Output power

[dhuebert]

All of the Fender amp schematics I looked at had 220K resistors here including the "Showman" that I based this amp on. I have seen the occasional instability but that was with both 100K and 220K. I didn't get too worked up about it, perhaps you'll understand when you see the snaps of the prototype (some long lines and a wooden chassis).

Don

[Shannon Parks]

All of the Fender amp schematics I looked at had 220K resistors here including the "Showman" that I based this amp on. I have seen the occasional instability but that was with both 100K and 220K. I didn't get too worked up about it, perhaps you'll understand when you see the snaps of the prototype (some long lines and a wooden chassis).

Don

Hi Don,

The first Showman schematic I could find - AA768 - shows 68K grid resistors. 68K seems like a good value.

Shannon

[dhuebert]

I based the amp on AB763. I wouldn't mind dropping those resistors to 100K or even 68K but I would have to find a ton more gain out of the PI to make up for the loading. As it is things are fairly tight, the output starts clipping just a few volts before the PI. Any suggestions as to how to maintain the good balance the PI shows and boost its gain would be gratefully accepted. Looking further at the Showman AB763 and AA768 reveals they used 12AT7 with 440 volts for B+. I am using 12AX7 with 250V B+, Hmmm interesting...

Don

[Shannon Parks]

I based the amp on AB763. I wouldn't mind dropping those resistors to 100K or even 68K but I would have to find a ton more gain out of the PI to make up for the loading. As it is things are fairly tight, the output starts clipping just a few volts before the PI. Any suggestions as to how to maintain the good balance the PI shows and boost its gain would be gratefully accepted. Looking further at the Showman AB763 and AA768 reveals they used 12AT7 with 440 volts for B+. I am using 12AX7 with 250V B+, Hmmm interesting...

Don

Hi Don,

I finally took a break to really look over your design. All the pieces are there, but your power supply needs to be rearranged a little. I'll try and get a fixed schematic posted. You can get your needed drive easily.

BTW, what is the DC resistance & current rating of your choke? Is it a Hammond?

Shannon

[Shannon Parks]

OK, Don - I like that circuit. I stand corrected on the 220K grid resistors.

Take a look at the AB763 schematics power supply:

Diodes->Cap->Choke->Cap (B+ here)->Resistor->Cap (PI Voltage here)->Resistor->Cap (Voltage Gain stage VDC).

You need to seperate the PI voltage source and the voltage gain voltage source - or else motorboating can occur. This is likely the source of your instability. Your PI B+ will go way up.

Shannon

[dhuebert]

I see your point about the combined power for the pre and PI, after reviewing the AB763 I will do that work this weekend. The pictures show this but the choke is: Hammond 157Q. Considering the Icat @ full power is 58 mA for a total of 232 mA, I might want to change this.

Don

[Shannon Parks]

http://www.diytube.com/forumpix/bench.jpg
http://www.diytube.com/forumpix/power-amp.jpg
http://www.diytube.com/forumpix/power-supply.jpg
http://www.diytube.com/forumpix/preamp.jpg

[dhuebert]

After a few more minor mods to boost gain, I took the amp as you see it in the photos to Joe's practice space. We plugged it into a bass bottom with his 1958 Fender bass reproduction (a new Fender). He noodled around on it and declared his love for the new amp (brag, brag). Power and sound quality were what he was hoping, so now it's time to package it. To that end I will be puting it in the toolbox you see on the floor in the "bench" picture. It will be tight but I think I can do it. One thing I was happy about was the very small buzzing at full volume, I was expecting alot more due to lack of shielding from the wooden blocks it's built out of. At anything below full throttle it was dead quiet.

Something of note: @ 55W, 1KHz sine, 20 mv RMS input, Icat is 60 mA; @ idle 30 mA, with the guitar the most I saw was 40mA.

He tells me his buddies want guitar amps in toolboxes too, Guitar amps are a whole different kettle of fish from bass amps. It's easy to build clean, but how to build good dirty?

Don

[Uncle Ned]

I based the amp on AB763. I wouldn't mind dropping those resistors to 100K or even 68K but I would have to find a ton more gain out of the PI to make up for the loading. As it is things are fairly tight, the output starts clipping just a few volts before the PI. Any suggestions as to how to maintain the good balance the PI shows and boost its gain would be gratefully accepted. Looking further at the Showman AB763 and AA768 reveals they used 12AT7 with 440 volts for B+. I am using 12AX7 with 250V B+, Hmmm interesting...Don

First thing, I'd use a 12AT7 or 5965/12AV7 instead of the 12AX7, unless you're going for the "Marshall sound" (Marshalls usually used 12AX7 as the PI). The extra gain doesn't do much good in the PI.

Blackface Fenders typically used 220K grid resistors for the 6L6-GC's, more than what's recommended, but good 6L6-GC's don't have a problem with it. Typically people think using the 220K resistors sound better, probably because they don't load the PI so much.

Lastly, the more juice (voltage) you can feed to the PI the more voltage it can swing before distorting. 250V is barely sufficient for PP 6L6's run at voltages Fender liked to run them at.

[dhuebert]

I do wonder about using the 220K grid resistors of the 6L6GCs, though

Blackface Fenders typically used 220K grid resistors for the 6L6-GC's, more than what's recommended, but good 6L6-GC's don't have a problem with it

I tried a 12AT7 in the PI and it threw the balance out. I would need to find new anode resistors, With the 12AX7 the 100K/82K combo gives perfect balance, I wouldn't want mess with success.

Boosting the voltage to the PI was one of the things I did to increase overall gain, it's now about 350V.

Once I get Joe's amp packaged, he will take it around to his various practice gigs and his guitarist friends will try it and give opinions. One fellow I talked to said he wanted clean power so he could better know what his pedals are doing.

As an aside...I have a friend who is a cabinet maker, when I asked him if he was interested in building speaker boxes for these amps he told me his first job out of high school was working for Garnet building, you guessed it, speaker boxes!

Don

[Shannon Parks]

Once I get Joe's amp packaged, he will take it around to his various practice gigs and his guitarist friends will try it and give opinions. One fellow I talked to said he wanted clean power so he could better know what his pedals are doing.

So when are you gonna let Gar hear it? Yellow_Light_Colorz_PDT_18

Shannon

[dhuebert]

Listen: Joe and I talked about this the other night. I'm undecided but Joe says he'd be glad to do the deed. Maybe we'll both go. It's not that I'm afraid or anything, it's just that I don't know if anything would be gained by shoving it in his face and saying "Ha, proved ya wrong!" I would much rather approach it in a more positive way and be able to say "You inspired me to do this". Either way I don't know if he'd much care. The future will bring itself.

Don