500B (needs love and 7591s)

[dfunghi]

I tried to skim through a few threads here and I have reached out to and heard back from J McShane.

Bryce resupplied me everything but the 7591 quad (most tubes included in purchase had issues).

Jim has nothing for me right now and also went on to discuss the trouble with the EH I am sure many here are well familiar with.

Since my 500B lacks the ability to adjust bias it was recommended I do:

"To use modern tubes the 500B needs:

1. Grid circuit resistance dropped from 330K to 200-220K, and

2. Either adjustable bias or bias circuit modifications to get a lower (more
negative) voltage to the tube grids"

But I am clueless. Is this a big job? Would I be better off just trying to find NOS 7591?

Friendly techs are welcome to PM me. I am willing to send this off if it can come back fully operational.

I refuse to allow my inability to work on gear keep me from owning stuff I want..... but I do not want it to become like a boat (a hole in the water you pour money in).

Right now I have a nice 500B with everything but the 7591 so it is somewhat worthless as receiver.

TIA,
D

 

[RS Steve]

I tried to skim through a few threads here and I have reached out to and heard back from J McShane.

Bryce resupplied me everything but the 7591 quad (most tubes included in purchase had issues).

Jim has nothing for me right now and also went on to discuss the trouble with the EH I am sure many here are well familiar with.

Since my 500B lacks the ability to adjust bias it was recommended I do:

"To use modern tubes the 500B needs:

1. Grid circuit resistance dropped from 330K to 200-220K, and

2. Either adjustable bias or bias circuit modifications to get a lower (more
negative) voltage to the tube grids"

But I am clueless. Is this a big job? Would I be better off just trying to find NOS 7591?

Friendly techs are welcome to PM me. I am willing to send this off if it can come back fully operational.

I refuse to allow my inability to work on gear keep me from owning stuff I want..... but I do not want it to become like a boat (a hole in the water you pour money in).

Right now I have a nice 500B with everything but the 7591 so it is somewhat worthless as receiver.

TIA,
D

With the issues of modern built 7591 tubes well known, it makes sense to install an individual bias adjustment mod that will allow proper settings for the output tubes.

 

[thornev]

D - If it makes you feel any better, I was a novice electrician (still am in some ways) when I purchased my 500-C. But I read a lot of threads in this forum, got excited about trying some electronic mods, and bravely ventured forward with the friendly help from forum participants. Now I have an even better sounding receiver after doing about 5 different mods including the bias adjustment mod referenced by RS Steve. You need the appropriate tools, especially learning how to solder, read up on safety and how to read a schematic, but it wasn't terribly hard and it was fun. And it was gratifying to hear the results of my work. There are a few restoration kits that can get you started with included parts and detailed instructions with pictures. So if you've ever been interested in trying your hand at electronics, my advice would be to dive in and join the party. Thorne

 

[dfunghi]

D - If it makes you feel any better, I was a novice electrician (still am in some ways) when I purchased my 500-C. But I read a lot of threads in this forum, got excited about trying some electronic mods, and bravely ventured forward with the friendly help from forum participants. Now I have an even better sounding receiver after doing about 5 different mods including the bias adjustment mod referenced by RS Steve. You need the appropriate tools, especially learning how to solder, read up on safety and how to read a schematic, but it wasn't terribly hard and it was fun. And it was gratifying to hear the results of my work. There are a few restoration kits that can get you started with included parts and detailed instructions with pictures. So if you've ever been interested in trying your hand at electronics, my advice would be to dive in and join the party. Thorne

Bad eyes and shaky hands........ or else I would delve in. Plus I have big fat fingers so fine work is tough.... I have thought of going to GW and buying stuff just to desolder practice.

 

[thornev]

D - Got it. Yeah, the tiny fonts on some of these parts lists and schematics requires even me with decent eye sight to have to wear reading glasses. And yes, some of the spaces in which to fit parts can get really really small. I can see why you'd rather farm out the work. Well, good luck.

 

[larryderouin]

The change of the grid return resistors from 330K to 220K is part and parcel with changing the output coupling caps from 0.047uf to 0.068uf. Cathode resistors(10ohm) are a must as are Screen stability resistors(100ohm) for ALL 7591's.

 

[Tim D]

The change of the grid return resistors from 330K to 220K is part and parcel with changing the output coupling caps from 0.047uf to 0.068uf. Cathode resistors(10ohm) are a must as are Screen stability resistors(100ohm) for ALL 7591's.

