102 as Lowplate Classic

[Max]

As best I know, that low plate classic with the .002 treble cap is Jackson Browne's(?) ODS that HAD loaned SRV while building amps for him.

The ".002 treble cap" of this transition generation low plate classic ODS might be a part of its mid boost circuit:

It's an interesting specimen, Low plate Classic with a Mid boost and a trimmer for the negative feedback. It does indeed have a .05 mid cap, a 100K slope and plates.

source: https://tubeamparchive.com/viewtopic.php?p=81594#81594

More details:

https://tubeamparchive.com/viewtopic.ph ... cannonball

https://tubeamparchive.com/viewtopic.ph ... 06&start=0 The discussion about this ODS starts in post #6 of this thread (talbany)

Date code on one of the power supply caps: "8117L". Source: https://tubeamparchive.com/download/file.php?id=8155

Happy 2015!

Cheers,

Max

[Charlie Wilson]

Max, so the linage is Jackson Browne, borrowed by SRV, Carlos Rios, and finally Ben Harper? Man, that thing gets around. I also would assume that Brandon would state that it had a mid boost because he was able to look at the back of the pots or how the switch was labeled on the front panel. In light of what Tonemerc said, I would suspect that it has a 2.7k negative feedback resistor with a trimmer because he ran out or did not have a 3.3k resistor which would be the appropriate value to go with the 270 ohm resistor. Which reminds me, it also looks like this amp has a 22k cathode resistor on the PI. If one were to reduce the 4.7k NFB resistor to 3.3k would it also be appropriate to reduce the cathode from 24k to 22k? I tried 3.3k and the 270 ohms and it seemed like the amp looses some gain.
CW

[Max]

I also would assume that Brandon would state that it had a mid boost because he was able to look at the back of the pots or how the switch was labeled on the front panel.

I don't think so:

- obviously this mid switch is labeled "deep" on the front panel: https://tubeamparchive.com/download/file.php?id=39120

- AFAIR a transition generation "classic" ODS with a mid switch mod but still a "deep" label on its front panel is rather usual.

- AFAIU this layout https://tubeamparchive.com/download/file.php?id=11313 .002 is a rather usual "treble cap" value in an ODS with a mid switch circuit.

BTW: #0124 is a perfect example for one of the 4th generation ODS amps with a mid switch (after the skyline mod) that's labeled "deep" on its front. AFAIR you'll find the first mid switches with a "MID" label on the front panel in the 5th generation ODS amps.

Cheers.

Max

[Charlie Wilson]

Max, thanks for the info. On my computer screen I can't really read how the switches are labeled on that photo. However, I guess my point was and maybe I wasn't very good at making it, that could it not be simply a Low Plate Classic with a .002 treble cap instead of 330pf? In other words, without seeing how the back of the switch is wired, it could have a deep switch function and simply a larger treble cap. I know that typically we see a .002 with a mid boost function. My interest in this was not necessarily historic but as a way to fill out the high end of a single coil type guitar and since this amp was used by SRV, maybe Dumble did it to make it a little fatter sounding for him(yeah I guess that is historic and pure speculation).
CW

[Charlie Wilson]

I tried the 33k grid stopper in my Low Plate Classic instead of the 22k and I love it. It takes that shrill edge off the high end with the Strat but still sounds great with humbucking pickups. Now I cheated a little. I used a big beautiful 1w Piher 33k to replace the 22k 1w Xicon but I still think the change in value helps with harshness. Did Dumble do it on purpose, we will never know.
CW

[aflynt]

Interesting. I'm still trying to figure out what happened to this amp when I converted it from Classic to Skyliner that resulted in loss of "sparkle." Not certain whether it's the Classic Stack design itself or the components I used. I'm guessing it's more likely in the parts somewhere given other people's related experiences. I suppose moving down to 22k grid might be worth a shot at some point, though.

I've got a bunch of vintage pots in mine: 2-25 '70s Fender Bass and Treble. 1M '70s Fender volume and Master (pull pot). '70s Fender ratio, etc... I put amp's old Mid pot (2-25 Fender) on the bass and a new 100k alpha in the middle for the Classic stack conversion. Most of them have static when turned while playing. Wondering if the loss of sparkle is in the pots somewhere.

