I have an AudioKit 845 - 300B - 6SN7 monoblock based on Cary Cad 805 schematics. When using Sylvania 6SN7GTB chrome domes the two amplifiers work perfectly. When I try Cossor 6SN7 tubes, one of the amplifiers produces very little gain. I have switched the tubes around and determined that the difference is in the one amplifier. My first thought was that the pins were not contacting so socket savers were implemented with the same results. The gain diminishes the longer the amp is on. What could be causing the result?
orca1993
orca1993
Why not post the schematic with relevant voltages shown. Without a schematic, there's not much to go on.
Maybe I'm confused but the schematic you posted calls for a 6SL7, not a 6SN7. Although they have the same pin-out, these are very different tubes and are generally not interchangeable.
With the old tubes it works fine?
Are the heaters lifted of the 6SN7 tubes?
Now i also see it... 6SL7 tubes...
Are the heaters lifted of the 6SN7 tubes?
Now i also see it... 6SL7 tubes...
There is very little gain overall with the 6SN7 tubes. Usually about 90 db of sound pressure at full volume. The original kit for this amplifier calls for the 6SN7 tube. A friend as said that the 6SL7 tube would create more gain but I have not tried one yet. The different tube does not explain why one make of tube is not happy in the application. Maybe the Audio Kit people were told to stop production of this amplifier because of patent infringement. They will not discuss the amplifier at all.
A circuit that was designed for a particular tube, be it a 6SL7 or 6SN7, cannot be expected to work properly when a different tube type is substituted. Doing so will almost certainly change the operating point of the tube significantly and can result in a variety of fault symptoms. One brand of tube may be more or less tolerant than another so what you are seeing really isn't much of a surprise.
I can only assume from this particular schematic that the component values are appropriate for a 6SL7, as indicated. Have you tried using a 6SL7 as the circuit calls for?
While it is true that a properly designed 6SL7 gain stage will likely have more voltage gain than a properly designed 6SN7 stage, you generally cannot just change the tube type without also changing the resistive components associated with the stage, which set its operating point. This is a common misconception among "tube rollers." A stage often has to be redesigned to use a different tube type.
I can only assume from this particular schematic that the component values are appropriate for a 6SL7, as indicated. Have you tried using a 6SL7 as the circuit calls for?
While it is true that a properly designed 6SL7 gain stage will likely have more voltage gain than a properly designed 6SN7 stage, you generally cannot just change the tube type without also changing the resistive components associated with the stage, which set its operating point. This is a common misconception among "tube rollers." A stage often has to be redesigned to use a different tube type.
Try a different brand of 6SN7. Like a GE or something made in the tube era. Don’t be afraid to try different tubes contrary to what the other poster said. If they match pin-wise and don’t have severly different plate impedances you wont harm a thing. Research on Frank pocnet. Tubes are for experimenting. Have fun. There are too many Debbie downers on here who know the ‘only way’.
Thanks for the input everyone. The kit actually came with the 6SN7 tube and it is marked on the tube install point. I suspect that that was done as a deception from the obvious ripoff. I will get a pair of 6SL7 and try them out. I am curious that there may be enough difference between manufacturing specs that one tube would work and another not. Maybe they are all working at a reduced level.
orca1993
orca1993
Of course anyone here can do whatever they want. It might work, and it might not.
But I too would encourage looking at tube datasheets for both the 6SL7 and the 6SN7. They are quite different, and their operating points will reflect this when plugged into the same circuit. It's possible that the operating point with the substitute tube will be far enough from its linear region to misbehave with some tubes but not others. I don't know whether that is the case here or not since I haven't simulated the circuit you provided.
Good luck.
But I too would encourage looking at tube datasheets for both the 6SL7 and the 6SN7. They are quite different, and their operating points will reflect this when plugged into the same circuit. It's possible that the operating point with the substitute tube will be far enough from its linear region to misbehave with some tubes but not others. I don't know whether that is the case here or not since I haven't simulated the circuit you provided.
Good luck.
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Did you look up the plate resistance of 6SL7 and 6SN7? Is that a mild difference or a severe difference?
And what exactly is a plate "impedance"? Is there a reactive component I missed?
Tubes are not for experimenting blindly ... if you know electronics that is a non sense , you should know that parts values must be changed for the other tube to be biased right 😀
6SN7 and 6SL7 are very different , almost as much possible for 2 small signal tubes
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2k resistors in an SRPP are a bit high for 6SN7s but the real mystery here is why one of the amps behaves differently with the Cossors, yet fine with the Sylvania's.
