Vibro Champ bypass cap recommendations

Hi friends,

1966 AA764 Vibro Champ.

Stock cathode bypass on V1A is 1k5 - 25/25 and V1B is 1k5 - 10/25

Only other mods are that I've reduced the NFB (20k in place of stock 2k7)

Currently has a 1950's vintage 8" alnico, NOS 6V6 and 5Y3, and Telefunkens in V1 and V2

Wondering what some of your various alternative recommendations are, with regard to setting the LF knee.

My goal is tightening up the bass and not throwing away so much power on useless things...yet without losing too much bottom. The amp is mostly played through the 8" speaker.

I'm at 1k5 / 2.2 on both of them now. It does sound pretty darned good, but MAYBE it seems like I took too much of a bite out of the bass - when I play a Tele through it.

I think it would probably be perfectly voiced for a Les Paul which, we all know, is the devil's guitar...

Questions:

1. double both of them? (4.4)
2. go back to 10 in V1B, leave V1A at 2.2
2. leave V1B at 2.2 and go with 10 or 20 in V1A

Your thoughts and experience are appreciated.


Cheers,

CBG

My first step would be to increase the filtering at the first stage to 30mfds. This will tighten up the low freqs. IF that doesnt' do enough for your purposes, then consider reconsidering that NFB loop resistance. That change you have made to the NFB loop resistance has increased the likelihood of 'flubbing' out in the output section, I would think.
The current draw is of interest here, too, ime. ONE can either try different
6V6's to find a different current draw....or change the bias resistor to a higher value....and then use the same spec'd 6V6 from then on in either of those scenarios, right? ORder a matched duo or quad and you are set for a long time.
IF you want to do a simple experiment as to what happens if you decrease the gain in the preamp...whcih is what happens when you increase that bypass resistance, right? .....you could simply put in a 5751 or a 12ay7 in V1.
I also would not be afraid to decrease the bass and mid tone stack capacitances if nothing else got me where I wanted to go.

Al of that said, the biggest change in the firmness and power of the low end in this type of Champ/VibroChamp circuit that I have heard is a speaker change. I have a vintage JEnsen P8RS that makes one of these amps get a lot 'bigger' with much more definiton in the low end. Thsi speaker's
Alnico magnet is twice the size of the 'normal' Alnico magnets in Champ/VC amps' speakers.

I would say keep the first stage at 2.2uF and change the other bypass back. Why not just try one and then if you need to do more just go back? Adjust till you are happy, not to say our opinions here are not worth gold.

Hi Wally,

Thanks for the great advice. I was fairly cautious about the NFB change, but with the NFB as it is now (the "tweed" champ 22k value) the amp is doing 300Hz at 5.8 watts at the onset of clipping. That seems about as good as I'm going to get out of a champ OPT and a 6V6.

I monitored the performance of the power supply with some impulse inputs. V1 and V2 voltage is rock solid. V3 showed only VERY slight sag, maybe 3-5 volts when hit with white noise impulse making 5 watts on the output.

I'm running the stock value 470ohm Cathode resistor, but 3w resistor, for reliability reasons. The original was 100 ohms out of spec.,(too high 540 ohms.) Getting the right resistor in there helped.

From my calculations, the V1 bypass capacitance change raises the frequency of the cutoff "knee". 2.2uF shows down 3db at 50 Hz., which seemed plenty safe to me.

I'm right with you on the Speaker. I have an early "big alnico" 8" which I had already substituted for the stock speaker. You are right - Much bigger tone.

I guess I was reluctant to increase capacitance on the 5Y3...thinking that the stock level is about max recommended. Have you gotten away with 30uF on B+1 on this design?

Thanks for the advice. I'll pick away at it some more tonight.


Cheers,

CBG

You do not have any more nfb actually, it's an open loop.
I would use 25 µf (for hum rejection) on V1A and 1µF on V1B as a starting point.

Yes, as I said in post 1 "I have REDUCED the NFB". Not to split hairs, but it's not "open loop" either, to be precise I have far less NFB that a stock VC. The stock 2k7 resistor is a voltage divider with the 47 ohm resistor at the junction of the NFB circuit and the elevated ground point of the cathode bypass for V1B. Increasing the "top" resistor in that divider network to 22k means that the NFB is reduced, but not totally eliminated.

25/1 is the stock "AA764 Champ" (not VIBRO Champ) value. Already been there. Not sure how going with 25uF for the cathode bypass capacitor is going to reduce hum, since going bigger there lowers the knee of the cutoff of the cathode bypass?

CBG

Because the bias point and the gain of tubes are varying with the hum, and it can be heard with the first gain stage where signal is low if the corner is not low enough to ground the hum.Less importance later in the circuit.
I did not calculate the exact nfb in this circuit, but most of the time in guitar amp it's only a few db, and you divided it by 8...

http://www.ampbooks.com/home/classic-circuits/fender-champ-5e1-negative-feedback/

Yes, I follow you conceptually, kleuck - but putting a 25uF cathode bypass cap on V1A LOWERS the cutoff frequency, not raises it, as compared to a 2.2uF which is what I currently have in there.

You did say "put a 25 uF in V1A and a 1 in V1B" in your post...right? Or did I misread it?


Cheers,

CBG

Just a shot in the dark, but how about a 220uf for the power tube cathode resistor to increase the bottom end,. On the PS filtering I have had great results with 40/20/20. On V1A&B cathodes try 2.7K/25uf combo.

Yes, I follow you conceptually, kleuck - but putting a 25uF cathode bypass cap on V1A LOWERS the cutoff frequency, not raises it, as compared to a 2.2uF which is what I currently have in there.

You did say "put a 25 uF in V1A and a 1 in V1B" in your post...right? Or did I misread it?


Yes, lower FC, and steadier voltage/bias/gain, i think reducing the second triode cap to 1µf is enough for your goal -did this way for my VHT SP6, along with a smaller cap on the input of the 6V6.
Every amp is different though, but in another one (more complex push-pull) i had good results with the stock 47µf cap on V1A, and 2,2µf for V1B & V2A.
Just like you, i do not like to waste power, nor muddy useless lows.
But my amps have 10" speakers, don't know what a 8 can give.

CBG, IIRC, the max spec for that 5Y3 is 30mfd. some folks also go to a 5V4 in these amps.
REgarding the 470 ohm bias resistor and current draw...it is interesting to get outside of that stock value....or get an array of 6V6's that yield different levels of current draw in the circuit....chagne the biasing, right? Changing to a 5V4 causes a rebiasing situation, too, as I am sure you know.
AS for controlling the low end beyond..or prior to....making any of these changes, I wil offer another solution. I have a home-built 'strat' that has a bass cut tone control in addition to the normal treble cut. The bass cut comes in really handy when one wants to push the amp hard and yet avoid that flatulent low end. Thsi approach to controlling the input signal structure allows one to have a richer low end in 'clean' situations and yet have the amp under control when pushed, ime. In other words, if I have an amp that is 'falling pieces' due to overdrive--excessive overdrive imho--when pushed, I can adapt instantly with the gutiar's controls.

There are many roads leading to Rome, right?

Generally, restrict the FR on the early stages and let it open up in later stages. That keeps the bass from overpowering later stages, and keeps the treble from getting excessive as distortion harmonics build up. So if you want to put some bass back in, do it in the second stage.

Also try playing with some psychoacoustics. Restrict the input stage to 3-5khz and see if you don't perceive an improvement in the bass.

So if you want to put some bass back in, do it in the second stage.

Don't agree, because of the hum rejection first, second cause in last stages, signal is of high amplitude, and can cause blocking distortion, particularly with lows, so the best place to cut them is not the first stage.

1966 AA764 Vibro Champ.

It does sound pretty darned good, but MAYBE it seems like I took too much of a bite out of the bass

Umm, if it sounds really good, stop tweaking & play yer guitar!

