SET amps, bass response and OPT's

[Kegger]

(Gordon W)
"One historically-proven way to get good sound, is to pick a good, middle-of-the-road-size transformer (big enough to not saturate, but small enough
to not slow the slew rate) and use a modicum of negative feedback to linearize the output. If done correctly, these can result in the best-sounding
reasonably-priced amps, IME."


(Mashaffer)
"You could try a bit of plate to plate feedback (output plate to driver plate)"

--------------------

Both of these are good advice and what I adhere to. I try an stay away from Global Feedback when possible but a little local FBK works rather well.

As to the Polk 10's, I run them on tubes and there decent with a good running 5 watt amplifier but do run Really Well on a 10-12 watt or higher amp.

 

[currituckco]

I agree with people who have said that you are not running optimal speakers for SET. I'd look into getting higher efficiency speakers, in addition to looking at the circuit of the amp. I have the feeling that replacing the OPTs is not the answer.

 

[mhardy6647]

I agree with people who have said that you are not running optimal speakers for SET. I'd look into getting higher efficiency speakers, in addition to looking at the circuit of the amp. I have the feeling that replacing the OPTs is not the answer.

Well, yeah, the canonical answer is to use high-efficiency (more accurately, high sensitivity) speakers with a low-power class A SE amp... but the more cogent constraint is usually to make sure you have as benign and amp-friendly of a load as possible; a flat impedance curve and relatively low reactance will make the speaker happier with the amp and vice-versa. Some of the classic speakers, of course, were designed for this. Ribbon and planar drivers are (IIRC) pretty purely resistive loads, and could be good with an SE amp even if not 100 dB-plus sensitive!

Case in point: the original DCM timewindow probably isn't terribly sensitive, but empirically matches quite well with the Paramours. Will the combination play house-party loud? No, but it sounds good at reasonable levels with the kinds of music I listen too most.

So, as long as I am pontificating about SE amps and speakers let me also say this: the canonical advice to use high(er)-impedance speakers is probably better stated thus: use as much of the secondary winding of your OPT as you can. For example, the secondaries on the Paramour's little OPT are 4 and 8 ohms. A SET-friendly, nominal 8-ohm speaker is fine here; in fact, I don't think I've ever tried a 16 ohm speaker. Steve Deckert's little SE-84 "Zen" SE pseudo-triode amp sounds awful with 15-ohm Lowthers; mostly (I think) because Deckert chose to use OPTs with 6-ohm secondaries to better match 'modern', low-ish impedance speakers. 15-ohms was just too much of a mismatch.

 

[jaymanaa]

I read a heck of an article somewhere about winding your own. Then it's affordable to do things like Gordon mentioned (multiple windings etc..). The old timer in the article wound them on old power TX frames. Too much math for this hayseed though.

 

[mhardy6647]

You can pay folks like Heyboer, Magnequest, or Electraprint to make you pretty much anything you want (if they're not making something you need already)!

 

[currituckco]

Steve Deckert's little SE-84 "Zen" SE pseudo-triode amp sounds awful with 15-ohm Lowthers; mostly (I think) because Deckert chose to use OPTs with 6-ohm secondaries to better match 'modern', low-ish impedance speakers. 15-ohms was just too much of a mismatch.

This is to say that there was only 1 secondary tap and it was 6-ohm? I can see why that would be less than ideal. I'm hoping to build an SET 45 amp (Gordon Rankin's Bugle) as my next project to mate with my 16-ohm Altec 604-D's. I wired my Dynaco MKIV's in triode and very much like how they sound, but am really keen to try out SE. I'm hoping to get some of those Magnequest Robin Hood OPTs...anybody have any experience with those?

 

[mhardy6647]

This is to say that there was only 1 secondary tap and it was 6-ohm? I can see why that would be less than ideal. I'm hoping to build an SET 45 amp (Gordon Rankin's Bugle) as my next project to mate with my 16-ohm Altec 604-D's. I wired my Dynaco MKIV's in triode and very much like how they sound, but am really keen to try out SE. I'm hoping to get some of those Magnequest Robin Hood OPTs...anybody have any experience with those?

1) Yes, you are correct, sir.

2) I can vouch for the Bugle; one of Gary Kaufman's many fine DIY projects was convertible version of the Bugle that can run 45s, 46s, or 300Bs; fine sounding amp. Hard to go wrong with 45s, I think, anyway. Oughta be dandy with 604s.
http://www.the-planet.org/Ragtop.html

Gary also once built the Angela Labs simple 45 amp design, too; also quite nice.
http://www.enjoythemusic.com/bostonbash99/

 

[Chrome]

Both of these are good advice and what I adhere to. I try an stay away from Global Feedback when possable but a little local FBK works rather well.

