Appropriate replacement OPT for Fisher 30-A with 2.5Ohm Secondary

[awroethlisberger]

Hi, I've picked up a nice Fisher 30-A amplifier. This is a mono push pull amplifier powered by two 6BQ5 tubes.

The standard Fisher 30-A has typical 4/8/16 Ohm output taps for the speaker connectivity. However, this particular unit is from a console that supported stereo output via one 30-A that was integrated into a preamp section, and the second 30-A(this unit) was chained off of the preamp but only has a single 2.5 Ohm output. I believe this combo was called the 610-T or something. And no, there are no other taps/leads from the output transformer on this unit that simply are not connected. It only has the 2.5 Ohm output.

So, I however do want to use this amplifier with an 8 or 16 Ohm speaker and therefore am thinking about changing the OPT. Unfortunately this is amplifier surgery I have yet to have experienced, so I have a very basic but important question about which OPT is the correct choice.

Looking at the datasheet for the 6BQ5 in a PP arrangement, the plate-to-plate impedance is spec'd at 8000 Ohm. This is also technically a 17W amplifier in this configuration.

6BQ5 pdf, 6BQ5 description, 6BQ5 datasheets, 6BQ5 view ::: ALLDATASHEET :::

I was looking at the Edcor CXPP series for PP given the relatively low cost and good reputation, and the model that appears to be right choice is the CXPP25-MS-8K.

EDCOR - CXPP25-MS-8K


So my simple question is: Is this indeed the correct Edcor model OPT for this application from a technical standpoint?


Thanks,
Al

[awroethlisberger]

Doing some research, it looks like another recommended "budget" OPT for EL84/6BQ5 PP amps is the Antek AT-020L. Both the Edcor and Antek are similarly priced. Does anyone have any experience/thoughts comparing the two? I can say that physically it appears that the Edcor would be the more appropriate replacement.

http://www.antekinc.com/pdf/AT-020L.pdf

Thanks,
Al

[HollowState]

Originally Posted by awroethlisberger

So my simple question is: Is this indeed the correct Edcor model OPT for this application from a technical standpoint?

Yes, it's a very good choice for PP 6BQ5s. Just tape up the screen taps and tuck them away neatly under the chassis. I wouldn't cut them off 'cause you never know what you might want to do with the transformer a few years from now.

The Sams Photofact schematic that I looked at says the output transformer was 7,000 ohms. But 8,000 is close enough for audio.
And Sams is not always right on the money.

Fisher Download Page - fisher-30a-schematic.pdf

PS, I would stick with the Edcor since I've only heard good reports about them. I even have some of their trannies and they build a nice product. I only wish they woudn't plaster their name all over the end bells.

[awroethlisberger]

Thanks so much for the detailed response, and the link to the Sam's is much appreciated as well.

Question: If the plate-to-plate is actually 7k Ohm, would an OPT from Edcor such as the CXPP25-MS-7.6K be a better fit?

Something like this: EDCOR - CXPP25-8-7.6K

I've heard that less(in number) secondary taps makes for better audio as well, so with the 7.6K I could have more options for a single 8 or 16 Ohm output where the 8k Ohm OPT only has the 4/8/16 option.

Would there be an easy and safe way for me to test the plate-to-plate impedance in the actual amplifier to verify the best fit?

Thoughts?

Thanks,
Al

[kevinkr]

Originally Posted by awroethlisberger

Thanks so much for the detailed response, and the link to the Sam's is much appreciated as well.

Question: If the plate-to-plate is actually 7k Ohm, would an OPT from Edcor such as the CXPP25-MS-7.6K be a better fit?

Something like this: EDCOR - CXPP25-8-7.6K

I've heard that less(in number) secondary taps makes for better audio as well, so with the 7.6K I could have more options for a single 8 or 16 Ohm output where the 8k Ohm OPT only has the 4/8/16 option.

Would there be an easy and safe way for me to test the plate-to-plate impedance in the actual amplifier to verify the best fit?

Thoughts?

Thanks,
Al

Hi Al,
It actually isn't that critical, choose the one that suits your needs best. The argument for lowering the number of output taps based on leakage inductance and other issues is valid, that said I have never heard a significant difference in a well designed transformer, and you loose flexibility if in the future you want to use it with a different speaker. IMHO for the money you can't go wrong with Edcors..

FWIW The other transformer brand you mentioned makes great power toroids and very bad output transformers. Do a little research here and you will find what I am talking about..

[HollowState]

[QUOTE]

Originally Posted by awroethlisberger

Question: If the plate-to-plate is actually 7k Ohm, would an OPT from Edcor such as the CXPP25-MS-7.6K be a better fit?

Very technically yes, but you will be limited to using an 8 ohm speaker with that transformer. And todays transformers will probably sound somewhat better with their superior materials then the older iron.

I've heard that less(in number) secondary taps makes for better audio as well, so with the 7.6K I could have more options for a single 8 or 16 Ohm output where the 8k Ohm OPT only has the 4/8/16 option.

Like Kevin says, I doubt you will really be able to hear a difference with a well made transformer like Edcor.

Would there be an easy and safe way for me to test the plate-to-plate impedance in the actual amplifier to verify the best fit?

You would need to get a turns ratio measurment first. Then square that number to learn the impedance ratio. Turns ratio is equal to voltage ratio. On the bench, disconnect all transformer leads and apply exactly 1 volt (AC) to the secondary and measure the primary output voltage. You could also do this in reverse. But in either event you will need an accurate good quality AC electronic voltmeter. Personally, I would just use my GR impedance bridge.

[awroethlisberger]

Originally Posted by kevinkr

Hi Al,
It actually isn't that critical, choose the one that suits your needs best.

No kidding....

I was reading the details of the Sam's regarding the OPT replacement options, which as mentioned above specify 7000 Ohm as the correct primary impedance.

But the listed replacement option transformers are interesting. For example, the Stancor replacement OPT is actually spec'd at 6000 Ohm. And the recommended Thordarson 22S68 is spec'd at 6500 Ohm. And the Triad S-24A is spec'd at 8000 Ohm, as well as the Merit A-3128.



Pretty interesting.

Thanks again.
Al

[no gas]

Originally Posted by awroethlisberger

No kidding....

I was reading the details of the Sam's regarding the OPT replacement options, which as mentioned above specify 7000 Ohm as the correct primary impedance.

But the listed replacement option transformers are interesting. For example, the Stancor replacement OPT is actually spec'd at 6000 Ohm. And the recommended Thordarson 22S68 is spec'd at 6500 Ohm. And the Triad S-24A is spec'd at 8000 Ohm, as well as the Merit A-3128.



Pretty interesting.

Thanks again.
Al

Remember, tubes have 'guidelines' not etched in stone specs like solid state stuff.

[Captn Dave]

Just my 2 cents, but I'd place more stock in the tube manual than the Sams in this particular case. Consider that there is nothing in circuit between the 6BQ5 and the transformer. The 8K is the traditional choice for the 6BQ5 because most tube manuals call out that impedance.

[awroethlisberger]

Originally Posted by Captn Dave

Just my 2 cents, but I'd place more stock in the tube manual than the Sams in this particular case. Consider that there is nothing in circuit between the 6BQ5 and the transformer. The 8K is the traditional choice for the 6BQ5 because most tube manuals call out that impedance.

Thanks Dave, that is very helpful advice. I was actually leaning toward the 7.6k Ohm version to "split the difference" between the 7k and 8k Ohm different specs. But if 8k Ohm is the more appropriate choice based on the tube specs, that makes sense.

Al