Heathkit AA-151

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Heathkit AA-151

Postby ecir38 » Fri Jun 05, 2009 8:41 pm

I just bought this amp for my first venture into push pull.

The intergrated has already been modified to work as an amp only. First thing I did was plug it into some 89 dB speakers to hear how it sounded since this was a working amp when bought . Right off the back I wasn't crazy about the tone of this amp and noticed around 95 dB at listining level a little crackling in the highs. Next step was to dig into it to see what was going on in it.

First pic is the factory schematic with the pre and tone controls removed and what I found is in the second pic.

Stock
Image

Modified
Image

Quite a few mods have been made to the original circuit (opinions?). I noticed a couple of other things going on. Resistance values in parentheses are out of spec actuals. The two caps have ? marks because the values are unknown. I measured them with two differant meters and could not get any readings. All other caps and resistors are within specs.

Here is the 6AN8 tube data sheet.
http://frank.pocnet.net/sheets/127/6/6AN8A.pdf

The can cap in the PS looks to have been replaced and checked out good with an ESR meter but the first section reads 85uF off the rectifier and the second section 33uF. The 4.7k resistor reads 5.3k.

Stock PS
Image

If the modified circut is acceptable (big if since I haven't seen a pp schematic before that resemble the mods) my plan is to replace the can cap with a 50+50uF JJ cap I have, then replace the resistors that are out of spec and the two questionable caps. I would also like to change the bias to a 200 ohm resistor and 150uF cap for a pair of EL84's. Right now the quad shares one bias circuit. Remove the hum pots from the H+ too.

I would appreaciate some help to get the best out of what I have to work with here. Something is telling me that the EL84's should sound better than what I hearing with this amp so far.

Brad
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Re: Heathkit AA-151

Postby kheper » Fri Jun 05, 2009 10:16 pm

ecir38 wrote:The can cap in the PS looks to have been replaced and checked out good with an ESR meter but the first section reads 85uF off the rectifier and the second section 33uF. The 4.7k resistor reads 5.3k.


85uF is too high for the GZ234. The max capacitance in the datasheet is 60uF.

If the modified circut is acceptable (big if since I haven't seen a pp schematic before that resemble the mods) my plan is to replace the can cap with a 50+50uF JJ cap I have, then replace the resistors that are out of spec and the two questionable caps. I would also like to change the bias to a 200 ohm resistor and 150uF cap for a pair of EL84's. Right now the quad shares one bias circuit. Remove the hum pots from the H+ too.


A 50uF cap after the rectifier should be ok.
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Postby TerrySmith » Fri Jun 05, 2009 11:04 pm

The 6AN8 circuit is easily adaptable to a 9A based tube. I did a AA-151 with a 7247/12DW7 in place of the 6AN8 and a 6AV6 in place of the 6AU6. Overall gain of the amp is slightly lower but sound quality is much improved! Slight wiring changes are necessary.

The 6AN8 to a 7247 is easy. Pins 1,2,3 stay the same. Remove and discard the .1 cap and 680k resistor at pin 7. Move connections from pin 8 to pin 7, and move pin 9 to pin 8. Other 9A tubes could be used also.
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Postby WA4SWJ » Sat Jun 06, 2009 5:24 am

Hi Brad,

Nice little amp!

I would definitely replace all the resistors that have changed values. And I agree that the electrolytic on the rectifier output is too big.

See that little R-C network to ground on the grid of the phase inverter? It creates a pole in the circuit that improves circuit stability, but it also rolls off the high frequency response. I've done a lot of Spice simulations on circuits like that and that little network can significantly reduce high frequency response. That resistor has drifted down to a very large degree according to your measurements so those parts definitely need to be replaced with the correct values. I don't advocate removing them, but if you have a scope you could look at the output with a square wave input to see if the amp remains stable with your speaker loads. Then perhaps it could be removed - you'll just have to check it and see. I'd probably leave it in with the right component values.

My preference would be to reconnect the ultralinear taps to the screen grids on the EL84's. It will give you a bit more power. But some folks like triode mode so it's really your call. You could put a switch in there to switch back and forth from triode to ultralinear modes (when power is off!). I'd also put a 100 ohm resistor in series with the screen grids no matter which way you decide to connect it.

Not sure I would remove the hum pots because if they work they probably help some. I'd do the corrections first and then see how you like it. After that change the hum pots if you feel the need.

Separating the cathode resistors on the EL84's is OK too, but I'd watch the tube bias to make sure the output tube currents stay reasonably matched (same is true if you leave it as it is). Not much you can do with the circuit arrangement as it is to balance it. Check out the ST-35 cathode circuitry on this site to see how Shannon did it to enable tube balancing. All tubes will drift over time. Not a big problem like it is unless you just want to fiddle with it.

