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Spitfire Transformer Info

2/23/1998 9:19 AM
Doc	Spitfire Transformer Info I finally got around to taking some measurements of the transformers used in a Matchless Spitfire. Power Transformer: WTI 9600 Horizontal mount, paper wound, 2-3/4"x 3-1/2" bolt pattern. Lamination stack: 3-7/16 x 4-1/8 x 1-9/16" Color code: HV- Red/R-W/Red 5vac- Org/Org 6.3v- Grn/G-W/Grn Primary- Blk/Brn Output Transformer: WTI 9601 Horizontal mount, paper wound, 2-1/4x 2-3/16" bolt pattern. Lamination stack: 2-3/16 x 3-3/8 x 1-1/8" Color code: Pri- Yel/Red/ Yel-Brn 0- Black 4- Org 8- Grn 16- Blu Anybody recognize who the manufacturer might be? Now here's the kicker...the load impedence: This was measured with 60 hz AC, using 0.1volts on the 8 ohm secondary, and measuring the stepped-up voltage on the primary. The results of this bothered me, so I ran other sets of measurements at the8 and 4 ohm taps, and measured from the CT to each primary half just to double check my results. I also used 1.0v, and 0.01v. The turns ratio, inferred from the voltage ratio, was 21.8:1. Squaring this and multiplying by the 8 ohms resulted in a plate-to-plate primary load of only 3800 ohms !! Not what I expected, and not what one normally sees in a 2xEL84 circuit. So this being the case, using a reclaimed 8k high fidelity transformer, and connecting your 8-ohm speaker to the 16 ohm tap will result in the 4k load this amp uses. Even using a 5k OPT for a pair of 6L6's would be close enough to the 4k. It's interesting that the closest Hammond is the 1650F, at 3400 ohms (which may be too low) and rated for 25w. Doc

2/23/1998 9:19 AM

Doc

Spitfire Transformer Info

I finally got around to taking some measurements of the transformers used in a Matchless Spitfire.

 
Power Transformer:    WTI 9600
 
Horizontal mount, paper wound, 2-3/4"x 3-1/2" bolt pattern.
 
Lamination stack: 3-7/16 x 4-1/8 x 1-9/16"
 
Color code: HV-   Red/R-W/Red
 
                   5vac-  Org/Org
 
                   6.3v-  Grn/G-W/Grn
 
                   Primary- Blk/Brn
 

 
Output Transformer:    WTI 9601
 
Horizontal mount, paper wound, 2-1/4x 2-3/16" bolt pattern.
 
Lamination stack:  2-3/16 x 3-3/8 x 1-1/8"
 
Color code:  Pri-  Yel/Red/ Yel-Brn
 
                     0-  Black
 
                     4-  Org
 
                     8-  Grn
 
                    16-  Blu

Anybody recognize who the manufacturer might be?

Now here's the kicker...the load impedence:
This was measured with 60 hz AC, using 0.1volts on the 8 ohm secondary, and measuring the stepped-up voltage on the primary. The results of this bothered me, so I ran other sets of measurements at the8 and 4 ohm taps, and measured from the CT to each primary half just to double check my results. I also used 1.0v, and 0.01v. The turns ratio, inferred from the voltage ratio, was 21.8:1. Squaring this and multiplying by the 8 ohms resulted in a plate-to-plate primary load of only 3800 ohms !! Not what I expected, and not what one normally sees in a 2xEL84 circuit.

So this being the case, using a reclaimed 8k high fidelity transformer, and connecting your 8-ohm speaker to the 16 ohm tap will result in the 4k load this amp uses. Even using a 5k OPT for a pair of 6L6's would be close enough to the 4k. It's interesting that the closest Hammond is the 1650F, at 3400 ohms (which may be too low) and rated for 25w.

Doc

2/23/1998 12:31 PM
Tony T	I'm building an amp which is basically an AC30 power section married to a London Power standard preamp. I've got a Hammond 1650N I'd like to use which has a 4300 ohm input. Based on the Spitfire information what do you think? THANKS.

2/23/1998 12:54 PM
Doc	A while back, there were quite a few threads on DIY AC30's. I believe someone posted the transformer characteristics there, but I don't recall what they were. The 1650N is rated for 60wrms, with a primary impedance closer to the textbook value for a quad of EL84's. It should give you a good, strong amp if you use it. I've heard others praise the gain switching preamp you're building. Maybe someone more familiar with VOX could post the spec's of an AC30 trans.

2/23/1998 12:54 PM

Doc

A while back, there were quite a few threads on DIY AC30's. I believe someone posted the transformer characteristics there, but I don't recall what they were.

The 1650N is rated for 60wrms, with a primary impedance closer to the textbook value for a quad of EL84's. It should give you a good, strong amp if you use it. I've heard others praise the gain switching preamp you're building.

Maybe someone more familiar with VOX could post the spec's of an AC30 trans.

2/23/1998 1:38 PM
J Fletcher	Primary impedance 4000ohms according to the schematic of the JMI version.

2/23/1998 2:18 PM
Tony T	Thanks Guys!

2/23/1998 9:06 PM
GVB	This is a real coincidence, but I'm working on REAL obscure amp right now, and it uses a Hammond 1664 OT. From what I can determine right now, it looks like it is a 3800 Ohm 60W. I don't think they manufacture this model anymore. Customer wants a nice "classic" tone. Humm... GVB

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