Bob Carver Cherry 180s

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jostber

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Bob Carver Cherry 180s
« on: 19 Jan 2014, 09:13 pm »
For my second hifi setup I have two Bob Cherry 180 monoblocks. These have a lot of tubes,  in total: Twelve KT88s, two 12AX7s, two 12ATfs and two 6AL5s. It will be a bit expensive to give dampers to all these. What would be recommended to start with here?



Herbie

Re: Bob Carver Cherry 180s
« Reply #1 on: 19 Jan 2014, 10:09 pm »
Start with the small-signal tubes first.

Steve Herbelin
Herbie's Audio Lab

jostber

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Re: Bob Carver Cherry 180s
« Reply #2 on: 20 Jan 2014, 08:54 am »
Thanks. According to the manual the 12AX7s are the input tubes, while the 12AT7s are the pre-driver stage. The 6AL5s are for DC restoration. So then would it be most effective with dampers on the 12AX7s?

Herbie

Re: Bob Carver Cherry 180s
« Reply #3 on: 20 Jan 2014, 03:06 pm »
If you want to start minimally, damping the input tubes would be a good place to start. I suggest starting with all the small-signal tube, though: input, pre-driver, and DC restoration. These tubes use inexpensive UltraSonic SS Damping Instruments, so the investment might not be prohibitive.

It's impossible to say which of these tubes would benefit most from damping instruments. All have the same potential for microphonics and micro-vibrational distortion, the degree of which will vary from one tube to another and from one component to another. I'm not sure of the circuitry design of the 180, but the DC restoration tubes might be the most essential to damp. Like with rectifier tubes in many amp circuits, the DC current they provide affects other tubes, sometimes all the other tubes in the component. You don't want a fuzzy DC current feeding the musical amplification.

Each stage of the vacuum tube circuitry works hand-in-hand with the next stage. Damping one stage will send a more faithful rendering of the music to the next, which can subsequently add microphonics right back onto the musical signal. Conversely, with an undamped, distorted signal going from one stage to the next, that distortion is then part of the music itself and amplified along with the music.

Damping the small-signal tubes in your amp will most likely bring out a significant improvement in the amps' musical potential. From there, damping the power tubes would be icing on the cake, if you're inclined to go a step further. KT88s are generally receptive to substantial improvement with damping instruments (either UltraSonic Rx-50 or HAL-O III-50). With a dozen tubes, that can be quite an investment, though worthwhile to bring out the full potential of the amps.

We've found, however, that damping just half of the power tubes in a multi-power tube amp usually provides substantially more than "half" of the improvement that you'd get by damping all the power tubes. Damping just every-other power tube instead of every one of them is recommended for the budget-conscious. (With monos, apply the damping instruments in matched stereo pairs.) You would probably find that damping half the power tubes is more than sufficient.

Steve Herbelin
Herbie's Audio Lab

Russell Dawkins

Re: Bob Carver Cherry 180s
« Reply #4 on: 21 Jan 2014, 12:05 am »
Could you not turn up the gain and lightly tap the various tubes to hear which ones were more microphonic?

Herbie

Re: Bob Carver Cherry 180s
« Reply #5 on: 21 Jan 2014, 12:35 am »
Hi, Russell. Tapping on vacuum tubes, capacitors, and other sensitive electronics will naturally induce a microphonic response. In a micro-vibrational environment, this is akin to whacking them with a sledge hammer. A more reasonable assessment of comparative influence to external vibration can be achieved by rubbing the tube glass lightly and briefly with a pencil eraser. This will give little or no indication, however, of the degree to which internally generated vibrations are affecting the tubes' performance, nor how the tubes' musical contribution is affected by the vibrational environment they are working in. The only real way to know is to audition the tubes with damping instruments.

Quite often, damping tubes that seem to to have no microphonics issues at all provides surprising benefit and listening satisfaction, because the excellent-sounding tubes then sound even better.

If you roll tubes in your gear fairly often, you will probably have a good idea of which particular tubes and/or stages in a component contribute most to subtle differences in the end result. Damping whichever tubes tend to influence the musical output most would be another criterion for choosing which tubes to try damping instruments on first.

Steve
Herbie's Audio Lab

jostber

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Re: Bob Carver Cherry 180s
« Reply #6 on: 23 Jan 2014, 02:11 pm »
Which tube dampers will go with the 6AL5 tubes? They are quite small.


Herbie

Re: Bob Carver Cherry 180s
« Reply #7 on: 23 Jan 2014, 02:54 pm »
7-pin miniature tubes like 6AL5 use size "7" damping instruments. I recommend trying an UltraSonic SS-7 on each of these tubes.

Steve
Herbie's Audio Lab