ss amp, no distortion... add a 2nd volume to get gain?

[darren]

i have an old heathkit ta-16 solid state amp that actually sounds pretty good, but id love to hear it with some distortion. my fender princeton 1/12 has two volumes- its called gain and master volume i believe. is it possible to add another volume pot into the mix to be able to get distortion without turning the poor thing all the way up and rattling the walls? if so, could the volume pot  be a push/pull pot for bright/warm?
Darren

[el nino]

you mean this?
i don't know your problem, but any senior in this forum will answer you

[J M Fahey]

Short answer: no.
The amp was never meant to distort and was designed accordingly.
Does not have the necessary gain *at all*, and there are no untapped gain stages which can be turned loose.
You can't even add a couple diodes just before the power amp input , because it works like a virtual earth mixer, and by definition there is not voltage swing there.
Build a good distortion pedal, that's what they were invented for, and it will sound 1000 times better than any distortion you might kludge into this.
Check GGG and other homemade pedal sites which provide full info, ready to transfer PCBs, etc.
The MXR Distortion+ is very good and simple to build.

[darren]

ok, thanks for the reply. kind of a bummer, but ill accept it.  and a pedal would be cool to have. too bad, i like the airy sound of this amp.

[darren]

the tremolo stopped working. neither the depth or speed knobs make any noise at all, the other pots are a bit noisy. actually this amp sounds extremely dark to me. i just did a side by side comparison with a les paul plugged into it, and then a princeton 112. the fender was much "crisper" and the highs all but disappeared on the heathkit amp. are there any mods or tune-ups to do on these amps? or are they just not worth the effort? i really like this old amp. i would like to clean up the sound, take away some of the bass and make it sparkle a little. having the tremolo would be a huge plus too!

[sim0n]

The tremolo section shows a lightbulb and ligh dependant resistor. The most obvious to thing to check would be if the bulb has died.

As for the crisper sound: it might not be the amp but actually be the speaker and/or cabinet responsible for this. So keep that in mind before you redo the entire thing.

[darren]

ah, makes sense! the cabinet looks to be chip board. and the speakers, there are numbers printed on them but no name that i can see. ill check for a lightbulb, but i think there is only one next to the on switch. anyone know why  the on switch is an on-off-on switch? i dont hear any difference between up on and down on. thanks for the tip simon

[bobhill]

Look on the schem to the left of Q13 (lower right) and you will see the bulb and ldr for the tremolo. If it is a separate bulb, just change the bulb, otherwise if it is a single unit wrapped together you will need a replacement for the unit. This is a common problem with old Fender blackface amps and easily fixed.

[Roly]

The light bulb in the tremolo circuit is in the collector of Q13.  If you follow the wiring of the tremolo depth control it should lead you to Q13 (presuming it isn't already marked on the PCB).  The collector of this transistor goes to the lamp, but it will be in a light-tight container of some sort with the LDR - Light Dependent Resistor, R56.

If you don't have any voltage on the collector of Q13 it's a fair bet the lamp has failed.

This is often a small plastic tube with two wires coming out of each end and in Fender amps is often called the "roach" since it looks a bit like one.  These sometimes have a brand name and type number, but more often than not you will need to do some detective work to determine a suitable replacement lamp, and a bit of creative handycraft opening it up, installing a new lamp, and making it light-tight again (I normally use black heatshrink tubing for this).  Be careful not to damage the LDR in the process or you will have two lots of detective work to do.


First thing I'd do is connect the amp to a different speaker cab and see if that changes the brights.

Two things strike me about this circuit, firstly the input impedance is quite low and this will tend to dull-off any passive guitar.  Secondly there is a cap at the input to the main amp, C16 0.1uF, across the audio path which may be "tone sucking".

To address the first I'd try a FET buffer between guitar and amp, or build some sort of grunge/buffer stomp box; and I'd be tempted to see what happens if the value of C13 is reduced, say to half, 0.047uF.  It's possible the amp may become unstable, but it's equally possible it may be just what you want.

The On-Off-On switch is a line reversing switch peculiar to un-grounded American amps (and removed everywhere else in the world).  It has the same function as reversing the plug in the wall to select the direction that gives the least noise pickup.  The rest of the world uses three wires to provide a safety ground on the exposed metal parts and minimise noise pickup.

HTH

[bobhill]

And for a distortion pedal you might enjoy with this amp, this circuit is based on Jack Orman's "Son of a too expensive boutique tube screamer" with the addition of a few other clipping arrangements. Without the clipping variations it is what Lovepedal "borrowed" for his Eternity model pedal. I have built a few of these over the last few years and it is voiced to sound pretty much exactly like my old Fender Bandmaster with everything dimed. It works well with both tube and SS amps.

[bobhill]

Mods, if you want to move this over to the Preamp and Effect section where it probably belongs...

[Roly]

Aww yeah, nice one Bob; high enough input impedance and a few different types of crunch and grind to play with (and I love J.O.'s name for it - DIY and save heaps).   :dbtu:

[J M Fahey]

Agree and add.
You have what you have.
This is a well made amp, designed by a serious company (Heathkit).
Problem is, it was not designed by longhaired hippies with guitars hanging from their necks but by short cropped hair, white short sleeved shirt dressed Electronic Engineers, with *TONS* of experience designing excellent Lab and Radio Amateur equipment and advanced Hi Fi amps, who probably listened to Henry Mancini , Mantovani and Sinatra at home. (google them)
So this amp is a Hi Fi amp, with tremolo and reverb added.
It's flat (none of the typical bright EQ of most guitar amps), has Baxandall tone controls, and designed *not* to distort.
Tweaking it will not solve that, use it as is (or with a pedal).
By the way, it's an *excellent* amp to reamplify any modern modeler pedal , or to play Jazz.
And yes, the very low input impedance kills sparkle ... but even if you solve that, the rest of the amp still does not help.
That .1 cap seems to be attenuating highs coming through R24/25 (both channels) but it *also* is the point that receives negative feedback (through R28) so it's a "virtual earth node".
I bet that .1 cap does not affect frequency response (at least within the audio band) but serves a stability purpose.
So , in a nutshell, keep it as is, experiment with pedals and use it to test them, it's the best for that.

[darren]

wow! thank you very much indeed, guys! very nice forum you have here. i believe i will try my luck with another amp, as this one doesnt sound like it was designed to fit the bill. my fender princeton 112 plus, is it mod-able? what, if any, mods do you gents prefer on this model?

[bobhill]

Oops, put the wrong layout with it. The one posted was my first attempt to add a bax style tone stack to it (that failed), so ignore. Here is the correct layout. Mods, could you delete the layout from the previous post? Pretty please :tu: