Amp Project, & PT used

[big_teee]

How about a thread to list needed and used Power Transformers for different projects.
Maybe this thread can prevent ordering a PT with too high or low voltages for the project.
IMO there is not enough documentation on PT Voltages, loaded and non-loaded.
Not to mention all the issues with todays higher AC Wall voltages.
Please list what the project is, rectifier & Tubes used?
What PT is needed to achieve the desired Voltage for the desired tubes.

For example I built a 6v6 Plexi.
I used the ClassicTone 40-18016 PT for a BFDR circuit.
I had 485v with diodes, and 455v with 5AR4 Rectifier, with 2 6v6s, 3 12AX7s.
I ended up with 435v, (which is higher than desired for my Tung Sol 6v6GTs) using a 100ohm resistor on the C.T.(center tap) to ground.
I really like amps with diodes and I think this PT would have been more suitable for my plexi type amp.
EDCOR - XPWR064
That would have kept me around 400 volts with diodes, and allowed the use of 6v6, 6L6, & EL34 Power tubes.

Please list any projects and PTs used here.
Maybe this thread can help others get the right PT part ahead of the build.

Or list what you want to build, and we can kick around here what would fit your application?
T

[tubedood]

Hi Terry, Perfect timing!! I'm just noodling ideas on a modded Princeton circuit, the Prince O' Wails. It'll have 2 6V6gt's and 2 12ax7's with probably 1N4007 diodes for the DC. I'd appreciate any advice on a good set of iron to use for this application. The project is on this forum under a Prince O Wails thread if anyone would like to provide input. I plan on starting a build thread as soon as I get started, it looks like a fun one!

Transformers I might try...

Allen TP24 : Allen Amplification - Parts Order Page
Classictone: Fender Power Transformer, Champ, Princeton, Vibro-Champ, Bronco, Harvard, 125P1B & 022772, 120V
Hammond 290CAX :http://www.hammondmfg.com/guitarLinePWR.htm
Dale

[big_teee]

Hi Dale:
I'm no authority by any means on the subject, but here goes!
The Important thing is to decide what B+ plate voltage you want, filtering cap size, and plan accordingly.
Also I would measure what your wall voltage AC is.
For your project, I think you want to keep the PT under 300 volts with diodes.
Maybe others will jump in?
T
** Here's an amp calculator that might help!
http://thesubjectmatter.com/calcptcurrent.html

[tubedood]

The calculator is a great tool thanks! That calculator says with the tubes I'm using the transformer with secondary voltages 325-0-325 will like to see 65.8 ma ..The classic-tone power transformer I was looking at is the 40-18019 it gives 650vac with a center tap.

[big_teee]

I'm calculating more than 450+ volts.
325 x 1.4 = 455.
Sounds pretty high to me.
If your wall voltage is above 120v AC, you can add more to that.
If anyone wants to jump in, please do.
T

[Chuck H]

There are several criteria that determine the final operating plate voltages. Duncan PSUD2 is an excellent program, but you NEED to plug in all the parameters. There's a preliminary step involved also. To do this you need to know the proposed PT's primary spec (115V, 120V, 125V, etc.). I used some Hammond 270 series PT's when they only came with 115V primaries. My wall voltage is 120V and pretty steady. So, lets say I have a PT with a 115 primary and a 300-0-300 secondary. 600/115=5.22. So, since I have 120V from the wall I'd use 5.22*120=626. So my 300-0-300 secondary is actually a 313-0-313 secondary. This would then be the figure that needs to be plugged into Duncan PSUD2. The program also offers a change to the default PT secondary resistance. Do change this to the actual resistance of the PT you want to simulate. That figure isn't often spec'd in sales lit so you may need to do further research, write an email or make a phone call. Once those two preliminaries are dealt with it's business as usual for the program and I've found it to be accurate to better than 10%.

