Kit or parts selection for Fisher 500B

[Sam08861]

notdigital,

I did indeed swap 13 with 14 and 15 with 16 and realized this just a few moments ago. I decided to go ahead and test the tubes in the tube testor since I'll be pulling them anyway before swapping the pairs as you originally requested. Will post results in just a few of both the tube tester results and the correctly swapped pairs.

 

[notdigital]

Well, no need. I had originally asked that the channel pair be swapped with the other channel pair but I see the same difference: Your channels are mismatched as are the tubes. With no other modifications to be made, your best bet is to get a fresh quad. You can probably balance them a little tighter with an IBAM or a balance board but you'd end up squeezing them anyway. Best bet is to simply start fresh.

You can play them the way they are but you won't be getting the most out of the receiver.

 

[Sam08861]

That's what I thought.

Tubetester put all the 7591 tubes at 20-24 or square in the middle of the 'replace' sections.

Guess a matched quad is in the works here. Any difference between the Jim McShane quads and the matched items available on tubesture, etc?

Anything else I should check out before putting the cover back on while waiting for the tubes and ibam parts?

 

[notdigital]

Well, McShane got a rep for consistently graded tubes, and service to match. Unless there is a stupidly outrageous difference in price, I go with the known.

I never like changing stuff to much in the beginning. So long as the unit is operational, I let it tell me what's ailing it, then address the problem. It might be a scratchy this, or an imbalance that. Before you know it, you'll be Done!

 

[larryderouin]

Go with McShane. His tubes are tested on a custom rig that tests them at actual operating voltages and loads. and they get burn in tested. And if they don't pass there, they go back to the factory. His main interest is the Citation series from Harmon Kardon, and they eat tubes like kids on an ice cream cone. If they can handle the voltages and loads for Citations they can handle easily the loads from any FISHER. His prices are in line with most everyone else. I would recommend the Tung-Sol Re-Issues and if you are on a budget, the EH7591's.

Set your tester for the 7591 settings, insert the tube and then crank the heater voltage to 12.6v in stages a few seconds between stages. Leave there for 3 minutes or so. Then back off to 7.5v. DO NOT push any test, or short buttons. Leave it like this for a few hours. Turn off the tester and reset the heater voltage for 6.3v, and turn on after 10 minutes. Test. . If they are really bad, they won't increase appreciably. But if they are just weak, they may climb 15-25 points and stay there. If so you can run them cautiously for a while. repeat for all 4 tubes If they don't come up some don't use them again except for test mules. But from the readings you are getting off pin 5 these tubes are lining up in the TEST MULE LINE.

 

[Sam08861]

Thanks All,

Ordered the McShane tung sols.

Larry,

I had no idea that tubes might be revived! Will try this when I get home. I have an Eico 625 tester that I've refurbed, so hope that it's up to the task.

Does anyone happen to have a parts list for an ibba for the 500b? Thinking I will need this before installing the new tubes.

If not, I will try and post a draft list for feedback this evening.

Sam

 

[larryderouin]

Revivals are in most cases done out of desperation. Some old 4 pin triodes etc. come up fairly well but it won't last long. I do it mainly to see if there is really anything left that I can squeeze out of them. It usually won't last long, but I've done 7591's that lasted a couple more weeks until the new one's showed up. There wasn't that much improvement. If you can get them to 40 on the EICO you're doing quite good. they won't bias up any hotter than 10-20ma @ about -10v which in the norml scheme of things is just too cold of a bias point at a too damned low bias voltage. The most I ever got out of a quad of Toshiba's that showed a 625 meter avg of 22 was about 35 for one and almost 30 for the others. They were quite literally shot, one foot in the grave and the other on a greased banana peel.

As for the 12a-7 types. None of them read above 62 on my Eico. Even brand new, straight from the facotry Sovteks, and EH's. 12au7 and 12at7 will read approx the same as 12ax7's. 6AQ8's, 6DT8's and other Hi or mid mU dual triodes will also read about the same.

 

[Sam08861]

First tube is on the 'rotisserie' and has been baking for about 90 minutes at 7.5 volts after being run up to 12.6 volts for 3 minutes. I'll go 2 hours total @ 7.5, let it cool, retest at 6.3vac, and report the results in the next 45 minutes or so.

