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Old 29th November 2012, 06:22 AM   #21
TVC is offline TVC
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Quote:
Originally Posted by Zen Mod View Post
that amp needs something as extended rework

try to find Ongaku schematic and compare general principle

problem is that - if you decrease voltage for SRPP - stage will not have enough headroom for driving 211 ; without thorough analysis I can't say more and I'm sorta short in time these days .......

one thing is sure - if you do what I wrote regarding additional xformer , heaters will be where they need to be
-------------------------------------

hello,

is it discuss for kondo ongaku? internet circuit had little mistake.

thx

Last edited by TVC; 29th November 2012 at 06:38 AM.
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Old 10th December 2012, 09:36 PM   #22
rrrs is offline rrrs  United States
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Hi, wanted to update a bit on configurations I tried and my findings.
I always kept one of the mono-blocks unchanged to have a clear comparison.
First I built a mu-stage with 5687 and D3a; this was very clear improvement over my original 3 stage design; much less gain (but almost enough for my setup) and way more transparent; thanks to Kevin to steer me from 3 to 2 stage concept!
Then I tried an interstage design using 5687 driver (180V@15mA), Lundahl LL1692 wired 4:3.5 and this was another improvement over mu-stage design; it just sounded cleaner.
Here Tungsol 5687 sounds a bit better (smoother in a good way) in my system then RCA.
Next I tried Amperex 7788 wired as triode (190V@20mA), IT wired 3.5:2; here overall gain was perfect in my system, but 5687 setup sounded better; 7788 had more pronounced HF, but sounded sort of "sharp"; maybe I had some parasitic oscillations, but was not able to find any on my scope?
Next was D3a as triode (150V@19mA), IT wired 3.5:2; I find this slightly better then 5687, but difference is very small. With D3a speaker appears to be "bigger, wider", but sounds a bit darker. 5687 has a bit more detail, but it sounds "closer and smaller".
See attached my latest schematics with D3a driver and 3.5:2 IT.
When looking on the scope at 1K sine wave I get 150Vpp max at plate of D3a before there is a clipping (only on one peak of the wave) at the secondary of 3.5:2 IT transformer/211 grid; with 5687 I get 170Vpp max before clipping at secondary of 4:3.5 IT transformer/211 grid.
In both cases there is no clipping at D3a or 5687 plate/IT primary, even when pushed up to 200Vpp!? Wondering if that means that it is IT that distorts the signal?
In any case, this happens at higher input signal then I would have (I got 2.4Vpp from my transformer coupled DAC chip), but would be interested to understand the cause of that distortion.
I still got C3g and WE396(would use this one in parallel configuration) tubes to try as a driver; will report once I get a chance to compare them to D3a and 5687.
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File Type: pdf 211 IT-update .pdf (33.7 KB, 87 views)
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Old 11th December 2012, 12:55 AM   #23
kevinkr is offline kevinkr  United States
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I would suspect grid current if the 211 is installed.
Check without..
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Old 11th December 2012, 01:54 PM   #24
euro21 is offline euro21  Hungary
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Quote:
Originally Posted by rrrs View Post
... D3a driver and 3.5:2 IT.
When looking on the scope at 1K sine wave I get 150Vpp max at plate of D3a before there is a clipping
1.) IMHO 19mA too much.
2.) My D3a drivers (green LED bias -1.95...2V-, 170..175V cathode-anode voltage, 10mA anode current, cascode -DN2540- CCS, capacitor coupling to next stage) produces even 300Vpp with moderate distortion.

Measured FFT samples: D3a drives A2 mode 300B -8W SET- with 95V RMS, and 10W with 105V RMS.
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File Type: jpg EH300B 8W gate 95V RMS.jpg (173.9 KB, 181 views)
File Type: jpg EH300B 10W, gate 105V RMS.jpg (233.3 KB, 173 views)
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Old 11th December 2012, 08:15 PM   #25
rrrs is offline rrrs  United States
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Hi, yes, it must have been a grid current. Without 211 tube I got 230Vpp on 5687 plate/200Vpp on 4:3.5 IT secondary without any visible sin wave distortion; was not able to try beyond that as I ran out of input signal voltage!
With D3a visible wave distortion happened at 200Vpp on plate/130Vpp on 3.5:2 IT secondary; here it was clear it had to do with not enough D3a grid bias as signal at that point was 2.8Vpp, so a limit for my -1.5V bias.
Will try a bit higher bias and lower current, as euro21 suggested.
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Old 11th December 2012, 10:03 PM   #26
kevinkr is offline kevinkr  United States
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I run 20mA, roughly 2.2V of bias and a plate voltage of 170 - 190 depending on the individual tube. Plate voltage is set by a CCS ahead of the IT. (Cap decoupled between CCS and IT B+ end of primary..) Works well and stays somewhat below max dissipation.. The IT is 1:1..
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Old 11th December 2012, 11:17 PM   #27
rrrs is offline rrrs  United States
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Hi, did some more testing and got a clear picture what is happening:
D3a: 208V plate,-2.3 bias, 19mA, measured gain-71 I get visible distortion on 211 grid (211 operating) at 160Vpp 3.5:2 IT primary/ 98Vpp on sec. (211 grid), 211 bias -50V
D3a: 175V plate,-1.9 bias, 16mA, measured gain-75 I get visible distortion on 211 grid at 160Vpp 3.5:2 IT primary/ 98Vpp on sec. (211 grid), 211 bias -50V
So, this two operating points give exactly same results, accept gain being a bit higher at second case (find this strange as expected gain to drop a bit with lower current)!
The second case looks good as it is ran more conservative and also G2 is under max spec that is 180V, so will leave it this way.

