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Post subject: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 11:40 am
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Fender graciously sent me the schematics for the four 68 Custom series amps. I've been comparing them with the 65 DRRI and 65 TRRI (AB763) schematics. One glaring difference between them is on the 68 Custom amps there is a series 10k resistor on the input to the control grid on both halves of all 12AX7 tubes except for V5, both halves of which make up the vibrato circuit. There is no 10k resistor on the DRRI and TRRI 12AX7's.

So, a) What difference in the tone/sound of the amp will this resistor make?
and b) is the resistor essential to the operation of the circuit or does it just change the tone/sound to make it different from the AB763 TRRI/DRRI?


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 12:56 pm
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Lynn, look like grid stopper resistors. Used to help stabilize the section. And yes, they effect the tone.

You could try running various resistors, in parallel, to the stoppers and see what that does to the stability. Ultimately, you want the least resistance that still keeps the section stable. Which maybe 10K-ohms, in these amps.

Can you scan those schemos & post them? Or email them to me?

Thanks! :mrgreen:


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 1:23 pm
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Along with what BMW said,
from what I just read on the internet, they might act as part of a high freq. low pass filter to roll off the very high freq, (being a series resistor with the input capacitance of the tube). Maybe that's why the 68 custom amps have a warmer tone to them, they help cut the very high freqs. If they do have a warmer tone to them?. It said for true filter the resistor might have to be a larger value. And the blocking distortion idea, what ever that is.


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 2:33 pm
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I'm not planning to modify my amp other than what I've already done which is:
a) Replaced the Celestion ten 30 speakers with Eminence Ragin' Cajuns
b) Replaced the GT 6L6es (Grade 4) with JJ 6L6es and rebiased them to give me more clean headroom.

Now my 68 CVR sounds more like a 63 Vibroverb, which I do like. Reminds me of a SRV kind of growl when the volume knob is above 6 on the Vintage channel.

I prefer the 65 Twin Reverb sound, but not enough to mess with this amp to that extent. It's a great sounding amp!! Just wish I could find a 40-50 watt 40 lb. 65 Twin reverb. I'm too old to be hauling a 64 lb. real 65 TRRI around!! I'm 67 years old and have COPD :|


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 2:35 pm
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By the way, BMW2002Ti, I PMed you my email address. I will email the schematics to you when you send me your address :D


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 09, 2014 7:29 pm
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Lynn,

Much thanks!

From what I've seen with those '68 Custom amps --- there are components added to the reverb section. Apparently to stabilize this section. Grid stopper resistor, ferrite beads, and snubber/bypass cap. Not quite sure why, as vintage Fender amps and even re-issued versions don't seem to suffer from an unstable reverb section. If anything, ppl complain that they are too weak.


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Fri Dec 12, 2014 7:46 pm
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BMW2002Ti wrote:
Lynn,

Much thanks!

From what I've seen with those '68 Custom amps --- there are components added to the reverb section. Apparently to stabilize this section. Grid stopper resistor, ferrite beads, and snubber/bypass cap. Not quite sure why, as vintage Fender amps and even re-issued versions don't seem to suffer from an unstable reverb section. If anything, ppl complain that they are too weak.


I've seen a couple DRRI's where anything above 7 or so on the Reverb at high volume results lots of feedback-maybe they're addressing this ?


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Mon Dec 15, 2014 7:40 pm
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Lynn Nicholson wrote:
Fender graciously sent me the schematics for the four 68 Custom series amps. I've been comparing them with the 65 DRRI and 65 TRRI (AB763) schematics. One glaring difference between them is on the 68 Custom amps there is a series 10k resistor on the input to the control grid on both halves of all 12AX7 tubes except for V5, both halves of which make up the vibrato circuit. There is no 10k resistor on the DRRI and TRRI 12AX7's.

So, a) What difference in the tone/sound of the amp will this resistor make?
and b) is the resistor essential to the operation of the circuit or does it just change the tone/sound to make it different from the AB763 TRRI/DRRI?


Lynn,
Have you posted the biasing test and adjust points anywhere in picture form?
I have both the 68 custom vibrolux and 68 custom dlx reverb amps and I want to test and adjust bias on both.
For the vibrolux I have a couple of matched pairs of SED winged C 6L6's that I want to try out.
For the dlx reverb I just want to test the bias and adjust to hear different tones at different bias settings.

On the hot rod dlx 3 the bias test point and adjust pot are really easy to access and adjust.
But on these 2 amps I'd like to see where the test point and adjust pots are.
If you have picts of those things I'd appreciate if you can either post them here or send me the picts.
TIA. :)

I'm thinking of trying a 15" speaker in the vibrolux like Fender did with the TRRI 15.
With the stock speakers the vibrolux has plenty of high end especially when the bright switch is engaged.
I'm thinking a 15" would be great to smooth that out and add some nice low end.


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Post subject: Re: 68 Custom Series 12AX7 10k grid resistors
Posted: Tue Dec 16, 2014 10:26 pm
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Rverb,

I haven't taken any photos of the bias adjust point on the amp. However, I will try to get a good shot of it and email it to you if you like. Just PM me your email address. If you don't have the schematics for your CVR and CDR I'll throw them in at no extra charge! Truth is that I don't know how to post a photo on these forums :roll:

I've edited this posting. I found the bias test point on the schematic. It's across D4, a 1N4006 diode, and labeled TP33 on the schematic and calls for +60 mVDC. I purchased a single-tube bias probe that has built-in plate-voltage and bias current meters from Eurotubes. It is essentially a porcelain octal tube socket wired properly to give you those measurements. You remove one 6L6 (or 6V6 or other octal tube) from its socket, insert the bias probe into the socket and then plug the tube into the bias probe. Turn on the amp, let it warm up and make the adjustment.

I will attempt to make a decent photo and email it to you if you PM me your address. But I will try to describe the location of the access hole for the bias adjust pot:
Turn the amp upside down so that it is resting on the carrying handle. Look at the chassis from the rear. First you have to remove the dang cage that they have installed around the power tubes. But it is attached to and comes off with the piece of thin board on the back of every Fender tube amp. The 2 power tubes are on the far right. From right to left they are V8 and V7. Behind V7, towards the front of the amp is a small access hole. If my memory serves me correctly, it appears to have some kind of brass ring lining the hole. I highly recommend that you pull the chassis out of the cabinet to make the adjustment. You will have easier access for your hands that way. I only pulled it out about 2" and had to reach over and under stuff to make the adjustment. In doing so, I apparently slipped out of the pot's slot, touched a wire and caused a spark! No harm was done, but I may have just been lucky.


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