Parading My Snazz: New 5C8 Ina Console Style

[rp]

Hooray, or sorry, but I'm proud of myself. And it's a long winded post, apologies for that too.

https://picasaweb.google.com/1041232856 ... directlink

Finally able to debut this as I wanted to, without the preemptive, imagined bugs of my pre-mature debut where I had jumped in an external PT and where the long jumpers had given me a big drop in the heaters, and where the plate V of the 6J5 was 1/2 the schematic. With Martin's help it was determined that the Fender schematic must be wrong or not updated as the curves and troubleshooting indicate the 6J5 is operating optimally. So, looked at it that way the amp actually booted up perfectly on the first try even though it didn't feel as thrilling as it should. The tube gods are sparing with their grace.

I did this Hifi style as it's for home or cousin's studio but it occurred to me that I could replace the walnut side panels with two pieces of 3/4 ply mounted flush and load this into a normal tolexed head, so it's not so precious and limited as it looks. The whole thing was to show off the Brimar and RCA coke bottles I've been sitting on for 20 years. I thought it was a shame to just stick them into a generic 5F6A or JTM45 head. I then looked for a fairly exotic early circuit to do them justice and settled on the 5C8. I figured the 6SC7 and 6J5 would help lessen the big tubes' culture shock upon wakening in 2012. I also had exactly the right number of .047 225 ODs wasting away that I figured the 5C8 was destiny.

BTW never fails to amaze me when a device that's been totally idle for 70 years fires right up and works perfectly - Vacuum tubes are beautiful things.

OT is a NOS Stancor A3851 I got off eBay. This week I found someone selling a clean 5C8 on eBay with detailed picts and it shows a stonking OT, #2485. It's a 25W amp on a good night, from a time when it was never intended to be cranked, so I've no idea why it should sport a Bassman sized OT and now I have second thoughts, but it's too late to go drilling holes in my fancy chassis and so far the Stancor sounds great, but that awaits further testing too.

I changed the 2 channels so that V1 is the standard grid-leak but made V2 cathode biased with one input straight in and the other with a 33K grid-blocker, both with a 1meg load. I can hear a clear difference btwn the two sides of V2. No stopper is brighter and more immediate, almost "gainier", that 33K sounds very standard 5F6A/Marshall. Makes a notable difference, maybe those of you building Expresses with no grid stopper might want to experiment here if it's too bright or squirrelly before attacking the tone stack.

Preliminary trials in my apt indicate the amp's terrific and flawless, but that claim awaits a real test flat-out with 2x12s. There is a steady low hum but I'm pretty sure it's 50hz (Italy) heater hum inherent in the octals. I hope my layout is not the cause, I think it's clean and minimal. This is my first amp with more than one octal pre and I was very worried about this after reading up on octals on the internet, especially given I was using an unorthodox layout. The hum's not excessive and with 4 octal pre's and a primitive circuit, not unexpected - sounds normal for 1953 to me. It doesn't change with respect to the input jacks being open or closed or with knob twiddling. As soon as a guitar is plugged in the hum either goes away, or more likely is obscured, and once a note is strummed it might as well be non-existent. The big hum test will be what comes out of a 2x12 or 15.

The 6SC7 are as microphonic as everyone says they are, particularly the metal ones, I have about 8 and rolling helped but there's no escaping 6SC7 reality. I like the metal ones more than the glass, grittier and brighter. BTW on start up I had no bottom plate, long jumpers on the PT, and my speaker right next to the amp. The 6SC7s were not happy, squealed, hummed more, and had me spooked but I've enough experience now to put that aside and didn't extrude a lizard. Moving the speaker far away and buttoning up the amp seems to have solved all the squealing. Hopefully that squeal to is not layout related. I left the amp on for a couple hours with a guitar plugged in its vol down but all amp controls up full and after 4 hrs both PT and OT were hardly warm - a good sign, I think.

