Bell 2200C conversion

[Ivabiggun]

Hi everyone. This is my first post here so I hope this is the right forum. I have recieved a Bell 2200C from a customer who wants it converted to the old Bassman 5B6 circuit. The are several areas I need to address, so I thought I would start with the PT first.

When I recieved it ithe amp was gutted except for the heater wiring on the tubes. I need some help figuring out the transformer wiring. The tube rectifier socket is still wired with two wires from the primary and two yellow wires going to the secondary. There are two black wires which must be the AC input and one other wire which may have been yellow and red at one time. The colors are faded so I am just guessing. Could I ground this wire along with one of the heater wires as in the 5B6 schematic?

The Bassman schematic shows a center tap on the recitifier and one heater wire to ground and the other to pin 7 on the first 5881. The 2200 had the heater wires to each side of the heater circuit with no center tap. What should I do here? Possibly make an artificial center tap, but with only one wire to the pilot light is that possible. I will deal with the OT on the next post. Thanks in advance for the help.

Iva

[Dave Curtis, dB AudioTech]

Originally Posted by Ivabiggun

Hi everyone. This is my first post here so I hope this is the right forum. I have recieved a Bell 2200C from a customer who wants it converted to the old Bassman 5B6 circuit. The are several areas I need to address, so I thought I would start with the PT first.

Welcome to the group, Iva.

Customer? Do I get a consulting fee?

I posted a similar reply to a PT question last week.

Originally Posted by Ivabiggun

When I recieved it ithe amp was gutted except for the heater wiring on the tubes. I need some help figuring out the transformer wiring. The tube rectifier socket is still wired with two wires from the primary and two yellow wires going to the secondary. There are two black wires which must be the AC input and one other wire which may have been yellow and red at one time. The colors are faded so I am just guessing. Could I ground this wire along with one of the heater wires as in the 5B6 schematic?

The wires on the rectifier socket are all from the PT secondary windings. The 2 yellow wires (on pins 8 & 2?) are likely the 5V rectifier heater supply for the 5U4G. The other two wires there (on pins 4 & 6?) are probably the High Voltage secondary winding. Get out your ohmmeter, set it to read low ohms, and start "ringing out" the transformer windings. The two black wires (primary winding) should show continuity only to each other. The same goes for the two yellows and the two greens. The other two wires on the rect should show continuity to each other. The other (R/Y) wire is likely the center tap of the HV winding and should read roughly half of the resistance of the whole winding. None of the wires should read any resistance to ground, unless the HV CT (R/Y) is grounded.

Originally Posted by Ivabiggun

The Bassman schematic shows a center tap on the recitifier and one heater wire to ground and the other to pin 7 on the first 5881. The 2200 had the heater wires to each side of the heater circuit with no center tap. What should I do here? Possibly make an artificial center tap, but with only one wire to the pilot light is that possible. I will deal with the OT on the next post. Thanks in advance for the help.

Run twisted pair heater string to the pilot light and all the tubes, and make an artificial center tap, if a heater winding CT isn't internally connected to the HV CT (which is possible). Grounding one side of the heater winding isn't recommended; you'll get more hum. You'll get the same result if the heater wires aren't twisted. Also, either keep them up away from other components or down flat against the chassis.

BTW, to keep your customer alive, don't wire the primary as it is on the Bassman schematic; it's unsafe. Use a 3-wire (grounding) cord, run the neutral directly to one leg of the tranny and the hot to the rear terminal of the fuseholder and then to the switch. Ground, of course, goes to the chassis near the entry point and it should be the longest of the the three. Don't use the "death cap".

[Ivabiggun]

Thank you for the excellent reply. Using your advice it was quite easy to figure out the PT wiring. I installed the center tap across the pilot light but I had to use a small terminal strip to attach everything. I have attached a pic of my work so far. Since the photo I have removed the .1uf cap from between V3 and V4. Fender just used P6/V4 as a tie point anyway for the OT center tap on its way to the B+ supply. I will use the last few turrets on the board to link the OT taps to the tube sockets.

I have found some discrepencies between the schematic and the layout diagram and I am wondering which one to follow. For example: the .1uf cap I just mentioned is not in the schematic but is shown on the layout. Please bear with me as I try to explain what I think is happening. The plates of V1-p2+p5 are tied together and connected through a dropping resistor to the last cap on the B+ line. They are also connected to the junction of two .1uf caps which feed the tone and volume pots respectively. The layout shows the first .1uf cap going to the wiper of the vol pot while the schematic shows it going to the input.

