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Old 9th June 2009, 11:17 AM   #1
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Default Bassman Schematic Alterations

Hi all,

Going for first valve project, figured start at a well known design, so I've gone for the Fender Bassman (5E6) circuit.

I have removed the speakers & presence pot from the image already. Just a few questions (see the slightly altered schematic below):

1. The power supply area, is the ground switch necessary? This design uses two 5U4GA valves for rectification, I plan to use solid state rectifiers (purely for simplification), where would I connect the diodes? I was a little confused by the direction of positive and negative voltage.

2. In regards to the 16uf capacitors to ground, would 22uf caps be an acceptable substitution? The 8uf cap to ground (near centre image) - how necessary is it to be at 8uf?

3. I plan to use solely 12AX7s in the preamp circuitry (thus replacing two 12AY7s with 12AX7s), are there any resistors or caps that have to be replaced to achieve this? I want to be able to achieve rather easy distortion, primarily on bass guitar.

4. The circuit shown is designed for two 6L6Gs, I plan to use four 6L6GCs, so what must be done to the circuitry to use the GCs and can I simply parallel a copy of the power section to achieve the doubling of 6L6GCs?

Many thanks, Nic.

Click the image to open in full size.
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Old 9th June 2009, 04:05 PM   #2
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Good choice Nic. The ground switch is not necessary. When changing to a grounded supply cord, you always disable the ground switch. You can just plug in the 12AX7s...no changes required. Did you notice the 820 ohm resistor on the cathode of the I tube and the 1.5K on the II ? Changing the 1.5K to 820 ohm will give you more gain and earlier breakup. Just be sure and adjust the bypass cap as well. If you change to diode rectifier, you will add headroom...don't know if that's what you want. Maybe just eliminate one rectifier tube. Fender added the second 5U4 to get more clean headroom. Also, check out the AB165 schemo...it already has a diode rectifier, and there are easy ways to mod them into mini-plexis.
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Old 9th June 2009, 05:54 PM   #3
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Default Re: Bassman Schematic Alterations

Quote:
Originally posted by psychocow21
2. In regards to the 16uf capacitors to ground, would 22uf caps be an acceptable substitution? The 8uf cap to ground (near centre image) - how necessary is it to be at 8uf?
You can use 22 uF in both cases. Afaik you could go up to 50 uF.

Good luck!
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Old 9th June 2009, 09:42 PM   #4
ChrisA is offline ChrisA  United States
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Default Re: Bassman Schematic Alterations

Quote:
Originally posted by psychocow21
Hi all,

Going for first valve project, figured start at a well known design, so I've gone for the Fender Bassman (5E6) circuit.

I have removed the speakers & presence pot from the image already. Just a few questions (see the slightly altered schematic below):

1. The power supply area, is the ground switch necessary? This design uses two 5U4GA valves for rectification, I plan to use solid state rectifiers (purely for simplification), where would I connect the diodes? I was a little confused by the direction of positive and negative voltage.

2. In regards to the 16uf capacitors to ground, would 22uf caps be an acceptable substitution? The 8uf cap to ground (near centre image) - how necessary is it to be at 8uf?

3. I plan to use solely 12AX7s in the preamp circuitry (thus replacing two 12AY7s with 12AX7s), are there any resistors or caps that have to be replaced to achieve this? I want to be able to achieve rather easy distortion, primarily on bass guitar.

4. The circuit shown is designed for two 6L6Gs, I plan to use four 6L6GCs, so what must be done to the circuitry to use the GCs and can I simply parallel a copy of the power section to achieve the doubling of 6L6GCs?

Many thanks, Nic.

Click the image to open in full size.
#1 Use two In1007 in series. Each can "only" handle 1000V and some transients can so that high. Use caps in parallel with diodes to equalize voltage accross each diode.

#2 The bigger the better if you are playing bass maybe even much larger (like 2X or 5X) unless you are trying to re-create the vintage tone, but you aren't

#3 Yes it will over drive. But may want to re-think how cathode are by-passed so as to get more bass sound

#3.5 Also re-think the caps on the tone stack as basses go lower. A four string goes to 41Hz and a five string even lower

#4 This is a major change. You have reduced the output impedance by a factor of TWO. And doubled the power. you WILL need the different output transformers with half the impedance. Also you will need a larger power transformer that supplies about double the current.

