Baldwin PC 40 Tube Amplifier Restoration/Rebuild

[Gtyler]

Hi All,

I recently came across these three amps, and decided to pick them up.

I am very new to tube audio and i don't know much, but I am hopping to learn quick, and be able to turn two of these into a set of mono blocks. Right now, I am not really sure how to do this so I am hopping that i could get some advice form the wealth of knowledge that is AK.

To begin with, two of the amps I have are the variety with the 12ax7 pre stage (I'm not even sure if it is a pre stage), and the other has the 6sl7. I have been told that the 6sl7 version is the better of the two, and I was wondering if it could be possible to convert one of the others so I could have two 6sl7 versions to work with. From the outside they look almost exact.

I am also curious about what I would use in line before these amps. Obviously they don't have any volume control, but i'm confused by the preamp tube (6sn7 or 12ax7), so can I use a regular preamp with them, or would this require that I remove the pre section in them?

Any suggestions or comments are welcome.

 

[Tinkerbelle]

As referenced in that previous thread, you can make changes to the 6SL7 or 12AX7, as well as other parts. Depending on your familiarity with these types of mods and the $ that you want to spend there are many options.

As a first step, you could try just getting these updated and fitted with I/O & power before doing major mods. That would give you an idea of the basic performance of the amps and would allow you to build on from there if you felt the need. The following pic gives an idea of a typical first step update, adding power connector, fuse, switch, indicator, input jack and speaker terminals. Several components underneath are updated (mainly caps): http://www.acousticpsychos.com/coppermine/displayimage.php?pid=379&fullsize=1

Depending on exactly which type amps you have, you might find that these amps already have a gain pot on that first input section to the 12AX7 or 6SL7. If so, you have some manual gain control, individual to each monoblock. You could try different preamps with these; others might comment on their experiences with any that they have tried.

If you have a Blueray, DVD player or CD player with variable output control, then you should be able to run that output directly into the amps and use the variable output control as a volume control (setting the gain control on the amp to a suitable level). Similarly, an MP3 player etc. should also work. For tone controls or phono input, etc. you will need appropriate preamp sections.

For more adventurous changes you could take a look at Kegger's suggested circuit or at Battradio's, or jasondace2b's, KT88 version.

 

[Gtyler]

As referenced in that previous thread, you can make changes to the 6SL7 or 12AX7, as well as other parts. Depending on your familiarity with these types of mods and the $ that you want to spend there are many options.

As a first step, you could try just getting these updated and fitted with I/O & power before doing major mods. That would give you an idea of the basic performance of the amps and would allow you to build on from there if you felt the need. The following pic gives an idea of a typical first step update, adding power connector, fuse, switch, indicator, input jack and speaker terminals. Several components underneath are updated (mainly caps): http://www.acousticpsychos.com/coppermine/displayimage.php?pid=379&fullsize=1

Depending on exactly which type amps you have, you might find that these amps already have a gain pot on that first input section to the 12AX7 or 6SL7. If so, you have some manual gain control, individual to each monoblock. You could try different preamps with these; others might comment on their experiences with any that they have tried.

If you have a Blueray, DVD player or CD player with variable output control, then you should be able to run that output directly into the amps and use the variable output control as a volume control (setting the gain control on the amp to a suitable level). Similarly, an MP3 player etc. should also work. For tone controls or phono input, etc. you will need appropriate preamp sections.

For more adventurous changes you could take a look at Kegger's suggested circuit or at Battradio's, or jasondace2b's, KT88 version.

Yes, I like the idea of first just bringing these up to operational condition and giving them a go. I was hoping to be able to use my Yamaha C-4 to drive them. It of course has all the needed phono stages and volume controls etc. Do you think this would work? Also, you mentioned that some minor changes need to be made to the original circuit (capacitors) to bring it up to date. What capacitors need to be changed and what are the values they should be changed to? Thanks for the help.

