I'm troubleshooting a tube amplifier (Music Angle XD850mk3).
It is a 300b SET amp with a 3A5 based input stage, 6SL7 SRPP driver.
The issue with the amp is that with intervals of some minutes give a weak little "ping" in loudspeakers. Somewhat like a tiny bell.
B+ for feeding the transformer of the 300b tube is 432 volts, B+2 is 360 volts for the driver stage and 144 for the input stage.
The first suspect was the input stage since it is very microphonic. However with a B+ of 144 volts, heater voltage of 1.4 volts, anode voltage of 84 volts and 3.6 mA cathode current it should be operating nicely within spec.
Next to be examined was the SRPP/Mu follower driver stage. Current measures approximately 0.8 mA, bias voltage of lower triode roughly -2 volts.
Cathode voltage of upper triode is at 170 volts. Anode voltage of upper tube is 360 volt. As far as I can see, also within spec. I noticed that the heaters are AC and there is a resistor from one of the transformer wires to GND. There is no resistor to any of the three available B+ voltages. Meaning that the heaters are ground referenced.
The 6SL7 has a maximum heater cathode voltage of +/-90 volts so the upper tube is therefore operating outside of spec.
Questions:
- Could the lack of heater elevation be the cause of the sounds? If not what are the effect of not elevating the heaters?
- Will the tubes be permanently damaged?
Follow up question. Since this is an amp without NFB I assume that the absolutte max peak input voltage of the driver stage should be 2 volts - If so, what is the purpose of the input gain stage? Reducing input capacitance maybe?
Thanks, Lars
It is a 300b SET amp with a 3A5 based input stage, 6SL7 SRPP driver.
The issue with the amp is that with intervals of some minutes give a weak little "ping" in loudspeakers. Somewhat like a tiny bell.
B+ for feeding the transformer of the 300b tube is 432 volts, B+2 is 360 volts for the driver stage and 144 for the input stage.
The first suspect was the input stage since it is very microphonic. However with a B+ of 144 volts, heater voltage of 1.4 volts, anode voltage of 84 volts and 3.6 mA cathode current it should be operating nicely within spec.
Next to be examined was the SRPP/Mu follower driver stage. Current measures approximately 0.8 mA, bias voltage of lower triode roughly -2 volts.
Cathode voltage of upper triode is at 170 volts. Anode voltage of upper tube is 360 volt. As far as I can see, also within spec. I noticed that the heaters are AC and there is a resistor from one of the transformer wires to GND. There is no resistor to any of the three available B+ voltages. Meaning that the heaters are ground referenced.
The 6SL7 has a maximum heater cathode voltage of +/-90 volts so the upper tube is therefore operating outside of spec.
Questions:
- Could the lack of heater elevation be the cause of the sounds? If not what are the effect of not elevating the heaters?
- Will the tubes be permanently damaged?
Follow up question. Since this is an amp without NFB I assume that the absolutte max peak input voltage of the driver stage should be 2 volts - If so, what is the purpose of the input gain stage? Reducing input capacitance maybe?
Thanks, Lars
Do you look for a mechanical source for this PING?
Maybe related to metal expansion by heat?
My 2 cents.
Maybe related to metal expansion by heat?
My 2 cents.
A mechanical source is/was a likely candidate. Stil, if it was mechanical expansion I would expect it to stabilize after a while. This does not seem to reduce in how frequent it appears nor in amplitude.
Are you shure the 6SL7 is the right one in there ?
With a 6SN7 the current is ~3mA (-Vg=7volts) and Vfk is 200volts max.
Like that the 3A4 makes more sense too.
Mona
With a 6SN7 the current is ~3mA (-Vg=7volts) and Vfk is 200volts max.
Like that the 3A4 makes more sense too.
Mona
The thought that the amp was actually designed for 6sn7 is new to me, however it makes sense!
Still it came with 6sl7 and advertised as using 6n9p / 6sl7 for the driver.
