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Post subject: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Thu Aug 07, 2014 10:13 pm
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Aspiring Musician
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I see there are some very electronics knowledgeable members here so I hope you guys can clue me in.
I'll post my impressions first and questions at the end.

I've played the 68 CDR and the 65 DRRI. I ordered the 68, and looks like it may be delivered tomorrow. :
Both are very nice sounding amps. The 65 had a more dominant reverb that has a bit more shimmer/liquid quality over the 68, and the 65's reverb is louder overall even when each amp is set to the same dial number.

65 is brighter over all, 68 warmer tonally.
Nicer, better imo, response with the 68. That really sold me.
I also preferred having verb and trem on both channels in the 68.
Bassman tone circuit is SWEET in the 68. I couldn't see myself using the first dry channel in the 65.

Got to crank both amps to around 8 and both have fantastic power tube crunch, with the 68 sounding a bit better to my ears, and the crunch comes in a bit earlier.
Conversely, the 65 has a bit more clean headroom.

The 65 has a Jensen and the 68 a Celestion.
Speakers in any amp with make a pretty big tonal difference.
I wonder how different the circuits and electronic parts are between the amps especially regarding the tonal differences, besides the speakers?

Tube compliment is the same, at last according to Fenders specs.
Are both amps using the same bias setting/range?

Interestingly, the 65 is US made and the 68 is made in Mexico.
Are both amp boxes made of the same 7-ply 5/8" birch plywood?
Fender lists the construction wood for the 65 but not for the 68.

Thanks for any info you can give. :)


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Fri Aug 08, 2014 6:16 am
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I have the 68 twin and its real wood for sure! not particle board


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Sat Aug 16, 2014 7:19 pm
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Aspiring Musician
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Anybody, regarding circuit/electronics differences between these two amps?

For all intents and purposes do tubes and mods for the 65 DRRI also hold true for the
68 CDR?


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Sat Aug 16, 2014 8:39 pm
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- Grid stoppers on both channels' first gain stages of the CDR, not on the DRRI.
- Different value cap for the mid resistor for the tone stack on the Custom channel of the CDR.
- No "bright cap" on the volume pot for the Vintage channel of the CDR, while the Vibrato channel of the DRRI has a bright cap.
- Both channels of the CDR have "Vibrato" and are in phase, while only the "Vibrato" channel of the DRRI has "Vibrato" and each channel is out of phase with the other. (aka "fritz mod" for the CDR).
- Negative feedback resistor is 1.5K ohm on the CDR, 820 ohm on the DRRI.
- Grid stoppers for the reverb driver tube, and the reverb recovery tube on the CDR not on the DRRI.
- An extra cap tied to the negative of the reverb tank jacks, and an extra cap on the foot switch circuit on the CDR, not on the DRRI.
- A fuse for the filament supply added to the CDR, not on the DRRI.
- Different Fender part numbers listed for the reverb tanks, though the tank numbers are not listed (such as 4EB3C1B, or whichever tanks are used).
- Some components are labeled differently between the two amps.
- There are some slight voltage differences listed on the schematics, but not much.

Other than these small changes, they are basically the same amp, maybe different speakers. There could be some other slight changes that I missed.

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Mon Aug 18, 2014 9:43 pm
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Aspiring Musician
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shimmilou wrote:
- Grid stoppers on both channels' first gain stages of the CDR, not on the DRRI.
- Different value cap for the mid resistor for the tone stack on the Custom channel of the CDR.
- No "bright cap" on the volume pot for the Vintage channel of the CDR, while the Vibrato channel of the DRRI has a bright cap.
- Both channels of the CDR have "Vibrato" and are in phase, while only the "Vibrato" channel of the DRRI has "Vibrato" and each channel is out of phase with the other. (aka "fritz mod" for the CDR).
- Negative feedback resistor is 1.5K ohm on the CDR, 820 ohm on the DRRI.
- Grid stoppers for the reverb driver tube, and the reverb recovery tube on the CDR not on the DRRI.
- An extra cap tied to the negative of the reverb tank jacks, and an extra cap on the foot switch circuit on the CDR, not on the DRRI.
- A fuse for the filament supply added to the CDR, not on the DRRI.
- Different Fender part numbers listed for the reverb tanks, though the tank numbers are not listed (such as 4EB3C1B, or whichever tanks are used).
- Some components are labeled differently between the two amps.
- There are some slight voltage differences listed on the schematics, but not much.

