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Vibrolux meets Bassman

9/11/1997 5:20 PM
Matt M.	Vibrolux meets Bassman I have been working on a S.F. Vibrolux for sometime doing various mods and the latest round involved removing the vibrato circuit, as I didn't ever use it anyway and using the now free tube for extra gain on the normal channel. The amp already had a 6G series phase inverter with a presence control and 56k in the feedback loop. After trying a number of different high gain circuits I ended up going back to a tweed Bassman (5F6-A), but with both sides of the free preamp tube paralelled with a 1k/25uf on the cathode and 100k on the plate. It's very cool and dynamic with a good biting crunch with my Tele wide open and the amp on about 5-6. To make a long story longer, when I turn the treble control up to 8 the amp starts to squeal, not feedback, but rather a low level high freq. oscillation. It does this whether my guitar is plugged in or not and its level varies according to vol pot rotation. Any suggestions as to places to start looking? There is one deviation from the 5F6-A circuit in that it doesn't have the 47pf shunt across the phase inverter caps, but the squeal happens whether it's in there or not. The amp sounds great and if worst comes to worst, I just won't turn it up to "8" but I'd like to figure this one out. Thanks in advance to any help for this little dillema. Matthew M

9/11/1997 5:20 PM

Matt M.

Vibrolux meets Bassman

I have been working on a S.F. Vibrolux for sometime doing various mods and the latest round involved removing the vibrato circuit, as I didn't ever use it anyway and using the now free tube for extra gain on the normal channel. The amp already had a 6G series phase inverter with a presence control and 56k in the feedback loop. After trying a number of different high gain circuits I ended up going back to a tweed Bassman (5F6-A), but with both sides of the free preamp tube paralelled with a 1k/25uf on the cathode and 100k on the plate. It's very cool and dynamic with a good biting crunch with my Tele wide open and the amp on about 5-6. To make a long story longer, when I turn the treble control up to 8 the amp starts to squeal, not feedback, but rather a low level high freq. oscillation. It does this whether my guitar is plugged in or not and its level varies according to vol pot rotation. Any suggestions as to places to start looking? There is one deviation from the 5F6-A circuit in that it doesn't have the 47pf shunt across the phase inverter caps, but the squeal happens whether it's in there or not. The amp sounds great and if worst comes to worst, I just won't turn it up to "8" but I'd like to figure this one out. Thanks in advance to any help for this little dillema. Matthew M

9/11/1997 8:00 PM
anonymous	Try soldering a 33pf cap pins 1 & 2 on your preamp tube. Marshall did this on their Master volume amps from the 70's and it controls that squeal you get when you turn up the volume. Hope this helps.

9/14/1997 3:57 AM
Steve Ahola	For the Dan Torres amp mod kits, he recommends a few different solutions to squeal problems. One). Add a small value cap (rated for 500v) in parallel with the plate load resistor (follow pins 1 and 8). He suggests using a cap between 500pf and .001; I usually start lower than that (maybe 50pf) and keep increasing the value until the squeal is gone. If you go too high you'll lose too much of the high frequency response. If that doesn't do the trick, maybe the gain is too high. You could try lowering the resistance of your load resistors a bit. You could also add a 470 to 10k resistor in series with the cathode bypass capacitor to lower the gain a bit. So where in the Bassman circuit did you add the extra tube stage- before the stock input stage or somewhere further down the line? I can read schematics but terms like "phase invertor" can throw me for a loop...

9/14/1997 3:57 AM

Steve Ahola

For the Dan Torres amp mod kits, he recommends a few different solutions to squeal problems.
One). Add a small value cap (rated for 500v) in parallel with the plate load resistor (follow pins 1 and 8). He suggests using a cap between 500pf and .001; I usually start lower than that (maybe 50pf) and keep increasing the value until the squeal is gone. If you go too high you'll lose too much of the high frequency response.
If that doesn't do the trick, maybe the gain is too high. You could try lowering the resistance of your load resistors a bit. You could also add a 470 to 10k resistor in series with the cathode bypass capacitor to lower the gain a bit.
So where in the Bassman circuit did you add the extra tube stage- before the stock input stage or somewhere further down the line? I can read schematics but terms like "phase invertor" can throw me for a loop...

9/14/1997 12:30 PM
Matthew M.	Thanks for the reply Steve, in answer to your question, I parralleled both haves of the extra tube for the first gain stage. The idea was to imitate a 4-input preamp where you run a jumper from the bright channel to the normal channel, but wired internally. It works quite well ,the squeal aside , the other high gain type circuits I tried where too much with very little clean range. This seems to give me that plus some nice crunch as I open up my guitar vol. I had used a pl. load bypass in one of my earlier versions but took it out when I went back to a stock circuit, I will try it again, Tahnks again. Matthew

9/14/1997 12:30 PM

Matthew M.

