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Vintage threads from the first ten years

Re: It works!! (now some questions)

1/7/1999 7:32 PM
MJ Harnish	Re: It works!! (now some questions) You are correct. I must have had something screwed up (probably grounding out) since when I reversed the wires the treble pot worked correctly though it was backwards; a switch of the wires back to their original position has things working correctly. So much for double checking everything... thanks. BTW - the amp is working very well now; I've got most of the minor problems worked out (thanks in part to everyone's help) and now I just have to tweak it some.

1/7/1999 7:32 PM

MJ Harnish

Re: It works!! (now some questions)

You are correct. I must have had something screwed up (probably grounding out) since when I reversed the wires the treble pot worked correctly though it was backwards; a switch of the wires back to their original position has things working correctly. So much for double checking everything...

thanks. BTW - the amp is working very well now; I've got most of the minor problems worked out (thanks in part to everyone's help) and now I just have to tweak it some.

1/9/1999 1:42 AM
Steve A.	MJ: Congrats! I was hoping to get my own SF Bassman to Tweed Bassman conversion done by the end of the year but... [insert excuse here] ("I didn't inhale"; "the dog ate my homework"; "my alarm clock didn't go off") Just a quick tip- when looking at an old Fender schematic, it is very helpful to have the layout drawing as well since it includes the taper and lead orientation for all of the pots. (I always seem to get the BASS pot backwards! [g]) Steve Ahola

1/9/1999 1:42 AM

Steve A.

MJ:

Congrats! I was hoping to get my own SF Bassman to Tweed Bassman conversion done by the end of the year but... [insert excuse here] ("I didn't inhale"; "the dog ate my homework"; "my alarm clock didn't go off")

Just a quick tip- when looking at an old Fender schematic, it is very helpful to have the layout drawing as well since it includes the taper and lead orientation for all of the pots. (I always seem to get the BASS pot backwards! [g])

Steve Ahola

1/9/1999 10:28 AM
MJ Harnish	Thanks for the encouragement. The amp works perfectly although I'm still working out some problems with excessive noise (hiss and hum). The noise isn't extremely bad but is very noticeable if I turn the amp up to 6 or 7. It's time to start fooling with the lead dress some, but overall I'm rather happy since I ran it for an hour yesterday with no real problems and the amp is completely useable as is. Right now my B+ is about 40v too high compared to the 5F6A schematic's voltages. I imagine you're going to have a similar problem if you're using the SF's PT. I've thought about using a zener to reduce the voltages some but right now the amp sounds good and so I'll try it as is for a while. I'm running a pair of EL34's (with the necessary mods) right now and they sound good. I have a pair of small bottle RCA 6L6's to try also. My next project may be to see if I can add reverb to the amp though I'm not sure if I want to go that route since it would involve an extra filament tranformer and quite a bit of reconfiguring, not to mention I'd need a new box since I don't think there's enough clearance inside the current one (the amp was once a SF Bandmaster) to accomodate a reverb pan. MJ

1/9/1999 10:28 AM

MJ Harnish

Thanks for the encouragement.

The amp works perfectly although I'm still working out some problems with excessive noise (hiss and hum). The noise isn't extremely bad but is very noticeable if I turn the amp up to 6 or 7. It's time to start fooling with the lead dress some, but overall I'm rather happy since I ran it for an hour yesterday with no real problems and the amp is completely useable as is.

Right now my B+ is about 40v too high compared to the 5F6A schematic's voltages. I imagine you're going to have a similar problem if you're using the SF's PT. I've thought about using a zener to reduce the voltages some but right now the amp sounds good and so I'll try it as is for a while.

I'm running a pair of EL34's (with the necessary mods) right now and they sound good. I have a pair of small bottle RCA 6L6's to try also.

My next project may be to see if I can add reverb to the amp though I'm not sure if I want to go that route since it would involve an extra filament tranformer and quite a bit of reconfiguring, not to mention I'd need a new box since I don't think there's enough clearance inside the current one (the amp was once a SF Bandmaster) to accomodate a reverb pan.

MJ

1/10/1999 1:51 AM
Steve A.	MJ Right now my B+ is about 40v too high compared to the 5F6A schematic's voltages. I punched a hole for a tube rectifier which should bring the B+ down a bit. But Andy Ruhl mentioned a trick that Carvin has been using that I may incorporate into the design: a "tweed/full power" switch that reduces the B+ just to the driver/PI (AKA "100w/60w" switch on some models). I haven't seen a Carvin schematic but I believe that you would branch off the existing power supply with something like a 10k power resistor and a 10-20uF/400+v filter cap for decoupling. Maybe someone out there can offer some ideas on how to wire the switch up... :hiss and hum at volume levels over 6 or 7 Since your amp doesn't have a mv (right?) it's good news that you only have a noise problem at the higher volume settings (since it means that the noise is coming from the first stage). Have you tried other tubes? Shielded cable from the jack to the grid (with a 33k resistor at the tube pin)? Input jack isolated from the chassis (run the jack ground to the common ground point for the first stage)? :extra tube socket? I have a leftover tube socket which I'm reserving for a tube buffered fx loop from Kevin O'Connor's TUT (complete with a series/parallel switch, parallel blend pot and level controls for the send and return signals- also a guitar level/line level switch so that you can even use stomp boxes. Since everybody seems to have an fx processor these days, I think it makes more sense to add an fx loop like this (for reverb, delay, chorus, eq or whatever) instead of just a tube driven reverb circuit (which seems like a lot more work!) Good luck! Steve Ahola

1/10/1999 1:51 AM

Steve A.

MJ

Right now my B+ is about 40v too high compared to the
5F6A schematic's voltages.

