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Old 16th April 2009, 02:22 PM   #21
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Quote:
Originally posted by rknize
Bottom row is in "300B mode", running Shuguang 300Bs (not mine) at about 28mA per tube.
300B should be biased at 60 to 75 mA (per Tubelab description). B+ voltage will drop as you increase the bias.

The blue glow looking like that is from the gas inside of tube as I was told. It shouldn't interfere with the tube's performance unless it floods the inside like fog which is said to be a sign of leakage (someone correct me if I'm wrong).

By the way, are those Sophia mesh tubes?
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Old 16th April 2009, 03:07 PM   #22
rknize is offline rknize  United States
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You are right about the bias. I was being very conservative with them, as they are not my tubes and I didn't want to have to replace them should something go wrong. This breadboard amp has at well over 100 hours on it now using 45s, so I guess I shouldn't worry.

The tubes are Shuguang's aptly-named "300BS". They are borrowed from a well-to-do friend who isn't using them anymore. He bought these after his set of vintage WE's got weak. He since bought a pair of the new WE's and so these were sitting idle. I was talking to him about my projects, so he offered to let me borrow the pair.

I've seen plenty of harmless blue glow in other tubes, mainly Sylvania beam tetrodes and a few others (SED EL34s are neat). I don't think it is gas if the blue glow is only projected onto the glass, which is the case here. I don't see any glow inside the open space of the envelope. I suspect that electrons are just blasting through the mesh in places and hitting the glass. I've just never seen glow from any DHT before....
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Old 16th April 2009, 03:23 PM   #23
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I've seen them in my Sovtek 2A3 and little bit in my 300B"S".
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Old 16th April 2009, 04:07 PM   #24
rknize is offline rknize  United States
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Ah OK, thanks. I guess I won't worry about it then. Just a bit of a DHT virgin, I guess.
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Old 17th April 2009, 04:36 AM   #25
w5jag is offline w5jag  United States
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Quote:
Originally posted by rknize


The B+ in "45 mode" is a bit higher than I was hoping, but it is within range of what a lot of folks run it at.
To keep the B+ down on my Tubelab SE, I use a choke input configuration ( 10Hy). That puts my B+ right at 275 volts with both 45 and 2A3, when I use a 5AR4 rectifier. My tranny is a flat mount Utah something I bought for a buck or two at a hamfest.

It is dead quiet with 45's on 96 dB/watt speakers.

I seem to recall George writing that the 2A3 sounds best in the low to mid 300's, but I haven't tried that.

I don't think 2A3's are all that. I have both RCA 2A3, and Sylvania 2A3W/5930, and just haven't found the right setting yet, I guess. I always seem to go back to 45's.

I don't own any 300B's.

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Old 17th April 2009, 07:45 AM   #26
rknize is offline rknize  United States
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OK, I put the spurs to the 300B's tonight and let rip for about 5 hours at 80mA per tube. Holy crap could this things get loud on my 98dB speakers. Serious wall shaking power. At more sane listening levels, I would say they have less detail than the 45s but it could be my ears ringing...

The 660VAC winding was pulled down exactly to spec: 660VAC. B+ sagged to around 370-375 VDC. Supposedly this is the ideal spot for the 300B. I suppose running the tubes well short of the rated plate voltage will help them last longer. Given the price of 300Bs....

After 5 hours, the transformer was a bit warmer than before but not hot by any means.

So overall I am happy with it. The only change I would make is to drop the B+ taps down to 250VAC for "45 mode".
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Old 18th April 2009, 01:13 AM   #27
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Quote:
I seem to recall George writing that the 2A3 sounds best in the low to mid 300's, but I haven't tried that..... I don't think 2A3's are all that. I have both RCA 2A3, and Sylvania 2A3W/5930, and just haven't found the right setting yet, I guess. I always seem to go back to 45's.
I guess that I haven't found the magic setting yet either. I have a few RCA's (all twin plate) a pair of Shuguangs and a pair of Sovteks. I tried to find something that I liked in the RCA's and Shuguangs, but they are my least favorite of all the DHT's that I have stuffed in my amp. I have tried B+ voltages from 225 to 375 volts with 3K and 5K OPT's. They definitely sound different than the other tubes. Yes, I tend to prefer high voltages and higher load impedances than most users, but 300 volts is over the line for most 2A3's.