I did all those changes to my Fisher KX-100 amp and love it. The KX-100 uses 7868 tubes which are almost the same tubes, but with a different base and pinout. I have peace of mind now that it is running right and it's a bit safer now too. Oh and it DOES SOUND GREAT!

(Note that I went with 0.1uF instead of the 0.068uF - it was close enough for government work..... There are threads that use the 0.1uF and some that show the 0.068uF - technically the 0.068uF is the more correct value. I'm told that the 0.1uF was used in the past because it was the closest value until the 0.068uF caps came along.)

I can say that I understand that the grid resistors are for protection from grid arc'ing which can be a problem with some output tube and power supply configurations. I can also say that the cathode resistors are used to measure the current through the output tube with a multimeter. You'll want to install those grid resistors to measure what is that current flow because you want the output tube currents to be very close to the same across all four tubes. Uou also want the current flows to be within a certain range (I don't know what that is for a 500B though. I'm running my KX-100 at about 42ma for each tube - but the KX-100 has a strange bias arrangement where I have to run the current higher than is otherwise needed to generate good sound).

I would suggest the mod that will provide for better current flow adjustment on those tubes than you have right now - Individual Bias Adjustment Modification (IBAM). Is it difficult? Well, I did it, but I am a bit handy. Not great with a soldering iron, but I get by. You'll have to be a judge as to how handy you are. Just be super careful about playing around when testing your changes. There is some seriously High Voltage DC (HVDC) under the chassis. If you don't think you can be safe, then have an expert do it for you.

BTW, I do have a 500B in the queue. It's going to be a while before I get to work on it though. I'll be watching to see what you do. I will be doing all of the mods that have been mentioned plus a few others.

 

[dfunghi]

BTW, I do have a 500B in the queue. It's going to be a while before I get to work on it though. I'll be watching to see what you do. I will be doing all of the mods that have been mentioned plus a few others.

I have a zero skill set so "I" won't be doing anything. I will likely run it with the tubes it came with (originals) until I can swing sending it for some upgrade/mod/rebuild attention.

 

[larryderouin]

Be very careful when running a 50+ year old piece of gear with no attention. Output tubes should be matched for Cathode draw and if way out of match, can redplate the tubes that are pulling heavier loads. Also the CAN CAPS on the top DO dry out and can short. Check them for excessive heating after 5 minutes of operation. If any are warmer than the surrounding components (radiant heating) or HOT, turn it off and send out. DO NOT RUN IT AFTER THAT. These are only TWO of the problems that can ocur with a unit that has not been overhauled. Would you take a '63 Vette that had been sitting in a garage for 30+years, pull it out of the garage and attempt to DRIVE IT to SFRAN or L.A.? You probably wouldn't get out of town without something major going wrong.

 

[dfunghi]

Be very careful when running a 50+ year old piece of gear with no attention. Output tubes should be matched for Cathode draw and if way out of match, can redplate the tubes that are pulling heavier loads. Also the CAN CAPS on the top DO dry out and can short. Check them for excessive heating after 5 minutes of operation. If any are warmer than the surrounding components (radiant heating) or HOT, turn it off and send out. DO NOT RUN IT AFTER THAT. These are only TWO of the problems that can ocur with a unit that has not been overhauled. Would you take a '63 Vette that had been sitting in a garage for 30+years, pull it out of the garage and attempt to DRIVE IT to SFRAN or L.A.? You probably wouldn't get out of town without something major going wrong.

But this unit was in daily operation by a fellow AKer that I bought it from so hopefully it is not going to exhibit as many issues. I plan to turn it on this weekend and run with original power tubes and the new tubes from Bryce. Will watch and listen closely.......

T,
D

 

[Tim D]

Just some food for thought. It might save you some grief.

Some items are costly and a little challenging to find parts to replace such as the transformers. The power switch is another costly repair if the caps go bad and burn the on/off switch. Just be careful you don't blow anything expensive if something like a Power Supply cap goes shorted/bad. Tubes can be somewhat easily replaced, but some are costly too. Having the right fuse will add a little protection. Fisher typically spec'd slo-blo fuses. I don't know what size is recommended for the 500B offhand.