One last thing - the feedback resistor that has moved up to 100K would increase the gain of the amp by reducing the negative feedback. I'd put it back to the proper value or perhaps put a pot in there and look at the output on the scope (if you have one) just to see how it does. Again check out what Shannon did with the ST-35.

Hope this stuff helps.

Good luck!! Be careful and have fun!

Regards,
Ed Long
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Postby jonnyeye » Sat Jun 06, 2009 7:43 am

The input stage has been converted from pentode mode to triode mode. This reduces gain somewhat and probably (possibly? I don't have 6an8 trioded pentode curves) also lowers distortion. This would normally be great all by itself, except that it is DC coupled to the phase inverter. The phase inverter must be biased around 1/4 of the supply voltage for the greatest linear swing. In this configuration, this bias is the plate voltage of the pentode stage - here 52V, which is far too low (by about 35V). I would try first replacing the plate resistor with the correct 220k, but you may need to decrease it further (say 200k or 180k) in order to get the correct bias (80-90V) for the phase inverter. Be sure to check the pentode's cathode voltage afterward and adjust the resistor for about 1.5V.

I agree that the common cathode resistor should be replaced with separate ones for each channel. Personally, I don't like the idea of EL84s with 370V on them in triode mode - the screens are going to be pushed too hard. UL is different - the screens may swing higher than the B+, but only when they aren't doing any hard work. The hum pots could stay if they aren't damaged.

Finally, the feedback resistor was raised to reflect the decrease in gain when the input pentode was trioded. That said, feel free to play with it - you have a gain control...

There's probably more that I've missed.
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Postby ecir38 » Sat Jun 06, 2009 5:47 pm

Don't know how to reply to so may suggestions at once but here is what I gather to try first off.

It looks like I am safe leaving the front end in triode as long as I get the volts up on the 6AN8 pentode side then rebias it.

Replace the can cap with the 50 + 50 uF but now am thinking if 33uF isn't too small for the first cap off the rectifier I could use the existing can cap with the 33uF being used off the rectifier and use the 85uF section to feed the B+.

Reconnect it for UL since I felt it was running out of gas with the 89dB speakers.

Things to try at a later date.

Play with the RC network and probably wire the feedback and cathodes for the EL84's like Shannon has the ST-35.

Once all of that is done play with a 7247/12DW7 like Terry recommended.

Should I be concerned with the voltage on the plate of the triode side of the 6AN8?

Thanks guys, time to get to work!!!

Brad
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Postby ecir38 » Wed Jun 10, 2009 7:48 am

Did what I could do with the what I had in the bin. Replaced drifted resistors, installed 50 + 50 uF ps cap and tied in UL taps. The 89 dB speakers sound great in UL, can see the potential now.

When I removed the pre amp it introduced some hum that the hum pots didn't handle so I removed them with good results.

Need to order some parts to finish tweaking the 6AN8. Here is how it looks for now.

Image

Image

Thanks, I am learning a good bit by playing with this amp. I'll post some pics tonight, it is starting to look like an amp now.

Brad
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Postby ecir38 » Wed Jun 10, 2009 7:38 pm

A couple of pics with my new camera.

Image

Image

I kept intact what I could of everything removed in case it was ever decided to be used as intergrated again.
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Postby Gingertube » Wed Jun 10, 2009 10:17 pm

Good Job so far,

If it were mine (I've designed and built) around 8 EL84 Push Pull Amps I would make the following changes.

Change #1 - also was suggested above.
Remove that 100 Ohm 7Watt cathoe bias resistor and the 50uF bypass capacitor. separate all of the EL84 cathodes and on EACH EL84 fit 390 Ohm 5 Watt resistors with 100uF/50V bypass capacitors. For a bit more top end you can also add a 1uF poly film capacitor across the 100uF electrolytic bypass caps.

Change #2 - longevity.
370V is above what is really recommended for EL84. Thats not a big issue, lots of folk run them at that voltage or even higher BUT most add a little protection by adding resistors between the EL84 screen and the Ultralinear Output Transformer taps. This is usually a good idea. Add 270 Ohm 2W metal film resistors in the feed to each EL84 screen. In Ultralinear Mode these resistors can also help suppress any high frequency instability and the correct place for those resistors is right at the tube socket.

Cheers,
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Postby ecir38 » Thu Jun 11, 2009 12:46 pm

Thanks Ian, I'll take all the feedback I can get.

Change #1
I may do that or use the circuit Shannon uses for the st35. Either route I have a stock of 1uF poly film capacitors for the added bypass cap.

Change #2
Ed metioned adding resistors between the EL84 screen and the Ultralinear Output Transformer taps too but recommended 100 ohm. I guess either or would work. Thanks for letting me know to install them as close to the tube pin as possible.

Brad
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Postby TerrySmith » Thu Jun 11, 2009 2:55 pm

That looks really cool! Pictures are worth a thousand words, I thought you were building a stock unit. I like reusing an old chassis, no holes to drill/cut/punch.