Through an Emi 1058 it sounds just great. Twangiest amp I ever heard, great chime. I think the main ingredient for these two are a cathode biased output, modest voltages and low gain, but that's a sophomoric analysis on my part. Grid leak bias adds an extra dimension too, not sure if it brings it's own twang or chime yet, but it adds a complementary clarity and incredible immediacy - string and finger noise are very apparent. Can't say what the octals themselves bring tonewise yet, as I've never A/B'd them with novals. One day I'd like to try some 9 to 8 adapters and see what I hear.

For the PT I found Raoli.it near Rome. One-off custom spec'd for 110€ shipped. He answered my emails right away promised it in 10 days and delivered. Nicely made, clean, true, and solid. Big, runs cool and acts well regulated. 120, 220, 230, 240, 330-0-330@200mA, 5@3A, 6.3ct@5A. Nice to have found someone as easy to work with and reliable as Heyboer was for me in the States. I've only dealt with the man once but he deserves a plug. FYI 660Vac give the called for 380Vdc on the plates with a 5U4GB and 250 ohm cathode resistor.

Next I have a DYI thingy on order to build a 6J5 - 6SC7 adapter so I can check the later 5C8 style with a paralled 6SC7 in V3. Maybe someone knows if the 6J5 version was only a prototype, all the ones I've found on the web seem to be with 6SC7s straight across. I like what I'm hearing now enough to not mess, like I said lower gain equals more chime, but I'm still very curious, after all it was the musicians that told Leo were to go with these things.

Wish me luck with high volume big speaker tests, hope my bubble doesn't burst.

Go forth and build something with octals and a grid-leak input.

And, big thanks for everyone's help.

edit: pardon that wire nut, I'm waiting on a lighted DPDT switch - gotta be proper in Euroland.

[JazzGuitarGimp]

Eccellente!

[martin manning]

Very elegant! Thanks for the detailed notes, and I'm looking forward to hearing about the 6J7 - 6SC7 test.

[Colossal]

That build is off the hook. Love the PTP layout and ye Olde new glass!! Really nice. Great tone report too.

[Koop]

Very nice!

[Structo]

Man I wish I had the guts to try a point to point build.

That looks great and the old style bottles really look great!

Good job!

[M Fowler]

She's a beaut

I had good luck with my 6SC7 knock on wood.

[rp]

Man I wish I had the guts to try a point to point build

Have no fear, you can always gut it into something else, right? PTP lessens the chances of problems like oscillations and other wiring issues. Your odds of success are actually better. PTP gives the amps a touch of added immediacy I think, even a lot depending how much cabling you manage to eliminate, though I would never even think of doing a BF Reverb or Dumble PTP. The real issue is mods / repairs are painful! I don't think I'd sell PTP amps, other than Champs.

[rp]

Ok, I've been sitting on this for 3 months. I tried assembling the adapter when I got it but kept f'ing it up and put it away (my brain seems to get stuck on inside-outside mirror image things). Finally got motivated to do it yesterday. I metered it several times then again in the morning and I was scared enough of a mis-wire that could blow up the amp that I used a DBL on boot. Came out perfect w/ no funny noises or oscillations which I also feared. I built it to included a 1k resistor and 220V cap inside that go in series with the cathode parts in the amp so it will allow fast A/B'ing. I realized later I messed up as the 220V cap is at the mid point of the two resistors, see drawing.

If anyone can tell me what's actually going on w/ the total bypass capacitance here I'd love to know, though my ears say it doesn't matters much, the fat gain is there. The resistors do meter 2500 total as expected.

Now, and this is important news here I might be the first, on the web anyway, to have A/B'd the two versions of the 5C8 (w/ V3 either a 6J5 or 6SC7) and I solved the question of the odd 6J5 plate voltage I got that didn't match the one in the Fender layout. I scoured the Marianas Trench of the web to find out about this and posted here too asking http://ampgarage.com/forum/viewtopic.ph ... hlight=5c8 but found nothing conclusive. So here it is resolved for posterity:

With a 6J5 the proper Vp will be ~80V (as MM had graciously calculated it should be) with a 6SC7 it matches the layout's 160V (I got ~170V). Therefore I'd say Fender measured the voltages on the later 5C8 with the 6SC7 for V3 but someone left 6J5 indicated on the service layout. I also claim dibs on solving how the 6SC7 for V3 was done, and I had posted trying to find this too as all there was out there was a tube chart image showing 4 6SC7s straight across but no real way to know if V3 was paralleled or one side unused and what Rp and Rk might be. Fortunately a 5C8 showed up on ebay with good gut shots to show the wiring and again Martin nailed that it must be paralleled. To sum up, V3 in the final 5C8 is a 6SC7 paralleled w/ 100K Rp and 2500 Rk & 250mf bypass.