Then changed 211 bias from -50 to -55 (drops 211 current from 65 to 60mA); hear I was able to get up to 185Vpp 3.5:2 IT primary/ 105Vpp on sec.; so clearly distortion is caused by 211 grid current, as Kevin suggested.

Interesting was the test of Amperex 7788: 172V plate,-2.6 bias, 21.6mA, measured gain-54 I get visible distortion on 211 grid at 162Vpp 3.5:2 IT primary/ 92Vpp on sec. (211 grid), 211 bias -50V; so 7788 in same conditions distorts earlier then D3a, although I was expecting 7788 would be able to handle more grid current (lower plate impedance and ran at higher current)!

Tested also 5687: 190V plate,-7.5 bias, 15.5mA, measured gain-16 I get visible distortion on 211 grid (211 operating) at 120Vpp 4:3.5 IT primary/ 105Vpp on sec. (211 grid), 211 bias -55V; so 5687 trough 4:3.5 IT can deliver exactly same max Vpp to 211 grid as D3a does trough 3.5:2 IT; was surprised a bit here as I expected D3a in combination with step-down IT to be able to deliver more current and Vpp to 211 grid.

Based on this it seems it would be best increase plate V for 211 (it is 930V-970V now, depending on my mains voltage fluctuation) to around 1100V and then be able to run higher 211 bias voltage while maintaining 60-70mA current.
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Old 12th December 2012, 02:24 AM   #28
kevinkr is offline kevinkr  United States
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Running -50V bias and swinging +49Vpk implies you are extremely close to zero bias in the 211 and I would expect some grid current to be flowing under these conditions.

One quick question, how are you biasing the D3A? Are you using grid bias, an LED or something else?

A quick look at a grid current graph indicates that grid current should be about 8mA at +25V (for a wide range of plate voltage) so something is not adding up.. (Maybe just in my head.. )

What is the dcr of the IT secondary, and what is the AC+DC component actually on the grid? Is the swing symmetrical about the baseline dc?

How do you return the cold end (AC wise) of the IT to signal ground?

Can you post pix or crude drawing of waveforms with amplitude? Circuit topology?
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Old 12th December 2012, 04:31 AM   #29
rrrs is offline rrrs  United States
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Here is the schematics of latest setup; see attached.
I am using just a 120R resistor on cathode; as this amp is not used below 80Hz I did not add bypass cap; measured -3db point is just under 30Hz without the cap.
I want to try battery bias (in series with the D3a grid), but choice of voltages are a bit limiting; button batteries seem to come in 1.55 or next is 3V.
IT, when wired 3.5:2 got primary DCR 790R, secondary is 200R.
From cold end of IT there is a 100K resistor, to the wiper of 20K bias adjustment pot, and there it is de-coupled to the ground.
Here is the waveform; sorry for really bad photo, but you could see the distortion that starts to happen at the bottom; it is not symmetrical; top stays clean.
This is happening with 211 operating, measurement taken at 211 grid, 211 running at -50V bias and AC on 211 grid is about +49 Vpeak (98Vpp) when this distortion starts.
Second trace is on the input signal taken on D3a grid resistor; of course they are in different volt/divisions settings.
Yes, 211 data shows that at +25V grid there should be 8mA grid current.
How much current should I expect to be able to get out of D3a; 8mA trough 3.5:2 IT should result at just 4.5mA on D3a plate?
Attached Images
File Type: jpg Trace.JPG (78.7 KB, 131 views)
Attached Files
File Type: pdf 211 IT-update .pdf (35.6 KB, 34 views)
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Old 12th December 2012, 04:48 AM   #30
kevinkr is offline kevinkr  United States
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I'd try a cheap red LED as the current 120 ohm resistor is raising effective rp to something over 10K which is not good for linearity with the IT.

In addition you are throwing away about 13% of your available mu which means for a given output level you have to drive the D3A harder which is pushing it towards grid current and linearity issues. I suspect this contributes to the lack of linearity as you drive the D3A towards the saturation region. (Unnecessarily early onset)

I would expect the D3A to be able to source at least 3 - 4mA current before clipping.

I think you may find that there is significantly more drive available with fixed bias or the led than with the unbypassed cathode resistor.
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