Another 5B6 build - couple issues

[man-bot]

Hi there - I'm just tweaking a 5B6 build and noticed there has been quite a bit of discussion over here on the subject!

Mine is finished and operating (built on the fender schematic and layout) - but I have a few bugs I'm working out:

- amp was very distorted, 6SC7 tube only had 117v on plates
* changed 250k dropping resistor to 47k and plate voltage is around 240v now
- amp was very bassy (go figure haha) changed 6SC7 bypass cap to 15uf from 220uf
* still could use a bit less bass, would you have changed the coupling caps instead?
- Amp is not as loud as I thought push/pull 5881's should be
* Volume wise is this normal? Sounds about thas loud as my brownface deluxe w/ 6V6s.

Really I want to tone the bass down a bit more and more volume would be great... it seems maybe cathode biased 5881s may not be as loud as I thought.

Anyhow - thoughts on if the low voltage on the 6SC7 at stock 250k dropping resistor makes sense? How would you approach the bass heavy response, at the bypass cap or at the coupling caps? Any thoughts on getting more volume?

Thanks!

[rp]

amp was very distorted, 6SC7 tube only had 117v on plates changed 250k dropping resistor to 47k and plate voltage is around 240v now

Not sure about distortion and others better than me will chime in but I think ~160 is correct here, you aren't that far off. From what I've seen octals pres seem to be run low. In my 5C8 I (correctly) have V1 (grid leak) 65V, V2 (cathode bias) 115V, V3 6J5 80V, PI 160V. Very different from what you'd usually see w/ novals. IME old octals can vary a fair bit too so you might be good with that tube. You should have ~2V on pin six so check your Rk for spec and Ra too. If good you just might have to roll 6SC7s, expensive if you don't have a couple. Seeing its a clone - ie a known quantity, I wouldn't twist it around to make it work, I'd try to get it to work as it should straight up first.

- amp was very bassy (go figure haha) changed 6SC7 bypass cap to 15uf from 220uf still could use a bit less bass, would you have changed the coupling caps instead?

I got one planned I might go .05 all around like the 5C8 from the start unless I decide I want it for a household bass pet.

Amp is not as loud as I thought push/pull 5881's should be Volume wise is this normal? Sounds about thas loud as my brownface deluxe w/ 6V6s.

Yes. My 5C8 is likely no louder than a Brown Deluxe. Bet in 1952 it had people covering there ears and running out of the room.

[Blackburn]

I built one of these last summer and really, really like the clean tone. Switching to .02s off the plates of the 6sc7 should make a BIG difference. I didn't bother with .1s there because I knew it would be too much. With .02s you can probably use the 220uf cathode cap. I used a very nice one and noticed something about the lows that was special. And the volume surprised me too. Very deceiving at first until you crank it, but still only about 25 watts at most. You'll probably get more having raised the preamp voltages whereas I kept mine stock with 270ks. Wiring in a 6sl7 instead of the 6sc7 will give you more volume.

As for the early distortion, Tino Zottola advises a .1 cap from the OT CT to one of the 6L6 plates to alleviate this, though you may notice a change in tone.

David

[man-bot]

RP:

Schematic shows 160v on the plates, but I would like a cleaner tone so I'm pushing the voltage there... I can always switch back when tweaking. Unfortunately I only have the one NOS 6SC7 (metal can) on hand...

I think I have about 3v on pin 6... it was in around 2.1 until I dropped the resistor/upped the plate voltage.

I also need more filtering up front as I dont have enough smoothing with the stock 4 x 16ufs as my B+ is wavering about 20-30v now that I have upped the plates. I am goign to swap a 40uf cap into the first postion to try and smooth that out.

I'll change the coupling caps too (and may have to up the bypass cap again as a result) to see if I can get a better guitar tone. I have some .02 on hand and have some .05 on their way.

Also I have a set of 6L6GCs on their way to see if I can get the volume up. Just seems weird to me to not have my ears ringing with it flat out!

Blackburn:

I'll try the .02s and see where I sit - I'm liking what you typed though! To be honest the tone is good but not blowing me away. I like it much more with the higher plate voltage only for the fact that i can actually play without the signal being completely distorted anywhere on the volume setting.

I'll try the .1 cap and it may allow me to bring down the plage voltage ont he 6SC7 again...

Thanks for the tweaks guys!! I'll let you know how it turns out.

Mike

[Blackburn]

I actually have RCA Blackplate 6L6GCs in there now. The big reason why the clean sound is so orgasmic... And one more... Check out PartsConnexion.com and look in the electrolytics section under BlackGate. I think you'll find what you're looking for at a spectacular price.

David

[rp]

I actually have RCA Blackplate 6L6GCs in there now. The big reason why the clean sound is so orgasmic... And one more... Check out PartsConnexion.com and look in the electrolytics section under BlackGate. I think you'll find what you're looking for at a spectacular price. David

Orgasmic! The hell with paying my back due gas bills, I'm gonna buy parts!

[Blackburn]

I know what you mean RP, but I didn't pay a dime for em.

A while back I gave a friend a nice pedal for Christmas or a birthday or whatever and the following Christmas he gave me an early 60s Baldwin organ that he got free from a neighbor. After hauling the solid hardwood monstrosity to my place I pulled over 70 12ax7s and the power section which was the same as the 5b6 including the 5u4.

David

[man-bot]

Okay things are coming along well - went with a .01uf coupling cap and the bass is much nicer. Still only have a 15uf bypass cap on the 6SC7 cathode and i like it.

Dropped down to a 1.5k cathode resistor on the 6SC7 as well and have cleaned up the tone quite a bit... still a bit high as it measures 2.6v and schem is 2.1v but I can play and figure where I want to be (need to get a decade box!).

The speaker my buddy wanted to use just ain't jiving though. He had an old Cerwin Vega 15" he wanted to use and it is just nasally or something... guitar sounds proper through some 12" oxfords but just sounds muffled with this Cerwin Vega so I'm fighting that a bit. As much as he want to use (It was one of his dad's from the 70's) I just don't think its gonna fly...

My biggest issue is I get loud hum when I turn my guitar volume down... no good!!

Any idea how to solve that issue?

Mike