Matchless Spitfire Questions

[mbratch]

I wasn't sure what forum to categorize these in, so I thought I'd pose them here. They regard the Matchless amp design:

• I noticed that there are several schematics kicking around, but the most common one I see is here. The hand-drawn schematic has an extra filter cap. Is this extra cap closer to the original design? Does it have any benefit over the more commonly implemented clones that I see, which has one less filter cap?
• From what I've read, I don't think hiss can be completely eliminated in an amp like this if you have the gain (volume knob) cranked up past, say, around 2:00. But how much hiss is normal, and how much is too much and can be fixed? (If you have a recording of a real Spitfire at idle with volume @ 3:00 and no guitar plugged in, as an example, that would help immensely!)
• I've seen one Spitfire clone that has a "mod" switch (see Turret Boards Spitfire clone) that pushes the cathode capacitance of the first 12AX7 from 25uF to 50uF. I've tried this mod and outside of an initial "pop" I don't hear any change in the sound of the amp. It's labeled "Fat Boost" in the layout, but I'm not getting it.

Thanks

[guitarmike2107]

-Both those diagrams show the extra cap? you can build it without, but why, to save a few extra dollars/pounds?

Because EL84 are high gain they can hiss more than other valves, just make sure you use a low noise valve in the first position. using the extra filter cap will help with any hiss causing grounding issues. this isnt a mega gain amp so hiss shouldnt be that bad.

That Mod wont do very much sonically that you would need, you are probably better using a small 1uf cap and a 25uf cap to switch in.

[mbratch]

-Both those diagrams show the extra cap? you can build it without, but why, to save a few extra dollars/pounds?

The first one shows two 33uF and two 22uF filter caps with two 22k resistors. The second shows three 22uF filter caps and three 22k resistors. The extra 22uF/22k set is on the connection to the two 100ohm grid resistors to the power tubes.

Because EL84 are high gain they can hiss more than other valves, just make sure you use a low noise valve in the first position. using the extra filter cap will help with any hiss causing grounding issues. this isnt a mega gain amp so hiss shouldnt be that bad.

Right. But I am having trouble determining if the amount of hiss in my amp is "bad" or not.

That Mod wont do very much sonically that you would need, you are probably better using a small 1uf cap and a 25uf cap to switch in.

OK, well that lines up with my experience. It did nothing discernible, at least to my ears. What does a 1uF in place of a 25uF do to the sound?

[mooreamps]

But how much hiss is normal, and how much is too much and can be fixed?

It's an old tube amp. Alot of hiss is normal. Can be fixed, but it would no longer be a matchless amp..

-g

[Jazz P Bass]

What does a 1uF in place of a 25uF do to the sound?

A 25uf & a 25uf in parallel will let more bass frequencies through.
If your setup has it available.
ie: pickups, guitar type.
A 1uf to a 25uf will be more of a "thin" switch.
The 1uf will not pass the bass.
If you are predriving the input with a pedal, the 1uf can help with a distorted sound.
Bass notes do not distort pleasingly.

[mbratch]

It's an old tube amp. Alot of hiss is normal. Can be fixed, but it would no longer be a matchless amp..

-g

So you're saying then that a real Matchless Spitfire will also hiss a lot? That's all I really wanted to know. If it was normal for this amp design. I certainly don't want to change the design. And I've already replaced the carbon comp resistors in the appropriate places in the pre-amp.

[mbratch]

A 25uf & a 25uf in parallel will let more bass frequencies through.
If your setup has it available.
ie: pickups, guitar type.
A 1uf to a 25uf will be more of a "thin" switch.
The 1uf will not pass the bass.
If you are predriving the input with a pedal, the 1uf can help with a distorted sound.
Bass notes do not distort pleasingly.

Cool, thanks.

Well, with my Hamer Artist and humbuckers, and playing through a 2x12 cab, I didn't notice a difference in bass response at all when I kicked it up to 50uF with the switch...

[imaradiostar]

So you're saying then that a real Matchless Spitfire will also hiss a lot? That's all I really wanted to know. If it was normal for this amp design. I certainly don't want to change the design. And I've already replaced the carbon comp resistors in the appropriate places in the pre-amp.