(Not recommending it, but I used a fast-blo of smaller amp rating than Fisher specified in my Fisher KX-100 amp. It gave me a sense of security just in case something ever goes wrong inside. I had a "Kill-a-Watt" meter from Harbor Freight which helped to measure the current draw at the power cord. That confirmed that I'm not too close to blowing my fast blo fuse choice. I think Fisher wanted a 3.2A slo-blo in the KX-100, I installed a 2A fast blo and I measured only about 1.35A on the cord. I figure if something shorts inside, that it will at least protect the Power Transformer, maybe the on/off switch, and maybe the output tubes depending on the failure mode. I also put in two CL-80 inrush current limiters (they are cheap). Now the current ramps up very gently and there is no sudden current spike that could blow that small fuse I put in. It takes a while for my KX-100 to warm up now, but I have never blown that 2A fuse - even with my fumbling around inside the chassis with my multimeter with everything hot. I have caused an arc or two when doing that even when I'm being vigilant.)

I think it was already suggested, but another way would be to get a capacitor kit from one of the known capacitor kit suppliers to replace the power supply can caps and then have a local tech install them. That might eliminate those power supply caps as a source of concern. Not real cheap to do if you buy all new can caps, but it would reduce the likelihood of some of the more common failure modes. (I stuffed my old can caps in my KX-100 with pencil caps and saved a bunch, but if you aren't handy, maybe buying new cans and paying someone to install them is your route.)

There are a lot of AK members in California too. You might think about hooking up with someone that could help with some basic upgrades. Whatever you decide I am sure you are going to love that 500B.

 

[Rockyhill]

I can appreciate the desire to "drive" one of these classics when they are initially acquired. However I resisted the temptation when I bought my 500B a few years ago. I never plugged it in until after implementing all the recommended upgrades. I used a Jim McShane kit. I also have 2 CL-80's to give it a slow start. I don't use the power switch to turn it on/off either, I have it plugged into a switched GFCI.

I had never overhauled something like this previously. With a lot of study and many questions I got it done. I'm glad I waited before powering it up. I have subsequently added the IBAM circuit.

 

[Rockyhill]

Here's the GFCI adapter I use.

 

[dfunghi]

Here's the GFCI adapter I use.

Yes, all my stuff is plugged into switched adapters and then into a power conditioner bar. I try to never use on/off switches on older gear

 

[larryderouin]

How about this to help alleviate our concerns. Take a few pictures of the underside and top of the unit. This way if there has been any work done we can tell and pass on info to you as to what still needs done.

 

[dfunghi]

How about this to help alleviate our concerns. Take a few pictures of the underside and top of the unit. This way if there has been any work done we can tell and pass on info to you as to what still needs done.

Thank you sir, a fine AK tech has contacted (actually a few) me and made same offer. I am about to go put all the tubes back in and turn it on. Seller assured me it was 100% working so I have little fear (but not none) about magic smoke. Another fine brother here offered to run some diagnostics on it if I bring it by. He also said he may part with a quad of 7591 for the 500B as well.

Lastly, I have been reading and hearing different things about the original output 7591 and that they could work pretty well even if testing weak. I will do a listen test and of course closely watch the heat factors.

D

 

[dfunghi]

OK, All tubes (upgraded and original 7591) are in and 500B has been powered on. I only have one speaker hooked up and an antenna with FM. Just going to let it run a bit and check the heat. Once I am confident all is well I will begin to add more sources and a second (left) speaker.

So far so good.

Thanks to all for the help and thanks to Bryce for the tubes. I am sure I somehow lost track of where you said to use which sets (correct type in correct place but some were tested to match) but I did my best.

FM reception is not as "solid" as the MAC 1900 but I think I may have knocked loose one of the FM alignment towers (sort of 4" tall aluminum box that looks like a heat shield sort of).

Be back soon......

 

[dfunghi]

So far so good...... can hold my hand above the 7591 for a long time and feel heat but not burn. Few of the other tubes are any more than warm. FM is not great. Going to plug in the AR TT soon.....

BTW I am listening via a pair (well so far just one) of Signet SL 260 BU.

 

[dfunghi]

Well, Sadly the 500B does not sound anywhere near as nice as it could/should. AR TT with a Shure V 15 V and a Coltrane LP known to be NM. If this is due to the poor output tubes or some other issue I will not know until I get this on someone's bench. I did buy this as fully functioning unit that "sounded great" and maybe something I have done or in shipping or ??? happened but clearly it does not sound great.

Will update progress as it is achieved. Right now budget is too tight to do much other than now get the 400 out and see how it sounds and then back to the MAC 1900 until I have working Fishers.

 

[dfunghi]

OK I switched the power and TT plugs polarity. Things sound much better. More work to do but this now sounds like a nice receiver.....