5F6A Complete - now what mods?

[waldner]

I just finished this Hoffman 5f6a, fired it up, no smoke and sounds great.  It's dead silent.  Had a couple of minor problems to correct like reversing output speaker leads for feedback not to squeal.  Also I didn't pay close enough attention to the difference between Doug's layout vs the Mojo Bassman chassis, and found that the mid and bass controls were reversed.  Rewired them and now everything's great. 

Observations:  It's not as loud as I would have expected.  Compared to my 5E3 which rips your head off at less then 1 on the volume controls, this has much more clean usable range before getting loud and then raunchy.  It may very well get louder overall, but can be very tame at lower levels.  I'm guessing this is how it's supposed to be. 

The only thing I would like to change is there is a little bit of ice-pick highs.  Not terrible, and it's not like there is too much treble, just a harshness at the top end.  Any mods for this?
I like running the presence up to get more raw breakup, but then the highs get harsh.

Any other cool mods worth trying? 

Thanks!

[sluckey]

The only thing I would like to change is there is a little bit of ice-pick highs.

Removing the 100pF cap from the volume control may be just what you want.

[waldner]

Cool.  I will give that a try. 

Thanks!

[EL34]

Looks good.


If the mod Steve mentioned does not do it for you, see if it is your speakers?


Some speakers have way too much high end that distorts unpleasantly

The only thing I see that concerns me a bit are the filter caps.
Just make sure they cannot move and short out


I usually tuck them down under the pots out of the way and use some red wire jacket over the positive leads

[Toxophilite]

Wow
How did I get under the mistaken impression the PS filter caps needed to be in their own space away from everything else??!
I can mount them right off the board/terminal strip the circuit is on?

[EL34]

Here's how I have done it several times on Fender type chassis

[EL34]

Tweed era chassis are a bit tight but I have still tucked them up under the pots like this
http://el34world.com/charts/filtercaps.htm

[waldner]

Doug, I know the filter caps look a little dicey.  I tried to mount them in such a way that even if they move, the leads can't hit anything, so I think they are ok.  The one on an angle looks crappy because the lead wasn't long enough to get where I needed it to go and I didn't want to add wire to it. 

Also good point on the speaker.  I have been testing this mainly through an orange 212 with Vintage 30s, which can be a little bright.  I'll try something else.
It's really otherwise excellent sounding even through the V30s.  The ice pick is minimal, but it's there.

[EL34]

They are probably ok
They only ones in the pic that I was concerned about were these two.
The + of one cap looks to be really close to the - of the other cap

[Willabe]

How did I get under the mistaken impression the PS filter caps needed to be in their own space away from everything else??!

I was trying to tell you that in your added mini chassis reverb.    (Go back and re-read it, you'll see it now.) It's in the grounding article by valve wizard too.

I can mount them right off the board/terminal strip the circuit is on?

Yes. (What would be a reason you couldn't?) Keeps each circuit 'loop' small. It's sometimes called 'distributed capacitance'.


                      Brad     

[waldner]

Ah, you are right Doug.  That is a little closer than I realized.  I just bent the single cap away quite a bit and I think gave it some nice safety margin.
Thanks for that observation.

[EL34]

That's one reason I like to slide a bit of red wire jacket over the + leads
The other reason is that as you are probing inside the amp, red means high voltage.


At least it does the way I use red leads in my builds

[HotBluePlates]

... Observations:  It's not as loud as I would have expected.  Compared to my 5E3 which rips your head off at less then 1 on the volume controls, this has much more clean usable range before getting loud and then raunchy.  It may very well get louder overall, but can be very tame at lower levels.  I'm guessing this is how it's supposed to be.  ...

This almost certainly has nothing to do with either circuit, but with the volume controls themselves.

Not all audio taper pots are the same; in theory they all have a logarithmic taper, but in reality a log taper is faked by manufacturers using generally 2 different regions of resistance increase vs. knob rotation. The early region might result in anything between 10-30% of total resistance from wiper to ground at 50% rotation, with the higher-percentage number sounding louder, quicker. If you accidentally use a linear taper pot, you get 50% resistance at 50% rotation, which is as-loud as a true-log pot at about 90% up.

The second issue which complicates the first is a Deluxe's volume controls are wired "backwards" to isolate channels. To some, this setup sounds like the volume comes up faster, but again that depends on the specific model pot you used.

Don't drive by the numbers: Turn each amp up until each just starts distorting by about the same amount (or turn them both full-up)... the Bassman will be quite a bit louder, regardless of what number on the dial that occurs.

[Toxophilite]

THanks for the confirmation EL34 and Willabe!!


Willabe, I did read the article and what you posted..Just in that particular case the PS was already a done deal and working well , also seeing it laid out and done really makes it clear. My apologies if I at all seemed to be ignoring your advice, it was appreciated and considered and I will likely do some rewiring of that reverb circuit for fun after I enjoy it a bit first.

[tubenit]

If you have a speaker that is NOT causing high end hash, then I would suggest using an "enhance" cap with a value between 120p to 250p across the plate resistor gointo into the LTPI.  And/or use a smoothing cap from plate to cathode such as is on Dumblish style amps (which I would try on the second gain stage).


Do a search on "enhance" cap or look at a D'Mars ODS schematic.


With respect, Tubenit

[Willabe]

Toxo, no need to apologize, sorry if I came off wrong.


            Brad   

[Toxophilite]

All is well and thanks again


I now return you to the previously scheduled programming!

[waldner]

Tubenit,

Any chance you could show me on Doug's layout.  I'm not so good at reading schematics- especially that one.  I can sort of tell what's going on when the schematic is colorful and like Fisher Price.

Thanks,

[tubenit]

Any chance you could show me on Doug's layout

I am not able to open Doug's PDF file to modify it.  However, I think you can figure this out.

You'd do a smoothing cap like maybe 220p from V2-1 pin to V2-3 pin.    You'd use a 220p for an enhance cap across the 82k resistor going to V3-1.

with respect, Tubenit

[waldner]

Thanks Tubenit.

[sluckey]

Did you ever remove the 100pF cap from the volume control?

[waldner]

Did you ever remove the 100pF cap from the volume control?

Yes, and I like the bright channel much better this way.  Thanks Sluckey. 
I haven't played this amp out yet, so I don't know if the slight harshness will matter anyway, but it's still there a bit on the normal channel, though admittedly through the 212 V30 cab.  So I will try the other tricks from Tubenit as well.

[tubenit]

I will try the other tricks from Tubenit as well.

Try the enhance cap first. My experience with an enhance cap is that it simply smooths out the harshness from the highs without eliminating the highs.  IF that gets you there then quit.   

If you still need more then with the  smoothing cap (V2-1 to V2-3), you can use something as small as 120p if you want.

With respect, Tubenit

[waldner]

I did the "enhance" cap using a 250p which was the closest I had laying around and I think it did enough that I will leave it as is.  Why does a cap across that resistor work for this purpose?  I'm used to thinking of using a smaller value cap paralleled with a larger value cap as a bypass to increase clarity in the high frequencies. 

Thanks Tubenit.

[sluckey]

Think of that cap across the plate resistor as a cap from plate to ground. The cap simply shunts high frequency to ground. The B+ node that the cap is actually connected to is a very good AC ground, same as chassis ground.