Help with 'bright cap mod'

[swleamon]

I have a '66 Deluxe Reverb which I absolutely adore. However I'm not a fan of the high-end fizz when using OD pedals at low volumes on the Vibe/Reverb channels.

Many have suggested simply clipping the bright cap on the back of the volume pot. I'd like to keep this option as I do love the sparkly tone of this channel at lower (practice) levels.

I'm thinking the best solution would be installing a pull-pot, allowing me to toggle between no-cap and cap.

Can anyone knowledgable of blackface Deluxe's please advice on what particular components (pot type) I may need for this?

A friend (with experience working on amps) is going to help me with the mod next week. I'd like to have everything ready for him prior to helping.

Also, if anyone knows of a better option, please let me know! I thought about a switch but there doesn't appear to be room, plus it would take considerably more work, I imagine.

Thanks in advance.

[CoolBlueGlow]

Hi swleamon,

But why install another switch? The "bright" switch on the front of your amp already bypasses that bright cap when in the "down" position. That bright cap only passes the HF around the volume controls at low volumes when the switch is "up"

If your amp is too "fizzy" and/or you want a darker sound at low volumes, and you are willing to install a push/pull pot somewhere, then I suggest installing a snubber cap across the 100/82k resistors at the tail end of the Schmidt phase inverter. wire it series to your switch and you're in business. (check the schematics - Original 1963 vibroverbs have a 100pf cap here.) Show your tech the schematic. Ask him to wire the switched snubber there, and try starting with a 680pf cap. I like silver micas for this application.

just my 2c

[CoolBlueGlow]

p.s. that cap needs to be rated at at least 400vdc.

[CoolBlueGlow]

Oh... never mind you said DELUXE Reverb. Sorry, I've been working on a Twin Reverb all day and I have bright switch on the brain. Yeah, sure, put in a push pull to give you a bright switch.

Personally, when I do this, I DON'T put the push-pull on the volume control. I pick a less used control like the "bass" e.q. on the normal channel, so that I'm not futzing around with the amp volume setting when I'm playing.

[swleamon]

Originally Posted by CoolBlueGlow

Oh... never mind you said DELUXE Reverb. Sorry, I've been working on a Twin Reverb all day and I have bright switch on the brain. Yeah, sure, put in a push pull to give you a bright switch.

Personally, when I do this, I DON'T put the push-pull on the volume control. I pick a less used control like the "bass" e.q. on the normal channel, so that I'm not futzing around with the amp volume setting when I'm playing.

No problem. At first I was like ... "If I had a switch I wouldn't...."

Good idea about using a different pot. One of the trem pots would be perfect!

So, please pardon my ignorance, but how would I go about this mod? I saw on another forum that I would need this:

AES part # R-VC1MA-PP, a 1 meg audio taper pot with push pull switch

If I'm using something other than volume I assume that this would complicate the mod more than I anticipates.

Thanks again.

[SoK66]

Just use a push/pull pot on the volume control for the Vibrato channel. Fender does this on the Pro Tube amps, works fine.

[zook]

Originally Posted by CoolBlueGlow

p.s. that cap needs to be rated at at least 400vdc.

There's very little voltage at that stage of the amp, you could get away with much less than that, although most caps that would be used there are rated higher.

Winnie

[zacmac]

Originally Posted by swleamon

I have a '66 Deluxe Reverb which I absolutely adore. However I'm not a fan of the high-end fizz when using OD pedals at low volumes on the Vibe/Reverb channels.

Many have suggested simply clipping the bright cap on the back of the volume pot. I'd like to keep this option as I do love the sparkly tone of this channel at lower (practice) levels.

I'm thinking the best solution would be installing a pull-pot, allowing me to toggle between no-cap and cap.

Can anyone knowledgable of blackface Deluxe's please advice on what particular components (pot type) I may need for this?

A friend (with experience working on amps) is going to help me with the mod next week. I'd like to have everything ready for him prior to helping.

Also, if anyone knows of a better option, please let me know! I thought about a switch but there doesn't appear to be room, plus it would take considerably more work, I imagine.

Thanks in advance.

I have the same issue with my 66' that my 74 SF DR did not have. I had thought about unscrewing the second input jack and sticking it back in the chassis (to simplify reinstallation if i desire to sell) and putting a switch there similar to the power switch on the rear of the amp.

After installing a JBL D120F in it, it is tough to bear with the treble much further than 2.5.

[RedRock]

1. You could add a series carbon comp resistor, 1/2 watt, to the bright cap
to pad it down. Try 220k or 360k.

2. Going into pin 7 of tube V4 is a 3.3m resistor with a 10pf across it.
You can clip a leg of this cap or pad it with a series resistor (470k or 1m).

3. Any combination of the above.

[Marky D.]

