London Power Tut 5 Power scaling in Standard amp

[Lindz]

I am taking a crack at building my first amp totally from scratch - layout, boards, chassis, cab etc.

Amp will be fixed bias and I want to scale it - probably will scale the whole amp since that seems to work well in my other builds with various scaling kits.

Still mucking with the design - I am not building the amp featured in the book per se but want to cherry pick some ideas from it and everywhere else. I know there are a few power scaling kits out there but I want to try building my own and fit the components on the board etc.

Bought the Tut series books and in Tut 5 Kevin has the standard amp build featuring power scaling and sag controls with detailed schem etc.

I am curious if anyone has implemented this particular type in a build and what they thought.

It seems to be fairly simple vs some of the newer kits I see and own (Dana vvr3, London Power PSK1) - does not seem to have a bias tracking pot etc.. It also seems to have only one mosfet while some higher power scaling kits I see feature 2 for higher power amps including the newer London Power kits.

Since that amp features switchable fixed/cathode bias I am not sure how the bias tracks and though I am reading and learning rapidly my chops are not there to understand the nuances well enough, so I though I would post here for comments

I'm looking for any helpful input on that particular scaling circuit or other home brew implementations that worked well for people

thanks!

[John_P_WI]

PM Ian / Gingertube he has built this. Kevin's newer ps kits are simpler than the "standard's". I believe that you can use any of his newer kits for fixed bias for either fixed or cathode bias applications.

[gingertube]

Yes,
I built one for my friend Neale.
Minor variations from the schematic in TUT5 ONLY.
1) I used a quad of vintage 6V6G (the coke bottle shaped) output tubes in to a spare 50W Marshall (Raa=3K4) Output tranny I already had. That impedance is a little low for a quad of 6V6 but worked well.
2) I powerscaled just the Output tube anodes and screens, left fuull voltage to the Phase Inverter.
3) Did extensive tests with different Reverb Tanks and ended up with the Accutronics 6 spring long delay.
4) The published switching card circuit has a problem in that with modern ultrabright LEDS the LED for the off channel does not go completely out, I did my own design for that board.
5) I changed the TRIODE/PENTODE and CATHODE/FIXED bias switching to have one each of these switches for each pair of output tubes. Neale regularly runs the amp with one pair of 6V6 in TRIODE Mode, CCATHODE Biased and the other pair in PENTODE MODE/FIXED Bias

6) Did not implement the "Body" Control but did do the SAG/COMPRESSION Control.

I did a speaker box for him using Kevin's Loose Tune box design.
Basically that just means its a closed box with baffle cut for 2 x 12" speakers but only one speaker fitted. Speker I used was a Weber P12B Alnico "Bulldog". The Cabinet dimmensions followed the HIFI box recommendation of 1 : 1.618 : 0.618 ratio.

CAUTION: Earlier copies of the schematic had wrong pin numbers on one of the 12AX7 triodes (pins 6,7 & where wrongly labelled.

Cheers,
Ian

[gingertube]

Here I am giving it it's final acceptance test before delivering it to Neale.

[gingertube]

And the final Amp.
Not a great pic.
Left hand side to right hand side
Switches are 10M/1M Input Impednace swictch to allow use of a ceramic passive pickup and the Clean/Lead Channel switch.
Two input jacks are the Guitar input - top and the footswitch input - bottom
Then there is the channel select switch - this must be in the Clean position for the footswitch to work.
VOL, BASS, MID, TREBLE, REVERB for Clean Channel across the top
GAIN, VOL, BASS, MID, TREBLE, REVERB for Lead Channel under those

MASTER VOL, DRIVE COMPENSATION, SAG, POWER SCALE Knobs for teh Power Amp Section at the Right.

Effects Send / Retun and Line Outs are on the back as are the Cathode/Fixed bias Switches and the Pentode/Trode Mode Switches

[gingertube]

It is not really a simple build.
Here is some of the guts during construction - Preamp Board part wired
I have more photo's of construction - yell if you want me to post a few more.

[Lindz]

Thanks for the reply and pictures (and PM)