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EQUIPMENT FORUMS >> MINI TORII   >> High Pitched Squeal from MT
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Message started by Ron on 04/22/13 at 02:32:41

Title: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 02:32:41

I have been using my MT since I opened it up and I love it. I went through a small phase of cold feet but have finally realized the true potential of this beautiful amp.

That said, my MT has developed sort of a high pitched squeal... my guess is somewhere around 16-18 kHz. As you could imagine it is quite distracting.

Any ideas?

The only tubes that are not stock in my MT are the EZ81 rectifiers because my JJ's didn't work. I am using RFT's I got off of eBay.

I have tried rolling in new signal tubes, new power tubes, new 0A3 and 0D3 regulators... sometimes I hear a  difference in frequency, yes, but it is never gone :(

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/22/13 at 02:58:30

I'm not sure if this is related to the RFT EZ-81, but I've experienced some sibilance with this tube and replaced it with the RFT EZ-80, and problem solved. I even prefer the EZ-80 sound contribution overall.

Having said that, I've never experienced any 'squeals' with the RFT EZ-81, just a loss of control on high pitch passages, which I presume may turn into a squeal if the particular tube involved is in worse shape than in my experience, or if your system tends to be on the bright side.

If you already replaced that tube with the same EZ-81 spec, I would urge you to try the EZ-80 (the tube sounds better anyway, IMO)

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 04:14:39

Thanks! I will give the EZ80 a shot for sure, would you stick with an RFT? Where would you get the tube from? Maybe eBay is not the best place?

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/22/13 at 05:13:42

As far as I know, German made RFT's are great tubes. I got them from Tubemonger, at a good price.

The EZ-81 has shown to be a little sibilant depending on the amp and tube complement.  It may work well with smoother drivers, but with my Brimar 12AT7's, it tends to misbehave in some high frequency passages.

The RFT EZ-80, on the other hand, is quite neutral and does not overdo it at those frequencies. These seem to create better synergies with my tube setup.

Alternatively, try to determine if you have a tube with high microphony (ringing sound due to some elements within that tube which resonate with vibration), in which case any spec tube in your setup could be causing this effect.

I'd try the EZ-80 first. If not, then replace each of the other tubes involved, one at a time. You need to discard a likely microphonic tube in that case.

Good luck!


Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 11:29:20

Thanks for the help...

Would I still hear anything if I have no music being played through the MT? Right now even if it is on it will squeal.

The frequency changes sometimes if I move the feedback/tone knob.

I have tried multiple 12AU7's, (RCA Triple Mica Black Plates & Stock)
I have stock 6v6's and Tung Sol 5881 Reissues
I have tried a 2nd set of OC2 (Raytheon), 0A3, and 0D3 regulators.

The only thing that is not stock and I cannot test is the rectifiers... so that is my start point and seems to align with your thoughts.

Steve is now shipping with the EZ90, are they backwards compatible as well? If so I will get some EZ80 tubes and EZ90 tubes as well to test.

Thoughts?

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/22/13 at 12:48:44

I have not yet experienced microphonic issues myself, so I don't know how these issues present themselves exactly, although a very microphonic tube (i.e., one that squeals), would probably still do it to a lesser degree, even at iddle, from vibrations coming from the trannies.

I understand the most common microphony scenario is the faulty tube's resonance with vibrations stemming from the sound waves created by the speakers, though. Also, the more the amp is under load, the more microphonic that tube should become.

Can you try to apply some dampers to both rectifiers and drivers, and see if the squeal issue improves?

I believe the official description on the Mini Torii keeps the old EZ-90 (6x4) rectifiers listed by mistake, since these have been replaced for quite a while now with the EZ-81's. Do not try those 6x4 (EZ-90's) in your recently revised model.

In case there is a more organic cause for this issue, I suggest you call Steve and ask him directly about it. I'm sure he will help you solve this in no time. In the meantime, get those EZ-80's, as one or both of your current EZ-81's seem at this point the most suspicious candidates for either spec sibilance (systematic) or microphony (non-systematic).

