Potential Tube Amp Coming in Trade

jdjohn

I found this ad on CL where a guy is willing to trade one of his custom-made tube amps for a pair of speakers. https://nashville.craigslist.org/ele/d/stereo-6bq5-el84-vacuum-tube/6568479535.html
Says it's modeled after a Harman Kardon HK-20, and he provides more technical information that I frankly don't know about from the tube world. I have a pair of vintage Infinity Column II speakers that might trip his trigger...he mentions wanting more bass. These speakers are just sitting in storage.

Does that seem like a decent tube amp? I don't really *need* it, but it caught my eye.

mhardy6647

WARNING, WARNING!! OPINION ALERT!!



Well -- it's kind of hard to go wrong with push-pull 6BQ5 (EL84) amplifiers, IMO.

I don't offhand know an HK-20 amplifier -- I thought the HK-20 was a loudspeaker model from them
I am also a little puzzled by the "fixed cathode bias" comment. There's fixed bias
and there's cathode bias, but they're not the same thing. My guess is it's fixed bias, which means that matching the four output tubes is important.

You're lookin' at about 12 to 14 wpc (15 is a little optimistic -- maybe 15 @ 10% THD), so the question is what (if anything) you'd use it for. One would need fairly sensitive full-range loudspeakers (90-ish dB SPL @ 1 meter per 1 watt input @ 8 ohms), or it could make a dandy MR/treble amplifier for a biamplified system.

The EH tubes are nothin' special; not a lotta value there.

I think the big question is what sort of output transformers he's usin' in it?
Rather more tedious to replace OPTs if they're -- not great. He says flat frequency response, but he doesn't say anything about frequency range, which has pretty much everything to do with the OPTs. I mean, it could be flat from 100 Hz to 10 kHz, you know?

The quality of the other passives (resistors, capacitors) is also of at least - academic interest.

Bottom line, as a DIY product/project, I would guesstimate ca. $300 to 400 of value there.

Even-steven swap for, e.g., a pair o' Klipsch Heresys
Were I to swap a pair of Cornwalls or Chorus speakers for one, I'd want some cash, too.

... and if I were to swap a real pair of Altecs or JBLs for one, I'd want quite a fair amount of real cash, too.

EDIT: Check the going price for the Column II (which, I will mention, is pretty much 100% horn-free )

YMMV, though.

END OPINION ALERT!

mhardy6647

son of a gun; the h/k HK-20 was a monoblock amplifier. Who knew?
https://www.hifiengine.com/manual_library/harman-kardon/hk20.shtml
It's a self-biased design (which is good in a way -- no need to make any adjustments unless changing output tubes... if there's a "balance" control inside the amp).


FYI and FWIW, here're the published specs for the h/k monoblock amplifier:

mhardy6647

https://www.hifiengine.com/manual_library/infinity/column-ii.shtml

Guy says he wants a fairly "efficient" (sensitive) speaker. I couldn't find any sensitivity rating for the Column II, but Infinity spec'd minimum amplifier power recommendation @ 15 watts "rms", so it's probably fairly sensitive... i.e., it might be just what the Dr. ordered for the guy!

jdjohn

Thanks for the input and information! The Column IIs have piezo tweets, which are technically horns, right? We've already emailed back-and-forth a bit, and he's willing to consider it. He's going out-of-town this weekend, but we may get together and audition both units at the same time next week. I honestly don't know how/where I would use it, but it did intrigue me...AND gives me a way to move those big speakers on to someone else.

mhardy6647

jdjohn wrote: »

Thanks for the input and information! The Column IIs have piezo tweets, which are technically horns, right? We've already emailed back-and-forth a bit, and he's willing to consider it. He's going out-of-town this weekend, but we may get together and audition both units at the same time next week. I honestly don't know how/where I would use it, but it did intrigue me...AND gives me a way to move those big speakers on to someone else.

Yes, you are right (and I forgot)! derp.

jdjohn

I'm not really sure piezos are the horns he had in mind, though

mhardy6647

mmm-hmmm.



Actually, the Motorola/CTS piezos* can be OK if (and only if, I'd opine) they're not allowed to "free range" down to their natural crossover point (ca. 4 kHz, if memory serves). They're not hard to manage -- the easiest way is to put an 8 ohm resistor across 'em, and then treat the result like a normal driver with a voice coil. Close enough for government work .

* The Brand X knockoffs are best avoided

Infinity did something to manage the piezos in the Column II -- but a quick look at the XO schematic from hifiengine & I couldn't quite grok what it was



And, of course, the redoubtable Dahlquist DQ-10 loudspeaker used the Motorola "lemon juicer" piezos in its rather -- complicated -- quasi-time-aligned array.

jdjohn

I *think* the original Column II piezos were also Motorola, but look different than those in the DQ-10. However, the mids look identical - Philips, I believe. Here are my Column IIs before I swapped the mids. I was testing them out in a side system.

Notice the glossy sealant on those mid cones. I think it was some type of repair. I found some OEM replacements on eBay from a parts unit. Here they are swapped-out.

I was actually surprised at the better sound...maybe I shouldn't have been surprised since there was clearly less mass on the cones! Btw, those front tweets are replacement horns. The rear tweets are still original.

You can't see in these pics (maybe they weren't installed at the time), but these speakers have removable glass panels on top. Razor blades not included, though

mhardy6647

Yes, Philips MR. Pretty good drivers.

The tweeter in the photo(s) above doesn't look like a piezo -- although there indeed were a bevy of morphs of horns (and at least one "cone-style driver) using the same piezo "motor"