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807 Tube Triode Connected
807 Tube Triode Connected
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Old 29th November 2011, 10:22 AM   #11
Alain Poitras is offline Alain Poitras  Canada
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oshifis,

You ask me in your message what is the difference between the same circuit with the "Quad style" connexion and the "usual" connexion like with a standard transformer :

Click the image to open in full size.

Much less input voltage is needed to reach the maximum power. The output power is slightly lower but the distortion is slightly higher except at 20Khz because there is more power loss at that frequency ...

Alain.
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Old 29th November 2011, 11:41 AM   #12
SY is offline SY  United States
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807 Tube Triode Connected
Just curious about a couple points:

1. Why parallel the cathode followers? The source Z of a single 6DJ8 CF is low enough to put the HF pole more than a decade higher than the transformer rolloff.

2. Why direct couple the CF to the driver stage and cap couple to the output stage? If you do it the other way around, you can have saner voltages on the 6DJ8 and make the amp much more resistant to blocking.
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Old 29th November 2011, 11:52 AM   #13
lcsaszar is online now lcsaszar  Hungary
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Quote:
Originally Posted by Alain Poitras View Post
oshifis,

You ask me in your message what is the difference between the same circuit with the "Quad style" connexion and the "usual" connexion like with a standard transformer :

Click the image to open in full size.

Much less input voltage is needed to reach the maximum power. The output power is slightly lower but the distortion is slightly higher except at 20Khz because there is more power loss at that frequency ...

Alain.
Alain,

Thanks for your efforts and the analysis. I will follow your advices and modify the circuit in the following order:

1. Decrease the plate voltage of the cathode follower. Fortunately, I just need to replace a single resistor in the power supply.
2. Put some LEDs in the cathode to get about 5 to 10 V cathode voltage. What type of LED is most suitable for this (green, red, high-efficiency, ...)?
3. Increase the grid resistor of the output tubes to 220K and/or increase the coupling capacitor (but I don't easily replace those nice Russian teflon capacitors...)

I will report on the results, may take some time until I implement them.

Here are some pictures of the amplifier:

https://picasaweb.google.com/lcsaszar/807_amplifier
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Old 30th November 2011, 05:47 AM   #14
Alain Poitras is offline Alain Poitras  Canada
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Quote:
Originally Posted by SY View Post
Just curious about a couple points:

1. Why parallel the cathode followers? The source Z of a single 6DJ8 CF is low enough to put the HF pole more than a decade higher than the transformer rolloff.

2. Why direct couple the CF to the driver stage and cap couple to the output stage? If you do it the other way around, you can have saner voltages on the 6DJ8 and make the amp much more resistant to blocking.
Only oshifis can answer those questions because he build the amplifier that way ...

He ask me to help him find some ways to improve it and this is what I am doing. But I don't think he like to modify it too much ... Just some few adjustments ...
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Old 30th November 2011, 06:01 AM   #15
Wavebourn is offline Wavebourn  United States
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807 Tube Triode Connected
Quote:
Originally Posted by Alain Poitras View Post
Only oshifis can answer those questions because he build the amplifier that way ...
I thought it was your design, and was going to ask the same couple of questions, by the way.
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Old 30th November 2011, 06:24 AM   #16
Alain Poitras is offline Alain Poitras  Canada
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Quote:
Originally Posted by oshifis View Post
Alain,

Thanks for your efforts and the analysis. I will follow your advices and modify the circuit in the following order:

1. Decrease the plate voltage of the cathode follower. Fortunately, I just need to replace a single resistor in the power supply.
2. Put some LEDs in the cathode to get about 5 to 10 V cathode voltage. What type of LED is most suitable for this (green, red, high-efficiency, ...)?
3. Increase the grid resistor of the output tubes to 220K and/or increase the coupling capacitor (but I don't easily replace those nice Russian teflon capacitors...)

I will report on the results, may take some time until I implement them.

Here are some pictures of the amplifier:

https://picasaweb.google.com/lcsaszar/807_amplifier
Congratulation Laszlo, your amplifier is very nice !

Click the image to open in full size.

Tonight, I made five more tests and I find the best you can do to improve it without doing too much modifications like you will see in my two next posts !

About your comments :

It was not a very good idea to use a LED bias for this circuit, the distortion would be too high ...

You cannot use a higher value grid leak resistor in a fixed bias circuit for the 807 ... The maximum is 100K, it is written in every 807 datasheets ... You will have to replace the 220nF with some good paper or film 2,2uF capacitors to get the optimum frequency response and the lower distortion ...

Alain.
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Old 30th November 2011, 06:48 AM   #17
Alain Poitras is offline Alain Poitras  Canada
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I will show my five tests results now.

But first, the two horrors ...

The results with a partial LED bias ...


Click the image to open in full size.

When I see the 2,48% distortion at full power, I stop this test right there ...

