Dynaco ST70 poor bass

[[email protected]]

Hi everybody, I have a pair of ST70 as mono driving my JBL Jubal L65
speakers. I´m not satisfied with their bass response, 500Hz bellow...
it seems that its very weak... what can be done to really resolve this
problem? or the ST70 is only good with de mids and high?

[Ian Iveson]

> Hi everybody, I have a pair of ST70 as mono driving my JBL Jubal L65
> speakers. I´m not satisfied with their bass response, 500Hz
> bellow...
> it seems that its very weak... what can be done to really resolve
> this
> problem? or the ST70 is only good with de mids and high?

http://www.curcioaudio.com/st7_mnl.pdf

gives performance specs, and how to connect for mono.

If it is connected up and working properly, and you are still not
satisfied, then you may wonder what impedance your speakers present at
the frequencies you are unhappy with.

cheers, Ian

[Jon Yaeger]

in article m,
at wrote on 10/10/06 12:53 PM:

> Hi everybody, I have a pair of ST70 as mono driving my JBL Jubal L65
> speakers. I´m not satisfied with their bass response, 500Hz bellow...
> it seems that its very weak... what can be done to really resolve this
> problem? or the ST70 is only good with de mids and high?
>


I had poor and distorted bass response on one St-70 that I rebuilt. I added
a little more filter capacitance and that seemed to help. Overall, I did
the following and was very satisfied with the results:


1. Removed the confusing "Center Speaker" terminal block and covered it with
a plate, leaving the outputs on the bottom two terminal strips.

2. Used acetone to remove the silk-screened info on the center channel, etc.
Marked the new terminals with Datak transfer letters and protected with
acrylic spray.

3. Rewired the output transformer, grounding the ground terminal to the
chassis. Per Welbourne Lab's suggestion, I connected the NFB loop to the 8
ohm tap instead of the 16 ohm.

4. Removed the speaker "Balancing" circuit and its associated 16 ohm, 20W
resistor.

5. Rewired the phase switch for the 2nd channel (an alternative would be to
do the phase circuit further "upstream").

6. Replaced the zip power cord with a 3 wire cord and ground the chassis (if
you don't do this, you run a 50% chance of making the chassis "hot").

7. Punched a hole in the chassis and install another multi-section cap (an
alternative would be to buy and install a "cap" board like Triode
Electronics sells). The stock power supply capacitance of the amp appears
to be inadequate - I was getting a lot of sagging and clipping at the higher
outputs. Note that the NFB loop is effected by the way the output trannies
are wired.

8. Replaced old coupling caps and electros, and any out-of-spec parts.

9. Lubed controls and switches; cleaned connectors with Deoxit.

Hope that helps.


Jon

[Bob H.]

How is the bass response with only one channel and then the other? How
do the output tubes check out?
Are the coupling caps original? Is the filter cap new, or in good
shape? How about the rectifier tube? Many things can affect the
sound.
My ST70 has great bass with the original cap amounts, plus another 20
or so Uf wired across the output tube filter cap.

Bob H.



wrote:
> Hi everybody, I have a pair of ST70 as mono driving my JBL Jubal L65
> speakers. I´m not satisfied with their bass response, 500Hz bellow...
> it seems that its very weak... what can be done to really resolve this
> problem? or the ST70 is only good with de mids and high?

[Bob H.]

wrote:
> Hi everybody, I have a pair of ST70 as mono driving my JBL Jubal L65
> speakers. I´m not satisfied with their bass response, 500Hz bellow...
> it seems that its very weak... what can be done to really resolve this
> problem? or the ST70 is only good with de mids and high?

Also, try reversing the wires on one of your speakers to make sure it
isn't out of phase with the other.

Bob H.