WTD Circuit for 10 W Mod

[Carl WA1KPD]

I am trying to build up a very small 10 watt modulator using tubes. I have looked at various circuits but can't seem to find what I need.

Specs I would like are:

10 watts of modulation for the plate
Preferably 2, but can accept 3 tubes
Prefer miniature tubes but can use one octal
Hi Z input
Have 400 V and plenty of current available.

Anybody have any thoughts of circuits?

Thanks and 73

Carl /KPD

[Todd, KA1KAQ]

Carl, have you tried any of the guitar amp/audiophool sites? I'm wondering about a pair of 12A** types, AX7, AU7, AT7, maybe P-P. Or perhaps a trio of 6U8, GH8, EA8s?

Should be something snaggable out there, not unlike driving a HB rig with the ol' PA or mono audio amp?

[w4bfs]

400V ...  seems made to order for 6bq5 pp .... can get up to 15 clean W with dese  guys
...a 6gh8 would make a pretty good gain stage / phase inverter ... there probably are other as good / better choices  ... judicious use of feedback to make clean...73..John

[W1AEX]

Hi Carl,

Along the lines of what Todd said, I steal a lot of audio ideas from here:

http://www.schematicheaven.com/index_HTML.htm

Lots of PA and guitar amp schematics using a variety of different tubes at various power levels. Should be something there that fits the bill.

73,

Rob W1AEX

[The Slab Bacon]

Carl,
       the fly in your ointment is the need to do it with only 2 or 3 tubes. That makes it kinda tough to get enough stages of amplification. You might want to look up a 6GW8 (IIRC). It is a triode / pentode audio amp tube. I believe the triode is of the high Mu variety and the pentode is an audio power amp. They were used in the old Lafayette HA460s and 410s. Also used in some of the last generation lower end audio stuff. A pair should make about 10w if you beat the snot out of them.

Use one of the triode sections an the speech amp, use the other ones triode section as the fuzz inverter and both pentode sections in shove yank as the outpoot. You may have to use a third tube (like the triode section of a 6AV6) for the mike amp, but that would do it with 3 tubes.

                                                       The Slab Bacon

[Carl WA1KPD]

Carl,
        You might want to look up a 6GW8 (IIRC). It is a triode / pentode audio amp tube. I believe the triode is of the high Mu variety and the pentode is an audio power amp.

                                                       The Slab Bacon

Frank,
Great idea- Take a look at this audio amp I found based on your suggestion of the tube

[N3DRB The Derb]

does the 47 pf feedback cap look a bit wimpy? I dont remember enough about feedback loops to remember that components exact function at petticoat junction. 

[Steve - WB3HUZ]

6AN8 or similar will give you a pentode section for lots of gain and a triode  section for a phase inverter. With high gain mic, carbon or crystal,  it might work. I'd say definitely with a carbon, but, of course it will sound like  crap.

[w3jn]

Pair of 6AQ5s for the output, or a 6360 dual tetrode would also be dandy.  The afore-mentioned 6AN8 pentode/triode should drive these well.  Remember you won't get much, if any, gain from the phase inverter stage.

Using the 6360 would halve your output tube count.  It was originally meant for VHF push-pull output stage but someone (Bill Kleneromous?) used it as the audio output in a HIHIFI R-390A audio mod.

[w4bfs]

400V is a little high for 6aq5 at full power .... they might not last too long but they are tough ....If you are space constrained and have extra B+ might consider some form of series modulation without extra iron ... If you are considering 10W of plate mod, that is ... this would fit in nicely with the 2 tube count ..(pass tube and gain stage/driver)   might need an additional fil xfmr ... interesting problem ...73  John

[Steve - WB3HUZ]

Three tubes.

1 - 12AX7 (one half is the mics amp, second is a voltage amp)

1- 6AN8 (pentode section is a voltage amp, triode section is phase-inverter)

1 6DZ7 or 6DY7 (These are essentially a pair of 6BQ5s in a single envelope. Running push-pull Class AB1, you could get 10-12 watts with just 300 volts on the plates. They will take 400 volts and produce close to 20 watts at max).

This set up will have enough gain for any reasonable mic. If you want only two tubes, skip the 12AX7 stages.