Amp advice sought

[Shadowbox81]

Thanks Dave/Geoff, it seems like your views pretty much sum up the body of opinion out there. I suppose the only way to find out would be to get a pair and have a listen.

Thanks Ian - I see that JJ valves also do a mid gain version of the Ecc83. I might consider that or the 5751 and see what happens.

On the other option of the internal trim pots, as I said, I am reluctant to mess with them. If I do adjust them however, could that cause any damage to the pre or power amps.

[dave2010]

From the tubestore website...

The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain.

I use a JAN GE 5751 though not for reasons related to gain, just think it's a fine tube. But as above, GC is your chap to help here.

if the gain ratio is 70/100 that represents a dB attenuation of about 10 log (0.7) or about 1.5 dB relative to the tube with the higher gain. There might be a difference if the ratio is power or voltage, but either way the likely attenuation is only going to be around or under 3 dB - barely audible to most of us.

[Firebottle]

From the tubestore website...

[I]The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70.

Be aware that unless special circuit techniques are used the actual gain achieved in a typical valve stage is only about 2/3 of the published figure for mu (gain).

So for an ECC83 stage the gain will typically be about x 60.

[Shadowbox81]

Thanks Dave/Alan, that is consistent with Shaun's comment earlier in the thread.

I'll drop Glenn a line but as specs on the Forte are hard to come by, I'll have to see what he suggests. In the meantime, I might give a pair of attenuators a go

[walpurgis]

The Forte is a lovely pre-amp. I had one.

It was a while ago, but I seem to recollect the trimmers were for the phono stage and had nothing to do with line output.

[Shadowbox81]

I think you're right Geoff. Looking at the boards, the trim pots are directly next to the LT1028 opamps with another set of opamps over the other side of the board. So it looks like the trim pots are indeed part of the phono stage section of the circuit. However, my Lenco is currently in pieces as it's in the middle of a rebuild and replinthing so it'll be a couple of weeks before I can hook it up to test that theory.

[walpurgis]

Yes. I think they are for MM impedance matching.

[Shadowbox81]

Interesting. Looks like I'm going to have to do some reading up on that subject before I get the TT back in the game