Frustrated about my 500c!

Switch 1 and 3 and see what it reads

If you switched the tubes and they read the same xcept in a different place, then it looks like you have 2 pairs of matching tubes. Unfortunately, they don't match each other.

Since you have a fixed bias, changing the resistor will just change the bias across the board. Substitute a single adjustable pot upstream, same result. If you don't want to do additional electrical work, i.e. 2 pot or 4 pot IBAM, then review post 50 in this thread and do what Larry suggested: check for tube dissapation on the hot tubes. After you do the math, if you're in the area of 75%, then leave it alone and enjoy. If you're running hotter, well, maybe a new matching pair; or fire up the soldering iron again.

Yup, that's where I find myself. I moved them and they stayed the same, just the two on each channel match better. Tomorrow I will check the plate voltage and see how that figures in to dissipation. If I am over the 75% I'll probably use the tubes but order a new pair. Am I risking a transformer if I run two tubes too hot? What would I be soldering at this point? That bias resistor? That would bring the current on the two high tubes into a good range, but wouldn't it also make the other two run too cool?

That's the reason I went this route to solve my bias problem:

ccheath said:

That's the reason I went this route to solve my bias problem:

Click to expand...

Yep, individual bias is the logical next step or buy a new matched quad.

That would bring the current on the two high tubes into a good range, but wouldn't it also make the other two run too cool?

Click to expand...

Yup and exactly!

Many, like CC & Bruce mention, go the IBAM route allowing you to manipulate the bias for individual tubes. Your other option is to get a pair that run cooler that will more closely match the other set. Very recently, in a post over at AA, Jim McShane counseled against running mismatched pairs, http://www.audioasylum.com/forums/vintage/messages/21/211802.html Really nice guy. If you got your set from him, (or even if you didn't,) maybe he'd consider matching a pair for you.

Here's a nice thread on Bruce's IBAM. Other examples from Larry, CC and the IBAM guru himself, Drew, are in there. Which direction you head will depend on your comfort level, time availability and considerations for long term integrity of the unit.

http://audiokarma.org/forums/showthread.php?t=417314&highlight=IBAM

Definately put the ibam in there 1st, and check the range of adjustment for each tube. I think you'll find that the two that are reading high will tend to have a higher bottom number than the other pair. That doesn't mean the other pair isn't good it just means they are more worn. (Keep them for spares). If the ranges overlap you can use all four and adjust to match. If not, then get a matched quad.(long run easiest and cheapest way to go) from Jim McShane. Jim WILL match 4 tubes for you. They will all be within 1ma when you get them. Save the boxes and mark them and the tubes to match, incase you accidently break one(BTDT), you send him the tube info and he can match another tube from that info.

You may have a pair that is fairly unused and a pair that is worn but servicable.

For example if your B+ is 430V with the higher tubes the tube drawing 42ma is putting out 18.06W. the 44ma tube is putting out 18.92W. WAY TOO HIGH! On the other hand the other two are putting out........32ma =13.76W and 28ma =12.04

The upper pair are putting out MAX (for all intents and purposes)dissipation, where the lower pair is putting out between 77%(32ma) and about 63%(28ma).

I used 430v as the B+ in the example as it's a good avg for the 500C. Higher or lower B+ will affect the ma but it won't be too far off what is there now.

With the IBAM you can get these all within 2ma which is ideal provided the tubes are in good shape and their ranges overlap. Plus you can adjust all the tubes Hot or Cold as you want. The IBAM is probably the cheapest mod I've seen to promote output tube longevity.

Larry

Whoa! Let's not lose sight that these are brand new EH's that have been red-platted! Don't waste your time trying to salvage them. Replace at least the two with weird bias readings.

IBAM or not it looks like at least two of the tubes are toast. If he measures them again I bet he will have different bias readings. Red-plated tubes normally will not hold a bias setting. Note that since all four source a common bias a drifting tube will affect all of them (though the bad one will be off the most). You cannot really identify a bad one without the IBAM in place.

I developed the IBAM because A) you have no chance whatsoever of using JJ's without it, B) most NOS tubes are really just good looking used tubes (no you cannot tell by appearance) and C) tube bias' drift differently as they age, I have yet to see a fixed bias Fisher with old tubes that match .

Even Jim will agree that the IBAM does allow for the use of unmatched tubes. His advice against running unmatched tubes is for the fixed bias, definitely the case.

Drew is absolutely right, these were brand new "matched" EH's when I started this project. They all have '34' written on their individual boxes, which I assume is their tested draw. The two that are giving me trouble are the two that were the first to light up, so I guess I toasted them. Is it going to hurt anything else if I use them for now, knowing they won't last as long as the others? I will also contact Mr. McShane, in the mean time I will read up on the IBAM and get a feel for the level of difficulty. Since my goal is to keep the 500C long term, it is probably a good idea to install one. Just surprised that the unit even needs one, you'd think the fixed bias that goes to all four tubes would be treating all four tubes the same. Some are more sensitive than others? Even when I had the rectifier wiring all screwed up, shouldn't they all have fried?