Keep us posted.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 13:03:32

I can't find a RFT right now. What about a NOB Tunsgram?

I will be sure to keep everyone posted.
Thanks again!

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/22/13 at 13:48:24

No problem, Tungsram rectifiers are reported as very good also.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 14:03:28

Thanks, I have ordered the Tungsram EZ80 tubes, should be here on Friday! I am also looking at some RCA Grayglass 6v6's or Tung-Sol 6v6-gt power tubes incase I need to swap those out as well.

Title: Re: High Pitched Squeal from MT
Post by grinnell on 04/22/13 at 15:03:27

I have a similar sound from the right side.  Its not the power tubes been swapping those around.  It will stop when i turn off that side and turn it on again.  And i have tube dampers on 2 tubes.

I suspect is a microphonic tube.  just havent had time to track it down.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 18:57:46

I was listening during my lunch break today.

Current tube setup:


  • JJ 6V6S
  • RCA Triple Mica Black Plate
  • Raytheon 0A3/Raytheon 0C2
  • RFT EZ81


So I noticed no sibilance at first then as the tubes warmed up it did get more pronounced. I am hoping that it really is the rectifiers, this makes sense here as the EZ81 rectifiers to get quite hot during use.

The Tungsram EZ80's shipped. Haven't ordered anything else yet, going to try this first. Might spring for NOS RCA 6V6GT grayglass or just go with Tong Sol 6v6GT reissue.


Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/22/13 at 20:38:29

Nice tube complement. I would go for the current production Tung-Sol 6V6GT's, unless you already have an overly bright system. These are better sounding output tubes than the JJ's, by far (pace, transparency.)

You did not specify if the RCA's are 12AU7 or 12AT7's. If the latter, these could tend to make the EZ-81's even more sibilant on high pitch passages.  I personally prefer 12AT7's in general, coupled with EZ-80's.

Discarding any more organic issues, the weakest link in Mini Torii's tube complement remains the rectifiers. These work hard, which explains some sibilance on EZ-81's when pushed too far (high frequency pitches), or when these tubes are not in top shape. Never use JJ EZ-81's in that position (don't last long).

I would stick to 6V6 output tubes for now. These sound the best in general (with some reported good exceptions out there), and won't press the rectifiers too much.


Title: Re: High Pitched Squeal from MT
Post by Ron on 04/22/13 at 21:52:33

Well I reached out to Steve just in case we were wrong... He wants me to ship it back to the mothership.

At least I have a nice pair of EZ80 rectifiers.

(The RCA Blackplates I have are 12AU7's)

Don't the 12AT7's give you too much gain?

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/23/13 at 01:01:29

I'm sorry it looks like there is something organically wrong with your unit, after all. I'm glad you called Steve, who will fix it in no time.

12AT7's do increase the gain, but not dramatically. The end result is less range used on the volume pots to reach your sweet spot listening volume, but not by much.

I can't generalize given my limited exposure to the vast variety of 12au7/12at7's out there, but the benefits I perceive from 12AT7's in my system, in general, is the improved openness and pace.

Some forum members enjoy the 7062 (slightly less gain than a 12at7), a driver that I will be trying myself eventually.

I'm sure you will like the EZ-80 rectifiers.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/25/13 at 01:22:34

So good news, I waited till I received my EZ80 rectifier tubes. Set the MT back up with the stock tube complement and swapped in my EZ80 tubes. Everything works perfect and it runs SILENT.

Clearly I got a bad batch of RFT tubes, maybe they were not as NOS as the seller said they were. Oh well. Lesson learned.

I have emailed Steve, want to ensure he is ok with me if I don't send it in unless I hear some squealing in the future. :)

Thanks all for the help!