Tung-sols have shipped as of this morning so I should have those by Friday or so.

As far as the remaining parts go, here's my planned order.

IBBA (based on the information in the 1st post of bhamham's thread here..... http://audiokarma.org/forums/index.php?threads/show-me-your-ibam-ibba-2016.713900/ ... Thank you bhamham!) ...
---------------------------------
4 of everything below + a perf board and some solid core wire in 4 colors. 14 or 16 gauge?
12K 1/2 watt metal film resistors
33uF 50v electrolytic
10k 1/2w potentiometer
ring terminals or solder lugs as test points

RIAA Mod Parts (per dgillespie's post here http://audiokarma.org/forums/index.php?threads/the-fisher-phono-preamp-and-riaa.600690/ ... Thank you Dave!)
---------------------------------
4 x 680pf silver mica caps (to make the 1350pf)
2 x 2M resistors (this plus below in series to get 2.13M)
2 x 130k resistors
2 x 350pf silver mica caps
2 x 270k resistors (since I'll be removing the PECs)
2 x 120k resistors
2 x 560pf silver mica caps
2 x 1" square pieces of perfboard

Current Inrush Protection
---------------------------------
CL80 Thermistor

Tuner Recap
---------------------------------
C210 - 1uf 350V film capacitor
C74 - 8uf 50V film capacitor

Screen Resistors (will rewire in the '7591 only' method.. I think this may have been your post Larry, but I can't seem to find it again from a quick search. Thank you however!)
---------------------------------
4 x 100ohm 1/2 watt resistors


Please let me know if I've got anything wrong here for a 500B.

Sam

 

[Sam08861]

So first tube results are as follows:

initial reading on tube tester after 10 min warmup. 24
After following Larry's process: 35

Not bad!

Will see what happens with the others.

Here's a shot of the next tube in the lineup lit up extra bright at 12.6V

 

[larryderouin]

IBBA wiring. 20ga is more than sufficient. 1/4w resistors and pots are fineand work well here, but it's your choice.

Screen resistors......page 2 of the "COMMON FISHER PARTS" sticky thread. With pics.

C74----> an 8uf 50V film cap will be HUGE! Use a Nichicon UKL (Low Noise) electrolytic here. space is kind of at a premium here......
10uf 50v radial 647-UKL1H100MDDANA

Just remember that the longer that the tube goes unused from the rotisserie the more the reading will drop. So they will probably be ok for initial testing and such, but for long range listening, not so good.

 

[bhamham]

Sam, the parts list above is for the IBAM not IBBA - there's a difference if you're unfamiliar with them. Here's a good thread that discusses the difference... http://www.audiokarma.org/forums/in...dividual-bias-adjustment-modification.601583/

 

[Sam08861]

Looks like tube results are a mixed bag. Tung sols will be here tomorrows so putting this in my box of soso tubes that I keep on hand for unknown reasons. Even with the reading so low, the unit sounded allright (but way more fatiguing than my 6v6pp amp, to which I can listen to for hours), so very curious what the new tubes will sound like. Will have to force myself to wire up the ibba or ibam before installing them.

Results...
tube----start----finish
1--------24--------up to 35, but drifted back to 24
2--------22--------steady at 35
3--------20--------22
4--------22--------34

Thanks bhamham, very helpful! I did use the wrong terminology there. The IBBA does seem like the preferred way to go, so looking into the parts/schematic for this.

 

[Sam08861]

So looks like for IBBA it's

2 10K resistors (all resistors1/4 watt, metal film)
4 15K resistors
4 10K potentionmeters (multi-turn)
4 10uf 35v caps (nichicon electrolytic)
5 lugs or ring terminals for test points and ground
1 piece of perfboard 2" x 2" or so
solid core 20ga

Let me know what mistakes I may have made here. Assume this would connect the same as the 500/800c in the thread.

