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FDP Forum / Amp Mods, Repairs, and Projects / Help/Advice. 68 DR-Vibrato channel too brite

jackostrato

usa

Apr 6th, 2012 11:24 PM   Edit   Profile   Print Topic   Search Topic

hi folks,

i'm hoping someone can give me some advice on this problem. about 12 years ago i made the mistake of installing a torres cap "upgrade" kit in my 68 deluxe reverb, it advertised that the vibrato channel would sound better. really, it made it both shrill and woofy.

2 years ago i had my tech restore the amp. the blue caps were gone but he removed the orange drops and now it looks like the picture in the link below . it sounds much better now but the vibrato channel is still brighter (in an unpleasant way) than the normal channel.

i opened it up and traced all of the components best as i could, comparing it to the layout and retracing the steps of the mod instructions. nearest i can tell, the caps and resistors in the vibrato channel are the same values as in the normal. the resistors on the bass pot and input jacks are the same, and the little 47pf "brite" cap was on the vibrato channel volume pot. i also double checked the wiring as best as i could to make sure that i didn't put in some other mod, it all looks good to me.

i removed that 47pf brite cap. that did warm up the vibrato channel but still the high end seems thinner to me in the vibrato channel than in the normal channel. i also swapped the 1st 2 12ax7's but that didn't help.

my tech is trying to think of what the problem might be but before i bring it to him and go on the $$$ clock, i'm hoping someone might have an idea.

could it be that even though these yellow guys are way better sounding than the spragues that some of the pink discs should be reinstalled? like maybe that little .02 short cap that's on the bottom edge of the board a few inches directly behind the normal cannel bass pot...?

i have no idea. anyone??

any help would be appreciated. thanks

Amp Chassis Now

(This message was last edited by jackostrato at 11:27 PM, Apr 6th, 2012)

AnalogPackrat

California

If it is to be it is up to me.
Apr 6th, 2012 11:52 PM   Edit   Profile   Print Topic   Search Topic

The vibrato channel in DRs is supposed to be brighter than the normal channel. It also has an extra gain stage. Assuming your amp is behaving normally, you might try lifting one end of the 10pF ceramic disk cap that parallels either a 3.3M or 4.7M resistor right next to the middle board mounting screw in your picture.

Did your tech replace the dual electrolytics (the black caps with two leads on one end and one on the other)? Looking at the solder joints I'd say not. If those get leaky or drift too much the amp can sound kind of lifeless. I believe you can still find dually replacements, but you can also just solder in two individual caps. What's the brown cap that was added on the reverb driver (12AT7 at V3)? Possibly part of the original Torres mod?

jackostrato

usa

Apr 9th, 2012 10:57 PM   Edit   Profile   Print Topic   Search Topic

is that the cap that looks like it's piggybacking the resistor that is mounted diagonally, immediately above the mounting screw? what does the cap and resistor do and what'll happen if i take the cap off?

anteriorl5

USA

Apr 10th, 2012 07:42 AM   Edit   Profile   Print Topic   Search Topic

Are those Sozo caps, the yellow ones? I would go ahead and have the (dual) electrolytic bypass caps replaced with 25uf/50 volt spragues. If they are really out of spec your low end will be weak, making your amp sound tinny. The vibrato channel will sound better/richer/warmer with volume high by the way. At 2 or 3 on the volume, it is a very bright amp by design.

jackostrato

usa

Apr 10th, 2012 08:49 PM   Edit   Profile   Print Topic   Search Topic

they are, i believe, illinois caps (ic mpw).

thanks but, for my tastes, it was the sprague orange drops that started this thread. they're not my personal fave. and i'm pretty sure those dual caps are new ones that my tech installed 2 years back. the amp has plenty of low end.

i did a gig with the amp (sans bright cap) and it's definitely better. likely, now that there is less high end accentuation because the cap is gone, the amp's overall tone is not as offensive to me. still, in a perfect world, the high end in the vib channel would be a little smoother and (dare i say) browner (i hate that word!!!).

maybe it's that extra gain stage? does the reverb or trem circuitry have any effect on the tone even when they're not being used (i do usually have the verb on 2.5/3)?

AnalogPackrat

California

If it is to be it is up to me.
Apr 11th, 2012 07:12 PM   Edit   Profile   Print Topic   Search Topic

"is that the cap that looks like it's piggybacking the resistor that is mounted diagonally, immediately above the mounting screw? what does the cap and resistor do and what'll happen if i take the cap off?"

Yes. It effectively provides a HF bypass for the main signal path on the vibrato channel before it is mixed back in with the reverb return. I removed this on my 77 DR and snipped the bright cap and I like it. I think it's worth trying.

The reverb on these amps can sound a little bright--just turn your reverb on and off while playing. Does the tone change significantly?

Also, we should have asked what guitar you're playing. Don't forget the tone controls on the instrument!

FDP Forum / Amp Mods, Repairs, and Projects / Help/Advice. 68 DR-Vibrato channel too brite




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