Bandmaster Normal Channel with 6V6

[Vdelli]

Hi,

a friend came to me with an old Carlsbro Hornet Vibrato Combo and asked me if I can build him the normal channel of the Fender Bandmaster + reverb and 6V6 power section into it. I never build fender stuff and don't find any similar. For the power section I think the Deluxe Reverb is a good point to start but in the DR  preamp voltage is around 160V and in the Bandmaster arround 270V. So did anyone tried something like this?

With best regards.

[sluckey]

For the power section I think the Deluxe Reverb is a good point to start but in the DR  preamp voltage is around 160V and in the Bandmaster arround 270V.

The differences in the preamp voltage are due to the value of two resistors in the power supply. The bandmaster uses 1K and 4.7K to get the higher B+ while the DR uses 10K and 10K to get the lower B+. Easy to change to whatever voltage you want.

Hoffman has a layout for a single channel AB763. It's pretty much what you describe ***EXCEPT*** it has tremolo. It's easy to drop the tremolo from the Hoffman plan.

[Vdelli]

Thanks Sluckey!

Yes, I saw this single channel Ab763 before, but I wasn't sure if it is okay to raise the preamp voltage to bandmaster specs. Is it a okay to raise the voltage of the reverb tubes too, or should I drop them to the original voltage (170V).

Best regards

[sluckey]

The AB763 circuit will tolerate a wide range of voltages. I would not be too concerned. If you really want Bandmaster voltages then get a Bandmaster power transformer and use 1K/4.7K resistors in the power supply. If you plan to use the PT from that old Carlsbro Hornet Vibrato Combo, you'll just have to accept the voltages you get.

[Vdelli]

Thanks Sluckey!

I think I build the single channel Ab763 without the Vibrato and when the amp works, I try to play wit the preamp voltage and listen what sounds good to me. I have no idea what voltages the Carlsbro Transformer has. The carlsbro is an transistor amp, have to measure. I want to use SS rectifier because of space. If I'm right, I can use a 18 Watt transformer with 300-0-300V, that gives ~420V with SS rectifier.

[sluckey]

I think I build the single channel Ab763 without the Vibrato and when the amp works, I try to play wit the preamp voltage and listen what sounds good to me.

That' a good idea. I would start with 1K/4.7K and tweak from there. 300-0-300 sounds like a good choice for PT.

I have a single channel AB763 with reverb that you may be interested in. Take a look...

     http://sluckeyamps.com/tdr/tdr.htm

[labb]

Take a look at Steve's AB763 lite.   http://sluckeyamps.com/misc/AB763_Deluxe_Lite.pdf   
Simple and a heck of a good sounding amp...Has most of the voltages you are looking for.

[92Volts]

The AB763 circuit will tolerate a wide range of voltages. I would not be too concerned. If you really want Bandmaster voltages then get a Bandmaster power transformer and use 1K/4.7K resistors in the power supply. If you plan to use the PT from that old Carlsbro Hornet Vibrato Combo, you'll just have to accept the voltages you get.

I was looking for info on power supply resistors and found this older thread: http://el34world.com/Forum/index.php?topic=10316.0 which suggests decent size caps (47uf) call for 3k resistors. I may have been doing that math wrong and a value like 1k with 47uf would be acceptable, though I'm still curious how much this matters in practice. I'm struggling a bit with low plate voltage on a build.

[sluckey]

I may have been doing that math wrong and a value like 1k with 47uf would be acceptable, though I'm still curious how much this matters in practice. I'm struggling a bit with low plate voltage on a build.

I would not even consider the cap value for determining the value for a B+ dropping resistors. The main objective here is to drop enough voltage to run the preamp and other lower voltage circuits with the desired voltage. All you need is Ohm's law and a handful of resistors. Heck, you don't really even need Ohm's law.

[shooter]

Heck, you don't really even need Ohm's law.

aaahh, you gotta make then try

[Vdelli]

Thanks for all the answers!

The Ab763 Lite schematic is very helpful!

Did somebody ever try a Bassman tone stack in a DR?
A friend bought a 68' DR Custom. That Custom Channel sounds awesome with pedals! I read that it is a DR with Bassman tone stack and higher plate Voltages.

The friend that gives that old Carlsbro combo to me, use always his Copicat tape echo with tube preamp (not the transistor version)  for some light dirt, like a pedal in front of the amp. That could be a nice option for my build.

Maybe there is a way to switch from Ab763 tone stack to Bassman tone stack.


Always when I begin to figure out what I want to build I get to many ideas and get no clear concept    

[ClippIn4tae]

Here ya go bud


http://el34world.com/Forum/index.php?topic=18505.0

[Vdelli]

Thanks ClippIn4tae!