If this is only going to be a power amp and you go with a ST35 type of circuit, you could use two 6AV6 or 6AT6's placed in the 6AU6 holes for the VA, and two 6C4's or one 12AU7/6CG7/6GU7 where the 6AN8's are located for the PI. That would fill up some holes and save the cost of 7247's.

6AV6=1/2 12AX7
6AT6=1/2 5751
6C4=1/2 12AU7

Is the chassis still sturdy without the reinforcement?
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Postby ecir38 » Thu Jun 11, 2009 6:45 pm

With the recommendations that you have given in this thread I can see now that this chassis would be great for breadboarding differant frontends for this output tube. Probably will keep it for experimenting in the future. As for right now it sounds pretty good and it still needs allot of work including the PS.

This will be my main amp for now but wouldn't mind picking up a couple more of these since they are so plentiful. She would make quite a nice intergrated amp if the effort was applied.

The chassis seems plenty strong although the top plate does move a little when pushing on the output tranny's. It is aluminum and .075" thick, height is 1 1/4". A piece of 3/4" x 3 1/2" hardwood painted black was mounted across the bottom front and rear which gives it a little more height and place for some rubber feet.

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Postby EWBrown » Sun Jun 14, 2009 3:33 pm

I started (but have not yet completed) a similar AA-151 re-design project a couple years ago, with an AA151 carcass from a flea market.

I stripped the chassis bare, replaces the four EL84 sockets, and GZ34 octal socket, and ended up rotating the output trannies 180 degrees so that the primary leads were closer to the EL84s. I used separate 400 ohm, 3W, 1% WW resistors that I had on hand, with 330 uF 35V bypass caps for each EL84.

For the power supply, I added two C-354 chokes, and made the filter arrangement a CLCLC for the EL84s' B+, and added a couple more RC sections for the VA and LTPI B+, so the PSU filter is basically a CLCLCRCRC arrangement. =:o

As I had 380VDC B+ at pin8 of the GZ34, I dexcided to add the chokes as a good way to "lose" the extra 20VDC, and lower the ripple at the same time.

FWIW, the 155VAC shown for the power trannie HV secondary leads is incorrect, I measured around 320VAC in my unit, with the EL84s and rectifier plugged in, and drawing around 140-150 mA total on the B+. IIRC, the trannie is rated 310-0-310VAC.

With today's somewhat higner AC line voltages, 320-325VAC would be about right.

My original intent for the VA/LTPI drivers was to use a couple of suitably modified Poseidon driver boards, but I have since come up with something else, at least on paper. So now it sits in the cellar in NC, at this point, waiting for me to get back on the project...

I re-inatalled the metal shield / partition on top of the chassis, for mechanical strength, (and it allows me to turn the amp uopside down to work on the underside of the chassis) and I used the original RCA input plate on the back for mounting the speaker binding posts - the RCA inputs and volume control will be mounted on the top / front of the chassis.

Then after that, I have two more fully complete AA151s to restore...

/ed B
Last edited by EWBrown on Mon Jun 15, 2009 1:22 pm, edited 2 times in total.
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Postby ecir38 » Mon Jun 15, 2009 1:08 pm

Haven't done much but do some listening and added a iec to earth the chassis. This thing don't sound bad at all as is.

Going to order some parts next for:

Upgrade PS as per Shannon's ST35 but use a 6H 200ma 150 ohm choke instead.
http://www.alliedelec.com/Search/Produc ... &MPN=C-14X

Purchase a variety of resistors to fine tune the 6AN8 first then some 12DW7 / 7247.

Install original RC network.

Add screen stopper's to the EL84's

Use Shannon's bias network for the EL84's.

Install a 100k trimmer on the feedback circuit.

Anyone know a part number or what they call the test port to check bias from top the chassis?

Brad
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Postby ecir38 » Mon Jun 15, 2009 5:01 pm

WA4SWJ wrote:See that little R-C network to ground on the grid of the phase inverter? It creates a pole in the circuit that improves circuit stability, but it also rolls off the high frequency response. I've done a lot of Spice simulations on circuits like that and that little network can significantly reduce high frequency response. That resistor has drifted down to a very large degree according to your measurements so those parts definitely need to be replaced with the correct values. I don't advocate removing them, but if you have a scope you could look at the output with a square wave input to see if the amp remains stable with your speaker loads. Then perhaps it could be removed - you'll just have to check it and see. I'd probably leave it in with the right component values.


Hey Ed, A friend measured the questionable caps for me since my meter didn't go that low. What I found was that the cap used in the RC network you described above was 220 pF. This in combo with the 2.7k resistor is the same RC network used in the Heathkit SA-2 which is basically the same circuit as the AA-151 so I think I am good with this one.
http://web.archive.org/web/200106220022 ... ematic.gif

Thanks, Brad
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