AB Lite question

[Beezerboy]

I'm gathering parts to build this amp. I'm looking at the layout diagram. There is a 220K. 3W resistor on the first power cap that goes to A (the OT), the choke, and pin 8 on the rectifier tube on one end... the other end goes to ground.  What does that do??   It's not in the schematic. Not in the Fender DX Reverb schemos either

[sluckey]

That 220K drains the filter caps when you turn the amp off.

[Beezerboy]

'doh... of course

[sluckey]

I'm looking at the layout diagram. There is a 220K. 3W resistor on the first power cap... It's not in the schematic.

That resistor is on the schematic on my website. Where did you get your schematic?

[Beezerboy]

not sure where I got it.... it's been on my desktop for a couple years

[sluckey]

not sure where I got it.... it's been on my desktop for a couple years

I asked because there is a copy of most of my files in Hoffman's tube amp schematics library. He put them there years ago when I had lost my website. But those files are stagnated. They are just sitting there with no way for me to update or add more files. The only way to get the most up to date files is straight from my website. My files get revised whenever some (like you) discovers a discrepancy or error. And sometimes when I'm bored, I update files just to make them prettier.   

[Beezerboy]

thanks!!

[Beezerboy]

another question....  "R14 270K"....  I see that resistor in the schematic but it appears to be a 220K in the component layout diagram. is that right? how much difference would that make (not much is my guess). also, R14 needs to go to ground.... where is a good spot? how about a jumper from R14 over to the neighbor pin that grounds the 47 ohm NFB resistor?

[sluckey]

That resistor should be 270K. Please post the layout you are using. I have two versions of this amp plus Hoffman has two outdated versions in his schematic library.

[Beezerboy]

I double checked at

http://sluckeyamps.com/misc/AB_Lite_II.pdf

do I want the 270K for sure?

http://sluckeyamps.com/misc/AB_Lite_II.pdf

[sluckey]

do I want the 270K for sure?

Yes. See note 2 on the schematic.

And the 270K should be grounded with the preamp ground, not the 47Ω NFB resistor. I've updated the file on my website to show these changes. Thanks for finding these irregularities. Love to see your amp when you get it built.

[Beezerboy]

OK... its mostly done but it has a problem with voltage on C and D. way too high. when I first fired it up I had a 12AX7 in the V2 inverter spot, the A node voltage was 400, B was 398, but C was 349 and D was 326. I put a 12AT7 in there instead and now its higher.... A=443, B=441, C=378, D=354.  I've rechecked resistances on everything that looks like it might be a problem, same with cap values (I have a Z meter) checked tubes (have a couple tube testers). it draws about 500mA total on the primary side so I think that is pretty normal.  it plays, but it way overdrives. I added another power resistor and it brought the voltage down but didn't help the main problem. I've got voltage readings for the tube pins... what other info will help?

[AlNewman]

What is your voltage at the grids of the power tubes?  Do you have a 1 ohm resistor to check bias of your power tubes?

[sluckey]

I never built that amp so the voltages on that AB Lite II schematic are fictitious. Your voltages are more realistic. Look at the voltages on my TDR. They are actual measured values. The bias was set for 24mA on the cathodes of the 6V6s.

If your 6V6s are biased cooler than my TDR that would explain your node A, B, C, and D being a bit higher than my TDR. And if you are running 6L6s then your voltages are about perfect.

[Beezerboy]

6V6 grids are 444-443. I haven't done the bias yet but I have the bias at max which is -52 as measured on pin 5 (I started at 1/2 the pot's rotation, then reduced. I didn't go to the "hot bias" side). I have a Bias Buddy thing to set the bias but it sounds like its not even close right now. there is something wrong. I mean at 1/4 on the Volume pot its going into breakup... there is no clean at all


so.... should I switch to 6L6s????

btw, I'm using a 5AS4A rectifier. and, as you can see (or not) I'm using parts from the bin. the power trans is a Classic Tone 40-18016 from long ago

[sluckey]

-52v on pin 5 is waaay to much for 6V6s. When the bias is properly set pin 5 will have about -36v. Adjust the bias for 24mV on pin 8. Reevaluate the sound. Your voltages will come down also.

[Beezerboy]

like I said, I started with the bias at mid control, but didn't measure, then I turned it to max. it made no real difference. I doubt that the bias will make any real difference since the thing blurts into overdrive right off the first sound it makes from zero volume. but, I'll try it tomorrow. start at maybe 20mA bias

anybody know anyone that has built this amp successfully?

[glass54]

Hi Beezer
A friend of mine built it, worked first up 
He made one change, No mid pot, just fitted the 6k8 (as per Note 1). The bias was at a ballpark figure of -35V.
He used a local (Australian) transformer and SS Bridge rectifier.
He was very happy with it.
Regards
Mirek

[Beezerboy]

actually... I think it is mostly a bias problem. that and a crapworthy speaker along with. its been a hectic last couple days with other stuff going on so I think I have been overlooking simple things. I'll get back on it here is a few days