There you go. Go build one.

... and I'm looking forward to hearing about the 6J7 - 6SC7 test.

You asked: on preliminary testing in my apt. there is just a HUGE difference btwn the two versions. The earlier is lovely, very jazz, country, like an old Gibson or Ampeg, and more 1940s, especially when played through V1 which is set up grid leak - extra nice for finger style, or picked out chords, and raking the strings. It's very twangy, bouncy and chimey which is a characteristic of lower gain and cathode bias. When you dig in it gets its hair up but this a a short haired dog and it doesn't sustain or scream much. A bit like the low input on later amps or sticking a 12AU7 into V1 or V2.

With the 6SC7 it flat out rocks, sustains and wails. It is much thicker, w/ plenty of mean to scream from just pick attach and sounds more like a later Fender tweed. The four octals sound huge. It's surprisingly vicious for 1953, with quick break up, maybe due to the smallish Stancor OT. BTW I also swapped out the coke bottle 6L6Gs I had in it (a nice, true NOS matched pair) with an equally nice pair of RCA (Tungsol?) 5881s and I much prefer the 5881s, just more complex and rich sounding (the current draw is about the same). Too bad as the coke bottles looked so cool. A 5U4GB gave 10 more volts on the power tubes than the ST 5U4G I had before too, and it helped nail the schematic voltages.

In retrospect I realize that Leo probably gave a prototype 5C8 with a 6J5 to the Bakersfield cats and they must have reported back that it needed more jam, and the engineers swapped the 6J5 for the paralleled 6SC7. You don't argue w/ the best ears around so I should have just wired for the 6SC7 from the start. Not so easy now as the heaters need to change and I've no slack, and the pin out's very different.

Next I want to test it full volume and in the hands of a couple people better than me to decide which way I'll leave it - sweet or nasty. Again, I think the 6J5 is very nice, especially with the smallish Stancor. Fender used a biggie here (#2485 I posted a pict somewhere) odd for the 25W this puts out, maybe he figured bass players or accordion players might want the Twin too. The Stancor A3851 looks to be about the same size as a BF Tremolux, Pro Rev, or Virbrolux so I went with it.

Right now I'm about 90% sure it'll get converted to the 6SC7 - a good amp should make you sit up and go Whoa! The second version does that.

Note: If you build one expect the 4 octals pre's to hum more than any noval amp. I'm familiar with old tweeds and mine is normal even with my different layout. You'll just have to live w/ it as they did in 1953 or go with DC heaters. I was going to try that until Welborne Labs ripped me off for the DC heater boards I had ordered. The other way to stop it from humming is to teach it the words

So, now's on the web there a couple of pretty detailed posts on 5C8 cloning. Watch someone start a business around it.

[Reeltarded]

Wow! So nice..

[martin manning]

That is just great, rp, and thanks for the follow-up! I had to wait until I got home to read this and see the pics on a bigger screen... now I'm dying to hear the difference! Any chance of that? Even a semi-crappy phone recording would probably catch the contrast you are describing. It's a beautiful amp for sure!

[Cygnus X1]

Bookmarked.
Awesome glass.

[LeftyStrat]

Bookmarked.
Awesome glass.

+1

That's too pretty to put in a tolex head cab.

[alvarezh]

"La Pietá" of guitar amps!

A plexiglass bottom cover would do it justice.

Congrats!

[tsutt]

That is gorgeous, nice work. How you guys figure out layouts like that is amazing. well done.