Traynor DarkHorse 15H Tonestacks

[Boots Deville]

Recently kagliostro posted a link to a collection of Traynor service manuals. (thanks for that!)

I've been looking over the Darkhorse 15H schematic:
http://www.traynoramps.com/downloads/servman/smdh15h.pdf

...and trying to make sense of the tone stacks.  The basic idea is this - there are two dual-ganged 100K pots (Treble and Bass) along with a three position toggle (on-on-on) controlling relays which change the tone stack component values and also its location in the signal chain.  They call the settings "Brit", "USA" and the middle position lifts both stacks for maximum gain which they call "Pure".

I'm intrigued by the tone stack as I haven't seen anything like this and I'd be curious to know how it works, from anyone who has played one.

To get my head around it, I redrew the schematic "dumbing down" the tone stack switching so I could see what was going on a little better.  I've attached that below.  I'm not 100% sure I got it right, but I think it's close.

So is this tone stack a new design, or does it exist elsewhere in amp history?  How effective is it?

Also notice the 100K resistor in series with the bright cap on the gain control.  What do you think the design goal is there?

[Geezer]

Also notice the 100K resistor in series with the bright cap on the gain control.  What do you think the design goal is there?

To reduce the effect of the HUGE 470p bright cap. The largest cap I use in that position is usually 120p, but more often 47p to 68p.

$.02

G

[Boots Deville]

Also notice the 100K resistor in series with the bright cap on the gain control.  What do you think the design goal is there?

To reduce the effect of the HUGE 470p bright cap. The largest cap I use in that position is usually 120p, but more often 47p to 68p.

Yeah, I had that thought too, but then why not just use a lesser cap?  Lesser parts count, etc.  Would that resistor alter the taper at all?

[PRR]

> 100K resistor in series with the bright cap on the gain control.  What do you think the design goal is there?

Resistor limits the _height_ of the boost.

> usually 120p, but more often 47p to 68p

Sets the _start_ of the boost.

Both _in conjunction_ with all other parts in the area.

Their values without the 100K would continue to rise in the highest octaves of guitar: ice-pick and hiss. 100K leaves the heavy boost in the harmonics zone without TOO much of the brittle hash.

Note that this network has no effect at full-up or full-down.

> Lesser parts count, etc

Phooey. What an over-stuffed PCB. Obviously they were not trying to reduce part-count.

[The_Gaz]

Thanks a lot for the graph, PRR. I think this is a really smart way to tame the brightness that goes a long with a bigger cap, while keeping the voicing the same. I just tried it on a Plexi's bright channel, and am really happy with the results. I used a 470k resistor in series with the typical 470p bright cap, and it cured the 'icepick' while retaining the overall tone.

I had previously tried to reduce the bright cap on the bright channel to 100p, which helped, but made the voicing to similar to the dark channel (where I had already added a 100p to get rid of some muddiness). The 470k in series with the 470p really did the trick. Thanks to the OP and PRR!

[HotBluePlates]

The tone networks are called "T-filters," named in a manner not unlike pi-filters.

The treble (high-pass) network is made of 2 series caps with a resistor to ground; the bass network is the exact opposite (has a cap to ground). To be more exact, the high pass T is a "bridged-T filter" as there is a cap shunting the resistors from input to output.

You see these types of networks in "engineered amps" from Gibson and some others. Treble/bass is normally all you get, and the networks are often fixed with a supplementary tone control.