# Heathkit TA-27 Tone Question



## Dasher (Feb 3, 2011)

I picked up an old, non-functioning Heathkit TA-27 (schematic) a few weekends ago and have been working on bringing it back to life. From what I can gather, these were sold as DIY amp kits in the late 60s to 70s. I was told that this specific amp was originally built in 1968...an oldie! Anyways, here's what I've done so far to the amp:

1. Replaced all of the electrolytic capacitors and beefed up the first filter cap (C104) and output cap (C22) to 4700uf. This removed power supply hum.
2. Replaced the first preamp transistor (Q2) and reverb amplifier transistor (Q5) with 2N5089s to remove the majority of hiss from the amp.
3. Replaced the power cord with a 3 prong cord for safety.
4. Replaced the no longer functioning bulb/ldr combo in the tremolo with an LED/LDR insulated in heat shrink. The tremolo doesn't work perfectly (it doesn't really kick in until the tremolo depth pot is around 5 on the dial) but it's good enough for me right now. 
5. The original builder grounded some of the shielded wire at both ends, creating ground loops and inducing some hum. I clipped one end of the ground shield where this was the case. The amp is now quiet enough for me!

So as it stands right now, the amp works pretty well. The transformer hums a bit, but not enough to warrant a replacement. This is my first amp with spring reverb, so I'm really digging that!

Anyways, on to my question. The bass control doesn't really have much effect on the sound of the amp. I've checked the wiring of the tone stack and everything seems to be correct. Does anyone have any ideas for some modifications that could be made to the tone stack to allow for more tonal versatility out of the amp? Thanks!


----------



## epis (Feb 25, 2012)

Hi, about your bass control, did you check out the pot ? Did you replace C9 and C12 ? Problem could be there.
When you replaced bulb/ldr did you include resistor in series with LED ? What type of LED did you use ? I would suggest bright led or original small 6V bulb, they still have it at radio shack. Cheers


----------



## Dasher (Feb 3, 2011)

Thanks for the reply epis. I haven't unsoldered the pot from the circuit to make sure that I'm getting the correct resistance across the lugs, so I'll have a look at that. I did replace C9 and C12 with 10uf electrolytics. 

As for the tremolo, I did try a resistor in series with the LED. I found that anything greater than 1k ohm caused the oscillator to shut off. Values less than 1k didn't have a noticeable effect so I just left a resistor out of the equation for now. I used a basic, medium brightness red LED...I'll have a look around to see if I can find a 6V 25ma bulb to see if I get better results with that.


----------



## mhammer (Nov 30, 2007)

Well, so far your instincts are good. An amp that age is bound to have some dried-up electros. I have a TA-16 and TA-17, and the input stages are indeed made less hissy with a change from the 3391 to a new 5089. THe bulb/LDR module was also fried on my TA-16, and still awaits proper replacement.

A couple of things to note. First, the spectral match between LDR and LED may not be optimal. A lot of LDRs seem to respond to shorter, rather than longer wavelengths, so red will be a little too long in many cases, yielding less responsiveness from the LDR. Yellow, or white may be better choices. Second, you may want to consider using a superbright LED (min 3000mcd), so that you can adjust the current-limiting resistance from some higher value down to "just right".

Finally, I have absolutely no idea if the LDR/bulb properties were selected to provide a particular feel. Bulbs take longer to turn on and off than LEDs, and LDRs can have particular dark/light change characteristics. In tandem, a bulb and LDR can yield a particular quality of light/dark transition that is the equivalent of a particular LFO waveform. The LFO itself may produce something more generic, but the bulb and LDR turn that LFO output into something different.

I don't know if that is the case with the Heathkit tremolo circuit, but if the LED gives you something unsatisfactory, it might be time to look for bulbs. Obviously, job 1 is to get the thing at least working. But keep the bulb-vs-LED thing on the back burner.

Is the amp's current sound too bassy, or too thin?


----------



## epis (Feb 25, 2012)

Hi Mark, I have very good experience with 5mm super bright leds, both red or white. I'm using it in combination with some old (no name) ldrs from Addison I got before they closed the store in Ottawa. They are <1K/>5M if I remember well, combo works fine as replacement for VTLs in channel switching circuits or in low voltage tremolo circuits.
I didn't play with it in blackface Fender tremolo circuit. You're welcome to get a few to experiment with it.
Dasher should try super bright led, I assume. This combo should give a proper load for his LFO as replacement for 6V/25mA bulb//ldr.


----------



## mhammer (Nov 30, 2007)

I've got wads of LDRs and superbrights of various colours to tinker with. The problem is that the "time" bin in my parts drawers seems to be empty. :-(

Too many projects. But thanks for the offer.

