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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6SN7 v 6N8P v 6H8C

Rosinante

Member
2010-11-02 8:58 pm
My amp has KT-88/98 output tubes (4), 12AX7 (2) tubes that can be called "input" tubes, and four 6SN7 tubes. The amp sounds a bit bright and is missing some midrange and some tightness in low tones. I hear about rolling output tubes, and I believe rolling the 12AX7's would also impact sound. Would rolling the 6SN7's impact the sound also? This amp can also accommodate 6N8P or 6H8C tubes in place of the 6SN7's. How do these other types compare with the 6SN7's?
 

Arnulf

Member
2009-02-02 9:41 am
Yes, roll them all down the nearest hill :)

Seriously, your question regarding "rolling of tubes" is pointless without a schematic and even with one, replacing tubes with identical tubes (slightly differing in operational parameters) would result in less a change than a redesign of your amplifier would.
 

Rosinante

Member
2010-11-02 8:58 pm
Thank you.

While I will not pretend that my speakers are flawless, I can report that the flabby bass, missing midrange and ultrabrightness was not present when they were powered by my POS solid state amp. So.....they are capable of the sound I am seeking.

The amp is Chinese, as are the tubes. Yaqin MC100B. According to many reports, rolling these tubes (like down a hill) would be a good thing.

Oh, and before someone suggests that I return to my solid state amp......I am not going to do that. I have started down the valve path because I have always GREATLY preferred the sound. Certainly as a musician. The finest-sounding home stereo I have ever heard was a McIntosh. By FAR. This current amp does a wonderful job in the upper half of the frequency range, with just a tad more brightness at the very top than I like. So.....as I say, I am looking to tweek this unit so that my tastes are satisfied. Like hotrodding a car. I think you guys understand......
 

Arnulf

Member
2009-02-02 9:41 am
So.....as I say, I am looking to tweek this unit so that my tastes are satisfied. Like hotrodding a car. I think you guys understand......

This amplifier has been discussed many times online so I think you really should look up the relevant information (just enter the amplifier model name into Google and you'll come up with heaps of threads about modifications to it) where people describe how they have already fixed similar problems to what you're experiencing.

It would seem this amplifier has been poorly designed so changes are far beyond the scope of replacing few tubes with identical tubes but the changes should be very worth your while.
 

Rosinante

Member
2010-11-02 8:58 pm
Those symptoms sound like the result of too high a source impedance. That's not trivial to correct. Tube rolling will not have much (if any) effect in that regard.

Thank you. Would this impedance problem be impacted by limiting AC input voltage. The limiting of input AC voltage to below 106 V was suggested in another thread, specific to this particular amplifier. Is this the "too high a source impedance" problem you mentioned, or would it be addressed this way. Again, I appreciate those who are patient with newbies like myself. I am ignorant, but not necessarily dense.
 

Rosinante

Member
2010-11-02 8:58 pm
Yes, I have been reading. The vast majority of posts have reported good results from things like simple tube rolling. Then there is the intriguing post by "Gonzalo" about three years ago here in which he suggests limiting AC voltage. So yeah, I am looking to learn what (reasonable) modifications can improve this unit. I do not find a widespread or consistent "poor design" conclusion regarding this amp any more than any other amp. But again, I am gathering information. Wildly radical re-wiring suggestions for this amp do not appear common, and are beyond my capabilities or agenda anyway. I am simply looking to make whatever improvements might make sense. If this amp turns out to be an expensive boat anchor, so be it.

All suggestions are appreciated.
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Thank you. Would this impedance problem be impacted by limiting AC input voltage.

No, it will have close to zero effect on source impedance. It may help reliability. The fixes for source impedance involve significant redesign, not just changing brands of tubes or capacitors or whatever. It's fundamental to the circuitry.
 

Arnulf

Member
2009-02-02 9:41 am
The vast majority of posts have reported good results from things like simple tube rolling.

Disregard the silly "tube rolling" posts, this is like suggesting an owner of VW Golf mk. 6 1.4TSI (160 HP) will get better performance by changing his car for another Golf mk. 6 1.4 TSI (160 HP) with same gear and everything. You can't get significantly different performance with essentially same parts. Yes, there will be some production differences but nothing like what you're after. "Tube rolling" is something you can tinker with when your amplifier performs well.

This is one of the threads I had in mind.

So the two problems you mentioned:

1: Amplifier sounds "bright" compared to what is presumeably SS amplifier with huge NFB (= very neutral sound). This is normal for tubes. If you want it to sound less bright, reduce the harmonic concent. The easiest way to do it is to use more NFB. The obvious way to do it (but not the easiest) is to change the operating points of the tubes so they operate in the more linear region.

2: "Tightness in low tones". I'm not sure what this is but I guess what you are trying to say was that the bass is somewhat lacking (not as "booming" as with your SS amplifier). Again this is something very common with tube amplifiers but unfortunately can't always be fixed as easily as the first part. This can be a result of too severe filtering throughout the amplifier (easily fixable), not enough subsonic filtering throughout the amplifier which saturates the OPT (note that is is directly the opposite of the first point ! also easily fixable) or a result of inadequate OPTs (not easily fixable).

Now what you really should do is try to locate the actual schematic of your amplifier. This will enable us to see where something extremely obvious pops out (and the thread I linked to above suggests such construction flaws, which is fortunate for you because it is rather easily fixable and very cheap - cheaper even than replacing tubes with the same type :D ).
 

Rosinante

Member
2010-11-02 8:58 pm
Oh. One more thing. Schematic:
 

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Rosinante

Member
2010-11-02 8:58 pm
NFB is negative feedback - you'll need a lot of homework to understand it well. OPT is the output transformer. Less homework.

Are you listening it in UL or triode mode?


Triode.

Thank you Stalker, ST and others. It will take me some time, with gentle developmental inputs from patient folks like you, to bring me to the point of at least asking meaningful questions. As you can imagine, I am still reeling from the discovery that these amplifiers are filled with wires, capacitors and resistors rather than fairies and magical clouds of rainbow sparkles.
 

Rosinante

Member
2010-11-02 8:58 pm
This is one of the threads I had in mind.


Well okay, I clicked on this thread which includes detailed descriptions of significant modifications of this amp by several enthusiasts. My head almost exploded and now I need to take a couple of aspirins and lie down.

On a lighter note, an interesting, vivacious, intelligent, warm, attractive, red-headed, young (she in her thirties, me in my fifties) female friend is coming to my home after work for libations and dinner. Unfortunately, she "bats for the other team," if you know what I mean. Still, she has been one of my very most interesting friends for several years now and it's always a pleasure.

Have a GREAT weekend, and accept my thanks for your help.