Blonde Bandmaster Sterile Vibrato Channel

shicks2525

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Greetings everyone :)

I have a recently acquired '61 blonde bandmaster, 6G7-A model.

I'm trying to figure out how this harmonic tremolo is supposed to work. It's the type that uses 3 tubes (well 2.5).

I understand the light dynamic resistor type(dump some signal to ground), and the bias wiggle type(vary output bias amount to lessen signal), but i'm not understanding how this is supposed to function. I get the first 12ax7 is the oscillator, and the last 7025 is taking the high and low frequencies and outputting them i guess out of phase for that phasey harmonic tremolo sound. I just don't understand the details of what's happening in the middle there. Can anyone shed some light?

The reason I ask is because the vibrato channel on this amp sounds very cold and sterile, but sort of harsh attack and mids. It is lacking that warmth, compressed, spongey, Spanky, bloomy sound that other fender amps ive played sound like. It just doesn't sound like a tube amp to me. It sounds decent enough, but both the actual sound and more the 'feel' of playing seems really lifeless and cold.

I'm pretty sure it's the tremolo circuit causing this, because if i pull the 7025 tube, and jumper the output of the .05 coupling cap coming out of the vibrato channel to the .001 cap right before the phase inverter, it sounds good

What would you folks recommend? Is this a normal result of the harmonic tremolo circuit? Could something be wrong with one of the caps in the tone splitting area of the trem circuit?

When i use the tremolo it sounds great, but when it's not being used and i turn the intensity to 0, it just is not pleasing.


Otherwise the amp seems fine, 3 prong chord, newer filter caps, hardly any idle noise or anything.
 

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shicks2525

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Thanks for the welcome!

And the linked thread. Reading through that was very informative. I found that patent document pretty easy to understand. I need to read through it again a few more times until i'm fully understanding.

So the first thing that seems a bit off to me, is that a few people in that thread said that the vibrato channel on the 3 tube harmonic trem circuits was extra mid scooped. Mine seems to have the opposite problem, where the mids seem a bit pronounced, with an almost harsh or hard sound. I'm going to go through later and doublecheck all components and take voltage and resistor readings to see how things are measuring up, but does anyone have any ideas why the vibrato channel would have what seems like an extra mid punch?

The other more general thing that is surprising me about this amp is it seems to lack compression, and be very touch sensitive. There is seemingly very little node bloom or sparkle or whatever words people use to describe the richness of fender tube amps. And note attack feels like a punch in the face.

I find this surprising, since i have a '62 tremolux which is absolutely amazing and sounds just like a dream. The bandmaster just sounds so different. Comparing the 2 circuits, they don't seem to me that they are night and day different. The tremolux is tube rectified and the bandmaster solid state, but it's hard to imagine that having such a great impact, because from what I've read, that type of voltage sag is only present during increased volume and I only play with the volume around 3 at the highest.

I'm hoping someone can shed some light or give me some ideas of where to focus on that might uncover why the amp sounds a bit cold. Or is this just how blonde bandmasters are?
 

shicks2525

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So I measured the bias last night, and hoping someone can double check my work since i'm a bit new to this.

2 Power Tubes, Sovtek 6L6WXT
OT centertap connects to standby lug


Inner Tube
Plate to OT Centertap(standby lug) Resistance = 80 ohms
Plate to OT Centertap(standby lug) Voltage = 2.525 vdc
Plate to ground Voltage = 505 vdc
Plate current = v/r = 2.525 / 80 = 0.0315625 amps or 31.5 ma
Plate dissipation = v*a = 505 * 0.0315625 = 15.939 watts

Outer Tube
Plate to OT Centertap(standby lug) Resistance = 70.7 ohms
Plate to OT Centertap(standby lug) Voltage = 2.3 vdc
Plate to ground Voltage = 506.5 vdc
Plate current = v/r = 2.3 / 70.7 = 0.0325318 amps or 32.5 ma
Plate dissipation = v*a = 506.5 * 0.0325318 = 16.47 watts

I believe I did this correct, as none of my numbers seem insanely off or anything.
But it does seem, based on the tube calculator and this tube sheet, that I am biased a bit 'cool'. I'm not sure if this would be indicative of the 'sterile' and not very exciting sound that I'm getting from the amp. I guess I will try to adjust the bias when I can, but there is no pot for that so I may do an install for this. Anyone have any thoughts they can share on whether this cool bias may be producing the symptoms I'm noticing?

Another thing that seemed strange to me was that the plate to OT voltage would fluctuate if i increased the tremolo intensity knob. I need to reread that patent doc again, but i would think that since this isn't a power tube bias wiggle trem, changing the trem intensity shouldn't have any effect on the power tube bias no?
 

corliss1

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Is the "sterile" sound only in the vibrato channel? If so, then the power section wouldn't have an issue since the normal channel sounds good.