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/25/13 at 01:57:52

I'm glad to learn your troubles are over. I'm very pleased with my RFT's, but I got them NIB from Tubemonger.  I agree with your conclusion about a possible bad RFT lot in your case.

Title: Re: High Pitched Squeal from MT
Post by Rivieraranch on 04/25/13 at 03:19:41

That is what happens when you buy from the bad batch of rejects out there.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/25/13 at 23:44:12

Fireblade: I am loving the EZ80 rectifiers! Thanks again for the recommendation!

Thinking about popping the TungSol 6V6GT Reissue power tubes in there, have you used the TungSol 12AT7 Reissue, what about the svetlana? I know they are both made by the same company in the same factory.

Just trying to find a less expensive new production tube complement that I enjoy listening to.

Title: Re: High Pitched Squeal from MT
Post by Ron on 04/25/13 at 23:48:02

Here was what I was looking at classified by tube model:

12AU7


12AT7


12AX7


6V6


6L6


Title: Re: High Pitched Squeal from MT
Post by Fireblade on 04/26/13 at 01:11:15

I'm no tube expert by any means, but my current views about tubes in the Mini relate to either my own experience or testimonials from experienced Mini owners, whom I trust.

I currently favor 6V6 output tubes almost exclusively in the Mini, as in my view, the amp was voiced against those. There are alternative exceptions, of course, but these are less likely to beat a good 6V6 option, and more expensive.

I have two output tubes I prefer so far, and both are relatively inexpensive: The black glass Reflektor Plant, a Russian NOS 6V6, and the current production Tung-Sol 6V6GT's.

The first is my favorite, as it has all the transparency and pace I expect, and at the same time offer a warm flavor without high/low frequency roll off.

The second choice has more power (volume) and is very transparent, but with less pace and may tend to overdo the highs, when pushed. Depending on the recorded material, I interchange them within the same complement setup.

If it was not for the still very affordable black glass Reflektor 6V6, I would stick to current production 6V6's, as the sound differences, risks and investment are less evident. My only caveat here is the JJ 6V6, a tube lacking pace and volume and dull sounding to my ears.

I have not tried current production Tung-Sol 12a(?)7 alternatives yet, since my current preferred drivers (NOS Brimar 12 at7's) keep me happy. I suspect these new tubes may not make much of an impact, though. The driver position in this circuit is very important for sound tuning, and maybe here a good NOS is worth the risks. Some forum members really like the current production Psvane 12at7's, but these are really expensive.  YMMV, so if you are curious, check the Tung-Sol drivers and let us know.

Title: Re: High Pitched Squeal from MT
Post by grinnell on 04/26/13 at 15:58:11

I have played with 3 different power tubes in the MT.  

The Reflector 6v6GT
Very nice all around sound


40's RCA 6v6g ST
These rock, got nice bass, good punch and top end, nice depth in stage

Art Valve 350
These are nice but mellow lack some bass, very different from the the other tubes  airy sound nice sound stage

Title: Re: High Pitched Squeal from MT
Post by TubeNut on 05/13/13 at 18:55:16

This is a very cool thread!! A positive outcome for Ron I hope...Fireblade, I have a MT on order as we speak...it's "on the bench". My palms are sweating due to anticipation to hear this little beast! I will refer back to this thread for your tube rolling suggestions if I decide to try it. It's comforting that we have helpful members like you who have experience with these issues...
I once had a girlfriend with a "high pitched squeal"...no troubleshooting needed for that, tho :P

Title: Re: High Pitched Squeal from MT
Post by Fireblade on 05/15/13 at 00:26:26

TubeNut,

Thanks for the kind words. You have to consider my particular opinions are a reflection of room, components and personal taste variables, and that my actual tube rolling experience is relatively limited.

Having said that, I do share the excitement of my fellow Decwarites, so whatever experiences and opinions we disclose in this forum are well intentioned and obey a common passion for good music and sound.

I'm sure you will greatly enjoy your new Mini Torii/DM945 combo. Welcome to the forum.

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