Weber 6A40 issues...

[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]

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.

[Regis]

Not to butt in here, but my understanding of the Weber Copper Cap is that it reproduces the characteristics of a tube rectifier, including sag, and voltage drop. Ted makes different ones that don't have these features so you might want to check with him and see what you have. You might not hear any difference with a real tube.

[lowell]

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

Hello,
I'm curious about the compression that you speak of wanting. You should definitely consider converting this amp to cathode bias if you want some "squish" to the amp.

[rmj134]

tubeswell,
That information seems very insightful, but its a little over my head. Any way you could translate for an amp builder newb?

lowell,
The compression I am referring to is the compression I seem to hear from pretty much all AB763 circuits, and even to a degree the silver face circuits. My Weber kit is the closest to the Super Reverb, so I've been referencing that.

You can hear the sound I am looking for all over youtube. Here is an example. Its the first 2 minutes I am referring to. Its a vibroverb, but I imagine mine should be able to get that sound too. http://www.youtube.com/watch?v=HuY29...e=channel_page

My amp breaks up just fine (although the bass gets pretty muddy past 5). It never really compresses though. I can post sound clips if you would like, and a PDF of the amp's voltages is on page 2 I believe.

Regis,
I think you are right about the Copper Cap. Ted Weber says that they are supposed to mimic the sag of a tube rectifier, which I guess is why when I switched to tube, I didn't notice much of a difference.

[bnwitt]

depends on which copper cap you have. They all have a different voltage drop. However they do not sag exactly like a tube rectifier.

[lowell]

Yes I agree... I swapped out a copper cap for a real tube recently with incredibly great results concering sag. Also I see negative feedback on the weber schem but not in a Vibro King. You can try removing that as well. Aside from a bit more breakup it'll feel spongier. Also the Vibro King has twice the filtering as far as cap values go... after the choke that is. That should tighten up the bass.

[tubeswell]

tubeswell,
That information seems very insightful, but its a little over my head. Any way you could translate for an amp builder newb?

Of the bit about rectifier tubes and Ht voltages. The choice of rectifier and the voltage of the HT winding (i.e. the secondary winding you connect to the rectifier anodes), in combination with the method of filtering, determines the B+ voltage and current you get. If you have a rectifier tube that drops a relatively bigger proportion of voltage and doesn't supply as much current (such as a 5Y3GT), you will get a lower voltage on the B+, as well as more sag and bloom, under high-current demand scenarios (like when you whack a power chord with the volume dimed) with a push-pull amp). So if you are using a gruntier rectifier (like a GZ34 or silicon diodes), you will get more B+ voltage and less sag, than if you used a 5U4G or a 5V4 or a 5Y3GT respectively.

On the tube question, the method of biasing affects the amount of compression and sag you can get. With fixed bias, you have more output power but less compression and a harder edge to overdriven sound. With cathode bias you get more compression, and if you want the most compression, then make the cathode bypass resistor small (but not no-cap -at-all)

[MWJB]

Post some pics of the chassis.

There seems to be more wrong than just brand & bias of tubes. In the meantime though try a 12AX7 in the PI?

"Anyway, anyone have any last ditch ideas before I find someone who likes this amp's sound, and go buy one that I like? If you don't like the sound of the amp, after all the work you have put into it, why do you think anyone else is going to like it?

Tubeswell wrote: "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?" You don't NEED 45-46mA to make a Super Reerb sound right, in fact at 45-46mA it won't really sound like a typical Super Reverb. Most 6L6GC available today are NOT 30W tubes.

[rmj134]

Well, the amp doesn't sound BAD, it just isn't what I was hoping for. I'm sure there are lots of players that would play it, and think it sounded great. That being said, I'd rather keep the amp and find a way to fix whatever problem it has.

I have experimented running the amp cold and hot (between 25mA and even 50mA). I didn't leave it running hot for very long, but I did try it out to see if I noticed a difference. The amp did get "warmer", but it didn't compress any more really.

I will post pics a little later today.

Thanks again everyone.

[lowell]

have you tried cathode bias yet?

[MWJB]

If you do try cathode bias, I'd make it switchable, you'll be knocking 10W odd the amp's output.

First off, I'd be looking to cure what is wrong with the amp, make sure it is reasonable example of type - rmj134 tried another Super Reverb that seemed to deliver the goods...no reason to think that other SR was cathode biased.