Unfortunately I don't have the skill to calculate currents for an SRPP, but according to mh-audio.nl - Home a 6SN7 is supposed to give me a gain of 17x (25dB) and output impedance of 1100 ohm, alas the calculator does not tell me anything about currents or voltages so I can't really say if it would be "plug and play" or not. (both cathode resistors are 2K7 with 10K from the grid of the "upper" triode to the "lower" anode.
300b tubes heaters are 5V DC with humbucking resistors, drivers are 6,? volt AC and 3A5 is 1,4v DC
Still it came with 6sl7 and advertised as using 6n9p / 6sl7 for the driver.
Unfortunately I don't have the skill to calculate currents for an SRPP, but according to mh-audio.nl - Home a 6SN7 is supposed to give me a gain of 17x (25dB) and output impedance of 1100 ohm, alas the calculator does not tell me anything about currents or voltages so I can't really say if it would be "plug and play" or not. (both cathode resistors are 2K7 with 10K from the grid of the "upper" triode to the "lower" anode.
300b tubes heaters are 5V DC with humbucking resistors, drivers are 6,? volt AC and 3A5 is 1,4v DC
Yes, it's PNP ! But the gain isn't 17x.An SRPP, without decoupled lower cathode resistance, gives only half of that.(was 35x with 6SL7)
Mona
Mona
Thanks Ketje, I will advice the owner to source 2 pieces and test!
BTW: Lower cathode is actually decoupled by 100uF / 50V, just did not mention it since it had nothing to do with the DC operating conditions.
Once I have the entire schematic traced (still missing parts of the PSU) I will post here for recommended upgrades. There are stuff going on that I really don't understand (like an 120K resistor between the upper cathode and the coupling capasitor for the 300b. It give a very high output impedance for the driver and form a voltage divider with the 120K resistor on the 300b grid!
BTW: Lower cathode is actually decoupled by 100uF / 50V, just did not mention it since it had nothing to do with the DC operating conditions.
Once I have the entire schematic traced (still missing parts of the PSU) I will post here for recommended upgrades. There are stuff going on that I really don't understand (like an 120K resistor between the upper cathode and the coupling capasitor for the 300b. It give a very high output impedance for the driver and form a voltage divider with the 120K resistor on the 300b grid!
> input stage since it is very microphonic. ... it should be operating nicely within spec.
I do not follow this argument. A tube can be "run within specs" and still be microphonic. Just like a 5-passenger car can be run with 4 passengers, and still have a muffler-clang.
Did you try different tubes?
> expect it to stabilize after a while.
Probably a valid argument. I'd still try different tubes.
OTOH, I would look around the house for coincidental electric activity. "Ping" is very odd, but we all know the buzz when a vacuum-cleaner is run near audio, I've had "CLICK"s from elevator relays, and stranger things. (Unlikely cause: old-old Browning CB radios would "ping" when transmission started....)
I do not follow this argument. A tube can be "run within specs" and still be microphonic. Just like a 5-passenger car can be run with 4 passengers, and still have a muffler-clang.
Did you try different tubes?
> expect it to stabilize after a while.
Probably a valid argument. I'd still try different tubes.
OTOH, I would look around the house for coincidental electric activity. "Ping" is very odd, but we all know the buzz when a vacuum-cleaner is run near audio, I've had "CLICK"s from elevator relays, and stranger things. (Unlikely cause: old-old Browning CB radios would "ping" when transmission started....)
Ketje: The supply for the driver is derived from the "main" 432 volts using a simple RC network - I assume changing to 6SN7 also require adjustment to the series resistor?
Poplin: Thanks for confirming - Will elevate to +90 volts. That way both halves of the tube will be right there at tthe outer edge of the spec.
PRR: The input tube is definitely microphonic, guess that comes with the DHT/low heater voltage territory. I'm hoping to kill some that with tube dampers/o-rings.
The ping sound however is appearing without (external) mechanical disruptions. Also amplitude and how often it appears seem to be unrelated to signal - although it is most notable when there is no music playing. Could still be the input tube, only not as likely.
leadbelly: Yes there is some type of delay by a 555 timer, though I've not been able to fully identify the mechanism and all involved component. Actually, I believe all it does is to change color of the LED on the front after all HT voltages are stable.