Here is what I want to do

DR normal channel with 68' custom reissue Tonestack
Reverb, no tremolo
Raw switch as on/off switch or Pot
3 way NFB switch
SS Rectifier and 300-0-300V 18 Watt Transformer

I read that there is a boost in gain if you remove the tremolo circuit. Is it okay to just put in the voltage divider from the Ab763 Lite, for "normal" gain structure?

I want to use the Hoffman Single channel Ab763 layout and swap out the tremolo. So I put that voltage divider between the .1 cap coming from pin 6  of V3 and the .001 cap coming from pin 5 of V5?!

Best regards

[sluckey]

I read that there is a boost in gain if you remove the tremolo circuit. Is it okay to just put in the voltage divider from the Ab763 Lite, for "normal" gain structure?

I want to use the Hoffman Single channel Ab763 layout and swap out the tremolo. So I put that voltage divider between the .1 cap coming from pin 6  of V3 and the .001 cap coming from pin 5 of V5?!

Hoffman's AB763 has a totally different tremolo circuit from the original AB763. Removing the trem circuit from the Hoffman AB763 will not result in a gain boost.

The gain boost refers to removing the tremolo circuit (specifically the 50K INT pot) in the original AB763 circuit. Hoffman has already "normalized" the gain boost by using R30 (47K) so you don't need the voltage divider that you see in the AB763 Lite. If you prefer to use the 220K/270K voltage from the "Lite" circuit you would need to remove R30 from the Hoffman circuit.

All the other stuff you want to do is very doable. However, the tonestack from the '68 Custom Deluxe Reverb is not the same as the 5F6A Bassman tonestack. It closely matches the original DR tonestack but has a couple extra resistors. Search for the schematic on the net. It's available.

[Vdelli]

Thats are good news Sluckey!

So it's very easy to swap out the tremolo.

Yes, for the 68' CDR style you must change mid cap from .047uF to .022uF. Between the bass pot and the 6.8k mid resistor you must add a 18k resistor as "bass floor" and change the NFB resistor to 1,5k.

[sluckey]

Sounds like you have done your homework. Here's a schematic that shows how to remove the Hoffman trem...

[Vdelli]

Thanks Sluckey!

I removed this! Today I started with the eyelet board. I never used eyelets. Very easy to work with.
Tomorrow come the other parts like chassis, caps... I will post some more pictures from that build in the next time!

With best regards

[Vdelli]

This is the circuit board and the filter cap board with rectifier, it's not finished yet because I have no 3w resistors (1k, 4k7), only 2 watt resistor. I don't know if this is enough for a dropping resistor in this amp. Fender use 1w resistors on the original schematic with 10k but I use the 1k per Hoffman schematic.
I downsized the circuit board a little bit because of that small chassis, so it is a little bit tighter.
The next step is to drill the chassis holes.

With best regards

[sluckey]

Looks good. 2 watt resistors are fine for the dropping resistors.

[Vdelli]

Hi,

today I fired up that amp for the first time and nothing burnt down 

But, I have a issue with my B+. I use a 18 Watt Transformer with 300-0-300V and SS rectifier. I have no bias Tap so I use one side of the 300VAC. Now my B+ is too low, it shows only 389VDC after the rectification. I have to use a 68k bias range resistor to get the right bias of 22mA.

Here are the voltages: of the power tubes:


V5
PIN 1: 0.022V
PIN 3: 389V
Pin 4: 384V
Pin 5: -35,49V
Pin 6: -35,55V

V6 is identical.

If I change the bias range resistor the whole B+ changes and the the AC voltage before the rectifier too!

Do someone know what happens?

Best regards

[sluckey]

Now my B+ is too low, it shows only 389VDC after the rectification.

I would not say that 389v is too low.

If I change the bias range resistor the whole B+ changes and the the AC voltage before the rectifier too!

That's probably quite normal. As you change the bias voltage the tube current changes, ie, the load on the power supply changes. This will cause the B+ and AC to change.

Set the bias pot so the negative voltage at pin 5 of the 6V6s is maximum negative voltage. What voltage do you measure on pins 5 and 3? Now pull the 6V6s and measure the negative voltage on pin 5. Is it still the same? Turn the bias pot all the way the other direction. What voltage do you measure on pin 5?

How does the amp sound?

[Vdelli]

Thanks for answer Sluckey!

  I would not say that 389v is too low

Okay, it's  good to hear that! I knew there would be a voltage drop, but never thought that would be around 35 volts. The fender voltage chart shows 405 V or 415V.

 
Set the bias pot so the negative voltage at pin 5 of the 6V6s is maximum negative voltage. What voltage do you measure on pins 5 and 3? Now pull the 6V6s and measure the negative voltage on pin 5. Is it still the same? Turn the bias pot all the way the other direction. What voltage do you measure on pin 5?
 

Maximum: Pin 5: -32,52V  still the same after swap out the tubes.
                    Pin 3:  376,3V

Minimum:  Pin5: -52,70V
                    Pin3:  425V


I never played a DR style amp, but the amp sounds good to me, but I played it only between volume 1 and 2 because it's 2 o clock in morning here.

