Klein&Hummel TS65 tube amp used in Canteen Electronic 160

Hello everybody!

I am currently restoring a K&H TS65 amplifier with the aim of using it in a small Hi-Fi setup.
After doing some work, I performed a series of measurements and found that the frequency characteristic is not linear with an increase after 1KHz and without showing signs of roll-off up to 20KHz. I performed the test with dummy 4 ohm load on each channel. Both channels behaved the same. Characteristic is attached here.
First I increased the feedback capacitor from 330pF to 1800pF and a slight roll-off appeared, but not flattened the characteristic which seamed caused by something very strong.
Since I suspected a phase shift on the feedback side, I also performed a test with an inductive load and the characteristic flattened out but not enough.
My question for the forum members is whether this characteristic has been observed by anyone else and whether it is embedded in the characteristic of the output transformer.

Best regards,
mircom

Hello mircom, is this graph showing the frequency characteristic of the power stage or overall amplifier?
Where did you connect the load? Note that the feedback is connected to the 0Ohm tap at the transformer while ground is at the 2Ohm tap. From the naming 0/2/4/8/16, I think the speaker should be connected 0Ohm. Then, the speaker current becomes part of the feedback. I have seen other very special K&H feedback concepts, this one is also new to me and I would be keen to figure out how it works and it`s pros and cons.
Rgds...Jürgen

PS: I found a measurement of the K&H TS60 using the same feedback concept. At that time, I did not pay much attention to the concept because all measurements were ok. The measurement shows the overall frequency response at different settings of the tone-control switches. The load was a 8Ohm resistor.



2 mal bearbeitet. Zuletzt am 26.03.26 09:12.

Hello Jürgen,
Thanks for the answer and for the attached frequency characteristic. I bypassed the first stage with the ECC83 tube and introduced the signal directly to the remote volume control connector, meaning only the final stage was tested.
I connected the 4 ohm load to the 0 and 4 ohm terminals, as well as the speaker. This type of connection to ground through a terminal other than 0 ohm, fragments the secondary into two coils offset by 180 degrees. It is a connection found in American Hi-Fi amplifiers (but also in jukebox ones) with which a third output is created for a center speaker where the signal is mono (L+R). This is useful if the R and L speakers are very far apart and the space between them needs to be "filled" somehow.
Your answer inspired me with an idea, namely checking the ground path on the test bench. I power the amplifier through an adjustable transformer with galvanic isolation, so it is floating with respect to ground, but the situation changes due to the signal sources (sound card or separate audio generator) and the oscilloscope.
The difference in the schematic between the TS-60 and TS-65 is not great, the latter has a few additional capacitors intended to make the amplifier more stable. I could say that the TS-65 has a more refined schematic.
Although in the schematic both amplifiers use the same output transformer, namely BV-4478a, my amplifier has the BV-4478b version installed. Could it be a transformer with a different characteristic?
I refuse to believe that K&H could have manufactured a transformer with a nonlinear characteristic just to be used only in a jukebox.
There are other simpler ways to adjust the characteristic and you can use a linear transformer in any amplifier, regardless of the field of use.
Best regards,
mircom

Hello mircom,
you are right, many Jukebox amps use grounded center taps for enabling mono bridge configurations.
What puzzled me is that the K&H 60/65 feedback wire is connected to the 0-tap while ground is at the 2Ohm tap. K&H had amplifiers with positive and negative feedback like the Telewatt Ultra. This made me think the 60 and 65 models are doing something very tricky here, but maybe just the overall phase is inverted at the transformer, otherwise this would be positive feedback in this circuit.
Could you find a grounding problem in your test setup?
Have you measured the amp in open loop?
Best regards...Jürgen

Hi Jürgen,

Yes, I found a problem with the grounding.
Now it's fine, but I still have a problem with the sound card.
I use the REW software together with an external sound card connected via USB (Focusrite Scarlet 2i2). This test setup is newer and I'm still learning to use it. But with the classic setup with generator, oscilloscope and bench multimeter everything is fine now.
The amplifier is very linear and I'm happy.
Obviously I still have work to do on it on the power supply side, then adjustments and maybe a modification to be able to adjust the bias, but I'm on the right track now. With the current fixed bias the amplifier works in class B and I would like to bring it to class AB.
The nonlinearity is from the software and the sound card, but due to the grounding problem it was also replicated by the classic test setup and that confused me.
Now I have to see the problem with the REW software, I think it's a digital filter included in the settings. This software is for acoustics but can also be used in audio, if you are satisfied with a cheaper solution.
Thanks for your help!

Best regards,
Mircea

P.S. The overall feedback is negative, no doubt for that, otherwise it wouldn't be so linear. The response to rectangular signal is good too.

Hello mircom,
here is my setup for floating measurements. I am using a homemade passive load box. The load can be switched between 4/8/16Ohm or real speaker. Attenuation 1 10 100 plus a switch float/ground.
There is an active (more professional) sound card interface on "Pete Millett's DIY Audio pages" but only Mono.
Rgds...Jürgen



1 mal bearbeitet. Zuletzt am 28.03.26 10:12.

Hello Jürgen,

Thank you very much for all the information!

Best regards,

Mircea