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Yet another High serial 400 rehabilitation......

What the heck were you thinking running any part of the air conditioner at all inside a buttoned up trailer in the middle of the summer time with a lot of hot vacuum tubes operating in there? You think you're supposed to be comfortable or something???? When I was a boy, we didn't have air conditioning, and............

Dave
 
I know, I know, No AC, We kept dimes in our penny loafers for emergency payphone calls, B&W TV (if you had one), What the hell is a MALL??, no such thing as "SNOW DAYS" for school. Walked 5 miles uphill both ways to get to school in 20-30 inches of snow, and the boys were expected to bring / keep their Boy Scout knives in their Jean pockets, not buried in someone's back! We rode Bicycles everywhere or rode a bus, (mom stayed in the kitchen), etc. etc. etc. You've only got a couple on me Dave IIRC.

I only use it on FAN. A.C. works but even on the lowest setting it's too cold for me now. Too many blood thinners. Which leads to being cold under 75*F. It's funny watching me bundle up like Nanook of the North in SEPT when it's 68-72* outside. Everyone else is running around in T Shirts and I look like i just spent a week on the tundra.

Short video HERE. Classical, so some may not like it. All controls centered except loudness is on.
 
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28 hours straight running. No problems. Temps are consistant with my 800-C's and TA-600. The tuner on this one is set up right. I don't anticipate any problems with it. It's good enough for the wife to comment that it does sound better to her. She heard it before the rehab.
 
Thanks Luis, Dave, glenbo, and everyone else for the kudo's. It means a lot.

I opened it up this morning and did some checks. With 117VAC downstream of the CL80's I got exactly (B+within 2volts) the same readings I did on sunday. Tubes are still drawing between 27.90 and 28.20ma with a avg. bias voltage of -14.5v and the bias pots maxxed out.

The output tubes are definately worn out. A new set should allow the B+ to increase some and run with a more realistic bias voltage @ 32-33ma.

It's back to running on test. I may cut it short @ 48 hours due to the condition of the output tubes.

Ya know, if I did this constantly I could justify a quad of good old stock 7868's to be used for testing the units. But at two 400's a year, and the cost of Old Stock tubes, no way. Now if EH would change the size of the pins on the EH7868's I'd drop a few dimes on a quad.

Larry
 
When I use an IBAM setup, I always put a pot in series between the bias capacitors and the IBAM module so that I can raise or lower the negative bias as a global adjustment for all four tubes prior to performing the fine tuning using IBAM at the individual tubes.

Since I have not seemed very fortunate at finding pairs of 7868s or 7591s that match closely, being able to select tube pairs that are close enough for both the global and fine tuning methods to work together has saved me from additional expense more than once.
 
Don, Sounds like a decent idea. I like the idea of a Global "rough range" adjustment for older tubes.

IMO these particular tubes are worn out to the point where they'd only be good for "TEST MULES" to prove out circuits. On these 7868's the lack of getter material left is indicative of overlong use, and more than a few episodes of being "rode hard and put away wet". I'm not comfortable with the current BIAS voltage(-14.36 to -14.80), and to get them to run @ 32ma would entail a BIAS Voltage of near -8 to -10V which is too damned low IMO. AFAIC -15V is too hot for any 7591-7868-6gm5 type tube. And with the balance drifting 2 or more mv, they've seen their 15minutes of fame and need to be put out to stud.

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I decided to cut short the 96 hour test due to the condition of the output tubes. Nothing wrong with the entire unit, but the output's are pretty worn and no use subjecting them to further "abuse", while TOM can get some more enjoyment from them.
 

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Does anyone have any direct experience with the Brimar EL506 as a replacement?

They were discussed in this thread:
http://www.audiokarma.org/forums/showthread.php?t=499026

@bhamham, I believe you may have tried these...?
@NOSValves, stated that they should be direct plug in replacements...?

Here is an ePay link (just as a point of reference... not sure I would want to deal with this seller...)
http://www.ebay.com/itm/1-Quad-BRIM...pt=Vintage_Electronics_R2&hash=item1e5a427f80

Thoughts, concerns, ideas...?
 
Larry -- Measure the screen voltage. 7591 class tubes (of which the 7868 is a member) will typically produce normal current flow levels in Fisher equipment with the control grids operating at a negative voltage level that is about 4.7% of the screen voltage. Therefore (for example), if your screen voltage is (say) 390 vdc, then normal, healthy output tubes will typically require about -18.3 vdc to produce current flow levels that are in the ball park of 32-35 ma per tube.

If it is taking more negative voltage than that on the control grid, then the tube in question is a hotter tube (requiring more bias voltage than normal). If it is taking a little less, then possibly the tube simply requires less bias voltage. But if it is a lot less negative voltage than 4.7% of screen voltage implies, then you can bet that the tube is quite well worn.

I hope this helps!

