Suhr amp with 12BH7 ?

[shocki]

John suhr published a new cool 5W amp which was designed by Jim Kelly (I think).
You can find a great demo here. Suhr Corso amplifier, demo by Pete Thorn - YouTube

Sounds really great. I find the tube setup quite interesting. (12BH7, 12AX7 & 12AU7)
Does he use the 12BH7 as a power tube? Never heard of this tube actually.

[MWJB]

12BH7 is like a longer bottle 12AU7, similar gain (mu), better headroom, 3.5W per triode, 0.6A heater draw. It can be used as a power tube, Kendrick have offered an amp called the So Lo 7 for some years now with a 12BH7 power tube (or 15W with 2x12BH7). But then 12AU7 can also be used as a power tube for low wattage amps, as can just about any twin triode if you can find a decent OT. I have used 6SN7, 6SL7, 12AY7, 12AT7, 12AU7, 12BH7...ECC99 are popular too.

A difference with using these tubes for low wattage outputs, as opposed to single-ended, tetrode/pentodes, is that they can be wired push-pull for different harmonics, thus a lot of builders bill them a "little amps that sound like quiet big amps". These kinds of amps often have a phase inverter & smoother harmonics compared to SE amps, though of course, just because you have 2 triodes to play with doesn't mean you can't use them parallel SE too, if you want.

Low wattage amps were for years just cheap, entry level amps, single-ended because it took less parts to get some noise out...these push-pull, mini amps may have a similar parts count to a medium sized amp, so are often a bit more costly than the cheapest SE amps...though a well built SE amp certainly has its charms, no better/worse, just different.

Lots of folks offer something in a similar vein now, like Blackstar, Marshall & others...

[shocki]

Very cool info. Thanks. So probably AX7 as preamp tube, AU7 as PI and 12BH7 as power tube? Does somebody have a schematic of such a power section. Or is there some kind of similar DIY project?
Found another amp with a similar output section.
http://www.hughes-and-kettner.com/pr...ter%205%20Head

[MWJB]

Well, could be 12BH7 as the power tube, certainly capable of 5W...but then the 12AU7 is too (just about), so I wouldn't get ahead of ourselves there ;-) It's likely but not certain.

The power section would be probably similar to any number of amps with respect to the PI (long tail PIs are quite common), the power tube could be wired like a 5E3 Deluxe's 6V6s, just no screen supply node, each 12BH7 plate feeds an end of the OT primary...you could even use a 5E3 style OT with 12BH7, just use a 16ohm speaker on the 8ohm tap.

Here's a schem from one of my amps, but ...just to confuse issues...this is a P-P phase-inverterless design, but you see the power tube wiring (6SN7 is essentially an octal 12BH7)...
http://i1083.photobucket.com/albums/...isAmpSchem.jpg

[shocki]

Here is also a PP schematic with 2W.
http://www.ax84.com/static/corepower..._Schematic.pdf

[MWJB]

Here's a variation of the harp amp above, for guitar (I used a 5F1 chassis & 1x12" cab with a Neo Celestion Century Vintage), moderate gain, imagine a tweed 5F8A twin at half the volume of a 5F1...



Again, phase-inverterless, so lower power than a PI driven amp, makes about 1/2W. I had an old GT Substitube (6SN7/SL7 to 12A#7) adaptor and could drop in 12AT7, 12AU7, 12BH7.

Check out the Weinbrock Hobo Wienbrock guitar amps - Welcome to the official website of Wienbrock Amplifiers

Mojotone have a 2W bedroom amp kit.

[shocki]

Thanks for the schematic. Other than more power. Will an amp with a PI sound bigger? The Mojotone amp kit looks interesting. But there is no schematic.

[MWJB]

The majority off these amps will have a PI, the amps they are often modelled on had PIs and they could be argued to contribute to the tonal signature, mine don't have PIs simply because I wasn't trying to emulate a specific larger amp & I didn't need buckets of gain, or max potential volume.

I know what you are asking, but I struggle with the concept of an amp sounding "big" or otherwise, louder/quieter, darker/brighter, tighter/saggier? Yes, but bigger/smaller...? It's counter intuative to me, a bit like something smelling grey, or tasting triangular! ;-) Many times a very large amp in the studio will sound tight & waspy, easily mistaken for little SE amp, whereas a smaller amp will be working the power supply harder & may be cathode biased, giving more envelope of sag & decay. So...I guess I would say a small amp recorded, with a PI, will sound as full & rich as just about any amp, so long as you assess the tone and volume at the speaker separately.

[shocki]

With sounding big I mean bass response mainly. If you compare a 20W, 50W and 100W Marshall they 100W Marhshall will have a "biggest" sound even at the same volume. It is hard to desribe. It is not the amount of bass but the way the bass sounds. This also has to do with the size of OT I think. Of course I dont expect a 5W amp sounding as "big" as a 100W amp. But this describes what I mean with sounding big.

I think the key is not choosing a too small OT, using 2 output tubes (in this case one preamp tube) and Push/Pull design. So do everything the bigger amps have but at lower volume.