Corksniffer's 5E8A Low Power Twin--Nearing Completion

[David Root]

I found an original '57 Fender 2818 OT, and the OT made me do it!

Not a replica, but the audio circuit is stock except I cut the 250uF first stage cathode bypass cap to 100uF, added a 470K gridstopper on the cathodyne PI, and also 56K gridstoppers on V1/V2.

Added variable fixed bias for both power tubes, with three 10uF 250V polyester film caps instead of three 33uF e-caps.

The chassis is a standard Mojo not-quite-right knock-off. I added rubber grommets on the preamp tube sockets as per the original design. If you are wondering what the extra piece of stainless steel bolted on the PT end is, that is a cantilevered additional support for the transformer end of the chassis. It bolts into the top of the cabinet. Bruce at Mission Amps sells them and I use them with all the tweed chassis amps I have built.

Tubes shown are a pair of Tungsol 5U4G, a pair of 1943 NOS Ken-Rad VT-115A (military 6L6G), '51 GE blackplate 12AX7, 5-star GE 6072, Raytheon Windmill getter 5751 in V2 and GE 3-mica JG-6072 in V1. I have some other rectifiers, '58 Sylvania 5931, '60s Tung-Sol 5U4GB, 1944 JAN RCA 5R4GY, and will be rolling power tubes also with 6L6GAY and Tung-Sol NOS 5881.

The speakers are a pair of Chicago Jensen P12Ns pulled from an organ, look like new, one is late '61, one is early '62.

The cabinet is a wonderful piece of work by John Mergili in Austin TX. Check his website, he does incredible stuff. He has also built more tweed cabinets than most of us have had hot dinners, and he had some antique pine that came from a grain elevator built in Wisconsin in 1883. The wood is very light, John gave me a sample piece and built the cabinet with this wood.

I have yet to put in the heater wiring, drill the cabinet and mount the chassis.

[martin manning]

That's top-notch David; really nice with the old glass and the custom cab! Looking forward to the report of the power-up and the tones...

[blue_dog]

wow that's an awesome find.

[vibratoking]

That looks like a great cab. How much, if I may ask?

[billyz]

John is the master of Tweed indeed.

Glad you found a good use for the OT. Nice work.

[Milkmansound]

Jupiter caps and domino micas... you know thats going to sound good!!

Just out of curiosity - how did you wire in the 470k grid stop? Kind of hard to tell from the photo

[David Root]

BillyZ, thanx again for the OT, without it I might not have gotten so motivated. If you look closely in the power end of the chassis you'll see a fuse holder in the ground switch slot. That will have a 200mA FB fuse connected to the OT center tap to protect it.

Vibratoking, $475.00 plus shipping. I think that was a fair price for such old wood. Below is a pic of the sample board John sent me with the cabinet. Hopefully you can see how straight the rings are in this quarter cut board, gives some idea of the size of the tree it was cut from.

Milkmansound, the red Jupiters are claimed to be reincarnations of the red Astrons. They had better have "the tone", they are NOT cheap. The 250pF domino is vintage correct, that is what is in the originals. The PI entrance cap is a Vitamin Q, would have been a Jupiter if I hadn't ordered only two .02uF instead of three, duh!

The 470K PI gridstopper is connected between the board and the tube by a single lug terminal strip I took out of an AB165 Bassman a while back. I saved everything from that amp that I didn't use in my 2nd gen. ODS build. There is a white wire from the 1M resistor to the single lug. It runs underneath the output jack but you can see both ends of it if you blow the pic up a bit.

[rp]

sandals with socks otherwise nice, love big bottles got a 5C8 almost all mise en place'd but got to do the move first.

please do a review of those Jupiters, my NOS stash is almost gone and I need a modern cap that has that swirly loose warm tone.

[Colossal]

Beautiful build David. I love the old glass. After we spoke, I've been on the hunt for old NOS for my EH-150 build. I've scored some great old coke bottles.

I'll bet that amp sounds killer.

[cbass]

Very nice.

