So I finally got around to putting in the correct 220pf caps based on the alternate "modern" schematic.
Final question for the community based on my observations. I'm trying to figure out where "flat" is with this. You can assume all parts are as in the schematic except for the previously mentioned bass pot being ~ 808K instead of 1M.
I had originally filed down the shafts at each's 15-20% mark to make them appear at 12 o'clock when half rotated (based on baxandall's being flat at around that range). Knowing that the bass pot is about 200K less than it should be, I'm starting to wonder if that really makes a difference.
Is the surrounding circuit just looking for a voltage divider of 900K / 100k (based on "flat" of 1M pot) or would I be still setting 10% of the 800K. In other words, is it 10% of the pots actual value or based on assuming they are each 1M? If it was the latter, I wouldn't be able to get 900K anyway on the bass even with it turned 100% down (which sounds too off).
Hope that makes sense. As it is now listening, it sounds most balanced with the bass at its 10% position (808,000 x .90 = 727,200K) which is about 9 on the clock face. The treble is set about the same (1,000,000 x .80 = 800,000) 20% and 1 on the clock.
Also to note, values are being measured from the left to center lugs on the pots.
Final question for the community based on my observations. I'm trying to figure out where "flat" is with this. You can assume all parts are as in the schematic except for the previously mentioned bass pot being ~ 808K instead of 1M.
I had originally filed down the shafts at each's 15-20% mark to make them appear at 12 o'clock when half rotated (based on baxandall's being flat at around that range). Knowing that the bass pot is about 200K less than it should be, I'm starting to wonder if that really makes a difference.
Is the surrounding circuit just looking for a voltage divider of 900K / 100k (based on "flat" of 1M pot) or would I be still setting 10% of the 800K. In other words, is it 10% of the pots actual value or based on assuming they are each 1M? If it was the latter, I wouldn't be able to get 900K anyway on the bass even with it turned 100% down (which sounds too off).
Hope that makes sense. As it is now listening, it sounds most balanced with the bass at its 10% position (808,000 x .90 = 727,200K) which is about 9 on the clock face. The treble is set about the same (1,000,000 x .80 = 800,000) 20% and 1 on the clock.
Also to note, values are being measured from the left to center lugs on the pots.
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+/- 20% tolerance is normal for commodity control pots. Passive tone control circuits were never known for accuracy. 'Flat' response could occur anywhere within about +/- 30 degrees from center of rotation. To find out where it is, you need a squarewave generator and scope, or at least a variable sinewave generator and AC voltmeter with flat response in the audio spectrum.
I have a mint condition AA-151 that I recap'd a few years ago. Running on 125 the EL84's run red plates. I use a 125 to 100 v step down transformer and it runs beautifully without making any modifications to it.
Yes the tone control positions are not flat at the 12 oclock position. I marked mine with a tiny piece of tape at the flat position.
Yes the tone control positions are not flat at the 12 oclock position. I marked mine with a tiny piece of tape at the flat position.
Thanks guys for the answers. Unfortunately I don't have access to a scope or square wave generator.
Are you able to speak to whether it's the ratio that counts or the resistance needed in the circuit?
Right now I'm wondering if my setting 10% of the 800K bass (and treble) pots is because that's just the percentage inward that I would need with baxandall, even if it were a 500K or 1M pot.
It makes me wonder how they came up with the 1M value in the first place. I'm sure there's more to it.
Also, Mark, are you still running all original P.E.C and 1M pots? If so, I'm curious to know where "flat" is for you
Are you able to speak to whether it's the ratio that counts or the resistance needed in the circuit?
Right now I'm wondering if my setting 10% of the 800K bass (and treble) pots is because that's just the percentage inward that I would need with baxandall, even if it were a 500K or 1M pot.
It makes me wonder how they came up with the 1M value in the first place. I'm sure there's more to it.
Also, Mark, are you still running all original P.E.C and 1M pots? If so, I'm curious to know where "flat" is for you
I've found SAM's Schematics tend to have many errors.
Did you ever resolve your 'flat' tone control issue?
x3workshop, I haven't really done much more with it. I believe my using an 800k pot instead of 1Meg might contribute to why the bass set at like 7-8 o'clock (way down) is where it sits best "flat".
This is why I had wondered above about initial values heath chose, and whether that 200k discrepancy had something to do with the bass not needing to be brought up more to be flatter.
This is why I had wondered above about initial values heath chose, and whether that 200k discrepancy had something to do with the bass not needing to be brought up more to be flatter.
I ended up rebuilding the pecs to the stock Heathkit schematic values and even then I had to back off the bass to a tad less than 11 O'clock. Otherwise it had too much bass.
Other than that, the little bugger sounded awesome.
Yes it's all original. The bass is in the 11:00 position and the treble is in the 1:00 position but I like the treble turned down a bit so I leave it at 12:00
and your right, it's one heck of a good sounding amp, I love it.