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My SE amps sound harsh without NFB

I have two DIY SE tube amps I built. One with 6V6 tubes and the other EL84. Both are strapped as triodes. I prefer the triode sound to pentode wired and don't need the extra couple watts. I have tried to listen to them with no NFB but they sound a bit too harsh and aggressive through Klipsch Forte's without some global NFB using a 10K resistor. I read about similar amps with no NFB and how great they sound. So please fill me in on what could be going on.
 
Troubleshooting a tube amplifier without a complete and accurate schematic, is like . . .
Blindfolding an Archer and asking him to hit the center of the Bulls Eye.

Please post a schematic of your amplifier, including the resistances, voltages, transformer primary impedance, etc.
(power supply too).
 
OK I knew you were going to say that. The EL84 amp is the familiar one but the plate is connected to the screen with a 100 ohm resistor. No screen voltage.
 

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I have two DIY SE tube amps I built. One with 6V6 tubes and the other EL84. Both are strapped as triodes. I prefer the triode sound to pentode wired and don't need the extra couple watts. I have tried to listen to them with no NFB but they sound a bit too harsh and aggressive through Klipsch Forte's without some global NFB using a 10K resistor. I read about similar amps with no NFB and how great they sound. So please fill me in on what could be going on.

Easy : your Klipsch Forte - as many Klipsch speakers - has certainly a very tormented impedance curve... See my Heresy I original Z curve below, as an illustration, where you can see a large hump in the 3K-5K region :

1694546022541.png


Given that you amp has no FB, it offers very poor damping (usually less than 1). So in the case of my Heresy I, it resulted in a barking, in-your-face tone. If you have harshness with your Forte, it is possibly because the hump is also high and placed more in the treble region.

Check with a modern SS amp, offering good FB (usually over 30-40) : the harshness should cease, the damping offered by the amp "tames" the hi-mid / treble hump of the Z curve.

If yes, then I don't see other solution than :

  • add enough FB to your amp to damp the hump.
  • modify the Forte X-over to tame that hump.

For my Heresy I, I chose the second solution. Here are the details below. Maybe testing a similar idea could work on your Forte ?

sPnnLb-heresy-F-and-Z-curves-13-11-2005-.jpg


T
 
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OT primary impedance is 5K,-should be 3.5K if the screen grid is connected to the anode via a 100R resistor.
EL 84 as a pentode require 5K,-in triode mode 3.5K as far as I know.
For max power output yes the lower primary impedance is better but using the 5k impedance you actually improve the damping factor slightly which in this case maybe beneficial.
 
Dave Gillespie is no slouch and that's a very carefully-designed circuit. He never modifies or designs anything that doesn't measure beautifully. Changing it to triode mode probably reduces the feedback and degrades the sound to some extent. Removing the feedback will only make that worse.

No-feedback amps typically use low-mu, low-impedance output tubes like a 2a3 or 300B. This amp is designed for a pentode and as such requires feedback in the design.
 
i built an all pentode single ended amp without global negative feedback....yes, it can be done....
do not get caught up in the triode vs. pentode debate bruhaha...there is much more enjoyment to be had...
 

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Dave Gillespie is no slouch and that's a very carefully-designed circuit. He never modifies or designs anything that doesn't measure beautifully. Changing it to triode mode probably reduces the feedback and degrades the sound to some extent. Removing the feedback will only make that worse.

No-feedback amps typically use low-mu, low-impedance output tubes like a 2a3 or 300B. This amp is designed for a pentode and as such requires feedback in the design.
we are only interested in a person's design not his personality.....
his design, that was how he wanted it, he has his reasons and we respect him for that though we may not agree...
 
Who said anything about his personality? He's a very skilled modifier and designer, to a limited extent. The amps he modifies or designs measure extremely well and are extremely stable. If you remove the feedback from that particular design it will not perform anywhere near as well. I'm not surprised that it sounds bad without it.

Whatever modifications are made to that amp may or may not not work as well with that particular output transformer. But it won't hurt anything to try. My guess is that returning to the original as designed will yield the best results.
 
Generalizations:

Triode Wired mode:
A Triode Wired 6BQ5 beam power tube with somewhere between 250V to 300V plate to cathode, about 40mA of plate current,
and with a 100 Ohm resistor from plate to screen, has a plate impedance, rp, of about 1700 Ohms.

Without the global negative feedback applied . . .
5000 Ohm output transformer primary / 1700 Ohms = Damping Factor of 2.9. But because of the output transformer primary DCR and secondary DCR, there will be an insertion loss of the transformer.
Expect a damping factor of about 2.3 to 2.7.

If you apply global negative feedback, the damping factor will increase;
But the gain of the amplifier will be reduced, along with a reduction of distortion.

Beam Power mode:
For the same 6BQ5 tube in beam power mode, and with No global negative feedback, the plate impedance, rp, will be about 15000 Ohms.
The damping factor will be 5000 / 15000 = 0.33, much less than unity (<< 1), and the distortion will be somewhat high.
Global negative feedback, or Schade negative feedback, or some other form of negative feedback that includes the output tube, will be required for a damping factor > 1. That will also reduce the distortion.

Ultra Linear mode:
An output transformer with the Ultra Linear Taps is required.
If there is another negative feedback that includes the output tube, in addition to Ultra Linear, the performance will be somewhere between beam power mode, and triode mode.
The damping factor and distortion will be between the values for Triode Wired, and beam power modes.

Power Output, generally . . .
beam power mode - highest
ultra linear mode - medium
triode wired - lowest

I hope I did not have a Typo or mistake.

Have fun!
 
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