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Old 15th November 2010, 06:42 AM   #21
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Quote:
Originally Posted by mltube View Post
A note about the 27's, I have rotated 9 pairs of them through and the globe mesh plates have the most delicate transparent sound. The solid plate st's sound slightly harsh to my ears.
Cheers, Mike

Hey,

Thanks for posting this. I've got a NOS 45 and 27 sitting in the box and have been planning a SE amp with. Good to hear someone using the 27. It seems to get a bad rap sometimes. Look forward to trying the circuit you posted out.

Am also looking for an OT for this project that has a 600r line tap in addition to the speaker taps. Rare item among modern iron. Hammond would do it - likely for the price of a James though. If anyone has such a beast they'd be willing to part with give me a shout.
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Old 15th November 2010, 07:08 AM   #22
mltube is offline mltube  United States
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Lasso,
I have removed the 1k plateload resistors on the 27, used seperate 10hy or higher choke to load the plates thru 27k resistor. After I have done this as per advise from kevin kennedy in a previous post, all is wonderfull. I thought, what is the big problem with the 27,... those using scopes to detect freq response curves. Most of the time the freq's are out of human hearing anyway or at least mine, are what people rave about. I build for myself and family, and have had zero complaints.
The mesh plate 27 to me sounds just sweet and shimmering. Use quality signal caps and good iron and it will put a smile on your face. Also, I have removed the paraphase arangement for the output tube, meaning I remove the choke to plate and connected B+ to the opt as for normal SE use.
When I can get to it, i'll post updated circuit. Mind you, I am no expert here just a diehard DIY'er using tubes for the pleasure of music.
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Old 16th November 2010, 12:52 AM   #23
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Sounds good. I would imagine the cross talk with the shared choke could be annoying at times. I have a pair of SE 6BQ5 amps that share a cathode resistor and I've concluded I don't like that arrangement.

One other thing of note - I noticed you're using a single filament winding for all four tubes. I've always heard "use two separate windings for 45s". Sounds like yours is doing ok though. Keep us posted.
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Old 16th November 2010, 01:09 AM   #24
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by lassoharp View Post
Sounds good. I would imagine the cross talk with the shared choke could be annoying at times. I have a pair of SE 6BQ5 amps that share a cathode resistor and I've concluded I don't like that arrangement.

One other thing of note - I noticed you're using a single filament winding for all four tubes. I've always heard "use two separate windings for 45s". Sounds like yours is doing ok though. Keep us posted.
Funny or not I came to about the same conclusion wrt to shared cathode resistors which are not too uncommon in PP stereo 6BQ5 amps either. Dyna ST-35, and SCA-35 are immediate examples that spring to mind.. My very first stereo SE amp as a teen was a repurposed magnavox with shared cathode bias.

I'd recommend separate filament windings for 45 with cathode bias, but if you are using fixed bias a shared winding is just fine except for the inconvenience involved in measuring plate currents. In this case the I think the cathode bias on the winding is helping to keep the 27 cathode to filament interface from misbehaving - much higher voltages though may cause the cathode to filament insulation to fail in these tubes - I learned this the hard way..

Normally I'd recommend the 27 filaments be biased about 20 - 25V above the cathode, but not substantially more than this. Using the output stage cathode bias to do this is probably ok as the voltage differential is still less than 40 - 50V which is where I experienced problems, and as long as the cathode bypass cap is a good one hopefully there will be no significant coupling through the 27 filament capacitance.

Heck, it works, and sounds good, all refinements can follow from a working prototype..
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Last edited by kevinkr; 16th November 2010 at 01:16 AM.
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Old 16th November 2010, 01:35 AM   #25
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Thanks for the heads up on the 27 cathode-filament interface. This will be my first time working with the 27. That sounds like some good information to know.

Quote:
Normally I'd recommend the 27 filaments be biased about 20 - 25V above the cathode
Something like this:? (note: meant to write as 20-25V instead of 40-50V)
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Old 16th November 2010, 04:48 AM   #26
kevinkr is offline kevinkr  United States
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Originally Posted by lassoharp View Post
Thanks for the heads up on the 27 cathode-filament interface. This will be my first time working with the 27. That sounds like some good information to know.

Something like this:? (note: meant to write as 20-25V instead of 40-50V)
Exactly.
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Old 16th November 2010, 09:15 AM   #27
mltube is offline mltube  United States
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Default Some EYE CANDY... :)

Just a sample of tubes I have rolled into this experiment...
These are matched pairs (quite close)..
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Old 16th November 2010, 10:56 AM   #28
jea is offline jea  United States
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These are matched pairs (quite close)..

And your favorite was?


Building a 45 set at the moment but don't own the tubes,...yet.

J
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Old 16th November 2010, 09:47 PM   #29
mltube is offline mltube  United States
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The majestic globe 27's w/mesh plate's for the input and Kenrad 45's are my current favs. I only have two pair of 45's, The kenrads and a pair of raytheons.
Cheers, mike
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Old 16th November 2010, 10:28 PM   #30
mltube is offline mltube  United States
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Quote:
Originally Posted by mltube View Post
The majestic globe 27's w/mesh plate's for the input and Kenrad 45's are my current favs. I only have two pair of 45's, The kenrads and a pair of raytheons.
Cheers, mike
Other's may sound better to your ears, and at varying operating points.
I am no expert here, just build for the pleasure of music and gratification of creating it myself. If I had the energy, I would build audio equipment for a living. Right now, I do not have the equipment to provide documentation of distortion or Db levels ect. My ears and common sense using mr. OHM and Kirchoff works for me.
It becomes difficult here, because most of my Iron have zero documentation, so at times it becomes a guestimate of sorts and over compensate and maintain strict saftey procedures. Yess, rambling of topic again, sorry.
Have a great day
Mike (mltube)
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