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!

[rmj134]

Anybody have a chance to check out my voltage readings?

[ahamay79]

How does the amp sound if you play through the regular channel?

Plus, have you tried pulling the vibrato tube?

[ahamay79]

Are you absolutely sure that all of you resistors in that section are the correct value? Have they all been checked with a meter?

[rmj134]

The regular channel and the vibrato sound very similar. No noticeable difference. Even with pulling the vibrato tube all together, still no noticeable difference.

I'm pretty sure the values are correct, but I will go through with a meter and make sure.

Based off the PDF, are there any stand-out values that could point to a problem?

[Five_E35]

I didn't see anything unusual. You can also test the AC operating voltages shown on the Fender schematic for the reissue version. They're usually close enough and that will show any stages or tubes that aren't pulling their weight.

http://www.fender.com/support/amp_sc..._Schematic.pdf

You need a dummy load resistor(50 watts @ whatever impedance the amp needs), and a meter that can read AC signal voltages(or scope). If you don't have a signal generator, there are some sample 1K sine waves online in MP3 format you can download and use - just loop them. See the instructions on the schematic for the test procedure.

It may just be a case of the real thing vs. a clone, but did you happen to notice what kind of tubes the original had - if it was loaded with RCA's, Sylvania's, and GE's that would make a big difference too, especially preamp tubes and a real tube rectifier.

[ahamay79]

What you posted is for a Super Reverb Reissue, not an AB763 like the guy built.

Double check the filter caps and resistors under the cap pan, too. Make sure they're hooked up correctly... check polarity.

[TD_Madden]

Don't forget that the plate voltage changes as you adjust the bias...that's why I use two meters at the same time when doing this, one for the plate-voltage and the other for the bias ma..

Just keep a calculator handy.

[Diablo]

I would work on the vibrato circuit first, because that's where your voltages are screwy. Trace back each resistor and capacitor and check values and voltages. It could be miswired too.

[rmj134]

You guys have been a big help. I feel like I'm really close.

The tubes in the original that I played were just your basic Groove Tubes. It did use a tube rectifier though, and mine is the Weber "Copper Cap" solid-state rectifier replacement (came with the kit). Do you think this could be the culprit? Also, I have no idea what the amp was biased at.

I will try and track down the problem in the vibrato circuit. Is there any way to "remove" this from the overall circuit as a quick way to see if it changes the tone of the amp, or is the amp's tone dependent on the vibrato circuit? Also, being a bit of a newb, how can I use my mulitmeter to check the values of the resistors AND the caps individually to insure they are all correct?

[MWJB]

"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)." There should be a 220K resistor between B+ supply and the vibrato tube plate 1, as you are only dropping 2v accross whatever resistor is there, its either much less than 220K or your cathode resistor at pin 3 is open?

It's a bit odd that your heater voltages seem to drop as you get nearer the preamp and that you have odd voltages at each leg? Should be constant wherever you measure.

If you played through 10 vintage Super Reverbs all 10 would sound a little different. But you seem to be describing a significant difference in tone. Do you have any pics of the circuit?

[bnwitt]

Originally Posted by rmj134

The tubes in the original that I played were just your basic Groove Tubes. It did use a tube rectifier though, and mine is the Weber "Copper Cap" solid-state rectifier replacement (came with the kit). Do you think this could be the culprit?

Yes indeed. Your complaint was a lack of compression. Lack of a tube rectifier would certainly deliver that effect.

[MWJB]

"I will try and track down the problem in the vibrato circuit. Is there any way to "remove" this from the overall circuit as a quick way to see if it changes the tone of the amp, or is the amp's tone dependent on the vibrato circuit? Also, being a bit of a newb, how can I use my mulitmeter to check the values of the resistors AND the caps individually to insure they are all correct?"

Pull out the vibrato tube altogether...it has no effect on basic tone anyway, just works the wobble.

Many of the resistors in the circuit are in parallel with other components (which will affect your reading), few of them will read their real value, the voltage check you did was more useful than trying this.

[rmj134]

So I did a little research on tube rectifier vs. solid-state rectifier. I knew that the rectifier was responsible for AC to DC conversion, but I never really heard the term "sag" before. It sounds exactly like what I am describing as the tonal difference between mine and the vintage counterparts I have been playing. I am going to order one today, and I will let everyone know how it works out. Are there any suggestions?

Also, although it doesn't sound like the vibrato is contributing to the tone of the amp, I would like to resolve that issue as well. I believe the 220k on Pin 1 is there (even though the Weber schematic says 22k and the layout says 220k). I will check to make sure the cap on Pin 3 is making a good connection.

Thanks again, you guys have been awesome!

