[GreatDane]

I recently picked up a new tube amp. The tube compliment is 4xKT88, 2x6SN7, and 2x12AX7. I've had the amp for about a week now, and I find that I am getting distortion (almost a "flutter") on the high end, especially cymbals and vocals. It only really seems to happen on "complex" or dense music, for example electronica or metal.

I am wondering if this is due to the apparently cheap, generic stock Chinese tubes that it came with, or is it indicative of some other bigger problem? I opened up the amp to look for any obvious issues (blown resistors, bulging caps, etc), and found nothing.

Any ideas?

[BobMajor]

My first thought is to clean all the connections including the tube pins and the sockets they fit in.

[GreatDane]

Checked that. Still no dice.

[Pete Schumacher]

If you can get access to an oscilloscope, it would make tracking down the problem much easier.

If you have a voltmeter, you could check the bias on the output tubes.

One thing I came across recently was an EL34 push pull that was marginally stable.  It was fine with resistive loads but oscillated when a speaker load was connected, which has an inductive high frequency characteristic.  The cure was to add a zobel network (series connected resistor and cap) of 10 Ohm and 1uF connected across the output terminals of the amp.  The sound improved dramatically as you might guess.

[raindance]

What amp is it? Make/model?

A lot of the time these amps are based on old fashioned designs and have too much gain for modern sources that output 10x more volts than the input stage was designed for.

Of course the bias is always the first thing to look at.

[GreatDane]

If you can get access to an oscilloscope, it would make tracking down the problem much easier.

If you have a voltmeter, you could check the bias on the output tubes.

One thing I came across recently was an EL34 push pull that was marginally stable.  It was fine with resistive loads but oscillated when a speaker load was connected, which has an inductive high frequency characteristic.  The cure was to add a zobel network (series connected resistor and cap) of 10 Ohm and 1uF connected across the output terminals of the amp.  The sound improved dramatically as you might guess.

I guess the oscilloscope will show distortion at certain frequencies?

I do have a voltmeter, and I checked the bias of the tubes. They're right at 40 mv, as the manual suggests. Actually the manual says anywhere between 35-40. I tried both, and still the same issue.

[GreatDane]

What amp is it? Make/model?

A lot of the time these amps are based on old fashioned designs and have too much gain for modern sources that output 10x more volts than the input stage was designed for.

Of course the bias is always the first thing to look at.

It is a Chinese amp, made by Shuguang (the tube manufacturer). It is an all point-to-point wired design. The construction seems top-notch, actually. The thing weighs over 50 lbs too!

Your suggestion of too much gain may be spot-on - I'm running a DAC (Schiit Audio Gungnir) straight into the aux input of the amp. Perhaps the amp's gain level is too hot for the DAC?

[dmatt]

I have a similar issue with my Melody I2A3 integrated (the old Melody amp - not the new production version.) 

When I ran my Channel Island DAC through the balanced connections, I would get distortion on peaks unless I dialed down the output of my Squeezebox Touch/SB3/Duet/Transporter by 5 dB.  Funny thing, it would not distort if I bypassed the DAC and ran the Transporter analog balanced outputs direct to the Melody balanced inputs or if I used the single ended inputs on the Melody with any source. 

I just chocked it up to a hot CI DAC balanced output and an overly sensitive Melody balanced input that didn't play well together.

Cheers

[GreatDane]

Interesting. I bet I'm running into the same situation.

I wonder if putting a tube buffer between DAC and amp would help? I know sometimes these things can smooth over gain/impedance issues.

[GreatDane]

Well I'll be darned .... I think you guys helped me sort this out!

I added a small pre-amp I have kicking around between the DAC and the tube amp, dialed the gain way down, and the distortion and sibilance is gone. So I guess that was it.

[raindance]

OK, now PM me and I will help you with a solution that takes the preamp out of the chain. If it is simple enough, we can share the details on here later.

Which model is it? I have the 300B amp that they make.

[raindance]

PS, a tube buffer will not help. It will make it worse.

[GreatDane]

OK, now PM me and I will help you with a solution that takes the preamp out of the chain. If it is simple enough, we can share the details on here later.

Which model is it? I have the 300B amp that they make.

PM sent!