This isn't the highest gain amp out there though there will be quite a bit going on with the volume above 2. I'm surprised that hiss is an issue. Have you tried different 12ax7's in the input position? Does the hiss stay the same with no cable or with the guitar plugged straight into the amp?

Gary has a good point- you could fix the hiss but it might not be the same amp anymore!

jamie

[guitarmike2107]

No You wouldn’t hear a difference between 25uf and 50uf, anything above ~8uf with that set up will fully boost all frequencies of the guitar signal, once its fully boosted, there no where left to go, unless you plug in a 7 string or bass.

The size of the cathode bypass capacitor affect the amount of bass boosted by the gain stage.

The value of the capacitor and the frequencies affected are determined by the operating condition of the valve. The spitfire already uses the coupling 0.0022uf cap to tame the bass before the volume control. That might not be enough though for your liking, so if you wanted the bass tighter then you can fit a cathode capacitor under 2uf say in place of the 25uf cap on the cathode, typical values are 0.68uf – 1uf. This is what is usually called a “bass boost” switch, but yes too much bass sounds bad when distorted.

I have been using these calculators recently for a quick idea as to how different caps work in a circuit.
Cathode Bypass Capacitor Calculator

if you don’t understand what actually going on go try it to see… erm hear the effect. You will probably find you like the tighter bass sound with the amp cranked up, but more bassy with it turned down.

[guitarmike2107]

The Hiss, What resistors have you used, if you had used metal film resistors I would expect the hiss with the volume knob turned down to be almost nil. If you have used carbon comp resistors then it could be as noisy as it likes. Also your grounding scheme and quality of pre amp valves is important for getting the hiss down.

get rid of the hiss, it will make it a better amp than a hissy Matchless.............

EDIT: there will always be some hiss with the volume turned up, you need to decide what is acceptable

[mbratch]

Thanks guys, that's helpful information.

I've been using an old RCA 12AX7 in V1. I did try a brand new Tung Sol RI but it was about the same. For me, the hiss isn't excessive. But our sound guy has occasionally switched on a noise gate, so it made me obsess a little over it. I will probably leave it alone.

I may try the switching in/out of a ~1uF v. 25uF for the V1 cathode cap. That sounds interesting.

Leaving the remaining question: why the different number of filter caps in the two different Matchless schematics? Would the extra (3rd 22uF) make any difference in sound? guitarmike2107, you mentioned it may do more to reduce hiss caused by grounding issues, or do you mean hum? My amp has a tiny amount of hum but very little (and it's the "4 filter cap" design ). And I'm still wondering whether a "real" Matchless Spitfire uses 5 filter caps or 4 (i.e., which of the two schematics shown in my link do they follow?).

And hey, guitarmike2107, that's a great online calculator, thanks!

[guitarmike2107]

Ok the one with the extra cap has the supply for the screens off it, It also has a choke fitted where the other diagram doesn’t.
The circuit with the extra filter cap will lower the screen voltage due to the high 22k resistor and will result in reduced clean head room.
But more importantly the 22k resistor it will lower the operating voltage of the screens down to safer limits for EL84’s.

So, on the same power transformer I would say with the extra cap arrangement the amp will distort earlier and the power valves will last slightly longer.

Matchless may have changed the circuit, removing the cap and choke will save a few dollars and the players would appreciate the added headroom, who knows which one is most correct?

cheers

Mike

[Diablo]

As has been mentioned above, if you switch to metal film resistors, you can knock down the hiss. Also you can increase the wattage of the resistors. Matchless originally used 1W carbon comp resistors - probably for reliability, because other manufacturers typically used 1/2 W in the same positions. Here's a link to Mr. Aiken's website that discusses resistor hiss:

Resistor Types--Does It Matter?

[mbratch]

Thanks guys.

Mike, I think both schematics show a choke. Good points about the clean headroom, etc.

[tubeswell]

I've been using an old RCA 12AX7 in V1. I did try a brand new Tung Sol RI but it was about the same.

I bought a NOS RCA 7025 a couple of weeks ago, and it is the hissiest pre-amp tube I've had (ribbed plates'n'all supposedly the finest 12AX7 ever made). Man was that disappointing. (No doubt it is just this particular dud)