Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on August 09, 2015, 11:00:56 pm
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My 1980 super reverb has a bright switch. It's a great clean amp and I like using the bright switch for jingle jangle and a more tactile sound.
On my Ab763 deluxe builds I used the same preamp as the super (including the mid pot) and added a bright switch. However I forgot to add the 10meg resistor I noticed going to ground
Does the 10 meg resistor just help to tame the amount of brightness added?
I've noticed the deluxe seems quite sizzly with that bright switch on, will putting the 10 meg resistor in there help?
thanks
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... I forgot to add the 10meg resistor I noticed going to ground
Does the 10 meg resistor just help to tame the amount of brightness added?
I've noticed the deluxe seems quite sizzly with that bright switch on, will putting the 10 meg resistor in there help?
Probably won't help. The 10MΩ is almost certainly there to discharge the cap so the switch doesn't make a 'pop' when you switch the cap in/out.
You could try to tame the sizzle by letting more lows through with the bright switch (make the cap larger) or by rolling off the highest highs somewhere. Lots of places to lose high end, but many/most involve either a small-value cap across a plate load resistor or a cap from grid to ground. Value is best found experimentally, maybe no bigger than 300pF (depending on where you apply it) down to a low of ~47-50pF.
Your speaker's high end response probably influences whether or not you think you need high end reduction. Let's say you decided to roll off highs in the long-tail phase inverter, and choose the 100kΩ plate resistor as the one which will have a cap across it. A 100pF cap across the 100kΩ would be -3dB at ~16kHz, a 200pF cap would be -3dB at ~8kHz. Or you might need a little bigger for a little more cut of highs.
You could also place a cap from one plate of the phase inverter to the other. Again, the value is best found experimentally.
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Thanks
That helps, I won't bother as it's not popping at all
Likely it's the top end of the K120 coming through too. I find I like the bright switch better on the super(with 4 Rola ceramic speakers I think) than I do on the deluxe with the JBL
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I don't know what the idea behind that 10M resistor was. I only see that on one late model Fender amp, the CBS ultra-linear 70W with master volume and pull boost. There's a lot of 'clever' new generation engineering stuff happening in that amp! This is not one of the more desirable models. Probably has a very clean, sterile sound.
http://el34world.com/charts/Schematics/files/fender/Fender_cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf)
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Haha
Desirable is relative! :icon_biggrin:
If you're looking to overdrive your amp and play da blues or rock leads (like everyone on youtube!!!!)
It's certainly not the amp for that, unless you put a pedal in front of it and then it does it really well!
I've a used 1980 70 watt CBS ultralinear Super Reverb professionally for the past 20 years or so playing very organic 30s 40s 50s 60s influenced original music (rockabilly/pop/jazz/latin/ cabaret) and people have always loved the sound.
Big, warm and sparkly when needed, Clear as a bell, and pretty as one too
I don't think it's meant to crank and distort
The funny thing about almost all the older Fender amps is that I believe that NONE of them was designed to distort. I'm sure the engineers are spinning in their graves. Wasn't that just a happy accident for rock music that they sound the way they do when pushed beyond their designed parameters??
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The funny thing about almost all the older Fender amps is that I believe that NONE of them was designed to distort.
I totally agree.
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I don't know what the idea behind that 10M resistor was.
Compare that resistor to similar-valued resistors in pedals, where an output cap is connected to a switch.
The path to ground keeps the cap from developing a charge when the switch is open, so that it doesn't pop when the switch is closed.
However, you might argue tens of thousands of Fender amps with bright switches got by fine without that resistor. I personally don't use bright switches, so it's a moot point to me. :laugh:
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I guess some of us are bright enough.
Other's need a little help sometimes :icon_biggrin:
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The path to ground keeps the cap from developing a charge when the switch is open, so that it doesn't pop when the switch is closed.
What charge? There's no DC involved and the signal is only about 100mv.
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What charge? There's no DC involved and the signal is only about 100mv.
You got me! Looks like I made the same mistake the Fender engineer did... No d.c. source to create a charge needing discharge.
I guess some of us are bright enough.
Other's need a little help sometimes :icon_biggrin:
Looks like Sluckey needs to install a bright switch in my head! Time to turn off the "Dumb Switch" :laugh:
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> Time to turn off the "Dumb Switch"
In retrospect, the Dumb Switch was in some CBS junior designer's head (or possibly corporate design policy).
There are many other situations where that cap *would* or could get stray charge, and POP when cut-in.
It is kinda a habit to throw 10Megs off all switched caps.
In DIY stomp-pedals, the cost of 10Meg is less than the "cost" of brain-work to show it isn't needed.
But these amps had some production. Somebody should have been Muntzing-out excess pennies.