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Baldwin Professional

Started by teemuk, October 18, 2007, 11:11:40 PM

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I've redrawed a schematic from a 1965/69 patent, which - judged by control arrangements - depicts model C1 Professional with "Supersound". I'm not sure how accurate the schematic is (for instance, I had to correct several obvious errors of the patent's schematic) and it's also missing the power supply section and transistor information. Anyway, I haven't seen any Baldwin schematics online so hopefully this one will help somebody. If someone can fill in the blanks the better.

Sorry for small size; I had to keep the attachment size small.

LJ King

Channel 1 is the second or lower preamp, the one with Supersound. Channel 2 is the first or upper preamp.

The decoupling resistors feeding all stages of the preamp from the +22V supply should be 3.3K, not 33K. 33K is correct for the tremolo oscillator.

The 33 microfarad cap feeding the channel 2 volume control should be .33 microfarad, same value as the post volume 330nf cap.

A production change to the 50 microfarad decoupling cap in the input stage of channel 1 was made in 1966, an increase to 100 microfarads to "reduce low frequency oscillation at certain settings of the controls".

Also as a production change at the same time, the 1.5K emitter resistors in the second transistors of the input stages of both channels was increased to 2.7K to "reduce the overall gain of the amplifier to value consistent with general requirements" (don't you just love engineer coverup talk?  :lmao:).

Along with this "decrease the gain" change, the 47 ohm resistor in the feedback/tone control circuit of channel 2 was changed to 100 ohm.

The Reverb section. The 27K bias resistor for the input stage should be 2.7K. The reverb control is a 50K pot and is wired backwards from this diagram - the pot's wiper connects to the output/mixer gain stage through an 18K resistor before the 1 microfarad cap. (Think about what would happen if the footswitch were used with full reverb - very muddy sound).

The dry signal from the input stage emitter connects to the output/mixer stage through series connected 120K resistor and a .082 microfarad isolation cap to avoid messing up bias - not a direct connection. The output cap of the mixer stage to the power amp is a 5 microfarad value, not a 1.

The output stage driver - the 33K decoupling cap feeding the voltage amp stage is 3.3K The Custom has triple totem pole output stacks, not dual. And the speaker impedance load is 2 ohms. The .47 ohm emitter resistors are fuse wire, not actual resistors.

Supersound. Just some minor things. The two chokes are .8h not 8h.

The lower choke (as it is drawn) is not connected to the right side of the other choke - it connects directly to the junction of the .68 cap and 27K resistor... similar to the 10K in the Mid II portion - not through the switch connections. With it switched as shown, there would be no bass portion feeding through when the Mixture switch is activated (Mix is a bass/treble boost).

Unfortunately, Baldwin didn't specify transistor information. All NPNs are the same with the exception of the reverb input/driver. All PNPs are the same. The only info on the schematics is that the types need to be selected in accordance with an internal procedure reference number that varies depending on their NPNorPNP-ness (S429-1 and S438-1), with yet another for the reverb driver (S429-2). I haven't seen any of these internal procedures.

The outputs (at least for the Custom) are designated on the schematic as "TRPV", with the V being a roman numeral. They are Germanium devices.

Also worthy of some note - of all these amps I've seen, maybe 12 or so - all are handwired point to point using perfboard for support.


Thanks, that even brought up few things that were not shown in the patent schema (especially concerning the reverb section). I'll post an updated schematic soon but at the moment I'm still tangling with getting that Supersound circuit right: The configuration you describe makes a lot more sense than the one copied from the patent (which is obviously incorrect) but some things are still unclear: For example, how are the switches wired? Obviously the last one "with mixing resistor" is preserved as is but how about the "dual" ones? Can you post a schematic showing that section?

Since you seem to have the original schematics can you verify the stage that has its gain modulated by the tremolo oscillator? - This one I had to modify quite extensively: In patent the 470K and 100K resistors were 470uF and 100uF capacitors instead (???) and the input transistor was collector-emitter bypassed with a 470nF capacitor (which is way too large). I replaced these with something that seemed more correct and behaved better in a SPICE simulation.

Concerning the preamp transistors, I believe all of them are silicon and rather low gain -type. Suitable models could be, for example, 2N5679 or similar. That is, however, merely a hunch. I agree with the gain modifications: All preamp stages I checked out with simulator seemed to have awful a lot of gain (thus the requirement for low-gain devices). I believe it wouldn't take much to overdrive them.

Never seen these amps live but they seem interesting and solidly built. Some amp porn can be found here:


Pictures show a massive heatsink for the output devices and the chassis is split to about three different modules consisting of preamp, power supply and the reverb. The latter is pretty interesting since it uses a "tapered" spring, which was Baldwin's solution for creating "full" reverb sounds without the need of multiple reverb springs. If you look at the model plate it has a reference to a corresponding patent.

I didn't know these were designed for a 2-ohm load. The patent showed a single 4-ohm speaker and only two pairs of output transistors. I believe this might be the configuration of the model D1 "Deluxe", which has a single 12" speaker and output power of only 30W. The pictures of 45W "Custom" model show two 4-ohm speakers and seven heatsinked TO3 devices.

How's the power supply? Is it just "conventional" RC filtering scheme or does it employ stuff like regulators?

LJ King

I must have skimmed past the tremolo modulation stage.

The 6.8K feedback resistor was changed to 3.3K as part of the gain modification.

