small crappy 10W SS amp conversion

[Paul1453]

I'm working towards converting a crappy 10W SS amp I had, into a rocking little 4.5W 6AQ5 tube amp.

Using almost all salvaged parts, except new ceramic sockets.

A couple of pics from the start.

[Paul1453]

My PS is salvaged from a Norelco R-R.

I have the circuit set up on my breadboard, and it is working pretty well.

The housing and speakers present a challenge to work out.

Attached is the schematic.  Any suggestions for improvements to the circuit?

I want to have that finalized on the breadboard before proceeding to the build.

No tube rectifier, using salvaged SS Norelco instead.
Also have eliminated the 1000pf cap at the pot.
A very simple circuit that sounds pretty good now.
It seems to like a 12AU7 better than a 12AX7.

[uki]

Oh this looks interesting !!  Did you add those side speakers?
I'm curious about how you match the impedance, how many ohms are each of them ?

[Paul1453]

Oh this looks interesting !!  Did you add those side speakers?
I'm curious about how you match the impedance, how many ohms are each of them ?

They are 2.5W and like 3.2 ohms each.  I had to add them because:
I needed to add them to match impedance with my salvaged OT to get near the 5K plate load the 6AQ5 wants at 250V.
I mentioned this in another OT thread.
This OT is smaller and will fit better than the other unknown Philco OT I killed.

[Paul1453]

There are considerable issues to be worked out when trying to cobble together odd bits and pieces to make an amp.

I must be getting better at it, though.
I managed to modify the Norelco PS for use in this.
I mounted it in the housing, and transferred my breadboard circuit to this housing using point to point wiring.
I wasn't sure on OT placement, or oscillation issues I might encounter, so some things are still not bolted down yet.
Wired everything up, in positions I hoped would work well, and fired it up.
No smoke, no whistling or hum, and she made sound on the 1st try.  YEAH!
No troubleshooting required thankfully.  That becomes much more difficult in the cramped confines of a small housing.

I'll post a pic later, if you promise not to make too much fun of me.
I'm definitely not a mscaggs quality builder, yet.   
I still need lots of quality tools, like a drill press.   
But it does work, and should fit in the housing as laid out now. 

Maybe some of you experienced guys could give some tips to optimizing this circuit?
It still has the issue of not liking the pot cranked to the max.
It doesn't seem to go into oscillation, current jumps if oscillating.
It gets scratchy, and both current and output drop.
Might adding a simple grid resistor after the pot fix that?   

[shooter]

here's a schematic that's close to yours, a re-worked Gibson kalamazoo.  look it over and see what they did diferent than yours, maybe that will help, or not

[Paul1453]

Well at least it worked.   

I definitely need better power tools.
Using a dremel to make socket holes sucks.

I tried that circuit on my breadboard Shooter.
It just has a little tweek for a tone stack.
I had whistling issues from my bench PS.
I don't know why but the 6AQ5 doesn't seem to have those problems.
I love that little output tube.   

The problem I'm having is it works great up to about 8 on the pot.
Then it gets scratchy, and current and output drop.
Back it off to 7 and all is well.
I've used at least 2 different pots, so I don't think that is the problem.
On the breadboard I used the 3.9K cathode resistor, series a couple together to get that.
In this layout I just used 1 3.3K resistor, it is within 20% tolerance, it's a metal film 1% resistor.
Both values give the same results with the pot maxed.
Maybe I should have gone up to the 4.7K single resistor, still near 20% variance?
Part of the reason for doing such simple circuits is so I can fiddle with component values easily and learn what happens.
Not many things to change on this circuit.
Any ideas why it doesn't like it when the pot gets maxed?
Same results with 12AX7, 12AU7, 12AT7, and mixed 7247 preamp tubes.

[shooter]

it gets scratchy

scratchy can be many things, solder connection, leaky dc into signal path, plate R's....
Is it true scratchy or SE overdriven grit!  about 1/2 my SE type builds get really *ugly* dimed (8-10 on pot)
Have you *tacked-in* a NFB for testing?  Do you have a scope to see what's up? like from pre, from PA, from OT, etc?

[Mike_J]

I definitely need better power tools.
Using a dremel to make socket holes sucks.

If you go to Harbor Freight and buy their step bits, a center punch and a 1/4" pilot point bit you can drill just about any hole in the chassis except for the PT and AC openings.  They can be used accurately with a battery operated drill.  You may be better off buying a higher grade step bit for every hole except the eight pin power tube holes if you plan on making a lot of amps.  They are expensive but last a lot longer.  The larger step bit is very expensive everywhere but at Harbor Freight so I would just plan on replacing that one periodically.  The Harbor Freight bits do not hold up as long as the more expensive bits and aluminum seems to be easier on the bits versus steel chassis.


You are a very patient man if you are making your socket holes with a dremel which is useful to eliminate any burrs after the holes are drilled but probably not ideally suited to making socket holes.


