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Old 17th March 2015, 11:21 AM   #1
forest 100 is offline forest 100
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Default Voice coil

Hi guys, newbie here,
Hope you can help me with my issue.Working on the vintage radio with a bad output transformer. 2 pp 42 tubes output, 8 watts. Have no specs on the tr-r. So, the primary impedance will be 10000ohms. My problem is the secondary. The voice coil reads 2.0 ohms DC and I do not know which way to go- 2 or 4 ohms impedance? Checked coil under the mag. glass- no shorts. Any idea?
Thanks in advance.
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Old 17th March 2015, 03:04 PM   #2
bear is offline bear
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How can you check the VC under a magnifying glass when the speaker is not disassembled??

What sort of "radio"?

Have you checked the tube manual(s) for the typical primary impedance for the B+ voltage you are running?? 10kohms seems surprisingly high. In which case the secondary impedance you wish to match will not match.

It will make sound, but the goal is maximum power transfer for a given load Z.

2 ohms DC seems to indicate <4 ohms, but "4 ohms" nominal is likely good enough for a radio.

Have you tested the speaker?
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Old 17th March 2015, 08:54 PM   #3
forest 100 is offline forest 100
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Quote:
Originally Posted by bear View Post
How can you check the VC under a magnifying glass when the speaker is not disassembled??

What sort of "radio"?

Have you checked the tube manual(s) for the typical primary impedance for the B+ voltage you are running?? 10kohms seems surprisingly high. In which case the secondary impedance you wish to match will not match.

It will make sound, but the goal is maximum power transfer for a given load Z.

2 ohms DC seems to indicate <4 ohms, but "4 ohms" nominal is likely good enough for a radio.

Have you tested the speaker?
Of course I disassembled the speaker,pulled out the field coil and armature to check the coil visually. It is push/pull with #42 tubes calling for 10k primary impedance. It is late 30' vintage tube radio. I can't get continuity to the CT of the primary, hence the new transformer is in order.
"Newbie" meant new for the forum.
Thanks.
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Old 18th March 2015, 12:14 AM   #4
opcom is offline opcom  United States
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Location: Texas
The 42 is a power Pentode very close to the 6F6.

AB1 pp service
Eb 315V
Eg2 285V
Eg1 -24V
Ib0 62mA
Ibmax 80mA
Load p-p 10K
Po 11W
THD 4%

My guess would be 4 Ohms, or 3.2. Measuring the impedance of the speaker at some reasonable frequencies could get closer to the truth. 400Hz is a safe bet usually.
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Old 18th March 2015, 02:40 AM   #5
opcom is offline opcom  United States
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Location: Texas
This little gem is real useful for checking speaker and general load impedance. Some say Raymer, some University. They show up on ebay sometimes.

They check 25V and 70V line wattage (by impedance) from 0.5 to 200W and speaker or transformed impedance itself from 1 Ohm to 10K. They do it at 100Hz or 400Hz.
Attached Files
File Type: pdf LWT EDS.pdf (121.4 KB, 5 views)
File Type: pdf Tech Bulletin 35 - LWT Tester Application.pdf (95.1 KB, 4 views)
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