Top Cat

A new amplifier design

inspired by Norman Crowhurst's "Twin Coupled"

   

A real Top Cat!

Top Cat was designed to be both high performance and low cost. Toward that end, 10GK6, a low cost 6BQ5 equivalent, and 6DT8, a low cost 12AT7 equivalent, tubes were used. Power comes from 'junk bin' power transformers and the chassis is a recycled ATR8000 (an Atari computer peripheral. Mine still works. This case came from a 'donated' dead one that looked like Thor had attacked the PCB). OPTs are original SPECO T-7010 line transformers (no longer made).

Of course, the Top Cat topology is just as applicable to any tube and OPT compliment you may like.

The "as built" amplifier stage is as follows.

Input comes through a rotary switch selector into a passive balance/volume control network designed for 'constant volume' as balance is adjusted. A concertina phase splitter was chosen for it's near perfect balance and the input triode is bootstrapped to maximize gain and minimize distortion. Concertina output then feeds the 6DT8 drivers bootstrapped from the OPT primaries. 47k grid stoppers, limiting current from the concertina, ensure a soft overload characteristic. Finally, the drivers feed the 10GK6 grids through low impedance 10k plate loads, allowing a nominal 'positive grid drive' for a smooth transition into 'soft clipping'. Since cathode feedback from the lower OPT does most of the work linearizing the output stage only a relatively small amount of global feedback is needed and that is drawn from the OPT secondary back to the cathode of the input triode.

Since line transformers are not gapped extra care is taken to balance DC current. The 10GK6s use a Blumlein Garter modified with a resistor side chain to reduce current through the low cost trim pot. OPT DCR is part of the bias circuit so a build out resistor is included to compensate for the differing resistance of the two primary halves. Since the drivers are bootstrapped off the OPTs, meaning their plate current also goes through the them, they are garter biased as well. Bias is adjusted by altering grid voltage, via the trim pots, so balance is easily determined by measuring cathode voltage, which will be equal when current is equal.

A Blumlein Garter inherently tends to balance, the trim pots being for 'ultra' trim, so it is envisioned the initial adjustment should be sufficient for the life of the tubes but it should be redone when they're changed. That includes moving them from one socket to another so if removed for any reason, such as transport, one must make sure to return them to the same socket, or re-trim balance.

 

Power Supply

This implementation was derived from a 'spare parts' amplifier that included supply voltages not used, nor shown, here.

The 24 V relay and associated timer provides a 15 second B+ turn on delay.

Q1 is a low drop, large RC filter, 'cap multiplier'.

With the addition of a small series R drop the rectified 36V conveniently provides DC voltage for series pairs of the 10GK6s.

The 12.6 V "heater pulse" section is explained in "Other Useful Circuits" on the home page.

The B+ transformer output was a little lower than normal so it's bootstrapped off the 10GK6 heater supply to get another 24V.

Of course, most builders would use a conventional supply.

 

Top Cap under construction

Shown is the right channel wired with the 'high side' OPT on the underside with fashioned wood standoffs to allow wire passage underneath. Red "splotches" are nail polish used as a 'loctite' substitute.

 

CAUTION

The current SPECO T-7010, shown on the left, despite having the same part number, in no way resembles the original, shown on the right, and is unusable. In addition to the obvious physical differences, smaller and less iron, it doesn't even have the same taps!

Revisting the lovely picture at the top of this page, the rear power transformers are flat mounted so their magnetic fields are orthogonal to the OPTs, eliminating magnetic coupling. That is not so critical with the distant Top Cat OPTs in-between the power tubes (which is where I wanted them in the first place) but the original, conventional, design, which was never put into service, had Ebay acquired Harman Kardon OPTs located in the adjacent rear corners (because they wouldn't fit between the tubes). Those OPTs were retrofitted into Stealth AX so it's original SPECO T-7010s could be used to complete Top Cat.

The power transformer, on the left, is a 480/240VAC to 120VAC step down rescued from an OEM's 'obsolete stock' junk bin. Running it backwards results in lower than nameplate voltage because the regulation factor over wind is then an under wind, hence the DC boost off the heater supply previously described.

The heater transformer, on the right, was salvaged from a Standard Power Inc. 12 VDC linear who's regulator board had self destructed. For reasons not entirely clear they used full wave center tap rectification so what would normally have been an 18VAC, with full wave bridge, was a 36VCT transformer. I wouldn't have bothered with DC heaters, especially on power tubes, but had plenty of rectifiers from dead PC power supplies so, with the 'big ticket' items essentially free, why not? Besides, the "Heater Pulse Regulator," again from mostly junk bin parts, was a lot of fun and using 10GK6s saved $16 over the 'normal' 6V types (same reason they're in Stealth AX). Next time I'll probably use the 6HB6 since it looks like a 6BQ5 with almost double the gm, is only a buck each, and has been sucessfully tested in the Stealth AX breadboard.

I like to plug in portable MP3 players and cell phones from time to time so one of the inputs, a 1/8" jack, is conveniently located on the front panel next to the input selector.

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