Change of circuit from triode to tetrode - diyAudio
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Old 23rd October 2010, 11:34 AM   #1
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Default Change of circuit from triode to tetrode

The original circuit has a pair of KT88's wired as triodes, wired for class B zero bias. Output is given as 100W. (Possibly originally tubes installed might have been 807's).

The original tube line-up for this amplifier (actually a modulator, not a high fidelity amplifier) is 6SJ7 (microphone amplifier) 6SN7 (voltage amplification) 6L6 (driver for KT88's) and of course a pair of KT88's.

The KT88's are driven via an interstage transformer, the secondary side is centre tapped and ends go to g2 first, and also g1 (through a 15K resistor). Cathodes of the KT88's were joined together and went to ground via a meter that reads "AUDIO WATTS".

Okay, I'm thinking about removing the interstage transformer and operating two output tubes (Possibly KT88's) as tetrodes.

I suppose what I want to do is perfectly feasible. But, I'm not entirely sure how to make the changes. I'd need a tube (say contemporary with the ones I've mentioned), anyhow, some arrangement to take place of the interstage transformer. (Or should I keep the interstage transformer?)

Any ideas what I'd need to do between the 6L6 and the output tubes. Would I even still need the 6L6 type before the finals? Thanks for any input.
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Old 23rd October 2010, 12:27 PM   #2
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Actually, I just remembered that I have a copy of the RSGB handbook 4th Edn. :c)

It talks of modulator design on page 9.20.

I see also it talks about zero bias triode connection on page 9.21, which was the original setup.

From the text I glean that fairly stringent requirements are placed on the power unit for tetrodes or pentodes, but not for triodes, particularly when wired for zero bias.

For a KT88 wired for zero bias about 7W drive power is required.

Much less than a watt no doubt if run as tetrodes.

What is doable, concerning a change from triode to tetrode connection, no doubt a function of what the PSU is capable of.

I'm busy reading this section on modulators to see the scope of my employing these KT88's as tetrode.

Last edited by richard8976; 23rd October 2010 at 12:29 PM.
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Old 23rd October 2010, 12:57 PM   #3
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Perhaps some here are radio hams as well as audio geeks. :C

I had this notion of running the KT88's as tetrodes rather than triodes. Just seemed the better thing to do. :c)

But, there would have been a reason to run the KT88's as triodes, and that might have been a matter of getting as much bang per buck of money spent on the PSU.

The problem I could have is that the PSU will have a higher voltage than one that would supply a tetrode. Put it this way, no point in trying to run a KT88 which requires (in tetrode) a much lower voltage than the PSU outputs. That's wastin'.

So, I may not have much option but to keep the triode connection.

Last edited by richard8976; 23rd October 2010 at 01:01 PM.
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Old 23rd October 2010, 02:04 PM   #4
DF96 is offline DF96  England
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Yes there are radio amateurs on here. You could try running the KT88s as normal tetrodes in Class AB2. This would require transformer drive, as you have now. You would need to rewire the KT88, and add a negative bias supply. However, you might find that triode operation would be a better match to the existing output transformer.

For less power, and probably less distortion, you could do Class AB1. This requires more or less no drive power, so you swap the 6L6 driver and transformer for a normal phase splitter. You could use either fixed bias from a negative supply, or cathode bias. This will be explained in the RSGB book you have.
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Old 23rd October 2010, 02:21 PM   #5
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The output transformer is the ubiquitous UM3 capable of a wide range of impedance matching options.

I don't want to use those GEC KT88's - I want to sell them off. :c)

I may use modern KT88's or even revert to a pair of 807's.

I don't quite know why, but I just feel like it's better to run as tetrode rather than a triode connected class B zero bias. If it's doable with the PSU.
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Old 23rd October 2010, 02:29 PM   #6
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Are you using this as an audio or rf amp? Any reason not to use the IS tranny? Does it have poor response/low inductance?
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Old 23rd October 2010, 02:51 PM   #7
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Quote:
Originally Posted by mashaffer View Post
Are you using this as an audio or rf amp? Any reason not to use the IS tranny? Does it have poor response/low inductance?
As an audio amplifier, as a modulator.

No, I guess I don't have a reason not to use the IS transformer. I thought it might not be required for tetrode connection and the much lower drive power requirement.

I presume the IS transformer (Chicago Transformer 7533-F) is perfectly acceptable for this modulator role.
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Old 23rd October 2010, 03:00 PM   #8
DF96 is offline DF96  England
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You could use 807, 6L6, 5B/254M (miniature 807) or 5B/255M (single-ended version) - these are all different versions of essentially the same valve. As you already have octal sockets in place I guess 6L6 is your best bet.
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Old 23rd October 2010, 03:35 PM   #9
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Actually, what I probably should do, is a modern implementation using the UM3 and the power transformer that I have.

That modern implementation would then not involve the original setup viz: 6SJ7 (microphone amplifier) 6SN7 (voltage amplification) and 6L6 (driver). Although maybe it's not a bad line-up. Maybe it is pretty decent, I don't know.

A modern implementation probably would involve some compression - whatever.
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Old 23rd October 2010, 04:59 PM   #10
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IMO, there's nothing wrong with the 6SJ7/6SN7 small signal complement. You should be able to set up nice Mullard circuit using those "puppies". Full pentode Class "AB1" operation yielding roughly 60 W. leaves some magnetic headroom for protection against GNFB induced core saturation. Don't forget to regulate g2 B+. The 6L6 could serve as the pass element in the regulator.

The best current production KT88 is the Saratov, Russia, made "reissue" GEC. However, they cost a good deal of money. Also made at Saratov is the ElectroHarmonix (EH) variant, which is less costly than the "reissue" GEC. The EH tube lacks a bit of bass extension, but is otherwise quite satisfactory, as the bass it does yield is "tuneful".
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