Heathkit TA 17 bass preamp

[Renegadrian]

I'm always after a good preamp, may it be for guitar, for bass or both.
I found the Heathkit TA 17 amp schematic, the bass part seems interesting and quite easy...now, I tried to replicate it up to the output after the tone control.
Subs for the transistor: 2n5088, 2n3906 and 2n3904.
I didn't wire the dual volume pot, so signal gets in at the 10µ cap.
unfortunately, the thing is dead silent...maybe it needs some changes?! maybe the biasing resistors!?
Any hint!?

[Mark Hammer]

believe it or not, I actually have one of those amplifier heads.  Unfortunately, there is a blown transistor in the power amp, so I've never actually heard it.  One of those post-retirement projects.

[Renegadrian]

Yes, I did read your posts on some other forums!  wish I could make this preamp work, it seems interesting!

[PRR]

> dead silent...maybe it needs

SO many possible ways to not-work.

Maybe we need some voltages?

However this thing, without the R608 R607 R101/102 network, has an awful lot of gain. It may be oscillating so bad it can't pass audio also.

[Mark Hammer]

I have the assembly manual somewhere, that might have voltages to check for.  It might be posted around too.

[Renegadrian]

The original have a voltage line of 60V, while those trannies I used cannot handle 60V - I just feed 12V

[PRR]

> I just feed 12V

At a glance, if Q6 goes "off" and rises to the full 12V, R104 R104 divide that to 0.27V at Q4 Base, which leaves Q4 "off", Q5 "off", Q6 "off", which is the initial assumption. You have a "stable bias" with all transistors OFF. So yeah, "dead silent".

You need at least 27V supply to wake things up.

> trannies I used cannot handle 60V

They do not get the full 60V. Q6 is the highest at 24V idle Vce. The Vce will rise with signal. Signal peaks on Q6 won't be over 50V due to loading. Your 2N3904 will stand 40Vce per spec, typically more in breadboard (not concerned with long life). I would expect 99% of them to stand 60V for days. A breakdown is NOT catastrophic since R109 limits breakdown current to 2.5mA and breakdown dissipation to 100mW.

Jack it up above 40V. It will begin to bias-up and won't break down even at 60V, though positive peaks may clip early.

[Renegadrian]

> I just feed 12V

At a glance, if Q6 goes "off" and rises to the full 12V, R104 R104 divide that to 0.27V at Q4 Base, which leaves Q4 "off", Q5 "off", Q6 "off", which is the initial assumption. You have a "stable bias" with all transistors OFF. So yeah, "dead silent".

You need at least 27V supply to wake things up.

> trannies I used cannot handle 60V

They do not get the full 60V. Q6 is the highest at 24V idle Vce. The Vce will rise with signal. Signal peaks on Q6 won't be over 50V due to loading. Your 2N3904 will stand 40Vce per spec, typically more in breadboard (not concerned with long life). I would expect 99% of them to stand 60V for days. A breakdown is NOT catastrophic since R109 limits breakdown current to 2.5mA and breakdown dissipation to 100mW.

Jack it up above 40V. It will begin to bias-up and won't break down even at 60V, though positive peaks may clip early.

ok thanks for the precious hints, i'll try tomorrow and report.

[PRR]

Plot of what happens as supply voltage is raised zero to 70V.
https://s24.postimg.org/ncvp0kf6d/TA17_bias.gif

Up to 27V, Q6 is "off" and its collector is "stuck" to the supply rail. No output possible. Above 28V supply, Q6 C is steady at 28V for any reasonable supply. If you plot a typical signal swing, you need to be well over 30V or the signal will just be slamming against the rail. 60V supply gives a real nice operating point, though this is not real fussy.

It is an odd bias scheme. It is mildly temperature sensitive. It does NOT tolerate low supply voltage. I wonder if the intent is to cut-off sound while power is ramping-up, to reduce start-up thump.

This is sonically a "op-amp". Raw gain is 100,000. Closed-loop gain is 1.5Meg/27K or 55. Excess gain is nearly 2,000 or say 64dB. This stage will be *clean*.

The tone network is unusual but too much for tonight.

R20, R113, R216, and R410 come together in a "virtual earth" node at Power Amp input. Pow-amp has raw gain over 1,000, so NFB resistor R318 "acts like" less than 1K, a near ground. To use the Bass channel alone, ground the right end of R113 and take signal voltage from the left end of R113.

[Renegadrian]

While browsing looking for bass amps schematics, I found that someone already had my idea...



Yeah some changes here and there in the transistor section and the last transistor has been moved after the tone section, but as you can see the tone control part is verbatim. I don't know if someone has already built this schem above...Gonna try it! Maybe it gets close to the original heathkit circuit...

Links HERE and HERE so you can read the BOM

[Renegadrian]

Tried the original schem at 50V - no sound. think I'll dismantle that board and try the last circuit I posted instead...