6GK5

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 3:28 pm

B7G, ra of 5.4k, mu of 81 !!! Is this valve as good as it looks????

any experience?

Right now I'm enjoying a little weird one called 7F8W - double triode, loctal high Mu and not bad anode resistance (partic. as I am paralleling it, though we all know, paralleling is not good.

pre65 · Post by **pre65** » Wed Sep 17, 2008 4:13 pm

Found this snippet !

6GK5 / 6FQ5A

The standard tube in the 2nd Gain Stage is the 6GK5, also known as the 6FQ5A (many are labeled with both numbers). RCA is the brand of choice here, being especially rich in midrange tone and detail. Though they can be almost too rich and caramel sounding, American made GE tubes are a close second. Beware of GE tubes that are made in Great Britain -- these are actually rebranded Mullards. The latter also show up under the Pro/Comm label, too. Examples by Mullard, Sylvania, Hitachi, Magnavox (which are rebranded Hitachi tubes), and others, are less musical and thinner sounding.

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 4:38 pm

hm, so a Mullard valve is not as good as a US valve; interesting.

bet they are dreadfully microphonic!

Andrew · Post by **Andrew** » Wed Sep 17, 2008 5:00 pm

Not tried one but I ought to, the D3A is good but has its flaws.

Got any to spare, Andrew?

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 5:02 pm

As a triode (cos its to drive a transformer) how does the D3a compare?

I am looking for the driver from hell!

Andrew · Post by **Andrew** » Wed Sep 17, 2008 5:13 pm

How much gain do you need?

All the high mu & high gm tubes take some taming, in my opinion. I suppose there's no free lunch

-- Andrew

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 5:39 pm

That's the problem - ain't it always! The 7F8W has a mu of 48, which I double to 96! by paralleling and halve the ra also so that's down to 7.25k. (hm, thinks, do I actually double the mu in parallel or am I fooling myself? - don't know)

The 7F8w goes into the primary of a lundahl 1660s interstage, which then drives 45s in push pull. There are NO capacitors in the amplifier but I am sacrificing some gain having removed the cathode cap on the 7F8W cathodes. I tried caps on the 45s but it sounds better without.

Amazing clarity,detail and dynamics etc. I'd love to match these ancient 45s, too. I'll probably switch to 2A3s to get a 'matched' output pair, then end up making a push pull pair of the 7F8W front end (that would give me double the gain) but I'd also be back with my friend 'Amity'!

I'd already concluded that I must do a three stage version to get gallons of power.

Anyway, let's face it, as you say, andyrew, 'there's no such thing such as a free lunch'.

ed · Post by ed » Wed Sep 17, 2008 5:51 pm

andrew Ivimey wrote:
I am looking for the driver from hell!

that'll be me then..I'm up for weddings, bar mitzvahs, funerals and the odd bank job....easy rates!

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 5:53 pm

I'll bear that in mind Ed, thanks

Andrew · Post by **Andrew** » Wed Sep 17, 2008 6:50 pm

andrew Ivimey wrote:That's the problem - ain't it always! The 7F8W has a mu of 48, which I double to 96! by paralleling and halve the ra also so that's down to 7.25k. (hm, thinks, do I actually double the mu in parallel or am I fooling myself? - don't know)

You second point was correct as I understand it, paralleling doubles gm but mu stays the same.

The 7F8w goes into the primary of a lundahl 1660s interstage, which then drives 45s in push pull. There are NO capacitors in the amplifier but I am sacrificing some gain having removed the cathode cap on the 7F8W cathodes. I tried caps on the 45s but it sounds better without.

Amazing clarity,detail and dynamics etc. I'd love to match these ancient 45s, too. I'll probably switch to 2A3s to get a 'matched' output pair, then end up making a push pull pair of the 7F8W front end (that would give me double the gain) but I'd also be back with my friend 'Amity'!

I'd already concluded that I must do a three stage version to get gallons of power.

What swing does a 45 need? Can't be more than 35v, can it? Does the Llundhal cause a step down?

Anyway, let's face it, as you say, andyrew, 'there's no such thing such as a free lunch'.

Sadly not.

-- Andrew

andrew Ivimey · Post by **andrew Ivimey** » Wed Sep 17, 2008 6:59 pm

the lundahl is 1 to 1+1 - that's okay!

Actually without all those (nasty) caps, right now, I think it is rather special. Oh I'll get used to it and decide otherwise, hack into it and do something else but right now... just right now, its very sweet indeed - wish I had matched 45s. use a CCS to pull things into lien a bit? CCS on the driver valve anyway? Amity? KARNA?

onwards

audio-talk

6GK5

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