Circuit & Layout Review for PP amp

[PRNDL]

I'm designing a low watt amp that uses two 6AQ5 output tubes for about 5 watts.

The design is based on the Matchless Spitfire with an extra gain stage, which is pretty much the same as a Fender Pro Junior.

I've built the amp without the extra gain stage and it sounds great for old rock tones, but I'd like to try it as a high gain amp. I'm also switching from point-to-point tag boards to a circuit board.

I'd love to hear your comments and suggestions!

PS
The layout sizes are not to scale.

[Chris - CMW amps]

If you want more gain I wouldn't use such TMB-style-tonestack (about 20 dB loss of gain)....

Maybe check out the Fender brown Deluxe 6G3 schematic for some inspiration?

[Bruce / Mission Amps]

Your voltages look funny to me and I'm trying to be a nit picker.
Is there really a 39v drop across the 390 ohm resistor?
That would mean your 6AQ5 screens and the two 12AX7s would be drawing abut +10ma. Is that right?
You have a 10v drop across your 5K resistor and I think a properly biased 12AX7 triode will draw about 1ma... I don't think yours will because you are using 2.2K cathode resistors.... 10v means your triodes are averaging about 500ma... that is cold and in some applications, could sound a bit harsh.
IMHO, for best tone, you probably should have the preamp tubes biased a little hotter and you'll see closer to a 20v drop there, not 10v.
Hmmm... the shared 1K on the PI cathodes will cool it a little too.
If the tubes were biased hotter, ... 5K and 4ma = a 20v drop.
If you want less gain going into the TMB, use a split load resistor on the plate of the first preamp tube and couple the TMB at their junction.
You could also do the same thing at the TMB recovery triode that drives the phase inverter.
By the way, I haven't heard this but I don't think you'll want a .047uF coupling cap to the PI... try something smaller to start... .0047uF to .01uF.

Also, I'd consider putting another filter section in there too.
R 26 can be 500@2w to 1K@2w, R27 could be 2k7@1w, then a new section with a 3K@1w to 5K@1w with a 10uF to 22uf filter cap.
Now take your first preamp tube's B+ voltage from the new filtered node.
The actual B+ voltage will not be that much lower but it will get the phase inverter off the same filtered node as the actual preamp.
And, you have no reference to ground for your filament supply so that will buzz too.
Since this is a cathode biased PA, put a couple 100 ohm resistors across the filament leads and connect those 100 ohm resistor's junction to your cathodes of the power tubes.

[PRNDL]

Thanks Bruce!

This is a modification of an old Dynavox amp that I converted to guitar. The amp worked and needed only a few changes, so I used the vintage caps/resistors/etc. I typed the voltages in during initial debug, so they are most likely incorrect.

The PT has a heater center tap, but there isn't one in the component with the schematic software I use.

I also changed the 6AQ5's to 6AK6's to drop the power -- it's still too loud, which is why I made a 6AK6 single-ended mini champ.

My plan is to put this amp into a 10" tweed combo, which means getting a new chassis (Hammond 1444-18) and making my first eyelet board. Hence the post.

The amp actually sounds fantastic, and I might decide not to change it (by adding an extra gain stage).

BTW
Is there a preference for either the G10 Garolite or old-fashioned black fiber board (similar to Fenders)?

It looks like the fiber board might be easier to work with (which might be good for a prototype), although it won't meet military specs.

[Bruce / Mission Amps]

Thanks Bruce!

This is a modification of an old Dynavox amp that I converted to guitar. The amp worked and needed only a few changes, so I used the vintage caps/resistors/etc. I typed the voltages in during initial debug, so they are most likely incorrect.

The PT has a heater center tap, but there isn't one in the component with the schematic software I use.

I also changed the 6AQ5's to 6AK6's to drop the power -- it's still too loud, which is why I made a 6AK6 single-ended mini champ.

My plan is to put this amp into a 10" tweed combo, which means getting a new chassis (Hammond 1444-18) and making my first eyelet board. Hence the post.

The amp actually sounds fantastic, and I might decide not to change it (by adding an extra gain stage).

BTW
Is there a preference for either the G10 Garolite or old-fashioned black fiber board (similar to Fenders)?

It looks like the fiber board might be easier to work with (which might be good for a prototype), although it won't meet military specs.

Ah-ha.. OK. I still use the greenish brown G10 for all my prototype boards... by the way the cost of the black "garolite" type material has gone up like 300% in the last year.
Except for tubes and a few minor component values, yours is almost a knock off of a newer project of mine, the three tubed 8-10 watt "Little Vixen" amp.
Just goes to show you how hard it is to come up with something new again.

If you want more gain, consider an extra input jack with small FET (J201) preamp running at about 18vdc in front of your first stage.
It is easy to get one to make about 12dB to 18dB gain.
Use a little 250k audio pot to drive the grid of your first preamp tube.
It will turn your phase inverter inside out.

[PRNDL]

Just goes to show you how hard it is to come up with something new again.

I agree!!!!

Every now and then I think about changing the first stage of the Matchless Spitfire into an Aikido, just to see how it sounds. Someone else tried it and stated that although it's great for HiFi, there's not much benefit for guitar.

At this point, I'm considering whether to use the Spitfire tone circuit, which will increase the gain (by lowering the insertion loss).

by the way the cost of the black "garolite" type material has gone up like 300% in the last year.

I'm wondering whether the tube guitar amp business is entering a phase like what happened in the '70s where increased costs caused prices to skyrocket. Major manufacturers cut costs with lower quality parts and many smaller companies went out of business.