In a direct coupled line driver, does the plate voltage of the first stage set the bias of the second stage?
So, for example, on my 6SN7 driven 6V6 Push Pull amp, the first triode of the 6SN7 has a plate voltage of 65V and a bias voltage of 2V = 67V
Shouldn't this then appear on the grid of the next stage? But its bias voltage is 6V, which should make the cathode voltage for that stage 67+6=73V, which should be the same for the plate resistor of 62K also. The current is then V:R= 73:62=1.2 mA, which is not very much....
However the plate voltage I am reading on the second triode of the 6SN7 is 130v - I'm attempting to reverse engineer the circuit and not having much joy
So, for example, on my 6SN7 driven 6V6 Push Pull amp, the first triode of the 6SN7 has a plate voltage of 65V and a bias voltage of 2V = 67V
Shouldn't this then appear on the grid of the next stage? But its bias voltage is 6V, which should make the cathode voltage for that stage 67+6=73V, which should be the same for the plate resistor of 62K also. The current is then V:R= 73:62=1.2 mA, which is not very much....
However the plate voltage I am reading on the second triode of the 6SN7 is 130v - I'm attempting to reverse engineer the circuit and not having much joy
Thanks Ian - I've trawled through quite a few of those schematics, but can't quite find anything like mine. I have also toyed around with some adapters to see how the plate voltages are affected with different tubes:
6SN7 (mu 20) - first stage; 65v, 2v bias, second stage; 130v, 6v bias
6DJ8 (mu 33) - first stage; 70v, 2.5v bias, second stage; 140v, 4.5v bias
E180CC (mu 47) - first stage; 90v, 2v bias, second stage; 100v, 3v bias
Not sure what all this means, but the E180CC definitely has a better sound, more full bodied with extended definition in the low frequencies. The gain is not double, though...seems only a little louder.
Is it worthwhile temporarily putting a capacitor between the first half's plate and the second half's grid, just to see what the voltage is on the 2nd half's plate before changing anything else?
6SN7 (mu 20) - first stage; 65v, 2v bias, second stage; 130v, 6v bias
6DJ8 (mu 33) - first stage; 70v, 2.5v bias, second stage; 140v, 4.5v bias
E180CC (mu 47) - first stage; 90v, 2v bias, second stage; 100v, 3v bias
Not sure what all this means, but the E180CC definitely has a better sound, more full bodied with extended definition in the low frequencies. The gain is not double, though...seems only a little louder.
Is it worthwhile temporarily putting a capacitor between the first half's plate and the second half's grid, just to see what the voltage is on the 2nd half's plate before changing anything else?
You must have got to this staruight away - when I realised I'd linked not just one of those schematics but the whole list I deleted the post.
The schematic is almost certainly a common cathode gains stage, direct coupled to a concertina (cathodyne) phase splitter. 62K in the anode and cathode circuits of the cathodyne. That is why the low anode voltage on the input stage, because of the direct coupling it sets the operating point for the cathodyne.
It will look very much like this with some different componenet values:
http://www.joeltunnah.com/images/jt_6SN7_6V6.jpg
If you intend to make mods I would recommend that you remove the direct coupling between the common cathode gain stage and the cathodyne splitter, like what has been done in the following example. It allows you to run the input stage at a better operating point because you no longer have to make sure the anode voltage is suitable for the direct coupled phase splitter.
http://webpages.charter.net/dawill/tmor ... 6W6_PP.gif
Hope this of some help to you,
Cheers,
Ian
The schematic is almost certainly a common cathode gains stage, direct coupled to a concertina (cathodyne) phase splitter. 62K in the anode and cathode circuits of the cathodyne. That is why the low anode voltage on the input stage, because of the direct coupling it sets the operating point for the cathodyne.
