I have bogen mo-200 it has been recapped through out the amp. It has eight 6550 power tubes and two 12dw7 tubes in the gain stage. It has been re-wired for stereo, although i like the tone and seperation the sound has some distortion. I thought i could solve the problem by swap the jj 12dw7's for some nos rca's but still distortion still exists. The power tubes are brand new jj's i dont know what is the problem is. I even check the pre amp and cd player no distortion there so i am sure it is the amp, please any advice would be helpful.:scratch2:
tube distortion
- Thread starter chefg1
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battradio
Electron trainer
Do you have a before and after schematic , Also it should be checked on a distortion analyzer before you shell out for new tubes .Find out what was done to correct the bias and drive problems . Even a signal generator and scope , to check out if it clips semetricly and is realy putting out the power it is suposed to .
JonL
Lunatic Member
Have you asked him what the problem might be?
Chefg1 -- If you are using 8 ohm speakers with the unit set up for stereo, they should be connected to the "25 volt" output tap. In this mode of operation, these taps represent a 6.25 ohm winding, which is the closest you will get to a true 8 ohm tap on this amplifier in stereo mode.
While each channel is in deed rated to deliver 100 watts RMS with stereo operation, this would only be true if a load of 6.25 ohms were applied to the 25 volt taps. When an 8 ohm load is used, it produces a power loss -- in this case, reducing power output to 78.1 watts RMS per channel. In actual practice, the slight loss is hardly noticeable, and many speakers -- nominally rated for 8 ohms -- have impedance dips below their nominal value anyway. Making sure you have the amplifier/speaker combination properly connected is the first step in tracing your distortion down.
As for the use of 6550s, they are indeed a very good tube substitution for this amplifier -- from the standpoint of output tube loading. In this regard, I am not sure what information drtechno is basing his information on with his recommendation of 6L6 class tubes, but they would be a very poor choice for this amplifier at the screen voltages this amplifier operates at. At 335 vdc, these tubes simply could not pull the peak currents required for the loading conditions presented, so that power output and distortion would both suffer significantly. On the other hand, 6550s are easily capable of the required currents, and more.
However, regarding drive voltage, Battradio's concern for having adequate drive available for 6550 tubes is well founded. The conversion from 8417 tubes to 6550 tubes will require about double the bias voltage, and therefore about double the drive voltage. While the original driver circuit is very capable for 8417 tubes, it is very marginal for 6550 tubes if full power is routinely required at low distortion -- especially on the low end of the frequency spectrum. The changes necessary to insure adequate drive for 6550 tubes principally involve raising the B+ to the phase inverter stage to increase its peak output capability. This should be rather easy to do within the framework of the original design.
Finally, since the Gm of a 6550 is about half that of an 8417, a change to 6550s would also require an adjustment to the NFB circuit to restore the original feedback level when 6550 tubes are used. While this would restore the original distortion specifications, it would also (approximately) double the drive requirement to the complete amplifier -- again, because 6550 tubes require approximately double the drive voltage to develop full power output in this amplifier.
As suggested, if you could provide a schematic as to the modifications made, it would then be very easy to see if the appropriate modifications have been made to properly use 6550s in your Bogen.
It's a great amp -- good luck with it!
Dave
While each channel is in deed rated to deliver 100 watts RMS with stereo operation, this would only be true if a load of 6.25 ohms were applied to the 25 volt taps. When an 8 ohm load is used, it produces a power loss -- in this case, reducing power output to 78.1 watts RMS per channel. In actual practice, the slight loss is hardly noticeable, and many speakers -- nominally rated for 8 ohms -- have impedance dips below their nominal value anyway. Making sure you have the amplifier/speaker combination properly connected is the first step in tracing your distortion down.
As for the use of 6550s, they are indeed a very good tube substitution for this amplifier -- from the standpoint of output tube loading. In this regard, I am not sure what information drtechno is basing his information on with his recommendation of 6L6 class tubes, but they would be a very poor choice for this amplifier at the screen voltages this amplifier operates at. At 335 vdc, these tubes simply could not pull the peak currents required for the loading conditions presented, so that power output and distortion would both suffer significantly. On the other hand, 6550s are easily capable of the required currents, and more.
However, regarding drive voltage, Battradio's concern for having adequate drive available for 6550 tubes is well founded. The conversion from 8417 tubes to 6550 tubes will require about double the bias voltage, and therefore about double the drive voltage. While the original driver circuit is very capable for 8417 tubes, it is very marginal for 6550 tubes if full power is routinely required at low distortion -- especially on the low end of the frequency spectrum. The changes necessary to insure adequate drive for 6550 tubes principally involve raising the B+ to the phase inverter stage to increase its peak output capability. This should be rather easy to do within the framework of the original design.
Finally, since the Gm of a 6550 is about half that of an 8417, a change to 6550s would also require an adjustment to the NFB circuit to restore the original feedback level when 6550 tubes are used. While this would restore the original distortion specifications, it would also (approximately) double the drive requirement to the complete amplifier -- again, because 6550 tubes require approximately double the drive voltage to develop full power output in this amplifier.