Remember, everything is connected. If the vib is drawing more current while it's vibing, that would show up in the power supply in general as a change in voltage, and that could affect the bias a bit.

Have you tried a simple tube swap in the vib channel?
 

shicks2525

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I guess I would say there's 2 issues:

Both channels seem to lack warmth, where there's hardly any compression and its very touch sensitive. It doesn't seem to have that great golden sound that ive heard/felt with other fenders.

The other issue is that the vibrato channel seems to have some sort of overly mid sound that's a bit harsh, and the attack sort of really jumps out and punches if that makes any sense.

I know a lot of this seems subjective, but other fenders I've played, both ss and tube rectified have all had that certain tube feel, while this one just kind of doesn't.



I'm going to go through tonight and trace all components to make sure everything's where it's supposed to be with the right values, and measure resistors.


I have tried multiple tube rollings and different set of output tubes, all seem to have same issue
 

JRapp

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I had a blonde Twin that was the same way---just harsh and brittle. I wasn't in the mood to figure it out and just passed it on.
 

Snfoilhat

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bandmaster_6g7a_notes.png

Check out the shaded areas. I'm not capable of offering a detailed analysis of this amp, but a few things jump out if we're asking why the vibrato channel sounds different and worse than the normal channel (or a related question, why does it sound different or worse than the normal or vibrato channels of other Fenders)

They sent a ton of signal to ground preceding the mixer stage. The Bandmaster and the Super (and perhaps some others) use this 100k:10k divider (green). The Pro (and perhaps some others) use the 100k:22k divider (inset, blue). That's a big difference. Why didn't they standardize across all the big, brown/blond amps w/ harmonic tremolo?

Another stage that might contribute to tone sounding off is the mixer itself. I think there are some people in the big harmonic trem thread who figured out all the filters, which might account for the mids. Also, why is the stage biased so cool?

Anyhow, it seems to me that the attenuation of signal before the mixer, the consequent need to turn up the preamp volume higher to make up for it, and the high headroom of the mixer might all help to account for the tones you are describing

Does that story make sense?
 

shicks2525

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Thanks a lot for that detailed explanation!
That mostly makes sense.

This is a noob question, but what is the mixer stage you're referring to? Are you talking about the last 7025 in the trem circuit?
 

Snfoilhat

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@shicks2525 I may be using the wrong word. I guess the 470k resistors are where the signals really get mixed. But I did mean the last stage of the trem where the split signals are amplified

Like maybe the cool bias is necessary for good function of the two triodes when processing the oscilating, tremolo-ed trem signal, but isn't necessarily what you'd pick for its good tone playing through the vibrato channel with the intensity set to 0. It's another guess
 

shicks2525

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Thanks, that's what I figured you meant!

When you say why is the mixing bias stage so cold what did you mean by that?

The bias i checked earlier up in the thread was the output tube bias, nothing with this 7025 tube
 

Snfoilhat

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See the note +3.5V at the cathodes of that tube? That looks like it might be cooler than the +1.5 at the cathodes of the other, normal gain stages. The supply and plate voltages for that trem tube are much higher too, so maybe that bias is just what you get and not a very good suspect for the origin of the sounds you're getting. This is out past the limits of what I understand about how tones are shaped across multiple gain stages. I know that when some stages are moved away from center bias you can get different sounds, but I don't know if it can help explain the tones your thread is about

There are some really knowledgeable brown/blond era people around I hope will chime in
 

shicks2525

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Thanks again for your input on this.

I finally got a bias pot installed, and set the bias to right about 70% dissipation and still no noticeable change unfortunately :/

Next thing I'm going to try is using an attenuator and see if maybe it will sound better and its just a low volume thing. Other than that I'll have to keep researching and investigating.

Hopefully some more folks will chime in
 

SoK66

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Greetings everyone :)

I have a recently acquired '61 blonde bandmaster, 6G7-A model.

I'm trying to figure out how this harmonic tremolo is supposed to work. It's the type that uses 3 tubes (well 2.5).

I understand the light dynamic resistor type(dump some signal to ground), and the bias wiggle type(vary output bias amount to lessen signal), but i'm not understanding how this is supposed to function. I get the first 12ax7 is the oscillator, and the last 7025 is taking the high and low frequencies and outputting them i guess out of phase for that phasey harmonic tremolo sound. I just don't understand the details of what's happening in the middle there. Can anyone shed some light?

The reason I ask is because the vibrato channel on this amp sounds very cold and sterile, but sort of harsh attack and mids. It is lacking that warmth, compressed, spongey, Spanky, bloomy sound that other fender amps ive played sound like. It just doesn't sound like a tube amp to me. It sounds decent enough, but both the actual sound and more the 'feel' of playing seems really lifeless and cold.