I got the Sovteks in response to an issue that a customer found. The Sovtek 2A3's arent really 2A3's they are mini 300B's with 2.5 volt filaments. To me they sound more like 300B's than the others. Like the 300B's these tubes sounded best when fed 375 volts into a 5K load. The filament current is higher than the 2.5 amp spec which annoys the regulator IC. It can also drag the input voltage to the regulator down near dropout causing hum.

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OK, I put the spurs to the 300B's tonight .... I would say they have less detail than the 45s but it could be my ears ringing...
That is my opinion too. The 45 is the best sounding tube that I have ever used. My only problem is its low power. The speakers in my lab are 87db so the 45 won't rock the house. The 300B gets them loud enough to get "turn it down" comments from my wife in another room.

Quote:
B+ sagged to around 370-375 VDC. Supposedly this is the ideal spot for the 300B.
I have run my Tubelab SE on a variable power supply and found that 370 to 400 volts all sounded good. I had a Tubelab SE with a big Hammond transformer (I don't remember the #) that provided 375 volts. I took it apart. I have one Tubelab SE that I use daily. It has a B+ of 320 volts and I run 45's or 300B's. I have about 10 45's of various flavors that I have collected over the years. They all seem to be happy on 320 volts. They make about 2 watts at 30 mA. 300B's make about 5 watts at 70 mA in the same amp.

I will order one of these transformers in the next few weeks. Despite the state of the company our division actually got a bonus check, so I finally got a modern TV. I gave away the 320 pound monster, and now I have a bunch of space where it used to be. This means that I need to put a tube amp there. My big "old radio cabinet" speakers have been connected to the TV set because there was no room for a real amp.

The speakers are two way, so a Tubelab SE with 45's for the HF is a given. LF is undecided yet. Maybe one of these transformers has enough juice to run two amps? I guess some experiments are in order. 5.1 with a few more old radios? Maybe. I didn't build a single amp during 2008, and only managed a few simple experiments, so its time........
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Old 18th April 2009, 03:37 AM   #28
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I decided not to try to cover the 2A3 with the heater as well because it feels like a compromise to me anyway. If I am going to make the effort to support a 300B and spend the money on a pair, why would I want to run 2A3s too? Maybe I'm just too simplistic and there is something lovely about the 2A3 I am missing...I dunno. I have heard at least one amp that uses them, but it wasn't a situation where I could compare it to anything else.

Quote:
Originally posted by tubelab.com
That is my opinion too. The 45 is the best sounding tube that I have ever used. My only problem is its low power. The speakers in my lab are 87db so the 45 won't rock the house. The 300B gets them loud enough to get "turn it down" comments from my wife in another room.
OK, so one odd thing about this amp with 45s...it is not obvious when the amp starts clipping. Maybe it is an SE-vs-PP thing, but usually an amp sounds good until I can start to hear the crunching. Perhaps my line stage needs more gain, but I can "dime" this amp and I don't get any of that. I do notice a drop in detail and sound stage before ear fatigue sets in, so maybe it is clipping and introducing harmonics, but it's not obvious.

It does have a hard time driving my BA A70s, which are acoustic suspension, when I start to push it with the 45s. The 300Bs can rock those suckers, though. The KLF-10s are driven to uncomfortable volumes with the 45s, so no problem there.

Quote:
Originally posted by tubelab.com
I will order one of these transformers in the next few weeks. Despite the state of the company our division actually got a bonus check, so I finally got a modern TV. I gave away the 320 pound monster, and now I have a bunch of space where it used to be. This means that I need to put a tube amp there. My big "old radio cabinet" speakers have been connected to the TV set because there was no room for a real amp.
Good to hear. The axe was poised to swing again here, but it hasn't at us just yet. Everyone is holding their breath...

