Croft specs - too much gain?

[Wegamus]

The 12AT7 is a lower gain valve and won't cause any damage if you try it. I suggest you do.

You’re sure it’ll work ok?

They are GE JAN 12AT7WB.

[Firebottle]

Yes absolutely. The 12AT7 is equivalent to ECC81.

[Wegamus]

Yes absolutely. The 12AT7 is equivalent to ECC81.

How long should I wait for the amp to cool down before removing a valve, is it dangerous for a long time after switch-off?

[Wegamus]

Right, I’ve changed the 7R audio valve from the ECC83 to a ECC81. Don’t think there’s any difference. Still hissing like before.

[Firebottle]

OK. Power the 7R without anything connected to the inputs.

You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.

[Wegamus]

OK. Power the 7R without anything connected to the inputs.

You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.

Right, I’ll try that now...

Should I put the ECC83 back in first? How long should I wait to do that if so?

[Wegamus]

OK. Power the 7R without anything connected to the inputs.

You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.

I got the hum like you said, but hiss was much less than before and was a bit hard to hear through the hum. There was minimal hiss.

It’s worth noting that the guy who sold me this amp told me it didn’t hiss at all when directly connected to his DAC with the DAC as volume control. The 25R he had didn’t have any hiss with another power amp he tried either. But when used together the 25R/7R had hiss, just like I’m finding. So individually the Crofts were fine, together they hiss.

[Firebottle]

There is another option with Croft gear, one that Glenn offers.

The output/line stage of the 25R (and other models) can be converted to a zero gain buffer (cathode follower). This removes the gain given by this stage normally and will allow further travel on the volume pots.

I have modified a Croft pre into this configuration previously.

[Wegamus]

There is another option with Croft gear, one that Glenn offers.

The output/line stage of the 25R (and other models) can be converted to a zero gain buffer (cathode follower). This removes the gain given by this stage normally and will allow further travel on the volume pots.

I have modified a Croft pre into this configuration previously.

But will that stop the hiss? Is it the preamp gain that’s likely causing it?

[Firebottle]

I believe it will and yes the gain is too high.