Booting Up a 5C8 Works! But Odd Low Heater & 6J5 Plate V

[martin manning]

No doubt in my mind about the 6J5. BTW, on the Gibson schematic you posted with the 6J5 PI the Ra's are 100k, but the shared Rk is 3k3, which looks like 6k6 to each triode. Accordingly the op point is more centered than your 100k/1k5.

Higher filament voltage will affect current draw slightly so there would be some effect on the HT voltages. I don't know how much, maybe it's not significant. Did you try powering the heaters using a different PT?

[rp]

No doubt in my mind about the 6J5. BTW, on the Gibson schematic you posted with the 6J5 PI the Ra's are 100k, but the shared Rk is 3k3, which looks like 6k6 to each triode. Accordingly the op point is more centered than your 100k/1k5.

Once it became clear to me that the 6j5 was operating correctly I first thought of changing Rk. Maybe Fender got that wrong or a revision that failed to get documented and the 160V is right? But 1.5K Rk is so Fender, I don't think Fender ever had 6.6K resistors in house And that would still leave the Rk V, which is spot on, unexplained.

Maybe I can build an all analog all tube telephone time machine and call up Fender tech support circa 1955?

Higher filament voltage will affect current draw slightly so there would be some effect on the HT voltages. I don't know how much, maybe it's not significant. Did you try powering the heaters using a different PT?

Figured something like that, maybe best hold off on the new PT. Yup, mentioned back in an earlier post, one of the first things I tried, Hammond with 5A taps on the heaters - no go, same 5.8V.

Played it for an hour today easy in the apt, turned up too but kept the fingers and picking in check. Rolled a few 6SC7s to find a pair that didn't sequel or rattle full at full throttle. For a slightly wacked amp it sounds splendid! Left on 2 hrs and both PT & OT are as cool running as off, and it's in the 90 degs out. That's always a good sign.

Sounds totally like 25W of cathode biased, bouncy, chimey, bright, clear, snappy Fender. Dig in and you get the hair up. Grid leak channel is the shizz! Very dynamic. Very detailed and fast, finger/string noise is very strong and forces clean playing. The treb and bass range are more useful and nice than typical. A hot sh#t tele picker (which I'm not) would go nuts with this amp. Once done I'll take it to cuz's studio and run it though my 2x12 Emi Black Mountains and try it on some local musicians. One dude here can country pick like the dickens - go figure.

Go forth TAGers and try grid-leak octal inputs.

[martin manning]

I'm still thinking that your unloaded filament voltage is low. A drop of 5-10% unloaded to loaded isn't unreasonable, so unloaded you should see something approaching 7VAC. You are not getting any overheating so the current draw is reasonable and the transformer isn't likely to be faulty.

Grid-leak bias has its charm, as do bearskins and stone knives...

[labb]

I like the grid leak biasing too. The last couple of amps I have built have been 5B3's. I have been using 6SL7's in place of the 6SC7's to get away from the microphonic problems with the 6SC7's. The last batch of 6SL7's I tried had mechanical rattling problems so I tried the Sovtek and Tungsol new production 6SL7's. The Sovtek was really close to the NOS tubes. The Tungsol sounded darker to me..Tube Depot gets $10.95 for the Sovtek's.

[rp]

FWIW- the last batch of NOS 6sl7's that I got had mechanical problems..After they heated up and then cooled down is when the problem started..The tubes would rattle.

I don't think tube defects are affecting my heater V as when I just run the pre's and PI the drop seems normal. Something else is up somewhere.

My limited stash of old octals have some issues but so far I got the amp up and stable, bit of a light rattle riding the notes in V1 but I need to try a big cab flat out to get it right. some 6Sc7s sure can squeal. Right now my test speaker is right next to the amp but I've got V1 & V2 shock-mounted on rubber grommets - doesn't help much. I would never use octal pre's in a combo.

In my limited experience w/ these tubes (here and 5C1) the glass ones are less microphonic but darker, and slightly muddier and seemingly less gainy. I like the metal ones a whole a lot, bright and immediate sounding, nice grind. I really want to get this amp done and give it a work-out. I got some metal 6L6 to try next - all metal amp, for when I'm driving through plutonium.

I'm still thinking that your unloaded filament voltage is low. A drop of 5-10% unloaded to loaded isn't unreasonable, so unloaded you should see something approaching 7VAC. You are not getting any overheating so the current draw is reasonable and the transformer isn't likely to be faulty.

I think it's low too, something's not right but it's not the PT as the new Hammond gave the same results. I might have to go back and try it again however with more patience. Maybe the hammond on one string, marstarn on the other - just for sh!ts and giggles.

Grid-leak bias has its charm, as do bearskins and stone knives...

You got it. I typed that phrase into google images and had that image in 5 seconds. The internets are a simply amazing series of tubes.

[rp]

I like the grid leak biasing too. The last couple of amps I have built have been 5B3's. I have been using 6SL7's in place of the 6SC7's to get away from the microphonic problems with the 6SC7's. The last batch of 6SL7's I tried had mechanical rattling problems so I tried the Sovtek and Tungsol new production 6SL7's. The Sovtek was really close to the NOS tubes. The Tungsol sounded darker to me..Tube Depot gets $10.95 for the Sovtek's.

