Various Tube Ampf Questions

[marnold]

Hey Tung, DVM, and all you tube-ampf-knowledgeable guys, I've just been pondering a couple of things and wanted to get your input.

1) What's the difference between using a master volume and an attenuator? Am I right in assuming that a master volume is basically an attenuator for the preamp and an attenuator/hotplate is for the power amp?

2) If someone were to hypothetically get a small tube ampf a la a Valve Jr. or a Blackheart Little Giant, I would assume the only way you'd get a metal tone from them is by hitting it with some flavor of metal or heavy distortion pedal. Will that pretty much eliminate any point of getting tube ampf? Will hitting the preamp with that much gain cause any weirdness?

3) Some people have commented on the noise of a Valve Jr., even modded ones. Is that a common problem with these little Class A ampfs in general or something specific to the Valve Jr.?

4) It is my understanding that tubes in a combo are more likely to fail given the fact that they will be subjected to things like greater vibrations and even magnetism from the speaker itself. Will having a separate head really help, even if the head is sitting right on top of the speaker cabinet? Are these vibrations really something to be concerned about in the 5 watt or less range?

[tunghaichuan]

1) What's the difference between using a master volume and an attenuator? Am I right in assuming that a master volume is basically an attenuator for the preamp and an attenuator/hotplate is for the power amp?


The master volume usually comes before the power tubes in the circuit. In a push pull amp there are two types: pre-phase inverter and post phase inverter. The phase inverter is usually a single tube that splits the signal into two equal but out of phase signals so that each tube in the push pull pair can take turns amplifying the signal. Post phase inverter types are thought to sound better as the phase inverter itself adds distortion to the signal.

In a single ended amp the master usually comes at the end of the preamp right before the power tube although it can come before a gain stage driving the power tube.

A master volume works by shunting the signal to ground when turned down and letting the signal pass when turned up. It does not affect the power amp.

An attenuator works by absorbing power and dissipating it as heat so that the signal sent to the speakers is not as loud. The amp is still working as hard as if the amp was producing full SPL levels.

So to make a long answer short, yes you are correct: the master mostly thought of as part of the preamp and an attenuator works with the power amp.



2) If someone were to hypothetically get a small tube ampf a la a Valve Jr. or a Blackheart Little Giant, I would assume the only way you'd get a metal tone from them is by hitting it with some flavor of metal or heavy distortion pedal. Will that pretty much eliminate any point of getting tube ampf? Will hitting the preamp with that much gain cause any weirdness?


Neither the BH or VJ will product metal tone cranked up. Single ended Class A amps like them do not have the aggressive tone as a push pull Class AB marshall. The BH and VJ produce distortion by using a relatively clean preamp driving the tubes into overdrive. Metal tones are usually done the opposite way: massive preamp distortion amplified by a relatively clean power amp. Examples of this are the Soldano SLO and its clones the Boogie Recto and Peavey 5150. Some metal tones can be achieved with Marshall style distortion: massive preamp distortion driving the power amp into overdrive.

So yes, to get metal tones out of a BH or VJ you're going to need some sort of pedal.

One advantage of using a tube amp with a metal pedal is that it will warm it up. You may or may not want this with a metal sound. I happen to like it. You could just plug a metal pedal into a squeaky clean solid state power amp, but this usually sounds terrible.

Hitting the preamp tubes with too much signal is generally not a problem. It applying too much signal to the power tube that can be the problem. If the signal is too hot, it can drive the grid positive and cause blocking distortion. The grid is where the signal enters the tube and must remain negative in relation to the cathode. This is particularly a problem with the EL84 as it has a very sensitive grid. It is a high gain tube that was invented for hifi and does not need a lot of signal to get full output out of it.



3) Some people have commented on the noise of a Valve Jr., even modded ones. Is that a common problem with these little Class A ampfs in general or something specific to the Valve Jr.?


The VJ and BH are single ended Class A amps, and hum is common in these amps to a certain extent. It is the single ended part that causes the hum. There is usually a small amount of hum in single ended amps as there is only one tube amplifying the signal. This hum is usually cancelled out in a push pull amp. My usual solution is massive power supply filtering. I've used as much as 250uF-350uF (stock Champs used 8uF to 16uF) to get the hum down. This usually kills the loose vintage feel of the amp as the power supply is very stiff with that much filtering. The last couple of single ended amps I've built have had massive filtering and are dead quiet.



4) It is my understanding that tubes in a combo are more likely to fail given the fact that they will be subjected to things like greater vibrations and even magnetism from the speaker itself. Will having a separate head really help, even if the head is sitting right on top of the speaker cabinet? Are these vibrations really something to be concerned about in the 5 watt or less range?

I would bet on the vibration. I'm not sure magnetism can affect a tube. I coudl be wrong though. It stands to reason that anytime the tube is subject to vibration, it could go microphonic. Some amp builders only built some of their circuits as heads for this reason. The physical isolation of the chassis and damping of the rubber feet can help prevent damage to the tubes.

OTOH, I've seen 40 year old Fender combos with the original tubes and are still going strong.

HTH,

tung

[marnold]

Thanks for the info, Tung. If I were ever to get one of those ampfs, I'd probably lean toward the Blackheart. The mini-stack vs. combo thing mainly comes down to convenience. The price is relatively similar. Of course, if I win the Premier Guitar giveaway and get Lynch's new Randall Lynch Box combo, I won't have to worry about it. :) Having said that, I could probably launch my house off the foundation with a 50W tube combo.

