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Old 9th August 2005, 06:55 AM   #1
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Default Precision Electronics Amp w/ 6GT5's, Convert to 6L6's?

I just bought a serious power amp that unfortunately utilises 4x 6GT5 tube in the power section. I want to know the feasability of converting it to a more common Octal base for use with a set of 6L6's or EL34's I plan on using it with Bass and Guitar mostly and possibly as a Tube PA. If conversion is not going to work, what are the chances of getting a matched quad of 6GT5's? Are there substitute tubes that would work with the Novar base? Are there adapters to use octal base tubes? Thanks in advance.

Jesse Mesa-Savage
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Old 9th August 2005, 11:21 AM   #2
SY is offline SY  United States
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This is a sweep tube, not too similar to audio pentodes like 6L6. My guess is that a bit of searching will turn some up, probably pretty cheap, though you'll likely have to overbuy and match some yourself.

Similar tubes would include 6JT6, 6JN6, 6JR6, 6GJ5, 6HB5, though all these have different pinouts.

The nice thing about sweep tubes (besides their generally low prices at surplus) is their ruggedness.
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Old 10th August 2005, 11:03 PM   #3
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> unfortunately utilises 4x 6GT5 tube

NOS 6GT5 is readily available at about $16 each. 6L6 cost $15 to $40 each. I say stick with 6GT5.

> what are the chances of getting a matched quad of 6GT5's?

About zero. BUT why do you need matched sets? 6GT5 was never available matched, so the amp must be designed to take about any motley set of 6GT5 that wanders in. Also a quad is less critical of balance than a pair. If you must obsess, test each tube alone and sort them into push and pull pairs so the total amp is balanced.

Can you find schematic or determine voltages and impedances? The general question "put 6L6 in place of any 6GT5 use" can't be answered "yes", but maybe the specific operating point can be handled by another tube.

> Are there substitute tubes that would work with the Novar base?

None of the Novars propspered outside TV set duty. You will not find them in matched sets or in new production. (If there is an exception, someone will hop on me.)
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Old 11th August 2005, 06:45 AM   #4
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Cool, thanks. I don't know anything about these tubes, but I'm curious what they sound like. What kind of poiwer output can I expect from the quartet. I will keep the 6GT5's for weirdness factor for sure.
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Old 11th August 2005, 01:59 PM   #5
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Without more circuit data, it's tough to predict power, but a reasonable WAG would be 50 watts or so.
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Old 11th August 2005, 08:57 PM   #6
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In the past, it was sometimes suggested to use 6GT5 as a sub for the 7868, with reduced screen voltage. Now that EH is producing the 7868, you might consider the reverse swap... requires much lower bias voltage, but the pinout is the same. And matched sets are available. The pins on the EH 7868s are slightly oversize, though, so 6GT5s probably won't fit afterwards - but then you didn't want to use them anyway?

But really, 6GT5s aren't hard to find. You would need to add separate bias adjustments, as you WON'T find matched ones easily.
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