[Question] 68 Custom Deluxe Mixing Resistors

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Bombacaototal
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[Question] 68 Custom Deluxe Mixing Resistors

Post by Bombacaototal »

Fender Blackface amps usually have normal and vibrato channels out of phase and connected via a voltage divider of 220k on each side before the Phase Inverter

The 68 Custom Deluxe connects both channels in order for them to have both reverb and tremolo. Because of this connection the mixing resistors are no longer needed and they remove one of them from the circuit (the 220k from normal channel)

This circuit now does not have a voltage divider between the V4B and Phase Inverter

Is it fair to say that the 68 custom has twice the voltage of a 65 DR between V4b and PI given the voltage divider is no longer there?

One question is that they still left one of the 220k of the previous voltage divider. Is it there just to filter high end oscillation and also to attenuate the gain a bit?

I know it is possible to jumper this resistor, which will increase the gain and high end. Will this cause potential oscillation issues? Any advice against or in favour of jumpering it?

Any reason to experiment with the value of this resistor?
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Last edited by Bombacaototal on Tue Feb 07, 2017 12:49 pm, edited 1 time in total.
Stevem
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Post by Stevem »

Answer your own questions by experimenting and report back!
Nothing your proposing will blow anything up!
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by JazzGuitarGimp »

Are you referring to R35?
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Bombacaototal
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by Bombacaototal »

JazzGuitarGimp wrote:Are you referring to R35?
Yes indeed
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by rooster »

Interesting. I think oscillation is off the table based on R120, RR121, and R118. This 10K resistive value does not exist in the original DR of course. I also think that the 220K resistor is there to buffer the .001 PI cap a bit and to smooth the top end of the two channels. Swap it out for a 10K resistor and you might not like what you hear, for example. I need to hear this amp I think. Why do I think Fender has reduced the feel of this classic circuit by adding the 10K resistors?

I also see that Fender has again used this clever $$ saver with the primary power supply cap, creating a 220uf/100VDC/100K and a 47uf/500VDC/470K capacitor stack. I think this is rather brilliant and they were the first to use this on some of their other amps. I have yet to try it. Total cost of power supply cappage: $2.33+$4.60 (220uf/100VDC + 47uf/500VDC) + 3X $2.55 (3X20uf @ 500vdc).

And BTW, I see that Cornell Dubler has purchased IC caps (Illinois Capacitor) - all Chinese or other places Asian.
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by Stevem »

Removing that resistor will not provide more gain, it will just make for more overall amp volume at a lower channel volume setting, and in turn push the PI and output stage into clipping faster!

Gain is related to the amplification factor of the tube and its plate load resistor, plate voltage , Cathode resistor and Cathode resistor bypass cap if used , and the screen set up if it's a Pentode preamp tube.
Once again , removing a voltage divider does not change the gain level of a previous stage!
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Not screaming like the passengers in his car!😊

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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by tubeswell »

Bombacaototal wrote: Tue Feb 07, 2017 12:37 pmFender Blackface amps usually have normal and vibrato channels out of phase and connected via a voltage divider of 220k on each side before the Phase Inverter

The 68 Custom Deluxe connects both channels in order for them to have both reverb and tremolo. Because of this connection the mixing resistors are no longer needed and they remove one of them from the circuit (the 220k from normal channel)

This circuit now does not have a voltage divider between the V4B and Phase Inverter
The 220k resistors from each input channel in the AB763 provides AC load buffering from each other input channel, but they are not a voltage divider per-se. They are mixing resistors - the 'dry signal' phase between the two input channels is out of phase at this point.
Bombacaototal wrote: Tue Feb 07, 2017 12:37 pm Is it fair to say that the 68 custom has twice the voltage of a 65 DR between V4b and PI given the voltage divider is no longer there?
The two channels in the AB763 are not in phase, so there is not 'twice the voltage' to begin with. The question of gain vs attenuation between the input channels and the phase inverter is complex. Remember that out of phase signals cancel each other out.
Bombacaototal wrote: Tue Feb 07, 2017 12:37 pmOne question is that they still left one of the 220k of the previous voltage divider. Is it there just to filter high end oscillation and also to attenuate the gain a bit?
The 220k after V4B provides a bit of overall signal attenuation prior to the phase inverter. It doesn't do much for HF oscillation.
Bombacaototal wrote: Tue Feb 07, 2017 12:37 pmAny reason to experiment with the value of this resistor?
Up to you.
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by Bombacaototal »

Thanks for taking the time to respond. Great clarification
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by Ten Over »

Is it fair to say that the 68 custom has twice the voltage of a 65 DR between V4b and PI given the voltage divider is no longer there?
Let's say that you only plug into the Vibrato/Vintage channel in both cases.

The first thing to consider is the mixer formed by valve 1B and 2B. With a signal only present at one on the grids, the gain is around half that of the standard stage by itself as in the AB763. Now if you strapped the channels in the 68 and gave both mixer grids the same signal, the gain would be about the same as a single stage by itself, but we're not doing that here.

The next thing to consider is the 220K situation at valve 4B. In the AB763 there is a pair of 220K's with one connected to the plate of valve 1B (Normal channel) and one connected to the plate of valve 4B (Vibrato channel). For this exercise, there is only signal at the plate of valve 4B. Under these circumstances, valve 1B has a low plate impedance compared to the pair of 220K's, so the signal going to the PI is cut nearly in half by the resulting voltage divider. Now if valve 1B had the same signal on its plate as valve 4B, each plate would look like a very high impedance to the other, but we're not doing that here. The 68 only has a lone 220K in series with the PI and the PI is a high impedance, so there isn't a lot of voltage drop across the 220K.

It looks to me as though it is a wash and the PI will see about the same signal in both amps.
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Re: [Question] 68 Custom Deluxe Mixing Resistors

Post by Bombacaototal »

Ten Over wrote: Thu Mar 30, 2017 11:18 pm
Is it fair to say that the 68 custom has twice the voltage of a 65 DR between V4b and PI given the voltage divider is no longer there?
Let's say that you only plug into the Vibrato/Vintage channel in both cases.

The first thing to consider is the mixer formed by valve 1B and 2B. With a signal only present at one on the grids, the gain is around half that of the standard stage by itself as in the AB763. Now if you strapped the channels in the 68 and gave both mixer grids the same signal, the gain would be about the same as a single stage by itself, but we're not doing that here.

The next thing to consider is the 220K situation at valve 4B. In the AB763 there is a pair of 220K's with one connected to the plate of valve 1B (Normal channel) and one connected to the plate of valve 4B (Vibrato channel). For this exercise, there is only signal at the plate of valve 4B. Under these circumstances, valve 1B has a low plate impedance compared to the pair of 220K's, so the signal going to the PI is cut nearly in half by the resulting voltage divider. Now if valve 1B had the same signal on its plate as valve 4B, each plate would look like a very high impedance to the other, but we're not doing that here. The 68 only has a lone 220K in series with the PI and the PI is a high impedance, so there isn't a lot of voltage drop across the 220K.

It looks to me as though it is a wash and the PI will see about the same signal in both amps.
Thanks a lot for taking the time. I appreciate the explanations
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