Sam

 

[Sam08861]

With everyone's input, my overall parts order is now as follows:

IBBA
---------------------------------
2 10K resistors (all resistors 1/4 watt, metal film)
4 15K resistors
4 10K potentiometers (multi-turn)
4 10uf 35v caps (nichicon ukl electrolytic)
5 lugs or ring terminals for test points and ground
1 piece of perfboard 2" x 2" or so
solid core 20ga wire

RIAA Mod Parts
---------------------------------
4 x 680pf silver mica caps (to make the 1350pf)
2 x 2M resistors (this plus below in series to get 2.13M)
2 x 130k resistors
2 x 350pf silver mica caps
2 x 270k resistors (since I'll be removing the PECs)
2 x 120k resistors
2 x 560pf silver mica caps
2 x 1" square pieces of perfboard

Current Inrush Protection
---------------------------------
CL80 Thermistor

Tuner Recap
---------------------------------
C210 - 1uf 350V film capacitor
C74 - 10uf 50V radial capacitor (nichicon ukl)

Screen Resistors (Thank you Larry, I see now it's posts #39 & #40 on the common parts sticky!)
---------------------------------
4 x 100ohm 1/4 watt resistors

Other
----------------------------------
grommets for phono tubes

I'll try and place my order late tonight or early tomorrow and thanks everyone for all your help, encouragement and guidance!

Sam

 

[larryderouin]

Unless you have done this yet already;
1.) Grid return resistors (R102-103-104-105) change from 330K to 200K-220K
2.) Output coupling caps. (C57-58-60-61) change from .047 to .1uf

The Tung-Sols, EH's and JJ's have a very hard time trying to survive and failing with the original value for the grid returns. The coupling cap change is to keep the R/C timing constant with the grid returns approx the same as the original config.

 

[Sam08861]

Thanks again Larry,

Jim McShanes kit did indeed include 200k 1/2 watt film resistors for r102-105 and .1uf 630V PIO caps (k40y-9) for the output coupling caps.

 

[Sam08861]

Mouser order placed and looks like tubes will be here tomorrow!

 

[Sam08861]

Tung sols got here yesterday. The matches set are labeled as follows:

Tube 1: 67.1, 9.25
Tube 2: 66.4, 9.15
Tube 3: 66.6, 9.25
Tube 4: 67.7, 9.25

Putting them in they Eico 625, they all measure about 56 or about 60% stronger than the strongest Fisher output tube that came with the unit. Guess the Eico measures low on everything. Adjusting the shunt to 20 (from 15) puts the reading in the 80's.

Couple of questions:

1. Any recommendations for pairing the tubes? (I'm assuming pair 1,4 and 2,3)
2. Should I wait for the IBBA before trying these out? (As I understand, the 200K resistors for r102-105 are there to help bring the current down and will help? Don't want to risk stressing these yet as they're brand new)

Thinking the best bet would be to will build and test the IBBA with the old tubes before swapping in the new?

All advice and opinions welcome!

 

[larryderouin]

It don't matter on them. They are close enough that it doesn't make any difference.
Install the 200K's and the IBBA, test it out with the old tubes 1st. Once you have it woring right, THEN swap the Tung-Sols in.

The grid return resistors are too high a value even for the old tubes. Back when these units were new 7591's were cheap (about $2 to $4.00each)and would last about a year or two with current conditions. There was a wattage war going on and manufacturers built these to beat the next guy. And most people expected to have to replace tubes. Now a days people think they last forever, they don't. But expecting a tube to last more than a year takes some changes and backing off the overclocking so they will still deliver and are able to survive more than a year or two. That's what this does to an extent. It drops the parameters back down to UNDER the maximum by a good 1/3. With the other mods this will make the unit pretty much bullet proof to anything short of a direct lightning strike, a tactical nuke, or the 2nd coming of Christ.

The EICO measures most Russian tubes about 25-50% lower than equivalent domestic tubes. Plus it's an emissions only tester. Or a GO- No GO tester. Drop your demarcation lines to 45 and 30 and it's pretty accurate at that point for emissions as to weak and replace for most tubes. Mine does the same thing. Put them on my hickok clone (philco 9100/Hickok 533a)and they tested out at about 200% of minimum good in gm.

 

[Sam08861]

Thanks once again, Larry. So glad you have a 625 to reference too!

The parts arrive on Wednesday and I'll post progress, photos and questions as I proceed.