CC Resistors in Pharmacist Overdrive

[micpoc]

I just ordered a Pharmacist PCB and found this gut shot of the Dr. Robert pedal:

I am curious which resistors on the Pharmacist PCB correspond to the carbon comps in the original layout. Some are obvious (both of the 18k and 33k as well as the 47k), but I cannot make out which of the 4k7, 10k, and 1M are CCs. I have some and may use them in my build. Any insights appreciated.

 

[Robert]

I should be able to pull up the trace images and let you know when I'm back at my computer.

 

[Feral Feline]

Shot in the dark is that Dr Robert has them in the signal path for "mojo", but if the component value is the same between carbon and metal-film, the result is the same?

 

[RetiredUnit1]

Shot in the dark is that Dr Robert has them in the signal path for "mojo", but if the component value is the same between carbon and metal-film, the result is the same?

Well, carbon comp are hissy noisy. You find this out after building an amp with them and then moving to carbon film. So maybe he's going for that true hissy vintage sound. But they also drift in value and crack or even break with age. I built one amp with them and after trying carbon film I sold all my carbon comp resistors. I built one amp with metal film and found it to be ice picky, sort of sterile sounding. I replaced the resistors with carbon film and it was much improved.

I know some people that only use metal film, but I don't like them in amplifiers except in non-signal locations. They're just fine there...

 

[Robert]

I am curious which resistors on the Pharmacist PCB correspond to the carbon comps in the original layout.

Carbon comp:

R1, R8, R9, R10, R11, R12, R15, R23, R24, R32

 

[micpoc]

Carbon comp:

R1, R8, R9, R10, R11, R12, R15, R23, R24, R32

Great; thanks Robert!

 

[micpoc]

Shot in the dark is that Dr Robert has them in the signal path for "mojo", but if the component value is the same between carbon and metal-film, the result is the same?

Well, carbon comp are hissy noisy. You find this out after building an amp with them and then moving to carbon film. So maybe he's going for that true hissy vintage sound. But they also drift in value and crack or even break with age. I built one amp with them and after trying carbon film I sold all my carbon comp resistors. I built one amp with metal film and found it to be ice picky, sort of sterile sounding. I replaced the resistors with carbon film and it was much improved.

I know some people that only use metal film, but I don't like them in amplifiers except in non-signal locations. They're just fine there...

I MIGHT end up socketing the resistors to see/hear if there is any difference. I actually have a Dr. Robert and am interested just how close it gets, then maybe diverging a little bit.

 

[RetiredUnit1]

I MIGHT end up socketing the resistors to see/hear if there is any difference. I actually have a Dr. Robert and am interested just how close it gets, then maybe diverging a little bit.

Remember that the signal in amps can run from millivolts to 500v *and more* for some tubes. I don't know if this effect would be noticeable at pedal voltages! And while the voltage is high, the current is low. The output transformer primaries are high voltage, low current, high impedance the secondaries are lower voltage, higher current, and lower impedance.

 

[pacealot]

Also, a note for "authenticity": I found that the original 7-series amps only used carbon comps in a very few locations in the preamps, notably the equivalents of R15, R19, R21, and R23 in the Pharmacist. The rest appear to have been mainly Piher carbon films. Just FYI and for what it's worth and all that...

 

[HamishR]

I'm a big fan of carbon film in amps but in pedals I generally just use metal films. I might try a version of my favourite OD with carbon films and see if I can hear a difference. But I have built my fave OD with regular 1/4W metal film and SMD resistors - whatever they are - and struggle to hear any difference. I'm no fan of CC resistors in amps but suspect they'd be ok in a pedal.

FWIW I once fixed up an original '65 Super Reverb which had hardly any volume. Turned out that most of the 100K plate load resistors were measuring around 1M. They would have taken more punishment than most of the resistors in that amp so I guess it's not surprising. Once I replaced them and the electrolytics the amp sounded about 100 times louder!

 

[music6000]

As others have noted, Carbon Film would be a better choice than Carbon Comp!.

 

[music6000]

FWIW I once fixed up an original '65 Super Reverb which had hardly any volume. Turned out that most of the 100K plate load resistors were measuring around 1M. They would have taken more punishment than most of the resistors in that amp so I guess it's not surprising. Once I replaced them and the electrolytics the amp sounded about 100 times louder!

They were Carbon Comp no doubt???

 

[RetiredUnit1]

They were Carbon Comp no doubt???

Fender used all carbon comp at that time. They're using carbon film in the reissues in the signal positions and metal film in the power positions. This is a Bassman RI board (not my pic)

 

[micpoc]

Also, a note for "authenticity": I found that the original 7-series amps only used carbon comps in a very few locations in the preamps, notably the equivalents of R15, R19, R21, and R23 in the Pharmacist. The rest appear to have been mainly Piher carbon films. Just FYI and for what it's worth and all that...

Interesting; thanks!