I installed the push-pull pot in the Reverb channel of my 65 last week. I got mine from Hoffman Amps. Make sure you get one with a solid shaft or it will wobble! I also installed a switch pot in the Vibrato Intensity spot to remove the vibrato from the circuit. With both of these mods going, I noticed more overall noise, so I unhooked 'em and it's back to normal.

Perhaps some attention needs to be paid to grounding when this is done. Ideas, anyone? Do you have a diagram you're referring to for the install? I downloaded one from MOJO, I think it was by Dan Torres.

1. You could add a series carbon comp resistor, 1/2 watt, to the bright cap
to pad it down. Try 220k or 360k.

2. Going into pin 7 of tube V4 is a 3.3m resistor with a 10pf across it.
You can clip a leg of this cap or pad it with a series resistor (470k or 1m).

Redrock: can you please explain what do these mods do?

[RedRock]

They roll off (decrease) the treble response of the amp a bit more or less,
depending on which you choose to do. They are reversible. You can
install them, and take them out if you want a different sound.

[bradpdx]

Originally Posted by Marky D.

I installed the push-pull pot in the Reverb channel of my 65 last week. Perhaps some attention needs to be paid to grounding when this is done. Ideas, anyone?

Redrock: can you please explain what do these mods do?

I've used a pull pot on the volume control of my 65 DR forever, it simply connects or disconnects the bright cap. No ground paths involved. Yanking the tremolo should likewise be easy and carry no penalty; simply have the switch disconnect the Intensity pot from the input to the phase splitter. To avoid "pops" you should reestablish a ground reference such as a 1 Meg resistor at the point where the Intensity pot is connected to the phase splitter. This will raise gain, which means more noise along with more signal.

Redrocks mods reduce the amount of treble boost added by the bright cap.

[Wally]

YOu might want to repalce the ceramic treble tone caps in with some silver micas.....this does smooth out the high end, ime....much more musical.
I also have to ask....at what setting are you putting your tone controsl when using this OD pedal. I ask because tone controls are gain controls. You are hitting the preamp with an increased gain situation....and this affects what goes on in that preamp relative to the gain in those tone stacks. IN other word, I would think about this aspect in addition to considering any mods to the amp. There may need to be some balancing act between the OD gain and the channel controls??

[swleamon]

Originally Posted by Wally

YOu might want to repalce the ceramic treble tone caps in with some silver micas.....this does smooth out the high end, ime....much more musical.
I also have to ask....at what setting are you putting your tone controsl when using this OD pedal. I ask because tone controls are gain controls. You are hitting the preamp with an increased gain situation....and this affects what goes on in that preamp relative to the gain in those tone stacks. IN other word, I would think about this aspect in addition to considering any mods to the amp. There may need to be some balancing act between the OD gain and the channel controls??

A few weeks ago I finally took my DR to the local tube amp guru ( Kye Kennedy ). That is exactly what he did with the bright cap. Sounds great! Highs are now more refined. And he also modded the Normal Channel. I now used an a/b switch with 2 discrete effects chains. Here is his description of exactly what he did.

"The Normal channel treble,middle and bass caps were changed,as well as the slope resistor was changed to 56k. You have 1k 5watt screen grid resistors which are more reliable and increase tube life without affecting tone. The Reverb channel is stock with a dipped mica bright cap and flameproof 1% plate load resistors."

[Wally]

swleamon, that is part of what I would have done for that amp. There is at least one more trick to do vis-a-vis the two channels---separate the cathodes and do some gain modification as well as the tone mods to the NOrm Ch, and there are tricks on down the line that are not invasive and can allow more versatility.
Kidos on getting a tech to give yiou more amp.

[jefrs]

I have an Epi BC30 which is Yet Another Bassman clone.

This too suffered from the over-bright response, and the usual mod is to lift the "bright cap".

However I found another simple solution to "open-up" the sound of the amp. The Epi BC30 has two 471 bright caps (for each channel), each in parallel with a 1Meg resistor. My solution was to fit a 20k resistor in parallel to the 1Meg (piggy-back), this bypasses (shunt) the bright cap, by dropping the shunt value to 19k6. I determined this value by experiment, ymmv. This alters the CR tone shaping which lifting the cap negates, and it allows more of the signal through, as on this amp it then feeds a voltage divider (in the form of the 1M vol pots for each channel).
I think this is the equivalent to shunting the 3M3/10pF (V4a) with around 68k, but you would have to experiment there, and it would only apply to the "Vibrato" channel.

However, looking at the circuit of the DR AB763 and AB868, I am not sure which cap you are referring to as the "bright cap". The circuits are quite different and you cannot shunt a DC-blocking cap with a resistor.

The caps in the T-bridge should not be ceramic, please ...
- they change value with the weather, they're for DC-blocking where the value doesn't matter.
- changing value of the treble cap is interactive with the slope resistor and the mid/bass caps, try modelling the effect with <name of program I cannot remember because I've apparently deleted it >
Tone Stack Calculator here http://www.duncanamps.com/tsc/