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Change of circuit from triode to tetrode

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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.
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.
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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.
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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.
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.
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.
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.
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".
Yes. I myself was thinking, what is a "modern implementation" regarding valve choice here with this modulator and really, is there much "wrong" or "missing" from the original valve line-up.

Some alterations here and there for sure, I think. An extra tube perhaps - different circuit arrangements, but perhaps the AF modulator might well be very nicely based on more-or-less the original tube line-up.

I need to look at this some more, come up with what I think, and see what folks say. Might take a week or two. :c) I'm just getting to grips with the issues. But, I think I would like the tetrode set-up rather than triode, class B, zero bias setup as original. I'm tying to get away from that. I am quite happy with a different pair of finals from the KT88 originals.
I suspect that maybe the line-up may likely to be 6SJ7 (mic. amp.) 6SN7 (AF Amp.) 6SN7 (Splitter) and then the final pair (KT88, 807 etc - whatever).

I won't need the 6L6, because that was for suppling appreciable drive power to the triode-connected KT88's. And the IS transformer is a component necessary for that arrangement.

I suspect I'll find a splitter circuit using 6SN7 somewhere.

"The 6L6 is of course not a zero-bias triode; it is a tetrode.
However, any tetrode can be made to operate as a zero-bias
triode, as explained later", or so CQ June 1956 begins to say...

Not sure they follow through on that promise, to really explain
how "any tetrode" can do it? They use 22K rather than 15K to
limit G1 current, but don't explain how this or any other value
automagically relates to zero bias?
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We are driving G2 with Voltage, at unspecified current.
We are driving G1 with current, at unspecified Voltage.

Even talking to account G1 forward conducting at 0V.
Don't see how this gets us a zero bias operating point?
Current of G1 should not be relevant? What gives?
The "zero bias" is the quiescent condition on g2, which is acting as the control grid of the 'triode'. g1 merely needs to be forward biased a little to help out, but without heating it up with too much current. The value of the g1 resistor is not critical. I suppose it may also limit any breakdown current on the negative swing, which is almost certain to violate the g1-k maximum voltage difference. You could simply connect g1 to the cathode, but you would get less output as the valve would not turn on quite so hard. Remember. all these big tetrodes were characterised for AB2 operation where g1 goes positive and gives you more anode current.
I guess it depends whether you are going to play music or speak through it!

You could try the homebrew section of the UK Vintage Radio forum. There are some very knowledgeable people there who, unlike me, would not be seen dead in an audio forum!

Well, that's it you see, I just want to speak through it. :c) And the spec to aim for, the circuit to aim for would be much lower than what the audio guys would suggest given their sense of audio quality.

The thing is, the AM forum is where the guys with modulators hang out, in contrast to where the guys with high fidelity amplifiers hang out.

And the twain shall never meet. :c) Well, some hams are audiophiles.
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The thing is, the AM forum is where the guys with modulators hang out....And the twain shall never meet. :c) Well, some hams are audiophiles.

And some audiophiles are hams. Well, they have met at least once. That was until I sold my old Globe King and defected to the dark side (Yeasu).

You said that the original tubes were 807's, which were replaced by the KT88's. Why was this done? Did someone modify the RF amp to produce more power and then try to coax more power from the modulator by swapping tubes? Cranking more power out of the modulator without increasing the RF power will cause overmodulation, which will make other users of the frequency band unhappy with you.

If the original modulator produced about 100 watts, it would be possible to get 100 watts from a pair of KT88's in tetrode mode. You wouldn't need a 6L6 to drive them. The original circuit did not need a lot of drive voltage, but did consume drive power. A "modern" circuit would not need much if any drive power but would require more drive voltage since the gain in the output stage is lower.

Modernizing the modulator circuit may not make a big difference in the way thr rig sounds on the air. The original modulator circuit has some crossover distortion at low levels. This is most annoying on music since your ears are listening to the details. Crossover distortion will be almost undectable on speech since your ears are listening to the peaks. On the lower HF bands the QRM will be far worse than the distortion!

Tubelab = KB4LRE
Hi. When I come to think about it, I don't know why my modulator has KT88's in it, rather than 807's. I need to figure that. :c)

The original instructions mention 807's. I presume that these were 807's without the top cap. Because the modulator has no provision for connecting to top caps. So, I just though the manufacturer one day decided to put in KT88's instead of 807's. But I don't have any hard facts on this.
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