Just for fun, I verify what results a "cathode bias only" would give :

Click the image to open in full size.

Ouch ! Over 7% distortion ... really horrible ...

No more comments about thoses two !
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Old 30th November 2011, 08:42 AM   #18
Alain Poitras is offline Alain Poitras  Canada
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Now, the good stuff !

I replace the 220nF coupling capacitors of the "version 2" with some 2,2uF and I redo the tests and there is a big difference. The maximum power is higher and the distortion at this power is much lower and the bass response is fantastic but unfortunately, the distorsion raise when the power is lower ...This is a strange thing some tubes do in push-pull because their "non-linarity" and the "operating point" chosen ...

I call it V2A and I start from for the two last ones.

Click the image to open in full size.

There is no ways to really increase the maximum power without using a lower load impedance, the 807 datasheets typical operations for push-pull with triode connexion said 15W can be reach with a 3k plate to plate load ... With a 5K load at this supply voltage, it is impossible to go much over 12W without "clipping distorsion" ...

I make a first test with a lower cathode current to see the effects of going deeper in class "AB1" ...

Click the image to open in full size.

Like you can see, with a 40ma cathode current, the maximum power is a little bit lower than with the V2A but at this power, the distortion is much lower, only 1,19% at 1Khz ... The problem is : The distortion raise to 1,57% at 9,376W and 2% at 6,34W and stay high at lower power ... I don't think that is what you want ...

So, I figure out it is better to go deeper in class "A" instead by raising the cathode current ... The 807 datasheets said the combined plate and screen grid dissipation should not exceed 25W in "constant service" (CCS) but since the dissipation in "intermittent service" (ICAS) is 30W maximum, it cannot hurt the tubes if it is slightly more than 25W ...

So the current can be raise to 60ma because the supply is 419,9V and 419,9 x 0,06 = 25,194W ...

Click the image to open in full size.

This is the circuit that give the higher maximum power, 12,818W ... Yes, at this power, the distortion is higher, 1,952%, but lower at 20Khz ... But what is very interresting with this circuit is : The distortion drop a lot at lower power, 1,67677% at 6,7667W and only 0,525741% at 3,5666W ... A very "normal" listening power with high efficiency loudspeakers ...

I think that is what you want !

So all what you have to do to improve your superb amplifier is :

1 - Lower the drivers supply to about 268,8V to protect them and the 807 ...
2 - Change your 220nF coupling capacitors for some good 2,2uF to 4,7uF
3 - Adjust the 807 cathodes current to 60ma ( but no more ... )

And like we said in Quebec : "Everything will be tigidou"

I will verify what is the distorsion of the "V2C" at 1W, 20Hz, 1Khz and 20Khz and tell you in my next post later. That is about the power I play my "vinyls" home not to disturb all the close neighbours ...

Alain.

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Old 30th November 2011, 09:08 AM   #19
Alain Poitras is offline Alain Poitras  Canada
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Quote:
Originally Posted by Wavebourn View Post
I thought it was your design, and was going to ask the same couple of questions, by the way.
This is not my design but I am very interrested using some 807 for my personnal home sound system with the "vintage" Eico ST-40 output transformers I got. Because they are good low cost tubes ... Maybe I will use some 6CA7 or 6550 but I don't decide yet ...

Of course, I will make the best design I can for that amplifier and a complete tubes preamplifier with tone controls for my magnetic turntable and also a monophonic output with a mixer and a lowpass filter for my 50 pounds 12" chinese subwoofer.

My simulation tests with the 6S19P triode is only for a "DIY project" design to put on my personnal website I am working on some times. But I have to modify it all because it was only in french and I will do it french and english to join more peoples all over the world, it is a lot of work ...

What I am really doing on DiyAudio beside having fun and practice my "english writing" is "PR" ( public relation ).

Alain.
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Old 30th November 2011, 11:24 AM   #20
Alain Poitras is offline Alain Poitras  Canada
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I just finish measuring the distortion and frequency response of the V2C circuit at 1W ...

Quote:
OSHIFIS V2C with a 90H K=0,99992 OPT

Total distortion and frequency response at 1W :

1Hz : 0,310959% - 2,875200dB
2Hz : 0,298093% - 0,814134dB
5Hz : 0,293531% - 0,134640dB
10Hz : 0,293112% - 0,032711dB
20Hz : 0,292358% - 0,007070dB
50Hz : 0,282578% - 0,000050dB
100Hz : 0,292749% 0dB
200Hz : 0,293065% 0dB
500Hz : 0,288049% 0dB
1Khz : 0,279339% 0dB
2Khz : 0,229143% - 0,003730dB
5Khz : 0,276340% - 0,030750dB
10Khz : 0,261454% - 0,261454dB
20Khz : 0,161489% - 0,526526dB
50Khz : 0,209793% - 2,604000dB
100Khz : 0,178489% - 6,318200dB
How about that ?

Alain.
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