Class A Trainwreck?

[mtheory]

If you don't care about what class your proposed 25W Express operates in, you consider changing the title of this thread.

I kind of like the title myself. Who knows? I may end up finding that pure Class A is the best way to go with this build, right? I'm reaching out for ideas here, and I certainly don't want to close out the Class A option.

Dig out your EL34 datasheets and figure out what you need to do.

What I was looking for was some insight into specific transformers that might be more suitable than the standard TW iron, as well as some insight into specific issues that you may have encountered in tackling this type of build yourself.

I don't do this for a living. I've tinkered a bit and built a couple of simple amps for myself, but nothing near this ambitious. That's why I decided to post my question here. I figured there might be a good chance that somebody else has attempted to do what I want to do, and could offer some valuable information that might save me some time, money, and hassle.

The little bit that I have gleaned over the past couple years is that the most significant decision in a build is transformer selection. Based upon what I've gathered, I don't know that the stock TW iron is the best choice for this type of build.

I don't know that simply tinkering with rail voltages will necessarily yield the best design in my quest for 25 watts from this amp. Maybe, but maybe not. Based upon what I've gathered thus far, everything matters, so it may be better to find different iron to begin with than to try to force a square peg into a round hole. Again, I've not done this myself, so I'm reaching out to those who have.

As for the 6v6 option, I wish I liked those tubes, because it would make things a whole lot easier. I just don't care for them much.

And, although VVR is certainly an option that's worth considering, I really was looking for something that could find 25 watts or so without added devices. I still may end up going that route, depending on what I learn in this information-gathering process, but that's not my first choice.

Thanks again for the responses. I'm going to read through the links that Ben posted. I tried to do a search for this, but didn't find what I was looking for. I appreciate the links.

[mtheory]

If you are using a TW style PT with the 300-260-0-260-300 HT secondaries, use the 260V taps for HT and this should get in about that ballpark as the plate voltage should be in around 340V. Chose the appropriate cathode resistor to bias it around 12W per tube and you should be good.

Excellent! That makes things a whole lot easier! I really wanted to be able to use the standard TW iron, because I know that there are excellent versions of it available, and I wouldn't have to hunt and peck for suitable candidates.

So, in your opinion, if I do happen to bias this at a point that qualifies it to be "Class A," or at least very close to, you'd feel that the stock PT would be sufficient to meet that added current demand, without overheating or causing problems?

I realize that this is a question that may be impossible to answer off the cuff, so your best guess, based upon what you know about that transformer will suffice.

Also, in terms of specific transformers, is there any particular reason to consider Pacific over Toneslut, or visa versa? Are there other options worth considering instead or as well?

[delux1959]

Dude, maybe you would get the answers you were looking for if you didn't come off like such a DICK. There are a lot of people that frequent this sight that KNOW what they are talking about, and its very apparent that you don't. Try a little humility.

[dartanion]

I thought you didn't care what class the amp would be operating in. I'm a bit confused now by your statements about not caring what class it would be in the end when you say:

"So, in your opinion, if I do happen to bias this at a point that qualifies it to be "Class A," or at least very close to, you'd feel that the stock PT would be sufficient to meet that added current demand, without overheating or causing problems? "

Anyway, you still would not be in Class A, but rather AB.

First I think you need to slow down and get a grasp of tube amps 101. Understanding how things work is crucial to getting your amps to be what you want them to be. You sound really Gung Ho about building, so that's cool. You need to harness that energy in a productive fashion. You can't fly a 747 after your first lesson, so custom building an amp takes knowledge and honed skills to accomplish. Besides, you can kill yourself pretty quickly if you don't know what you are doing. No sense in that.

The recipe (more of a guidline):
What you should do is this, follow the power supply from the Liverpool, use Express for everything else except use the 260V taps of either the Toneslut or Pacific PT. Use either again for the OT, 6.6K is my preference for the Express circuit. Start with a 250 ohm 10W or above cathode resistor and calculate the dissipation. Adjust as needed to get you into the 13W range for each tube by raising or lowering the Rk value on the power tubes. The advantage of fixed bias is that you can adjust easily using a trim pot to tweak your bias. With cathode bias, you have to swap resistors, so you'll be unsoldering and soldering to get there.

If you are dead set on class A, then you have a good deal of work ahead of you. Understand the fundamentals first, then chase your ideas.

[benoit]

Dude, maybe you would get the answers you were looking for if you didn't come off like such a DICK. There are a lot of people that frequent this sight that KNOW what they are talking about, and its very apparent that you don't. Try a little humility.