Thanks for your help! 

[Vdelli]

Today I played it loud!

The amp sounds a bit stiff to me and with a humbucker on the bridge it doesn't sound good... It's like farting and crackling if you hit the strings.

The plate voltage on the PI is about 220V (100k)  and 200V (82k)

The preamp tube are all at 240-250V.

I think I try a bigger dropping resistors to lower that voltage.

[sluckey]

Why don't you have the negative feedback connected and the 820Ω resistor on the board?

[Vdelli]

On this picture is not in there,  but I soldered in a 1k resistor before I fired up the amp.

[Vdelli]

 I put a 22k resistor instead of the 4.7k in. Now I have around 190V-198V on the preamp plates. PI is still around 204V (100k) and 224V (82k). The amp sounds good until I get over 2 of volume. When I hit the strings,  there is a bad distortion for 2 second or so.

[sluckey]

Did you reset the bias for about 22mA?

Which speaker are you using?

Disconnect the negative feedback wire. Does the amp get a bit louder? Sound any better?

[Vdelli]

Hi Sluckey,

Yes I set it back to 22mA. For me it sound like that this harsh Distortion that come if you hit the strings "lay over" the good sound.


I use one 8 ohm Rola G10-60 and one 8 Ohm WGS ET10 wired to 4 ohms into the 4 ohm input. I try it also with my 4x12 V30 cab... No difference in this Distortion.

Yes, when I disconnect the NFB it gets louder and the amp sounds more sponky.... I have to test it at loud volumes today, because that amp is stupid loud for playing it at night  ;)


Thanks and Beste regards

[Vdelli]

I discovered that I use the 4 ohm tap instead the 8 ohm tap for the NFB ist that important? And what is with that R45 100R? I don't know where but I read that you must change that to 47 ohms if you change the output is that right?

[sluckey]

Fender used a 47Ω resistor with an 8Ω OT/speaker. He used a 100Ω resistor with 2Ω or 4Ω OT/speaker. But if you like the way it sounds with some other resistor value, that's OK too.

[Vdelli]

Thanks!

Now I tested the amp with volume on 10. With no NFB it seems to be all good! There is distortion but in a musically way!

[Vdelli]

It's really frustrating! I thought that I found the problem!
But I found out that the issue is still there if I waiting a few minutes after fire up the amp.

There is something I don't understand. I measure on pin 4 a higher voltage (393V)  than on the plates (389V). On Pin 5 - 36,2V.

Muste be the bias circuit I think.

[shooter]

don't understand

G2 will often measure higher than plate, basically there is little current flowing through G2, way less than plate, so voltage drop will be less.

[Vdelli]

Thanks shooter,

so that's not the problem.

The amp sounds quite good now but I get this cross over distortion or ghost notes  (I don't know how to call it) at volume over 4.

How do you guys set up a bias circuit if you use the high voltage tap of your PT? Do you go down from (in my case 300V AC) to 50V like the normal bias Tap should be?

[sluckey]

How do you guys set up a bias circuit if you use the high voltage tap of your PT? Do you go down from (in my case 300V AC) to 50V like the normal bias Tap should be?

The only difference is the value of the bias range resistor. A 50v bias tap would use a small resistor, maybe 470Ω to 1KΩ. Using the high voltage would require a much larger bias range resistor, maybe 100K to 220K. You have a 220K on your board.

There's nothing wrong with your bias circuit. You have a range of -32v to -52v and are able to set the bias current to a normal value.

[Vdelli]

Thank you Sluckey!

I think I solved the problem! I try different resistor values between the 300V AC and the diode of the bias circuit. I came up with 82k.

Now I have 383V on the Plates, 384V on Pin 4,  - 36V on Pin 5 and 22mA across the 1 ohm resistor.
The Preamp voltage is about ~200V maybe I lower this and hear what's sounds good to me.


I found out that I get a oscillation from the reverb if I cranked it. When I soldered a second 100 ohm resistor over R45 to go nearer to 47 ohm it was gone.

My first fender style build and I learned a lot.

I hope tomorrow it still runs, when I try it on volume 10!

Can't wait to bring it back to my friend, he is a German Guitar builder and I'm very proud that he let me build a amp for him :)

[Vdelli]

Sorry guys,

But can you please listen to that little recording...

I build in a trainwreck type-3 post phase inverter master to get the volume cranked to record that sound with my mobile phone. Sounds like a sub octave.... What to you think? I hear the same without that master  volume build in, but it was just to loud to record this.

https://clyp.it/tawifv2o

[Vdelli]

So this octave sound came out of one bad 6V6...theyre new ones... When I knock on this bad tube, there  is something that sounds like a fart. I tryed some 6L6 in that amp and it sound so good. The shop where I buy this tube will replace it. All that trouble because of a new bad tube....