HW A-7 Amplifier to Speach amp

[ke7trp]

I got this complet POS HW A-7 Amplifer for free.  I would like to use it as a Speech amp.  The Both rigs already have a 8 ohm to Grids Transformer so I need 8 ohms out.  Currently I am using a small Solid state Amp, I really dont care for it and thought this might sound better and be more buzzardly


Here is the idea, This mono amp has two inputs.  I would like to use one for a D104 and the other for my audio rack.  I could then switch back and forth depending on mood. 

Can you please take a few moments to suggest circiut changes to the amp or is it good to go after a recap/cleaning and three wire cord?


Here is a picture of the amp so you get an idea of what it looks like. I will make a screen cover.

http://www.kta-hifi.net/projects/amp_page/a7amp/a7.htm

Here is the schematic.  I thought about a complete redo using one of the speech amps in my handbook..  Oh and I have it working on the variac now.

[W7TFO]

Good idea.

12A6's in PP are a good design.

73DG

[ke7trp]

This ones got some nice 12v6s in it.  I am cleaning it up now.  Will replace the caps.  When I have some idea as to what values to use. I have been looking over some speech amp designs.

C

[ke7trp]

Ok. New orange drops.  Checked the resistors and replaced as needed.  Tubes where good.  Three wire cord also.  She fired right up. Using it for the audio amp on the SP600 now for a test.  She sounds wonderfull.  My only concern is the transformer runs SUPER hot.  Can only hold my hand on it for about 1 second before you have to Draw back.  I tripple checked the voltages and resistance.  Can it be normal to run this hot??

Now to start thinking about conversion to speech amp

C

Good idea.

12A6's in PP are a good design.

73DG

[W7TFO]

12A6's are the right tube, a 12V6 gobbles up a lot more heater current.

Also check the cathode bypass cap if not replaced.  Can really raise the plate current if leaky.

73DG

[ke7trp]

150ma versus 225ma on the heaters.. Big deal?  I can get some 12a6s I guess.  I hooked up a D104 to the input and tested it PA mode.  MAN does it sound warm and rich.

C

[KA2DZT]

Power xfmr should not be running that hot.  Some will run hot after some length of time running, especially xfmrs with a copper magnetic shorting band.

I would recheck everything and I think 12V6s will draw more filament current than 12A6s and probably far more plate current.  I'll check the tube manual....BRB....I'm back.  12A6 draw .15amp fil, 12V6 draw .225amp fil.  Zero signal plate current 12A6 is 30ma and 12V6 is 45ma both with 250V plate.

So the 12V6s are a little heavy for the power xfmr.

Fred

[ke7trp]

Ok.  They are $2.80 each at Tubesandmore and they are down the street. I will order some in the morning.   

What I really need help with is the inputs. I dont have a manual and cant find one.  I would like to know what the impedance is.   I have found that the one that bypasses the first tube is low.  That much I know.  I would like to convert the first one to run a D104 (probably phono now).

The last thing I need help with is the DAP310 has a Balanced output that right now feeds a balanced input on my Solid state amp.  This thing has no balanced input.  I need help here.  I of course do have a balanced to unbalanced converter but hate to have to use that.



C

[KA2DZT]

From the schematic,  the input impedance of the second tube is 1meg  (look at the grid resistor/volume control).  The first tube (12SL7) is 27K (look at the grid resistor).  You need a much higher grid resistor (5-10meg) for a D-104.  Also, the second stage has some sort of tone circuit, probably for a phono.  You may have to remove some of those parts.

For a balanced input, you'll need a 600ohm to single grid input xfmr.  This is not that easy to come by and usually worth some big bucks if you can find one.  A UTC A-10 is one you can use but I've seen these going for a lot.

My HB xmtr uses a balanced mike input but I have a special UTC mike input xfmr that I pulled from an old RCA studio mixer.  They can be connected for 150ohm or 600ohm balanced input.  I've seen these xfmrs going for a few hundred bucks on ebay.

I think you don't need all the stages of amplification for the output from the DAP310.  I'm not familiar with the DAP but most of this audio stuff runs at line level (usually 1 volt at 600ohms).

I'm thinking that the balanced input xfmr can be connected to the second tube's input.  The first tube can be used for the D-104 and those two stages may be too much amplification but you do have the volume control on the third stage.

Fred

[ke7trp]

THanks.. I have it up and running on the bench.  Both Inputs now work.  The input 1 (higher gain stage) never worked as someone had a mistake when they built the unit.  I followed each lead down and found the mistake and rewired it.

