Magnatone Custom M10 - REVERB

[TelRay]

Hello all,

fixed this one with your help after it arrived to "the family" about a year ago in very bad shape (no POWER TRANSFORMER, no REVERB TANK, etc)
I wanted your help to understand the REVERB circuit and see how could I increase the WET SIGNAL's TREBLE
Firstly, none of the components in the REVERB CIRCUIT have been replaced, so I would gladly take your advice about checking existing (old 1964) capacitor and resistors values.

This is what I see:

- there's a 12AU7 TUBE that is driving the REVERB TANK
- the REVERB TANK is a NEW MOD, 4FB3A1B, Long Decay, 2-Spring
- the RECOVERY is made by a 2N2614 TRANSISTOR

I believe the REVERB is STRONG enough, I just think it's bit too DULL

SIMPLIFIED detail of the REVERB CIRCUIT



FULL SCHEMATICS: https://www.magnatoneamps.com/schema...natone_M10.pdf

Thank you in advance

[Helmholtz]

- the REVERB TANK is a NEW MOD, 4FB3A1B, Long Decay, 2-Spring

Are you sure the tank is a correct replacement? I think a 4FAXXXX would suit the low impedance recovery circuit better.

Anyway, with the given tank you might try to

1) increase the value of the 5k reverb pot to maybe 100k and wire a 2.2nF across the output transducer.
2) wire a 1nF or 2.2nF cap across the 33k rev mixing resistor.

And don't forget: Circuit redesigns may have unexpected side effects.

[TelRay]

thank you as always for your time and valuable suggestions Hr Helmholtz!

Are you sure the tank is a correct replacement? I think a 4FAXXXX would suit the low impedance recovery circuit better.

The suggestion of the 4FBxxx tank came from the discussion on the Magnatone Custom M10 repair post #21 https://music-electronics-forum.com/...860#post791860
I think I have currently enough REVERB in terms of volume and it's also long enough (maybe even too long)
What would be the differences heard by switching a 4FB tank for a 4FA?

Anyway, with the given tank you might try to

1) increase the value of the 5k reverb pot to maybe 100k and wire a 2.2nF across the output transducer.
2) wire a 1nF or 2.2nF cap across the 33k rev mixing resistor.

And don't forget: Circuit redesigns may have unexpected side effects.

thank you sir for the suggestions, i will try to understand so that i can also benefit from your reply to increase my knowledge

5K REVERB POT the schematic shows it as a DOUBLE POT (in lack of a better term), marked as "1" in the photo below and this is a bit confusing to me and in case I need to change it for a new 100K pot I'd need to understand what kind of pot this is
will changing the pot only affect reverb volume (the amount of wet reverb)?

ADDED CAPACITORS what is the effect produced by each one and why in those locations?

4 uF NON POLARIZED CAPACITOR (marked as "2") what is this acting as a High Pass Filter? I've found 4.7 uF non polarized electrolytics as the closest value for replacement. or should I go for a 4 uF film cap (exact value and 10x more expensive)?




thank in advance, sir

[Helmholtz]

What would be the differences heard by switching a 4FB tank for a 4FA?

The 4FB has an output impedance of 2.25k, the 4FA has 500 Ohm. Both at 1kHz.
As a general rule in audio, the load impedance should be at least 10 times the source impedance to not unnecessarily load down the source.
As the reverb transducer is essentially inductive, it's impedance roughly increases proportional with frequency. So the loading increases with frequency.
I would expect the loading effect to be similar to connecting a guitar PU to a low impedance amp input.
Fender typically uses a 220k input impedance with their 4AB tanks.

ADDED CAPACITORS what is the effect produced by each one and why in those locations?

A suitable cap in parallel with the rev output transducer forms a parallel resonant circuit which can increase treble response if correctly tuned.

A cap across the 33k mixing resistor results in a high pass filter.

4 uF NON POLARIZED CAPACITOR (marked as "2") what is this acting as a High Pass Filter? I've found 4.7 uF non polarized electrolytics as the closest value for replacement. or should I go for a 4 uF film cap (exact value and 10x more expensive)?

The 4µF coupling cap is terminated by the 56k input impedance/resistance of the recovery stage. The resulting bass cutoff frequency is 0.7Hz, so no high pass filter effect.
(BTW, less bass doesn't generally mean more treble.)

[g1]

I think part of the reason that tank was recommended was practical. 'A' variant output impedance is almost impossible to find.
I don't think I've ever come across a tank that was anything other than B or C output impedance. If you were going into production, they would probably make some for you.

[TelRay]

thank you gentlemen,

the following questions are a consequence of my lack of knowledge and pursue of enlightenment

REVERB TANK IMPEDANCE:
understood, we bough 4FBxxx because 4FAxxx wasn't available and not because it was the right output impedance match (the input is OK)
What makes for the impedance, is it the transducers used in the tank? if yes, can them be "easily" swapped by more adequate ones?
just a thought

5K REVERB POT the schematic shows it as a DOUBLE POT (in lack of a better term), marked as "1" in the photo below and this is a bit confusing to me and in case I need to change it for a new 100K pot I'd need to understand what kind of pot this is
will changing the pot only affect reverb volume (the amount of wet reverb)?

