Output tube plate resistors

[heisthl]

clean - all knobs straight up, single coil guitar (Tele) in middle position.

[Max]

Hi,

as this is the Dumble Discussion section of The Amp Garage forum it may perhaps be of interest for some here to know how Alexander Dumble himself recommeded to adjust these trimpots which he calls and - if they are adjustable on the back of one of his 6550A and KT88 amps (see attched pic) - labels "dynamic balance": https://tubeamparchive.com/viewtopic.ph ... 211#145211

Cheers,

Max

[greiswig]

Right, Max, and this is the other reason I wanted to try to do this in a less subjective way than by ear. We've been round and round this adjustment on this forum, and it may well be that I
-don't have the right PI tube
-don't have as good an ear as I thought
-don't have some of the details needed (loudness?) to do the ear adjustment right

In any case, I haven't heard the sweet spot yet. And it seems like doing it by ear puts a lot of variables into play (pick attack, pick position along the string, etc.) that could influence the outcome.

I'm hoping to get a local expert at some point who can show me how it's done. Once I hear it, I'll have a better idea. Even using the scope, I wasn't able to see the amplitude changing as I adjusted the trimmer, so clearly I'm doing something wrong.

[greiswig]

clean - all knobs straight up, single coil guitar (Tele) in middle position.

Again, do you want the volume up to the point that the strings are on the verge of feeding back already, and you're just listening for the point at which the maximum feedback occurs, or is it something else? Just seems like how hard and where you pick will have a big influence on this...

[Structo]

George when I have done this before you want the amp up pretty loud but not at where it squeals from feedback.

You want it loud enough to excite the strings.

This is how I did it.

I used a strat with singles.

Tone pots at noon, presence at noon in clean mode.

Lightly brush the open strings.
Give the trimmer pot about 1/8 turn.
It takes a second for the change to settle in due to the caps I think.

When it is set right the notes will want to bloom into harmonic feedback.
Like second and 4th order.

As mentioned, some tubes won't do it at all.
I spent almost a whole day before trying different tubes and settings.
Then the other day I had a problem and I changed the tubes because I thought the problem was tube related. It wasn't and now it isn't set at the optimal setting anymore.....

The other thing, as the tube wears or other things change, I'm not sure how long your setting is good for before it changes.

I wish I had some way of matching triodes, that would help things.

[greiswig]

I wish I had some way of matching triodes, that would help things.

Tom, I've seen you post this before, but thanks for the additional info about some tubes not doing it at all. Gil has said the same.

On the matched triodes... I guess I'm left with a "why" question? This sort of seems to fit into the "why bother with the AC balance technique Ampeg recommends if all that matters is how it sounds?" My theory with trying to do the AC balance thing with my scope (failed) was that if I get it balanced at first, the optimal tone is probably a little to the side of that. So instead of just starting at some random point of "definitely out of balance," I'd be starting at a quantifiable ideal and deviating from there.

But, as I said above, I couldn't detect any changes in the wave amplitude as I adjusted the trimmer. So that didn't work.

But getting back to the balanced triodes...why bother if the trimmer should be able to get the output balanced anyway? Isn't all you need a pair of triodes that are in the ballpark, but don't have to be perfectly balanced?

[Max]

Hi greiswig,

based on the way Alexander builds the power amps of his big amps - some I know with full volume specs (THD as an example) in the range of a first class hifi amp - I doubt that he thinks about the power amps of his amps as "tone generators" that shall produce certain kind of harmonics caused by an intentional misbalance of the power amp stage. And IMO it would not make much sense to call a pot "dynamic balance" and to recommend to adjust this by the "SVT method" if it is meant to adjust the amp for an intentional misbalance.

But please don't understand me wrong. Of course anyone should feel free to do with a Dumble (style) amp whatever he thinks is best for the music he wants to use it for - even if this means not to do what the builder recommends. As far as I know the guitar players in the sixties and seventies did many things with their amps that have not been recommeded in the user manuals. So why not with a Dumble (style) amp?

But I personally - and based on my personal experiences with these amps - can recommend that if you read (or know of) a recommendation - like on the back of this ODS 150W - to adjust the idle current for two 6550A tubes to be 125 mA (or .125V at the 1 ohm test points), just to do it like this. I was always happy with the results - and from living room to earthquake levels.

All the best to all you guys here!

Max

[Structo]

The method I posted was what Gil and Tony and perhaps Scott have recommended to me personally on setting the balance.

[greiswig]

clean - all knobs straight up, single coil guitar (Tele) in middle position.

Okay, I may have gotten it. I used the Tele middle position, clean and loud, with hearing protection on. Just doing hammer ons on a few notes on the 2nd string, I started to hear some subtle differences.

I think I was looking for a magic moment when suddenly it would start feeding back, and if I'm right that I got it, that isn't what to look for. I could get it to start to feed back almost in any setting at this volume, and just adjusted the volume until it would start but not really feedback.

As I got close to one setting, I started to hear the pitch of the feedback change; at first it is strange harmonics..."out of tune." Move it a little more, and you start to get that first octave up feeding back. Move it a bit more, and you start to get the second octave up sounding through.

Is that it? Is it more about "tuning" the harmonics, and are those the harmonics to look for?

-g

[Structo]

For what it's worth, it took me a few tries on different days before I figured it out and what to listen for.
Everybody is going to do it slightly different.

Personally, I don't want my amp taking off into a harmonic every time I sustain a note so I don't adjust it that radical.

But pleasing overtones is where it's at and this amp does it pretty well.
You just gotta find the right tube and setting.

[Max]

Is it more about "tuning" the harmonics, and are those the harmonics to look for?

Hi greiswig,

according to Alexander Dumble (at least if did I understand him correctly, but I am rather sure about this): No.

According to Alexander Dumble it is the best overall "dynamic balance" or symmetry of the operation of the complete power amp section and over as large a range of its operation as possible to look for. This is what he recommends and not to use the "dynamic balance" pot as a device for adjusting the power amp to produce a certain kind of harmonic distortion.

The tone generator part of a Dumble amp is IMO the preamp with all its sophisticated possibilities to fine tune the harmonic content (the "trigger" one of them - just as an example.)

IMO one should think about the power amp stage of a Dumble amp as being the canvas on which the preamp shall paint the colors of the tone. And the "whiter" and "brighter" the canvas, the better and the more pure these colors can be perceived.

To adjust the power amp of a Dumble amp to intentionally produce "harmonics" (harmonic distortion) is IMO a misaplication of the special kind of tone generation tool (at least most of) Alexander Dumble's amps (especially the ODS/ODR amps) IMO are.

IMO the use and adjustment of the power amp stage for the intentional generation of harmonic distortion would be a correct application of Jim Kelly's technological approach to tone generation (IMO great amps BTW!) - just as one example for a different kind of tone generation tool.

Cheers and a nice weekend,

Max

[greiswig]

Is it more about "tuning" the harmonics, and are those the harmonics to look for?

Hi greiswig,

according to Alexander Dumble (at least if did I understand him correctly, but I am rather sure about this): No.

Max, I hear you! I find it interesting that several people on this list whose opinions I respect as much as your own come down on the "adjust by ear" side of things, though. So as usual I have to try it on my own and hear for myself.

But I'm still not sure I am doing it correctly, which is why I was describing what I heard and asking if it was what Henry, Scott, Gil and others are describing.