GM-70 Amplifier

[randy warren]

thanks

[SoundMods]

You're very welcome!

[Roscoe Primrose]

B[color=#BF0000]ut don't power tubes like to "see" a high load impedance. It seems to me that most production tube amplifiers with output transformers have a compromise transformer to help deal with different speaker loads -- hence the different taps at 4, 8, and 16 ohms. The nominal impedance of my Altecs is 8-ohms, but the sound really opened up when I started feeding them from the 16-ohm taps. My 845s seem to like a higher plate load.

Paul Elliott's Magnepans have a nominal impedance of 4-ohms and yet when we ran them off of the 8-ohm taps of his them Monsoon amps. they opened up in a similar fashion. Paul's KT-88s liked the higher plate load.

When you connect an 8-ohm load to a 16-ohm tap, or a 4-ohm load to an 8-ohm tap, the output tube(s) is (are) seeing half the nominal primary impedance... Triodes tend to like a higher load impedance, pentodes tend (to a point) to like a lower load impedance.

Roscoe

[SoundMods]

I had to dust off my brain. Yes -- but actually transformers do not have an inherent impedance per-se but are "looking" for the reflected impedance of the load to provide a load the tubes like.

That said -- my guess is that our success with the higher taps may have been a coincidence in terms of the particular speakers and what the amps. like to see.

It comes down to transformer design essentially and the industry practice. My previous Music Angel amp, my Radio Craftsman amps, Paul's Monsoons, and my Carys sound best using a higher tap. ??

OH WELL! I am always experimenting (that's what a rabid DYI kind guy/gal does -- right?) so I run into things that seem to violate every rule in the "books" and yet the results are different (better) than what would be theoretically expected.

[Don Diego]

I had the opportunity to hear David's latest creation yesterday and was blown away on all fronts. The sounds and synergy coming from these amps and speakers are awesome. Intricate detail, powerful dynamics, perfect tonal balance, and very musical made my listening experience extremely enjoyable. Like Stuart said earlier, these amps are huge, each monoblock looks to be over 100 pounds. As with all of David's projects the workmanship is superb. David never fails to impress and these GM-70s have to be seen and heard.

[David McGown]

Thanks for the review. I do look forward to showing these amps off sometime soon.

With respect to the interesting discussion on power tube cathode bypass caps, I do have space under the "bonnet" to add a rather large composite film capacitor, several hundred uF. Though it sounds pretty good with the 50uF I have in right now.

It would be fun to test out a gm70 and even bring over an amp to Dave B. for testing. Though transporting one of these amps will be an effort given the weight.

Not sure if I mentioned it, but the output transformers are beautiful Tamara 10K 50W units, using the 8 ohm taps, which match the nominal impedance of my BassZilla Platinum Edition Mk2 speakers.

David

[Guy]

David, I have an engine hoist we can use to get them to DBs.

[TubeDriver]

I used to hear that BG electrolytics sounded better than most metalized film caps in the cathode bypass position? Has that not been your experience?

Metalized film. We are not dealing with much current to speak of and the larger values caps (even polycarbonates) can get massive when compared to their electrolytic brothers.

When upgrading cathode bypass caps you arefaced with installing 10 lbs of crap into a 1 lb bag -- as it were.

[SoundMods]

Black Gate capacitors are still aluminum electrolytics and may sound somewhat better than the usual aluminum cap because of the carbon damping. As far as electrolytics go my fave is the Elna Cerafine.

BTW all -- I forgot to mention that my Carys came from the factory with 50-ufd polypropylenes across 1,125-ohm cathode bias resistors for the 845s.

That put the 845s at 4.5-Hz.

So -- it's Dennis Had -- renowned designer and manufacturer of Cary products that does that as well to name just one manufacturer -- it's not just me.

Just saying.

[David McGown]

I put in a 60uF/250V Dayton PP cap in parallel with what I already had as a cathode bypass capacitor. Significant improvement in the definition of the bottom octave. I have another like it to put in, but liking what I am hearing.

David