New build, OD problem, Pics added

[Synchu]

I am not sure that I understand what red marked parts of the schematic attached mean, but if you've skipped the LNFB networks, snubbers and put the HRM - the overall voicing, to my experience will be quite trebly and brittle.

Niki

[ToneMerc]

I am not sure that I understand what red marked parts of the schematic attached mean, but if you've skipped the LNFB networks, snubbers and put the HRM - the overall voicing, to my experience will be quite trebly and brittle.

Niki

Exactly, those are the things that stand out for me as well.

1. Preamp voltages too low at the PI
2. No LNFB
3. No snubbers
4. Too much loading at V2b

I think it should sound brittle........small things matter.

YMMV



TM

[Bob-I]

I am not sure that I understand what red marked parts of the schematic attached mean, but if you've skipped the LNFB networks, snubbers and put the HRM - the overall voicing, to my experience will be quite trebly and brittle.

Niki

Exactly, those are the things that stand out for me as well.

1. Preamp voltages too low at the PI
2. No LNFB
3. No snubbers
4. Too much loading at V2b

I think it should sound brittle........small things matter.

YMMV



TM

1) I too was concerned about the low voltages at the PI but since the clean sounds good I put that aside. Perhaps I should make adjustments in the dropping string first.

2) yes, but on other amps I have a switched LNFB that I never use, so I left it off. Easy enough to add

3) I put 250pf snubbers in

4) I'm not sure what you mean by too much loading. Please explain.

Thx for the help

[Bob-I]

How far up are the masters set? That 15pF bright cap can be pretty brittle sounding at low levels.

I took the bright cap off

[Bob-I]

I am not sure that I understand what red marked parts of the schematic attached mean, but if you've skipped the LNFB networks, snubbers and put the HRM - the overall voicing, to my experience will be quite trebly and brittle.

Niki

I added 250pf snubbers but not the LNFB, I will add that next time I have a chance.

The schematic is not 100% what I built, and I didn't write it up. Being colorblind I have no idea which parts are red

[ayan]

I just looked at this and have a couple of comments, which hopefully may help -- and my apologies in advance if this was already discussed and I missed it:

1. You use carbon film resistors throughout. While I don't believe this is the cause of the problems you've experienced, you may want to consider using metal films at least for the plate loads and cathodes, like Dumble did.

2. I don't know how you've wired the board, but looking at it, you have for V1 150K then 220K for the plates, however, on the overdrive you have 220K first then 150K. I'm sure you know that both stages are wired the same in the actual amps. If you've indeed wired this backwards, I've never tried that but I wouldn't think it would cause the lead channel to sound bad. However, what could cause things to sound bad would be if you've paired the plate load resistor with the "wrong" cathode resistor. I.e, 220K goes with 3.3 and 150K goes with 2.2.

Good luck to you, hope you get this resolved soon!

Cheers,

Gil

I am not sure that I understand what red marked parts of the schematic attached mean, but if you've skipped the LNFB networks, snubbers and put the HRM - the overall voicing, to my experience will be quite trebly and brittle.

Niki

I added 250pf snubbers but not the LNFB, I will add that next time I have a chance.

The schematic is not 100% what I built, and I didn't write it up. Being colorblind I have no idea which parts are red

[Bob-I]

I just looked at this and have a couple of comments, which hopefully may help -- and my apologies in advance if this was already discussed and I missed it:

1. You use carbon film resistors throughout. While I don't believe this is the cause of the problems you've experienced, you may want to consider using metal films at least for the plate loads and cathodes, like Dumble did.

Yes, I have another Dumble style amp with CF on the plates and I like it, a tad mellower. I do have some MF here so I may try that if this doesn't get resolved.

2. I don't know how you've wired the board, but looking at it, you have for V1 150K then 220K for the plates, however, on the overdrive you have 220K first then 150K. I'm sure you know that both stages are wired the same in the actual amps. If you've indeed wired this backwards, I've never tried that but I wouldn't think it would cause the lead channel to sound bad. However, what could cause things to sound bad would be if you've paired the plate load resistor with the "wrong" cathode resistor. I.e, 220K goes with 3.3 and 150K goes with 2.2.

I've labored over this issue several hours. I have 220K/3.3K on V1a and V2a, 150K/2.2K on V1b and V2b. From what I see on the layouts and schematics this is correct.

Good luck to you, hope you get this resolved soon!

Cheers,

Gil

Thx for looking Gil.

[Bob-I]

Well daymn, I put the scope on it last night, P.O. I'll bet when I find and solve this it'll sound great. P.O. Is a pain to troubleshoot and I've never had it before in a dumble type amp.

[ayan]

Curious to see what could have gone wrong, one always learns something new when building an amp. Hope you get it resolved soon and that it sounds great when you're done with it!

Gil

Well daymn, I put the scope on it last night, P.O. I'll bet when I find and solve this it'll sound great. P.O. Is a pain to troubleshoot and I've never had it before in a dumble type amp.

[talbany]

Bob
Are those the cheap Xicon CF's ...If they are? for me personally I thought they sounded OK in clean style amps,however I found they can be somewhat inconsistent sounding (some sounded pretty bad) in the overdrive amps like the ODS..I am not sure about the QC on those resistors but will say they can be all over the place with respect to tone and harmonic content..Once we switched to the KOA's (and NOS Draloric style) my amps were much more consistent ,smoother and more musical sounding..I am not in any way saying that you should replace those in the amp and that this is your problem,but thought I would mention my experience with those types of resistors (in ODS type amps).. Filter caps can have a similar effect..Some sound OK in clean type amps but can again get harsh and non musical used in an overdrive stage..I don't recognize those caps you used?
I really cannot say why or how these resistors and some filter caps react in these amps..I never had the time and resources to thoughtfully test and document them over long periods of time..When I find something that works/sounds best or better I move on and don't really look back..Hope you find the problem..SOON!!

Best of Luck!!!

Tony

[Bob-I]

I'm starting to think I need to do some major changes on this one. I grabbed the filters from mouser but I've never seen them used before. The xicons were laying around and another amp sounds fine with them. Now I'm seeing the oscillation issue.

I think my approach is going to be to build a new filter board with well known caps, then replace the CFs with KOAs. I'd rather rework this now than to run into issues later.

Thx

[Charlie Wilson]

Hello Bob I, to start I agree with all of the comments regarding components although I probably "hate" the Xicon carbon films less than some of the other guys. The thing is you still have not found the problem and I don't think your component choice is the reason your OD sounds the way it does. Sure you can rebuild the amp but them you will never know what the problem was. I still think you should investigate possible connection, wiring, and grounding issues before you move on. My background is more of a repairman than builder and Willie Whitiker (Lord Valve) used to tell me "find the %$%* problem before you start messing with all of the other stuff".

[talbany]

Just to be clear..
I never said I thought Bob should rebuild the amp right away with different type components,it's just a possibility we should consider.. I've had similar issues with the overdrive side of some amps while using these resistors (and some caps)I also have heard this from other respected builders as well....I too think we should continue to search for the golden nugget first and if all else fails go to the extreme of a rebuild if you are convinced that is where the problem is through process of elimination..

Again Good Luck!!..Bob

[Bob-I]

My biggest concern is for the filter caps. I've never seen these little stubby radial caps before.

I'm going to try to find the source of the oscillation. If I can resolve that I'll bet the OD will sound much better, then I can address the components.

[Charlie Wilson]

Tony, maybe I was misunderstood. I was not trying to be critical of the idea of replacing components or doing some rebuilding. I just wanted to suggest that Bob I might feel a little better about doing that after he figured out what the issue is.
CW