70s build - some questions (new - added some clips)

[Bob-I]

Glad you figured it out. I've been burned by that one as well. It's also one of the first things I check when any amp with a loop is acting out, especally wenh guys tell me the volume fades in and out.

I'll be interested in the full report once you're done.

[wjdunham]

I'll post my results and hopefully some clips when I'm done. My playing style is definitely not the type that is most often heard in clips here (ie, not anywhere near as talented ) I tend to play much more heavy handed, which is why I prefer a guitar with single coil pickups, I tend to just muddy everything up with a humbucker equipped guitar. Sort of why I wanted to try a different take on this build. I've had a lot of stupid problems with this one, sorry for asking so many dumb questions. Too many late nights working on it. Back on track now though.

[mdroberts1243]

Sure enough, dumb problem - the return jack was not making good contact - emphasis on "good", it was making SOME contact. 5 minutes with the scope and problem was solved.

I've had this problem too... with brand new switchcraft jacks... always suspicious of those built-in switches now.

I've had someone tell me recently they treat all their new pots and jacks with 'stabilant' before use... but I'm reluctant on the pots, I like the 'feel' of the pot control out-of-the-box.

[wjdunham]

Brand new jack in my case too - cost me a few hours chasing my tail. This was caused by slightly bent contacts, not oxidation buildup. This particular jack seemed to have lost it's spring (or it never had any).
I have an old Marshall JCM900 which used to cut out every once in a while, very intermittent problem and I took that amp in to two different shops with no success. I finally figured out it only happened when I had it at home with nothing in the loop, I always used a rack at gigs. Stuck a cable in the loop and never had any problems since.

[Structo]

Yes those switching jacks can be problematic.
When I get a new Switchcraft one I generally bend the contact a bit more to make sure it has enough spring in it to make good contact.

You can't really do that on a Cliff style jack.

[crbowman]

Hey Bill,
Glad to hear you got it going. I'm really interested in hearing your overall impressions of this thing as I use primarily single coils as well. how do you like the cleans??

[wjdunham]

I'm finishing up the FET board so we'll see how that sounds with this configuration. This is the first time I've tried the FET circuit so it will be interesting. I haven't been impressed with the cleans on this amp so far, they seem a bit dull, and with the lack of gain in the PI and a fair amount of GNFB it's hard to push it near the edge which is how I like to play. I can tell the OD on this is going to really work for me though, I'm already getting a lot more bloom and sustain at lower drive levels than with 80's configuration. Overall seems a lot thicker with single coils, someone with more experience can comment on why. I suspect the larger coupling caps all around? I only have 5uF cathode caps on V1.

Can't wait to finish the FET which is supposed to really be nice in the early style amps.

[wjdunham]

Here is a quick wankage clip of my amp. I'd love to get some feedback. I've gotten a lot of help from Brandon (thanks!) after I heard those clips on TGP of Lance wailing on one of his 70's style builds. The clip is with my strat, mostly using the bridge pickup which I've found very thin on skyliner type amp. It's nice and fat with this one, and it seems to suit my heavier style of playing. Right now it's pretty much as in the #40 schematic, I played with the voltages some and found lower V1 to really smooth things out. I also upped the NFB resistor some to get some gain back into the output section, and I left the OD level pot instead of the fixed divider shown in the schematic. I'm loving the FET input, this clip is all FET, all OD. Currently have the FET trim set to give 3db of gain.
The only thing I haven't been able to do is really get it to sing, it seems on the verge but just wont quite give up the goods. I tried a few different PI tubes and played with the trim, I have something like 275/281 on the PI plates, sounds best with the trimmer set all the way to one side. Is it just a matter of finding the right tubes or is there something else I can play with? Otherwise I'm liking this one a lot.
Bill

[crbowman]

Sounds really cool Bill!
Are you using a 12AT7 or 12AX7 in the PI? I've been looking over the schem, layout, and pictures of #40 and think I have found a few minor discrepancies. Mainly in the fixed output network from the OD channel (which you aren't using anyway), the wiring of the deep boost, and as already mentioned somewhere here, the grid resistors being 56K on #40 as opposed to 68K. Did you come across these as well or am I just looking at things wrong?

[wjdunham]

I put an AT7 in late last night, too late to crank 'er up. I checked the voltages and it dropped the PI plates to something in the 225 range, and V1 down to 160. I haven't done any research yet on how to really bias with a 12AT7. Should I up the cathode resistor some to get the idle current down a bit? Would I also have to adjust the plate resistors? I didn't really want to mess with the PI too much as I'm told it's an important part of the sound and feel of these amps. At very low volume I didn't really notice a difference between the two tubes types.
Right now I have 51K for both V2 grid resistors since I didn't have any 56K. 56K is the correct value. I saw in another thread where Brandon indicated the divider values in #40 schemo were correct, I may try that to see how it sounds, although I like the flexibility of the OD level control. He also posted that the OD entrance is different (220K, 500P, 1.2M, 33K), which is what I have.

[wjdunham]

A few more updates - I added a mid boost to see how it sounded, .002 in series with 350pf to give me the stock 300pf treble cap value. With the mid boost on it seems much less pronounced with this amp. At first I didn't really hear a difference, but the texture of the OD was improved IMHO.
No other comments on the quality of the sound? Was it that bad, I can take it...
Bill

[Funkalicousgroove]

if you have an audio taper treble pot you'll have to turn it up quite a bit to hear a mid boost-

[Structo]

Isn't the Treble pot normally linear in that spot?

[Max]

early stack good with single coils, later stacks good with humbuckers.

Statements like this can often be found at ampgarage.

My own experience and the one of a l l Dumble users I personally know is completely different:

Me and the other users I know really never met a n y original Dumble amp that we liked with certain guitars and disliked with others.

Some have been more to my personal taste, some less. But that always had to do with the special "voice" Alexander had given to this amp and had nothing to do with "humbucker or single coil friendly".

If I liked the voice and feel of one of his amps, I always liked it with all kind of guitars. (BTW: For my taste there have always been enough switches and pots on his amps to fine tune them to the different sounds I am looking for when switching from e.g. a Strat to a Les Paul.)

O.K. all that may have to do with my personal taste and the personal taste of the other users I know.

But how can it be explained, that I o f t e n read here some statement like the quoted one but n e v e r read statements like:

"What a great sound RF gets with his tele but what boring one with Larry Carltons Goldtop".

The sounds LC and RF get from there amps are always discussed here as a point of reference. And these two very often change between humbucker and fender style single coil guitars (LC sometimes adds a P90).

But all discussions here always talk about "the Larry Carlton tone" or "the RF sound" and I can't even remember a single post here where someone raved about the tone Larry got from e.g. his Strats and complained about the tone he got from his Special or ES 335.

And when Gregor Hilden posted the clips he did with #094 and very different guitars (single coils and humbuckers) all raved about the great guitar tones and there was not a single post that said "great amp with this guitar, not so great with the other one".

So if these statements like the one above I quoted would be valid:

Can someone explain, why most if not all here say LC, RF and Gregor get great tones out of all their guitars?
Obviously these three don't switch from an "early stack" to a "later stack" when they change from a single coil to a humbucker guitar?

Have a good time

Max

[wjdunham]

Max, can you give me some pointers as to how to actually make my amp sound as you describe. How about some Ohms and Ferads? How did my clip sound in your ears relative to others you have heard?
Thanks,
Bill