Other Stuff => Archived Manufacturer Circles => Herbie's Audio Lab => Topic started by: pstrisik on 20 May 2014, 05:50 pm

Title: Dampers for 6FM7
Post by: pstrisik on 20 May 2014, 05:50 pm
What is the appropriate model for the 6FM7 tube in a preamp (three tubes)?  These tubes are somewhat microphonic with audibility from the speakers with very gentle tap on the top of the tubes.  Not sure if they are considered hot running or not.

Also, I've heard mixed opinions about which tubes will benefit most with input/drivers top of the list but less enthusiasm about power tubes and rectifiers. 


Title: Re: Dampers for 6FM7
Post by: Herbie on 20 May 2014, 06:40 pm
6FM7 tubes use a size "30" damping instrument. I recommend an UltraSonic Rx-30 for each of these tubes which run fairly hot.

Inducing a microphonic response by tapping on a tube of itself does not indicate any problem with the tube and is just a natural response, just as if you tap on a capacitor or other sensitive electronic part. There's no reason to tap on a tube. Tube microphonics occur in the music as fuzziness, glare, linearity and tonal irregularities, loss of imaging focus, and other anomalies. Distortions may be extremely subtle or quite profound but have little or no relation to what you get by tapping on the tube.

Input/driver, power and rectifier tubes all have generally equivalent susceptibility to microphonics. Degree of microphonics will vary from one tube to another and from one component to another. Small-signal, and especially tubes in a phono stage, amplify smaller signals so oftentimes appear to be more microphonic as they are amplifying proportionally more of the low-level and electronic "noise floor" information. With some components, however, damping the power tubes achieves the most significant improvement, so there's no one-size-fits-all rule that fits every audio system. Damping the rectifier tubes is often the most significant improvement made with an amplifier because the rectifier tube typically affects all the other tubes in the circuit--if you have a fuzzy rectified current, you'll have fuzzy amplified music along with other distortions.

Though all vacuum tubes are susceptible to some degree or another to microphonics and micro-vibrational distortion, it usually makes sense to start out with damping just the source tubes and amplifier input/driver tubes if not going all-out to properly and adequately address all the tubes.

Steve Herbelin
Herbie's Audio Lab

Title: Re: Dampers for 6FM7
Post by: pstrisik on 20 May 2014, 08:09 pm

Thanks for the thorough answer and tutorial!

I'll order dampers for the 6FM7's as I know they will stay in the preamp.  Probably for the pre's rectifier as well.  My amp, on the other hand, is a tube roller's dream.  I have one of your hi temp dampers on that input tube (6H30).  But I will be trying various power tubes from 6V6, EL34, KT120, KT150 and the recommended rectifier that would match up with them.  So I guess I'll see about settling into a single configuration before getting dampers for those.

The preamp's rectifier is a 5Y3 (NOS RCA).  What do you recommend for that tube?

I guess I should consider a damper for the 5AR4 that is in the power supply of my Cary SLP-05 (other preamp).  It is enclosed within the chassis and mounted sideways.  Any particular considerations there?  I will have to make sure about clearance around the tube for one.

Good to hear that the noise through the speakers is not of concern.  Otherwise, sound quality is getting quite nice!

Thanks again!
Title: Re: Dampers for 6FM7
Post by: Herbie on 21 May 2014, 12:25 am
I recommend an UltraSonic Rx-30 for the 5Y3 tube (or HAL-O III-30). No special considerations are needed for placing a Herbie's Audio Lab damping instrument on a sideways-mounted tube. I haven't looked inside an SLP-05 power supply, but sometimes horizontally mounted tubes have very little clearance along one side; in such case, just place the damper on the tube so that the open portion of the C-ring faces the close side. With UltraSonic Rx dampers, the little isolation pads can also be repositioned along the titanium C-ring if needed to clear any obstacles in close proximity. The Cary SLP-05 power supply has a pretty substantial, full-size component chassis, so you'll most likely find that you've got plenty of room around the 5AR4 tube. (Most 5AR4 tubes use a size "35" damping instrument; UltraSonic Rx-35 or HAL-O III-35 would be recommended.)

Herbie's Audio Lab
Title: Re: Dampers for 6FM7
Post by: pstrisik on 21 May 2014, 02:34 pm
Thanks again Steve.  I'll have an order for you this week.