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211/845 tubes in a 805 based amp ?

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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:

MC3008-A5B15D.jpg


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

An externally hosted image should be here but it was not working when we last tested it.


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. :)
 
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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
 
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).
 
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.

s201131622635216.gif



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.
 
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 :D. 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.
 
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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
 
805 572b 811 chnage for negative bias

The 572-3 is now very expensive have not been made for many years; they are a 572-160 with the grids wound to produce a tube similar to the 845, with 6.3volt heaters.
The Pp circuit shows how to change from + Bias to - Bias. Instead of the cathode resistors going to earth + bias; for negative go back to the output of the bias supply.
Phil
 

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The 845 worked OK in the MC3008-A circuit with a 1477R cathode resistor, which lifted the cathode ~133 volts meaning the +24V grid is approx. -110V compared to the cathode. Anode voltage is ~940-950V so the tube had ~810-820V across it. Bias was approx 91mA with this cathode resistor.

But the problem was low output power because of the low operating point the first tube, the 12AX7, which has only 63V across the tube and 1,6mA bias. It has been set for the original circuit and input sensitivity to give very long tube life propably.

At first the output with 845 tube was limited to ~3,5Vrms because the input tube 12AX7 was clipping. Then I lowered the 150k anode resistor of the 12AX7 to ~70k, which raised the voltage accros the it to ~103V and the bias was also raised to ~2,6mA. With this setup I could get almost 5V before the 12AX7 clipped, meaning almost 3dB more juice. I will have to continue testing and raise the operating point higher still.

FYI: the next tube, 12AU7, has ~150V across it and the driver tube 300B has ~370V across it. I did not make notes of their bias. They might be up for the task to drive the unsensitive 845, but the first tube needs a higher operating point definitely for the 845. 211 has 6dB higher gain than 845 so it's easier to drive.

I will report when some progress is made. :)
 
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