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Old 5th December 2003, 08:56 AM   #1
exurbia is offline exurbia  Australia
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Default type 27 pre

Does anyone out there have a schematic for a type 27 pre amp

i've got the 27's and assorted bits n pieces. any help is most

welcome.
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Old 5th December 2003, 12:06 PM   #2
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Hi,

You can take a look here for inspiration:

#27 PREAMP

Cheers,
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Old 5th December 2003, 02:04 PM   #3
exurbia is offline exurbia  Australia
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thank you Frank, i will have a bash at that over the weekend. I have seen a design for a 26 that has choke loading of the anode, what are the benefits of this ? . Would the 27 be better if choke loaded.
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Old 5th December 2003, 02:27 PM   #4
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Hi,

Quote:
I have seen a design for a 26 that has choke loading of the anode, what are the benefits of this ? . Would the 27 be better if choke loaded.
The 26 and 27 are very similar so you could look at circuits using that too.

Burnedfingers had a thread running on that valve as well.

Choke loading offers a much higher impedance AC-wise than a resistively loaded design.
Final results will depend on implementation but in theory it should be better to use a choke.

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Old 6th December 2003, 12:07 AM   #5
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Some people say the #27 holds the same promise the #26 does. I cannot comment on this as yet. I just received a 2.5V transformer today and will wire it in tomorrow for a try. The circuit will have two 150H chokes in series to the plate for each tube. I will try AC on the heaters first with the hopes that it might work. Looks like I will aim for 135Von the plate for the first go around.

Will post more tomorrow.



Joe
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Old 6th December 2003, 11:05 PM   #6
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Default Update

Wired in the heater transformer and fired it up today. It doesn't quite live up to the #26's performance. It is however very natural sounding without any microphonic problems. Its worth the time to play with. Running AC on the heaters with very little hum. Running 135volts on the plate and 9volts cathode voltage. Sounds very nice with 2) 150 H chokes in series to the plate.


Joe
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Old 7th December 2003, 07:58 AM   #7
RS is offline RS  Germany
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Default 27 Line Amp

26 and 27 are very close regarding , Ia and Rp, but 27 is pretty easy to heat. Just AC heating and it works fine. For 26 you will need a regulated DC supply in most cases.
Running with 135v Ua and -9V Ug the Rp of 27 is 11kohms.

For 11kohms Ri a choke starts to work series with 150H, it delivers -1db at 45Hz and - 3dB at 12Hz.

Regards

Ralf
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Old 8th December 2003, 11:24 AM   #8
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Default Drop the chokes or keep them?

RS

Is it your feeling the chokes should be dropped in favor of a plate resistor? That is the notion I get when I read your response that it is 3db down at 12HZ.


Anyone else have any feelings on this?







Joe
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Old 8th December 2003, 11:12 PM   #9
RS is offline RS  Germany
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Default Re: Drop the chokes or keep them?

Quote:
Originally posted by burnedfingers

Is it your feeling the chokes should be dropped in favor of a plate resistor? That is the notion I get when I read your response that it is 3db down at 12HZ.
I like plate chokes and interstage coupling pretty much. But with circa 10k Rp the critical value is reached. With very high inductive chokes you can underrun the cut-off at bass frequencies, but one should take care about parasitic capacities of the windings.


From my point of view -3dB at 12Hz is still fair; in most cases the complete system including speakers can not perfom on this level.

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Ralf
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Old 9th December 2003, 01:06 AM   #10
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Default Re: Drop the chokes or keep them?

Quote:
Originally posted by burnedfingers
Sounds very nice with 2) 150 H chokes in series to the plate.
Quote:
Originally posted by burnedfingers
Is it your feeling the chokes should be dropped in favor of a plate resistor? That is the notion I get when I read your response that it is 3db down at 12HZ.
Anyone else have any feelings on this?
Keep the chokes. With two in series, you have 300H, not the 150H of RS's calc so your f3 should be around 6Hz not factoring in the 7k4 winding resistance of the two of them.
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