Weber 6A40 issues...

[rmj134]

Hello everyone, and thank you in advance for your responses.

I built my amp about a year ago (my first and only build). I am not a professional, but with the occasional phone call to my father (electrical engineer), I was able to get the thing running without killing myself.

Although the amp does sound better than my previous amp (Hot Rod Deville), it doesn't have that Fender AB763 black-face sound. Until now, I've just let it go, but the other day I played into a vintage super reverb, and it really showed me how different my amp's sound is from what it is supposed to be.

My amp doesn't make any noise, and has no audible indications that something is wrong. It just doesn't sound right, and it really kinda bums me out. The bass sounds muddy, and it doesn't have that classic "compression" sound of the back-face amps. I just replaced the tubes, and that didn't help much. When I check the bias, tube 2 is at 21.25 and tube 1 is at 19.73. Are they supposed to be that far off from each other? I get similar results when swapping the tube positions. Also, the plate voltage is 459. If I turn the bias pot all the way up, the most static dissipation I can get is 26.07. Theoretically, shouldn't I have the ability to go above the max dissipation of a 6L6 tube (not that I ever would)?

If anyone out there in amp building world can help me out, I'd really really appreciate it.

Thanks

[ahamay79]

The bias sounds low. What kind of plate voltages are you showing?

[ahamay79]

Sorry, I missed the 459 volts. Get them up to 33-35 ma each.

[rmj134]

If I turn the bias pot all the way up, the most I can get is 26 mA. Even then, it doesn't seem to have any more "compression", it just breaks up more. Also, the specs say my tube's max dissipation is 30. Do you still think I should be shooting for 33-35?

[ahamay79]

The way to figure out your bias setting is to take your max dissipation (25watts is standard on a 6L6) divided by your plate voltage (459 volts). The gets you .054 amps, or 54 ma.

Multiply that by .70 (70%) and you get .038 amps, or 38ma. Try any setting that sounds good between 30 and 40 ma. That's 30 - 40 ma on each tube.

459 volts x .035(35ma) = 16.065 watts.... well within limits.

Is there a 27k resistor on the bias pot? Clip out the 27k resistor on the bias pot and install a 33k in it's place to give you more usable range.

Or, try changing out the 10k bias pot with a 50k cermet pot. That's what I do. Very precise pot.

[rmj134]

Ok sorry, I misspoke (miswrote). The max dissipation for these tubes is 30 watts. With my plate voltage (459 volts), I am currently at 46.3 and 43 mA. This puts me at 21.25 and 19.73 static dissipation. I have heard the rule of 70% before, so that it was I was shooting for. That being said, it sounds kinda flat to me. If I turn the pot all the way up, the most static dissipation I can get is 26 watts (56 mA). I know this is not recommended, but I wanted to hear the difference (I changed it back). All that happened was that the amp broke up much sooner.

Also, the bias pot resistor is 22k.

[ahamay79]

Okey doke. Now we're on the same page.

Hmm.... Ted's kits usually get pretty good reviews, but you can always improve on them.

You may try upgrading the capacitors..... his amps usually come with pretty generic types. Maybe change out to Spraque Orange Drops and/or Mallory 150's?

Maybe try some carbon comp resistors, too?

[rmj134]

So that's it? The caps?

I swapped out the original caps in the preamp stage of the reverb channel with oil filled caps a few months ago. To be honest, I don't really hear a difference between the reverb channel (oil filled) and the normal channel (original caps).

Is it odd to have the two tubes be 3mA apart? Could I be missing something somewhere? Some magical ingredient?

[ahamay79]

I wouldn't worry about the 3ma. Not enough to make a difference.

Hmmm.... no real difference in types of caps huh?

How are all the other voltages in the amp? You can check them against the Fender Super Reverb / Vibroverb values if you have a layout diagram of those amps. You can find thm at www.schematicheaven.com/fender.htm

[rmj134]

I have the schematics and layout from the weber website, but they don't have the voltage values. Thanks for sending the link!

Is the amp on or off standby when checking the voltages?

[Five_E35]

JMHO but try biasing the tubes at 35-36ma.(37-38 if you're including the screens) or more like 60%.

Hot biasing BF Fenders beyond a certain point makes them loose their sparkle and chime, break-up quicker, and do pretty much what yours is doing. You may loose some volume at 35ma, but my guess is you'll have a lot more presence and punch and you can turn up the volume control a bit to compensate. If that makes it too bright, try going up in ~2 ma. steps.

[ahamay79]

Very good point. That's why I like 20 turn 50k cermet pots. You can dial in exactly what you want.

And the amp is "on" when checking voltages.

[rmj134]

Ok, so I biased the tubes to mid-upper thirties. This helped a little bit in getting back tone, but unfortunately, not what I was hoping for.

So what I decided to do was check the voltages on the whole amp. Took me two and a half hours, but I was able to make it through the whole thing with voltage readings on everything that is in the original schematic. I have the values with what the spec is supposed to be in an excel file. Should I post it? I don't really know what the margin of accuracy is supposed to be, so I can't really tell if the amp is within spec or not.

[Diablo]

Yes, post the voltages. What you are looking for is something that is way off from the trend. For example, I would expect most of your voltages to be about 10% higher than the Fender schematic.

[rmj134]

Below is a link to a PDF that has all my voltage readings and what they are supposed to be. Clearly, there is something wrong with my vibrato, but I have no idea what it is, and if it would impact the sound of amp either way (turned off / on). It was stupid of me to not mention this earlier, but I forgot about it because I never turn vibrato on.

http://storage.bobbyjuncosa.com/6A40_voltages.pdf

Thanks for everyone's help!