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Author Topic: Need Help To Brighten 60's Heathkit Amp-Schematic Included  (Read 7852 times)

dajamc

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Need Help To Brighten 60's Heathkit Amp-Schematic Included
« on: November 10, 2012, 02:22:53 PM »
I have a Heathkit TA 27 that has a great clean with very little treble, but general lack of brightness greatly effects any slight overdrive or fuzz used.  The problem gets worse when to achieve anything close to brightness, the bass must be turned all the way down, killing any "chunk" or low end dynamics.  I'm looking for any possible mods to brighten up the amp but not lose the dark warm tones already there, and allowing more range of use with the bass and treble.  I have a link to the schematic below.




http://ctgelectronics.weebly.com/uploads/3/1/6/6/3166248/ta-27_schematic.pdf
« Last Edit: November 10, 2012, 02:24:30 PM by dajamc »
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J M Fahey

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #1 on: November 10, 2012, 09:06:47 PM »
Add a "bright" capacitor.
.022uF (22nF) from volume control "hot" lug to centerpoint lug.
Solder it directly on the pot lugs .

phatt

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #2 on: November 11, 2012, 07:46:57 AM »
What about a Fet buffer in front? Might improve the input Z.
Phil.

Roly

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #3 on: November 11, 2012, 08:08:31 AM »
Whatthe?  22k in series with 24k?  Ye gods I've seen outputs with higher impedance than that!

Absolutely Phil, an AMZ FET buffer would transform this one.



To give it some serious stoke I've been using R2 = 8.2Meg and R3 = 4.7Meg with MPF102 JFET's.  This would go right after the join of the two 22k's at the input, and R5 100k can be omitted.  It'll run off the +16V supply just fine.   :dbtu:
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Kaz Kylheku

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #4 on: November 11, 2012, 11:19:03 PM »
This design is from the 1960's?

How large would that 2500 uF coupling cap to the speaker have been in 1960-something?
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Kaz Kylheku

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #5 on: November 11, 2012, 11:58:57 PM »
Adding  FET buffer to this thing is a waste of effort. Just let it be what it is: a power amplifier with EQ, tremolo, reverb, and a line-level input.

You need something before this anyway if you want distortion or other effects. That something can provide the instrument-level input.

On a different note, that stupid C103 capacitor by the power switch should go, along with the stupid DPDT power switch.  A
three-prong cord should be used.

What the hell is with that? In one position, the cap is connected to hot, in the other to netural. If that cap fails short, you have a problem: one position of the switch will connect your circuit's ground to hot. It is a form of  "death cap" that should never be used in modern designs.
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Roly

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #6 on: November 12, 2012, 11:27:13 AM »
dajamc - this amp uses a type of tone control called a Baxandall, and while these are almost universal in stereos they are not popular with guitarists, and you may be rediscovering why.

From the circuit alone there is no obvious reason why this amp should lack trebles, and I have to wonder about the speaker you are using with it.

There is also a potential problem unique to Heathkit anything - they were owner built, and as such it is quite possible that there is a wiring error or component in the wrong place that won't be apparent from just looking at the circuit.  Unlike commercial amps you need to get the circuit and a bright light and go over it in detail and check that it actually has been built correctly.


Kaz - you seem unaware that the "death cap" and three position mains switch were standard kit on American guitar amps for many years, certainly in the 60's when this was built.  Most of the Fender amps I have worked on in Australia have a three position mains switch, albeit modified on import for our LEN system.

I don't know where you get the idea this is a line level input amp since the original spec for this and its mate the TA-16 show an input sensitivity of 25 and 35mV on the normal and reverb channel respectively.

http://ctgelectronics.weebly.com/uploads/3/1/6/6/3166248/1498555_orig.jpg?256

A FET buffer is a very simple mod and would turn this into something resembling a real guitar amplifier.
If you say theory and practice don't agree you haven't applied enough theory.

dajamc

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #7 on: November 12, 2012, 01:45:31 PM »
Thanks!  I will put the buffer in shortly.

I have a wild question to ask:  Would it be possible to add the reverb and tremolo units in the Heath to a 5e3 clone? 
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Enzo

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #8 on: November 12, 2012, 06:29:39 PM »
Consider that perhaps the lack of treble is a problem needing to be repaired rather than a modification to the circuit being required.   If the tweeters were blown in your PA stacks, you COULD increase the high end on the graphic to try and compensate or you could fix the tweeters.  Same here.

Worth checking.