Other than these small changes, they are basically the same amp, maybe different speakers. There could be some other slight changes that I missed.


That's great info, thanks!

I don't know what "grid stoppers".
What is their function, and what effect do they have on tone?

With both channels being in phase on the CDR would that allow both channels to be used at the same time?


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Mon Aug 18, 2014 10:12 pm
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Grid stopper resistors basically stop instability issues in stages of the amp. They are tied from the preceding section to the grid of the section's tube. And yes, they may effect the tone. So you need a resistance that does the job & has least effect on the amp's overall tone quality. Here is a little blurb from Randall Aiken's old tech site:

Function of a grid stopper resistor:

" It helps prevent high frequency parasitic oscillation in the tube itself. It helps prevent radio frequencies from getting into the input stage. Where they can be rectified. Plus, low pass filtered (AM detection) and both may become audible at the amplifier output. It can limit grid current when it drives the tube into the positive grid region, which helps in preventing "blocking" distortion."


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Tue Aug 19, 2014 6:37 am
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Thanks BMW2002Ti. Maybe I should have just said grid resistors.

Rverb wrote:
...With both channels being in phase on the CDR would that allow both channels to be used at the same time?


Yes. You can use both channels even if they are out of phase with each other. With channels that are out of phase with each other, you can get cancellation of the signal, low volume, depending on how the knobs are set between the two channels.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Tue Aug 19, 2014 10:03 am
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yeah - I used to do that in the old days with my Pro-Reverb! Plug into both channels, set the volume and tone controls the same and listen to the sound disappear - then the reverb and vibrato effects were really intense - great to get some really psychedelic effects playing organ!


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Tue Aug 19, 2014 7:13 pm
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Aspiring Musician
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shimmilou wrote:
Thanks BMW2002Ti. Maybe I should have just said grid resistors.

Rverb wrote:
...With both channels being in phase on the CDR would that allow both channels to be used at the same time?


Yes. You can use both channels even if they are out of phase with each other. With channels that are out of phase with each other, you can get cancellation of the signal, low volume, depending on how the knobs are set between the two channels.


Thanks.
Phase cancellation I do understand from the practical side of things working with audio and recording gear. Just like with getting the +/- wrong when wiring speakers.
Get it wrong and bass goes away along with other frequencies that are 180 degrees out of phase. On a guitar amp I can see how that could be even worse. :)

I'll have to experiment with inputs to the 2 channels.
I read an interesting mod/trick one can do with a deluxe reverb and I'm thinking it could work with the custom dlx too.
You take the reverb return and instead of plugging it in where it normally goes you plug the return into one of the channels.
That way you can use the channel eq to eq the reverb.
You'd have to keep the reverb off on the channel where you plug the verb return into, otherwise I'm sure it would start to feedback on itself.

Does removing the trem/vibrato pre tube offer positive benefits, and does it work on the custom dlx?
I read where removing the trem tube can help clean up the tone a bit on the DLX reverb.
I've played around with the vibrato, but I really have no need for it.


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Tue Aug 19, 2014 7:55 pm
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" Does removing the trem/vibrato pre tube offer positive benefits, and does it work on the custom dlx?

I read where removing the trem tube can help clean up the tone a bit on the DLX reverb.
I've played around with the vibrato, but I really have no need for it. "


Best test, try it on your amp. Posting your opinions of this tube removal would be nice.

Thanks! :D


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Post subject: Re: Circuit/electronics differences 68 CDR vs 65 DRRI
Posted: Tue Aug 19, 2014 8:50 pm
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Aspiring Musician
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Joined: Fri Jun 20, 2014 2:07 pm
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BMW2002Ti wrote:
" Does removing the trem/vibrato pre tube offer positive benefits, and does it work on the custom dlx?

I read where removing the trem tube can help clean up the tone a bit on the DLX reverb.
I've played around with the vibrato, but I really have no need for it. "


Best test, try it on your amp. Posting your opinions of this tube removal would be nice.

Thanks! :D


Will do. :)


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