Could be a fault condition in the amp that the Sylvania's are less susceptible for.
Perhaps a bad connection on R8?
Could be a fault condition in the amp that the Sylvania's are less susceptible for.
Perhaps a bad connection on R8?
I'll check the solder points today, Parafeed813. That is my true puzzle. It may be that this amplifier really does want a 6SL7 even though the labels are 6SN7. Hopefully it is just a bad solder though. Thanks.
The Cossor tubes are working quite happily in my Yaqin B-100.
Curious that you mention the effect on bias. With the Sylvania tubes the highest bias for the new 300B tubes is between 40 and 45 mv. I should be able to adjust between 40 and 70 for the 300B but nothing above 45 is available.
orca1993
The Cossor tubes are working quite happily in my Yaqin B-100.
Curious that you mention the effect on bias. With the Sylvania tubes the highest bias for the new 300B tubes is between 40 and 45 mv. I should be able to adjust between 40 and 70 for the 300B but nothing above 45 is available.
orca1993
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You are not using the right units, this is confusing.
In the schematic where it says 40-70mA (not mV), this should be the effect, not the measurement at that point.
This is the bias voltage, which should be around -80V per schematic. To be adjusted to give between 400 to 700 mV across the 10ohm resistor under the 300B.
When you say between 40 and 45 instead of 70, what exactly do you mean?
The input tube should not have significant impact on the current through the 300B, unless C12 or the schematic is faulty.
In the schematic where it says 40-70mA (not mV), this should be the effect, not the measurement at that point.
This is the bias voltage, which should be around -80V per schematic. To be adjusted to give between 400 to 700 mV across the 10ohm resistor under the 300B.
When you say between 40 and 45 instead of 70, what exactly do you mean?
The input tube should not have significant impact on the current through the 300B, unless C12 or the schematic is faulty.
You are correct Parafeed813. The bias should be between 40 and 70 mA. I am using teflon sockets with pcb connectors on the pins. As per your R8 question, I bridged the pin and pcb with a wire to ensure proper connection and lo and behold the Cossors are working. Thank you for your input. Cannot trust the pcb's trace.
Orca, your OP question is a very good one. But without full knowledge of what you have there, it's a meaningless exercise and likely a frustrating one.
You need to start with a circuit schematic that is known good. And by good I mean one that has been documented to work well (by a reliable source, not some eBay seller). Or one that you have simulated in Spice and verified it works well. Then you need to verify that your actual circuit is per schematic. Then it's a simple matter of measuring voltages everywhere and verifying operating conditions. First with one tube, then the other. Done.
If you don't do things in that order, you'll likely be here for the next 963 posts and frustrated.
You need to start with a circuit schematic that is known good. And by good I mean one that has been documented to work well (by a reliable source, not some eBay seller). Or one that you have simulated in Spice and verified it works well. Then you need to verify that your actual circuit is per schematic. Then it's a simple matter of measuring voltages everywhere and verifying operating conditions. First with one tube, then the other. Done.
If you don't do things in that order, you'll likely be here for the next 963 posts and frustrated.
I have attempted to get a proper schematic from AudioKit in Italy, but they refuse to respond to my inquiry. It is as if they have orphaned this design. When we began retracing the amplifier we found that the schematic from the Cary Cad was spot on where we took the measurements. Those hand written voltages were from our actual measurements.
Parafeed813 actually pointed me to the problem with the tube not working. That part is now sorted out. Thank you.
Parafeed813 actually pointed me to the problem with the tube not working. That part is now sorted out. Thank you.
Since the schematics called for 6SL7 tubes I got a pair of them to try.
Got a pair of Tungsols and the amplifiers have come to life. Volume overall is better with full spectrum of sound. I believe that these AudioKit amplifiers were a direct copy of the Cary Cad 805 with the 6SN7 used to appear different. I see now that the Anniversary edition is using two 6SN7 tubes in place of the single 6SL7. Thanks everyone for the help.
Got a pair of Tungsols and the amplifiers have come to life. Volume overall is better with full spectrum of sound. I believe that these AudioKit amplifiers were a direct copy of the Cary Cad 805 with the 6SN7 used to appear different. I see now that the Anniversary edition is using two 6SN7 tubes in place of the single 6SL7. Thanks everyone for the help.