[David Root]

rp, sorry about the sartorial lapse but that's what I wear around the house. In summer I even go out like that!

Colossal, I have been utterly seduced by the coke bottles, they are so cool. I just hope they don't rattle when I crank the amp. If they do I'll try a couple of silicone damping rings on them.

I have the chassis all finished now and will check it out this afternoon. It's in my amp cradle now. I still have to put some 16 ga wire on the speakers and solder in a plug. Existing speaker wire is only 18 ga.

[rp]

I have been utterly seduced by the coke bottles

Me too. I've been wasting needed money on 6V6G's. I like the non grey glass ones - more glow for your dough. I actually think the small glass ones sound more complex, a little fuller, the STs a little thinner, brighter but I hate A/B'ing so who knows, but you can't beat big bottles for pure show - like chrome rims.

Here's my new 5F1 with a big bottle made ~70 years ago and working like a day hasn't passed since 1940.

Time to turn out the lights and play in the dark.

[Reeltarded]

That is a great looking amp!

[David Root]

Classic simplicity! I like the gray-to-blackglass ones as they tend to be older. The more of these I buy the more I think the best quality was made in the depths of WWII.

I have seen statements that the big coke bottles ST16 eg 6L6G are "more musical" than the smaller ones ST12, and the exact opposite. I have a quad of Sylania 6L6GA in my Bluesmaster D-build that sound great, better to my ears than NOS Tung-Sol 5881. Don't have a matched quad of 6L6G to compare with unfortunately, you have to buy a lot of 6L6G to make one up.

In my 5E8A I CAN make that comparison, and 6L6GAY, aforesaid NOS Tung-Sol 5881, and RCA blackplate 6L6GC too. Only 6L6 type I don't have is EL37. Unfortunately my best NOS dealer source with any depth in his inventory just upped his price for a NOS pair from $395 to $425.

[David Root]

Now in its cabinet, and sounds wonderful. Haven't tried detuning the power tubes from a less than 0.5mA match yet.

Changed Tungsol 5U4GYs to RCA JAN CRC 5R4GYs, 1944. Running higher than schematic voltages because the two Ken-Rad VT-115As need bias voltages of
-52.6V and -62.2V in order to run at 68% of 19W max. plate dissipation, 31.5mA each cathode with maybe 1mA on the screens, at idle. The Fender schematic shows -42V. I cannot imagine what these tubes would do at that bias, I think they would redplate & croak.
My bias supply has no 3K3 resistor feeding the diode and gives a bias range of -47V to -74V, using two 50K pots in parallel and a 47K tail resistor, from 55VAC off the bias tap.

In the preamp I have V1 & V3 '55 GE blackplate 3-mica 6072, V2 is a '60s (I think) GE 5-star 2-mica 6072A, and V4 is a '51 GE long black plate 12AX7 with the gain of a RI Tungsolski 12AX7. Some of these early 12AX7s have very high gain. Its pin 6.7.8 triode is somewhat microphonic but that is the PI stage so it doesn't matter.

The screens run 2V above the plates at idle, with 470 ohm screen resistors.

Aside from a very slight mains hum, not audible at 3 ft. from the speakers, the amp is dead quiet all the way up. There is no shielded cable used. This has made me a bit more conservative about using coax on lower gain preamps as it doesn't seem to be necessary if you get the grounding system right.

I used a Dumble type grounding system, with the preamp tubes and pots etc. going to ground off the ground of the V1 volume pot, using a pot ground lug.
The cathodyne (the 12AX7) has its own chassis ground further down the chassis, directly from its B+ node. I also grounded the bias pot at that node. That is different than the Fender schematic, which grounds the cathodyne with the power tube and transformer grounds.
All the heavy current grounds, and the bias supply grounds went to a third ground point on the power transformer end of the chassis.
The AC cord ground is also on the PT end of the chassis but 4" away from it.

The input jacks and the output jack are grounded directly to the chassis as in the original design.

I hope this detail is useful to anyone contemplating a 5E8A build.