FET Bias

[martin manning]

I have a sim of this circuit running with a 2N4416... at 285V, and your component values (22k and 2k7) Vcc is 21, Vd is 12.9, Vs is 2.2, so you look good. You should be getting around 19 dB gain with the 10k trimmer all the way up. There must be some other error, so I thought you could start by isolating and making sure that the FET amp is working.

[David Root]

OK, thanx again Martin, I'll try it because it is not happening, something must be going to ground somewhere.

[markr14850]

If all else seems correct, you might want to try replacing the FET. When exposed to over-voltage / over-current, I've had some FETs not completely fail, but just lose most of their gain.

[David Root]

Thanx, I'll keep that in mind. I don't think I ever exposed it to >30V but it's possible.

[David Root]

Changed the 2K7 to 2K5, now have 19.0V, 10.0V and 2.25V.

Must have had a bad joint before because now it works. Gain pot set mid-way and clean is similar to normal input, not identical, but ODS is woooooo!! Way more gain than normal input, sounds awesome. Slightly fizzy, but not real noticeable.

Changes from strict #124 are now (besides FET):-

ODS entrance resistor 470K, trimpot 310K

ODS snubbers 500 pF each (should change these to about 390 pF I think)

220K resistor on PI entrance

ODS exit bleeder 470K/.001uF, set to lowest treble bleed.


I had the ODS entrance trimpot set at wiper 169K to ground. I had been using 74K to ground, maybe not enough gain although I have seen a number of folks say 74K or so is about right. Will have to play with that I guess.

Now I have the two final tweaks to try.

1) Change used RCA blackplates to NOS 7581As (Mesa STR-415s)

2) Connect amp to new Avatar cab with a pair of old 16 ohm Altec 417s in parallel I got from Tony.


Thanx for all your help folks!

[Structo]

IIRC, most set the OD trimmer wiper to ground at 24K-32K.

On the OD entrance, are you saying you have a 470K instead of the 220K there?

What does the 220K on the PI input give you?

[talbany]

220K on PI just helps with the loading.. It will also shave off some high end and smooth things out a bit..

Dave how is the OD tone.. Is it dark and lose or bright and tight or are you diggin it.. No big deal just curious!!

The STR'S THUMP!!


Tony

[Structo]

At one time I had a 150K on the PI input.
That was before my D'Lator.

Right now I am trying to figure out how to brighten the OD a little bit.

I have 4.7uF bypass and .01uF and .005uF couplers on OD.

At one time I had a 1uF bypass on OD2, I may revisit that.

[David Root]

Tom, yes, 470K instead of 220K. Cuts the gain a bit, 310/780<310/530 but it was recommended to help with fizz control. Guess that's why I am running the trimpot at 169K to ground, to make up for that.

Tony, the OD is better now than before, with the standard #124 settings, but some fizz is still there. Seems to be less noticeable the more gain I use which is odd, I guess it becomes less noticeable in the overall distortion level. Not perfect yet though. The fizz noticeably dropped after I rerouted the power tube grid leads a la Dumble. I am wondering if I should take out the individual bias wiring, there is a lot of wire both sides of the power tubes, which might be making parasitics, and use the single bias control a la Dumble.

I'll report back after I put the new tubes in and try it with the new cabinet.

[David Root]

I think I prefer the old RCA blackplates to the Mesa 415-STR for cleans, about the same for OD. Those old Altecs in the Avatar cab are to die for.

Unfortunately one of my old blackplates is going bad on me. It works fine for four or five minutes after HT is applied, then suddenly stops conducting. I get a little *click* in the speaker and it's gone. Must be temperature related. Well it's almost fifty years old and well used. Only paid $10 for it so I can't complain.

I had six, four for this amp and two for the next one ('70s ODS combo build). So now I'm short one.

I do have a pair of NOS '40s Sylvania JAN CHS 6L6GAs which I was going to use in a EH150 build, I guess I could use them but I won't get 50W out of them with Bassman iron, and I don't think they could take 430V on plates and screens anyway....anyone have any opinions on that? Fender seemed to not use anything higher than 410V that I can find.

[David Root]

This is in my Princeton chassis 50W '70s ODS. DC side seems more or less OK, but I'm getting AC output to the 4.7 uF cap to the trimmer input, and with a 1.5VAC signal in, but nothing out of the trim pot and (update) no VAC on the FET case, that is just in the air and registers when the meter is grounded to the chassis.

Found the problem, one wire too many between the input jacks.