HELP! Amp Feedback? (Motion-Sound tube amp)

[Dave Pierce]

Hi KCers,

 

I'm having a problem I've never seen before with my amp. It's a Motion-Sound KBR-M. The clean channel works great, no problems. The rotary channel, however, is doing a behavior that seems a lot like feedback. It emits a high-pitched feedback sound (seems to be around 1.5k or so). This can be eliminated if I turn down the overall volume of that channel, particularly if I turn down the post-gain.

 

The rotary side does have tube pre-amp. Is it possible that this is "bad tube" behavior? I don't have much experience or expertise in tube amps...

 

Thanks in advance for any help!

 

--Dave

 

[Dave Pierce]

Ah! Gotta love Brits. I had googled before posting this, and didn't find anything obviously useful. Then I remembered -- Brits say "valve". So I googled "amp valve replacement" instead of "amp tube replacement" and found this on Marshall's web site:

 

Where pre-amp valves are concerned, the most common fault is microphony : a high pitched whistling noise which is apparent all the time, whether anything is plugged into the amp or not.

 

Duh. I guess I'm going to go buy some, uh..., tubes right now.

 

--Dave

 

[JMcS]

You might try finding some old 50's/60's era GE, Amperex, RCA or Sylvania's etc. instead of the Russian or Chinese tubes being made today. Look for some used ones on eBay to get an inexpensive variety of types (long/short plate, grey/black plate, box plate etc.) to see what tones you like best.

 

Good Luck.

[Dave Pierce]

The Motion-sound site recommends Electro Harmonics tubes, so I got that. It's not clear to me where those are made, but hopefully they will sound good.

 

--Dave

[JeffLearman]

eh, all tubes sound the same.

 

Transistors: make sure you get the ones made on Wednesdays.

 

[Dave Pierce]

eh, all tubes sound the same.

 

Transistors: make sure you get the ones made on Wednesdays.

 

 

Hmmm. I'm not a tube expert or analog circuits expert. But even I know that tubes do, in fact, sound different. They are fundamentally analog devices, and small differences in manufacture make significant differences in performance characteristics.

 

Obviously, this is NOT true of transistors.

 

--Dave

[Pale]

They are fundamentally analog devices

 

Transistors are also analog devices.

[Dave Pierce]

They are fundamentally analog devices

 

Transistors are also analog devices.

 

Ah right, yes of course they are! It's been way too long since I was in school...

 

--Dave

 

[JeffLearman]

I guess you didn't see the wink, Dave.

 

Of course, talk to Tube Screamer afficionados, and they'll tell you that the transistors in the original one sound significanly different than the ones made today using parts whose specifications are the same, but made using different technologies.

 

Yup, those old trannies sound better. Especially the ones made on Wednesdays.

 

Notice the wink this time. But the stuff about what the tube screamer afficionados say is true. (I've never compared them, and the new ones work fine for me.)

[Pale]

The truth is transistors (the same kind) often vary a lot in their characteristics. I remember when I made a clone of MXR 45 phaser, cycling through 3-4 transistors (on protoboard) would give dramatic results in terms of different phasing quality. Ok, that's because you need paired transistors to get nice phasing. The same goes for the time I made dyno-my-piano preamp clone. I made 2 circutis (one for myself, another for a friend), both preamps sounded different, despite using same components.

 

Factory made stuff usually has less variation because of... well, because it's factory made (machines made less mistakes, components are made with less characteristics fluctuations etc)

 

[MoodyBluesKeys]

Very early transistor circuitry - transistors were made using germanium with deliberate impurities; later transistors are made with silicon as a semi-conducting base (also with deliberate impurities).

 

First, there is a very significant difference in germanium and silicon - voltage drops across the circuit, non-linearities in the circuit, etc. Second - the early circuitry had much more variation between transistors that had the same manufacturer and part number than later.

 

Tube wise - this is one area in which present-day technology is very definitely inferior to the technology during the hey-day of tubes (1950 through 1970 especially). Back in the earlier day, millions of tubes were being manufactured, mostly by some of the largest companies in the various countries (such as Mullard or Amperex in Europe; or GE, Sylvania, RCA in the US). The factories were capable of very large production to extremely tight tolerances. Even back then, Mullard in particular was held in very high esteem in the audio world (both home and professional).

 

Over time, as solid-state circuitry took over almost all market areas (except early electronic organs, guitar amplifiers, ultra-high end high fidelity amplifiers like McIntosh, etc.) the large companies got out of the business - no longer profitable to keep the huge plants designed to make millions of tubes going, when only thousands were being made. At that point in time, service personnel like myself began to use what was available - I well remember the first couple of tubes that I bought from a supplier in white unmarked boxes (except type). Inside, the bube had "Made in USSR" stamped on it. At that time, the cold war between the US and Russia was still going - I didn't DARE tell my customers where the tubes were being made. As time went on, the Russians also went to solid-state and phased out tubes. Then the Chinese became the only source.

 

In more recent times, even the Chinese went out of tube making (on large scale basis), and small boutique companies were started to produce a very limited number of different tube types that were still in demand, such as 12AT7, 12AU7, 12AX7, 6L6GC, 6550. These companies produce tubes on a much smaller scale. The manufacturing process for really high quality tubes does not scale downward well. Tube production required very precision materials, spacings, and was always a bit of an art instead of completely a science.

 

A set of brand-new in the box "NOS" (new but old-stock) output tubes for a Marshall Plexi guitar amp are increasingly hard to obtain and very expensive. They will way outlast and produce a "better" sound quality than anything available in new stock. The same is very much true of the 6550's in a Leslie 122 or 147 amplifier.

 

Equipment such as the the Hammond Suzuki organs that use one or two preamp level tubes for "warmth" or controlled distortion - it will very much make a difference what kind of tube is put in the unit.

