First amp build - 5F1 - seems to run a little hot

[robthomp]

Hi,

I'm new to this forum and tube amp building. I just completed my first build - a 5F1.  I didn't really use a kit, I made my own chassis, bought parts from Hoffman and TubeDepot, and used point to point wiring, with no board. (I didn't use P2P because of any belief system, I just thought it would be simple and a little cheaper. In fact it was harder than I thought and if I build another amp I'll probably use a board.)  I prepared by watching a lot of Uncle Doug videos and reading a lot of Robrob's site and also Aiken amps.

 I made one mod - I put in a 10k-60k variable negative feedback resistor.  Overall, I think it was a success.  The amp works, however I have one concern - it seems to run a little hot.

Here are the voltages I measure while idling:

B+1 386v
B+2 338v
B+3 289v

JJ 6V6S:
pin  2   3.1v
    7     3.1v
    3     358v
    8     22v
    4     339v
    5    .026v

JJ 12AX7:
pin 1   181v
     2   0v
     3   1.49v
     4/5   3.1v
     6   180v
     7   0v
     8    1.43v
     9   3.1v

JJ 5Y3:
pin 8   386v
     4    347v
     6   347v
     2-8   5.0v

Should I change the 470 ohm cathode bias resistor to a larger one? Try something else?  What do you think?

I have attached a few pictures.

Rob
 

[robthomp]

I forgot to mention - the above gives an estimated plate dissipation of 15.73w. The JJ 6V6S is rated for 14w.

[shooter]

how's the sound?
It's a SE running 112%, close enough 

you got a couple choices;
you can raise Rk, but plate volts will go up also since you're sucking less I, or you could add a PS tap, REC ->cap & R -> PA.  that will lower plate Vdc, which will change Vk, which will spin your head 
tweaking the bias on an SE gets you "there", where that is is subjective to how much tube abuse vs cost vs sound.
So do all the "tricks" until your happy, or give up 

[PRR]

~~5% of cathode current goes to screen. I make it as 14.1 watts in plate.

Are you going to work it 24/7 in the wilderness, no spares? Then 14.1W is on the hot side.

Are you going to play a few hours on weekends? I would not be real concerned. The tube will last for years. Set aside a few cents an hour, not even beer-change, and you'll buy a new 6V6 when the time comes.

[robthomp]

Thanks for the replies. So if I understand, you're saying the only issues are 1) does it sound ok, and 2) I might have to replace the power tube more often than if it was cooler.  And there's a tradeoff between these two things?   The JJ 6V6S cost less than 10 bucks so if I had to replace that every year or two that wouldn't be bad.

As for the sound, I think it sounds ok, but I can't tell for sure because I don't have a decent speaker yet.   I've ordered a Weber Alnico Signature 8S, but for now all I have to test it with is this 8 ohm 12" speaker from this cheap little Fender Frontman.  I put it in parallel with a 7.5 ohm resistor to get the impedance right,  but I'm imagining when I put the real speaker in it will sound a lot better.   

 If I keep the amp for myself, then it won't get a lot of use, because I'm not actually a guitar player, I'm just horsing around.    But there's a good chance I'll give it to one of my sons, and they really do play, but not professionally, so it still won't be played all the time.

There's also a little hum and buzz at higher volumes, but it's not terrible, so I have to figure out if that's in the range of normal or not. 

[robthomp]

I take back what I said about hum and buzz.  I think that was all coming from the guitar.   The guitar is a Fender Strat and it hums in whatever amp you put it in, in 3 out of the five pickup switch positions, and not the other  the other two (I think I understand the reason for that). 

With the guitar unplugged the amp is dead quiet at all volume levels.   

Playing the guitar sounds pretty darn good to my untrained ears.   The sound is pretty clean, it doesn't start to break up until you get to a volume which is probably louder then I'll get away with the neighbors and my girlfriend.   That thing is loud!    The NFB pot has a definite effect, I would call it subtle, but when I turn the resistance up there is an increase in distortion, so I overall I would say that mod was worth the little bit of extra work.   

[shooter]

the amp is dead quiet at all volume levels.

In troubleshooting that's "half splitting"
 

That thing is loud! 

let your kid play it through a Marshall stack.

[nandrewjackson]

In the 2nd image the green cloth wire looks like velvet.


New secret mojo part idea: velvet covered wire. 


 

[robthomp]

In the 2nd image the green cloth wire looks like velvet.


 

It's that 'vintage' cloth covered 'pushback' wire.   I've never used it before.  I'm new to tube amps, but not new to electronics - I'm accustomed to pvc.  I didn't really like it.   For one thing, after pushing it back it would slide back in place and obscure the joint I was trying to solder so I couldn't see what I was doing.   And then when I was done, the frayed fabric would get in the way and I couldn't really visually inspect the joint.

[nandrewjackson]

Don't get me wrong,  I'm not ragging on it. I've seen/used cloth wire before.  I was just saying how that specific picture made the wire look velvet.


 
Your build looks nice!

[shooter]

I personally use 22 or 24ga stranded with Teflon, but for;

the frayed fabric

extra step, slid a piece of shrink wrap over cloth, hold wire end with needle nose and shrink. kinda sorta keeps things in place

[sluckey]

Or use the stuff that Doug sells.

[robthomp]

Don't get me wrong,  I'm not ragging on it. I've seen/used cloth wire before.  I was just saying how that specific picture made the wire look velvet.


 
Your build looks nice!

Thanks!  I spent a lot of time preparing for this project before I actually started working on it.  Next step - make a cab.

Or use the stuff that Doug sells.

This is Doug's wire, and I'm sure it's top quality, I wasn't complaining about that. I'm sure if worked with it a little more I would get the hang of it.  But I think next time, I'll go for Teflon.

I personally use 22 or 24ga stranded with Teflon,

 

Is 24 ga sufficient?

Rob

[shooter]

Is 24 ga sufficient?

haven't flamed out in ~ 8yrs, well except when I put it to the wrong turret 

[Willabe]

Is 24 ga sufficient?

haven't flamed out in ~ 8yrs, well except when I put it to the wrong turret