Poplin: Thanks for confirming - Will elevate to +90 volts. That way both halves of the tube will be right there at tthe outer edge of the spec.
PRR: The input tube is definitely microphonic, guess that comes with the DHT/low heater voltage territory. I'm hoping to kill some that with tube dampers/o-rings.
The ping sound however is appearing without (external) mechanical disruptions. Also amplitude and how often it appears seem to be unrelated to signal - although it is most notable when there is no music playing. Could still be the input tube, only not as likely.
leadbelly: Yes there is some type of delay by a 555 timer, though I've not been able to fully identify the mechanism and all involved component. Actually, I believe all it does is to change color of the LED on the front after all HT voltages are stable.
the AC from the secondary is rectified by a RCA 5u4-G, feed to a capacitor (size unknown, probably approx 100uF). Then there are individual Drossels for left and right channels (size unknown) finally feeding 150uF/450 volt capasitors. Measured DC voltage on the caps is 430 volt.
Probably yes.
360v on the anode gives a drop of 70v at (2x ,common both sides?) 0.8mA with 42k resistor.
With a 6SN7 the current is 2x 3mA.For a drop of 70v the R is ~12k.
If you keep the original resistor, no harm done.The current drops to 2mA pro SRPP and the voltages go down to something like 270v anode and 130v halfway.
And the capacitor after the rectifier has to be not bigger than 40µF (datasheet).
Mona
This project has been "sleeping" for a while.
Now, finally there are 6SN7 tubes in the mail and I will hopefully have them within a week for further experiments.
While waiting for the alternative driver tubes, the owner of the amp figured it was time to take some serious actions.
We removed the common cathode 3A5 input stage volume contral and input selector all together, instead feeding the SRPP stage directly from a pre-amp.
Using my Emotiva pre - sound was horrible and completely lacking of any base. When using the owners Audio GD pre sound was restored, and most importantly the "pinging" noise in the speakers were gone. Thankfully, the Audio GD has a "high gain" button giving an extra 6 dB boost to the output signal and thus compensating partially for the lost gain stage.
This leaves me with two questions:
1 - Is a SRPP stage a more difficult load for a pre-amp compared to a common cathode sage?
2 - Not discussed so far; The 300B has a grid stopper of 10K and a 220K resistor to ground. On the output from the SRPP (just after the coupling cap) there is a 120K series resistor. This effectively form a voltage divider with the 220K grid resistor "buring of" some 3dB gain. What would be the purpose of such a resistor? I am guessing something to do with stability or current limiting to avoid the 300b "Miller cap" starving the SRPP of its weak 0,7mA standby current.
Your views on these two issues would be appreciated.
EDIT: Yes, using 6SN7 as a single stage SRPP will give very little gain. If the SQ is improved with the new driver tubes, the initial plan is to make a new input sage based on one of the "leftover" 6SL7 tubes.
Now, finally there are 6SN7 tubes in the mail and I will hopefully have them within a week for further experiments.
While waiting for the alternative driver tubes, the owner of the amp figured it was time to take some serious actions.
We removed the common cathode 3A5 input stage volume contral and input selector all together, instead feeding the SRPP stage directly from a pre-amp.
Using my Emotiva pre - sound was horrible and completely lacking of any base. When using the owners Audio GD pre sound was restored, and most importantly the "pinging" noise in the speakers were gone. Thankfully, the Audio GD has a "high gain" button giving an extra 6 dB boost to the output signal and thus compensating partially for the lost gain stage.
This leaves me with two questions:
1 - Is a SRPP stage a more difficult load for a pre-amp compared to a common cathode sage?
2 - Not discussed so far; The 300B has a grid stopper of 10K and a 220K resistor to ground. On the output from the SRPP (just after the coupling cap) there is a 120K series resistor. This effectively form a voltage divider with the 220K grid resistor "buring of" some 3dB gain. What would be the purpose of such a resistor? I am guessing something to do with stability or current limiting to avoid the 300b "Miller cap" starving the SRPP of its weak 0,7mA standby current.
