Two bad tube experiences

[Fred]

Amir:

1. Sounds like Peachtree doesn't select their tubes and put crappy ones in. The better tube companies routinely reject 2/3rds of the tubes they get in

2. Vis a vis the tube amp: while it's clear a tube amp could have crappy bass, that the bass is totally MIA sounds to me like a mismatch somewhere eg. cables, impedance between line stage and amp (though usually it's a tube preamp with ss amp), speakers, etc. What cables and speakers were you using with this amp and did the amplifier have enough power for the speakers? Was wondering if the amplifier also sounded dynamically restricted too?

I'm with Myles on this one. It would be interesting to know exactly what tube is in the Nova. But it sounds too that other potential issues need to be explored.

 

[amirm]

Here is the write up on Nova: http://www.soundstage.com/equipment/peachtree_nova.htm

Says the tube is "Russian-made Electro-Harmonix 6922EH."

 

[MylesBAstor]

Here is the write up on Nova: http://www.soundstage.com/equipment/peachtree_nova.htm

Says the tube is "Russian-made Electro-Harmonix 6922EH."

While many co's use the 6922EH, every manufacturer I know of selects the 6922EH they put in their unit.

BTW Amir, did you listen to the Peachtree w/w/o the tube in the circuit? Did it sound gritty in both situations?

 

[amirm]

Yes, they let you switch the tube on and off using the remote! The switching time is a few seconds so it take a bit of work to compare. But the fidelity was far better with the tube *off*. Hence my comments in my opening post. The best sound though was with an external async USB. The internal one is just "good." With the adapter, it becomes very nice. Not state-of-the-art but surprisingly nice.

Just to be clear, none of the grittiness was there without the tube.

 

[mep]

Its very linear in its normal operating point but really spits out the odd harmonics a whole lot. Tom

Can you clarify the above sentence as it appears to be at odds with itself the way that it's written?

 

[MylesBAstor]

The 6922 IMO is about as close to "transistor sound" as you can get with a tube. Its very linear in its normal operating point but really spits out the odd harmonics a whole lot. Gritty can be applied to this tube in many circuits that use it. It is not tube sound if you are searching for that, if you want strictly high specs, it will deliver. IMO, if you use quite a bit of feedback this tube does tame down.

BTW, I am not knocking solid state or transistors, just saying this tube is more like solid state than tube IMO.

Tom

And how does the 6922 (and the 6DJ8s tubes as a whole) susceptibility to microphonics affect this linearity and sound? That's the reason many designers eschewed their implementation.

As I said, the rejection rate of these and most currently produced tubes is anywhere from 33 to 67 pct or so. And most designers, for instance cj, use a zero neg (that's local and global) feedback design.

And others would say that the "premiere" neutral 6DJ8 type is either a Tele CCA or if you're slumming, an E88CC. But it would be silly to use a $300 tube in the Peachtree.

 

[fas42]

And most designers, for instance cj, use a zero neg (that's local and global) feedback design.

You're kidding me, aren't you? Talk about adding tone controls to a system ...

Frank

 

[MylesBAstor]

You're kidding me, aren't you? Talk about adding tone controls to a system ...

Frank

Not if implemented correctly Frank.

And as they say, the proof is in the pudding. Go listen to a cj GAT linestage first and tell me what you hear. Then come back.

 

[amirm]

Here is the measurements with our without the tube. The cyan+magenta are with the tube:

Seems like you get to have all the transistor distortions plus tube! See the second harmonic added while the other ones (as best as can be seen) are not reduced.

 

[fas42]

Not if implemented correctly Frank.

And as they say, the proof is in the pudding. Go listen to a cj GAT linestage first and tell me what you hear. Then come back.

I was referring to the comment about local feedback, it would be almost impossible to do a circuit without such being effectively part of the workings, and tonally the amplifier would be a complete mess. Global feedback you can get away with not having, but as you say, you really need to know what you're doing.

The trouble is feedback is such a dirty word in the audio community, and such is totally unwarranted ...

Frank

 

[fas42]

... revel in details they never heard before.......accuracy I fire up the solid state chain.......but feeling like groovin and care free....fire up the SET and get that sing songy slightly reverberant and "room pressurization effect..even in headphones".

Tom

The point is, Tom, that when a speaker system is working properly you get all of those things in the one bundle: you get detail by the bucketful, you get musicality, you get accuracy, you get groovin', you get sing song reverberant room pressurisation -- it all comes in a single package. Just like live sound, in fact ...

Frank

 

[microstrip]

And how does the 6922 (and the 6DJ8s tubes as a whole) susceptibility to microphonics affect this linearity and sound? That's the reason many designers eschewed their implementation.

As I said, the rejection rate of these and most currently produced tubes is anywhere from 33 to 67 pct or so. And most designers, for instance cj, use a zero neg (that's local and global) feedback design.

And others would say that the "premiere" neutral 6DJ8 type is either a Tele CCA or if you're slumming, an E88CC. But it would be silly to use a $300 tube in the Peachtree.

We should remember that the 6922/6DJ8 family is a very high transconductance tube developed for VHF and UHF TV tuners. Its wide bandwidth and high gain are very promising but make it very sensitive to RF signals - in the worst scenario some tubes in the preamplifiers can become oscillators if the designers do not take precautions to limit its bandwidth. These VHF oscillations can not be seen in normal oscilloscopes, but they strongly affect the audio performance.

Even the old Audio Research SP10 suffered from this problem - solved in the mk2 version with the addition of 1kohm stopper resistors in series with the tube grids.

 

[microstrip]

Here is the measurements with our without the tube. The cyan+magenta are with the tube:

Seems like you get to have all the transistor distortions plus tube! See the second harmonic added while the other ones (as best as can be seen) are not reduced.

it seems they are deliberating adding even order harmonic distortion (may be to try to create some "warmth" ) or may be something is faulty in your unit. Anyway, an example of something that should not have happened.

 

[JackD201]

You guys think the tube section is just a switchable buffer stage? The graph makes me think it.

 

[amirm]

it seems they are deliberating adding even order harmonic distortion (may be to try to create some "warmth" ) or may be something is faulty in your unit. Anyway, an example of something that should not have happened.

This is from the stereophile review. I did not have mine long enough to measure it.

 

[amirm]

You guys think the tube section is just a switchable buffer stage? The graph makes me think it.

From what I have read, yes, it is a buffer stage.