 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
136.175.75.28
Wondering if any SRPP experts can comment on my present state operating points for my e80cc SRPP which feeds a KT88 (tt22) in tetrode mode. I have one dual triode per channel.Present state is: one e80cc per channel
Top of totem B+. =425 volts
No resistor for top triode's annode
No resistor between bottom Anode and top grid
1k resistor between top grid and bottom anode
Bottom triode grid leak 10k (amp's input (and input impedance))
Bottom Grid stopper - 850k lossy ferrite bead
Bottom cathode: One 866ohm resistor bypassed by 220ohm black gate, then finally a 150ohm unbypassed for NF insertion point
Impedance of next stage: the KT88 has a 47k grid leakMeasurement: getting about 4.7 mA plate current, as calculated from voltage measurement on cathode bias resistors.
Heaters elevated to 115v
Are these good operating points?
My questions:
A. do I gain any sonic advantage of trying to go up in B3+ (top of totem)... or is it too big a risk to the heater to cathode leak issue...?
B. Do I gain any advantage of biasing for an increased plate current to the 'recommended' 5.8 mV current? Will it sound better than my present state 4.7mA?
C. should my 'totem' resistors be equal? or is there an advantage of lowering the middle one (top cathode to bottom anode) by 10% relative to the lower cathode bias resistors?
D. Should I raise my next stage KT88's grid leak from 47k to say 100k to make it an easier load for the e80cc?
E. The controversial question: Should I use one dual triode for both channels upper and another single dual triode for both triodes of the bottom two triodes? I understand that this makes it easier on the heater to cathode voltage issue (I do have multiple 6.3v secondaries- or i should say i have one secondary for each tube), but would I lose sonically in doing this? In other words, advantages of using one tube per channel top and bottom vs one for both channel tops and one for both channel bottoms.
E. part two: Keeping it one tube per channel as in the present state, what heater elevation voltage should I shoot for- should it just be exactly halfway between the upper triode cathode and bottom triode cathode?
F. will i ever enjoy the sound of a 12au7, 12bh7, or 6sn7 ever again? Or has this Tungsram e80cc ruined them forever?
G. How to reduce sibilance a bit?
Thanks in advance for the replies!
Edits: 08/25/23Follow Ups:
(Pictures of voltage gain circuit should be side by side not top to bottom)
Triode Kingdom- I'm taking your advice and going to abandon the SRPP. Should I just use a couple of good-old voltage gain sections in cascade such as this (pictured)?
Edits: 09/12/23
Ok, I'll leave the negative feedback there. I do have enough gain using a 3k3 resistor (and 330pf) for the neg feedback loop. Not sure how much Nf I need, or how to calculate, but based on volume it seems to be 6 db or so...
I would like to keep the SRPP... I have this belief that it will sound better and be simpler than changing to cascode.
also- I threw out the bypass-less SRPP idea after calculating the gain. With the bypass it seems to be 15v of gain, and without it calculates to 2.5v...
Also calculated that makes sense to leave the current totem resistors there, and add a 1k at the top anode. And increase the voltage to 475... and increase the heater elevation to 122...
"Not sure how much Nf I need, or how to calculate, but based on volume it seems to be 6 db or so..."
Drive the amplifier with a 1kHz sine wave so it produces a couple volts at the output. Now measure the output level with and without the GNFB loop connected. Convert the resulting voltage ratio to dB, and that's how much FB is being applied. 6dB isn't nearly enough to tame distortion, but maybe you actually have more. In the end, I suspect you'll find that applying the proper amount of feedback (probably somewhere between 14dB and 18dB) will leave the amp very short on gain.
"I would like to keep the SRPP... I have this belief that it will sound better and be simpler than changing to cascode."
Not cascode, cascade , one after the other. This will probably produce too much gain, but it can be adjusted downward with a nested loop. That will also improve driver distortion and headroom. This really isn't the right place for SRPP topology, especially with a mu of only 27.
Very good.
I measured NF at three different frequencies because I don't know how accurate my multimeter is at the midrange frequencies
I'm getting the following NF:
60hz 6db
500hz 9.5db
1000hz 11.5db
But also just learned that I can get an additional 6db of voltage (and possibly better sound) from my Schiit Gungnir by using pin 2 and 3 of the XLR rather than the 'summed" RCA...
So, considering the current Gain is just fine, I can probably get the extra gain from the XLR out on my Dac and increase the NF another 6db.
But what about all these designers that swear that NF is bad, and they just leave it out completely- even on a SE kt88?
Also- why would Audio Note design this amp with the SRPP if it wasn't optimal...
