Questions about building AB763 out of Bogen amp...

[martin manning]

With typical plate and screen voltages, 4600 will be fine for a pair of 6L6. 4000 is the usual primary impedance for Fender 6L6 amps.

Just to be clear, which leads did you use to get the 12:1 voltage ratio? If you applied 12VAC across half of the primary, from CT to one plate lead, and measured the 1VAC on an 8 ohm secondary, then I agree with your result.

[chikov]

With typical plate and screen voltages, 4600 will be fine for a pair of 6L6. 4000 is the usual primary impedance for Fender 6L6 amps.

Just to be clear, which leads did you use to get the 12:1 voltage ratio? If you applied 12VAC across half of the primary, from CT to one plate lead, and measured the 1VAC on an 8 ohm secondary, then I agree with your result.

I applied variac generated AC across the whole primary ( centertap was taped) and stopped increacing voltage when across whole secondary ( from black ground wire to green 16ohm) i detected 1V AC. At this point my variac was sending 12V ac. I followed Gerald Weber's video instructions. Plus, i learned a lot from Uncle Doug on youtube.

[martin manning]

That would say you have a 2k3 primary. If the voltage ratio is 12, then the impedance ratio is 12^2 or 144, 144(16)=2304. You would need to miss-match the secondary (put an 8 ohm load on a 4 ohm tap, say) to get 4k6.

[chikov]

That would say you have a 2k3 primary. If the voltage ratio is 12, then the impedance ratio is 12^2 or 144, 144(16)=2304. You would need to miss-match the secondary (put an 8 ohm load on a 4 ohm tap, say) to get 4k6.

Thank you Martin for helping me with that. Instead of 2,4,8,16 ohm outputs I will install rotary selector (i got an extra one with my Ceriatone kit 2 years ago) and wire 2,4,8 ones as 4,8,16 ohms outputs. Next step is to buy the parts I do not have. Oh heck, I think I will try to add the spring reverb to it - see what happens

[chikov]

I have been buzy with work lately, but finally came around to drawing the schematic of the Bassman based amp with reverb, that i want to build. Here it is:

[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/image_zpsmseyjhhv.jpeg[/img]

I was not sure if my reverb connections will work. Could any of you look through it and see if there are any mistakes? I am not sure what size grid leak resistor i should install on V2b... Plus, power supply resistor between the chocke and the node 3 seems to be a little too small...

Sorry about the crudeness of the drawing- i could not install on my Mak the schematic building programm for some reason...

[Smokebreak]

I'd be inclined to use a more traditional insertion, but it can't hurt to try your way.
I would just add the reverb circuitry before and after the 220K coming off the added coupling cap, and just leave the 220K//.001 to ground. But I'd be curious to hear it both ways.
Also, check out the one tube reverb over on the Hoffman forum, el34world. Lots of good ideas and helpful info.

That "??" resistor isn't the grid leak, the way you have it drawn, but the mix resistor, and that's what looks a little odd to me.

[chikov]

I'd be inclined to use a more traditional insertion, but it can't hurt to try your way.
I would just add the reverb circuitry before and after the 220K coming off the added coupling cap, and just leave the 220K//.001 to ground. But I'd be curious to hear it both ways.

Just to see if I understand you correctly, you are talking about the 220k resistor between the plate ( not counting .1 coupling cap) of V1B triode and the grid of a V2b one?

[Smokebreak]

I'd be inclined to use a more traditional insertion, but it can't hurt to try your way.
I would just add the reverb circuitry before and after the 220K coming off the added coupling cap, and just leave the 220K//.001 to ground. But I'd be curious to hear it both ways.

Just to see if I understand you correctly, you are talking about the 220k resistor between the plate ( not counting .1 coupling cap) of V1B triode and the grid of a V2b one?

Correct. Check out the ab763 scheme for the idea. In that one, its a 3M3 resistor. 220K should be enough to get the job done, and it's native to your amp, so it should work fine, without altering the amps tone(too much)

[chikov]

Hello again, I finally finished putting my glorious hybrid amp together. The good news - I did not have any shorts. the bad news - my Fixed Bias negative voltage is not working. From the schematic above, you will see that I was going to copy AA864 bias circuitry. But, I get 240vDC before the diode, but nothing past it (I did soldered the ring side towards the 470 Ohms resistor). I thing, may be because I used the voltage doubler from my Bogen amp, I should copy its bias circuit. But they do not have the 10k pot that I want to have. If I will copy the Bogen's circuit, which resistor should I replace with the pot? Thank you.
here is the schematic:
[IMG:640:381]http://i409.photobucket.com/albums/pp171/alexjohnne/Bogen_chb100_zpshv367tvz.gif[/img]

[chikov]

I have a good news. The amp is alive! I copied the negative voltage bias circuitry from the Bogen amp and I got about -27VDC on the grids of 6L6s. However, when I calculated the currant - it was almost 94mAmps (ouch) and with 410V on the plates it made about 38W tube dissipation - way too much. I have to figure out which resistor to turn into variac and then raise the negative voltage on the grids. Any help is welcome.

[chikov]

I'd be inclined to use a more traditional insertion, but it can't hurt to try your way.
I would just add the reverb circuitry before and after the 220K coming off the added coupling cap, and just leave the 220K//.001 to ground. But I'd be curious to hear it both ways.
.

I can barely hear the reverb, so I will try more traditional insertion as my next step.

[martin manning]

I have to figure out which resistor to turn into variac and then raise the negative voltage on the grids. Any help is welcome. :?

Increase the value of the 15k between the 33k and the diode. Better yet, replace the 15k and the 33k with 25k's, and you should be pretty close.

[chikov]

I have a good news and the bad news. First of all, I followed your advice and adjusted the negative Fixed Bias voltage to about -46DC and now both 6L6s draw about 50mAmps and plate voltage is about 471VDC.
The bad news is that all of a sudden the amp started blowing fuses. One thing I did was adding 460Ohm resistor between the Choke and the Screen resistors (as an experiment). Second, I changed Long Tail resistor value from 22K to 10k (to add some gain). Another thing, I found that in my V1 tube the filament wire came loose from the pin5 - i re-soldered it back. Another thing I did in order to quiet the low friq hum, I attached the screen of my grid wire on pin 2 to the plate on pin1 on V1. The tube went bad, but I am not sure why, may be the filament wire had something to do with it too...
The fuse does not blow instantly, it takes a few seconds. Before it blows, the whole amp is very microphonic - anywhere I tap it with my bamboo stick it sounds like the tapping on the wine glass... What can it be? I checked my Output tubes on my tube tester - everything seems ok, just one of them is a borderline on the grid emission scale...
Any ideas on what might be wrong?
thanks

[chikov]

Ah, I just checked the rest of my tubes and found a bad one on the Reverb circuit -12AT7. It is way in the red zone on my tube tester. Am I giving them too much Plate Voltage? I wonder what is a normal voltage? I pulled all my preamp tubes and every plate has about 471VDC on it... weird

[martin manning]

The wineglass sound is likely due to a microphonic tube. The high voltage on plate pins is normal. When no current is flowing you don't get the voltage drop in the power supply resistors or the plate load resistors. For normal voltages, some of the old layouts have them, and they are marked on the reissue schematics.

I wouldn't connect the shield on the input grid lead to the plate. Some people do it, but I don't look the idea. If the insulation fails that could destroy the tube. Just ground the shield at one end and leave the other end unconnected.