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No question, the KB85 was marketed as an “EL37 amp”. Even if original design iterations were 6L6 based, I wouldn’t expect Allied to reveal this to the public in any form whatsoever. What you see in the catalog and lit is what you get. Regarding optimization, the opt primary impedance is...

If you’re disagreeing that the EL37 is interchangeable with a 6L6 type, your disagreement lies with Mullard. See portion of Mullard cut sheet below. Regarding the KB85 specifically: Contrary to speculation here, pin 1 of all finals are chained together and grounded to the chassis at the...

Yes the tester “tests” for gm among the typical parameters. It tests an EL37 as a 6L6 . . . they tend to test quite healthy. Despite the differences in operating principle and specs, a 6L6GC subs just fine for the EL37 in the KB85: meets specs/sounds fine with either. Probably runs cooler...

While EL37 specs appear closer to an EL34 than a 6L6, back in the day the 37 was promoted as a premium upgrade to the 6L6 . . . Even though the 37 is a true pentode and the 6L6 is a beam tetrode. I have at least one vintage tube tester that says “use 6L6 setup/test data for EL37”.

Having restored and used both amps, I'd opine that the KB85 is not all that similar to the 240. To my ears the 85 is more like a somewhat less "sweet" Marantz 8B . . . which is still pretty good company. Both the 240 and the 8B put out more power than the 85. In this regard the 85 more...

This is an example of the GE 5AR4 I was referring to. Renown GE etched dot-code readily apparent. Date appears to be 17th wk of (19)66 . . . About right for this design. Notice large cathode, the telltale copper anode and 6V6GT sized envelope. Image from web.

The pictured tube is definitely a Sylvania 5AR4. As pointed out elsewhere, it’s a great version of the type. Always struck me as having a damper diode heritage with huge cathode and really extended warm-up. Its only possible drawback is its physical size. The other US mfgr was GE whose...

Russian origin. “Flying saucer” getter holder is a major clue.

Tube rolling for extended life is new reason to roll (for me). I’d guess that rolling 6550s into a 6L6 position would accomplish that goal all else equal. In the good ol days I would have been more inclined to roll in the US mfg 35W 6L6 variants like 7581A or 7027A if extended life was a...

It’s good you had a positive experience successfully rolling 6550s into your MC30s. It's also good you have hard data confirming proper performance. While it’s a fact that the basic signal circuitry of the 30 is quite similar to one channel of the 240, there are enough significant differences...

All of those types are really not up to the rigors of the Mac amps. But I’ve also seen those and other lower rated 6L6 types apparently surviving fine in them. So you probably could get away with using them . . .while keeping vigilant. I stick with GC types either NOS or new mfg.

No, the schematic shows 430V and my experience is 425-475V depending on line voltage and how hot finals are biased. This range applies to all vintage Mac “consumer” amps I’ve encountered. Don’t know about MC3500 and MI200 is far higher than 500V.

While 6550s may “work” in a 240, the amp wasn’t designed for them, esp wrt filament current needs. A quad of 6550s will increase filament needs by ~ 3A, not a trivial increase. My experience with 6p3se is that it’s nearly a 6L6GC but with plate dissipation closer to 25W vs the 30W rating of...

“Not really sure what the right type is though honestly.” I’ve successfully used high temp “red” silicone RTV rubber as a cap adhesive. I allow the RTV to cure 24-36 hrs prior to soldering.

Sure, it’s a valid means of improving the probability of obtaining an acceptable “match” between two randomly selected resistors or composite resistors. It doesn’t assure the absolute quality of match, only that the odds of achieving an acceptable match are improved. OTOH, if a more precise...

Can't speak for Kegger's motivations, however . . . one potential benefit of paralleled equal value resistors is improved match without measurement where matching is necessary for proper performance. ex. Say you're working on a split-load PI design that calls for 47K resistors for cathode and...

To be sure, even with lube and partial insertion, the pins on Russian 7868s (and Russian vertical sweep tube subs . . . Forget designation), are big enough @ 0.050” compared to std 0.040” that socket contacts will be permanently stretched. After this, the only 7868s you’ll be able to use are...

The oldest repair I can recall is ~10-12 yrs for plate cap on a 6BQ6GTB used in an old Kepco reg DC supply. I used the red RTV here. This supply gets only occasional use. The second oldest is where I used black automotive RTV on a #80 rect base that I rebased to octal to look like a 5Y3G...

I tried hi temp refractory or furnace cement on a tube base. It adhered but dried hard and brittle and caused the glass to crack at the base/glass interface. One benefit of the silicone RTVs is that they’re somewhat compliant.

That parallels my experience with epoxies. I’ve found the “red” high temp RTV silicone rubber to be durable, heat resistant and strong enough to stand up to the rigors of plate cap adhesive service including the smaller ones on some horizontal sweep tubes. Most silicone RTVs seem to have an...

If the resistor is 1 Meg:1/2W, that 1/2W would occur with ~700V across it. 10M:1/2W ~ 2.2KV! And that’s at rated power, not grossly over powered. Where would voltages like that come from? Clearly something is seriously amiss here. The resistor has to be (was?) a lower value or something...

While it is indeed unfortunate that Jim is retiring from the tube business, his knowledge and advice don’t depend on messed up supply chains and global politics. I’m sure he’ll be gladly welcomed whenever he chooses to participate in communities like this one. Not gone and certainly not forgotten.

If the voltage and current measurements are correct, wouldn’t the resistance be 20/0.045=444 ohm? Maybe a little high depending on rated primary impedance.

My transformer meas method is essentially the same as yours . . . And so are the results. IOW, my unloaded or open circuit tests usually duplicate the mfgr’s specs wrt turns ratios. I believe the open circuit test is a standard for determining turns ratios. Loading the transformer brings...

The Tube Store (an AK advertiser) shows stock for EH, JJ and TS reissue so new mfg is apparently still avail for a price. If you’re willing to come up with socket adapters, there’s also the option of NOS 6GM5 that sometimes can be a little more avail and maybe less costly than than NOS 7591(A)...