Hi guys, I am bumping this up because I have new information in addition to the leftover questions from my last post. I went home for lunch today and turned the 500C on to let it run a while. After about 20 minutes of FM I checked the bias to all four tubes again, -19.8V. I checked the draw across the "fusistors" again, still running at 28, 32, 42 and 44. Don't know why I think they might have changed. Just for giggles I also checked the voltages at Pins 3 and 4, for the first time. I am getting 456V at Pin 3 and 415V at Pin 4. Isn't that high, if Larry is assuming about 430 on average for the high B+? Could that be part of my problem too? I certainly don't want to buy a new pair of tubes and fry them also. And is my tube that's at 28 ma too cold?

The IBAM idea is a learning moment for me - is the idea that we are taking the -19.8V coming in (or going out, whatever) and trimming it up or down to get the tube draw the same for all four tubes, at a reasonable level (say, 32 ma for all tubes)? Am I right in thinking that we may lower the bias for the two hot tubes (say, to -22 or -23V) in order to get the tube current draw where we want it? Is that the actual goal here when we say we are "biasing" a tube? Newbie questions, I know, but I'd like to understand the concepts. Not quite understanding yet what is really going on inside the tube. Can these adjustments be made by changing a resisitor elsewhere in the system, or does that one resistor in series, in my case 8.2K, really drive the bus?

You are correct, lowering the bias voltage (making it more negative) will lower the current draw of the output tubes. However, -20 VDC for the bias is fine. You shouldn't have to go any lower. And, worse, if you lower it, you'll drop the 28 mA and 32 mA output tube so low that they'll be running too cool, and your performance will suffer.

Try this just for giggles...swap the 28 and 44 tubes, and swap the 32 and 42 tubes, and then recheck the currents.

Plate and Screen voltages are IDEALLY 430V(pin3) and 375V(Pin4) by the schematic. There are some variables involved. Line Voltage for testing at the factory is 117VAC and all the values on the schematic are based on that voltage. There should be a note on the schematic stating that the voltages can vary as much as 15% due to variances in components, tubes, etc. or words to that effect. Unfortunately the 500C schematic I have doesn't note it. Also the plain voltages (not in parentheses or brackets) are taken with the Selector in the FM mode with NO SIGNAL AT THE ANTENNA LEADS(in notes on schematic). In other words, no antenna, and off station (static only). The voltages given are less than 10% off from ideal. Check the voltages again with the antenna disconnected, tuner between station and volume at minimum. If you have a variac, use it for testing purposes. Set at 117VAC by meter. Turn on the unit and reset the variac(if needed). Let it warm up a minute so it stabilizes, and check the plate and screen voltages along with the bias voltage, then the individual tubes.

OUTLET VOLTAGE xxxVAC
Plate Voltage (Pin3) xxxV
Screen Voltage(Pin4) xxxV
GLOBAL Bias Voltage -xxxVdc
BIAS V-8 xxxma
BIAS V-9 xxxma
BIAS V-10 xxxma
BIAS V-11 xxxma

Cut and paste the format above. It makes it easier for us to read you results.

With 456 on the plates, with the numbers above(-19.8V, 28,32,42,44ma) your tubes are putting out roughly,
12.76W(28ma), 14.59W(32ma), 19.15W(42ma), 20.064W(44ma).
33ma would put you @ 15.048W and 34ma gives you 15.50W 75% is 14.25W 80% is 15.02W.

Incidently WHICH 2-3 redplated??? You DID mark them, right?

Here are the lunchtime numbers for today...

Outlet voltage - 121v
Plate voltage - 450v
Screen voltage - 415v
Global bias - -19.7v
Bias V8 - 28ma
Bias V9 - 32ma
Bias V10 - 44ma
Bias V11 - 41ma

I don't have a variac yet, I use the dim bulbs to start things. Based on what I'm understanding, the two tubes that got to glowing during my trials with the rectifier are the ones that are giving me the problem now. Yes, the ones that are now in positions V10 and V11 were red for about 20 seconds before I shut it down. Not hot cherry red, but a dull deep red glow. My bias has consistently stayed between -19.2 and -19.8. so the question remains, is it safe to use the unit while I save up money for a new matched pair? Things are very tight right now and she who must be obeyed would not understand...:nono:

I will look more into the IBAM, but in the meantime...Is there another way to adjust individual bias to get those too high tubes down and the low tube a little up? Add/subtract a resistor somewhere?

Everyone is correct, there is a possibility that the two tubes that red plated are damaged. However, since they didn't get beyond the dull red stage, that is by no means a given.

In the meantime, swap V8 and V10, swap V9 and V11, and see what current readings you get.

Yeah, I tried the swapping route, that's how I got to where I am now. It didn't change anything. It sounds like the only risk I am taking is that these tubes might expire prematurely, but since the consensus seems to be that they are likely done anyway, considering the high current draw, I will just enjoy them while they last. One little sidebar - I do have one old original Fisher 7591 from when I bought it. It still tests good. Would it make sense to pop it in for one of the high draw tubes, just to see if it changes anything? Maybe the 44 ma one? Just a thought. I'm at the point now where I would like to button it up, put it back in its case and listen for a while. It's hooked up to A-25's right now, with Forte II 's waiting in the wings.

If the high current draw moved with the tubes, then it's the tubes. Sure, try the old tube. Swap the old tube for the 44 and see what happens.