It will look very much like this with some different componenet values:
http://www.joeltunnah.com/images/jt_6SN7_6V6.jpg
If you intend to make mods I would recommend that you remove the direct coupling between the common cathode gain stage and the cathodyne splitter, like what has been done in the following example. It allows you to run the input stage at a better operating point because you no longer have to make sure the anode voltage is suitable for the direct coupled phase splitter.
http://webpages.charter.net/dawill/tmor ... 6W6_PP.gif
Hope this of some help to you,
Cheers,
Ian
Fantastic - that is exactly what I have been looking for, that schematic is almost spot on - the b+ at the input stage is 280v, so it gives me a lot to go by.
The second schematic you linked isn't quite working, I think the hyperlink is missing, are you able to re-edit?
Thanks Ian
By the way - I see you are in Adelaide...me too! What are the chances?!
The second schematic you linked isn't quite working, I think the hyperlink is missing, are you able to re-edit?
Thanks Ian
By the way - I see you are in Adelaide...me too! What are the chances?!
After a little more testing, I think I’d like to do the mod which Ian has suggested - remove the direct coupling between the common cathode gain stage and the cathodyne splitter, which means we can run the input stage at more optimal operating conditions.
However, before this – I’ve taken a few measurements to see if anyone is able to advise whether such a mod would upset the stages further along in the amp.
Data is as follows:
6SN7 pre-stage tubes:
Plate voltage on the first stage is 65v
Plate voltage on the second stage (splitter) is 130v
GRID 1 = audio input from dac/preamp
PLATE 1 = resistance from power supply is 100k, note this pin is also directly coupled to PIN 4
CATHODE 1 = resistance shown is 920R (curiously this pin is also linked to the 16 ohm tap on the output transformer, possibly for feedback purposes??)
GRID 2 = directly coupled to PIN 2 (PLATE 1)
PLATE 2 = resistance seen is 62k, noting that this pin is also directly linked to the 0.22uF capacitor which leads to the 6V6
CATHODE 2 = also directly linked to a second 0.22uF capacitor, and the resistance seen is 62k, via a second 62k resistor.
6V6 stage:
The voltage at the output transformers of the amp is 289.5v
The voltage over the 6V6 tubes (plate-cathode) is 267v.
The bias voltage of the 6V6 tubes is 15v.
In relation to the bias resistor for the 6V6, it appears a single resistor is being used to bias both tubes for each channel – is this considered normal practice?
So, if we were to insert 0.1uF between plate 1 and grid 2 of the 6SN7 tubes, what would the other necessary changes be? Presumably we can change the resistor at plate 1 to get a higher plate voltage?
But also, 130v on the 6SN7 in the cathodyne splitter is not optimal....
However, before this – I’ve taken a few measurements to see if anyone is able to advise whether such a mod would upset the stages further along in the amp.
Data is as follows:
6SN7 pre-stage tubes:
Plate voltage on the first stage is 65v
Plate voltage on the second stage (splitter) is 130v
GRID 1 = audio input from dac/preamp
PLATE 1 = resistance from power supply is 100k, note this pin is also directly coupled to PIN 4
CATHODE 1 = resistance shown is 920R (curiously this pin is also linked to the 16 ohm tap on the output transformer, possibly for feedback purposes??)
GRID 2 = directly coupled to PIN 2 (PLATE 1)
PLATE 2 = resistance seen is 62k, noting that this pin is also directly linked to the 0.22uF capacitor which leads to the 6V6
CATHODE 2 = also directly linked to a second 0.22uF capacitor, and the resistance seen is 62k, via a second 62k resistor.
6V6 stage:
The voltage at the output transformers of the amp is 289.5v
The voltage over the 6V6 tubes (plate-cathode) is 267v.
The bias voltage of the 6V6 tubes is 15v.
In relation to the bias resistor for the 6V6, it appears a single resistor is being used to bias both tubes for each channel – is this considered normal practice?
So, if we were to insert 0.1uF between plate 1 and grid 2 of the 6SN7 tubes, what would the other necessary changes be? Presumably we can change the resistor at plate 1 to get a higher plate voltage?
But also, 130v on the 6SN7 in the cathodyne splitter is not optimal....
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