Hi guys,
I want to increase the glassy tone of my amp.
How could I tweak an AB763 circuit to implement a pot or switch to increase the "glass" tone of a Fender (actually a Vibroverb clone based on a 69 bandmaster).
Which components are involved in the tone stack for that mod ?
Any suggestion ?
Thank you
Need advice for a mod
Moderators: pompeiisneaks, Colossal
Re: Need advice for a mod
Are you referring content in the 4KHz to 6KHz range? I see from your other posts you were considering modifying a same/similar amp. Can you add a pic of your circuit board so we can see what has been changed already? Is this a bone stock 1969 Bandmaster Reverb circuit or has it been modified?
There are a few circuit elements that effect glassiness in a Fender reverb-style amps:
If you have added a master volume this may impact tone as well.
Please let us know what you have now.
There are a few circuit elements that effect glassiness in a Fender reverb-style amps:
- the 10pF capacitor in parallel with the 3M3 resistor in the normal channel mixer
- the bright cap hanging off the volume pot
- the negative feedback resistor in the phase inverter
- the input grid stoppers (68K)
If you have added a master volume this may impact tone as well.
Please let us know what you have now.
Re: Need advice for a mod
Hi ,
Thank you for your reply
!
Currently the amp is a dead stock 69 bandmaster with his paktron components and white Mallory (although not sure the Mallory are stock). The amp sounds clean and « modern » not my taste , I miss that blackface sound.
My goal is to convert it in a Vibroverb clone.
I have a brand new Soursound OT bassman wired in 8ohms ready for this project. And I kept the 15 Altec originally mounted on my previous real Vibroverb.
Yes I plan to adapt a PPIMV as per Tyler grind design, the results on his SR is perfect to me.
I want to make the tone stack more versatile. As the normal channel and trem will be disconnected I plan to make the SRV usual mods.
My old Vibroverb was equipped with some Draloric and El Menco components and sounded so close to SRV vibes and frequencies.
But when pushed ( from 8 to 10) the amp was increasing the mids and was throaty. Can you explain and how to avoid it ?
In my BM I want to get a sound not as throaty when pushed and adding a « glass » knob. It could be interresting.
I would like advice on kind composition or brand of components to get that old BF tone .
Regarding speaker , I got the idea to mix a 15’ and a 10’ in a cabinet to get best of both world. But never seen before so was wondering if it’s a good idea ?
Thank you
Thank you for your reply
! Currently the amp is a dead stock 69 bandmaster with his paktron components and white Mallory (although not sure the Mallory are stock). The amp sounds clean and « modern » not my taste , I miss that blackface sound.
My goal is to convert it in a Vibroverb clone.
I have a brand new Soursound OT bassman wired in 8ohms ready for this project. And I kept the 15 Altec originally mounted on my previous real Vibroverb.
Yes I plan to adapt a PPIMV as per Tyler grind design, the results on his SR is perfect to me.
I want to make the tone stack more versatile. As the normal channel and trem will be disconnected I plan to make the SRV usual mods.
My old Vibroverb was equipped with some Draloric and El Menco components and sounded so close to SRV vibes and frequencies.
But when pushed ( from 8 to 10) the amp was increasing the mids and was throaty. Can you explain and how to avoid it ?
In my BM I want to get a sound not as throaty when pushed and adding a « glass » knob. It could be interresting.
I would like advice on kind composition or brand of components to get that old BF tone .
Regarding speaker , I got the idea to mix a 15’ and a 10’ in a cabinet to get best of both world. But never seen before so was wondering if it’s a good idea ?
Thank you
Re: Need advice for a mod
I also like the modifications that Tyler Grund makes to Super Reverbs. As this is a service that he currently provides, and he does not currently share his recipe, I will point you to his web page:
https://tylergrund.com/gear.htm
Your plan to 'blackface' your Bandmaster Reverb and perform some SRV Vibroverb modifications should get you close. One modification that you did not mention is the substitution of a solid state rectifier. This raises the voltage and eliminates sag.
Disconnection of the normal channel and vibrato intensity control will provide you with extra gain that can be tapered toward your preference for additional 'glass'. I would point you toward the 3M3 resistor and 10pF capacitor as a point of exploration. Try increasing the value of the 3M3 resistor to something between 4M7 and 10M and then experiment with capacitors between 10pF and 100pF. An external potentiometer wired as a rheostat in series with the capacitor with a value between 4M7 and 10M should provide some level of adjustability.
I find the Paktron signal capacitors to be fine for the task, but strongly encourage replacement of the white Mallory cathode bypass capacitors. If your circuit is an AC568 revision, a full blackfacing will include removal of the output tube cathode network and reversion of the bias circuit to bias voltage adjustment from bias balance.
The Bandmaster Reverb has a midrange control which is an addition to the Vibroverb tonestack. This will provide some flexibility. Experimentation with midrange capacitor values between .01uF and .047uF is recommended.
