Croft 25 Cathode Follower?

[fiddlemaker]

About 8 years ago, Glenn upgraded my Micro A preamp to 25RS spec, with a separate power supply box. We didn't discuss much technical detail - he just said he'd take it to the current 25RS spec, with the phono stage circuit from the RIAA RS. I've always been delighted with it and have no intention of parting with it. However, I am thinking of downsizing from my 7R monos to something a bit more compact - probably solid state. So I'm wondering about power amp compatibility. I understand that the "cathode follower mod" that Glenn used to do reduces the 25 output impedance, helps with that. Is there any easy way I can tell if the work carried out by Glenn included the cathode follower mod? Is it by any chance related to the use of the 12bh7a/ecc99 in the line stage?

[Macca]

You want a power amp with as high an input impedance as possible. Seems like the 7R is 470Kohm so something in that ballpark.

[fiddlemaker]

Thanks, Martin. My understanding is that the 25RS line stage circuit is different from the 25 and 25R, using an 12bh7a valve rather than an ECC83.

Does this also mean that the output impedance of the line stage in the 25RS is lower (because modified cathode follower circuit is implemented?) than 25/25R? And the question is how can I check? It would greatly increase my range of power amp choices if so.

I'm not aware of any SS amps with input impedance as high as 470k.

[Macca]

Thanks, Martin. My understanding is that the 25RS line stage circuit is different from the 25 and 25R, using an 12bh7a valve rather than an ECC83.

Does this also mean that the output impedance of the line stage in the 25RS is lower (because modified cathode follower circuit is implemented?) than 25/25R? And the question is how can I check? It would greatly increase my range of power amp choices if so.

I'm not aware of any SS amps with input impedance as high as 470k.

I could not find any info on the output impedance of the 25, 25R or 25RS. It will be high as with all valve pre-amps which is why the 7R has such high input impedance.

Really, for the sake of removing one not very large box I'd stick with the Croft Monos, they're designed to work optimally together. The other option is to sell the lot and go to an all solid-state integrated. That way you'd ditch two boxes. Never any problem to sell Croft, someone will have your arm off for it.

[Barry]

Most solid-state power amplifiers have an input impedance around 20KOhm. Thus you need the output impedance of any preamplifier that feeds it to be at least 10x lower: that is no more than 2KOhm, and preferably lower still.

I can't find the output impedance of a Croft 25RS preamp fitted with a cathode follower, but it is likely to be a couple of hundred Ohms, so in theory should satisfactorily drive a solid-state amplifier.

[Firebottle]

I have modified a couple of Croft preamps to cathode follower output.

Just because the preamp uses the 12BH7A/ECC99 doesn't mean it is a cathode follower, unless modified accordingly.

[fiddlemaker]

I have modified a couple of Croft preamps to cathode follower output.

Just because the preamp uses the 12BH7A/ECC99 doesn't mean it is a cathode follower, unless modified accordingly.

Thanks, Alan - that's very helpful. I guess I can assume that Glenn didn't implement this mod when my pre-amp was upgraded as I didn't ask him to do it at the time.

Could I ask what the CF mod involves, how much it changes the output impedance and are there any downsides?

Thanks again

John

[Barry]

I have modified a couple of Croft preamps to cathode follower output.

Just because the preamp uses the 12BH7A/ECC99 doesn't mean it is a cathode follower, unless modified accordingly.

Then you are the best person to give some indication of the lower output impedance. It doesn't have to be an exact figure: I think it would be in the region of a couple of hundred Ohm. Would you agree?

[Firebottle]

Then you are the best person to give some indication of the lower output impedance. It doesn't have to be an exact figure: I think it would be in the region of a couple of hundred Ohm. Would you agree?

Yes I would agree with that Barry, I might even have a measured value jotted down somewhere.

[Firebottle]

Thanks, Alan - that's very helpful. I guess I can assume that Glenn didn't implement this mod when my pre-amp was upgraded as I didn't ask him to do it at the time.

Could I ask what the CF mod involves, how much it changes the output impedance and are there any downsides?

Thanks again

John

The output valve stage needs to be reconfigured from a common cathode circuit to a cathode follower implementation.

The anode load resistor has to be moved to the cathode plus a B+ connection made to the anode (I fit an anode stopper resistor to ensure complete stability).
A blocking capacitor has to be fitted in the grid connection from the volume control and a suitable bias network added.
The cathode to heater rating of the valve needs to be checked against the new bias voltage to ensure safe operation.

The output impedance is reduced considerably, by a factor of 7-10 times depeding on the valve type.
The only downside is the loss of gain that the output stage was originally providing, usually around 6dB.