I know, I do the same thing. Sometimes I have to be reminded to let the iron cool off & get to know my amp.

Don't agree, because of the hum rejection first, second cause in last stages, signal is of high amplitude, and can cause blocking distortion, particularly with lows, so the best place to cut them is not the first stage.

Do What you want, think what you want, but if you control the hum correctly, then you don't have to compromise your signal path to control it. fix the problem at its root, don't just put a bandage on it

That's not a bandage, and you can't always avoid issues or the tone shaping you want with a single trick, but rather you have to fine tune each stage.
And it's (hum rejection and blocking distortion issues) not my own advice only, but Merlin Belcove's one too, and my pro tube amp tech friend's too.

However, where possible the
first stage of an amplifier should always be fully bypassed so as to maximise
rejection of heater and power supply hum.

A further problem with using a large cathode bypass capacitor is that the
stage may have too much gain at low frequencies. If the stage begins to overdrive the
following stage, too much low frequency content can lead to blocking distortion [see
chapter 2], which is most undesirable.

http://valvewizard1.webs.com/Common_Gain_Stage.pdf

Yeah, I am one of Merlin's proof readers. and my statements still stand.

O.K.

Wally, I get your point about managing the e.q. at the input stage. I definitely do that with a vengeance in my rig... so I'm in total agreement that this is a great and oft overlooked trick to getting great tone. Also, I'm going to add 10uF on B+ #1, per your suggestion. Can't hurt, and I can definitely see a bit of a sag there with 20uF on it.


No offense, Kleuck - appreciate your engaging my question too, but I'm going to have to side with Celeste on this one. If hum is present, then fixing it at the power supply is job 1. Second, I'm trying to follow you here, but try as I might I'm just not getting how RAISING the minimum cutoff frequency of the cathode RC in V1A is going to make it and the following gainstages MORE susceptible to power supply hum OR blocking distortion. That seems opposite to the physics of the thing?

Maybe I'm missing something really obvious here, but it seems beyond obvious that the EARLIER in a cascaded class A voltage amplifier design that you restrict the LF with an RC filter on the cathode, the less possibility there is of LF (including 120Hz power supply hum) ever getting in there in the first place.

I'm open to learning where I'm missing it, though.

Cheers,

CBG

O.K.

Wally, I get your point about managing the e.q. at the input stage. I definitely do that with a vengeance in my rig... so I'm in total agreement that this is a great and oft overlooked trick to getting great tone. Also, I'm going to add 10uF on B+ #1, per your suggestion. Can't hurt, and I can definitely see a bit of a sag there with 20uF on it.

No offense, Kleuck - appreciate your engaging my question too, but I'm going to have to side with Celeste on this one. If hum is present, then fixing it at the power supply is job 1. Second, I'm trying to follow you here, but try as I might I'm just not getting how RAISING the minimum cutoff frequency of the cathode RC in V1A is going to make it and the following gainstages MORE susceptible to power supply hum OR blocking distortion. That seems opposite to the physics of the thing?



First i must say i tried both ways in two amps, and finally found that i prefer the sound of a later filtering (both amps used with power tube distortion, one -a PP- was prone to a harsh distortion before tuning) and in the case of the VHT, lowering the first triode cap lent to a slightly more audible hum (that was before i put a choke, did not try after that)

Why : because we are not talking about the hum that your axe picks up, which of course would be attenuated with a higher FC, but the residual from the power supply, varying a little the bias point of the triode by varying the voltage on the cathode and plate where the signal is at it's lowest.
With a lower FC, the 50/60 hz cycle is "grounded through the cap" and has no more effect on the bias point.
If you raise the fc over 50/60 hz, the bias point can be slightly "floating" at 50 or 60 hz (excuse me if i'm not clear, my English is not so good)

For the blocking distortion, i suppose that the exact place where you cut lows doesn't matter.

If you're not after sag, i'd recommend you modify the power supply for the use of a choke, they do wonder in this case.