[Diablo]

Originally Posted by rmj134

So I did a little research on tube rectifier vs. solid-state rectifier. I knew that the rectifier was responsible for AC to DC conversion, but I never really heard the term "sag" before. It sounds exactly like what I am describing as the tonal difference between mine and the vintage counterparts I have been playing. I am going to order one today, and I will let everyone know how it works out. Are there any suggestions?

Make sure you wired up the rectifier socket correctly to use a tube - see Ted Weber's schematic/layout for directions. If you want to get a "good" tube, try a NOS 5U4GB for the rectifier. They're widely available for around $20, and more reliable than current production. Any US brand will do fine. The 5U4GB will drop the voltage a little and give you some compression.

[Daedalus]

What kind of 6l6s are you using? The chinese ax7 and at7s that come with those kits are pretty good, not so sure about the 6l6s.

[rmj134]

Thanks Diablo, I will order one of the NOS 5U4GB rectifier tubes today. Is installation as simple as adding fillament power to the socket?

I have Groove Tubes in there now, but I also have Sovtek 6L6WXT+ ready to put in. The preamp tubes are a mix between Tung Sol 12AX7, Tung Sol 12AT7 and JAN 5751.

[MWJB]

I doubt that the Super Reverb you tried had a 5751, not a stock fitment to these amps.

Which Groove Tubes do you have in there? Russian, Slovakian, Chinese...Groove Tubes sell the same tubes that everyone else does for the most part. If they are the Russian 5881WXT (std fitment to current 6L6 Fender amps), these are typically not a good choice for a Super Reverb, on the stiff/sterile side.

To run a tube rectifier, if already running a copper cap, just connect up the yellow 5V wires to pins 2 & 8 of the rectifier.

[rmj134]

Yeah, you're right, the Super didnt have 5751 preamps. At this point, I've put 12AX7 tubes back in, replaced the rectifier with 5U4GB, and swapped the Groove Tube 6L6s with Sovtek 6L6WXT+. Although the rectifier did help, unfortunately, it wasn't nearly as drastic as I was hoping. I'm pretty much at a loss. I have the amp biased at about 35mA (the Sovteks are matched much better than the GTs were).

The only time I've really heard that blackface compression sound come from my amp was when I put Tung Sol 5881 tubes in. I had the amp biased way hot for those tubes, and they fried within two weeks, but it was the best two weeks of my life (haha).

Anyway, anyone have any last ditch ideas before I find someone who likes this amp's sound, and go buy one that I like?

[tubeswell]

Hmmm... The PT on the weber schematic says its a W022798 - and when I look that up its says 325-0-325 on the HT winding. Compare that with the BF amps (is the BFSR ab763 the closest schematic? Was that the amp you liked the sound of? - that has 360-0-360 HT going through a GZ34 giving 460 plate voltage - and bearing in mind that wall voltages in the '60s might've been a bit lower. Nevertheless, double check all the component values in the BFSR and compare them to what you have actually put in - did you put in a wrong resistor somewhere?)

That aside, you say your getting 459V plate idle voltage? With 6L6GC that needs 45-46mA (per plate) for 70% idle Class AB1 operation. Did you tweek the bias circuit to get you in that zone?

AFAICT the 018343 OT in the weber amp is a 4k2 primary Z - this is not too far off the 4k BFSR spec - so that shouldn't be the cause of much difference

According to R.G.s theory about where CCs make the most difference, the 100k plate resistor feeding the driver before the PI, and possibly the 82k and 100k resistors supplying the LTP, might make the most difference (if you were going to throw some of them into the mix).

The other thing I can think of is that the pre-amp tubes affect most of the voicing in the amp. If the pre-amp tubes (esp V1 tubes) are good NOS tubes, you should get a pretty nice voicing. Modern pre-amp tubes just aren't in the same league.

[Diablo]

Originally Posted by tubeswell

Hmmm... The PT on the weber schematic says its a W022798 - and when I look that up its says 325-0-325 on the HT winding. Compare that with the BF amps (is the BFSR ab763 the closest schematic? Was that the amp you liked the sound of? - that has 360-0-360 HT going through a GZ34 giving 460 plate voltage - and bearing in mind that wall voltages in the '60s might've been a bit lower. .

The Weber W022798 PT has two output tap choices - 330-0-330 and 360-0-360 for the HT. The higher voltage taps give around 460V DC - that's what I get with my 5F6A clone using that transformer.

Besides tubeswell's suggestions for NOS preamp tubes, you may want to try 5881 tubes biased to around 35mA each. I've got NOS Phillips 6L6WGB (same as 5881) in my amp and they break up nicely. About $50/pair for the Phillips on fleabay. Folks also like the new production Tungsol 5881 - about $30/pair.