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Author Topic: Speech Amp / Driver  (Read 23873 times)
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N2DTS
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« Reply #25 on: February 26, 2015, 10:57:17 PM »

I do not know, the ls 22 might do well reversed so its a step down.
Ideally you want something that matches the plate to plate load of the 6L6's and gives the needed voltage swing on the 811a grids.
The problem is the 811 grids are class B, and not a steady load.
To drive class B grids you want a very low plate resistance tube operating into a step down transformer so the load the grids hit on peaks do not effect the driver.
Tetrodes make poor drivers of class B triodes.

An 8 ohm solid state amp into an 8 ohm to 5000 ohm center tapped transformer works real well as the solid state amp makes MORE power if it works into a lower impedance load.

I am not sure how much distortion and peak power limiting you are willing to put up with, or how a pair of 6L6's as drivers for 811's will work. You do not have to sound like a broadcast station to sound good.

 
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W9ZSL
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« Reply #26 on: February 27, 2015, 03:38:45 PM »

I did manage to score the Stancor A-4416 driver iron.  Primary is 3K to 10K CT at 40 MA / side to P/P class B grids.  I'm going to pass on the LS-22 but am interested in the 6B4 tubes.
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N2DTS
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« Reply #27 on: February 27, 2015, 08:54:33 PM »

$20.00 each plus $5.00 for shipping?
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Opcom
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« Reply #28 on: February 27, 2015, 11:26:25 PM »

lots of discussion, very fun. 6B4s for cheapish prices, it is good. be sure to stock up some spare too.

1. The 6B4 (or 2A3, 6A3) can be driven by an RC coupled circuit and uses grid bias. The 6N7 is a zero bias dual section tube. For power amplifier applications the 6N7 will work in place of two 6B4s, up to the maximum power output allowed for the 6N7, which is 10W. The 6B4 pair can make 15W.

2. 6N7 in push pull is a zero bias class AB2 situation and the grids require a little current, so a transformer is needed to drive it instead of a RC coupled phase inverter. Due to this requirement the tube manual offers a simple class A Triode driver made of both sections of a 6N7. Alternately a small dual Triode could be cathode coupled to drive a 6N7, if the extra complexity is not minded.

3. That Altec circuit: You can use screen grid tubes for class B grid drivers well, but you need to shoot for a quite bigger amp that that 20W Altec, so that at its half-power level you can still swamp 50%+ of your drive in a resistance, which will improve the regulation. So a 4x size amp is a bare minimum*, if it is a regular screen grid tubes design like that Altec. 40-80W minimum to avoid distortion with that setup, based on 10W into the 811 drive xfmr, for generous losses. 40W cuts it close.

4.) All wattage ratings for screen grid tube amplifier apply to closely-specified nicely driven and polite little resistive loads, and are immediately un-useful for class B driver discussion. For discussion of class grid drive via push pull screen grid tube amplifiers, the output has to be considered in terms of:
a.) peak voltage and peak current at the crest of the audio half cycle.
b.) strict regulation of the drive voltage regardless of a current waveform that is not of a linear relationship but exponential
 b1.) with added weirdness.
c.) Solid state will do well but that's the chicken way out.)


I use a Stromberg-Carlson 150W PP-PAR 6550 public address amp to drive a pair of 3-500Zs with 25W, When the drive to the 3-500 grids is 25W average during speech, the amp is making only about 50W average for 100% modulation (incl. 25W to the swamping load resistor), and is not called upon for its full current except when the grid current and voltage together reach their peak at the crest of the half cycle. This reserve has the effect of driving from a very well regulated source. It is a brutish way to do it but very simple.*

No you do not need to sound like a broadcast station, but you must be free from amplitude distortion over the entire half-cycle to sound clean and clear and natural.
Choice:sound like space shuttle or sound like speaking in the room.
I prefer to use a switch to select the space shuttle sound (as desired) and not be limited to it by design. Especially if spending precious time building something.

The proof of this advice for those who use screen grid tubes to drive class B grids is what my transmitter sounds like. Grin Ask anyone who has heard my transmitter on 7160 what it sounds like or more specifically if it sounds clear and natural and easy to understand every word. Some of them are on this board.  Grin

I am way too smug in my audio, but worked a lot on it to learn and verify these truths, which are not new but are all implied in the Ancient Volumes.

*power reserve can be reduced to about 2x with a custom class B driver transformer, by using tubes with extreme current reserve and a special cathode feedback circuit to lower the output Z as shown here: http://www.montagar.com/~patj/aph1050.htm

Lastly, for a simple package modulator, the standard Altec 1570B circuit is an extremely simple, cheap, and well proven way to deal with a pair of 811's. It is only 175W, but in experiments I bypassed the filter choke for C input and obtained 255W by raising the plate volts to 1200. manual: http://www.dvq.com/hifi/images/1570a.pdf
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Radio Candelstein
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« Reply #29 on: February 28, 2015, 12:09:54 PM »

Yes, lots of clean audio to dump on the carrier is not so easy to come by.
Class B triodes seem easy, and they are with solid state amps and good iron, not so easy with all tube drive.

