audiophoolery?

andrew_k said:

Junction said:

As mentioned by PRR, it is quite common to experience feedback problems in an EF86 combo guitar amp design

Click to expand...

I'm experiencing this with a recent EF86 guitar amp build, even after mounting sockets using rubber grommets between socket and bolt, as well as between socket and chassis. Don't know if they still do, but Dr Z amps used to ship their EF86 amps with thick heat-shrink on the EF86. My attempts at using EF80 have been even more fraught with oscillation, despite having ~70 to choose from.

Not audiophoolery, a pain in the butt!

Click to expand...

Andrew, do they say EF80 on them? Not E80F? An EF80 is not a substitute for an EF86, according to its data sheets it's an entirely differnt tube... transconductance is 7-8 for instance, compared to about 2 for the EF86 and E80F, which are suitable substitutes for each other.

I am having the same problem with the EF86 stage I put into my JCM800 clone (no channel switching yet - just getting the pre right). Just for fun I put in a siemens ef80 (because it was so pretty!) and my noise and oscillation - not to mention sound quality - problems increased dramatically, as expected. I have a Telefunken ef86 and e80f and the e80f seems to have the least problems. Its data sheet says it is a "special quality" tube and it has higher limiting values than the ef86, looks better and has gold pins. Even so, the JCM800 is a loud mofo and I have some slightly bothersome microphonics problems. I used a Russian tube socket which came with a tall metal tube sheild which helps some too.

I am thinking of using the "trampoline" mount as shown in the Jones "Building Valve Amplifiers" book where the tube socket is isolated from the chassis by using some elastic material to essentially lace its socket mount to the rest of the chassis so that the tube(s) mounting more or less "floats" and the compliance of the whole thing can be experimented with - different elastic material, stiffness, tigtness of the lacing, etc.

BTW, which ef86 pre did you build? I used the one that Voxs, I guess like the AC15 used, which I got from the Lightning clone web site, because I was working on my lightning clone at the time.

HTH,

Kiira

Kiira,
I was actually talking about two different pentode pre's in one amp - one designed around EF86 (from the Dr Z Route 66- PM me if you want the schematic) and one designed for an EF80/6BX6 (loosely based on the AX84 Moonlight). The PI and PA sections are provided by a pair of 6BM8. [Picture] I can see how my previous post led you to think I was attempting to substitute an EF80 for an EF86.

Regarding vibrations and socket mounting, I went with the cheapest, easiest approach I could come up with:


Softer rubber would be a worthwhile improvement.

Thanks for the advice

Hi Andrew,

andrew_k said:

Kiira,
I was actually talking about two different pentode pre's in one amp - one designed around EF86 (from the Dr Z Route 66- PM me if you want the schematic) and one designed for an EF80/6BX6 (loosely based on the AX84 Moonlight). The PI and PA sections are provided by a pair of 6BM8. I can see how my previous post led you to think I was attempting to substitute an EF80 for an EF86.

Click to expand...

Ah ha, ok I see the confusion. Oh well.

Regarding vibrations and socket mounting, I went with the cheapest, easiest approach I could come up with:

Softer rubber would be a worthwhile improvement.

Click to expand...

Fer sher. I snarfed the tube mounts out of an old Tek scope once for a high gain pre (a NYDave design)... they used shock mounting for that weird tube they started using, it was some mutant thing that looked like a small blue button or something.

I work in a hardware store... the gasket that is between the tank and the toilet bowl is large and can be gotten in several grades of softness, some are like spongy neoprene. Just a thought.

Thanks for the advice

Click to expand...

You're very welcome! Good luck!

Kiira

IMHO the application on that picture will do nothing to prevent the shock/vibration reaching the valve unless the screws are isolated from the chassis with additional rubber gaskets and not tightened as much as they are.  Of course the ultimate would be to use compression springs betwen the socket and chassis and with no direct screw connection.

Thanks Sahib, there is another gasket between the nut and chassis, I will try your suggestion of loosening them.