Fisher 800c tube replacement

[kandetvara]

Hi there,

just a quick question. I have a Fisher 800C which is working more than fine and I have a lot of fun with it.

I got a matched set of 4* 7591A Tungsol tubes last week and I wonder if I can just swap the old tubes with the new ones - just for testing and having a look if it sounds different.

Is there anything specific to watch out for or is it just swapping them?

Thanks for your help and let me know.

Kandetvara

 

[larryderouin]

What output tubes are in it now? Has it been under the knife to your knowledge? Do you have any experience working on tube gear. Your answers will decide what we can do with you
getting your 800c up to snuff. Or sending it out for work.

You're probably gonna need to do some mods (more for reliability and safety) if they haven't been done already. The Tung-Sols run "HOT" so the stock set up is going to burn them up quickly without some sort of adjustments. They will more than likely redplate without doing anything to the unit, then you have 4 tubes that are only good for plinking with a .22 rifle.

Minimum mods in no specific order.
1.) Screen stability resistors (100ohm 1/4w)
2.) change grid return resistors to 220ohm vs. 330 ohms.
2a) change coupling caps from .047uf 400v and 250v to .068uf 400v
3.) Build and install either an IBAM or IBBA (Individual BIAS ADJUSTMENT Module or Individual BIAS-BALANCE Adjustment)boards.

This is the absolute minimum you need to run the tung-sols Safely (improves the safety and reliability of ALL 7591(a) tubes).

Take and open the bottom of the 800c. Grab your camera, and take closeup pictures of each quarter of the bottom. We can tell you what exactly (up to and including a full rebuild) what you'll need in addition to the above.

Larry

 

[kandetvara]

Hi Larry - thanks for your answer!

It has been under the knife according to my knowledge. And no - I don't have any knowledge / experience on working on tube gear. Sending it out to work will be an issue - since I live in Germany and the Fisher is hardly known...

I attach a couple of pictures - perhaps they help...?

According to the place I bought it at the tubes are matched and fine...

So there is actually no need to change it - I am just curious...

thanks

kandetvara

 

[larryderouin]

I see some of the work I listed above. The one thing I would check (as the pic is somewhat faded andI can't tell what color the bands are) is the value of the grid return resistors (stock 330K Orange-Orange-Yellow), and the value of the coupling caps (stock .047uf ). If they are stock, you could replace them with 220K resistors and .068uf caps. These will help keep the tubes happier, and if you should need to use the Tung-Sol's the only thing you'd need to do is to find someone to build the IBBA board (fairly simple to build) and install.

But as is the Tung-Sol's wouldn't last too long.

 

[kandetvara]

Ok - please help... I can post another picture via external link for better resolution. Which one of the attached (above) would allow you to see what you are asking for / need to know?

thanks!

 

[AlTinkster92]

Just a question, have you tested the present 7591's? They look almost like new. I'd get them tested before going to the Tungsol's.......just my 2 cents.

 

[kandetvara]

Hi Al - they are just fine and great - I just wanted to play around with the Tungsol and swap them to see if there is any change in sound... But Iearned now that i shouldn't do that without any precautions....

So I will leave the Tungsols in the cupboard and wait until the others dont work anymore...

Thanks

kandetvara

 

[larryderouin]

I arrowed the resistor's in question There are two pairs in a "V" configuration, and the coupling caps are connected to the same "T" Strip. I need to know the values of those please. If you can tell me the color of the bands on one of the resistors and the value on the side of the caps, that will suffice.

Larry

 

[kandetvara]

Hi Larry - thanks again. I am currently travelling quite a bit - so I can't open the Fisher at the moment. The pictures on my HDD are not sharp enough. Will have to take the unit out once back at home. Now that I know what to look after, this will be easier. So at the moment I will not change anything!

Thanks for the advise and i will come back!

kandetvara