
- The filter caps in the PSU are MKP polypropylene. Those weren't too expensive so I replaced the electrolytics by those.
- The output tubes are not supplied by the choke because the Fender one is only rated for 50mA, that's not enough for the 100mA of the EL84 in the Vox circuit.
- I replaced the EF86 pentode circuit by the first half of th 12AX7 normally used in the Brilliant circuit, as it's done in some AC30s. I decided to do it after reading Merlin' Blencowe's book and to avoid microphony problems.
- I added a master volume pot, as implemented in the latest AC15 designs.
- I removed the "low" input jack, as I do not plan tu use it and as I don't want to drill holes in the enclosure.
Unfortunately, it doesn't work fine. It seems that there is a problem with the output tubes but I'm not sure, that's why I'm looking for some help : I don't want to buy a pair of EL84 and fry them in a few seconds.
Here are my experimentations until now :
When I first switched it on, I noticed that the EL84's plate became red hot, and after I measured the idle current I found out I had an idle current of 113mA for both tube + the screen grid current. I also noticed that I had a non negligeable voltage at the control grid : about 15V ! This value seemed to increase with time, as well as the grid/cathode voltage decreases, and thus idle current (deduced by measuring cathode voltage) increases. I didn't let the tube run like this during a long time, hoping I didn't let them long enough to fry them...
Taking a look at the PSU, I measured 391V right after the full wave rectifier (instead of the 440V without load). I'm a bit surprised that the trafo is already on its knees as it is rated for 120mA (from Hammond's equivalent datasheet, http://www.hammondmfg.com/pdf/EDB290UX.pdf ). I measured 374V after the series resistor (B+), which is about the voltage Vox runs its EL84 in the AC15, so that shouldn't fry the tubes.
Another thing that surprised me is that the zener diodes I added in series (1N5364B, 33V) only drops about 10V instead of the 33V expected.
As the control grid is the only place where some current can flow for a voltage to take place, I deduced that I had some control grid current (15v/220k so about 68uA). Does that come from faulty tubes ?
Then I tried to increase the cathode resistor. I had a 470R/5W so I put it in place of the 150R. Now the tubes run a lot cooler but I still have about 15V at the control grids, so those are still sinking current. I tried to plug it in a speaker (for now I ran the amp through a 7,5R/50W dummy load). The amp isn't as loud as a 15W tube amp should be, and I can hear hum. The normal channel seems quite fine, but the brilliant channel starts oscillating as I increase the gain. The frequency of the oscillations is about 20kHz. That occurs with the dummy load as well as with the speaker, so that doesn't seems to be related with microphony
I then disconnected the output tubes to check if the oscillations take place already in the preamp. It doesn't : everything seems just fine without the EL84, frim the input stage to the output of the PI.
Another thing : when I run a test signal into the amp with the EL84 in place and the dummy load, I can slightly hear it as I increase the output level ! Is that normal ?
Do you see any mistake in my wiring ? I triple checked the layout against the schematic, and the build against the layout but I might have made some fundamental stupid error in the design process... I'm always looking for learning and understanding what's happening so any explanation on those phenomena would be appreciated !
Best regards.
Eric
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