As to the Polk 10's, I run them on tubes and there decent with a good running 5 watt amplifier but do run Really Well on a 10-12 watt or higher amp.

I should have said what you said when I said I dislike feedback in amplifiers.

Local, ok, here and there. Global...... Run!

My RTA-15s kick some serious ass with my SET creations. And I'm talking everything from 1.5 wpc 6bm8 amplifiers, all the way to Frankensteined paralleled 6v6's (10 wpc) After that it's P-P with me

And they are only rated at 90dB....

The low end is always there, however, I don't listen to rap

 

[currituckco]

2) I can vouch for the Bugle; one of Gary Kaufman's many fine DIY projects was convertible version of the Bugle that can run 45s, 46s, or 300Bs; fine sounding amp. Hard to go wrong with 45s, I think, anyway. Oughta be dandy with 604s.

Yes, I've seen that page. It looks interesting, but I don't know if I'd have enough space for the extra 5-pin sockets wired in parallel, though it's certainly an interesting idea. I don't think I'd try for the 300b since even the new production of those are quite pricey.

 

[mhardy6647]

I think the 45 sounds much better than the 300B (MNSHO, of course). The 300B SE amps I've heard (with one exception, Kevin Kennedy's) are a little sterile sounding to my taste.

The 46 is fun tube, though... cheap, too (last time I looked). I have a small stash and most of the parts to build Gary's amp (more or less). Someday...

 

[currituckco]

if i have enough space i'll definitely try the 46 thing too...nice to have options. The 46 is dual grid though - how would one wire it in parallel to the 45? Would I leave out one of the grids, or wire them together or something?

 

[jaymanaa]

if i have enough space i'll definitely try the 46 thing too...nice to have options. The 46 is dual grid though - how would one wire it in parallel to the 45? Would I leave out one of the grids, or wire them together or something?

Triode strap it and you even gain a little power.:thmbsp:

 

[currituckco]

So that means strapping the two grids together, or strapping the 2nd grid to the plate?

 

[Thespeakerdude8]

http://simpletube.com/databank/SingleEnded/EL84_SE_ECC81.gif

Try this kind of circuit. I have used it on several SE console amps so far and have gotten such magnificent results that my standard now is to gut SE console amps and replace the stock circuit with this.

 

[markus]

http://simpletube.com/databank/SingleEnded/EL84_SE_ECC81.gif

Try this kind of circuit. I have used it on several SE console amps so far and have gotten such magnificent results that my standard now is to gut SE console amps and replace the stock circuit with this.

that is great news . . . I'm doing exactly that with a little SE Sylvania console amp. It's always nice to know what you're attempting has already been successful for someone else :thmbsp::



Eventually, I want to try and build the RH 807 amp too . .. I have some 1625 tubes (essentially the same as a 807 w/ different pinout)

 

[mhardy6647]

Markus... have you tidied up your PM inbox lately?

 

[markus]

Markus... have you tidied up your PM inbox lately?

yup - did last night after your bumped message

got another question for ya too

 

[mhardy6647]

all righty then...

 

[Kegger]

http://simpletube.com/databank/SingleEnded/EL84_SE_ECC81.gif

Try this kind of circuit. I have used it on several SE console amps so far and have gotten such magnificent results that my standard now is to gut SE console amps and replace the stock circuit with this.

That is simular to what I've been doing for a while now to.

BUT, I've been using Single Ended Outputs that have UL taps when running the
pentode output tubes, so I run them either Triode wired or in UL, and the plate
to plate/grid feedback resistor is larger for less feedback when I run in this mode,
roughly twice+ the size resistor/say half- the feedback, simple good working circuit.

(Here's an example of the latest spud amp)
http://www.audiokarma.org/forums/attachment.php?attachmentid=114314&d=1224803151
Lately for more gain from the driver I either bypass it's cathode resistor, or use an LED instead.

 

[hypertone]

Do your SE amps employ any negative feedback? If not, besides the issues with small OPTs limiting LF linearity, the output impedance of the amp will be pretty high (i.e., very low damping factor) and as a consequence, the speaker and amp will interact considerably.

I've learned a lot about tube amps since originally posting this. I triode strapped one of the single ended amps, and while I was in there, I noticed it had no NFB loop! I think that was the reason for the weak bass. Triode mode made a world of difference in bass response on this amp. It's still a bit light, but it's far more balanced and the bass is much tighter. Gotta love tubes! :yes: