Super Reverb

[Joe L]

For the vibrato circuit, replaced the existing treble 500p mica cap with 250p and 10p in parallel, which measured ~270pF on my DMM. I didn't have anything else to get close to 330pF.

Here's where I think I made a mistake -
On both the normal and vibrato channels I measured the slope resistor in circuit at 104K ohms.
In the vibrato circuit, to reduce the slope resistance down to about 100K ohms, I added a 2M ohm resistor in parallel. But surprise, after the addition the slope resistors measure 109K ohms (in circuit).

First off, adding caps in parallel of different values can cause some weird results. I was trying one of Ken Fisher's designs and part of the sound was paralleling two different types of caps feeding the phase inverter. A lot of people in the forums seem to like the results but I found the combination had a comb filtered type sound. Now this is with my ears and my caps. Someone else might have loved the result.

As far as your resistor values acting strangely, one of the traits of carbon composition resistors is heat sensitivity. When you touched that 100k resistor with your iron, you made it drift up in value. A reading of 104K is too close to worry about with tube circuits that call for a 10% tolerance. And looking at the schematic, an in-circuit reading would be accurate due to one side of that resistor having no DC connectivity to anything else in the circuit.

[Steve A.]

First off, adding caps in parallel of different values can cause some weird results. I was trying one of Ken Fisher's designs and part of the sound was paralleling two different types of caps feeding the phase inverter. A lot of people in the forums seem to like the results but I found the combination had a comb filtered type sound. Now this is with my ears and my caps. Someone else might have loved the result.

While experimenting I may tack one or two ceramic disc caps onto the original ceramic cap. When I get the results I was looking for I will replace that hodge-podge of caps with a single cap of the value I liked. I have noticed no difference at all going from multiple caps to a single cap. Another example is the very common boost switch added to a tone stack which adds a cap in parallel to the treble cap. (I am talking about ceramic disc caps usually under 1000pF but sometimes going up to 4700pf.)

As for mixing ceramic caps with mica or mylar cap in certain parts of a circuit it seemed to me that it added together the characteristics of each rather than subtracting them. But I can see how in other parts of the circuit the characteristics might be subtracted from each other.

As you suggest it could have to do with what we each are looking for in the sound and response of a guitar amp. I prefer low gain overdrive sounds- many people might not even consider that to be overdriven but the original signal is distorted in ways I like it so it technically is distortion. However in a high gain amp those characteristics could be a liability not a benefit.

Thanks!

Steve Ahola

P.S. It would be interesting if someone analyzed the KF "tag team" with a scope to look for something that might be causing the comb filter you mentioned.

[Joe L]

Hehe, that is why I said "my ears" Steve. I was farting around one day with a signal generator and headphones and noticed the pitch of a tone was slightly different in my left ear compared to my right. That and above 13k I'm toast.

Getting oldish sucks.

[Steve A.]

Hehe, that is why I said "my ears" Steve. I was farting around one day with a signal generator and headphones and noticed the pitch of a tone was slightly different in my left ear compared to my right.

That would have been the .0022uF cap between the 2nd and 3rd stage? Although the dual-cap arrangement was reportedly seen on some of the Express's the reverse-engineered schematics at the Amp Garage don't show it so perhaps it wasn't that common- maybe other people noticed what you noticed.

FWIW I've heard that the 3rd gain stage with its 10k cathode resistor produces assymetrical distortion which some people like and some people don't.

Steve

P.S. I think that of all the passive components in a guitar amp nothing compares to the heated discussions about caps!

[pbiagi]

Some results -

When you touched that 100k resistor with your iron, you made it drift up in value.

That appears correct - after removing the two 2M resistors in parallel, the original resistor now measures 115K ohms (which before modifications measured 104K ohms).
I went ahead and replaced it with one that measured 103K ohms, however after soldered in, it reads 107K ohms.
I need to replace the tip on my soldering iron - its dialed too hot!

The bass/middle/treble controls appear to work as they should now (good).
However the sound is still raspy. Now I test without reverb which makes the issue more obvious.

Rigged up a patch board and connected the amp to tweed bassman cabinet.
I originally was convinced that the issue was a bad speaker however the bassman cabinet simply makes the bad raspy distortion more obvious.

I noticed that on the rectifier socket, one pin socket has a bent side. The tube pin is not making a good solid connection.
Could this be the cause of the raspy sound?
In any case I'll have to order a new socket.. (Tried reforming the connectors but looks like it's a goner.) It'll be another week or so before progress on that.

Haven't had a chance to measure voltages but I'm fairly certain I did a few months ago and everything was inline with expectations.
Will do again by this weekend..

[edit]

Are you working on an eyelet board, a turret board or a pcb?

Eyelet board - this is a super reverb from 1964, real one.. However the electrolytics and a few other capacitors have already been changed out by someone before I got the amp..

[pbiagi]

To let readers know how this all ended..

The rectifier socket change out obviously helped hold the tube but did nothing for the raspy sound, with amp giving less volume, which seemed to get progressively worse..

Measured all the voltages and found the PI plates way out while the rest of the tubes measured very close to spec.
I didn't keep the readings but pin 1 was ~60V but should be 230V.
The 82K plate resistor gave total resistance, as in no circuit connection, dead. I imagine over the years it was connecting on/off with voltage surges, which was giving the amp a sort annoying knocking effect. (I could be wrong about that, but is what I imagine.)
Anyway, replacing the resistor did the trick and the amp sounds wonderful, better than ever since I bought it, like it should and as good as others I've liked so well.

Steve, On the negative feedback loop I added in series the 25K linear pot, replacing the extra speaker jack. Fits there nicely.
Works great and livens up at low volume, even set at low value. Haven't yet had a chance to play the amp out, and live in small apt., so no chance to turn up yet.
Question: When set to 0 (the neg. feedback pot), the amp is very quiet. But as soon as it's turned past 1, there's a small amount of white noise or hiss introduced. The hiss gets a little louder when turned to 10, but not much. Overall the hiss - like tape hiss - is so quiet it doesn't really bother me, and I can certainly live with it.
However I wanted to ask if there is something to look into to reduce or remove it.?

Thanks for all the advice, great forum!

P.S. This is a real vintage blackface amp however the few mods are easy to change back and I've kept all vintage parts.

[soundguruman]

To let readers know how this all ended..

The rectifier socket change out obviously helped hold the tube but did nothing for the raspy sound, with amp giving less volume, which seemed to get progressively worse..

Measured all the voltages and found the PI plates way out while the rest of the tubes measured very close to spec.
I didn't keep the readings but pin 1 was ~60V but should be 230V.
The 82K plate resistor gave total resistance, as in no circuit connection, dead. I imagine over the years it was connecting on/off with voltage surges, which was giving the amp a sort annoying knocking effect. (I could be wrong about that, but is what I imagine.)
Anyway, replacing the resistor did the trick and the amp sounds wonderful, better than ever since I bought it, like it should and as good as others I've liked so well.

Steve, On the negative feedback loop I added in series the 25K linear pot, replacing the extra speaker jack. Fits there nicely.
Works great and livens up at low volume, even set at low value. Haven't yet had a chance to play the amp out, and live in small apt., so no chance to turn up yet.
Question: When set to 0 (the neg. feedback pot), the amp is very quiet. But as soon as it's turned past 1, there's a small amount of white noise or hiss introduced. The hiss gets a little louder when turned to 10, but not much. Overall the hiss - like tape hiss - is so quiet it doesn't really bother me, and I can certainly live with it.
However I wanted to ask if there is something to look into to reduce or remove it.?

Thanks for all the advice, great forum!

P.S. This is a real vintage blackface amp however the few mods are easy to change back and I've kept all vintage parts.

a 7pf 1000V cap between plate and grid of V1A will stop the hissssssssssssssssssssssssssssssssss. Used by Fender and Marshall on factory amps.
Quite successful, done it many times.

[isaac]

Have fun with it.
(Fender Authorized Technician, for Life)

If you notice he said these little pieces of advice came from a (more than likely incompetent) fender tech, it makes more sense and we can just laugh about it... :-) Although, I once emailed Magnatone, and I got an email back from Obeid Khan, and he is a great amp guy, so Magnatone has a good email tech. ;-) Couple people I know have talked to Marshall, I'm not sure of their satisfaction with the service though. The one I saw most recently was from Marshall's chief tech (or whatever he is called). lol

[blearyeyes]

I would like to know first
What imaginary privilege do you have to attack, berate and swear at other members of this forum?
which I may add, you do pretty often...

I am under no obligation to acquiesce, concerning your bullying.

Concerning the fact that others, with musically trained ears, hear or detect musical qualities that you don't hear.
(or for that matter, can't hear)
I think essentially what I may be inferring is that you are not a musician, and that you are tone-deaf (maybe).
And that you wish to assert your superiority to others by wholly rejecting the observations of others more musically qualified than yourself.

My first recommendation is that you sign up for guitar lessons, or ear training.

I still want to know how it works. If you don't know just say so. No big deal.
Enzo just isn't a diplomat.

What he should have said is: "How can that possibly be so? It seems as if it is utterly absurd but I acquiesce to your deep understanding of such things. Help me understand how it is possible please."

So instead of this personal attack perhaps some empirical science is needed to back up your highly trained ears?

[Wombaticus]

a 7pf 1000V cap between plate and grid of V1A will stop the hissssssssssssssssssssssssssssssssss. Used by Fender and Marshall on factory amps.
Quite successful, done it many times.

You're better off adding a (larger -- maybe a couple hundred pf -- since it won't be multiplied by the Miller effect) capacitance between grid and cathode or grid and ground. That way there's no risk of coupling in power supply noise. This trick works best in conjunction with reducing the size of the grid resistor to around 10K or so, since the Johnson noise from that resistor is the major source of hiss in most amps. For my own amp builds I now usually use a 10K/220pF combo for setting the input low-pass, although the high gain guys I've worked with prefer more roll off, and often opt for 10K/500pF or even 10K/680pF. That's a bit much for me, but it works for that sound.