Valve Question

[Jimbo]

I have been using a GE BH7A 5751 valve in my Croft 7 power amp for the past 12 months. I used this to lower the gain as it is not rated the same as a ECC83. It worked fine and sounded good but I did have to crank up the volume a bit and indeed since I had it I have had to crank up the volume more and more. This begs the question, do valves loose their ability to provide gain over their life span?

I ask this as I put in a NOS GE Jan 5751 this weekend and although it also low gain it sounds soooo much more dynamic and powerful. I went through a whole bunch of stuff this weekend and in every case the recordings sounded superb, the clarity and detail came through so well I could follow much more clearly what was happening in the recording. In fact I would go as far to say it is the best I have heard my system sound since I put it together!

I am not a techy so could someone give me an explanation for this?

[lurcher]

Gain tends to stay constant over a valves life as its controlled by the physical layout of the cathode grid and anode. What actually happens is emission fails, this raises the anode resistance, and as gain is constant, gm (transconductance) falls. This is why you will see gm used as a measure of valve condition. The lower gm and higher ra means that the actual gain produced when the valve drives into a load will fail. But the gain (mu or µ) of the valve stays the same.

[Jimbo]

Gain tends to stay constant over a valves life as its controlled by the physical layout of the cathode grid and anode. What actually happens is emission fails, this raises the anode resistance, and as gain is constant, gm (transconductance) falls. This is why you will see gm used as a measure of valve condition. The lower gm and higher ra means that the actual gain produced when the valve drives into a load will fail. But the gain (mu or µ) of the valve stays the same.



Thanks for the explanation Nick. How does this translate to SQ and what would you expect to hear as the valve fails to drive into a load?

The valve in question here is the only one used in the input stage of the Croft series 7 power amp.

[fiddlemaker]

A 5751 isn't significantly lower gain than an ecc83, but a 12bh7a is considerably lower.
In my 25r/7r valve rolling extravaganza I found that I didn't like the 12bh7a in the power amp input stage at all. I do, however, like it very much in the preamp line stage. You should stick it in there and see what you think.

[Jimbo]

A 5751 isn't significantly lower gain than an ecc83, but a 12bh7a is considerably lower.
In my 25r/7r valve rolling extravaganza I found that I didn't like the 12bh7a in the power amp input stage at all. I do, however, like it very much in the preamp line stage. You should stick it in there and see what you think.

I thought the 5751 has a lower amplification of 70 mu as opposed to a 100 mu of a ECC83/12AX7. I thought the 12BH7A was also about 70 mu or is it lower ?

[fiddlemaker]

I thought the 5751 has a lower amplification of 70 mu as opposed to a 100 mu of a ECC83/12AX7. I thought the 12BH7A was also about 70 mu or is it lower ?

Yes, the 12bh7a has a mu of less than 20. With a log taper volume control you won't notice much difference at all between a 5751 and an ecc83 (in terms of dial position for equal loudness).

[Jimbo]

Yes, the 12bh7a has a mu of less than 20. With a log taper volume control you won't notice much difference at all between a 5751 and an ecc83 (in terms of dial position for equal loudness).

Wow thats a lot lower than I thought, no wonder I had to crank up the gain! However I have done a bit of research and found that as a valve ages you do loose HF and the soundstage becomes more 2D which is what I was finding.

Which valves did you use in the phono stage of the 25R?

[fiddlemaker]

I have Mullard cv4004s in the phono stage and in the input stage of the 7r.
In my experience/opinion the most important valve to get right is the power amp input valve, followed by the line stage.

[lurcher]

Its not lego though, while the pinouts of the ECC83/2/1 range are the same, and they will work, substituting a ECC82 for a ECC83 will put it in a very poor place in operating terms, so yes, you will get lower gain, but you will also get loads more distortion. ecc83 -> 5751 is ok, as they are designed for similar operating points. The Guitar amp folk can get away with it as distortion is not a bad thing in that space. Also depending on the circuit you may stress following stages as well.

[fiddlemaker]

Its not lego though, while the pinouts of the ECC83/2/1 range are the same, and they will work, substituting a ECC82 for a ECC83 will put it in a very poor place in operating terms, so yes, you will get lower gain, but you will also get loads more distortion. ecc83 -> 5751 is ok, as they are designed for similar operating points. The Guitar amp folk can get away with it as distortion is not a bad thing in that space. Also depending on the circuit you may stress following stages as well.

That's what I would have thought, but Croft reckons it's ok in this particular circuit.

[lurcher]

That's what I would have thought, but Croft reckons it's ok in this particular circuit.

For some value of "ok"

[Jimbo]

Must say my 12 BH7A did not really work well at all in the Series 7 input stage even if it did lower the gain. Might have wasted a year listening to this set up when I could have used the GE 5751 all along. Did not realise how much influence re SQ this input stage in the Series 7 was, thought the phono stage of the pre was more important.

[awkwardbydesign]

However I have done a bit of research and found that as a valve ages you do loose HF and the soundstage becomes more 2D which is what I was finding.


I have to crank mine up as I am losing HF as I age! :(