"Mini" Push-Pull Amps - Limitations of Triodes as Power Tubes

[rozrosner]

Steve, are you pretty sure that in your opinion, based on what you personally like a guitar amp to sound like, that the 6G6 is not a good choice as a power tube for a very low powered PP guitar amp?

Worse, in your personal subjective opinion, given what you like to hear, than a triode amp as discussed above?

I was going to say volume and power and Fletcher-Munson issues notwithstanding, but you have already explained why you can't do that. But are there technical spec reasons other than its puny wimpiness that you are referring to?

[Steve Conner]

Rozrosner, I have no real opinion either way. If someone asked me to design a low-powered guitar amp with 6G6s, I could make it sound just fine. Probably better than a triode amp, it would break up with more crunch, as we discussed earlier. There have been plenty of amps made with little low-powered pentodes like the 6BM8, 6K6, 6AQ5 and so on.

The only thing it couldn't do would be loud and clean like a Twin, given the low power available. It would be a little dirtball of an amp. Some people (Neil Young?) put 6K6 tubes into the old Fender Deluxes to reduce the power, and I expect it would be much like that but quieter still.

Now if someone asked me to make a 300 watt tube bass amp, well then I think the 6G6 would be a poor choice of tube. You'd need about 50 of them.

[Steve Conner]

PS, here is a site with some interesting info on the 6G6:
http://www.frontiernet.net/~jff/pips...uitar_amp.html

Looks like you'd need nearer 150 of them than 50, to make 300 watts.

[zhyla]

I listened to the recording of that 6G6-based amp. Based on the recording I'm not impressed. I'm not good at making up adjectives for the infinitely complex concept of "tone" but it just sounds kind of one-dimensional to me.

Speaking of lower-powered power tubes... how many are there? Have they all been tried by amp builders?

[gaul23]

Mini Vox AC30

I just built a 2W push pull amp, based on the AC30 gain stage (an EF86 pentode). It has a passive Baxandall tone stack. It uses 12au7's for a long tail pair. And then 12au7's as power tubes, through a Hammond 125d and a Celestion 12" speaker.

I'm ironing out bugs. And I've never heard an AC30 in person. But it's tone has a lot of flexibility. I can get a growl or a mellow chime from it.

So, it can be done. You can have my LTSpice model of the amp if you tell where to send it.

[GibsonLover]

Don't forget about speaker combinations too. I wanted to hear how "clean" my Fender reverb driver was, so I connected it directly to the 12" speaker in the cabinet ('67 Deluxe Reverb). After pulling the fourth preamp tube, I was left with the initial gain stage, EQ, Reverb driver, and speaker. Given the reverb driver input has a corner frequency of ~318Hz, I was still surprised how much bass I got out of a parallel 12AX7 in SE. The distortion was very much second order and singing like a triode in heat. A bit curious for a reverb driver, which I imagine would have more 'sparkle' and definition if rewired to a cleaner P-P design (the purpose of my experiment). Even with the EQ flat (set to 0) there was distortion. Given that adds ~20dB attenuation after the first gain stage - surprising. Leo...

[cservonyec]

This is the trap. One dives into the amp, diddling with this and that, looking for "tone", when one should really be practicing. The activity is very satisfying, but "tone" is a beast that morphs frequently, so the quest becomes endless pursuit.

Is "tone" as subjective as I make it out to be? The whole concept of good and evil sounds is a bit ridiculous, giving weight to my argument, but music is a language full of idioms. Scales, tone, volume, etc. all carry emotion in a way that is both shared and arbitrary. Major chords sound "happy" and minor chords sound "sad". Hard to argue that that isn't an idiom. Especially on this forum, you can see attempts to speak in the voice of another whose voice is admired. There's a great deal of attention paid to the overdriven voicing of early Fender amps, though, as you pointed out, the tone was pretty much randomly generated initially, from the collective mis-behavior of clouds of electrons, crummy power supplies and carboard speaker cones, and the frequency response of several configurations of lots of wire wrapped around a magnet, influenced by the personal taste of designers who were trying to make a different noise. That voice has picked up "rock" connotations, and become an idiom. Some idioms are sonically unpleasant, but still have their ideals. Saxophone, violin, banjo and ukelele come to mind. It's possible that "great overdrive amp tone" is another example in this class, and that the sweet/sour balance is a matter of taste. So it's possible that the tone quest is a search for remembered idioms one likes, but that's subjective too.

Bob.
I am sorry to butt in. I was just reading the various threads, because I am interested building a good sounding amp, and while I am not an expert, I have some experience building valve amps.
I found your comment almost poetic. It struck to the core. It is very true, tone is very subjective. For instance I like very much the tone of Mick Taylor's guitar in the Rolling Stones free Hyde Park concert starting Midnight Rambler, or some of the early Chicago's Terry Kath's 'Southern California Purples' to mention a few. Of course they are all different, as the instruments amps and players are different. You cannot duplicate these tones, the best you can do is get near the tone you want, and be happy. Going down that path of forever tinkering with the amp leads nowhere, you have to find a good design, and get it sound the best and forget tweaking it.
There are lots of low wattage push-pull amps on youtube to check out the sound of them. Firefly comes to mind.
Keep searching! But practice more as Bob said.

[Capacitordeath]

Here's a link to someone who builds full tube amps that run 12v. They have some 24v ones as well. The little ones run downwards of 1/8w.

Sopht amps - Home

I would figure that although some of a guitar's tone has to do with the tubes, I think the lions share has to do with the other components. This is coming from a very green amp guy, but look at it this way...how can the guys at runoffgroove.com make convincing stompboxes that emulate awesome amps...with zero tubes? Analog circuitry, and the have 2 marshalls, a tweed, a supro, a mesa, and 2 voxes, I think. If I was to think on this personally, I'd say build a 12 or 24v tube power amp, and then build one of these preamp circuits.

[Steve Conner]

I tend to agree, I think people overstate the importance of tubes in determining the tone of tube amps. They do affect it, to be sure, but they're only one piece of a puzzle that has maybe 100-200 pieces. I've had very good results with hybrids and entirely solid-state circuits similar to the JFET stuff at runoffgroove. (although when you say "awesome" and "convincing", who's saying that apart from the runoffgroove members themselves?)

I'd say the most important parts of a guitar amp were:
1: The player
2: The guitar
3: The topology (you can build a '59 Bassman or a Trainwreck out of the same pile of parts, but they don't sound the same!)
4: The speaker
5: The cabinet

[unparalleled]

So why not build a Firefly and use that as a basis to improve on? It's a reasonably simple push-pull triode design and won't really cost you much to throw together. You'll get better answers from doing a build and tweaking it (try different transformers, different tubes, etc) than from hypothetical discussions. It seems like you're really interested in this kind of thing so it will be time well spent to get something on the bench to play with.

Try plugging a 12BH7 in the output PP tube socket and connecting your wirefly to a real speaker. You will be stunned! It will develop 5 Watts ( unless your PT HT can't handle the extra current)
I may gain a reputation for this comment!

[tedmich]

Try plugging a 12BH7 in the output PP tube socket and connecting your wirefly to a real speaker. You will be stunned! It will develop 5 Watts ( unless your PT HT can't handle the extra current)
I may gain a reputation for this comment!

I concur, I built up my FF in 2 days from my parts bin, and that includes etching my own TT PCB! I am using a NOS HP 5963 for power tube and it sounds very good, not too fond of cascode boost though, think Mason Atom's AX84 LII pre
http://www.ax84.com/static/corepream...tom_LeadII.jpg
+kt88 PP:
http://www.ax84.com/static/corepream...tom_LeadII.mp3
with 5963 x 2-> 12ax7 should sound sweet into the FF's 5963 output. I'll post clips when done...