-Aaron

[bluesfendermanblues]

Interesting. I'm still trying to figure out what happened to this amp when I converted it from Classic to Skyliner that resulted in loss of "sparkle." .........
.......... Most of them have static when turned while playing. Wondering if the loss of sparkle is in the pots somewhere.

-Aaron

the static could stem from a leaky treble/mid cap, leaking dc into the tone stack.
and a bad mid or treble cap could has drifted in value causing treble loss.

[aflynt]

Interesting. I'm still trying to figure out what happened to this amp when I converted it from Classic to Skyliner that resulted in loss of "sparkle." .........
.......... Most of them have static when turned while playing. Wondering if the loss of sparkle is in the pots somewhere.

-Aaron

the static could stem from a leaky treble/mid cap, leaking dc into the tone stack.
and a bad mid or treble cap could has drifted in value causing treble loss.

Good point. I'll check for DC across the pots.

-Aaron

[Charlie Wilson]

Aaron, I recommend you investigate those pots you used. One at a time starting with the volumes, swap them out with new ones, you can put the old ones back if no change in sound. I started a post Master Volume static and I started having issues with my amp after I used a vintage Fender 1 meg for my master. It not only caused static but rolled off the high end. I got used to it but I remember thinking that my amp was kind of nasal sounding. It also veiled the fact my GE PI tube had gone bad and was shrill sounding. Your amp should be very bright regardless of what V1 grid stopper you are using. Yeah caps can go bad but I would suspect forty years of wear and tear on a pot before a new cap.
CW

[67plexi]

I would take a closer look at the plate resistors used in real Dumble
amps 4th generation and 5th generation they are not Dale they are
Sprague or Mepco-Electra. Brown Sprague 1% or Sprague Q-line 2%
or Mepco-Electra 1% I'm no expert but it does make a difference
I learned the hard way also.

Steve.

[Charlie Wilson]

Yeah, but the store has been out of those for about 20 years now. I think the guys that do not have access to those part are doing the best they can with what is available.
CW

[aflynt]

I'll mess around with the pots and check for DC tomorrow.

I'm using NTE Flameproof on the plates and cathodes right now. When it was highplate I had Mepco/Electra on there, but it doesn't look my old source for those (Marcoparts) has those available anymore. I do have a whole bunch of giant old 1% Mepco 100k precisions that I think might be from the '60s though. I was considering trying those, but they'd be a tight squeeze on the plates.

-Aaron

[Max]

I guess that is ... pure speculation

Indeed - just as most of what I read in the internet in regard to the intentions of Alexander Dumble.

And IMO one should perhaps be aware anyway, that AFAIK up to now the circuits of perhaps somewhere between 5% and 10% of all the existing (all custom made) Dumble amps - for sure still untouched by other people - have been inspected in a scientific way.

And because they are based on such a small statistical database IMO all the generalisations you can read in the internet in regard to "the" specs of "Dumble circuits" in general (e.g.: "the" 2nd generation ODS circuit, "the" Dumbleland circuit, "the" SSS circuit, "the" Big Tex Reverb circuit etc. etc. are pure speculation, too, and far away from what I would call facts.

But on the other hand: Sometimes an educated speculation might be a good starting point for scientific research.

Cheers,

Max

[Charlie Wilson]

Yes but a lot of good sounding amplifiers have been constructed on speculation. You probably play through one yourself.
CW

[aflynt]

I cured the dullness issue. None of the couplers were leaking DC that I could detect (crappy meter reads 0 - .1mv DC with nothing connected). I also realized that only the presence pot is scratchy. The rest are actually fine.

As it turns out, putting in a JJ ECC803S PI, finding some better matched power tubes and setting the trimmer by ear brought back the sparkle. The funny thing is that moving the trimmer just an 1/8th turn either way makes the amp sound dull again. I didn't try setting it before because it figured it was pointless without matched tubes.

Now the amp sounds better with master bright cap disconnected when using the D'Lator. I also lowered the resistance on the HF Taper quite a bit (71k) to even out the channels more at higher gain settings.

-Aaron