One more thing. You can't simply parallel the tubes. Well you could if they were 100% matched impossible tolerances. You need to add so low value power resistors. If not one tunbe will draw more current then the other. Series resisters prevent that. Look at the Fender 135 watt bassman as an eaample


I'm building the exact same amp you are. It will be a bassman with four 6L6 tube. I just finisheded etching power supply PCB just last night. Most of the amp will be built on a turrent board but I'm doing the PS on a PCB because I have some radial lead caps. I went with a 110uf on the HT power supply but so far no choke. This can change later if need be
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Old 10th June 2009, 04:54 AM   #5
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Default Re: Re: Bassman Schematic Alterations

Quote:
Originally posted by bereanbill
Good choice Nic. The ground switch is not necessary. When changing to a grounded supply cord, you always disable the ground switch. You can just plug in the 12AX7s...no changes required. Did you notice the 820 ohm resistor on the cathode of the I tube and the 1.5K on the II ? Changing the 1.5K to 820 ohm will give you more gain and earlier breakup. Just be sure and adjust the bypass cap as well. If you change to diode rectifier, you will add headroom...don't know if that's what you want. Maybe just eliminate one rectifier tube. Fender added the second 5U4 to get more clean headroom. Also, check out the AB165 schemo...it already has a diode rectifier, and there are easy ways to mod them into mini-plexis.
Cheers Bill. Yes, I thought it was a bit odd, I hadn't seen a ground switch before. I'm still not entirely sure of where the positive and negative outputs from my rectifier will go. From what I can figure off the Bassman 135 schematic, the negative will go to ground and the positive straight into the standby switch etc? I'm also a little confused by the line coming off a tap (I think) on the transformer almost directly onto the cathode of the 6L6, through the 1500ohm resistor you mentioned.

Quote:
Originally posted by Miniwatt


You can use 22 uF in both cases. Afaik you could go up to 50 uF.

Good luck!
Thanks Miniwatt, cheers.

Quote:
Originally posted by ChrisA


#1 Use two In1007 in series. Each can "only" handle 1000V and some transients can so that high. Use caps in parallel with diodes to equalize voltage accross each diode.

#2 The bigger the better if you are playing bass maybe even much larger (like 2X or 5X) unless you are trying to re-create the vintage tone, but you aren't

#3 Yes it will over drive. But may want to re-think how cathode are by-passed so as to get more bass sound

#3.5 Also re-think the caps on the tone stack as basses go lower. A four string goes to 41Hz and a five string even lower

#4 This is a major change. You have reduced the output impedance by a factor of TWO. And doubled the power. you WILL need the different output transformers with half the impedance. Also you will need a larger power transformer that supplies about double the current.

One more thing. You can't simply parallel the tubes. Well you could if they were 100% matched impossible tolerances. You need to add so low value power resistors. If not one tunbe will draw more current then the other. Series resisters prevent that. Look at the Fender 135 watt bassman as an eaample


I'm building the exact same amp you are. It will be a bassman with four 6L6 tube. I just finisheded etching power supply PCB just last night. Most of the amp will be built on a turrent board but I'm doing the PS on a PCB because I have some radial lead caps. I went with a 110uf on the HT power supply but so far no choke. This can change later if need be
I have some 22uf caps, so I'll see how it sound with them.

I'll have a look at how the cathodes are bypassed once I've put it all together, first thing is to get it to work

I'm considering just using 5U4GAs in the rectifier, may end up being easier. Are you using solid state or tube rectification?

Would you have any recommendations of what values to change the caps in the tone stack to?

I am about to order the power transformer, once I have finalized the circuit I will use, it will do 500mA at 400-0-400, which I believe should suffice. I might just take the power section from the Bassman 135 and drop that in, don't have time now but I'll have a look this evening.

In the Bassman you're building, which Fender model are you working off? Are you going straight from the schematic or have you made alterations? Would you share the schematic?
You say you are going without a choke, what did you change so as to omit it?

Thanks, Nic.
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Old 10th June 2009, 05:21 AM   #6
eyoung is offline eyoung  United States
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look at fender twin reverb later schematics for a quad 6l6 I'm thinking more on the lines of 425 to 450 vdc and depending on the sound you like you might consider EL34 or 6550's with some minor adjustment to grid screen dropping resistors and a beefier output tranny...


Regards, Elwood
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Old 10th June 2009, 06:41 PM   #7
ChrisA is offline ChrisA  United States
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Default Re: Re: Re: Bassman Schematic Alterations

Quote:
I'm considering just using 5U4GAs in the rectifier, may end up being easier. Are you using solid state or tube rectification?