Labb, did you A/B 6SC7s and 6SL7s? Was their a difference of any note? I wanted to build it straight up first to see what the original was like. If I can't control the microphonics once cranked maybe I'll go to 6SL7s. Are you comparing the Sovteks to metals or glass? Do old metals sound different to old glass to you?

*** Hey all: is it at all reasonable that there could be some flaw in my heater windings: a stray strand, a stretched-out, over-spun, too thin Teflon jacket, bad layout, that would not show up in the unloaded voltage but show up under load? And not show up testing for resistance to ground or across the windings? It's max 24" of wire not 300'! How's this possible??? Please everyone say NO before I start redoing the f%$#ing heaters in a finished PTP build.

[martin manning]

I'm running out of ideas on the filament voltage... You expect about 5% drop loaded-unloaded and you are seeing 10%. You also said that you used a lower voltage primary and got higher filamet voltage but only temporarily (that would imply that you were seeing a drop of around 20% at that point?). Trying another transformer and swapping tubes didn't work either. I hate to say it, but the only constant is the heater wiring. When you measure heater voltage, are you measuring at the transformer leads or somewhere else? Does it matter?

[labb]

Labb, did you A/B 6SC7s and 6SL7s? Was their a difference of any note? I wanted to build it straight up first to see what the original was like. If I can't control the microphonics once cranked maybe I'll go to 6SL7s. Are you comparing the Sovteks to metals or glass? Do old metals sound different to old glass to you?

*Yes I A/B them in the same amp. There is some difference in the three as you would expect. The NOS SC's and SL's were metal. Have not tried any glass NOS SC's or SL's. If you can get as set of NOS SC's that are good then that,to me, is the best. Really all three sound good, just some difference.

[rp]

It will most certainly turn out to be a simple thing to fix, it's just the journey getting there that is aggravating.

You called it, Tom. I got little hair left after pulling it all out.

I was going to shelve this but then decided what better way to spend a ~100deg afternoon w/o ac than to redo all the heater runs in a finished PTP amp. Guess I was in a good mood. Took CDs 1 & 2 of Haggard's - Untamed Hawk to do, that's ~60 songs, so it was a chore. I reran them in a daisy chain just to be safe as that has always worked in the past. Looks nice, cleaner and shorter than before, don't know why I didn't do it this way when I started? Sometimes I'm just in a dumbass loop.

GUESS WHAT??? Got 5.8V as I knew I would! Hahaha! But I must have been in a very good mood (the Haggard?) 'cause instead of breaking down in tears I got the notion that maybe the jumper leads were causing it - Too long? I then swapped the 24" jumpers to some 5" ones I made and raised the PT up closer and wop-bop-a-loo bop there was exactly 6.3V! Why this notion had to hit me at the very end rather than the beginning...? But then none of you caught it either

Tom, TAG should have a Wiki (maybe it does already) where tips like this can get posted. Save some other poor bastard a heap of aggravation. At least this post is out there for others to Google.

Martin seems to to have laid the 6J5 Vp to rest so I really did it hit out of the park. Worked first time, and had the all the voltages perfect

Thanks all for the trouble-shooting effort.

Now I need to source a stand-up PT and I'll be all set, but there are none anywhere - I turned the internet inside out. Looks like I'll have to go euro-custom, which is a time and money bummer. I think RJ had a 650CT 120/240 Edcor but he's off for a while. If anyone knows of a PT in Euroland that meets my needs please let me know, I could prolly leave out the 230V taps.

~180VAC,
Primaries 120v, 230V, 240V
Secondaries:
330 -0- 330V (660V CT) ~200mA
5V - 3A
3.15-0-3.15V (6.3V CT) ~5A

[martin manning]

Hey, great! I tried to imagine how a resistance in the heater string could be causing your problem without showing itself by getting very hot, but I guess if you have two 24" resistors they can drop voltage and remain pretty cool. In our collective defense, you didn't mention that you had such long leads feeding the filament string, did you? Glad to hear you've got it solved, and that you are still smiling!

[tubeswell]

I want to clean up, get my kitchen table back, I want to order it's dedicated PT but would like to be certain of the voltages first.

BTW going (hopefully one day) from 5.8 to 6.3V shouldn't affect the plate voltage should it? I can safely order a 330-0-330 PT?

My wife would kill me if I used the kitchen table to build tube amps.

[rp]

My wife would kill me if I used the kitchen table to build tube amps.

There are a few advantages to being single, though I have to cook my own food to eat at the kitchen table. My neighbor saw I had no furniture and gave me this giant sturdy table, best work bench possible, I could do 3 amps at once! If I was more of a philistine and this wasn't Italy I'd leave it as a work bench and eat on the couch. Does your wife let you eat on the couch? Ah, the bachelor life...

[tubeswell]

Ah, the bachelor life...

I dimly remember something about it