Having been through Tweed many times, I gather there is nowhere local to buy components. Where do folks in the greater Belleville area go for parts?


----------



## epis (Feb 25, 2012)

mhammer said:


> The problem is that the "time" bin in my parts drawers seems to be empty. :-(
> 
> Too many projects. But thanks for the offer.


You're welcome. And I do understand you completly, after you reach 50s, that " bin" is getting empty every next year faster and faster


----------



## Dasher (Feb 3, 2011)

Thanks Mark and Epis.

I have a couple of red super bright LEDs (no other colours of the super bright variety in my parts box right now, unfortunately) so I may give one of those a whirl when I have some time. I'm not too concerned about the tremolo right now though...I rarely use it.

What I'd like to focus on right now is getting a bit more bass out of the amp. Could this be achieved by increasing the value of C102?

As for where I go for parts, nowhere locally. I buy everything online .


----------



## mhammer (Nov 30, 2007)

While they have limited stock compared to some other places, I recently dealt with DIPMICRO in Niagara Falls, and was very pleasantly surprised by the low cost and speedy delivery. Of course, if you need pots, oddball germanium transistors, stompswitches, etc., you'll need to go elsewhere, but they're impressive for what they do and carry.

As for the bass, you might want to consider increasing the value of C102, but my guess is that a) it was appropriately chosen in the first place, and b) has not declined in quality, in comparison to the various electrolytics. Consider replacing those first two 10uf units, C7 and C8 (looks like an 8, but could be a 6). Heck, for that matter, consider springing the big money ( http://www.dipmicro.com/store/C9E22-35-105 ) for replacing C22, that big 2500uf cap on the output. Note that this cap, by virtue of its age, is going to be several times (I'll estimate about 4x-5x) the size of its modern replacement. A 2200uf/35v electro is likely to not be much bigger than you thumb...if that big. I'm guessing the one you have is about the size of a tin of tomato paste or a small soup tin. This may be problematic for installation, but still it would be a likely culprit.


----------



## epis (Feb 25, 2012)

Hi Dasher, before you start tweaking tone stack, make sure that cabinet/speaker is capable to deliver amount of bass you were looking for. Try it out with different amp or try your amp with different box. 
This could help you as well : Heath ta-16


----------



## Dasher (Feb 3, 2011)

Thanks Epis! I feel a bit silly...this whole time I've been testing, the speaker has been outside of the cabinet and on my work bench. I loaded the same speaker into the cab and bingo bango, it sounds much better . As a bonus, I placed the chassis in the cabinet as well and now the tremolo is much more responsive. It looks like my heatshrink wrapped led/ldr must have been letting in enough light to affect its functionality.


----------



## epis (Feb 25, 2012)

Cool, I'm glad you solved the problem. Enjoy it now. :banana:


----------



## Dan Harmon (Aug 14, 2021)

Dasher said:


> I picked up an old, non-functioning Heathkit TA-27 (schematic) a few weekends ago and have been working on bringing it back to life. From what I can gather, these were sold as DIY amp kits in the late 60s to 70s. I was told that this specific amp was originally built in 1968...an oldie! Anyways, here's what I've done so far to the amp:
> 
> 1. Replaced all of the electrolytic capacitors and beefed up the first filter cap (C104) and output cap (C22) to 4700uf. This removed power supply hum.
> 2. Replaced the first preamp transistor (Q2) and reverb amplifier transistor (Q5) with 2N5089s to remove the majority of hiss from the amp.
> ...


----------



## Dan Harmon (Aug 14, 2021)

Built the head and speaker cab for TA-17 decades back. I was extremely disappointed because of the hissing (frying) noise from all the channels. Tried transistor substitutions with no joy. I'm excited by the recommendations . I still fiddle with the electronics and play with it, although it's not my gig amp. I'm gonna get the 2N5089's and try em.


----------



## Dan Harmon (Aug 14, 2021)

Dan Harmon said:


> Built the head and speaker cab for TA-17 decades back. I was extremely disappointed because of the hissing (frying) noise from all the channels. Tried transistor substitutions with no joy. I'm excited by the recommendations . I still fiddle with the electronics and play with it, although it's not my gig amp. I'm gonna get the 2N5089's and try em.


Additionally, I did blow out the drivers and power transistors because I accidentally shorted the amp's outputs and replaced them while at Spain😡


----------



## Latole (Aug 11, 2020)

Hiss may come from noisy resistors. After so many years, some resistors can retain moisture and become noisy, they should be replaced.
Mostly the ones at transistors collector.
I'll replace input jacks by ones with normaly close contact to ground


----------

