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Old 26th November 2010, 11:09 PM   #1
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Default Tube ID help needed (EL34?)

Guys, I have just been handed a non-working mono amp for which I have no schematic or other info. It looks like the bias pot has gone open on the wiper and the output tubes have done a meltdown (plates are buckled). As they have no ID marks (other than RCA made in USA and 63EX3 on their bakelite base), and are non-functional apart from heaters, I am having trouble working out what they were. Pin connections suggest 6550 or similar, as pin 1&4 connected together and pins 5&6 connected together, the rest standard EL34 type.
I have attached a couple of pics of one next to a EL34 for comparison. They are the same height as most EL34s but fatter at 1.5 inches across the glass.
The internal structure looks similar to EL34.
I don't have much experience with the bigger output tubes and have looked at various pics and data sheets and not found one quite the same.

Edit: some other info: the amp is PP (both tubes appear dead), but the OPT (no info on that either) appears intact. I did a voltage ratio test and got a ratio of 28.6:1, which would make it about 6.5k:8 if my calcs are right? There is no indication on the amp as to what impedance speakers were used. The OPT looks big enough to probably be 50-100 watts in PA use.

Thanks
Gary
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Last edited by rotaspec; 26th November 2010 at 11:36 PM.
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Old 27th November 2010, 12:11 AM   #2
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by rotaspec View Post
Guys, I have just been handed a non-working mono amp for which I have no schematic or other info. It looks like the bias pot has gone open on the wiper and the output tubes have done a meltdown (plates are buckled). As they have no ID marks (other than RCA made in USA and 63EX3 on their bakelite base), and are non-functional apart from heaters, I am having trouble working out what they were. Pin connections suggest 6550 or similar, as pin 1&4 connected together and pins 5&6 connected together, the rest standard EL34 type.
I have attached a couple of pics of one next to a EL34 for comparison. They are the same height as most EL34s but fatter at 1.5 inches across the glass.
The internal structure looks similar to EL34.
I don't have much experience with the bigger output tubes and have looked at various pics and data sheets and not found one quite the same.

Edit: some other info: the amp is PP (both tubes appear dead), but the OPT (no info on that either) appears intact. I did a voltage ratio test and got a ratio of 28.6:1, which would make it about 6.5k:8 if my calcs are right? There is no indication on the amp as to what impedance speakers were used. The OPT looks big enough to probably be 50-100 watts in PA use.

Thanks
Gary
I'm almost certain this is a 7027 based on your description and what it looks like.

Here is a link to the data sheet: http://scottbecker.net/tube/sheets/049/7/7027.pdf
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Old 27th November 2010, 01:53 AM   #3
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Thanks Kevin, that makes sense. I thought of 6550, 5881 and KT88 but they are all different shaped to this. Now that I look at the 7027 data sheet, it seems most likely.
7027 wasn't overly common in amps here, most were KT88 at that power level.
The amp is actually quite well built, but whoever designed the bias adjustment didn't put any safeguards in. It's a 120v transformer, 1/2 wave ss rect/filt into a pot to ground with wiper straight to the grids. When I fired the amp up with no tubes, the grid voltage was zero. Wriggled the pot a few times and got voltage.
I'll just use it as a source of iron.
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Old 27th November 2010, 12:22 PM   #4
DF96 is offline DF96  England
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In a well-designed amp the bias slider coming off the track won't fry the output valves, as there will be a highish value resistor connecting the slider to the negative bias supply in order to protect against this predictable fault. Not all amps are well-designed.
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