Fisher k 10 Spacexpander - Reverb Transformer

Hi,
I have a K-10 with broken reverb transformer (T2 on the schematic).
Does anyone knows the specifications for this transformer?
Here's the schematic:

smilan said:

Hi,
I have a K-10 with broken reverb transformer (T2 on the schematic).
Does anyone knows the specifications for this transformer?

Click to expand...

A bit of retro-engineering. The 2nd section of 7247 is similar to a 12AU7. In cathode follower mode, it wants to see about 10-20 kohms. That would be the impedance on the primary side. For the secondary, check the input impedance of the drive coil. If the tank is a Hammond/ Accutronic, check the 2nd digit of the reference
Digit #2 - Input Impedance
A 8 Ohms 10 Ohms
B 150 Ohms 190 Ohms
C 200 Ohms 240 Ohms
D 250 Ohms 310 Ohms
E 600 Ohms 800 Ohms
F 1,475 Ohms 1,925 Ohms
It is very likely a low-Z type so the secondary should be 8 ohms.
I would bet the type 1750A from Hammond or the standard Fender replacement would work fine there.

It is odd, agree. But if the tank is 2 Ohm to 10 Ohm input, I would use the standard replacement for Fender Combo Reverb (not Stand-Alone reverb), which is also as cheap as these things get.

Just for picture and price-zone: Hoffman "Reverb output transformer for Fender combos. Used on all reverb heads and combos." 022921 Typically driven with 2-half of 12AT7.

Or: ClassicTone 40-18034.

The "Output for Champ and Reverb units" is 5K-7K and really meant for a bigger tube, 6F6(6V6), and twice the price. And much more than this amp needs.

Yeah Ive seen the standard fender reverb transformers for as little as 8 dollars a piece, should do the job nicely.
Apparently the idea of the fisher spring was you added it to your hifi system so everything sounded like it was recorded in a metal hangar, what a f**king hideous concept , still used judiciously in a studio setting or with a guitar amp might sound quite good .

Heres the link, $8.95

https://www.tubesandmore.com/products/transformer-fender-reverb-pro-reverb

Thanks guys,
The 2nd digit on the reverb tank is F  = 1,475 Ohms on type 4.
In this case, would a standard fender reverb transformer will be good or should I get some other transformer?

smilan said:

Thanks guys,
The 2nd digit on the reverb tank is F  = 1,475 Ohms on type 4.
In this case, would a standard fender reverb transformer will be good or should I get some other transformer?

Click to expand...

Quite surprizing! The standard Fender xfmr would be totally inadequate. Can you look at the xfmr's wire size? If the secondary was low-Z, the wire would be much thicker than the primary.
It looks like a very specific xfmr that would be something like 10-20k at the primary and 1.5k at the secondary.
Hammond types 143N and 145R seem to be good candidates.

Both weirs looks the same size.

smilan said:

Both weirs looks the same size.

Click to expand...

I meant the inner wires; you may have to take off some of the insulation material to look at it.

I've checked the dc resistance of T2 on another k-10 with a working transformer. So on the primary I measured 310 Ohms and 185 Ohms on the secondary.
Does it helps to know the specifications for the new transformer?

smilan said:

I've checked the dc resistance of T2 on another k-10 with a working transformer. So on the primary I measured 310 Ohms and 185 Ohms on the secondary.
Does it helps to know the specifications for the new transformer?

Click to expand...

That would indicate a ratio of about 3:1, which would be consistent with 15k:1.5k
Maybe you could measure the actual ratio, measuring the AC level on both sides. Anyway, the ratio is not critical at all.

Apologies Smilan,
The fender transformer wont suit the high-Z tank in the Fisher.

abbey road d enfer said:

That would indicate a ratio of about 3:1, which would be consistent with 15k:1.5k
Maybe you could measure the actual ratio, measuring the AC level on both sides. Anyway, the ratio is not critical at all.

Click to expand...

AC level goes from 0.060V on primary and 0.015V on the secondary that bring us to ratio of 4:1?

smilan said:

AC level goes from 0.060V on primary and 0.015V on the secondary that bring us to ratio of 4:1?

Click to expand...

Exactly. Looks like either of the aforementioned Hammond would do.

Thanks, I'll get me one of those.
I have two more questions regarding the schematic:
1. On the PSU section, what is the meaning of the "V" sign between the +6V to 0?
2. What kind of switch is "S2"?

The < symbol represents the tubes filaments

S2 is whats known at DPDT ,dual pole dual throw , so the two sets of contacts in the middle connect with either of the two pairs of outside contacts .

I'm think that the measurements I've done yesterday was wrong.
Anyhow I've applied 1KHZ with voltage of 1V to the  primary and measured 0.437V on the secondary.
So 1 divided to 0.437  equal 2.288, this should be the loop ratio (2.288)?
Now 2.288*2.288 = 5.236, this should be the impedance?
When multiply 5.236 with the input impedance of the reverb tank (1475) it's equal to 7723.
I've checked the plate resistance of the 7247 tube that feeds the reverb tank and it's 7200, so it  make sense.
Now, does this testings helping me to find the right transformer?

smilan said:

Now, does this testings helping me to find the right transformer?

Click to expand...

Yes.
Hammond 143N, nominal 10k:2k. -> ratio=sqrt(10/2)=sqrt(5)= 2.25
Loaded with 1500 ohms, 7500 ohms reflected at the primary

Thanks

Digging this up for a power transformer question. I need a new one, only thing I can find is 115v primary 215v at the secondary. This would give me about 302vdc at the first measurement marked 209 on the schematic with a 115vac input. Would that be too much for the tubes, considering I might even get a bit higher voltage input on the primary?