Your views on these two issues would be appreciated.
EDIT: Yes, using 6SN7 as a single stage SRPP will give very little gain. If the SQ is improved with the new driver tubes, the initial plan is to make a new input sage based on one of the "leftover" 6SL7 tubes.
Last edited:
Did some testing and got some numbers. To any owner of this amp, I will say that switching from 6SL7 to 6SN7 improves SQ - even without any HW changes.
I hope somebody can chime in with some further advise (ref my questions at end of this post).
As mentioned in my previous post, the input stage and the weird 120K series resistor at the driver output was removed. Thus, my preamp is now feeding the SRPP driver directly. Further modifications where carried out in 3 steps. No distortion measurements were done, but subjectively, each modification bring an improvement to the sound quality.
Step 1: Replace 6SL7 for 6SN7 as per Ketje's advice - Lost a lot of gain, but with the high-level output of pre-amp it is still possible to get full volume. B+ for SRPP dropped to 258 volts and tube current per tube is 1.85mA (5 volt drop over cathode resistor).
Step2: Each channel has separate resistor and capacitor for cleaning up B+ to the SRPP. Each capacitor is 58uF. Reducing series resistor for each channel from 100K to 50K increased B+ to 323volts and the current to 2.3mA (6.5 volt cathode voltage drop).
Step3: Cathode resistors where reduced from 2K7 to 1000 ohm. The B+ voltage dropped down to 252 volts and tube current increased to 3.8 mA (3.8 volt across cathode resistors)
So to my questions:
a) Should I increase the bypass capacitor value to compensate the lowered cathode resistor? It is currently rated 100uF/50V
b) Should I attempt to increase the driver B+ back up to approximately 320 volts? Given that SRPP is known to have poor PSRR I am afraid that reducing the series resistor will give ripple related issues.
c) Will my current configuration be able to fully drive the 300b output tube? 300b has 440 volt B+, 72 volt bias across 860ohm cathode resistor (84 mA). 300b grid stopper is 10K and grid leak is 100K.
d) The input tube has a grid stopper of 22K and the "upper" tube has a 10K resistor between its grid and the lower anode. Any tweaks I could do to make improvements? Can I safely reduce these?
I hope somebody can chime in with some further advise (ref my questions at end of this post).
As mentioned in my previous post, the input stage and the weird 120K series resistor at the driver output was removed. Thus, my preamp is now feeding the SRPP driver directly. Further modifications where carried out in 3 steps. No distortion measurements were done, but subjectively, each modification bring an improvement to the sound quality.
Step 1: Replace 6SL7 for 6SN7 as per Ketje's advice - Lost a lot of gain, but with the high-level output of pre-amp it is still possible to get full volume. B+ for SRPP dropped to 258 volts and tube current per tube is 1.85mA (5 volt drop over cathode resistor).
Step2: Each channel has separate resistor and capacitor for cleaning up B+ to the SRPP. Each capacitor is 58uF. Reducing series resistor for each channel from 100K to 50K increased B+ to 323volts and the current to 2.3mA (6.5 volt cathode voltage drop).
Step3: Cathode resistors where reduced from 2K7 to 1000 ohm. The B+ voltage dropped down to 252 volts and tube current increased to 3.8 mA (3.8 volt across cathode resistors)
So to my questions:
a) Should I increase the bypass capacitor value to compensate the lowered cathode resistor? It is currently rated 100uF/50V
b) Should I attempt to increase the driver B+ back up to approximately 320 volts? Given that SRPP is known to have poor PSRR I am afraid that reducing the series resistor will give ripple related issues.
c) Will my current configuration be able to fully drive the 300b output tube? 300b has 440 volt B+, 72 volt bias across 860ohm cathode resistor (84 mA). 300b grid stopper is 10K and grid leak is 100K.
d) The input tube has a grid stopper of 22K and the "upper" tube has a 10K resistor between its grid and the lower anode. Any tweaks I could do to make improvements? Can I safely reduce these?
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