AB1 is great, like my 4x150 mod deck, just voltage needed to drive the grids, but you need bias adjustment for each tube, stiffly regulated screen voltage, and a variable plate supply to closely match the load to the modulator.
Can do 600 watts, blowers needed for the tubes.

Screen modulation can be great (no iron, phase shift or frequency response limitations) but it takes big tube blowing off a lot of heat to get big power output.

Why can we not have our cake and eat it too?

I have tried them all, and I think the AB1 modulator sounds better then the class B, and the screen modulators sound best with the right tubes.

Two or three 4-400's screen modulated would be ideal I think.
About 500 watts carrier.


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Steve - K4HX
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« Reply #30 on: February 28, 2015, 01:12:18 PM »

I guess it depends on what you mean by a big tube. A pair of 828s in AB1 will produce over 300 watts in AB1. No air required. These tubes were designed for AB1 audio service and feature low idling current (IIRC, 50 mA per tube) and low distortion. I had a GE BC TX that used a pair of these as modulators. I had never heard of 828s before I got that TX. They are quite unknown in ham circles. The down side is that they are not widely available. Too bad. The 828s are an excellent tube.
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W9ZSL
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« Reply #31 on: February 28, 2015, 05:09:03 PM »

I think the power problem could be satisfied if I could score a Stancor A-4763 driver iron.  It's 30 watts and would probably match anything.  The only solid-state amp I have is a Realistic STA-77 receiver and it's good for 16-22 watts per channel output.  I certainly could run it into a hi-fi plate to VC transformer which would probably work.  For now, I'll put the modulator deck together and experiment.  I'm not even going to mount the Kenyon line to grid transformer on the chassis...which is my alternate driver.  I'll just stick a 3-terminal barrier strip on back for the drive input and play with it.  Sigh.

Meanwhile Brett, what do you have for tubes that would work for drivers besides the 6B4's?  At this point 6N7's, and 6L6's are in the running.  Do you have any 811A's?  I have one and need another for certain...maybe a matched pair?

Oh,and as far as broadcast quality audio is concerned, I was a broadcaster for over 35 years and great audio habits are hard to break!  Then there is the recording studio thing.

Mike


* WOBT 1963.jpg (465.63 KB, 912x1664 - viewed 403 times.)

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* FCSS All.jpg (435.6 KB, 1536x2032 - viewed 387 times.)
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Steve - K4HX
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« Reply #32 on: February 28, 2015, 07:46:17 PM »

You're saying that this isn't your first rodeo?  Wink
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W9ZSL
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« Reply #33 on: February 28, 2015, 07:56:30 PM »

Nah. Smiley
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N2DTS
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« Reply #34 on: February 28, 2015, 10:31:21 PM »

I have some 6n7's, 6sn7,s (lots of those, new), lots of 6080's, and all sorts of hifi tubes, kt88, kt90's, 6ca7's, and all sorts of other tubes.

Audio amplifiers are dirt cheap, all you need is 20 or 30 watts and the right transformer, Hammond sells good ones.
A PA amp will even have a mic input/preamp, there are amps on ebay very cheap, kits, and pawn shops sell equipment as well as music stores.
I use a symetrix 20 watt per channel one rack unit high amp I got at a pawn shop cheap, its a bit light in power, but its small and its been working for the last 25 years or so.
A radio shack PA will even work, and likely be cleaner then anything you could build using tubes without a lot of effort and money.
Its fun to build, but I have never been lucky with building a really clean, quiet HIFI audio amplifier that takes a D104 input and gives 300 to 600 clean watts out using all tubes.
I did build a really great sounding 2 tube screen modulator, 12ax7 and a 6de7.
It modulates a pair of 4x150's at 100 watts carrier output and sounds fantastic.
That is the best all tube audio I ever built.

Much depends on if you are looking for the best sound, the easy fast way, or the very hard, complex all home brew all tube way.

In my book, almost everyone on the air sounds quite good.
Some ssb type rigs sound very nasty, some sound good, but most vintage, homebrew, sdr and class e rigs sound very good.
If you want really fantastic audio on the air, there are about 3 ways to do it, sdr, a modified low level modulator into an amplifier and class E.
A broadcast rig will do on 160 and 80 if you have a lot of space in the garage and own a fork lift.
 
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W9ZSL
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« Reply #35 on: March 02, 2015, 01:32:49 PM »

I checked out some PA amps and they certainly are going for cheap!  I take it a plate to voice coil transformer would be used in reverse as a driver?  I've actually done this before for screen modulation.  I know I have a 70 volt transformer lurking around somewhere, I assume using that 70 volt tap would work also?  As for the tubes Brett, I have to dig through my jonque boxe and see what I need.

