Any reason not to use a B8A EF40 in an EF86 application?

[bikerhifinut]

Hi gents.

I am in the final throes of collating the parts to build a pair of monobloc power amps based on the Leak TL25+ circuit.
It uses an EF86 in the first stage and a bit of homework suggests that the earlier EF40 would be a direct substitute, given the different pinout and valve base. The reasoning is that they seem plentiful as NOS and are also substantially cheaper than even a current eastern european manufacture EF86.
My Chassis has holes punched for B9A sockets and I believe this is the same diameter as for the B8A socket.
So based on that information, I see no good reason why I shouldn't go for the EF40? I do have a few used EF86 but prefer to keep them safe for spares in my Varislope2 stereo and Point One Stereo Leak preamps on the odd occasions I give them a run out.
And on the same tack, is/was there an equivalent double triode in B8A to the ECC81/12AT7 which is used as the LTP phase splitter in the TL25+? Just trying to think slightly "out of the box" although I have a couple of NOS RFT 12AT7.
I did originally intend to use the older Octal 6SJ7 pentode but the chassis holes are very close to the edge which makes enlarging them at best tricky.

Thanks.

Andy.

[G6Tanuki]

It should work just fine: audio amplification's not difficult for a small-signal pentode!

The B8A double-triode of the 'series' is the ECC40: http://www.r-type.org/exhib/aaa0954.htm

http://www.r-type.org/pdfs/ecc40.pdf has examples of using the ECC40 as a phase-inverter in audio amplifier duty.

[I'm more familiar with coming across these by the hundreds in monostable/bistable/divide-by-two type circuits in 1950s scientific instrumentation]

[bikerhifinut]

Thanks for that. I figured the EF40 would substitute for the EF86, just needed reassurance.

i had a quick google of the ECC40 and it doesnt have a direct equivalent in the ECC8** series, so I wont consider it for this purpose. Plus it's still possible to get new/NOS 12AT7/ECC81 for less than a fiver each so I will stick to what I know will work.
I think the biggest problem will be finding a couple of B8A sockets! I am going to the Tonbridge Audiojumble so I may find some there.

thanks

Andy.

[Leon Crampin]

A 6BR7 is a bit cheaper than an EF86 and has the same base - but you would need to rewire the connections. No value changes are needed, and I find this valve works just as well as the EF86 with low levels of hum.

The lowest noise option is the 6BS7 - but it has a top cap connection to g1 and may not be as convenient to fit. The audiosheep don't seem to know about this valve (yet).

Leon.

[frankmcvey]

No problem with the valve bases - Roy at Electrojumble does the skirted chassis-mounted type that engages with a little tit on the valve, and Peter at ECD Online does a chassis-mounted B8A socket without a skirt (the only method of registration - orienting the valve - is by visually aligning a mark on the base with the valve tit). Both provide excellent service.

You'll also find PCB-mounting B8 bases on Ebay under the description "Rimlock". Here, for example.

Alternatively, I could just sell you a few EF86s!


Cheers,

Frank

[bikerhifinut]

Thanks for the info.
I'm at Tonbridge on Sunday so will be keeping a lookout.
I found supplies of new B8A bases, as you say, they aren't that rare.
I do have a few used EF86, I was mainly keeping them for my Leak preamps as spares. I have found in my experience that the EF86 in my possession don't seem to be very long lasting and go gassy or noisy in the leak preamps. Maybe mine are just about worn out including the stash of used ones. but I remember buying a batch of "NOS" Mullards CV types as well as EF 86 from Jim Fish a few years back that went the same way. This was one of several experiences with "NOS" Mullard valves that made me suspicious as to their provenance. I have discussed this with more experienced friends who seem to think the same. i.e. that a lot of valves that shoud have stayed in the "reject" bin somehow made their way back onto the dealers shelves. Which is why I tend to stick with known and trusted valve sellers especially when going for older types.
I found that the EF40 seems to have risen in value. Not that many years ago there were lots around for a pound or two but I think maybe someone else cottoned on to them when the price of NOS EF86 went silly. Even the Russian new ones are at least £15 each from the bigger suppliers.
That all said I can still get NOS EF40 for around a tenner or less so they are still an option.

The original plan was to use an Octal pentode RCA6SJ7 as they are plentiful and cheap, and russian ones are for pennies (Almost literally), but the Chassis I want to use are punched for the small bases and a bit too close to the chassis edge to open out safely.
And it might alter the symmetry of the amp but that's just aesthetics. I have no intention or need to stray from the ECC81 LTP on the original design and those are plentiful at reasonable prices still, new manufacture or NOS.

AS a parting shot, was there an equivalent Octal for the ECC81/12AT7? The nearest I could get to when researching for this project was the good old *SL7 series in terms of gain at any rate. I think there's no high gain octal equivalent of the ECC83 type which seems to be out there on its own, and probably explains its ubiquity in Valve RIAA stages.

Mods I may have addressed two separate issues here, do feel free to edit or split it.

Andy.

[Refugee]

My policy is to make an adapter and try it out.
In my example the heater cathode voltage on an ECC33 was suspect causing hum on a single triode phase splitter.
It enabled me to find another source of the hum in the power supply.

[G8HQP Dave]

6SL7 is roughly equivalent to 5751, which is a lower gain version of ECC83/12AX7. The same triode design is found in 6Q7, EBC90/6AT6 etc.