    I punched a hole for a tube rectifier which should bring the B+ down a bit. But Andy Ruhl mentioned a trick that Carvin has been using that I may incorporate into the design: a "tweed/full power" switch that reduces the B+ just to the driver/PI (AKA "100w/60w" switch on some models). I haven't seen a Carvin schematic but I believe that you would branch off the existing power supply with something like a 10k power resistor and a 10-20uF/400+v filter cap for decoupling. Maybe someone out there can offer some ideas on how to wire the switch up...

:hiss and hum at volume levels over 6 or 7

    Since your amp doesn't have a mv (right?) it's good news that you only have a noise problem at the higher volume settings (since it means that the noise is coming from the first stage). Have you tried other tubes? Shielded cable from the jack to the grid (with a 33k resistor at the tube pin)? Input jack isolated from the chassis (run the jack ground to the common ground point for the first stage)?

:extra tube socket?

    I have a leftover tube socket which I'm reserving for a tube buffered fx loop from Kevin O'Connor's TUT (complete with a series/parallel switch, parallel blend pot and level controls for the send and return signals- also a guitar level/line level switch so that you can even use stomp boxes. Since everybody seems to have an fx processor these days, I think it makes more sense to add an fx loop like this (for reverb, delay, chorus, eq or whatever) instead of just a tube driven reverb circuit (which seems like a lot more work!)

Good luck!

Steve Ahola

1/10/1999 9:59 AM
MJ Harnish	Since your amp doesn't have a mv (right?) it's good news that you only have a noise problem at the higher volume settings (since it means that the noise is coming from the first stage). Have you tried other tubes? Shielded cable from the jack to the grid (with a 33k resistor at the tube pin)? Input jack isolated from the chassis (run the jack ground to the common ground point for the first stage)? No MV. Yes, it's definitely the 1st stage now. The hum is virtually gone (barely audible at 10) if I pull the first tube and is TOTALLY gone if I pull the 2nd as well. I have to fool with the 1st stage today. I am using shielded cable. I'm going to try disconnecting the input cable 1st to see if I can isolate if it's the input jacks or the something beyond. No, the input jacks are not isolated but they may need to be if that's the problem. :extra tube socket? 5 extra sockets! Yes, you read correctly. 2 extra power tube sockets (one is now taken up by the rectifier), and 3 extra holes punched for preamp sockets. The guy who owned the amp before me butchered it in an attempt to turn a nice SF (a '68 which cosmetically was in great condition) into a Marshall. It has an additional 6 holes drilled for controls including one right through the word "Band" on the faceplate. That's ok though since I needed that hole for the presence control and I've put many of the other holes to good use for an impedence selector, etc. At the moment the controls for the "normal" channel are unused since I just used the bright swtich to allow me to switch in or out the "jumpered" channels trick; much easier and more useful. The FX loop might be a very good idea though rather than reverb.I don't actually own an FX processor at the moment (I just use some delay and chorus from pedals) though I'd like a Replifex....

1/10/1999 9:59 AM

MJ Harnish

Since your amp doesn't have a mv (right?) it's good news that you only have a noise problem at the higher volume settings (since it means that the noise is coming from the first stage). Have you tried other tubes? Shielded cable from the jack to the grid (with a 33k resistor at the tube pin)? Input jack isolated from the chassis (run the jack ground to the common ground point for the first stage)?

No MV. Yes, it's definitely the 1st stage now. The hum is virtually gone (barely audible at 10) if I pull the first tube and is TOTALLY gone if I pull the 2nd as well. I have to fool with the 1st stage today. I am using shielded cable. I'm going to try disconnecting the input cable 1st to see if I can isolate if it's the input jacks or the something beyond. No, the input jacks are not isolated but they may need to be if that's the problem.

:extra tube socket?

5 extra sockets! Yes, you read correctly. 2 extra power tube sockets (one is now taken up by the rectifier), and 3 extra holes punched for preamp sockets. The guy who owned the amp before me butchered it in an attempt to turn a nice SF (a '68 which cosmetically was in great condition) into a Marshall. It has an additional 6 holes drilled for controls including one right through the word "Band" on the faceplate. That's ok though since I needed that hole for the presence control and I've put many of the other holes to good use for an impedence selector, etc. At the moment the controls for the "normal" channel are unused since I just used the bright swtich to allow me to switch in or out the "jumpered" channels trick; much easier and more useful.

The FX loop might be a very good idea though rather than reverb.I don't actually own an FX processor at the moment (I just use some delay and chorus from pedals) though I'd like a Replifex....

1/11/1999 11:21 AM
Steve A.	MJ: To compare notes here's a link to the layout drawing for the SFBM to hotrodded 5F6A conversion I'm doing. At this point it is still just a "virtual" amp, but it does sound great playing air guitar: http://www.techaccessinc.com/blueguitar/sftwdbm1.pdf" TARGET="_top">http://www.techaccessinc.com/blueguitar/sftwdbm1.pdf Steve Ahola P.S. It is in Adobe Acrobat PDF format and if you need the plug-ins, et al, go to my home page for a link to Adobe.

1/11/1999 11:21 AM

Steve A.

MJ:

To compare notes here's a link to the layout drawing for the SFBM to hotrodded 5F6A conversion I'm doing. At this point it is still just a "virtual" amp, but it does sound great playing air guitar:

http://www.techaccessinc.com/blueguitar/sftwdbm1.pdf" TARGET="_top">http://www.techaccessinc.com/blueguitar/sftwdbm1.pdf

Steve Ahola

P.S. It is in Adobe Acrobat PDF format and if you need the plug-ins, et al, go to my home page for a link to Adobe.

1/11/1999 7:49 PM
MJ Harnish	Cool. What exactly are the boost and the warm switches doing? I like the idea of the switchable rectifier although right now I would expect a SS rectifier to make my B+ voltages even higher which isn't really desireable.

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