Quote:
Originally posted by tubelab.com
The speakers are two way, so a Tubelab SE with 45's for the HF is a given. LF is undecided yet. Maybe one of these transformers has enough juice to run two amps? I guess some experiments are in order. 5.1 with a few more old radios? Maybe. I didn't build a single amp during 2008, and only managed a few simple experiments, so its time........
Two might be pushing it, but I guess it depends what demands the 2nd amp will have. The heater winding would be overtaxed at the very least....
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Old 19th April 2009, 06:40 AM   #29
rmyauck is offline rmyauck  Canada
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Are the Electra print's worth the extra $, as George mentioned them as tops especially for the OT's? What about Lundahl's?
Has anyone tried to wind their own? I want quality , but can't afford to spend lot's of $. I ran across a thread to Susan Parker's designs on winding Trans. She also has quite the simple Amp design! Could hand winding be as good as factory if one has the time?

Thanks Very Much

Randy
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Old 19th April 2009, 11:15 PM   #30
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Quote:
OK, so one odd thing about this amp with 45s...it is not obvious when the amp starts clipping. Maybe it is an SE-vs-PP thing
If you have access to an oscilloscope it is an interesting exercise to hook one up to the speaker outputs. Most SE amps can be run well into cliping on transients before the distortion is heard. It is not an SE vs PP thing. It is usually a feedback or no feedback thing.

In an ideal zero feedback design the amp simply limits the output during cliping. The total amplifier gain drops slightly since the increased input does not produce more output. When the transient passes everything returns to normal. Small signal details are lost during the overload period since the output is slammed against the rail (clipped). The ear usually does not hear fine detail during loud sounds (the masking phenomenon that brought us MP3's), so the lost detail may not be missed.

In an ideal amplifier with large amounts of global negative feedback (typical SS amp) the amp may have an open loop gain of 60db. This may be reduced to 30db by applying 30db of NFB. When the transient pushes the amp into clipping the feedback loop causes the gain to increase until the excess gain is used up. The amps gain will aproach the open loop gain in an attempt to avoid clipping. This can drive multiple stages to their rails leading to long recovery times, IMD distortion and a bunch of other ugly sounding phenomenon.

Obviously there is a lot more to it than this, but it is possible to build a push pull amp with similar clipping behavior as the Tubelab SE. My 300Beast can be pushed well into clipping before it becomes obvious, but it is a class A all triode design with no feedback. It unfortunately no longer works, and will be rebuilt, but that is a new story yet to unfold.

Quote:
Good to hear. The axe was poised to swing again here, but it hasn't at us just yet. Everyone is holding their breath...
We hear similar rumors. Something unplesant happens coincident with the quarterly earnings (not) report.

Quote:
Two might be pushing it, but I guess it depends what demands the 2nd amp will have. The heater winding would be overtaxed at the very least....
Yeah, I am just dreaming out loud for now. I want to experiment with some more push pull stuff before I build the final design. I have come to realize that no amp is ever completly done. I keep taking them apart and rebuilding them. I only have two amps that have been left alone since they were built. The Lexan Tubelab SE and the "industrial" Simple SE. The industrial amp is a bit too small and it gets too hot especially since I have it set at 100 mA per tube. I decided that I would rebuild it when it blows up. It hasn't missed a beat since I said that, so it remains untouched.

Quote:
Are the Electra print's worth the extra $, as George mentioned them as tops especially for the OT's?
I got a pair of "10 watt 5K SE OPT's" made by Jack a few years ago for $100 each. At that time I decided that they were the best transformers that I have owned. They are on my Lexan amp which is the only other amp that I made that I liked well enough to leave alone. That amp still sounds very nice especially with 45's in it and it has been to a few auditions where it held up well next to some real expensive amps.

The Electra - Print transformers are considerably more money now, so I can't say whether they are the best value for the money today. I have not tried Lundahl's or any of the other higher priced OPT's. If I had to pick an "under $100 OPT today I would go directly for the big Edcors (CXSE series). I got them after the Lexan amp was done so I have never done a direct comparison with the Electra-Prints yet.

Quote:
Has anyone tried to wind their own?
I have tried to wind my own, but so far all of my attempts (3) have sounded bad. There are people on this forum who have made their own transformers though. I must say that a SE OPT especially one of a fairly high impedance ratio is one of the hardest transformers to get right.
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