Line Amp Design review

[DougL]

I have been working on a 6H30 SRPP line amp.

I wanted to ask if there were any comments.

My goals were a moderate gain, simple circuit to drive a Tube power amp.


Thanks.

Doug

[SY]

What gain did you calculate for this circuit?

I've never used this tube before, but the specs look interesting. They're expensive, but not horribly so. I'm curious about anyone's experiences with these as regards microphonics, reliability, tendency toward oscillation, or any other watch-out-fors.

[DougL]

What gain did you calculate for this circuit?

I got about 7 (17 db) un-bypassed and about 10.5 (20.5 db) bypassed from Tube CAD.

I was originally looking for a circuit with gain between 3 and 7.
after reading an article claming that un-bypassed cathode resistors degraded tube parameters, I decided to try it both ways.
I am also looking at trying 3 red LED's instead of the cathode resistor. I have never seen it done, and might unbalance the circuit, but it was a thought.

Doug

[sparkle]

Hi!
My humble opinion- looks nice but I would like to use some bleeder resistor on Your 47uF cap and bypass the cathode of the lower tube for sure (like You did).
Also watch for the potential of Your heater- it is not on the ground.
regards
daniel

[SY]

after reading an article claming that un-bypassed cathode resistors degraded tube parameters

That's not correct. Unbypassed cathode resistors increase degeneration, not degradation. Leaving off the bypass cap reduces distortion.

[x-pro]

Originally posted by SY


That's not correct. Unbypassed cathode resistors increase degeneration, not degradation. Leaving off the bypass cap reduces distortion.

However, at the same time the distortion structure changes, increasing the number of harmonics, so it is (as with any NFB) a double-sided effect and could actually degrade the sound quality.

x-pro

[SY]

Originally posted by x-pro


However, at the same time the distortion structure changes, increasing the number of harmonics, so it is (as with any NFB) a double-sided effect and could actually degrade the sound quality.

x-pro

That's absolutely correct- see Baxandall's excellent articles, for example- but distortion reduction with degeneration is still true as a general statement.

The art of design is being able to make a judgement as to where the most appropriate use of the feedback tool can be made. If I had, for example, a circuit which is producing 1% of 2nd HD and vanishingly low third and higher, yeah, I'd be hesitant to throw in some degeneration. But more often, one is starting with a circuit that has appreciable amounts of third and fifth to begin with, so the use of feedback can be very appropriate.

[ekaerin]

Hi folks,
I'm not an expert on tubes but I have designed some tube-stuff.
If you dont need the excess gain at least make options to easily
remove your bypass cap and even make provision for a film type in
parallel with the electrolytic. Then you can listen and compare.

Also I would put in some low pass noise filter at the input to avoid
RF crap into the circuit.

And maybe a small film cap for decoupling near the tube.

What does Tube_Cad say if you pick your signal at the lower halfs anode. ?

See my old post: When the 6922 married the 2sj49/2sk134

/ Mattias

[ekaerin]

Just want to add.
I did the simulation of my hybrid amp with Synopsys Saber software
and I think I reached the lowest output impedance by picking the signal
at the top of the bottom anode. Otherwise i don't remember any other
difference.
It would be nice to here some opinions on this from you tube-dudes.

/ Regards / Mattias

[DougL]

use some bleeder resistor on Your 47uF cap

Good Catch.

Also watch for the potential of Your heater- it is not on the ground.

Also excellent point. A voltage divider set to 1/3 b+ should do it.

And maybe a small film cap for decoupling near the tube.

Yes. I will do it.

What does Tube_Cad say if you pick your signal at the lower halfs anode. ?

I'll run it tonight.
My WAG at this point is gain about mu (15) and Zout about Rp (2k) rather than gain 7 and Zout about Rp/2 (1K). But I'll check.

SY, thanks for the sanity check on cathode bypass. I am going to pick up a BlackGate and try it as a learning experience.
The reports of the 6H30 have been positive, but it’s prone to oscillate. I may need to throw a ferrite bead on the upper plate.
I'll report back with my experiences.

Thanks for the responses

Doug