The 100K and 470K resistors are actually capacitors - values are 100pf and 470pf. The emitter-collector cap is correct at 470pf. The oscillator and modulator driver stages are correct.

Backing up to the channel 1 tone controls, the 82K and 150nf should be 8.2K and .015 micro farad.

I remember the NPNs are silicon. I only replaced a few, and I'm thinking I used 2N3391 or 2N3391A - it's been a while.

The "massive heatsink" is simply sheet metal, aluminum IIRC approximately 1/8 inch thick - I'm guessing as I never measured it, maybe slightly thinner. The early versions it was flat and in later versions it got a right angle bend at the bottom of it, pointing towards the back of the amp and the TO-3's were mounted on the bottom horizontally with the transistor case facing downward.

Power supply is conventional - center tapped transformer, full wave rectification, dual +/- 43 volts at the first electrolytics. No regulation, just RC filtering to provide the other voltages.

There is one strange aspect though. The positive side has a separate leg of filtering that isn't directly used in the amplifier, separate from the filtering that provides the +32 volts (390 ohm/500 microfarad). After a 150 ohm 5 watt to a 500 microfarad electrolytic paralleled with a 3.3K 1 watt... it's split into TWO resistive voltage divider sections, looks to be a 700 ohm 5 watt (print is bad) that is grounded with a 2.2K 1 watt.

Here is the strange part - the voltage at the junction of the voltage dividers are simply designated "Y" and "Z". These sources are connected to what would be a "ring" connection on the "high" input jack of each channel with a .01 microfarad cap to ground. ("Y" to channel 2, "Z" to channel 1).

It could be a phantom supply, but I've never heard it advertised as such. I'm thinking it's purpose is to load down or reduce the +43 supply voltage (and the +32 which is derived) when the high gain inputs are in use... since it would be grounded when a normal T/S input plug is used.

That seems kind of contradictory... as that would seem to me to make the high gain distort earlier, but that's what the schematic shows.

Here is the Supersound portion of the schematic. The 68 micro input to the bass "format" is really a .68 micro - I missed that before I think.


Thanks, for the info. About that supply you mention – I did some research: It is indeed a phantom supply for some Baldwin guitar preamps. Another thing that came up was that some of the later models fitted a light bulb in series with the speaker load for some protection.

LJ King

Yes, I have seen the light bulb in series with the speaker in later models. Doesn't make much sense to me as the back is enclosed making a short highly unlikely... But we can always assume the action of a cosmic ray from a galaxy a zillion light years away as being the cause of a possible short!!

Interesting about the phantom supply, as the voltage available would seem to be quite high. Maybe the Baldwin/Burns guitar that had a preamp capable of using this voltage would also have some means of further filtering... and voltage reduction.

At any rate, hope this helps!


An update.

As a word of warning: I think the "Supersound" portion is still erroneously drawn - or the operation of that circuit is just extremely erratic. Anyway, there's nothing I can do about that since I don't have access to Baldwin Supersound that I could test or reverse engineer, sorry. This schematic now follows the schematic posted by LJ King.

Thanks for all the info.


I'm new here, but, have scans of the original Baldwin schematics and a lot of NOS Baldlwin Amp parts. My day job is in tech support for USA built Baldwin organ products. I've restored a few of these amps in the last 10 or so years. If someone wants to try building a similar amp, I have NOS power and Interstage transformers. Also, PNP Germanium TO-3 output transistors, all from old Baldwin stock.
If anyone needs it, I can email the larger scans.
Mr. Lynn


I have recently purchased a Baldwin Model C-1 Custom. The original speakers are 4 ohms. They need replaced, but I cannot find any 4 ohm 12" loudspeakers. Any suggestions?   


" I have recently purchased a Baldwin Model C-1 Custom. The original speakers are 4 ohms. They need replaced, but I cannot find any 4 ohm 12" loudspeakers. Any suggestions? "

The best thing to do is have them reconed. 4 Ohm accordion edge speakers are hard to find and the amp wants to see a 2 ohm load to get full power transfer.
Hope this helps,
Mr. Lynn



Mr Lynn,
i PM'd you. thanks



there is a slight hum with my Baldwin Professional amp so i thought that i'd try replacing the filter caps in the power supply. I used new high quality USA made separates rated at 200V since i couldn't find the old multicaps with the correct values. my problem is that it still hums, and after its been on awhile, the 39 ohm resistor burns up!!!! any ideas? someone please help me as i can't figure this out! (the original caps are rated at only 50V)

i re-hooked up the old caps, and installed a 39ohm resistor and the hum is still there but the resistor doesn't burn up!??? i can't for the life of me figure out why the new caps would cause the resistor to smoke as they are brand new and made by cornell dubilier and spragues. The new caps' values are: 2000uf@200V, 470uf@450V, and four 470uf@ 200V.

any help would be greatly appreciated.............


J M Fahey

Thanks for sharing such a beautiful piece of history. There's a very minor error in the schematic drawing: in the 6 output transistors, emitters and collectors are swapped, that´s to say, emitter "arrows" should be touching the R .47's , and pointing "upwards" towards the "-43". Mr Martínez pictures were also very good.



The bigger filter can is a weird one, as I now know [I have 2 of these amps to fix]- it is common POSITIVE, unlike 99% of can caps.  You installed the caps backwards here, and they leaked internally, burning the resistor.

Try it again with the caps positive grounded.  It has a PNP power amp after all, which requires a negative supply voltage........