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Rispoli 12th May 2006 06:50 PM

bypass tone controls on heathkit sp-2
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Looking for steps to take in removing tone controls from my preamp circuit. Just rebuilt a 1959 Heathkit SP-2 pre. I removed the loudness control and would like to bypass the tone controls, but not sure how to proceed. From schematic: signal comes from 1/2 of the 12AU7 to the base/treble control circuit and returns to a 1/2 of the 12AX7 on the output side. The manual states that this (1/2 of 12AX7 boosts the signal 20db to make up for loss in the tone circuit). Included schematic

cogsncogs 12th May 2006 09:03 PM

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Hi Rispoli

Welcome to diyAudio!

After a short glance the easiest way would be to bypass the tone amp (V2B-12AU7) at C21 .02uF ceramic disc, pin 7.

Remove coax from selector switch side of C21
Connect coax to pin 2 of V3A via a .047uF or so cap.
Place a 470k resistor from pin 2 of V3A (grid) to ground.

Now the icky part. The Record Out needs to be buffered because of the phono eq amp and the individual level pots on the aux inputs. That was one of the duties of V2B. And the tone control was outputted thru the record out. That 1/2 of 12AU7 can be reconfigured as a cathode follower (CF) as a buffer for the record out. Of course if you don't want or need a record out, then you can leave it out!
Note that after doing this your output will be shifted 180 deg. This is of no major concern tho as a lot of amps/preamps do this and the recording/mastering studio; you'll never really know!
There are more complicated ways of doing this but not necessarily better.
I'm posting a clearer schematic for all to see better!


Rispoli 13th May 2006 07:29 PM

cogsncogs: thanks for the welcome and thanks for the info. I am a bit confused by your mention of bypassing C-21. This is the record out part of the circuit. Would'nt I just connect the cap from C-22's hot side of the coax (from plate of 12AU7) and connect it to the coax hot coming out of the tone control and back to the grid on the 12AX7? Then add the resister from that grid to ground? (or am I beyond a "bit confused"). Thanks for any follow-up in advance.....Rispoli

cogsncogs 13th May 2006 09:43 PM

C22 is part of the record out. The tone amp (12AU7) also feeds the record out. Meaning if you turn the bass up, the level of low freq. will go up at the record out jacks. C21 comes from the selector sw. It's there (C21) mainly because of R26, R27, R28 which biases the 12AU7 and sets the gain. Bootstrapped. You can do as you've stated but you may have too much gain.

I'm going to draw up a few diagrams for you to help explain and give you some more alternatives.


cogsncogs 14th May 2006 01:44 AM

2 Attachment(s)
Ok, here is one I fudged up in micro-cap.
I made as few changes as I could!
R46 and R45 were added to drop the signal down as would occur with the insertion loss of the tone controls, giving about 21db of gain as seen at the grid of V2B to the main pre-amp output jacks.
If lower gain is needed, just drop in a 12AT7 in place of the 12AX7 and/or remove C24 cathode resistor bypass cap.

Gain at the record out jacks = 10db.

Changed some capacitor values to give better low end response.

R35 and R44 together comprise the Level control (R35). Was using that combo to determine overall gain; just forgot to change back to single resistor of 250k!

I have C30 (.02uF) going directly to grid of the last 12AX7 (CF) as I don't have a complete schematic after the R45 (270k) and before C30. Those little arrows point to sumthin' but what? :D

C23 (2500p) is a HF compensation cap to make up for high frequency loss caused by, you got it, the Miller capacitance of V2B in series with the 500k level trim pots and the high impedance tone controls with V3A. That'll put a damper on your highs fer sure.

I raised the value of C22 from .02uF to 1uf as most tape decks today have an input impedance of 50k -> 100k. Back in the 50's and 60's most tape decks (recorders) had an input imp. ranging from 1meg to 220k or so! Even transistor decks! Having a .02uF cap going into a 50k load, -3db would be around 100Hz!

Preamps back then weren't meant to drive loads much below 250k. So the tape out amp/buffer really isn't up to snuff with today's loads. A buffered amp would be more desirable. The 10 db of extra gain provided by V2B is to compensate for the loss in the individual input Level trim pots. And the tone control circuitry. But why output it to the tape out? Easy! It's cheaper and to compensate for poor recordings, different eq's (pre-emphasis) of some LP's and radio broadcasts.

I have to be honest, I'm not too impressed with this Heathkit. There are better Heathkit preamps but guess what... they are MONO units! ;)
If it were mine and in very good condition, I would restore it to near original condition; not so good condition I'd ripp every-thing out and start over. :angel:


I forgot to ask what are your input sources, turntable...
Outputted to; Toob-amp, SandyState Amp, Tape Deck...

Rispoli 17th May 2006 02:01 AM

Wayne, thanks for your feedback. I think I will give it a whirl. I currently have this hooked up to 2 B&K ST-140s in a horizontal bi-amp configuration. Rotel CD player and tuner/pre (used for the tuner. Digital Satelite takes up the 3rd line input. The speakers are Vandersteen C2-e's, which I have not grown tired of in 13 years.....I have replaced every cap and every resister in the heatkit. It had old mullards for tubes and I fired them up with them in place. This, I'm sure is not all of what tubes have to offer, but the sound is far better than the Rotel SS tuner/pre, which I paid $450 for in '93. (I have about $235 into the Heathkit) The thing that was most noticeable were the high's. The cymbals sound like cymbals, which was not formerly the case. Guitars have a much better 'air' as well. I am considering tube amps. Want to go inexpensive as I can and still drive the C2-E's with some authority. Any thoughts???? thanks, again....

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