'Throaty' can have a lot of meanings. Please clarify...
I think the addition of a smaller speaker in parallel with your 15" speaker is a wonderful idea. be afraid to go even smaller than a 10" speaker. I like 8" alnico speakers as 'guitar tweeters' for 15s. Search Google for tweeter capacitor. A capacitor between 2uF and 10uF should help accentuate the highs and protect the 8" speaker from low frequencies.
A quick search brought me to this page:
https://electronics.stackexchange.com/q ... s-with-a-f
https://tylergrund.com/gear.htm
Your plan to 'blackface' your Bandmaster Reverb and perform some SRV Vibroverb modifications should get you close. One modification that you did not mention is the substitution of a solid state rectifier. This raises the voltage and eliminates sag.
Disconnection of the normal channel and vibrato intensity control will provide you with extra gain that can be tapered toward your preference for additional 'glass'. I would point you toward the 3M3 resistor and 10pF capacitor as a point of exploration. Try increasing the value of the 3M3 resistor to something between 4M7 and 10M and then experiment with capacitors between 10pF and 100pF. An external potentiometer wired as a rheostat in series with the capacitor with a value between 4M7 and 10M should provide some level of adjustability.
I find the Paktron signal capacitors to be fine for the task, but strongly encourage replacement of the white Mallory cathode bypass capacitors. If your circuit is an AC568 revision, a full blackfacing will include removal of the output tube cathode network and reversion of the bias circuit to bias voltage adjustment from bias balance.
The Bandmaster Reverb has a midrange control which is an addition to the Vibroverb tonestack. This will provide some flexibility. Experimentation with midrange capacitor values between .01uF and .047uF is recommended.
'Throaty' can have a lot of meanings. Please clarify...
I think the addition of a smaller speaker in parallel with your 15" speaker is a wonderful idea. be afraid to go even smaller than a 10" speaker. I like 8" alnico speakers as 'guitar tweeters' for 15s. Search Google for tweeter capacitor. A capacitor between 2uF and 10uF should help accentuate the highs and protect the 8" speaker from low frequencies.
A quick search brought me to this page:
https://electronics.stackexchange.com/q ... s-with-a-f
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Re: Need advice for a mod
Sorry I forgot to mention about the SS rectifier 
It’s planned in my build .
As I will not use the channel 1 and to avoid butchering the faceplate all the pot from channel 1 will be replaced by the master / mid / maybe a presence as per Rob Rob mod and the bright switch could be a « glass « switch but I like the idea of a rehostat, it’s to be defined later.
Regarding « Throaty »
When I played at 6 the sound was well balanced with chimey low mids and crunchy clean very sparkle.
But when pushed , the tone turned high mids and a little muddy , the sparkle faded .
Maybe at 6 it’s what some call « the spot »
What difference if paktron would be charged by OD 6P for example ?
Or some brown El Menco ?
Sometimes I see some capacitor replaced by mica or ceramic disc . Why? What does it do ?
It’s planned in my build .
As I will not use the channel 1 and to avoid butchering the faceplate all the pot from channel 1 will be replaced by the master / mid / maybe a presence as per Rob Rob mod and the bright switch could be a « glass « switch but I like the idea of a rehostat, it’s to be defined later.
Regarding « Throaty »
When I played at 6 the sound was well balanced with chimey low mids and crunchy clean very sparkle.
But when pushed , the tone turned high mids and a little muddy , the sparkle faded .
Maybe at 6 it’s what some call « the spot »
What difference if paktron would be charged by OD 6P for example ?
Or some brown El Menco ?
Sometimes I see some capacitor replaced by mica or ceramic disc . Why? What does it do ?
Re: Need advice for a mod
Regarding 'throaty', my experience has been that two things are at play. The first is the bright switch. The 120pF cap is parallel the signal and wiper lugs. As you turn up the volume control the resistance between the two lugs is reduced and consequently the effect of the bright cap. The second is that overdriving a tube has the tendency to flatten the frequency response.
The brown Paktron signal capacitors in your '69 are polyester film/foil which are the same construction method as PS series Spragues and the El Menco. Differences are subtle and I need to change more than a few if I'm going to here a difference between polyester caps from those three manufacturers from that era. Polypropylene film foil (715P) will have more definition in the upper register. They are considered a 'more forward' sound. I don't like them with my nose stuffed into the speaker, but love them from the third row back from the stage.
What about leaving the Paktrons for last?
The brown Paktron signal capacitors in your '69 are polyester film/foil which are the same construction method as PS series Spragues and the El Menco. Differences are subtle and I need to change more than a few if I'm going to here a difference between polyester caps from those three manufacturers from that era. Polypropylene film foil (715P) will have more definition in the upper register. They are considered a 'more forward' sound. I don't like them with my nose stuffed into the speaker, but love them from the third row back from the stage.
What about leaving the Paktrons for last?