I'm pretty sure it's the tremolo circuit causing this, because if i pull the 7025 tube, and jumper the output of the .05 coupling cap coming out of the vibrato channel to the .001 cap right before the phase inverter, it sounds good

What would you folks recommend? Is this a normal result of the harmonic tremolo circuit? Could something be wrong with one of the caps in the tone splitting area of the trem circuit?

When i use the tremolo it sounds great, but when it's not being used and i turn the intensity to 0, it just is not pleasing.


Otherwise the amp seems fine, 3 prong chord, newer filter caps, hardly any idle noise or anything.
In my experience that is actually how the vibrato channel sounds on the brown/blonde six (actually 5 1/2) preamp tube Fender, there's most likely nothing wrong with your amp. Use the normal channel unless you want that vibrato effect, which is mighty fine.
 

schmee

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In my experience that is actually how the vibrato channel sounds on the brown/blonde six (actually 5 1/2) preamp tube Fender, there's most likely nothing wrong with your amp. Use the normal channel unless you want that vibrato effect, which is mighty fine.
I have to agree. The Brown and Blondes in some forms are cleanish and a little more sterile than their BF counterparts. Pushed really hard/loud that can be a good thing, but at lower volume playing less so.
 

Jasonpatrick

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Thanks again for your input on this.

I finally got a bias pot installed, and set the bias to right about 70% dissipation and still no noticeable change unfortunately :/

Next thing I'm going to try is using an attenuator and see if maybe it will sound better and its just a low volume thing. Other than that I'll have to keep researching and investigating.

Hopefully some more folks will chime in
Just out of curiosity, what type of signal caps are in it? Blue or yellA? Also, 70 is too fn high for these blonds n browns..
 

Euphonica

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I have to agree. The Brown and Blondes in some forms are cleanish and a little more sterile than their BF counterparts. Pushed really hard/loud that can be a good thing, but at lower volume playing less so.
I'm not nearly as experienced as you all, but I do have four real Fender piggyback blondes and I have not noticed this channel sterility at all. The Showman, Bassman, Bandmaster and Tremolux all sound wonderful and rich, no matter the channel. All I've really noticed is that sometimes that the non-trem channel is a bit louder and fatter. (Obvs the Bassman channels sound totally different.) The two blonde Twins and the brown Super are the same as the piggybacks, nothing close to sterile from either channel. I also have a '64 Showman that sounds waaaay more sterile through both channels, just like the other '64 Showman I just traded.
 

Timbresmith1

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Thanks for the welcome!

And the linked thread. Reading through that was very informative. I found that patent document pretty easy to understand. I need to read through it again a few more times until i'm fully understanding.

So the first thing that seems a bit off to me, is that a few people in that thread said that the vibrato channel on the 3 tube harmonic trem circuits was extra mid scooped. Mine seems to have the opposite problem, where the mids seem a bit pronounced, with an almost harsh or hard sound. I'm going to go through later and doublecheck all components and take voltage and resistor readings to see how things are measuring up, but does anyone have any ideas why the vibrato channel would have what seems like an extra mid punch?

The other more general thing that is surprising me about this amp is it seems to lack compression, and be very touch sensitive. There is seemingly very little node bloom or sparkle or whatever words people use to describe the richness of fender tube amps. And note attack feels like a punch in the face.

I find this surprising, since i have a '62 tremolux which is absolutely amazing and sounds just like a dream. The bandmaster just sounds so different. Comparing the 2 circuits, they don't seem to me that they are night and day different. The tremolux is tube rectified and the bandmaster solid state, but it's hard to imagine that having such a great impact, because from what I've read, that type of voltage sag is only present during increased volume and I only play with the volume around 3 at the highest.

I'm hoping someone can shed some light or give me some ideas of where to focus on that might uncover why the amp sounds a bit cold. Or is this just how blonde bandmasters are?
Less sag with a solid state rectifier, so less compression.
 

schmee

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I'm not nearly as experienced as you all, but I do have four real Fender piggyback blondes and I have not noticed this channel sterility at all. The Showman, Bassman, Bandmaster and Tremolux all sound wonderful and rich, no matter the channel. All I've really noticed is that sometimes that the non-trem channel is a bit louder and fatter. (Obvs the Bassman channels sound totally different.) The two blonde Twins and the brown Super are the same as the piggybacks, nothing close to sterile from either channel. I also have a '64 Showman that sounds waaaay more sterile through both channels, just like the other '64 Showman I just traded.
Yeah, I definitely wouldn't go so far as to say they are 'sterile' sounding, just indicating they lean a bit more toward that maybe compared with the BF. Maybe like the difference between trem and non trem channels, but not even that much. Great amps.
 




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