For now I'm going to build the modulator deck first then decide how to drive the bugger.  Did I mention I will need at least 1, but preferably a matched pair of 811A's?
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N2DTS
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« Reply #36 on: March 02, 2015, 02:15:17 PM »

I never bothered about matched pairs in modulator service.
I am not sure that is important.
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W9ZSL
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« Reply #37 on: March 02, 2015, 03:38:26 PM »

I have 1 x 811A Brett so I need at least 1 more.  If I had a larger driver iron, the 6B4's might be desirable so I'm putting a hold on that.  I would like to score some 6N7's.  I'm actually building two modulators; the 200 watt and a 50 watt using a UTC S-20.  That will have either 6L6's or 5933's driven by a 6SN7 using the Stancor A-4416.
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N2DTS
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« Reply #38 on: March 02, 2015, 04:10:16 PM »

I use 811's and have some Russian ones, they work but the base and plate cap glue was no good.
I have some old US made 811's (not A's) and some 812's and 812A's and could actually use some good US 811a's myself.
6n7's I never used and maybe I can send you some if you are going to use them.

Hammond sells good transformers, 8 ohm to whatever, 5000 or 10,000 ohm center tapped, 15 or 20 watts would work very well for 811 grids, and if you wanted to, you can get a 20 watt IC chip to drive it, or any old solid state audio amplifier.

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W9ZSL
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« Reply #39 on: March 02, 2015, 05:40:51 PM »

Heck, am I dumb or what?  Sometimes the obvious doesn't register. If I don't run into problems with RFI, I could use my studio amp as a driver!  It's a big honkin' Yamaha home theater amp with DSP.  I'm running around 175 watts/channel stereo into the main speakers BUT it also has outputs for surround speakers at 75 watts a pop.  That pix is old and things have been updated including a pair of 15" MTX speakers. The ham shack is in the studio to the left front of the big mixer. That would rock!  After all, I've been wanting to integrate the two for years. 1,000 lashes with a wet spaghetti noodle.


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Opcom
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« Reply #40 on: March 02, 2015, 07:38:05 PM »

I take it a plate to voice coil transformer would be used in reverse as a driver? 
 I assume using that 70 volt tap would work also?

The reversed output transformer may work. W5FRS uses one rated 20W to drive 808 grids modulating an HK54. The difference is the 808 takes negative bias and less grid current than the 811 under similar conditions.

70V transformer, does it have a CT? What peak voltage is needed? It could work if the voltage is adequate.
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Radio Candelstein
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« Reply #41 on: March 02, 2015, 08:54:15 PM »

Opcom, forget the 70 volts.  I'm thinking driving the 811A's through an 8 Ohm iron might be the way to go since so many of you seem to favor the "old buzzard" approach!
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N2DTS
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« Reply #42 on: March 02, 2015, 09:10:55 PM »

I do not think I would run a 175 watt amp into a 20 watt transformer.
If you just want to get on the air with some good audio, the 8 ohm to 5000 ohm center tapped to the grids of 811a's is a good and easy way to go.
Run a vx2000 or some such cheap mic processor into a small audio amp and away you go.
The S22 transformer will be more of a limitation then the audio equipment.

If you like building, an all tube setup would be fun, but might not work as well.
It takes a LOT of parts and much time to build.
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W9ZSL
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« Reply #43 on: March 02, 2015, 09:24:23 PM »

Brett, of course not, but I could run one of the 75 watt outputs into the transformer at reduced volume.  The Yamaha has multiple outputs. The amp would be less than loafing.  It would be dozing!  I have more than enough parts to build anything but until I get my cataract surgery done, I'm literally shooting in the dark where construction is concerned!
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KA2DZT
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« Reply #44 on: March 02, 2015, 09:29:13 PM »

I used a single 6L6 with a 25 mod xfmr to drive 811As.  I used the primary for the 811 grids and the stepped-impedance secondary for the plate load of the 6L6.  I think (IIRC) the stepped secondary was 3K, 5K and 10K.  I used the 5K for the 6L6 and the 10K for feedback to the 12AU7 that drives the 6L6.  I used a lot of feedback so it takes about 5 volts RMS to drive the 12AU7.  My audio stages before the 12AU7 were able to supply that much audio.

Seems to work well.  I designed the driver using 100-200 ohm load on the driver xfmr in place of the 811 grids.  I think the ratio was a step-down to the 811 grids from the 6L6 plate.  Using a lot of feedback lowers the plate resistance of the 6L6 which what you need when driving class B grids.

Fred
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W9ZSL
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« Reply #45 on: March 02, 2015, 09:35:26 PM »

I was wondering about that concept Fred.  I also have a 50 watt modulator in the works using an NOS UTC S-20.  Could that be used as a driver?  Imagine a modulator driving a modulator.
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KA2DZT
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« Reply #46 on: March 02, 2015, 09:51:59 PM »

May not have a low enough secondary impedance to drive the 811 grids.  It may work, you would have try it using a step-down ratio going to the 811 grids.  A S-20 is a universal type mod xfmr.  It has four windings, each with a tap.  The taps are not in the center of the windings, they're offset from center.  Maybe using the taps from center (both secondary windings in series) to drive the 811 grids.  May work, you'll have to try it.

Fred
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W9ZSL
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« Reply #47 on: March 02, 2015, 11:06:51 PM »

That's what I was thinking.  Impedance on either side of the tied-together  terminals would not be the same on either side.  It was just an idea.  Frankly I've tracked down an NOS Stancor A-4763 30 watt driver iron cheap. I probably will pick that up and build around it.
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