Schematic discrepancy in Matchless Spitfire output section

[Potato Cakes]

Hello, everyone,

I in the process of reworking some amp builds and one of them is a Matchless Spitfire. Going over the schematics I discovered that there is a different cathode bypass capacitor for the output tubes used between the hand drawn and reworked versions that I have. The hand drawn has a value of 250uF and the reworked one has 25uF. I have not seen a value over 25uF used in this position before so I looked at some other schematics from Matchless and found that the Lightning uses a 220uF in that same position. I do not know if this difference has to do with the version being either 115V or 230V mains, but I don't see why that would affect the cathode bypass capacitor if the voltages on the secondaries are the same. Has anyone here used values this high for this position in their amp builds?

Thanks!

Paul

 

[Tubetec]

Its almost identical to a Marshall 20W ,
they use a 50uF ,
Let it be a matter of personal choice rather than worrying about right or wrong value ,
I wouldnt expect to hear much difference at low to medium volumes , when you crank it up the low end should hold together a bit better with the larger value cap

 

[Tubetec]

Note how the phase invertor is wired in the Marshall , I dont like it
I changed mine to standard long tailed pair by adding the two extra resistors,
made a big difference how the amp handles the peaks .

 

[Accelerator]

Hi Paul @ Potato Cakes and greetings from Greece.

A bigger Cathode Capacitor value gives you better bass response (as a filter with the Cathode Resistor) and more gain aka bigger Power Tube Saturation – Distortion if something like this is desirable.

The Matchless Amps (Chieftan, Clubman 35, DC30, Hurricane, Lightning 15, Spitfire, Tornado) use 250μF / 50 Volts Cathode Capacitor as standard.

Orange AD15 & AD30 amps use 100μF / 50 Volts Cathode Capacitor as standard.

Marshall 1974X 18watt and 2022 20watt amps use 50μF / 50 Volts Cathode Capacitor as standard.

Vox AC15 amps use 50μF / 50 Volts Cathode Capacitor as standard.

Vox AC30 amps use 220μF / 50 Volts Cathode Capacitor as standard.

Bad Cat Hot Cat amps use 220μF / 100 Volts Cathode Capacitor as standard.

Divided by 13 CJ11 and FTR37 amps use 50μF / 50 Volts Cathode Capacitor as standard.

As you can see I wrote all of these Cathode Capacitor values are standard values by the factories, because every modifier out there can modified any Cathode Biasing Amplifier with any value from 50μF to 500μF Cathode Capacitor value as he/she likes…

 

[Potato Cakes]

I meant to add the schematics as a reference. Here they are.

 

[Potato Cakes]

I am aware of the effect that the capacitor value has when used as a cathode bypass. My concern was with that big of a value the low end being pushed through the output tubes would cause and undesirable amount of over saturation as I had previously only seen 25uF used in that position. Clearly I have been overlooking this as I have seen schematics for those other amps but my mind for some reason did not pay attention to the actual value listed.

Thanks!

Paul

 

[Tubetec]

The guitar setup plays a big part there ,
Neck pickup on a Les Paul can be very bass heavy , bridge on a tele is all midrange and top ,
So again the value you choose depends to some extent on what your putting in .

 

[Accelerator]

Hi Paul @Potato Cakes
The first page of your schematic comes from Hofman Amps and as you may expect Hofman Amps are more “oriented” to “American voicing” of Fender Tweed Deluxe 5E3 amp. that use 25μF with 270Ω Cathode Resistor for Cathode Biasing of a pair of 6V6-GT Tubes.

The second page (the hand drawn has a value of 250μF…) comes “direct from the lion’s mouth” from Mark Sampson (Matchless amps. Bad Cat Amps, Novo Amps) himself…

In a guitar amp’s output stage there 3 important parameters that you have to “count” and to put attention about the “voicing” of you amplifier:

1. The characteristics of your power tubes…

(Let’s say EL84 vs. 6V6-GT, EL34 vs. 6L6-GC, KT88 vs. 6550...)

It’s one pair of the tube(s) has similar RL with the other, but they have both total different tube characteristics and different voicing…

2. The characteristics and the voicing of your Output Transformer…

Let’s say…

The Heyboer – Mojotone VOX Style AC-15 Output Transformer

https://www.mojotone.com/VOX-Style-AC-15-Output-Transformer

Vs.

Heyboer – Mojotone Tweed Deluxe 5E3 Output Transformer with 4, 8 and 16 OHM Secondary

https://www.mojotone.com/Tweed-Deluxe-Output-Transformer-w-4-8-16-OHM-Secondary

They have both the very similar 8KΩ (a-a) RL impedance, but also they both have different windings and different voicing…

3. Speaker(s) & Cabinet(s)…

Let’s see some characteristics and the differences in this “British voicing vs. American voicing” comparison:

Weber Alnico Blue Dog

https://www.tedweber.com/blue12a/

Vs.

Weber 12A125A

https://www.tedweber.com/12a125-a/

The total summary of all of these characteristics and the differences in the parameters is a general “map” that will help you to choose which value of the Cathode Capacitor will be “theoretical” more “right” to you, but the most important “factor” in this personal “choice” it is you, your personal likes, your personal preferences, your personal choice about your guitar setup which plays a big part there, your personal style of playing etc., etc., etc….

 

[merlin]

The hand drawn has a value of 250uF and the reworked one has 25uF. I do not know if this difference has to do with the version being either 115V or 230V mains,

It has nothing to do with the mains, it is purely about tone. You can use any value you like -just try some. Once you're above about 100uF you probably won't notice much further difference though.

 

[Flatpicker]

They have both the very similar 8KΩ (a-a) RL impedance...

Mojotone VOX Style AC-15 specs say 6K ohm, not 8K ohm.

 

[CJ]

it don't matter, since this is a push pull amp, the degeneration will cancel unless unsymetrical wave forms

i like 25 uf because you can get a higher voltage rating without the can getting too big,