On the other cap the schematic shows it going to the input of the tone pot with the wiper to ground. The layout shows nothing on the input, the cap going to the wiper and the output tab to ground. Did Fender do this a lot in the old days?

Lastly, the PT appears to have two sets of windings on the primary. One is attachted to P3 on each power tube with the center tap. The other is on pin 8 of each tube with a center tap. On the ones to pin 8 I measured 43R between the two pins and an average of 22R on the CT. The ones on P3 measured 423R and 211R average on the CT. What are the taps on pins 8 used for?

I have attached two photos so you can see the progress. Thank you for help.

Iva

[Dave Curtis, dB AudioTech]

Originally Posted by Ivabiggun

Thank you for the excellent reply. Using your advice it was quite easy to figure out the PT wiring. I installed the center tap across the pilot light but I had to use a small terminal strip to attach everything. I have attached a pic of my work so far. Since the photo I have removed the .1uf cap from between V3 and V4. Fender just used P6/V4 as a tie point anyway for the OT center tap on its way to the B+ supply. I will use the last few turrets on the board to link the OT taps to the tube sockets.

I have found some discrepencies between the schematic and the layout diagram and I am wondering which one to follow. For example: the .1uf cap I just mentioned is not in the schematic but is shown on the layout. Please bear with me as I try to explain what I think is happening. The plates of V1-p2+p5 are tied together and connected through a dropping resistor to the last cap on the B+ line. They are also connected to the junction of two .1uf caps which feed the tone and volume pots respectively. The layout shows the first .1uf cap going to the wiper of the vol pot while the schematic shows it going to the input.

On the other cap the schematic shows it going to the input of the tone pot with the wiper to ground. The layout shows nothing on the input, the cap going to the wiper and the output tab to ground. Did Fender do this a lot in the old days?

Yeah, it looks like they made a mistake on the control panel layout. I've seen it before. There isn't really an "input" or an "output" on a pot; although the volume pot's wiper can be considered an output, the tone pot doesn't have one. It's fuction in this ckt is to bleed off (shunt) high frequencies to ground, so in effect, it's output is to ground. (It'll work wired either way). I'd disregard the Control Panel layout and wire the pots per the schematic. I don't know what's up with the .1 between the OP tubes; it looks like it doesn't belong.

Originally Posted by Ivabiggun

Lastly, the PT appears to have two sets of windings on the primary. One is attachted to P3 on each power tube with the center tap. The other is on pin 8 of each tube with a center tap. On the ones to pin 8 I measured 43R between the two pins and an average of 22R on the CT. The ones on P3 measured 423R and 211R average on the CT. What are the taps on pins 8 used for?

You mean the OP tranny. See the attached schem of the 2200B; it's a type of cathode bias for the OP tubes. Remove the wires from pins 8, find their CT wire and insulate them; you'll be biasing this with a cathode resistor/bypass cap and won't be using them.

You should change your AC power wiring; lose the cap on the power switch and run the hot AC feed through the back of the fuseholder, then through the switch, then to one side of the PT primary. The neutral goes directly to the other side of the PT primary. Ground wire is obvious. You are going to use a 3-wire cord, right?

[Ivabiggun]

Hi, did I say PT, I meant OP. I was just checking to see if you would catch that. Thank you for the schematic. I was not able to find one. I will use a 3 wire AC cord. I suppose I could not use it just to see if the guys' hair will smoke when he touches the chassis I have been in music for about fourty years now, but only in the last three or fours years have I been seriuosly studying electronics and guitar amps. I really enjoy doing this kind of work. I have learned quite a lot from nice folks on forums like this. Thank you for the assistance.

Robert

[Dave Curtis, dB AudioTech]

Hey, glad to help. Just a few more things; the CT of the PT should tie to ground at the same point where your filter cap's ground is attached. Where you have it probably won't be a problem, but it's best to have it go through as little chassis material as possible. It's long enough, I'd move it over. The artificial CT for the heaters is fine where it is. Use a different bolt for the AC power ground, and keep it the longest of the supply leads so that if the cord somehow gets yanked out, it will be the last to come undone. That way if the hot hits the chassis, the breaker/fuse will (hopefully) trip at the panel. Safer that way.

