|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
108.168.121.47
In Reply to: RE: RH84 : tried higher B+ posted by deathtube 667 on January 21, 2015 at 14:14:39
Ah well you tried.
Let us know when you go at it again.
On the other hand, while you have the B+, why not try the first version of the RH84 without the zener diode and LM317?
Follow Ups:
I measured the operating temperature of the LM317 w/heatsink.
It was 85F, the extra B+ voltage did not seem to make much difference.
The voltage between out and adjust across the 27 ohm current set resistor was only .985V instead of 1.25V
Something is affecting the CCS operation of the LM317 when the B+ is at 300V.
I sleep on it and try a bit more tomorrow.
No need to sleep on it.
Bias voltage too low = cathode voltage too low.
In order to function properly, LM317 needs a voltage drop across it. This is not a low drop regulator. Drop voltage plus reference voltage equals cathode bias voltage.
Depending on Ug2, Ua, tube condition, etc. this might be too low for the LM317 and the best sign is reference voltage below spec (1.25V).
Probably your tubes are old or Ug2 is too low?
Did you remove the cap from g2 to ground?
Needless to say, all this affects performance. Spice sim is ideal, but you might have other issues in your test circuit like old worn out tubes, leaking caps, faulty zeners...
******
http://rh-amps.blogspot.com/
First, I removed the cap bypassing the CCS.The B+ and other voltages appeared to stabilize.
Measured 1.25V on the output tap of the LM317.
Removing the cap from g2 to ground did not have any effect on stability.
I will re-measure distortion/spectrum at 1W to see if improvements are realized and post findings.
Thank you for your assistance.
dt 677
Edits: 01/22/15
The cap from g2 to ground is superfluous and while it does not affect DC performance, it probably does affect AC performance (bandwidth).
The leaky or otherwise bad cap was probably influencing your output power vs distortion. Besides, at this power levels, 40-50V across the tube affect performance more than you might imagine.
Last but not least, 5k is not the same as 6k, particularly for a pentode.
******
http://rh-amps.blogspot.com/
i thought the lm317 needed to be bypassed with an SE circuit?
After removing the CCS bypass caps, I measured with ARTA at 1W and got horrid distortion , THD = 42%.
I didn't realize that the bypass cap was needed for correct circuit operation since I never tried LM317 as a CCS before.
I took out my scope and looked at the output of the amp, the resulting waveform was like a sine on the positive and a weird looking distorted square wave on the negative. It was really odd looking.
Next, I checked the driver stage and it was function normally.
After checking the EL84 plate, I saw the odd distorted signal again, so I knew it was probably the CCS.
But it was not until I read your post that I decided to bypass the CCS with some 22uF caps on jumper leads that everything worked again.
I'll have to try another set of 100uF bypass caps and see if anything changes.
Needless to say, not the most fun evening but it was informative, LOL.
Of course it does.
But the above is about DC performance improvement after elimination of leaky cap. The cap must be good. As for voltage value, twice the bias voltage I.e. cathode voltage is probably the best choice.
******
http://rh-amps.blogspot.com/
With B+ off the rectifier (UF4007 full wave) of 300VDC :
Total power output before clipping : 3W RMS
Gain = 6.36
THD = 2.43% @ 2.75W RMS measured with ARTA
2nd harmonic = -34.64dB, 3rd harmonic = -36.62dB ,
4th harmonic = -48.47dB, 5th harmonic = -55.28dB
60 cycle below -80dB, 120 cycle below -60dB
I failed to match the designers benchmark of 1% distortion at 4W RMS.
Perhaps deviations from the original design or using cheap AES PT-31 OPT's is a contributing factor.
That is it for now.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: