how to make tube amp sound layback and mellow?

[hagtech]

thanks for spreading "the cat's" ideas here

Uh, this ain't no special secret. It only goes back a hundred years. Or at least way before either caT or I were born. Just look at the plate curves, it doesn't take a rocket scientist to see where to bias for euphony.

Many others do this too. Like the Paia tubehead, for one.

And if you check back, you'll see I was the one who pointed this out to the caT. Sure, he found his own sweet spot, I merely tried to help explain it.

jh

[lndm]

Originally posted by Dave Cigna
Even the plate resistor will have a noticeable impact on the sound.

Surprised it has barely been mentioned, I think changing resistors for carbon comp's is one of the easiest and foolproof ways to get that warm and mellow sound - even works well with crossovers.

[triode_al]

1) It has also been noted that an ECC83 will very easily give a warm sound 'out of the box'.
2) Often the anode resistor is taken as 3 times the Rplate. Why? pragmatics! There is a balance of distortion vs efficiency.
Now an interesting element: in my Revox Modell 40 amplifier the plate resistance is equal to the plate resistance (200 k) in a schema where there is NO kathode resistance bypassing) and consequently the tube draws very little current (0,7 mA or something like that). Then of course an output stage is needed. I thought this would never be OK (aka old thinking) but it did sound very nice and .... 'warm'.
3) Also it has bee reported that increasing the current gives too much controlled sound, maybe coming a bit forward...?
So, instead of going to 25 mA go to the low side, increase the Rp.
Yes, some say you tune into distortion, but, well, it's a trade off; and you're looking for some mellowness, aren't you?
4) Yes, I agree that having a high pass roll off (at 200 kHz) on the input grid would also potentially give the sound more depth.

Hope this helps in confusing you a bit (then it might be called Zen).

[arnoldc]

You are right bright is the word with the 12b4.
it is a brett 12b4 linstage.
I have try changing the the bais form 15ma to 19ma to 25 ma.
change the from 265v to 300 v
change bypass cap to black gate.
try 3 other SET amp, they all sound bright.
can it be the out cap ? .22 beewax.
Thansk again for all the help.
John

I don't know how different is Brett's to Brian's which I changed a bit and fdegrove changed a bit which became...



The 8k2 plate resistor is a Kiwame. And ignore the Auricap on the output as I downgraded to a 0.5uF Electrocube. B+ is 180V.

In my system, and with my amps, it is not bright. It does not have the coloration of 6SN7 preamps that I've tried and I like it better.

I'd second the recommendation about the passive first. If it yields the same "brightness" then it might not be the preamp.

ps.

My PSU is solid state- 4 x FR107 > 100uF > 1K > 100uF > 1K > 100uF + 47uF decoupling for both channels. All electrolytic

pps.

Sun Audio 2A3 attached...

It also exceeds the maximum capacitance for a cap-input PSU. I'd also change the 750 Rk to 880 (or 820 available at Farnell).

And since this amp has too much gain, I don't know why a preamp is still needed.

What speakers are you driving John?

[Johan Potgieter]

Hi all!

I get the impression that "mellow" and "laid back" mean different things to different people, like other terms such as sweet, musical, bright and shouty mentioned here. In that sense it would be very difficult to steer any person to what he wants by the mere (subjective) experiences of others.

All the advice given broadly applies, but what is actually desired? More 2nd harmonics would give what can be described by a richer sound (everything an octave higher added) - that would make most music sound better. 3rd harmonic "extras" are regarded to make things like violins and brass instuments sound crisper. I am not one for changing an amplifier's sound by biasing to add "extras" (another term is distortion, but that usually upsets folks).

For my money, to begin with, I would like to see a frequency response of what is desirable and what not, then one can work in somewhat less of a twilight. For example switching to an unbypassed cathode resistor or lowering the plate current can give the same sonic result, but the latter method could also increase 2nd harmonic distortion. It would appear to me, JJ2, that you can at best experiment, and it might be a thoroughly time-consuming journey!

Regards.

[lndm]

I like your point about the frequency response Johan. I once mused that due to Fletcher Munson et al, we really need to design for the levels we listen at. Like a continuously variable loudness control ganged to the volume control.

Maybe it is a little bass lift wanted here?

[jj2]

Originally posted by arnoldc

I don't know how different is Brett's to Brian's which I changed a bit and fdegrove changed a bit which became...



The 8k2 plate resistor is a Kiwame. And ignore the Auricap on the output as I downgraded to a 0.5uF Electrocube. B+ is 180V.

In my system, and with my amps, it is not bright. It does not have the coloration of 6SN7 preamps that I've tried and I like it better.

I'd second the recommendation about the passive first. If it yields the same "brightness" then it might not be the preamp.

ps.

My PSU is solid state- 4 x FR107 > 100uF > 1K > 100uF > 1K > 100uF + 47uF decoupling for both channels. All electrolytic

pps.

Sun Audio 2A3 attached...

It also exceeds the maximum capacitance for a cap-input PSU. I'd also change the 750 Rk to 880 (or 820 available at Farnell).

And since this amp has too much gain, I don't know why a preamp is still needed.

What speakers are you driving John?

thanks arnoldc
problem solve ,it was the cap.(.22 beewax)
the 12b4 was bright when I have it with beewax .22uf, but after I change it to 3.3uf the brightness are gone.
I have use 6x4>46uf>2k>4uf>8k2 plate resistor.
my speakers are Avantgarde UNO,ZU Druid and Omega super 3.
amps DRD45,sun 2A# and Cary 805C

arnoldc what have you done to your sun 2a3 and what improve over the stock unit ?

Thanks John.

[HiFiNutNut]

jj2,

I built Brett's 12B4A preamp and I would not describe it bright. Natural or analytical perhaps. It sounds very good. In an A/B test it destroyed the Audio Research LS8 comfortably. The ARC has a very sweet and clear tone.

I used the cheapest possible 2W metal film resistors (in parallel to make up the Watts) since I couldn't source any resistors of famous brands in Australia.

220R grid stopper, 680R cathode (no cathode bypass), 5.8k plate, 4.7uF ICW Clarity SA Cap (630V polypropylene), cheap 50k volume pot.

I have compared using the 12B4A preamp to without using a preamp (with a 845, LS3/5A, since the 845 has a volume control) and adding the preamp actually improved the overall sound. Without the preamp, the sound was clearer and cleaner but much thinner. Adding the 12B4A preamp added much more depth to the music. It may be due to the "good" distortion introduced by the 12B4A or the 12B4A provides better drivability / impedance matching.

I agreed that the 0.22uF was way too low. As far as parts are concerned, your electro caps have the biggest influence on the sound.

I think to get mellow and warmer sound a tiny change on the speaker crossover will have a much bigger impact than changing parts on the preamp.

Tube rolling? I have had Westinghouse, Sylvania and GE JAN. The differences in noise and microphonics exist (can't say which brand is better because I had at most a pair or two for each) but you would push yourself hard to find differences in sound.

I would be interested in knowing your result comparing your now "rectified" 12B4A preamp (with 3.3uF output cap) to your passive preamp. I would also be interested in your findings on various bias points on the sound of the 12B4A.

Kind regards,
Bill

[arnoldc]

Hi John,

It's not mine. But the minor mods include:

1) lowering the first cap to 20uF (so as not to exceed 5U4-G limit)
2) the extra 47uF used with another choke (lower effective B+, a bit)
3) last filter cap paralleled with small value film (0.01 RTX)
4) changed 750 ohm Rk to 820 ohm