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Tent Labs autobias circuit
http://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3078
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Author:  ajcrock [ 05 Mar 2011, 14:56 ]
Post subject:  Tent Labs autobias circuit

Pros:
1) Eliminates the need to bias your tubes again
2) Syncs the bias of 4 tubes
3) Tighter bass response
4) Increased detail retrieval

Cons:
1) Cost
2) Long lead times
3) Not very responsive to email

Last October,m I began a search for an autobias circuit. Through this forum and other web site it appeared that TentLabs was the only available option. However because of the cost, about $215 for 4 tubes, no one had ordered one. On November 9th I placed my order, about $430 for 8 tubes.
After many promises of delivery, I finally gave up and filed with PayPal in January to get my money back. It turns out the Tentlabs ships via surface postal which has a habit of getting lost as the mail comes by boat, no I am not kidding and DHL is not an option. However, in late January the boards showed. So I paid Tentlabs and installed the boards only to find a transistor was missing from one of the boards. Needless to say this was not amusing. However, I contacted Tentlabs and after several attempts they responded by shipping a new transistor and providing a part number so I could order the transistor myself. After a week the transistor arrived with and SACD which was a nice gesture. I say all this because you need to be aware that the lead times can be lengthy and communications are not the greatest.

When installing the boards you will need to find space. My amplifier is an Audio Research VT110 (preamp is Cary SLP 50B, SACD player is Marantz, and speakers are Mythos STS) which would have had space if I had not replaced all the coupling capacitors with Mundorf silver and oil capacitors which are about 4 times the size of the original capacitors. Be prepared to do a fair amount of rewiring. It took me a whole day to get the boards in. Also be prepared to file down your 16 gauge wiring so it can fit in the plated through holes. They are not a standard US size. Finally, the instructions have you set the bias without the tubes, good idea, and then goes on to state that you should check again after tubes are in place. Highly recommended! I set my bias to 550mv (it uses small 10 cathode capacitors, don’t worry they do not get hot). Yet when I put the tubes in the bias showed 620mv! I reset the bias and all was ok after that.
So how does it sound? Before the addition of the autobias circuitry I had very deep black levels and great sound staging. From the music server there is little if any change. Part of this is because the limited resolution from lossless wave files ripped from CDs along with wireless transfer.
When playing an SACD, I now notice a soft click coming from the speakers when I change the volume on my preamp. The volume control is a stepped attenuator with an Elma switch and Takman metal film resistors that I selected and hand assembled to match the gain of the pre-amp, amp and the sensitivity of the speakers. This soft click is evidence that the auto bias has now increased the detail and dropped the noise level enough to hear this change, again before there appeared to be no noise at all. I have also noticed a tightening of the bass and increase in transient speed and the lower frequencies. This is interesting in that my speakers are Def Tech Mythos STS which have a built in power subwoofer. However, that signal still must begin in the VT110 with the autobias circuitry. Finally, I have noticed a little more warmth from the SACDs. This is not a bad this in that I had worked to eliminate all “Tubeiness” to the point that the system was sounding solid state and not tube. I also believe that the sound is more consistent but that is more difficult to verify.
Would I recommend this? That depends on where you are at with your equipment and what you are looking for. Silver and oil filled coupling capacitors will give you a bigger bang for the buck, use Mundorf gold and oil if you can afford them. A stepped attenuator is also a change that is a bigger back for a buck. But If you have already done this and worked on your shielding of the transformers and cabling inside your equipment then you are in a position to get the most out of this change along with the added benefit of no more biasing.
l

Author:  pavlikkkk [ 07 May 2018, 02:34 ]
Post subject:  Re: Tent Labs autobias circuit

ajcrock wrote:
Pros:
1) Eliminates the need to bias your tubes again
2) Syncs the bias of 4 tubes
3) Tighter bass response
4) Increased detail retrieval

Cons:
1) Cost
2) Long lead times
3) Not very responsive to email

Last October,m I began a search for an autobias circuit. Through this forum and other web site it appeared that TentLabs was the only available option. However because of the cost, about $215 for 4 tubes, no one had ordered one. On November 9th I placed my order, about $430 for 8 tubes.
After many promises of delivery, I finally gave up and filed with PayPal in January to get my money back. It turns out the Tentlabs ships via surface postal which has a habit of getting lost as the mail comes by boat, no I am not kidding and DHL is not an option. However, in late January the boards showed. So I paid Tentlabs and installed the boards only to find a transistor was missing from one of the boards. Needless to say this was not amusing. However, I contacted Tentlabs and after several attempts they responded by shipping a new transistor and providing a part number so I could order the transistor myself. After a week the transistor arrived with and SACD which was a nice gesture. I say all this because you need to be aware that the lead times can be lengthy and communications are not the greatest.

When installing the boards you will need to find space. My amplifier is an Audio Research VT110 (preamp is Cary SLP 50B, SACD player is Marantz, and speakers are Mythos STS) which would have had space if I had not replaced all the coupling capacitors with Mundorf silver and oil capacitors which are about 4 times the size of the original capacitors. Be prepared to do a fair amount of rewiring. It took me a whole day to get the boards in. Also be prepared to file down your 16 gauge wiring so it can fit in the plated through holes. They are not a standard US size. Finally, the instructions have you set the bias without the tubes, good idea, and then goes on to state that you should check again after tubes are in place. Highly recommended! I set my bias to 550mv (it uses small 10 cathode capacitors, don’t worry they do not get hot). Yet when I put the tubes in the bias showed 620mv! I reset the bias and all was ok after that.
So how does it sound? Before the addition of the autobias circuitry I had very deep black levels and great sound staging. From the music server there is little if any change. Part of this is because the limited resolution from lossless wave files ripped from CDs along with wireless transfer.
When playing an SACD, I now notice a soft click coming from the speakers when I change the volume on my preamp. The volume control is a stepped attenuator with an Elma switch and Takman metal film resistors that I selected and hand assembled to match the gain of the pre-amp, amp and the sensitivity of the speakers. This soft click is evidence that the auto bias has now increased the detail and dropped the noise level enough to hear this change, again before there appeared to be no noise at all. I have also noticed a tightening of the bass and increase in transient speed and the lower frequencies. This is interesting in that my speakers are Def Tech Mythos STS which have a built in power subwoofer. However, that signal still must begin in the VT110 with the autobias circuitry. Finally, I have noticed a little more warmth from the SACDs. This is not a bad this in that I had worked to eliminate all “Tubeiness” to the point that the system was sounding solid state and not tube. I also believe that the sound is more consistent but that is more difficult to verify.
Would I recommend this? That depends on where you are at with your equipment and what you are looking for. Silver and oil filled coupling capacitors will give you a bigger bang for the buck, use Mundorf gold and oil if you can afford them. A stepped attenuator is also a change that is a bigger back for a buck. But If you have already done this and worked on your shielding of the transformers and cabling inside your equipment then you are in a position to get the most out of this change along with the added benefit of no more biasing.
l

When using the AB-Q auto-bias, the lead times are shorter and Cost is substantially lower.
Pavel (audioamp.eu)

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