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211/845 tubes in a 805 based amp ?
211/845 tubes in a 805 based amp ?
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Old 8th April 2015, 02:24 PM   #1
Legis is offline Legis  Finland
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Default 211/845 tubes in a 805 based amp ?

I have been wondering if I could make 805 based amps into a wondorous tube rolling machines by also allowing use of 211 and 845 tubes in the circuit with couple of switched. One switch would change different cathode resistors and the other switch would change the grid voltage.

The 211 and 845 will require negative grid voltage whereas the 805 needs positive grid voltage. I think it could be possible to make the negative DC-supply out from the existing parts or almost as easily with a new diode bridge. With a switch one would choose whether the grid would be biased with negative or positive voltage in accordance to the used tube.

The amps are Mingda MC3008-A, the schematic of the newest model is presumably pretty close to this:

Click the image to open in full size.

Here is a 211/845 amp from another chinese manufacturer BEZ. Tubes are changed with one switch in this:

Click the image to open in full size.

Could I implement the 211/845-compatibility to an 805 amp with such a way or would the driver stage need some tuning also? All help is much appreciated.

Last edited by Legis; 8th April 2015 at 02:27 PM.
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Old 8th April 2015, 08:45 PM   #2
multi is offline multi  Australia
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THE 845 needs enormous drive voltages the driver stage may not be up to it.
The 805 on paper may not be the ultimate Audio tube, but any problems with the high plate impedance are more than made up by the ease of driving them. I have just made an 805 PP amp
Sounds wonderful to me, also made a RCA 8000amp Mu of 16.5 same circuit and bias circuit one positive the 8000 negative. Just a small change to the bias circuit fior the 8000. I think the 845 has a mu of about 3 the 211 is 10.
Phil
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Old 9th April 2015, 05:51 PM   #3
ColinA is offline ColinA  United Kingdom
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Because the Chinese do it in their amps.
Is NOTa reason to try anything!
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Old 9th April 2015, 08:41 PM   #4
goldenbeer is offline goldenbeer  Germany
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You can't do it in a reasonable way. The 805 needs positive bias and quiescent grid current. The 845 and 211 don't. (211 needs some grid current on large signals to be driven in class A2, if you want to harness it's full potential)

Compare the schematics. The 805 is driven by a dc coupled cathode follower. The 845/211 are driven by a RC coupled driver gain stage (off the plate).
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Old 10th April 2015, 04:30 AM   #5
Legis is offline Legis  Finland
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Quote:
Originally Posted by goldenbeer View Post
You can't do it in a reasonable way. The 805 needs positive bias and quiescent grid current. The 845 and 211 don't. (211 needs some grid current on large signals to be driven in class A2, if you want to harness it's full potential)

Compare the schematics. The 805 is driven by a dc coupled cathode follower. The 845/211 are driven by a RC coupled driver gain stage (off the plate).
Thanks for the responses!

Here's another schematic I found regarding the 211. It looks a lot more similar to Mingda, also a direct connected cathode follower. The grid of the 211 has positive grid voltage but the 750R cathode resistor lifts the cathode's potential to around 56V so the grid is minus ~40V compared to the cathode.

211/845 tubes in a 805 based amp ?



I think the grid voltage is around ~15-20V also in Mingdas, at least in the same ballpark. But there is very small cathode resistor for the 805 that does not rise cathodes potential so the grid stays positive compared to the cathode. I have to receive them first in order to be able to measure.

Maybe I could rise the cathode's potential compared to the grid also in the Mingdas just by using different cathode resistos for different tubes. For example ~1800R for 845 and ~750R for the 211.
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Old 10th April 2015, 03:00 PM   #6
FullRangeMan is offline FullRangeMan  Brazil
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Legis,
As you know these tubes have different gain(211=12, 845=6, 805=50) the final stage cant be the same.
I would dont put my time on this project, unless its a order pay in advance.
Chances of bad sound are many.
Good luck.
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Old 10th April 2015, 03:28 PM   #7
Legis is offline Legis  Finland
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Quote:
Originally Posted by FullRangeMan View Post
Legis,
As you know these tubes have different gain(211=12, 845=6, 805=50) the final stage cant be the same.
I would dont put my time on this project, unless its a order pay in advance.
Chances of bad sound are many.
Good luck.
Hi, I have bought those amps and as a DIY-oriented person I want to try to make them more all-round compatible with different tubes just for the ***** and giggles. 211 might work better than 845 given that it needs the cathode to be raised less and has higher gain than 845. But I don't mind if the output is less than max potential that could be obtained with the specific tube.

I planned to put the global negative feedback loop also behind a switch or remove it altogether. I'm not allergic to a couple percent of distortion . But I do like good, tubey sound very much, amplifying my ~105dB/1w horn speakers (incl. horn loaded bottom octave). They are made for SE-tubes.

Last edited by Legis; 10th April 2015 at 03:32 PM.
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Old 10th April 2015, 10:23 PM   #8
hpeter is offline hpeter  Slovakia
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Location: Electrostats or bust
lundahl plate choke , ct is input from driver cathode, one end gnd.
other end to grid-ac coupled-add resistor to bias voltage
you end up with 1:2 stepup, and dont forget pray for miller capacitances pulling down treble much
no guarantee whatsoever
------
or genuine stepup driving trafo
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Old 10th April 2015, 11:44 PM   #9
multi is offline multi  Australia
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Should be easy to change between 805 and 211/845.
You need to change the bias circuit from + to -; I have just done this in a push pull circuit.from 805 to 8000 similar to 211 Mu of 16.5. The 8000 needed only 12 volts - bias at 650 volts
I think the ming Da runs at about 800 volts for 50 watts out the Out put trnasformer recomended for the 805 is 3500 ohms, the recomended out put transformer for the 845 at 800 volts is 3500 ohms.
The 845 at 800 volts gives about 15 watts maybe more with cathode drive; Only problem could be the drive voltage and if there is sufficient bias voltage for the 845.
I will try and find a circuit for the changes you need to go from + to - bias.
Phil
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Old 11th April 2015, 02:06 AM   #10
FullRangeMan is offline FullRangeMan  Brazil
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Legis, If I be you I would like to built a 2 in to 1 amp with the wonderful 572-B tube:
Its works with B+500V to 3W and 900V to 20W.
It would be nice to have a 3W and a 20W amp in the same chassis.
The 3W sound would be more detailed;
http://www.mif.pg.gda.pl/homepages/f.../s/SV572-3.pdf
Just a idea.
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