7868 tubes, what's everyone using?

[Ziradog]

Oddly, I got the exact same email from Jim McShane about 3 weeks ago when I inquired about a matched quad.

This reply from Jim M. says it all.

Hi Steve,


Thanks for the note.


Un fortunately there is a shortage of 7868 tubes right now. I can’t get replenishment from the distributor and I only have one quad left. That quad is too “hot” (meaning high bias current) for a stock 400. If you have changed to adjustable bias or are willing to do so (I’d really recommend it!) then the quad I have will work.


The only other suggestion I have is to see if possibly another vendor can help. The supply of 7591A EH and Tung-Sols and EH 7868s is in trouble right now. So if you can find a suitable quad grab it. I hate to lose a sale but I don’t want you to be without tubes if you need them!!


Let me know if I can help – and my apologies for the hassle!


Jim McShane

McShane Design

 

[Dave451]

I ordered a quad of Brimar's from the auction site for the Sherwood S8000 IV that's waiting in the wings now. (Right now, up to my elbows in an SCA-35 doing a complete tear down and restore.) Will be my first work on a Sherwood. The Brimar EL506's tested well and were economical. As you can see, they have the larger pins, as was noted. They are the right size and shape. I asked Jim McShane if he was familiar with them and he was not. Tested them for mutual conductance at the same settings as 7868's and no issues there. Anyone else have experience with these?
Dave

 

[gadget73]

No, but I've been considering a set as spares. Where did you get them? Were they already matched, or are those your numbers on the box? My S-7000 doesn't have individual adjustments, just a single pot that I put in there for adjusting all 4 in one go.

 

[Ziradog]

 

[Dave451]

Here's the link: http://www.ebay.com/itm/1-Quad-BRIM...-Sound-EL34-/130363326336?hash=item1e5a427f80

Those are their numbers on the boxes. Here's how they say they test them:

Will test / select on one power amplifier, at Va=370V, Vs=370V, Vg=-13.5V


Record the Ip(mA) value within +/-3%

30 mA at these conditions seems pretty good. I tested them on my B&K 707 and got similarly close numbers for mutual conductance. I haven't been able to find much about folks' experience using them, but thought it would be worth a try. Fall back on EH if my experience is not good. I'll post a thread on them once I try them out. I don't want to put them in my Fisher 400 because I have a nice set of NOS 7868's in it and don't want to stretch the pins. Might consider an adapter to try, as some have done.
Dave

 

[gadget73]

Cool. Honestly at least half of my reluctance to buy a set is the seller's location. Getting stuff from overseas can be a crapshoot. I've seen them listed a bunch of times and almost bought a set more than once.

 

[Dave451]

Yep--they came from Hong Kong, but very well packaged and in good time. Maybe just got lucky? We'll see after I power them up, but they look good so far.

 

[Dave451]

Oops--sorry. From Taipei, Taiwan.

 

[gadget73]

The other thought was that they weren't the real deal. I guess its probably easier to fake a solid state device though, and the pics don't make me believe they are re-marked EH tubes. The price they go for wouldn't leave much margin so I don't really see the point.

 

[Dave451]

I agree. I wondered about that, too. The boxes sure look authentic, the tubes look well made so, for $20 per tube, why bother? The proof will be in the use, I guess.

 

[gadget73]

Fair enough. And considering that I ordered a set. We'll see.

 

[Ziradog]

 

[gadget73]

Off topic, but I was at a local ice cream stand / food joint and noticed on the wall "try our new caramel popcorn flavor ice cream". Suddenly I wish to try it.

Dave, how long did it take your tubes to arrive?

 

[Dave451]

One week. By the way, I haven't been able to find a complete data sheet for the EL506, but comparing pictures of the original to the ones I have shows them to be authentic to my eye at least. Web lore says the EL506 was an attempt by Brimar to upgrade the 7868, although some say it was a pin-for-pin copy. Attached is what I have been able to find for tube data (a Brimar ad in September 1966 Wireless World).
Dave

 

[Ziradog]

Did other Brimar tubes of the mid-1960's use the oversize pins? The pins size (and the location) seem cause for concern.

 

[Dave451]

Not sure, but what's written about the EL506 says they, of course, did it on purpose. Other commenters have noted that the 7868 should have had the magnoval base (with the bigger pins) and I agree for a power output tube. Use of these pins and base makes the EL506 configured like the new production EH 7868, with the magnoval base instead of the novar base (smaller pins) used on NOS 7868's.

 

[gadget73]

I got my EL506's in today. Haven't checked them in the tester or in the amp yet but eyeball impressions are that they are what the auction claims. If someone cloned these, they look to have done it a lot of years ago with great attention to detail. These look to me like a 40 odd year old tube. The boxes have the right old cardboard look, and one even has a couple of small spots like it was sitting against something steel a long while back.

If these check out well, I may buy another set as spares for my other 7868 Sherwood. Four low hour / no hour sets between two amps that don't get heavy use should last me an extremely long time.

 

[RS Steve]

I got my EL506's in today. Haven't checked them in the tester or in the amp yet but eyeball impressions are that they are what the auction claims. If someone cloned these, they look to have done it a lot of years ago with great attention to detail. These look to me like a 40 odd year old tube. The boxes have the right old cardboard look, and one even has a couple of small spots like it was sitting against something steel a long while back.

If these check out well, I may buy another set as spares for my other 7868 Sherwood. Four low hour / no hour sets between two amps that don't get heavy use should last me an extremely long time.

I will be waiting to see how they perform.

 

[Dave451]

Me too. Have a quad waiting for a Sherwood 8000 IV when I restore it . I saw the same things. Looking forward to the report!

 

[gadget73]

Looks like one of the tubes is a dud. Tests OK, but in the amp it draws massive current and has a nice blue-pink glow inside. I'm testing this in my Sherwood S-7000 which was converted from 7591 tubes to 7868. All power supply caps were fresh about 5 years ago, all coupling caps and resistors verified good at that time.

Testing as best I can, with only two of the Brimar in place, the other channel loaded with Electro-Harmonix tubes. This is just a real quick power and THD test, I haven't actually listened to it. Being as they bias so differently, I'm not going to. With two Brimar and two EH in there, the EH tubes run near 50ma when the Brimar are at 34ma.

Full power test at 1kc with both channels driven. 8 ohm dummy load. The tests with EH tubes were with all 4 installed in the amp.

EH tubes: 22 watts, 1.8% THD, 34ma per tube bias, -19.6v at grid

Brimar tubes: 20w, 1.5% THD, 34ma per tube, -16.9v at the grid

Initial impressions are that the Brimar bias closer to what old stock 7868 should. The factory schematic on this Sherwood S-7000 calls for -17v at the grid. I also suspect the slightly reduced power is because of the higher current draw on the EH tubes. Voltages were probably just a wee bit lower, though I did not confirm this.

Same fat pins as the EH tubes though, so you're either stretching the old sockets or using Magnoval sockets. Once you use these, there is probably not a path back to US production.

I'll contact the seller about the dud. Things happen.