COnsidering the age of these things, and I have one next to me here, it is entirely possible a fistfull of new caps might bring it back to its sparkling glory.

dajamc

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #9 on: November 13, 2012, 10:42:50 AM »
I've also heard the ceramic Jensen reissues can be nasally.  How much could that speaker be involved, and what would be your speaker suggestion?
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J M Fahey

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #10 on: November 13, 2012, 01:43:18 PM »
The Jensen is a bright speaker, your problem does not lie there.
That Heathkit is "not a guitar amp" but a "hi Fi" amp with tremolo and reverb added.
Input impedance, Tone Controls and Equalization are all wrong for a "Rock" amp.
Simple as that.
Yet (as you already know), it works for Jazz type sounds.
In fact, the Polytone amp is *exactly* that: clean flat preamps, a Baxandall tone control, a flat Hi Fi type SS power amp, a guitar speaker.
Although it adds a Bright/Flat/Dark switch and has much better 330K input impedance.

By the way, did you try the bright cap I suggested?

Kaz Kylheku

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #11 on: November 13, 2012, 02:17:19 PM »
Kaz - you seem unaware that the "death cap" and three position mains switch were standard kit on American guitar amps for many years, certainly in the 60's when this was built.  Most of the Fender amps I have worked on in Australia have a three position mains switch, albeit modified on import for our LEN system.

You're right. I know about death caps, but I don't know all their specific configurations like this DPDT switch. In any case, it's not up to modern safety standards. Safety first, then input impedance, and all the rest.

I heard stories about how players used to be electrocuted by their guitar amps in the olden days, which should never happen.

Quote
I don't know where you get the idea this is a line level input amp since the original spec for this and its mate the TA-16 show an input sensitivity of 25 and 35mV on the normal and reverb channel respectively.

Sorry, I should have said line impedance. The level is really neither here nor there; with the volume attenuator we can adjust the output of the first stage all the way down to zero.
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Kaz Kylheku

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #12 on: November 13, 2012, 02:46:52 PM »
The Jensen is a bright speaker, your problem does not lie there.
That Heathkit is "not a guitar amp" but a "hi Fi" amp with tremolo and reverb added.
Input impedance, Tone Controls and Equalization are all wrong for a "Rock" amp.
Simple as that.

I think the Baxandall tone control could work, if it was "primed" so that it has a mid-scooped EQ curve with a bit of emphasis on the treble, when the controls are in the middle position, and also playing with the location of the knee frequencies. The skew of the controls could be achieved by splicing some series resistors with the pots. For instance, the middle could be set up so that it cuts extra deep when turned left, and only goes to a flat midband when turned right (which, for guitar, sounds like a honky mid boost).

Another thing that could help this amp might be a bit of current feedback in the power section. It has only a voltage feedback line. That alone will kill a bunch of sparkle and life. Current feedback will add significant high end.

Speaking of feedback, a presence control could be added in the feedback line for extra control over the high end.
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galaxiex

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #13 on: January 01, 2017, 11:08:16 PM »
Happy New Year!  :)

... and sorry for necro bumping such an old thread.  :-[

I have a Heath TA-16 amp that has the very low input impedance just like the TA-27 in this thread.

Finally got around to tinkering with this one.

I added the input buffer almost exactly like our dear departed Roly suggested. (Post #3)
(actually 2 of them, one for each channel.)

This thing came ALIVE!  8)
Almost have to fight it off with a stick!  ;)

Mine has a couple of Fender replacement speakers that I thought were sh*t.
That obviously was not the problem.
Amp sound great now.
I also replaced all electro caps, and put box film caps in the preamp section that had a half dozen 10uf electro's in the audio path.
Got rid of the "death cap" and the line reverse function. Added a 3 wire grounded power cord.

There are several other forum threads around on these old Heath amps.
A common problem seems to be a non-functioning tremolo due to a burnt bulb in the potted LDR assembly.
Thankfully mine still works!
The reverb tank was dead however.
I replaced it with a MOD tank that matched the type numbers on the old tank.
Reverb sounds quite good.  :)

This one's ready to Rock!

Edit;
added a few pics, and forgot to mention, it came with the original foot switch!  :o
oh, and I replaced the tattered and stained grill cloth with some Fender style that I had on hand.
« Last Edit: January 07, 2017, 01:56:17 PM by galaxiex »
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joecool85

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Re: Need Help To Brighten 60's Heathkit Amp-Schematic Included
« Reply #14 on: January 06, 2017, 01:19:26 PM »
Happy New Year!  :)

... and sorry for necro bumping such an old thread.  :-[

No worries.  Also, this came out great!  Too bad no one makes a kit like this anymore.
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