Something Stirs in the Spare Room.

[Cressy Snr]

Since the solid-state amp with NVA boards is the main amp in the living room system these days, it has made it easier to play around with valves without running the risk of ending up amp-less if something were to go wrong.

The SEP sounded so lacking compared to the transistor job that I didn’t have a lot to lose by seeing if I could at least make it sonically presentable in the solid-state company.

So, rather than keep making new topics, it seemed better all round just to collect all the experimentations together in one thread, similar to the “something stirs” threads by Mark and Steve (vinylvalves.)

The SEP was torn down; out went complexity, back came simplicity in the form of triode strapped EL34s, 6550s, KT88s and experiments with different drivers. A PM and a few valves in the post and we were away. I’m enjoying messing with the glowy stuff again. Nothing new or exciting in terms of the devices being used or the circuit topologies but whatever. It’s fun and not too serious.

So here’s a nice little combo:

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Unusual UHF triode as driver, the E88C, which is not half of an E88CC, but a completely different device, introduced in 1961 and meant as a grounded grid aerial amplifier for frequencies up to 900MHz. I found this pair in my small signal valve box. Gawd knows where they came from but there they were. http://www.r-type.org/exhib/aaa0220.htm so why not? I thought.

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[Cressy Snr]

Schematic:
These little buggers probably oscillate in their boxes, but I took the usual precautions to prevent spurious behaviour. I even sent a text and phoned the missus with the iPhone placed on the chassis while the amp was operating. Nothing. The valves are some of the quietest I’ve come across. Gain = 70, current through them is 12.5mA. The music emerges from black silence.

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There'll be other drivers substituted over time so we'll see what differences happen.

This iteration sounds excellent. Nice expansive soundstage and plenty of filigree detail; subtleties are probably set free by the super low noise of these small valves. Quite a sophisticated presentation that sings with jazz, big band, crooners, classical choral and classical instrumental. Not for rockers I suspect.

[simon]

Interesting looking valve Steve, handy gain and still relatively affordable NOS. Wonder what the drawback is?!

[Cressy Snr]

There’ll be one somewhere. No doubt I’ll find it pretty soon and say, “Ah, that’s why nobody uses them.”

[Cressy Snr]

One possible source of problems with EC88s is that they have five grid connections. FIVE - I mean why? That’s ten in total for stereo and ten to deal with in order to try and prevent the whole pair going off on one. I suppose one could cut off the unused four socket tags but then you’d be farked if you wanted to try out anything else.

I mean the question might be why fook about like that when there are easier, more conventional alternatives. That, I suppose is why nobody can be arsed with them. There is very little online about them and nothing about how they sound. For all I know, mine could be oscillating at some analogue TV frequency but my scope does not go up to 1GHz or anywhere near. The simple act of using a scope probe might set them off anyway, so testing might be a bit of a lottery.

The only reassurance I have is that there is silence from the speakers, there is no volume control rustle, there is no distortion on well recorded, hard struck, single piano notes and there is no pickup of mobile phone tower pinging. Can’t really do much else. They do sound lovely driving EL34s though, and they’re cheap as chips, though not quite as cheap as EF184 pentodes.

[Nick]

The five grid connections mean they can be used in parallel to reduce connection impedance. For audio, just use one and ignore the rest. Looks like a nice valve to me.

[Cressy Snr]

The five grid connections mean they can be used in parallel to reduce connection impedance. For audio, just use one and ignore the rest. Looks like a nice valve to me.

Sshhh Nick, I was trying to put people off.
And thanks for the explanation of the multiple grid connections. I can imagine these valves being useful as the first stage in a phono amp, given their high gain and ultra-low noise characteristic.

[simon]

I see there are E88Cs and EC88s - I haven't played Top Trumps with the datasheets but they look to be quite similar, the EC88s have the 5 grid connections and a gain of 65. Also affordable.

[Cressy Snr]

OK!
A lot of work done today on the amp from the other thread (6AS7 SET.)
I added a 4H choke to the part of the supply that feeds the E88C drivers and in addition I choke-loaded said drivers with budget Hammond 150H jobs. The buggers’ll easily swing the required volts for the 6AS7 now: providing of course that one can get enough welly up them from the source; no problem for a CD player or streamer.

Well pleased with the results. The E88C sounds superb when choke-loaded but don’t tell everybody.

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[Cressy Snr]

Audio circuit schematic:

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HT to the drivers could do with losing a few volts in order to bring the current down from 10 to 8mA. This will bring it into spec for the Hammond 156C anode choke.

[simon]

Nice. What sort of voltage gain are you getting now the E88C is choke loaded?

[Cressy Snr]

The mu is 70, so I guess somewhere near
that value.

[simon]

Very useful

[Cressy Snr]

Aye, they are useful valves by the looks of it.

Power supply:

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[Paul Barker]

Just a quicky on gain. The data sheet I found mu is 65 not 70.

Actual gain is load over load plus internal resistance. The data sheet says 4k8 Ri. If we bypass Rk which you have. Im making a judgement youre hammond choke represents 20k

65x20k/24k = A52

Youre cathode bypass cap 1,000 would satisfy a 1 hz - 3db.

No bypass would give you another 13k2 Ri 65x20k/38k = A of 34

Shared cathode resistor = .63 x 65 = A of 41

I like shared cathode resistor and I like no bypass for the feedback sound in the valve stage of an unbypassed resistor.

So you can decide. I’ll leave it to you to see what gain you need and try sound difference.