Thanks
Mike

[Paul1453]

I did get an oscope recently.

I could redo this on my breadboard and use my scope on it.
Much easier to test and change things on the breadboard.

No scratchy anywhere else on the pot.
It could be SE overdriven grit, I'm not sure how that sounds.
I was thinking about adding NFB to see if that helped.
I was glad to hear that many of your SE builds get ugly dimed.
Made me feel better that maybe it wasn't just me screwing up something.

I didn't really make this to sell, but you know that anyone who would test this would instinctively dime the pot.
I could just use this as it is, aware of it's limitations.

[Paul1453]

I definitely need better power tools.
Using a dremel to make socket holes sucks.

If you go to Harbor Freight and buy their step bits, a center punch and a 1/4" pilot point bit you can drill just about any hole in the chassis except for the PT and AC openings.  They can be used accurately with a battery operated drill.  You may be better off buying a higher grade step bit for every hole except the eight pin power tube holes if you plan on making a lot of amps.  They are expensive but last a lot longer.  The larger step bit is very expensive everywhere but at Harbor Freight so I would just plan on replacing that one periodically.  The Harbor Freight bits do not hold up as long as the more expensive bits and aluminum seems to be easier on the bits versus steel chassis.


You are a very patient man if you are making your socket holes with a dremel which is useful to eliminate any burrs after the holes are drilled but probably not ideally suited to making socket holes.


Thanks
Mike

I don't know about that patient thing, maybe it's the opposite, I can't wait to get the proper tools to do it right.
A man's got to do what a man's got to do, even with all those Macgyver workarounds.   

[shooter]

many of your SE builds get ugly dimed

I'm not a player so it's ugly to me, when my Son and crew plug in, dime it, magic happens and they are quite impressed Go figure

[Paul1453]

many of your SE builds get ugly dimed

I'm not a player so it's ugly to me, when my Son and crew plug in, dime it, magic happens and they are quite impressed Go figure

Magic is happening until about 7 1/2 

Then it all goes to pot, and sounds like shit.
I don't think ugly is a strong enough word for it.
I don't think they would still be impressed with mine dimed.

Now if the pot maxed gave the sound of 7 1/2, and that was all you could get, I'd consider the problem solved!
I think then if somebody else tried the amp and dimed it, they would be quite impressed.

Maybe a 150K resistor added between the wiper and grid would make the pot act like that, and not change the sound too much?   

[shooter]

if the pot maxed gave the sound of 7 1/2

Been there also, had one guy say "it sound great till about 8".  So I ohmed the pot at 8, figured out what || (parallel) R I needed to make the pot at max = the value at 8 and walla, happy camper, again, go figure!

but If you have a scope, probe the signal path and i'll bet you find too much gain/clipping somewhere.  Make sure your signal source is set to the same value as your guitar at the input, or close, otherwise you're mixing apples and mangos

[Paul1453]

I can slap that circuit together on my breadboard rather quickly now.   

OK, started off adding 150K between the pot and the grid.
That stopped the pot issue, but changed the sound.
100K better, 47K better yet, 4.7K better yet, and down to 470 ohms now seems best yet.
No more pot issue, but even down to 470 ohms the circuit doesn't overdrive nearly as hard as without any resistance there.

How low can I go and still tame the pot issue, or just more trial and error to figure that out myself?
I can obviously go all the way to zero, as that is what it was with the pot problem.
I'd like to go as low as possible to tame the pot problem, and still keep as much overdrive as possible. 

[Paul1453]

As low as I could go, with a resistor measuring 12.5 ohms tames the pot problem.

It also tames the gain too.  She won't quite growl like she used too.
Tube rolling the preamp, now.

Maybe more growl needs to come from a pedal on the input instead?

The 12AU7 which sounded best to me with no resistor in, is no longer the best.
My different 12AX7s do lend a slightly different flavor.
Probably a toss up, or individual preference between the Bugleboy and the 12AX7WA.
12AX7 is definitely the preamp tube to use with the grid resistor in.

If I found a 1 ohm resistor to put in there, do you think it would still tame the pot problem?
A 1 ohm should give it as much gain as possible, without removing it completely again.   

[shooter]

That stopped the pot issue

I believe that why most here consider an R on the grid as best practice, I'm still learnin to, my current understanding,( which changes often ), the grid R helps stop some parasitic problems and maybe miller effect issues

[Paul1453]

That stopped the pot issue

I believe that why most here consider an R on the grid as best practice, I'm still learnin to, my current understanding,( which changes often ), the grid R helps stop some parasitic problems and maybe miller effect issues

Well my testing shows there is something going on which a little resistance appears to reduce or eliminate.

Now I'm wondering if I would get the same results moving the resistor to the grid of the 6AQ5.
Would that give the 12AX7 back it's growl, while still fixing the maxed pot issue?
Or is this problem related to having the signal amplified twice in the same tube?   

Back to the lab for further testing.