.. In the Bassman you're building, which Fender model are you working off? Are you going straight from the schematic or have you made alterations? Would you share the schematic? You say you are going without a choke, what did you change so as to omit it?
5U4GA easier? I think two IN4007 are a direct drop in replacement. But some thing the tube rectifier sounds better. I don't know yet. But it's hard to beat the ease of use of a SS diode.

I'm copying the power supply from the Ampeg VT120 because I am starting with a salvaged Ampeg VT120 and have all the parts. Basically Ampeg used a pair of 220uF caps. because in 1989 the caps were cheaper than the choke. In 1959 Fender used the choke because in 1959 big caps where very expensive.

I'm using the power supply and power amp section straight from Ampeg. But it looks just llike the Fender with 4 GLG tubes.

For the input section I'm starting with a copy of the Bassman 5F6A.

Values for the tone stack: Using stock 5F6A but I may modify it. But then I may add an equalizer stage using about 8 slide pots.

Hope to have some of the power supply built this weekend
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Old 11th June 2009, 06:05 AM   #8
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Default Re: Re: Re: Re: Bassman Schematic Alterations

I made a few alterations to the circuit, any critique (ie places where I buggered up) would be very much appreciated.

I replaced the valve rectification with solid state (they're 1N4007 diodes), are they the right way round?

Removed tone stack, I'll put it back later, first thing is to make a working amp.

I'm still not entirely sure how I should go about doubling the 6L6GCs, any thoughts?

Click the image to open in full size.

Quote:
Originally posted by eyoung
look at fender twin reverb later schematics for a quad 6l6 I'm thinking more on the lines of 425 to 450 vdc and depending on the sound you like you might consider EL34 or 6550's with some minor adjustment to grid screen dropping resistors and a beefier output tranny...


Regards, Elwood
I'll have a look into some of those schematics tomorrow. Cheers.

Quote:
Originally posted by ChrisA


5U4GA easier? I think two IN4007 are a direct drop in replacement. But some thing the tube rectifier sounds better. I don't know yet. But it's hard to beat the ease of use of a SS diode.

I'm copying the power supply from the Ampeg VT120 because I am starting with a salvaged Ampeg VT120 and have all the parts. Basically Ampeg used a pair of 220uF caps. because in 1989 the caps were cheaper than the choke. In 1959 Fender used the choke because in 1959 big caps where very expensive.

I'm using the power supply and power amp section straight from Ampeg. But it looks just llike the Fender with 4 GLG tubes.

For the input section I'm starting with a copy of the Bassman 5F6A.

Values for the tone stack: Using stock 5F6A but I may modify it. But then I may add an equalizer stage using about 8 slide pots.

Hope to have some of the power supply built this weekend
I put the 1N4007 diodes in, is that what you meant? (see above)

So would that mean I could just put in two 220 uF caps (in series?) and not have to deal with the choke?

Could you send me the schematics you're using, I'd like to have a look at the output section.

Let me know how you go with the power supply, would love to hear about it.

Cheers, Nic.
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Old 11th June 2009, 05:02 PM   #9
ChrisA is offline ChrisA  United States
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Default Re: Re: Re: Re: Re: Bassman Schematic Alterations

Quote:
Originally posted by psychocow21
I made a few alterations to the circuit, any critique (ie places where I buggered up) would be very much appreciated.

I replaced the valve rectification with solid state (they're 1N4007 diodes), are they the right way round?

Removed tone stack, I'll put it back later, first thing is to make a working amp.

I'm still not entirely sure how I should go about doubling the 6L6GCs, any thoughts?

Click the image to open in full size.



I'll have a look into some of those schematics tomorrow. Cheers.



I put the 1N4007 diodes in, is that what you meant? (see above)

So would that mean I could just put in two 220 uF caps (in series?) and not have to deal with the choke?

Could you send me the schematics you're using, I'd like to have a look at the output section.

Let me know how you go with the power supply, would love to hear about it.

Cheers, Nic.
I'm using this
http://www.schematicheaven.com/ampeg...mpeg_vt120.pdf

I'm building a pretty much exact copy of everything on page two. Ampeg build all this on a PCB but I'm using a small PCB for the PS and the power amp is going to be hand wired.

About the caps vs. choke. So long as you control power supply ripple, I don't think it matters.
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Old 11th June 2009, 06:01 PM   #10
radtech is offline radtech  United States
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When going from tube to solid state rectifiers you have to remember that a solid state diode has a much lower voltage drop than a tube, so for the same transformer you'll end up with higher voltage.

Going from 5U4G to 1N4007 might get you an extra 100 volts, which is not insignificant!
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