 

[battradio]

Hi Tyler ,
You need to replace all the filter caps , and the two .1uf coupling caps to the 6L6s , the 390 ohm and the3.9K ohm resistors in the feedback circuit should be changed to metal film 1% resistors to insure equal gain of both amps , if you don't need the level controls replace them with 47K ohm resistors .The hole that the level control was in makes a good spot for the RCA input connector . The first filter caps needs to be 5uf at 600 wvdc , the second cap should be 50 uf or larger at 600 volts , i used two 470 uf 350 volt caps in series to give a 235 uf cap at 700 volts , the third cap should be 50 uf or larger at 500 wvdc , use a 100 to 250 uf cap at 50 wvdc across the cathode resistors of the 6L6's .

 

[Gtyler]

Hi Tyler ,
You need to replace all the filter caps , and the two .1uf coupling caps to the 6L6s , the 390 ohm and the3.9K ohm resistors in the feedback circuit should be changed to metal film 1% resistors to insure equal gain of both amps , if you don't need the level controls replace them with 47K ohm resistors .The hole that the level control was in makes a good spot for the RCA input connector . The first filter caps needs to be 5uf at 600 wvdc , the second cap should be 50 uf or larger at 600 volts , i used two 470 uf 350 volt caps in series to give a 235 uf cap at 700 volts , the third cap should be 50 uf or larger at 500 wvdc , use a 100 to 250 uf cap at 50 wvdc across the cathode resistors of the 6L6's .

I can do that! I guess I better order some parts. Would you recommend metal film for the rest of the resisters too? I imagine that i should just go ahead and replace everything including wiring. There are not that many components and it'll make sure that there are not any resistor that have drifted a lot, or shorted capacitors. Does anyone have a favourite source for components?

 

[Tinkerbelle]

... i used two 470 uf 350 volt caps in series to give a 235 uf cap at 700 volts...

Hi Mark; I'm assuming that you used balancing resistors across those two 470uF caps to help ensure a reasonably even voltage split across them.

I can do that! I guess I better order some parts. Would you recommend metal film for the rest of the resisters too? I imagine that i should just go ahead and replace everything including wiring. There are not that many components and it'll make sure that there are not any resistor that have drifted a lot, or shorted capacitors. Does anyone have a favourite source for components?

There are many good resources: Mouser, Digikey, AES (tubesandmore.com), justradios.com and others.

 

[battradio]

I can do that! I guess I better order some parts. Would you recommend metal film for the rest of the resisters too? I imagine that i should just go ahead and replace everything including wiring. There are not that many components and it'll make sure that there are not any resistor that have drifted a lot, or shorted capacitors. Does anyone have a favourite source for components?

They used quality parts for the most part , just ohm the resistors anthing off more than 10 percent replace , it wouldn't hurt to put 1 watt 220K ohm resistors across the 470uf series caps as Tinkerbelle mentioned .The wire in mine was fine just look it over good , if it been wet you might want to replace it .
http://angela.com/
http://www.tubesandmore.com/
http://www.mouser.com/search/default.aspx

 

[Gtyler]

They used quality parts for the most part , just ohm the resistors anthing off more than 10 percent replace , it wouldn't hurt to put 1 watt 220K ohm resistors across the 470uf series caps as Tinkerbelle mentioned .The wire in mine was fine just look it over good , if it been wet you might want to replace it .
http://angela.com/
http://www.tubesandmore.com/
http://www.mouser.com/search/default.aspx

Sounds good. I think that my units have seen some harsh conditions for electronics, so if in doubt I think I'll ere of the side of replacement. I'm very excited about these and looking forward to working on them, but I am also in second year engineering and coming up on the first set of midterms, so don't lose faith in me if I am slow to report progress. Thanks again for the help.

 

[jasondace2b]

I can do that! I guess I better order some parts. Would you recommend metal film for the rest of the resisters too? I imagine that i should just go ahead and replace everything including wiring. There are not that many components and it'll make sure that there are not any resistor that have drifted a lot, or shorted capacitors. Does anyone have a favourite source for components?

evil bay is one of my favorite sources for caps. try to use all polypropylene capacitors if you can. oil caps will work for coupling caps. dont use all metal film resistors use a combination of metal carbon film and carbon comp. so as to not give it a specific signature in the sound. you'll need a sheet metal drill bit if your gonna change your 12ax7's over to 6sl7s.

 

[928GTS]

I think the preamp would be fine left alone at least for gain reasons. I've run MP3 players directly into 6SN7 preamps and it has worked just fine. As mentioned before you'll need to build a preamp if you're going to be using sources without a volume control or need the EQ curve for a turntable. Those amps look lovely and seem to have nice iron.

I'd replace the components, install a grounded cord, bring it up slowly on a variac and see what is what.

 

[Gtyler]

I'd replace the components, install a grounded cord, bring it up slowly on a variac and see what is what.

Just to be sure, I just need to attach the ground cord to the chassis right? Also, I don't have an variac. Is it worth getting one for this, or will it probably be ok without it? If not, are there other ways to accomplish the same thing without the variac?

 

[Gtyler]

I have just noticed something that is concerning. As I said before, I would like to convert one of the 12AX7 versions to a 6SL7 version, however, I just noticed that the transformers on these are slightly different. The transformers on the 6SL7 versions have numbers like this: PT: 512-14512E, Choke: 512-6F, and OPT: 512-14633D. The Transformers on the 12AX7 versions have numbers like this: PT: 512-14512F, Choke: 512-13926D, and OPT: 512-14633E. What do you think? Is it better to just stick with the 12AX7 version, or is it still ok to do the change?

 

[battradio]

I would take both the 12AX7 amps and use them.The outputs and chokes look different between the two versions of the amp, get them working and leave the 12AX7 to 6SL7 mod till last on the two of them .
PM me anytime will give you a helping hand

 

[928GTS]

Just to be sure, I just need to attach the ground cord to the chassis right? Also, I don't have an variac. Is it worth getting one for this, or will it probably be ok without it? If not, are there other ways to accomplish the same thing without the variac?

Yes, the ground is attached to the chassis. A variac is very important when working with vintage electronics as it will slowly bring the unit and the components inside it up from their long slumber. This is a much more sympathetic treatment and will allow you to at least evaluate the amp and its function for a short while before you start restoring it.

 

[Gtyler]

I finally had a chance to crack these babies open. Here is a pic of the insides. The one on the bottom is the 6SL7 version, and top is the 12AX7. I'm surprised at the differences between the two. I though the differences would only be in the pre section, but there are some other differences.

 

[Gtyler]

Pretty Transformer

My Capacitors are officially on the way, and I can't wait for them to arrive.

In the meantime i had a bit of time to beautify one of the transformers that desperately are in need of attention. Here is the result.

Amazing what a little paint and some rust removal can do eh? Take a look at the first post to see what they used to look like.

One down, 5 to go.

 

[battradio]

Looking good ,the they used higher wattage droping resistors in the 6SL7 versions of the amps .

Mark

 

[Gtyler]

Looking good ,the they used higher wattage droping resistors in the 6SL7 versions of the amps .

Mark

Actually I think it must actually be the time that the amp was made the dictates the wattage of the dropping resistors, because I believe that yours are the 6SL7 version and they look the same as my 12AX7 version with the large dropping resistors. My 6SL7 version has much smaller resistors, and in fact it doesn't even have the two large dropping resistors that are mounted to the side of the chassis.

 

[Gtyler]

Mark, do you still have yours? If you do and if they are the 6SL7 version, I would be very interested to know what the numbers on the transformers are. Would you mind taking a look for me?

 

[Tinkerbelle]

I don't have mine anymore but the PTs were 512-14512-D (-E on another one) and the OPTs were 512-14633D