Thanks for the reply Steve, in answer to your question, I parralleled both haves of the extra tube for the first gain stage. The idea was to imitate a 4-input preamp where you run a jumper from the bright channel to the normal channel, but wired internally. It works quite well ,the squeal aside , the other high gain type circuits I tried where too much with very little clean range. This seems to give me that plus some nice crunch as I open up my guitar vol. I had used a pl. load bypass in one of my earlier versions but took it out when I went back to a stock circuit, I will try it again, Tahnks again. Matthew

9/14/1997 6:09 PM
Mark Cameron	COOL,I did the same thing to my SF bassman it does sound vary good. It sounds like we did the same thing but I used 1.5k/.68 and switch in 200mf/25v with the deep switch(used the bass side for this and left the normal ch. stock) and the other thing is I mounted the tone caps to the pots to get them away from the 1st stage. This is not a hi-gain thing so dont put tone sucking caps on the plates of the pre tubes, try shilded wire for the input to each gain stage,check to see that you dont have a wire from the 1st stage to close to one from last stage. I think DR.Z also does the parralle thing in some of his amps

9/14/1997 6:09 PM

Mark Cameron

COOL,I did the same thing to my SF bassman it does sound vary good. It sounds like we did the same thing but I used 1.5k/.68 and switch in 200mf/25v with the deep switch(used the bass side for this and left the normal ch. stock) and the other thing is I mounted the tone caps to the pots to get them away from the 1st stage.

This is not a hi-gain thing so dont put tone sucking caps on the plates of the pre tubes, try shilded wire for the input to each gain stage,check to see that you dont have a wire from the 1st stage to close to one from last stage.

I think DR.Z also does the parralle thing in some of his amps

9/16/1997 12:12 PM
Matthew M.	Great! I'm glad someone else tried this, I think it's a easy way to spice up an otherwise unused channel. I did put a slight tone sucking cap on the plate resister (900 pf.) and the squeal disappeared. This also allowed me to bump up the treble cap to 510 pf. to get back some of that brightness. All in all it sounds really good, and not bad for a retrofit circuit without the aid of a scope or anything like that to aid in design. It would probably be helpful to go back now and clean things up a bit, as it can make a huge difference in noise and such depending on how its laid out. Thanks.

9/16/1997 12:12 PM

Matthew M.

Great! I'm glad someone else tried this, I think it's a easy way to spice up an otherwise unused channel. I did put a slight tone sucking cap on the plate resister (900 pf.) and the squeal disappeared. This also allowed me to bump up the treble cap to 510 pf. to get back some of that brightness. All in all it sounds really good, and not bad for a retrofit circuit without the aid of a scope or anything like that to aid in design. It would probably be helpful to go back now and clean things up a bit, as it can make a huge difference in noise and such depending on how its laid out. Thanks.

9/17/1997 1:46 AM
Steve Ahola	__ Dan Torres has schematics for similar projects on pages 39 and 40 of his "Inside Tube Amps" book. He uses a switching jack to bypass the added gain stage and run directly into the stock preamp. __ The schematic on page 39 uses a 750pf cap paralleled with a 220K plate load resistor coupled with a .01 cap and 330K resistor going to a 500K pot used as a gain control for this stage. This schematic uses no cathode bypass cap, but just a 2.7K resistor. On the input jacks, he replaces the stock 68K pair with 33K for more gain. __ The schematic on page 40 uses a .001 cap in parallel with a 120K plate load resistor. He lists a 330K to 470K resistor going to a 250K gain pot. Their is switch to turn on a 1.0uf cathode bypass cap in parallel with a 1.8K resistor. For the inout jack, he uses a 470K resistor to ground compared to 1meg on page 39. __ If you think a 1.0uf cathode bypass cap would produce a "wimpy" sound compared to a stock 25uf, I should mention Dan's "Triple Killer" mod. You use a mini toggle to switch between the stock 22uf, no cap and 0.47uf. The 0.47 cathode bypass cap produces a very "Marshall-like" sound without a lot of watts wasted producing low frequencies that are going to be drowned out by the bass player anyway. With no bypass cap, the volume is much lower for a more traditional Fender sound. __ Incidentally, on the clean channel of his Super Texan mod (which is the same as his Reverb King amp reviewed in Guitar Player's latest roundup of "boutique" amps), you can switch on and off the 22uf bypass cap for a boost switch (which is actually an "unboost" switch because the lower gain setting removes the cap normally used in the circuit). __ Good luck on your next project... it sounds like you already have this one nailed down!

9/17/1997 1:46 AM

Steve Ahola

__ Dan Torres has schematics for similar projects on pages 39 and 40 of his "Inside Tube Amps" book. He uses a switching jack to bypass the added gain stage and run directly into the stock preamp.
__ The schematic on page 39 uses a 750pf cap paralleled with a 220K plate load resistor coupled with a .01 cap and 330K resistor going to a 500K pot used as a gain control for this stage. This schematic uses no cathode bypass cap, but just a 2.7K resistor. On the input jacks, he replaces the stock 68K pair with 33K for more gain.
__ The schematic on page 40 uses a .001 cap in parallel with a 120K plate load resistor. He lists a 330K to 470K resistor going to a 250K gain pot. Their is switch to turn on a 1.0uf cathode bypass cap in parallel with a 1.8K resistor. For the inout jack, he uses a 470K resistor to ground compared to 1meg on page 39.
__ If you think a 1.0uf cathode bypass cap would produce a "wimpy" sound compared to a stock 25uf, I should mention Dan's "Triple Killer" mod. You use a mini toggle to switch between the stock 22uf, no cap and 0.47uf. The 0.47 cathode bypass cap produces a very "Marshall-like" sound without a lot of watts wasted producing low frequencies that are going to be drowned out by the bass player anyway. With no bypass cap, the volume is much lower for a more traditional Fender sound. __ Incidentally, on the clean channel of his Super Texan mod (which is the same as his Reverb King amp reviewed in Guitar Player's latest roundup of "boutique" amps), you can switch on and off the 22uf bypass cap for a boost switch (which is actually an "unboost" switch because the lower gain setting removes the cap normally used in the circuit).
__ Good luck on your next project... it sounds like you already have this one nailed down!

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