Easy killer, this has been said more or less earlier in the thread, and I don't think anyone in the thread intended to be a dick. It is frustrating to be looking for answers and to not get them, and even if it's not an appropriate response to come back upset, it's the natural human one. It's the internet, don't take it too seriously. The discussion has reached the point of productivity again, so let's keep it there.

[JamesHealey]

make an 18w liverpool.. surely this is the way to go for a low powered wreck?

[dartanion]

EL34 datasheet says you'll need around 250v on the plates, so a PT of around 190v secondaries should right if you stick with SS rectification. You'll want to bias hot too, about 90%.

[funkmeblue]

I looked into this a while ago.....edcor has a 180-0-180@250ma power transformer, that should put you in the ball park. Not sure what bias resistor. Then all you need is a 5k or so 15 watt output. Do you plan on building this without negative feedback loop? If so, that extra gain stage is going to screw things up for you. Check out alessandro's black and tan, "blackface" preamp into a true class a push pull ultralinear cathode bias no nfb loop el34.

[mtheory]

The recipe (more of a guidline):
What you should do is this, follow the power supply from the Liverpool, use Express for everything else except use the 260V taps of either the Toneslut or Pacific PT. Use either again for the OT, 6.6K is my preference for the Express circuit. Start with a 250 ohm 10W or above cathode resistor and calculate the dissipation. Adjust as needed to get you into the 13W range for each tube by raising or lowering the Rk value on the power tubes. The advantage of fixed bias is that you can adjust easily using a trim pot to tweak your bias. With cathode bias, you have to swap resistors, so you'll be unsoldering and soldering to get there.

A PT designed for 6bq5s can supply enough current for 6ca7s?

If you are dead set on class A, then you have a good deal of work ahead of you.

Like I said before, I'm not at all set on Class A for this. I don't really care what class it is. If it takes Class A to get it to where I want it, I'm fine with that. If it can be done in Class AB, that's fine, too. I have no personal preference. I had actually seriously considered parallel SE as well, but shifted away from that idea and towards cathode bias push-pull instead.

make an 18w liverpool.. surely this is the way to go for a low powered wreck?

Yeah, this would be the easy way to go about it, for sure. However, there are two issues at hand...I want more than 18 watts, and I want 6ca7 output.

Do you plan on building this without negative feedback loop? If so, that extra gain stage is going to screw things up for you.

Yes, I was intending no negative feedback, and your comment was a concern. I'm obviously not going to have an Express when I'm done, but that's really not what I'm after. I'm hoping to capture the overall voicing, gain, and responsiveness of that amp, with lower horsepower. I know that there will be tradeoffs along the way. Hopefully, what I end up with is a chunky, articulate, screaming 25-ish watter.

Anybody know what H&K is doing with their Puretone? They're claiming 25 watts from a pair of EL34's.

Thanks again for the responses. Well...MOST of them anyway!

[funkmeblue]

You may not be able to build it without a nfb loop or at least going ultra linear unless you lower the gain some where else in the amp. It may "freak out" or just be really noisy. I built one based on half a liverpool but running at about 313 volts no nfb loop and ultra linear. Way too much gain and unwanted noise.......added the loop and presence control and now this thing just screams. Tried it UL and normal, not sure what I like better. But your going to need some kind of nfb or reduce the gain.

[mtheory]

You may not be able to build it without a nfb loop or at least going ultra linear unless you lower the gain some where else in the amp. It may "freak out" or just be really noisy.

Yeah, I think that may be the case. I'm sure this will entail a good deal of trial and error...hopefully more trial than error!

[roberto]

Anybody know what H&K is doing with their Puretone? They're claiming 25 watts from a pair of EL34's.

I'm interested too. (PSE?)

[Zippy]

I never intended to open an discussion over the clinical definition of Class A amps....

You were the one that wrote the opening post.

The definition that I referred to is one that I got from a book I'd read....

You need better books - and you might consider changing your moniker, "Mister Theory".

I wasn't trying to pooh-pooh anyone... I just wanted to keep on topic.

Next time, try being a little nicer. Consider "I've never played an Express and have no idea how loud they can bee, however I need an amplifier that puts out 25W."

G'luck with your build. Treat the natives nicely and you'll find we are a generous lot.

[benoit]

G'luck with your build. Treat the natives nicely and you'll find we are a generous lot.

Man, you gotta read the thread first. Your post is prob third or fourth post to say the exact same thing, and the snippy portion of the thread died days ago, let's let it lie and give this new friend a break.

[Zippy]

Nice to have the thread sheriff on the case.

Thanks.