[olddawg]

I think people get a bit over concerned with the "line voltage" issue. I might be concerned if I was constantly using collector quality vintage stuff. In that case I would probably have a variac in a locked box, lol. But.. I play a lot of clubs all over town. The house voltage is all over the place. I actually have a large digital display line voltage meter on my pedal board. The other night I had 110vac. The gig before that 121vac. Beach clubs and old buildings... The other day I had 100vac at a dive bar on the 101. I installed the damn thing a couple of years ago after I played a gig and our PA kept shutting down after 10 minutes. Finally....I ran a big extension cord from another mains outlet on the other side of the room and everything was copasthetic. That one stage outlet strip (that everyone uses) was at 80vac! So now at set up I can look down and know what's going on. My point is.. You can drive yourself nuts with the line voltage thing. It's really all over the place unless you stay in one new place. I just end up changing tubes more often. But I torture them anyway...that's what they are for.

[Chuck H]

Great point. I was just trying to help with establishing a baseline! A 115V primary on a vintage correct PT for a design that uses a bridge rectifier can put you 40V over spec. Not a big deal if your spec is low enough to accommodate the extra volts. But if you're buying a shelf PT from Hammond (that also has a lower secondary DCR) for something like a Fender DR clone... Well now you have 488V on the plates of your 6V6's! That's the sort of thing I was hoping to warn and educate against.

[bob p]

I think the over voltage thing is important to recognize, but it is not as critical as most people make it out to be.

Over voltage on heaters? That matters. You have to compensate for it.
Over voltage on B+? Not a big deal. Just adjust bias, prevent excess screen dissipation, and move on.

The reality is that tubes don't die from high B+ at idle, they really die from excessive current under load at that high B+. If you've got a transformer that runs B+ too high, then adjust bias accordingly. If screen dissipation gets too high then bring it down. By all means don't let the B+ run high, while keeping the bias hot and turning a blind eye to making other adjustments... if you do nothing then you're just asking for trouble.

My point is that there are many things that you can do to make sure that the amp will function properly even when the B+ turns out higher than you expected. Backing off on the bias and paying attention to dissipation goes a long way. It's not always necessary to go yanking the transformer out and looking for a replacement.

[big_teee]

What I was shooting for when I started this thread?
Is real numbers and real PT and tube combinations that work.
What voltages are too high, or too low, for certain applications.
All of you guys build amps?
What PT did you use for what tubes?
Real numbers would be great!
Chuck H did some of that.
Thanks,
T

[Chuck H]

Well then... A 2Xel84 / 3X12ax7 amp using a Hammond 270EX that only offers the 115V primary, a full wave diode rectifier and a 150R series resistor, biased hot to the tune of 10.5W (total screen+plate each tube, 150R cathode resistor) will result in about 360Vp.

[big_teee]

I've not done anything with EL84s?
What is the sweet spot voltage needed for EL84s.
T

[bob p]

What is the sweet spot voltage needed for EL84s.

EL84 are an interesting tube because they have a sort of biphasic response. If you keep the voltages low, say 300-320V or so, they have that sweet bell-like chime. But if you push them for power, with voltages that run up to 390-400V, then you can get more of a big-bottle sound out of them with a lot more volume, but IMO they lose their charisma when pushed that far.

Many people will split the difference, running the plates at 350 or so, to try to get a hybrid of the chimey tone with longer life vs. high power output and volume.

that's how I see it, anyway. I'm sure others will have different opinions.

[The Dude]

That's my experience, too. I will add that El84 amps are my favorite amps. I really like that "bell-like chime" as you describe it. I often refer to it as "glassy". I also like the sweet overdrive you can get at lower volumes.

Edit: The thing I don't like is that many of the modern cathode biased EL84 amps are biased way to hot. IMO, the beautiful sound of the tube is lost when they are run that way.

[bob p]

Chuck, I'm wondering if you ever dropped down to something smaller like the 270DX. Might just fit the bill...