Dave
 
Dave, thanks for posting that test - that's good to know.


tnsmnichols said:
Does anyone have any direct experience with the Brimar EL506 as a replacement?

They were discussed in this thread:
http://www.audiokarma.org/forums/showthread.php?t=499026

@bhamham, I believe you may have tried these...?
@NOSValves, stated that they should be direct plug in replacements...?
Click to expand...

I've not tried my quad of EL506s yet. But I did take another look at the tube data for the EL506 and the 7868 and think they would be a direct plug in. I know of one AKer who replaced his 7591s in a KX-200 with EL506s and reported good results.
 
They have the same pin configuration and similar electric characteristics BUT the socket is a magnoval not novar

Look here:

http://www.r-type.org/exhib/aaa0711.htm

Regards

Luis

bhamham said:
I've not tried my quad of EL506s yet. But I did take another look at the tube data for the EL506 and the 7868 and think they would be a direct plug in. I know of one AKer who replaced his 7591s in a KX-200 with EL506s and reported good results.
Click to expand...
 
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So the options are:

-Old, original 7868 tubes (have to trust seller's testing)
-Electro Harmonix 7868 (with magnoval socket pins)
-Brimar EL506 (with magnoval socket pins)

If you choose to go with the larger magnoval pins...
I wonder which would be surperior...
the 7868EH or the EL506?

Decisions, decisions, decisions... :scratch2:
 
Me personally? I'd go with the EH7868's. They have a good proven track record (with the exception of the pin size). I used them daily avg 6-7 hours a day for two years in my 400 until I sold it to an AK'er near Audiodon, and they never gave me a minutes grief, stayed balanced and bias was checked every 6 months and never had to adjust them. And they sound pretty good, compared to the old stock tubes.

I figure in another 10 years or so the Old Stock 7868's are gonna be gone or so expensive that you'd need to mortgage your house. And you can't guarantee how good they are inthe 1st place. Practically the same with the Brimars. Those Hong Kong Brimar tubes, I'd wager a good amount they are fakes made in China.

When the old stock runs out, you won't have much choice but to get the EH7868's (unless someone comes out with a novar version in the mean time). You could always put octal sockets in it and run 7591's if push came to shove.
 
larryderouin said:
Those Hong Kong Brimar tubes, I'd wager a good amount they are fakes made in China.
Click to expand...

Seems a bit harsh when you've never tried them. I think we need to consider all sources until they're proven not good. AKer 'tube.guy' put them in a KX-200 and likes them. The ebay seller from Taiwan (not Hong Kong) has good feedback and has sold quite a few. At $74 a quad it's an attractive option.
 
Well, you know what they say... "Knowledge is power."

Knowing that McShane...

-is a trusted retailer with a proven track record
-tests and matches his 7868EH quads
-runs his tests at 425V

...I'll go with McShane.
 
bhamham said:
Seems a bit harsh when you've never tried them. I think we need to consider all sources until they're proven not good. AKer 'tube.guy' put them in a KX-200 and likes them. The ebay seller from Taiwan (not Hong Kong) has good feedback and has sold quite a few. At $74 a quad it's an attractive option.
Click to expand...

Bruce;

Didn't mean to sound harsh. The big question here to me is when were they made. Is BRIMAR still in business, and still manufacturing tubes? And where did this seller get them from?

To me the tubes looked too new, and the boxes, too good looking like they were recently made. The carton also didn't show age or staining that cardboard goes thru over the years. I don't think BRIMAR would have hermetically sealed them in plastic. Gut feeling they look to me to be of recent manufacture. I could be wrong too, but my gut is telling me these particular tubes are not vintage.

Until I see quite a few more than 1 actual use recommendation for a substitute tube in the 400 I'd still recommend the EH7868 or old stock 7868's.

Larry
 
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Hi Larry,

I'm suggesting that we need to try them out before we condemn them is all. There's such a limited supply of 7868s and any new comers would be welcome - provided they're proven good.

I know Electro-Harminex makes a good 7868 of recent manufacturing, and I'm not saying to use these EL-506s over the EH ones. EH's are a sure bet and I would recommend them to anyone - especially to new guys.

I'm not at home at the moment or I would try the ones I have. I just wish more guys who have used these EL-506s would chime in.

Bruce



larryderouin said:
Bruce;

Didn't mean to sound harsh. The big question here to me is when were they made. Is BRIMAR still in business, and still manufacturing tubes? And where did this seller get them from?

To me the tubes looked too new, and the boxes, too good looking like they were recently made. The carton also didn't show age or staining that cardboard goes thru over the years. I don't think BRIMAR would have hermetically sealed them in plastic. Gut feeling they look to me to be of recent manufacture. I could be wrong too, but my gut is telling me these particular tubes are not vintage.

Until I see quite a few more than 1 actual use recommendation for a substitute tube in the 400 I'd still recommend the EH7868 or old stock 7868's.

Larry
Click to expand...
 
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