Input 1 with a D104 sounds tinny and thin with no bass.  I removed the 27K resistor and placed my Eico resistor box there.  10 Meg sounds the best, However, Its still really thin with no bass.  Its not right.  I think its an impedance issue.

Input 2 with the D104 also works and sounds about the same, Thin.  Just has lower gain and requires ALOT of volume knob.

I then tested both inputs with my 600 ohm D104. This one has the k7Dyy Board in the bottom. This gives you 600ohm D104 so you can run the mic into an audio rack or modern radio.

With this mic, The sound is incredible.  Super smooth, clear tone with good low end.  Very hifi.  This mic works with both inputs 1 and 2 and has similiar sound on both. Just  input 1 has a TON of gain.

I do have a 500ohm Merit transoformer, variable ratio, to class B grids.  I also have a small balanced to unbalanced transformer for audio rack gear.

I will have to experiment more.  Its really sounding good with a low impedance input, Just complete crap with a D104.

The Dap310 is designed to drive a BC rig directly via the balanced 600 ohm input.

C

 

[Pete, WA2CWA]

I got this complet POS HW A-7 Amplifer for free.  I would like to use it as a Speech amp.  The Both rigs already have a 8 ohm to Grids Transformer so I need 8 ohms out.  Currently I am using a small Solid state Amp, I really dont care for it and thought this might sound better and be more buzzardly


Here is the idea, This mono amp has two inputs.  I would like to use one for a D104 and the other for my audio rack.  I could then switch back and forth depending on mood. 

Can you please take a few moments to suggest circiut changes to the amp or is it good to go after a recap/cleaning and three wire cord?


Here is a picture of the amp so you get an idea of what it looks like. I will make a screen cover.

http://www.kta-hifi.net/projects/amp_page/a7amp/a7.htm

Here is the schematic.  I thought about a complete redo using one of the speech amps in my handbook..  Oh and I have it working on the variac now.

You state A7 but you show a A7-E schematic. There was the A7, A7-A, A7-B, A7-C, A-7D, & A7-E. A7, A7-A, A7-B, A7-C used the same output transformer, part #51-13. D and E models used part #51-21. In the D manual, there was an addendum which indicated that 12V6's were being supplied instead of the 12A6's due to supply issues. In concert with that change, the cathode resistor was changed from 470 ohms to 330 ohms. I didn't compare all the various schematics so there may be other changes, although the A7-B schematic only shows 5 tubes.

[ke7trp]

Pete. This is just an A-7.  Mine has a 470 ohm 2 watt.  Its late, I am tired but I think there is a mistake on the 10 UF cap.  Someone has it from  negative to the chassis and the pos to the cathode.   The schematic shows negative to cathode and pos to the 470K.  In the morning, after coffee, I will sit down and trace this out. 

C

[KA2DZT]

Pete. This is just an A-7.  Mine has a 470 ohm 2 watt.  Its late, I am tired but I think there is a mistake on the 10 UF cap.  Someone has it from  negative to the chassis and the pos to the cathode.   The schematic shows negative to cathode and pos to the 470K.  In the morning, after coffee, I will sit down and trace this out. 

C

You're wrong, schematic is correct, positive to the cathode and negative to ground.

Fred

[ke7trp]

More mistakes. The 10 UF was in backwards. I put in a new 10 UF 50 volt the correct way.   

Last night what I was looking at was the 20 UF on the schematic.  That one shows a Negative symbol right on the schematic.  At least thats what it looks like.

C

[Pete, WA2CWA]

More mistakes. The 10 UF was in backwards. I put in a new 10 UF 50 volt the correct way.   

Last night what I was looking at was the 20 UF on the schematic.  That one shows a Negative symbol right on the schematic.  At least thats what it looks like.

C

That's not a minus symbol. The amp uses a 4 section electrolytic can. The symbol shows you which terminal on the can is 20 mfd. The same symbol should also appear on the can adjacent to the terminal. Look at the symbols in the schematic for the three electrolytic caps used in the power supply. Each one of them also has a symbol. They're all part of the same can.

[ke7trp]

I know about the symbols on the can. I did not realize that minus sign was indicating that.  The amp is playing nice now. I need to come up with a good high impedance input for the d104 now. It sounds great with an audii input. But horrible with tge d104

[AJ1G]

Mention of the A-7 brings back memories...I used one to dirve a pair of 807s for an outboard modulator I built and used with an Apache back in my WB2ZPS days in high school around 1969-1970.  As I recall, I used a mod transformer from a Navy TDQ VHF transmitter.  The TDQ  modulated the two halves of an 829B in parallel with the 807s.  Don't know if the impedance match was even close to what was needed for the Apache.  I stole the HV for the 807 plates from the Apache and had it wired in to modulate the 6146s  of the Apache in lieu of the built modulator and mod transformer.  Sounded OK, used to play Led Zeppelin II as "test audio" into the dummy load with it, and the mod waveforms lookd pretty good on the old Hickock scope I had at the time, including the Zepplin bass lines from Heartbreaker.

[ke7trp]

Neat story!.  I was not even alive.  LOL

I found the Bias resistor was measuring at 700 ohm.  I replaced it with a new 470.  Heat is much lower with all the resistor and cap changes. the one output tube is solid blue. Waiting on new tubes. 

Can someone help me understand what the circuit is from the plate of the first half of the 12sl7 to the grid of the second half is for?   I see other schematics online showing just a coupling cap there.  The 0035 cap was very leaky. I replaced it and now the sound is very different.   I removed it and get a BIG increase in gain.  I tried different values for fun.

Thanks

C

[KA2DZT]

If that input was for a phono then that circuit was probably a frequency-compensating network used to balance the way music, etc was recorded on records.  You can remove most all of it,  just leave the grid resistor (680K) and the (.05ufd) coupling cap,  of course leave the 100k plate load resistor.

Fred

[ke7trp]

How about the 470K in series with the .05?   

TFO came by and gave me a set of metal 12a6s and a super nice little 600ohm balanced input transformer.  I will clip lead that in for a test. This should match the DAP310 nicely.  Then the other input will be for those D104 nights.

C

[KA2DZT]

How about the 470K in series with the .05?   

TFO came by and gave me a set of metal 12a6s and a super nice little 600ohm balanced input transformer.  I will clip lead that in for a test. This should match the DAP310 nicely.  Then the other input will be for those D104 nights.

C

Remove the 470K, it only reduces the signal much the same way a volume control would.

FB on Dennis stopping by with the tubes and input xfmr.  He was suppose to stop by my QTH with those two BC rigs he has in the pick-up.

Fred

[ke7trp]

Fred.   I fixed another mistake made the builder and I sorted the 20 UF cap for the 12a6 tubes.  They run hot. Super hot. I guess thats normal. The transformer heat is down now and the sound is clear.  I gutted the entire Phono circiut.  I kept the 470K. There is WAY to much stage gain now.  Even if you flip to that input with no music, Its hot with static. I need to tame it down a bit.

I need to figure out how to match this input 1 to the D104. It sounds like complete crap.  I tested three D104s.  All highs and no mid or low. Sounds Like I am being called to the principals office.. LOL

Keep in mind that this same number 1 input sounds wonderfull with any low impedance mic hooked up.  I tested several. Its very smooth and clear.  It also sounds great with a line out form my ETON SW reciever. 

Currently, On the Grid of the 12ls7, I have a 1 meg to ground and a .05 in series with the mic.   Is this tube just not happy with the high impedance input of the D104?  Should I swap this tube out for another type and redo the front end?

C

[KA2DZT]

For input 1,  D-104 needs to look into the highest resistance possible.  You need at least 5-10meg grid resistor.  Do not use any cap on the input.  Connect the D-104 directly to the grid with the 5-10meg resistor.

I thought there would be too much gain.  You can add another gain control at the grid of the second stage.  It would be in place of the 470K and 680K resistors.  If you don't want to add another control , you can leave in the 470K and reduce the 680K grid resistor to 100-200K.  They form a voltage divider.

Fred

[ke7trp]

I put the 470K back in. I will lower the 680K when I have time.  For now, The amp is playing great.  Transformer is cool.  Tubes are hot. All voltages are with correct.  The INput 2 is hooked to the output of my Eton XM1 playing FM to a hallicrafters speaker. Input 1 is hooked to a D104 with a 10 meg resistor.  Still no low end. All highs. Not much of anything below 1k. Even with max bass and min treble, the D104 is thin.  I wonder if its a negative feedback issue? 

Tomorrow I will install the fuse holder, OFF/ON lamp and test into the 304Tls.


C

[W7TFO]

Hey CT,

Just where is that 10 meg resistor?  In series with the mic line, or from grid to ground?

To do it properly, you should've removed the original grid resistor and subbed it.

Just a hint...

73DG