Dogzilla Amp Logo
Finally done!

Dogzilla is my second "big" project; a large (over 200W RMS) valve amplifier primarily designed for maximum flexibility as a guitar/instrument amplifier. Description, design specs and in-process photos on this page, view performance specs, download the full schematic, or jump to the section-by-section article using the navigation bar:

Part 1:
Part 2:
Part 3:
Part 4:
Part 5:
Part 6:
Part 7:
Part 8:
Part 9:

Original block diagram
Original Block Diagram

Progress to date:

(November, 2001)
The chassis layout has been designed, and the 17" x 14" x 3" steel chassis is ready for drilling and hole-cutting. There's no backing off now, Dogzilla will live!

Dogzilla ready for drilling
Dogzilla Chassis ready for drilling

(Later that month...)
Most of the holes are drilled, including the pesky big round meter hole. Couldn't resist stuffing a couple parts in just for the picture.

Sea of Holes
Sea of Holes

(December, 2001)
It's actually starting to look like a piece of gear. Most of the chassis parts mounted, except for the iron. Those pretty gold vent-hole screens came from the local paraphernalia shop. Sure got a funny look when I asked for 20 of them....

Filling the Holes
Filling the Holes

(January, 2002)
The iron and the metering switch box are now mounted. Far left: 5V filament transformer for rectifiers, and the B+ power supply filter choke. Center: balanced line input transformer. Right: two Hammond 1650R output transformers (the old style, before they started putting those stupid knock-outs in the bells). Also the 6V filament transformer, and the wee "small amp emulator" OPT (hidden by the meter box). Underneath are four more transformers (filament supplies for pass regulator, control supply, and auxiliary B+ and negative bias supply).

More mechanical additions
This thing's getting heavy!

(February, 2002)
Finally, almost all of the ruddy mechanical stuff complete. I was getting tired of metal filings and shavings everywhere. Underneath, there's a dozen 8-prong terminal strips, the bias control board, and barrier strips for power and speaker signal distribution, ready to start the major task of wiring it all up. Here's a sneak preview of what it will look like when complete. The only thing I'd change in the layout would be to move the input section to the other side, away from the HT power supply. (I'm hoping to get away with it by shielding and insulating the input stuff within an inch of its life.)

Mechanics Done
Mechanics Done! (whew)

(April, 2002) - DogZilla Lives!
All the mechanics, and much of the wiring complete. Opted out of the 866AX rectifiers originally planned, mainly because my filament transformer arced over and I couldn't be bothered to replace it. Also dropped the individual cathode bias adjustments, which turned out to be more of a nuisance than they were worth. In their place is a small "monitor" speaker scrounged from a PC "multimedia" speaker, with a large dropping resistor to aid in debugging while on the big dummy load.

All the power supplies, metering, PA, and driver sections complete and tested. Burn-test (one hour at continuous 200W RMS into a dummy load) successful, no smoke or flames!

Click on the "start" link below to follow through the circuit descriptions. While I think it unlikely that anyone will ever build an exact copy of DogZilla, there are many circuit details and other information that may be of use to DIY amplifier builders. If you'd like to see more pix of Dogzilla, click "more pictures" button.

(May, 2002) - Tone Control and Limiter done
Recent addition to the article is Part 6, detailing the active tone control section. Block diagram modified slightly (not updated yet, as is still subject to possible change).

Also started on the performance measurements of the portions completed to date. Click here for a report.

(August, 2002) - Small Amp Emulator, Compressor, Marshall Tone Stack and Line Amp done
This is detailed in Part 7, detailing the small-amp emulator and compressor option. Part 8 details the bypassable "Marshall" tone stack, and line amplifier.

Also updated the schematics to indicate maximum peak output swing (in blue) and required input level (also in peak voltage) for full output (in red).

(September, 2002) - Input stage finished, project "done"
Part 9 covers the variable-mu pentode input stage. Scrapped the planned lo-z input; even a shielded mic transformer on the chassis picks up so much hum that the notion was written of as a Bad Idea.

While the project is nominally complete, there is the inevitable fine-tuning left to do. Particularly, the amp still has hum issues; I suspect I'll have to add mu-metal shielding, since much of it appears to be the result of magnetic induction from the power transformer and filter choke.

(November, 2002) - Some updates
I found that the doubled-up output transformers really isn't necessary. Dogzilla works just fine with "just" a single Hammond 1650R. Just goes to show how conservatively rated these things are; I'm still getting well in excess of 200 watts out of a single transformer rated at 100 watts (albeit for hi-fi use), with no sign of overheating. Woohoo! Part 4 has been updated accordingly. Freed up chassis space! (Hm... maybe a tremolo or other effects board might fit there...) Added shielding around the dual preamp board, and on the tubes on that board.

Dogzilla as of November 2002
Dogzilla as of November 2002

Part 7 has been updated somewhat, to reflect the use of a dual 100k pot for the "Timbre" control. Previously, there was some bleedthrough from the small-amp emulator, even with the control turned all the way to "clean".

The full schematic has also been appropriately updated.

I have also updated the chassis layout diagrams. If you want a copy of these for reference I'd be happy to send them to you. While it's unlikely that the layout would work exactly for your available parts, it could give you a starting point for your own project.

More Pictures Article Start!
More Pictures Article Start!


You may contact me regarding this project at I'd be happy to hear from you, and any bug reports or suggestions for improvement are always welcome.

To find out what other kinds of trouble I get into, check out my sites listed below:

Back to Tube Projects Main Page
My main Dogstar Music site
Fred's ZX81 retro computer page
My Theremin Exploits

Original music on
... and on Soundclick. Nice bio.
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