Ok, truth be told, this amp was originally a 12ax7 SRPP with an additional non-gain 6sn7 buffer stage for the input... but the buffer stage was not needed and I dont like the sound of 12ax7/5751... let alone the 12at7. 12au7 and 6sn7 are better but even less gain, so the e80cc seemed logical... and please beleive me when I tell you- it IS enough gain...
But I'll take your advice- I'll look into increasing the NF.
I think the pain point for me right now is that the 'ideal' load for this SRPP with e80cc is 10k. Which is a factor of 22 less than the original design of 220k, but the 220k was no good for the kt88... i beleive it was maybe degrading my cathodes...
So now increasing the coupling cap: but also I use a sub, and have it all digitally crossed over (80hz) before my dac (i use two dacs- one for main and one for sub) but I'm fine with a -3 point at 50hz, because my tube amp does the highs and the rolloff is fine, and so as long as i dont need full-range, seems .47uf works...
"But what about all these designers that swear that NF is bad, and they just leave it out completely- even on a SE kt88?"
It's inconceivable to me that any educated designer would do that. The distortion products that result from this practice are highly invasive and easily measured. It's really not appropriate except for a guitar amplifier.
"Also- why would Audio Note design this amp with the SRPP if it wasn't optimal..."
You would have to ask them. All audio companies bow to marketing pressure, and SRPP was the rage for years. It was never intended for this use though, and it doesn't provide push-pull functionality with these circuit values. If you search, you'll find this has been discussed at length here on the Asylum and elsewhere. I'll just add that I have seen situations in which SRPP provided very good distortion numbers, even though it wasn't "properly" loaded. Perhaps AN made measurements and confirmed some benefit in this application.
TK, can you comment on the idea of removing the power tube cathode bypass cap? And would the resulting local NF from the cathode resistor(s) tame some of the power tube's intermodulation distortion?
I'm not sure what happens with a pentode or a beam power tube but with a triode the plate resistance goes up and the relationship between the load impedance (output transformer) and the tube's output impedance gets unfavorable and HD goes up.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Can you speak a little more about the needed load resistance when running without the cathode bypass cap? Without this cap would I need a transformer with more resistance or less resistance?
I'm asking because my transformer is already a bit short on primary resistance. If removing the cap makes this situation better that's good but if it makes this low impedance trefo situation even worse then the cap is definitely staying in until I can get higher primary trefos.
I calculated my needed trefo primary using the equation Z=Va^2 / Pa and for the e80cc I get 8k. But my trefo is 2.6k because it was calculated for the HT of 425 and not the 585 I'm running...- but someday I'm ready for the Hasimoto with the 7k primary when I can afford it...-
But the irony now is nit that I optimized my SRPP (today) the 6sn7 sounds so much thicker and musical than the e80cc and I'm gonna go with it. So, now optimizing for it. And going to give up on e80cc - it's sound is just too intense and bringing for prolonged listening
Without the cathode bypass cap there will be "local current feedback" applied to the tube and that increases the plate resistance of the tube. The output transformer would need to have a higher primary impedance to keep the relationship between the output impedance of the tube and the input impedance of the transformer (the load impedance for the tube) happy. Otherwise the load line (for the tube) will be rotated towards the vertical. That will lower the gain of the stage and increase the harmonic distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I measured NF at three different frequencies because I don't know how accurate my multimeter is at the midrange frequencies
I'm getting the following NF:
60hz 6db
500hz 9.5db
1000hz 11.5db
I've never seen a multimeter exhibit such a wide variation in readings. Even cheap models that aren't "true RMS" should be flatter and more linear than this. Can I assume you re-verified the before and after numbers at each frequency? Feedback does vary with frequency of course, but these numbers are extreme, especially the 2dB difference between 500Hz and 1kHz. I think you will need to determine exactly what's happening with this before the numbers will be meaningful.
Yes, three separate measurements with and without NF.
I don't think that design has enough gain. How much feedback are you using (dB)?
Triode Kingdom, I had just enough gain with the NF connected... but now just discovered that I like how it sounds without NF... so plenty of gain now.
Just read that these are most linear when the load is about equal to or a bit smaller than the anode resistance (actually calculated 6.8k for ideal load but really not sure if I will have enough voltage in this config), so today I'm going to change the AC load resistor- (aka the next stage grid leak) to 13k and see if there is still enough voltage gain...
the other gain experiment will be to see if I can run it in an unbypassed configuration- removing the electrolytic and adding a top of totem 2k resistor between the power supply and the top anode to balance it out...
A single-ended pentode output with no NFB will produce a huge amount of IM distortion. Every effort to create a high quality amplifier will be defeated by this. I would suggest that you abandon the SRPP concept, configure the e80cc in cascade with a nested loop to control gain, and reinstall GNFB.
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
