Question About The 6J5 In The 5C8 Twin

[rp]

The only schematic I've ever found shows the 6J5, which is a medium mu triode with an amplification factor of 20 from the sheets I found on the web. It's half a 6SN7.

Then I found this pict of an odd 5C8 label showing a straight line of 6SC7s in the amp. The 6SC7 is a hi-mu triode with a factor of 70.

Any ideas what Fender was doing with the change? Obviously goosing the amp up a bit and wasting half a triode unless there was a really big change in the design and an extra stage was added, but I don't see this happening in 1955. I like gain, I preferred the 6AV6 in my Harvard clone to the 6AT6. I might go with the all 6SC7 style. Anyone familiar with the two versions of this amp?

Is the 6SC7 a direct drop in or is there some rewiring of the socket I must think about or changes to the plate and/or cathode resistor? Think the plate resistor was changed to 220k?

[Jack Hester]

No, you cannot replace the 6J5 with the 6SC7. Here are the tube specs to compare the pinout of each:

http://tubedata.itchurch.org/sheets/127/6/6J5.pdf

http://tubedata.itchurch.org/sheets/127/6/6SC7.pdf

Rewiring is a must, for this change. I can't think of one, right off hand, but I'm sure a high-mu single triode was probably made that will have the right pinout. Hopefully, someone else knows of one and will post.

Jack

[Firestorm]

My Twin (mid '54) has the same 5C8 chart, but is a transitional model with the 5D8 circuit board, but 5C8 chassis (no Presence control, punched for octal tubes, but holes adapted for novals). So my guess is the chart was used for a long time despite circuit changes. The earliest models probably did have the 6J5, but Fender must have gone for more gain midway through the wide panel series with the 6SC7. Fender circuit changes seem to happen most reliably prior to the summer shows, so the 6J5 probably goes away in early '53 at the latest (or even in '52); serial number 200 or lower (just guessing). Anybody have one that old?

[rp]

My Twin (mid '54) has the same 5C8 chart, but is a transitional model with the 5D8 circuit board, but 5C8 chassis (no Presence control, punched for octal tubes, but holes adapted for novals).

Firestorm, next time you go poke around in your twin if you can note the changes from the older 6J5 schematic and can post here, it'd be much appreciated, or even just detailed innard picts. Maybe you can draw up a whole schematic and send it off to the Hoffman database.

Even if it's many months from now, feel free to revive this thread.

I just checked the pinouts and I would have to rewire, so unless I get some news on how the all 6SC7 one is wired I'll have to go with the schematic I have and maybe mod it later. IME with Fender revisions, the higher gain version is usually the one to have. I could also try combining the 5C8 and 5D8 on my own, might be good experience, I've never soloed before just painted by numbers.

Anyone else been inside an octal 5D8?

[Firestorm]

Firestorm, next time you go poke around in your twin if you can note the changes from the older 6J5 schematic and can post here

Sure, but it'll be of limited use to anyone trying to get a handle on the octal versions. Mine was built immediately before the introduction of the 5D8 so it was made with 12AY7s and 12AX7s using the 5D8 circuit, but stuffed in a 5C8 chassis.

I'd love to compare the circuit with the earliest Bandmaster, which is stupidly rare, and for which no schematic seems to exist...

[rp]

Firestorm, it'll give me an idea what they were doing in the transition, maybe yours is wired as a straight 5D8 or maybe something in between. It's looking like I'll have to guess at the all 6SC7 version, and make it up, shouldn't be too hard to pull off with help from people here. I was being lazy but now compared the 5C8 to the 5D8 and they did add the extra gain stage with a cathode follower.

I guess I can figure that out and there must be other Fender octal amps using a cathode follower, I'll need to check all the 2 6L6 tweeds.

Anyone familiar with the sound of these the two octal versions of the tweed twin, with and without the cathode follower? I'm sort of getting into the old octals amps with the grid leak bias as a different sound. I usually prefer higher gain but I don't need an other amp that sounds like a 5F6A, maybe the 6J5 version might be the more interesting/unique one to build.

[rp]

Went through all the Fender octal 6L6 tweeds and there are no 4 octal preamp tube amps except for the Twin in the version in the picture I posted. So now I really have to find someone who has seen one and knows what's going on with that 3rd gain stage.

[jon]

The Bandmaster and Twin are almost the same. If you look at the 5C8 schematic you will see that each input gets its own triode, in grid leak bias. As the Bandmaster only has three inputs you take the extra triode and use it for the gain stage in lue of the 6J5. I built a 5C8 a while back and I have built a 5D8, As far a tone goes the 5D8 walked all over the 5C8. The D8 has a smoother tone with more pleasing breakup. Additionally, the tone controls seem to be better tailored to amp in the D8. While the 5C8 has a very cool vibe I think that the D8 is a better amp.

[Structo]

I have never built any tweed amps and really don't know anything about them but in comparing the 5D8 and the 5E8 I can see the 5D8 has cathode biased power tubes vs fixed bias on the 5E8.

I would guess the 5D8 has a nice growly overdrive tone once the volume is high enough.

Do you recall if you added any modern touches such as grid stoppers on the power tubes or screen resistors?

[rp]

Discovered what Fender did when they dropped the 6J5 for a 3rd 6SC7. There's presently a 1953 5C8 on eBay pretty much stock. From the picts they just rewired the base of V3 for 1/2 a 6SC7 and left everything else totally unchanged. They must have decided it was better with more gain, 20 vs 70.

Here's picts from the eBay amp and the schematic again.

Do I have it right? This might prove helpful to future 5C8 builders.

Bummer for me as mine's ptp and I'll likely not suffer to change it, especially the filaments. On the bright side one less 6SC7 to deal with.

http://www.ebay.com/itm/1953-FENDER-TWI ... 0861684952?

[martin manning]

The cathode resistor on the third (from right) tube looks like 2k5, same as the shared Rk on the 6SC7 PI. I can't tell from the pic but perhaps they paralleled the two triodes?

[rp]

The cathode resistor on the third (from right) tube looks like 2k5, same as the shared Rk on the 6SC7 PI. I cant tell from the pic but perhaps they paralleled the two triodes?

red, green, red, pretty clear! I missed that one, you are absolutely right! But why did they keep 100K as the plate resister, that's clearly visible too? That would exactly explain the low voltage problem I had on the 6J5 wouldn't it (I had an earlier post about it)? Fender changed things and didn't relabel the tube on the layout. Begs the question, did they really want 100K / 160V on the 6SC7 or really 80V but they spaced the resistor?

https://tubeamparchive.com/viewtopic.ph ... 8&start=15

now I'm really curious to hear the change. Big bad rats, about the filaments being different, that's a lot of work to change.

Edit: I removed the schematic and replaced w/ the layout which is more useful here.

[martin manning]

If you do the 6SC7 load line using the 270V B+ from the layout, a 200k plate load (shared 100k), and a 5k cathode resistor (shared 2k5), the operating point is at about 175V, and it is pretty well centered. From the grid curves, it looks like it will also add some significant 2nd order harmonic distortion. I say go for it!

[rp]

I say go for it!

easy for you to say. the heaters would be a bear, I'd have to flip the socket around and rerun V1 too. now why couldn't the dang 6J5 have the same pinout as a 6SC7

sounds really good once I got the head far away from the cab. the lower gain makes it twangy as heck. For once I'll temper my itchy solder finger and force myself to live with it for a good while, get to know it, then mod it.

[martin manning]

I can see how rewiring that would be a bit of a pain. Nice looking point-to-point, BTW. Now, if you had an adapter socket thingy, all you'd have to do is change the Rk...

Why does the amp not like being close to the cab?