Newbie question, Half Power?

[Stone]

Basically, I'm looking for info on how to make a 50 watt design into a 25 watt. Is it as simple as transformers and power tubes? Or is there more involved?

Thanks in advance.

[galibier_numero_un]

Basically, I'm looking for info on how to make a 50 watt design into a 25 watt. Is it as simple as transformers and power tubes? Or is there more involved?
Thanks in advance.

You'll get some good answers here, but just to prime the answer pump a bit ...

There are some relatively no-brainer solutions, depending on operating points and architecture of the amp.

For example, a Tweed Fender Champ, Princeton, and Deluxe can roughly halve their power by swapping 6K6's in place of the 6V6's.

Not all swaps are that easy as you have already surmised.

A while ago, I was pondering a 6V6-based Fender Bassman (instead of 5881 / 6L6). This would have (as you suspected) involved some iron swapping.

Basically, if you're thinking of a design in the public domain, you can generally approach it modularly (as preamp and power amp) combining designs and adjust your power supply and iron (both p-s and output) accordingly.

By no means is it trivial, but neither is it rocket science.

I'll bet you'll get some great (and more specific) answers if you throw out a design that has caught your fancy but which has power in excess of your requirements.

Also, I haven't messed with triode switches in guitar amps. 99% of my electronics experience is in hi-fi. From what I've read on this forum however, it seems as if most folks don't think much of running the common guitar tubes in triode mode.

OTOH, if you're building your own amp, it's literally a DPDT switch and a couple of inches of wire ... then, you can decide for yourself.

There ya go ... making me wonder about this all over again

Cheers,
Thom

[Jaicen]

Galiber, do you have any further details on swapping a 6K6 for a 6v6???
It's something i'd be pretty interested in I think! Does it sound the same but quieter, or does it change the character of the distortion?

[galibier_numero_un]

Galiber, do you have any further details on swapping a 6K6 for a 6v6???
It's something i'd be pretty interested in I think! Does it sound the same but quieter, or does it change the character of the distortion?

I do know that for the 5F1/5F2 Champ/Princetons, that the cathode bias resistors are such that your safe in just plugging in a 6K6. While the Rp (plate impedance) of the 6K6 isn't identical to a 6V6, it's close enough to work with the impedance reflected to it without an output transformer change.

I can't recall if with the 5E3 Deluxe, whether a cathode resistor swap is required to properly bias the 6K6.

Cheers,
Thom

[Stone]

Thanks for the reply galibier,

I'm looking at trying a Plexi type design in a lower wattage.

[Rob Mercure]

Stone,

While the above advice is generally sound why do you want a "25W" amplifier? The reason I ask is that a 25 W amp isn't 1/2 of the "volume" of a 50W - but perhaps you know this. Electrical power is based on simple arithmatic principles while the human ear has a logarithmic response to sound levels. As a "rule of thumb" you have to increase electrical audio power level by 10 to double the sound level so your amp won't be "1/2" as loud. This is why a Champ - at about 5 W - can be "heard" (at least somewhat) when played against a Twin. But if an amp around 20W is what you are looking Deluxe type circuits are generally a good bet.

Rob

[Stone]

Stone,

While the above advice is generally sound why do you want a "25W" amplifier? The reason I ask is that a 25 W amp isn't 1/2 of the "volume" of a 50W - but perhaps you know this.

Rob

Rob,
I understand the "percieved" volume thing. It's more of a curiosity/experiment than anything else. I'm wondering about how reducing original power designs affects the "feel" of the original.

[galibier_numero_un]

Rob,
I understand the "percieved" volume thing. It's more of a curiosity/experiment than anything else. I'm wondering about how reducing original power designs affects the "feel" of the original.

I too was thinkin' that he was trying to learn a bit about what elements contribute to the sound.

When I was pondering a Bassman build, I was considering ways to make a 15-18 watt version - having concluded that the preamp, and especially the long-tail phase splitter contributed strongly to the Bassman's sound.

While studying the preamp sections of the 5F6A and it's contemporaries in the Fender lineup, I realized that the Bassman's phase splitter was unique to this era's Tweed amps ... surmising that a low-power Bassman could indeed share many characteristics with the original.

Of course, I came upon the book analyzing the Bassman and the fact that the JTM-45 is almost a direct knockoff (with higher value plate load resistors in the preamp section - for more gain)

http://www.pentodepress.com/index.html

From this perspective, it would be interesting to try a Plexi style preamp driving something that puts out flea-power ... some 12AU7's for o/p tubes maybe??

Cheers,
Thom

[famp69wv@yahoo.com]

If you're going the 6L6/6V6 route

Guys

Perhaps because of my age I've been building "old man" amps for some years now. You know; heads & cabs anywhere up to 35 watts. I've tried the usual topological suspects and have found that the JJ6V6 does a really good job of staying stable even where biased fairly hot. Additionally it seems, at least in my amps, a little more 6L6 like in tone.

I believe you'll find that tube would be a good lower wattage substitute for 6L6s in a 5F6A or JTM45 mock up. In a similar but higher gain amp I currently have a pair of them biased (quiescent) at 11 watts plate dissapation (approx. 73%) and have experienced no problems over a fairly long time span.

I only suggest these tubes as an 'Occam's Razor' alternative because you could build either type amp with very little change from the original beyond a power transformer that yields a little lower B+ voltage (or dropping B+ with a Zener network). (I like them at about 390 volts on the plates)

After that I guess I should say that I have no financial interest in JJ or any tube seller or re-seller...

[Chris333]

According to JJ's web site, their 6V6 can take up to 500V on the plates and 450 on the screens, so you may not need to mess with the power supply of a 6L6 amp unless you like the tone better with lower voltage. The New Sensor Tung-Sol reissue 6V6 is rated to 475V on plates. They both sound nice in the 6L6 amps I've tried them in, but they don't seem to lower the volume of the amp much...

[bob p]

Philip, my experience with the JJ 6V6 has also been that they're very robust, but in some respects they seem to be more of a small bottle 6L6 than a real 6V6. That can be good or bad, depending upon what you're looking for.

[bob p]

When I was pondering a Bassman build, I was considering ways to make a 15-18 watt version - having concluded that the preamp, and especially the long-tail phase splitter contributed strongly to the Bassman's sound.

The 6V6 Plexi is pretty well documented. Google for Mark Huss 6V6 Plexi and you should find it.

If you haven't already, get a copy of the AES paper about the cathode follower and soft limiting in guitar amp circuits. IMO it explains why the combination of the Class A triode + CF driving the tonestack plays such a large role in giving the Bassman its signature tone.

Going back to the Bassman variants, some useful deisgns out there are the Huss 6V6 Plexi, Casey's 4x6V6 bassman, and my personal favorite, Bob P's Bassman/Brown Deluxe hybrid.

[famp69wv@yahoo.com]

Bob- Thats pretty much my take on them...

Bob

Nice to know my ears aren't completely fubar. IMHO 'more like a small bottle 6L6' is a very apt description of them. In my experience, while they sound very good, they lack some of the characteristics that many folks look for in a 6V6. Most notably they don't seem 'creamy' to me.

My earlier suggestion was premised on the one thing they do have in common with the 6V6. They are 'supposed' to be in the 15 watt max dissapation area. That is why I thought they might serve Stone's proposed 50 to 25 watt reduction. Especially where the amps Thom was suggesting used tubes from the 6L6/5881 family to begin with.

It occurs to me that they would be a nice change for a vibrolux reverb as well, eh.