Are those PS caps non-polarized? They look it (crimps on both ends). I've never used NP caps in the PS. I guess they should be OK. The first caps are in parallel, so you could just use a single 33uF or a 47uF.

I'm sure you know this, but you want to get all those bits of metal out of there, or they'll cause problems later on. I find it best to do this as they're made. I've worked on a few Marshalls where the factory had left some tiny (like 3/8") pieces of bare wire floating around inside; one had shorted B+ to ground and made a small crater in the board. Just sayin'.

Good luck, and keep us posted.

[Ivabiggun]

Hi, those are polarized caps. I was wondering that at first also. I will clean out the chassis. I was in a hurry to take the photos at the time. I will post some more as I progress.

Iva

[zhyla]

Iva, this is the 5B6 bassman circuit, yes? I built one recently (well... still tweaking with it). I really like it.

[Ivabiggun]

Hi, yes it is the 5B6 circuit. Did you use any input grid resistors? The schematic doesn't show any. It does show one resistor connected to the plate junction which goes to the B+ supply but it does not give any value. 10K possibly? I was wondering if I should add some plate resistors, such as two 100K's across the junction of the two .1uf's.

Iva

[zhyla]

Here is my marked up version of that schematic:

http://zhyla.net:4000/files/dev/bass...m_modified.jpg

The value for R4 is missing on the original schematic but it's in the layout page as 250k.

The marked up schematic reflects what I have implemented so far except for C1 which I've been adjusting to modify the bass response of the preamp. I'm using it primarily as a guitar amp and I want to limit the bass response. But I'm probably going to revert that change and redo the tone stack as a bass rolloff.

[Ivabiggun]

I have been discussing this with another tech who is very well versed in this stuff. He reccomends 100K plate resistors across the tone caps ala 5F6 or JTM45 style. These I have installed, along with a 1M strap to groung resistor and two 68K's on the grids. The cathode Rk/Ck is 2.5k/250uf. Perhaps I could go down to 1.5k or even 820R. My client wants some gain, not a super clean amp I think. I did remove the two caps seen in the photo. The ones across the power switch and the one on the end of the board.

I am debating whether to go with the PI as it is on the schematic, or go with the true long-tail-pair as on the 5F6/JTM45 circuit. Could there be any problems going with the LTP as the PI tube is an octal 6SL7GT. Oops, I forgot you know that as you are already building one. DOH!!!

Right now I have the V1 and the tone caps and pots wired. I made a ground bus for all the wires which I think will help keep things neat. If I decide to install a fixed bias circuit I should tap of off the rectifier tube at pin 6, correct? From there I could use either the Marshall or Fender type bias circuit. Any thoughts here?

[Dave Curtis, dB AudioTech]

Since you mentioned ground buss I thought I'd steer you to an Aiken link on grounding:

http://www.aikenamps.com/StarGround.html

Scroll down for buss grounding.

Enjoy!

[zhyla]

Iva, how's this Bassman build going? I tried out a bass tone control and liked it a lot, definitely an improvement. I'm interested to see if you find bass control necessary or not.

[Ivabiggun]

Hi, thank you for the continued interest in this project. I have completed all the wiring except for the power cord and the PT primaries to the power tubes. I changed the center tap setup after reading some of Randall Aikens paper on grounding. Where before the heater wires went to a turret strip and then to the 1st 5881, I have changed to the more standard set-up of heater wires to the pilot lamp-to pins 2 & 7 on the first power tube. And also the two 100R resistors to ground for the artificial center tap.

The original schematic used only on side of the heater wiring to the filaments and the other to ground. I was advised to connect both wires to the tubes and make an artificial center tap. To answer your question; I have not tried a bass control as the client wants to keep expenses down as much as possible. So therefore I will stick with the stock set-up as much as possible without neglecting modern safety standards of course.

I took a few quick pics of the progress. Have a look and tell me what you think. Oh, the last two are of a Stromberg-Carlson hi-fi amp I bought off of Ebay for $50.00. That should be a fun one.

Ivabiggun

[zhyla]

Looks too clean... there's no way it will work . I look forward to your impressions once you fire it up.

The bass control I used was simple, 1 pot, 2 resistors, and a cap. Maybe you won't need it but it's funny how much bass comes out of bass amps!