[rockadanny]

I see 50/60 Watt PP tube amps out there, some with only two output tubes (e.g. Quicksilver Mid Mono), some with six (Cary Six Packs).

Why have so many tubes? (sound? cost? circuit complexity?)

In general, what is the advantage of one over the other?

In general, do you personally prefer one over the other? Why?

[Thebiker]

Why so many tubes?  Depends on the output power of the tube to determine the number of tubes needed to accomplish 50-60 watts and yes, they do all sound different.  There are a lot of other variables as well including the voltage that you are pushing to each tube. 

There are a lot of folks on this site that can give you more info than you probably want concerning amplifier topology....I'm not one of them, I just know what I like and I let my ears lead the way.  That said, I have one amp using EL84's, 4 per channel, yielding 50 watts per channel and another using KT88's, two per channel, outputting 80 watts per channel in PP.

I run both in the triode mode which gives me 25 per side and 40 per side....once again that's what my ears like.

Go out and listen, take your own music, have fun and let your ears lead.

Walt

[mfsoa]

And on the other hand, my VAC amp uses 2 KT88/ch but makes 105 watts/ch

You'd think the tubes wouldn't last but that doesn't seem to be the case.

I'd be curious if VAC is being optimistic about the power rating, or if they are pushing the tubes really hard.

-Mike

[mitch stl]

The number of tubes needed to achieve a particular power output on an amp will vary with the choice of output tube. The specs for any given model of tube vary widely.

For example, an EL34 is a common output tube and is widely beloved for its midrange sonic quality. However, one tube won't handle as much power as a single KT88, for example.

One can look at tube specifications at a web site such as:  http://tdsl.duncanamps.com/tubesearch.php.

You could see from these pages that the maximum plate dissipation is 25 watts for a single EL34 but it is 42 watts for a KT88.

At this point, the design of the amp also comes into play. Is it a single-ended amp? A push-pull? Triode mode? How hard does the amp designer want to push things? An amp with plate and other voltages pushed to the max may generate more power, but at the risk of rising distortion or stability.

50 or 60 watts is a pretty easy push for a single pair of KT88s or 6550s in a push-pull mode, but you'd need at least two pair of EL34s to get there.

So, there are lots of variables to deal with and we're back to the point: how does it sound to you?

[Frihed91]

An advantage that the Quicksilver Mid-mono has over other PP amps in this size range is that it can take any ("almost any"?) octal power tube, including among others, the El34/KT77 and 6550/KT88 family, up to the KT90.  However, the power output does not vary by as much as a previous poster suggested.  According to their web page, "Power is 50 watts with the standard EL34s; 55 with KT88s; and 60 with KT90s".

This is a fabulous sounding amp. in every gear.  If you look underneath the hood, the design is very traditional, but the implementation is spectacular at any price. 

[jrebman]

In addition to what's already been said, the output transformers also play a role in how much power, and the quality of that power that a given amp can produce.

You can also get 50 watts from a single tube in single-ended mode if you want to use something fairly exotic like the Russian GM70, and I'm just waiting to see what happens when and if somebody ever designs a SET amp with a GM100 tube.

Right now I only know of two amps that use the GM70 -- the DeHavilland at about $11k for a pair of monoblocks, and the Lamm ML 3.1 (and I'm not even sure if this one is actually available or not) at something like $130k .

-- Jim

[jeffreybehr]

"An advantage that the Quicksilver Mid-mono has over other PP amps in this size range is that it can take any ("almost any"?) octal power tube, including among others, the El34/KT77 and 6550/KT88 family, up to the KT90.  However, the power output does not vary by as much as a previous poster suggested.  According to their web page, "Power is 50 watts with the standard EL34s; 55 with KT88s; and 60 with KT90s"."

Fri, the previous postist was discussing the range of power from amps designed specifically for those various tubes.  When an amp uses sort-of-all of them, it has to be designed to the lowest common denominator, for instance the lowest screen-to-plate Voltage that ANY of them can tolerate.

rock, others have discussed this, but to summarize, the differences inlude:
1. Each tube's basic ability to disipate power,
2. How close the designer got to the tube's maximums,
3. Whether the tube is a pentode/tetrode or a triode,
4. Whether the pentode/tetrode is used that way or as a triode, with the latter having about half the power of the former,
5. Whether they use output transformers or are output-transformerless (OTL) like the Atma-Spheres, and
6. One of the most obvious, whether they're used singly or in pairs.  And pairs can be used push-pull or in parallel, so we can have a single-ended-triode amp that uses 2 (or more) output tubes.  Roger Modjeski of Music Reference uses paralleled putput stages in his little EM7-12 SET poweramp.

So...altho EL34/6CA7s can produce up to maybe 50 Watts per pair in ultralinear (a pentode/tetrode push-pull operating mode), if run in triode and at perhaps-lower Voltages, we have something like the Cary Six Pack.  And didn't Tim DeParavacini (of EAR) build a poweramp that used MANY pairs of 12AX7s, a Voltage-gain tube?  Joseph Lau of Antique Sound Lab has designed another somewhat-odball, a push/pull amp, the Cadenza, that uses a pair of directly heated triodes, the 845, for 60 Watts... http://www.divertech.com/aslcadenzadt.htm

I hope this helps.

[mitch stl]

As jeffreybehr noted, amp designers certainly need to be respectful of any given tube's operating specs. To get 50 watts from a pair of EL34s is to essentially run them at their maximum plate dissipation. Many amp designers aren't quite as comfortable pushing things to the edge like that.

However, Quicksilver is not alone in this. Conrad Johnson claims 55 watts a channel from a pair of EL34s in their MV60. (I owned one of those for a while. CJ doesn't claim as wide a bandwidth as Quicksilver.) Both are well regarded companies and I am sure their design engineers took full account of the various specifications of the output tubes they utilize. However, just be aware this type of usage is running that particular tube at, or even just beyond, its maximum published ratings.

[Imperial]

I would like to point out that a tube is but a "valve"...
It's still the powersupply + the valve that IS the amplifier...

The number of tubes isn't even half the equation as a tube can be tied into the circuit a number of ways..

Imperial

[rockadanny]

Thank you all for your responses thus far. There is certainly much to digest. I am sorry I cannot add anything more to my response at this time as I am currently out of town with extremely limited computer access.