Heathkit AA-161 EL84/6BQ5 PP mono integrated amp mod - diyAudio
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Old 8th April 2012, 05:28 PM   #1
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Default Heathkit AA-161 EL84/6BQ5 PP mono integrated amp mod

I have a pair of Heathkit AA-161 mono integrated amps using EL84/6BQ5/7189 in ultra-linear push-pull, with 6AN8 as driver, 6AU6 as line stage, and 6EU7 as phono stage, as illustrated in the schematic below. I have worked on other Heathkit models using 6BQ5 such as UA-2, AA-151, EA-2, EA-3, before and sometimes have reconfigured them radically using different driver circuits. But this time I want to keep it simple and closer to stock form. It's a work in progress. Nothing urgent and just want to have some fun with it. Currently one amp has more hum than the other, and it will need filter cap replacement, etc...

As many of you already know that with vintage gears is that modern household AC is typically 120vac or higher so when a vintage piece power transformer's primary was designed for typically between 105vac to 110vac so the extra voltage creates problems for amps that was already designed to push the tubes on the edge, maxing the total plate dissipation. I like to run my tubes conservatively and they sound more relaxed that way, too. I am not a big fan of ultra-linear at high voltage when the screen-grid is the same or higher than the plate voltage, leading to shortened tube life and tense sound when tube is pushed to the edge.

Long story short, I want suggestions on elegant ways to reduce the high voltage, either on the B+ or at the power transformer. Obviously I don't want to have to use a variac all the time when powering up these puppies.

Here is a stock schematic:
Click the image to open in full size.

Here is the modded schematic so far. Not using half of 6EU7 as part of the line stage, only using the 6AU6. Phono using both halves of 6EU7. Using "Tuner" and "Crystal Phono" as line inputs, deleting all the junk in the signal path before going to the volume control. Other than that, everything is still in stock form. I like the tone controls so I would like to keep them if quiet enough as they can add some flavor to the sound. (Yes, I like coloration and tone so no lecture on "neutrality," please.)

Click the image to open in full size.

When slam with 120vac the supposedly 390vdc B+ goes all the way to 430vdc! Not good.

Click the image to open in full size.
.
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Old 8th April 2012, 05:31 PM   #2
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Calculate a drop down resistor on the promary winding of the mains transformer, must be a high wattage type. This is propably the simplest way

Stef.
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Old 8th April 2012, 05:46 PM   #3
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One classic solution is called a "bucking" transformer, typically a filament transformer with its secondary in series with the Heathkit's primary. With the right polarity (check carefully!) the Heathkit will see 6 or 12 or whatever volts less.

This could be done safely in a separate box with a cord for the male end and a socket for the female end. 125 volts in, 115 volts out, more or less.

All good fortune,
Chris
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Old 8th April 2012, 06:09 PM   #4
dgta is offline dgta  United States
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Quote:
Originally Posted by directdriver View Post
Long story short, I want suggestions on elegant ways to reduce the high voltage, either on the B+ or at the power transformer. Obviously I don't want to have to use a variac all the time when powering up these puppies.
Long story short, the Variac is precisely the best solution and it's not at all obvious that you don't want to do that. Quite the opposite.

You set the Variac to the voltage of your choice and leave it there. You DON'T play with it every time. Simple, infinitely adjustable, doesn't waste power as heat (like a resistor would) cheap and you don't have to modify the amp.
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Old 8th April 2012, 06:31 PM   #5
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Quote:
Originally Posted by dgta View Post
Long story short, the Variac is precisely the best solution and it's not at all obvious that you don't want to do that. Quite the opposite.
The problem is that if I loan them to friends who don't have a variac or use them at a different location, I don't want to haul around a variac all the time. I only have one variac and I need that for other projects. Long story short, using a variac at ALL time is not practical, at least for me. Thanks for the advice.

Also thanks for other members' advices above.
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Old 8th April 2012, 07:12 PM   #6
dgta is offline dgta  United States
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If you don't want to modify the amp, see if you can find a rectifier with a larger drop. The EZ81 looks like only 15V drop at 100mA.

If it doesn't bother you to modify it, a drop resistor and extra cap is simplest.
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Old 8th April 2012, 08:27 PM   #7
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I'd do the bucking transformer too. It will lower the B+ and filament. On some of these older amps, now a days you'll find the filaments are running in excess of 7 volts. With the bucking transformer, the power transformer will run cooler, the bucking transformer will just get warm, and they are usually cheap, 12.6 volts at 1.2 amps at radio shack is $9.49. RS's 3 amp is $10.99. 12.6 volts will drop voltages inside 10 - 11%; 400 volt B+ is now 360, 7 volt filament is now 6.3. For less drop use a 6.3 volt transformer instead of a 12.6.
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Old 8th April 2012, 08:34 PM   #8
rmyauck is offline rmyauck  Canada
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You don't want to put a resistor in there of 21 ohms or more as you will raise the DCR of that nice PS hurting the sonics. A choke of 21 ohms would be better, and Edcor has affordable ones in there new series.

Tube DIY Asylum

Another solution is to convert to fixed bias using Dave Gillespie's EFB mod. You can set the bias to 25-27 mA for long tube life. It only cost $10 to implement or you could do it with the conventional small add on transformer for fairly low $ .

The 5th post down has a link to his paper.

diytube.com :: View topic - Improved SCA-35/ST-35 Performance



Biasing


I agree the bucking trans would be better than a big resistor too.

Add ICL's too the input of PS trans too.

Those output transformers are the same on the UA amps and the AA-151 right?

Randy

Last edited by rmyauck; 8th April 2012 at 08:41 PM.
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Old 8th April 2012, 10:34 PM   #9
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Quote:
Originally Posted by rmyauck View Post
I agree the bucking trans would be better than a big resistor too.
I think a bucking transformer is probably the best route. This way, I can use the extra winding for, perhaps, fixed bias supply if I ever decided to go that route.

Thanks!

Quote:
Add ICL's too the input of PS trans too.
What is "ICL"? Sorry, I am not good at abbreviations.

Quote:
Those output transformers are the same on the UA amps and the AA-151 right?
Yes, same output transformer (51-29), also the following amps:

mono amps
, power transformer (54-63)
UA-1
UA-2
EA-2
EA-3
AA-161

stereo amps, power transformer (54-93)
AA-111
AA-151
SA-2

All of these amps use similar circuit, ultra-linear, 6AN8 driver, etc...

.
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Old 8th April 2012, 11:12 PM   #10
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ICL is a input current limiter. Mostly we use GE's CL series. You can find them at mouser, look for CL-90. An input current limiter has a high resistance when cold and much lower resistance when hot, it gets hot when running. Like a CL-80 is 50 ohms when cold, or unit is off. When first turned on, you have 50 ohms series resistance on the incoming AC, in 30 seconds or so the resistance has come down to something like 3 ohms. So when hot a CL-80 will drop something like 3 volts AC. The filaments, and rectifying diodes won't get a high surge current through them.
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