Re: warmed-up metal tone. That'd work for me to a point. I prefer the 80s metal tone to the 90s scooped thing.

Is it safe to assume that both the Valve Jr. and Blackheart are going to be more Marshall-like than Fender-like? I base that mainly on the choice in tubes.

[tunghaichuan]

Thanks for the info, Tung.


You're quite welcome.



If I were ever to get one of those ampfs, I'd probably lean toward the Blackheart. The mini-stack vs. combo thing mainly comes down to convenience.


My main amp setup is a Valve Junior head (heavily modified, natch) run into two 1x12" speaker cabs. The advantage of the Blackheart is that you can plug it into either two 8 ohm cabs or two 16 ohm cabs due the the parallel output jacks on the back. The VJ only has one jack per impedance out and I had to make a parallel box to get it two work with two cabinets. Not really that hard though.



The price is relatively similar. Of course, if I win the Premier Guitar giveaway and get Lynch's new Randall Lynch Box combo, I won't have to worry about it. :) Having said that, I could probably launch my house off the foundation with a 50W tube combo.


Yeah, a 50w amp will be at hearing damage levels by the time it is loud enough to get it to sing :dude:



Re: warmed-up metal tone. That'd work for me to a point. I prefer the 80s metal tone to the 90s scooped thing.


I'm with you, I like the warmer 80s tones as well.



Is it safe to assume that both the Valve Jr. and Blackheart are going to be more Marshall-like than Fender-like? I base that mainly on the choice in tubes.

I would say that the VJ is more aggressive in the distortion, and more Marshall-like in that regard. The BH is more Fendery as it stays cleaner longer and never really gets distorted. The three band tone stack in the BH sucks up a lot of gain which makes the amp stay cleaner, longer.

If you're looking for a small low wattage tube amp to run pedals into, I'd recommend the BH over the VJ as the BH sounds pretty good stock. To tell the truth, I thought mine sounded better with the stock Chinese tubes than the JJ and EH tubes I put in, so I put the stock tubes back in. The BH doesn't need much in the way of mods to make it sound good, it sounds pretty good out of the box. It has almost no breakup unless it is cranked all the way up.

Conversely a modded VJ sounds very good cranked.


HTH,
tung

[sunvalleylaw]

FWIW, I was really impressed by Spud's Little Giant head he has under a desk in his office, sitting on top of one of those MF blow out cabs a while back. Really nice sounds for such a small, inexpensive set up. I don't recall getting him getting into metal territory while we were messing around, but it sure rocked for a cute little amph!

[duhvoodooman]

Tung covered all your questions superbly, Rev, but I'd just throw in the comment that I find my modded VJr takes pedals very nicely. But for sure, you'll never get serious metal tones out of that little sucker by itself.

Haven't played a BH, so can't comment there.

[marnold]

If you're looking for a small low wattage tube amp to run pedals into, I'd recommend the BH over the VJ as the BH sounds pretty good stock. To tell the truth, I thought mine sounded better with the stock Chinese tubes than the JJ and EH tubes I put in, so I put the stock tubes back in. The BH doesn't need much in the way of mods to make it sound good, it sounds pretty good out of the box. It has almost no breakup unless it is cranked all the way up.
Actually, I think the extra clean headroom would be A Good Thing for me, ironically enough. I've heard that one limitation of these little tube ampfs is a lack of clean headroom. You can always add more grit via a Tube Screamer, metal pedal, etc., but you can't take it away once the amp's already spitting it out.

Why did I even bring this up? Now I want one of these. OK, I did before too.

Having said that a number of the questions I had piqued my interest anyway. I like knowing "why" things work instead of just being content to know that they work. That's especially true with electronics since it's all so much mystical voodoo to me.

[tunghaichuan]

Actually, I think the extra clean headroom would be A Good Thing for me, ironically enough. I've heard that one limitation of these little tube ampfs is a lack of clean headroom. You can always add more grit via a Tube Screamer, metal pedal, etc., but you can't take it away once the amp's already spitting it out.


Exactly. I've built a number of tweed Princeton clones: one 6V6 one 12AX7. The original had a 5Y3 tube rectifier, but I usually use SS diodes. I bring this up because these have *no* clean headroom. They start to overdrive on about 3 and just get crunchier and crunchier from there. This is great if you like fat power tube distortion, but it definitely isn't a metal sound. Much more bluesy.

Having more clean headroom in the BH amps is a nice feature. Just to confuse things even more:

http://www.blackhearteng.com/bh15h.html

The 15W Handsome Devil head. Probably way to much wattage to crank up at home, but it might make a nice platform for warming up pedals. I haven't heard one in person, but the clips I have heard lead me to believe that there isn't a whole lot of gain on tap.



Why did I even bring this up? Now I want one of these. OK, I did before too.


Unfortunately, the BH series just went up in price. They were a pretty good deal, but now not so much so. Still, they are not a rip-off either.



Having said that a number of the questions I had piqued my interest anyway. I like knowing "why" things work instead of just being content to know that they work. That's especially true with electronics since it's all so much mystical voodoo to me.

A lot of it is mystical voodoo to me as well. There are large gaps in my knowledge. After 10 years of building them, I think I'm starting to understand some of the very basics. It's been a steep learning curve for me since I had no formal training in electronics.

tung