ADDED CAPACITORS in order to prioritize I think I will
1. add the 2.2 nF capacitor across the reverb tank output (as I have understood this will "create" treble)
2. depending on how "1." goes, add the 1nF cap across the 33K resistor (this will create a high pass filter. so, maybe, by reducing the low frequencies I would need more "wet" signal feed to the recovery stage to keep the amount of reverb, maybe this is what the change of REVERB POT from 5K to 100K is doing)

Should I use MULTI LAYER CERAMIC DISC caps or go for FILM (WIMA)?
would 100 VDC an appropriate rating?

4 uF NON POLARIZED CAPACITOR (marked as "2") I've found 4.7 uF non polarized electrolytics as the closest value for replacement. or should I go for a 4 uF film cap (exact value and 10x more expensive)?
I think that by using a 4.7 uF I will be lowering the bass cutoff frequency from 0.7Hz to 0.6 Hz. Using a non polarized electrolytic of 3.3 uF (the other available size) would change to 0.86 Hz.
I'm thinking the 3.3 uF would be fine, WDYT?

thank you!!!

[Helmholtz]

ADDED CAPACITORS in order to prioritize I think I will
1. add the 2.2 nF capacitor across the reverb tank output (as I have understood this will "create" treble)
2. depending on how "1." goes, add the 1nF cap across the 33K resistor (this will create a high pass filter. so, maybe, by reducing the low frequencies I would need more "wet" signal feed to the recovery stage to keep the amount of reverb, maybe this is what the change of REVERB POT from 5K to 100K is doing)

Should I use MULTI LAYER CERAMIC DISC caps or go for FILM (WIMA)?
would 100 VDC an appropriate rating?

1) The cap across the rev output transducer may improve treble response, but I think with the 5k pot its effect will be insignificant.
2) The cap across the 33k resistor will increase treble response (add treble) independent of other changes. Try this first. If it's sufficient - fine.

Cap technology (ceramic, film etc.) doesn't matter here. Operating voltages are very low, probably around 1V or so, so voltage rating can be anything.

4 uF NON POLARIZED CAPACITOR (marked as "2") I've found 4.7 uF non polarized electrolytics as the closest value for replacement. or should I go for a 4 uF film cap (exact value and 10x more expensive)?
I think that by using a 4.7 uF I will be lowering the bass cutoff frequency from 0.7Hz to 0.6 Hz. Using a non polarized electrolytic of 3.3 uF (the other available size) would change to 0.86 Hz.
I'm thinking the 3.3 uF would be fine, WDYT?

Why do you want to replace the 4.7µ cap? If it's bad, use anything between 1µ and 10µ, independent of technology.
If you want to reduce bass response, you won't notice a difference until the cap value is lower by a factor of 10 or more.
But lowering bass response this way would affect both dry and rev signals.

And - as said before - less bass doesn't result in more treble.

And don't forget, the lowest frequency of a (standard tuned) guitar is 82Hz. So any changes in frequency response at lower frequencies should be inaudible.

[TelRay]

thank you Helmoltz!

clearer for me now:

1. add the1.0 or 2.2 nF cap across the 33K resistor
2. if that's not enough add the 2.2 nF cap across the transducer and change to a 100K potentiometer

I want the change the existing 4 uF 15V capacitor because I believe it's a 56 years old non polarized electrolytic (black cylinder with +++ on both ends)

thx!

[trobbins]

Is the dry signal from the vibrato channel ok for you? Do you notice any difference between channels 1 & 2 when vibrato and reverb are disabled on channel 1, and both channels have the same settings ?

[TelRay]

hi trobbins and thank you for jumping in

I have not compared the two channels lately but I remember this is something I have done in the past with an A/B Pedal Switch. I recall making them sound similar but cannot assure the settings were the same.
In any case, CHANNEL 1 had an odd component which was a .001 uF GLASS CAPACITOR which seemed to be coming from a previous repair. I have recently replaced it along with a couple of components in that channel.

I'm curious about why are you asking



My intention is to continue replacing the components of CH 1 with new ones and leave CH 2 as is

[Helmholtz]

That "GLASS CAPACITOR" is actually a polystyrene ("Styroflex") capacitor. These have excellent electrical properties.

[TelRay]

That "GLASS CAPACITOR" is actually a polystyrene ("Styroflex") capacitor. These have excellent electrical properties.

I'll keep it in the "museum-of-old-part-taken-out-of-vintage-amps"?

[Helmholtz]

I'll keep it in the "museum-of-old-part-taken-out-of-vintage-amps"?

They're typically very reliable and precise (quite the contrary of ceramic disks). They just don't like high (soldering) temperature.

So if works, no reason not to use it somewhere. The HiFi guys love them.
Red ring means 160V.

[TelRay]

in that case i’ll “treasure” it
i’ve read about them and thought it was more a matter of industrial reliability than good sound (i know hifi guys go crazy about Wima and orange drops)
thank you for adding to my knowledge, sir

[trobbins]

I was just asking in case there was something before the reverb stage that may be attenuating treble more than expected. I don't have roll-off measurements for my maggie clone, but if the triode grid following each vibrato stage has increased leakage then that could change the frequency response from the vibrato stage as the frequency moves away from the nominal 1kHz mid-band response (when no vibrato phase shift occurs from the special varistors).

Wrt the styro caps, our locally made vintage styro's are very reliable and remain accurate in value if the 'glass' encapsulation is not cracked. I would always keep them in an amp, although I think the hifi aspect is about aestetics, but underlying that is that they are just a good 'invisible' capacitor.