 

[JMcS]

Tube wise ...

 

Equipment such as the the Hammond Suzuki organs that use one or two preamp level tubes for "warmth" or controlled distortion - it will very much make a difference what kind of tube is put in the unit.

 

+1

 

I've been watching this company and waiting for their tubes to come on line:

http://www.techtubevalves.com/about_us/index.php

 

They have a whole new design that looks interesting.

[Pale]

Very early transistor circuitry - transistors were made using germanium with deliberate impurities; later transistors are made with silicon as a semi-conducting base (also with deliberate impurities).

 

Do I smell a colleague EE?

[MoodyBluesKeys]

Pale:

I did go to NC State for 2 years studying to be an EE, but did not complete. However, I have serviced electronic equipment since I was 17, and am 67 now. Ham radio operator, former broadcast TV chief engineer, two-way radio, electronic musical products, computer networking.

I was 7 when the transistor was invented.

 

[Pale]

I did finish, but I am considerably younger than you are.

 

Actually, my specialty is RF engineering, so your area of work really overlaps with mine a lot (you mentioned TV, ham radio etc).

 

Btw, being 7 when the transistor was invented sounds impressive. You got to see all the changes from tubes to solid state circuitry which must have been quite interesting.

[Dave Pierce]

I guess you didn't see the wink, Dave.

 

I saw it, but misread what you were winking.

 

Sometimes there's just no substitute for real-time, face-to-face conversations!

 

--Dave

[JeffLearman]

Equipment such as the the Hammond Suzuki organs that use one or two preamp level tubes for "warmth" or controlled distortion - it will very much make a difference what kind of tube is put in the unit.

Most of these are "starved plate" designs, where the tubes are run at much lower voltages, to get the overdrive effects at low levels and avoid the need for voltages supplies of 400V and more. It just ain't the same thing, and don't let nobody tell ya otherwise. Not that it doesn't have its uses.

[JeffLearman]

I guess you didn't see the wink, Dave.

 

I saw it, but misread what you were winking.

 

Sometimes there's just no substitute for real-time, face-to-face conversations!

 

--Dave

I guess I needed to use parentheses, or factor the smiley back in. Algebraic smileys!

[JeffLearman]

Even digital circuitry behaves quite surprisingly, when you run it outside the design envelope -- which is what we're doing with tube distortion.

 

I read about an experiment in the use of genetic algorithms for designing circuitry. Rather than teaching the computer how the gates worked, they just let it try different combinations (using real hardware) and test it against the desired circuit behavior.

 

The best engineers always needed 16 gates to get the desired logic. But the computer came up with a design that only needed 14. Nobody could quite figure out how it worked from the schematic. They realized that the computer had broken the normal rules for digital circuitry; applying variable signals to inputs that were intended for power or ground, and involving feedback loops that drove logic gates intended for "on/off" behavior to ambiguous middle states.

 

No doubt it would have failed with a change of manufacturer for the gates.

 

Not surprisingly, it shared a lot of traits with other mechanisms presumably designed by genetic algorithms, like being freakin' hard to figure out!

 

... What were we talking about?

[JMcS]

Equipment such as the the Hammond Suzuki organs that use one or two preamp level tubes for "warmth" or controlled distortion - it will very much make a difference what kind of tube is put in the unit.

Most of these are "starved plate" designs, where the tubes are run at much lower voltages, to get the overdrive effects at low levels and avoid the need for voltages supplies of 400V and more. It just ain't the same thing, and don't let nobody tell ya otherwise. Not that it doesn't have its uses.

 

The XK-3 does not use a starved plate design, it is a high voltage (250V) circuit. To my knowledge, the XK-3c while quite a bit different and more tweakable is also a high voltage design. It's been a while since I had mine open to swap the tubes.

 

I'm not sure how many preamps of any type operate on 400 volts. The Technical Data sheets I have for old Raytheon and Sylvania tubes show the typical operating voltage to be 250 volts with the test limit/design maximum being 300 to 330 volts. Power amps maybe.

[Pale]

Speaking of Xk and tubes... has anyone made a preamp for xk1. While I really like the sound of tonewheels, I don't really like the sound of the digital preamp (is it digital, or is it analog solid state?)

Anyway, I would like to either buy a nice tube preamp, or maybe even do a little diy project. Any recommendations?

[Dave Pierce]

Speaking of Xk and tubes... has anyone made a preamp for xk1. While I really like the sound of tonewheels, I don't really like the sound of the digital preamp (is it digital, or is it analog solid state?)

Anyway, I would like to either buy a nice tube preamp, or maybe even do a little diy project. Any recommendations?

 

Yes:

 

http://www.speakeasyvintagemusic.com/products/preamps-and-effects/speakeasy-vintage-tube-preamps/

 

Enjoy!

 

--Dave

 

[JMcS]

Speaking of Xk and tubes... has anyone made a preamp for xk1. While I really like the sound of tonewheels, I don't really like the sound of the digital preamp (is it digital, or is it analog solid state?)

Anyway, I would like to either buy a nice tube preamp, or maybe even do a little diy project. Any recommendations?

 

These:

http://www.artproaudio.com/products.asp?type=79&cat=1&id=58

 

and these:

http://tinyurl.com/BlueTube

 

sound pretty good. The Blue Tube is no longer in production.

 

I haven't tried one of these, but I've been told they sound good as well:

http://www.artproaudio.com/products.asp?type=79&cat=1&id=1

 

Good Luck.

 

 

[mate stubb]

Every time this conversation comes up, I must again point out:

 

The iconic grinding leslie overdrive sound comes from POWER TUBES being overdriven, and is influenced only a little by PREAMP TUBES being overdriven.