Fisher 500B Iron?

[Blair]

Anyone know this output iron? Specs?

Someone sent them to me, and said they were the "C" version, but the #s are clearly the "B" output transformers. I used my SS preamp and a signal generator to push 1K sine into the 8 ohm tap and got 23v across the primaries. 23:1 ratio, or roughly 4.2-4.3K A-A impedance. Is that about right?

I have these tubes to use: 6L6GC, KT77. The 77s would be much "harder" on the filament windings, but I think I can make it work.

Thanks for any info,

Blair

[Tom Bavis]

Give me a week and I'll let you know the impedance from 20Hz to 100K or so - I'd guess they're 7K or so. I have access to an impedance analyzer, and have been measuring some transformers on my lunch hour. 500B transformers, Pioneer SX-2000, some Stromberg iron are yet to be done.

[Blair]

Wow! Thanks Tom!

Will this be hosted anywhere for people like me who manage to always get old "junk" from folks?

I sure appreciate it! The issue is that running a sweep of the signal yielded results all over the place.

Thanks again!

Blair

[battradio]

Fisher 500B's use 7868's and would have had 6600 CT transformers most likely

[Blair]

Thanks Mark,

Shortly after I posted this, I did look at the datasheet, so I am sure you are right as well as Tom's suggestion that it is close to 7K.

I'm going to do an "Oddblock" variation with Bruce's 317CCS and a 12AT7 driver.

I'm going to try to do all this in a ST-35 style chassis:) Or at least try.

I suppose the CCS will help with space.

Blair

[Blair]

For the oddblock design to use 6l6gc tubes, will I need to adjust the CCS for closer to 80mA total?

I figure ill have a B+ of around 410-415V since the original was 395.

I emailed Bruce, and despite his earlier designs with the 317T, he suggests the 317HV regardless of the tube compliment, so I guess I will not be wiring today. Here is what I did yesterday:



Blair

[Tom Bavis]

Don't have the measurements in front of me, but it was 6.2K loaded with 8 ohms (this includes copper loss - you'll get a lower value if you only take the voltage ratio).

I will eventually put the transformer measurements online - maybe Google Docs - one more bit of software to learn...

[Blair]

Thanks Tom?

This is perfect! Where are you located? Would you be willing to test an output transformer I have if I ship it to you? This would be something that there is no hurry at all for. I started the project a year ago, and it may take a few more years:)

It's for a Bogen MO-100/200. Just in case you have tested one:)

Thanks a bunch Tom!

Blair

[Tom Bavis]

Actually I have access to a MO-200, though I doubt my friend who owns wants to haul it in to work - if he's willing to remove a transformer, we can measure it. I have a few smaller amps that would be easy to measure the transformers in place -that Bogen - not so much...

[Blair]

Ha! I doubt he will want to pull one. It's a pain in the rear! Lots of secondaries!

Mine are sitting unbolted on a top plate. I just want to square this all up before looking at circuit design. I've seen a much debated, "it's four 8417 tubes, so it has to be ~2K" and "you are wrong. David Bogen ran the quads into a higher 5K load with a higher B+, etc. to run the tubes more conservatively"

On a ratio type test, I get around 4.5k

Thanks!

Blair

[Blair]

So,

I was emailing with Bruce about his design and he said that my output transformers need to be able to handle 120mA or 60 per side, but preferably 200mA or 100mA per side.

Is anyone experienced enough with Fisher iron to know if these are OK or not?

I looked at the datasheet for the 7591 and under the parameters in the original 500B, it looks like it idles around 88 mA, but hit max current at 145mA.

Fisher would have been a bit conservative with this right?

I really do not want to burn these up if I can help it.

If not, then fixed bias is the way to go.

Thanks!

Blair

[kheper]

From the service manuals and comments on websites:

Fisher 500C = 35-37 wpc = 430V plates = 375V screens = 35ma bias current per tube

Fisher 500B = 32 wpc = 393V plates = 348V screens = 35ma bias current per tube

35ma is stated by refurbishers as the recommended idle current for both the Fisher 500 C and B, but this figure is not given in the service manuals.

So, 70ma per transformer is the minimum current drawn. I suspect 140-150ma is the max current drawn at the rated specs per transformer. The max is over 120ma per transformer, but less than the ideal 200ma. Fisher probably built the transformers to handle more current than the above max. The weak link, according to refurbishers, is the 7591 tubes. They back off the idle current to 30-32ma.

[Blair]

Thanks,

That's the way I read it also. Like Bruce said, "150mA will work, but 200mA is preferred".

I already have the LM317HV coming, so ill try his design.

He also suggested lowering the bias point to 50mA vs. 62mA per tube. I can only assume the fishers current capability, but I would think like you suggested, that they probably designed them conservatively and they are really rated for 175-200mA.

You think there is any harm in backing it off to 45mA?

Thanks!

Blair

[kheper]

Are you building the Oddwatt 225? The idle current draw is 60-65ma per tube. With a PP Class A amp, the idle and the max current draw will be close (no more than 10ma per tube greater in the datasheets). 50ma as Bruce recommended sounds risk-free for a max of ~120ma per transformer. You can increase to 55-60ma, if the transformers do not run hot. You may want to drop your B+ to 350V, too.

[Blair]

Thanks!

Yeah, or a variant using a 12AT7 driver vs. 6sl7.

Sounds good. I have a choke I can drop in to drop the B+ a touch.

Blair