As suggested, if you could provide a schematic as to the modifications made, it would then be very easy to see if the appropriate modifications have been made to properly use 6550s in your Bogen.
It's a great amp -- good luck with it!
Dave
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I dont know that much about tube amp , but the guy who perform the work comes highly recomended . when i first ask the ? i thought it could be a simple fix. As far the work that was done, yes he did re-wire the amp for 6550's i was thinking that it could be dirty sockets so i was going to clean them to see if that helps.:scratch2:
I agree with the above.
In addition:
(My 2 cents worth discounted to free.)
Are both channels distorted, equally?
Is the distortion most noticeable at low levels, high levels, both?
Does it improve as it warms up?
Distortion on low level signals might indicate excessive neg grid bias for the operating conditions (crossover distortion).
Distortion at moderate to high level signals might indicate insufficient driver voltage swing capability. (This would be my guess if I had to make one).
Filament currents are equal, so insufficient filament would not be the prob.
Do the plates glow any at all?
Good luck sir.
Rickey.
Chefg1 -- When converted to stereo mode, the original tap labeled as "8 ohms" should now be considered as a 16 ohm tap. So converted, it is in fact a true 16 ohm tap should you have speakers of that impedance.
For your 8 ohm speakers, leave the strap in place between the "Com 1" and "Gnd" terminals (the strap simply grounds the speaker system), and this connection also becomes the negative side of your speaker connection. The positive side of your 8 ohm speaker should connect to the terminal marked "25 V". Do not use the "25 VCT" terminal, as this represents a 1.56 ohm tap in stereo mode, which would only be useful for possibly two 4 ohm speakers connected in parallel.
For what it's worth, in stereo mode, the impedance between the "Com 2" and the "70 VCT" terminals represents 12.25 ohms if you find that useful at all.
Dave
For your 8 ohm speakers, leave the strap in place between the "Com 1" and "Gnd" terminals (the strap simply grounds the speaker system), and this connection also becomes the negative side of your speaker connection. The positive side of your 8 ohm speaker should connect to the terminal marked "25 V". Do not use the "25 VCT" terminal, as this represents a 1.56 ohm tap in stereo mode, which would only be useful for possibly two 4 ohm speakers connected in parallel.
For what it's worth, in stereo mode, the impedance between the "Com 2" and the "70 VCT" terminals represents 12.25 ohms if you find that useful at all.
Dave
Ok the following is the way he hooked the speakers up on this thing. 1. pos going 25 ct 2. pos going to 8 3. two minus twisted together on one ground terminal with the strap hanging . please forgive electronic stupidity , for i know more about food than i do about things of this nature.
No problem Chef. The problem is that the original amplifier appears to have only ONE output strip, where both of the amplifiers had their outputs connected together at so as to give the output taps as shown on the amplifier. When the amplifier was modified so as to split the two internal amplifiers apart, it sounds like whoever did the work simply continued to use the one output strip, and used it to make the connections for (now) two separate amplifiers. As a result, who knows what anything means regarding the indicated taps now! (unless you were given specific information regarding this by whomever did the work)
So lets go at it this way: The output transformers from each amplifier have many wires that originally connected to the output strip. Likely, the tech removed many he/she felt was not necessary, taped them off, and just left the ones connected that they felt you would use. To that point, for an 8 ohm connection, you will want your speakers connected to the BLACK and YELLOW leads from each transformer. The black leads represent the negative lead to your speakers, and would originally be connected to the "COM 1" terminal. This terminal should also be grounded by way of the terminal strap to the "GND" terminal, which should internally be grounded. The negative lead of both of your speakers would connect to the black transformer leads connected and grounded through these terminals. The YELLOW leads represent the positive lead -- one to each speaker. Likely one is still on its original 25V tap, while the other yellow lead is now connected to a different terminal representing the same 25V tap for the other channel. Possibly, it is now occupying the 25 VCT terminal, but you will have to look underneath to be sure.
If you can look under the amplifier (with it off!) and verify if these are the leads you are in fact connecting your speakers to (or which terminals the black and yellow leads are in fact now connected to), it will cut through all of this guessing real quick.
I hope this helps!
Dave
So lets go at it this way: The output transformers from each amplifier have many wires that originally connected to the output strip. Likely, the tech removed many he/she felt was not necessary, taped them off, and just left the ones connected that they felt you would use. To that point, for an 8 ohm connection, you will want your speakers connected to the BLACK and YELLOW leads from each transformer. The black leads represent the negative lead to your speakers, and would originally be connected to the "COM 1" terminal. This terminal should also be grounded by way of the terminal strap to the "GND" terminal, which should internally be grounded. The negative lead of both of your speakers would connect to the black transformer leads connected and grounded through these terminals. The YELLOW leads represent the positive lead -- one to each speaker. Likely one is still on its original 25V tap, while the other yellow lead is now connected to a different terminal representing the same 25V tap for the other channel. Possibly, it is now occupying the 25 VCT terminal, but you will have to look underneath to be sure.
If you can look under the amplifier (with it off!) and verify if these are the leads you are in fact connecting your speakers to (or which terminals the black and yellow leads are in fact now connected to), it will cut through all of this guessing real quick.
I hope this helps!
Dave
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