Seeking advice on cap/resistor relationship in a tone control

[Bunkey]

I'm rebuilding a 1982 A&R Cambridge A60 amplifier which started popping fuses (likely from one of the psu caps leaking electrolyte across the board!)

I have a photocopy of the hand-typed service manual which details an upgrade ARCAM offered when servicing these amps back in the day. It's mostly updating caps & resistors in the signal path to poly and M/F types that would have been prohibitly expensive to spec back in the 70's.

I intended to replace the caps only as per these recommendations; a common sense improvement 'whilst I'm in there'.
There is one change to the standard cap values however which sees the 3n3 Treble control cap replaced with a 2n2. I had no intention of replacing resistors at the same time but noticed R32&33, the 2k7 resistors to either side of the connected tone pot, were spec'd to be replaced with 6k8 resistors as part of the upgrade.

I want to assume the lower cap value is just an update to the frequency response of the treble control itself and can be treated seperately from the resistors but I don't fully understand the design of the tone stack and how the relationship between these values will affect its function.

My question is: could I go ahead and replace that cap for a 2n2 without having to do these resistors at the same time as I don't have any 6.8k's to hand, or is this going to have an adverse affect on the frequency response of the amplifier?

Thanks

[Bunkey]

It's notable that the 4k7 resistors, R27&28, at the bass end of the stack have also been revised to 10k in the upgrade.

[antonis]

The design of the Tonestack is a Baxandall configuration and you can find caps/res frequency relationship here: http://bitsavers.trailing-edge.com/components/national/_dataBooks/1980_National_Audio_Radio_Handbook.pdf, page 2-46

[Bunkey]

That's excellent, thank you.

[PRR]

> amplifier which started popping fuses ...... an upgrade ARCAM offered .... I intended to replace the caps ....

No. Fix what is broken and verify it stays fixed.

If you combine "repair" with "upgrade" you are introducing potential new troubles faster than you are fixing known troubles.

I'm not sold on the idea that cap leakage blows fuses. Salt-crap does not conduct that well. It could point to perished caps which "can" pop fuses. IMHO more likely is blown rectifiers which abuse both caps and fuse.

When it works good.... the new R-C values give less impressive treble boost/cut. I suspect the original was over-dramatic and they toned it down (no pun). But it has been acceptable(?) for 50 years so no rush to go with new fashion.

[PRR]

The resistors 2.7k/6.8k set the max boost/cut. The cap 3n/2n sets where the boost/cut starts. They can be picked independently.

The mod changes +/-6dB at 3kHz (a pretty shrill/dull curve) to +/-3dB (more like what you want with pre-EQ-ed records and tapes).

[Bunkey]

Yes it could just as well be an impedance match issue on the speakers (8ohm output into 6ohm load) or the fact I pulled it from a skip 

But I am doing a complete overhaul and replacing the near 40 yr old regulators, psu caps, electrolytics, rectifiers and transistors across the board.

I don't see an issue with swapping out ceramic caps for film whilst I'm at it.

From the equations in the material above I got a threshold of 1760hz original and 2640hz using the 2n2 films I have, keeping the resistors the same. It looks like upgrading as per the manual keeps a similar boost/cut starting point but has less range like you say, though it also introduces an upper ceiling of 10khz compared to the standard 17khz which leads me to believe the treble control becomes more useful in the upper mids without getting shrill on the top when boosted. The graph seems to support this?

I'll hold off on that particular change til I have the supporting resistor values and get the thing working in the meantime.

Cheers




I doubt you'll have seen any of my builds Paul but I have a tendency to ridiculously sized components... this one's no exception.
Pretty though aint it!

[Bunkey]

These couple the source signal from the input selector to the preamp stage.

They were generic 1uF electrolytics til I had my way with it