Low-voltage AC30 PT

[dehughes]

Hey all,

So, I'm looking to run my AC30 at a lower voltage permanently (r.e., no VVR) and was going to replace the Mercury Magenetics PT with a Heyboer one. I've settled on the specs overall, but can't seem to decide where I should land on the B+ voltage. Normally you'd find about 280v per side of the B+ winding of most replacement AC30 PTs, but the amp (as we all know) runs the EL84s pretty darn hot when you run them at proper AC30 ratings (and yes, I know that's part of the "magic"...but run with me here...).

My goal is to run the plates of a quad of EL84s, with a shared Rk of, oh....62 ohms or so...right around 300-310V, using a 5AR4 rectifier. So, my calculations show that somewhere in the 250-ish range should be where I'd want to land on the B+ winding.

Am I close?

[Zippy]

By "B+ winding", do you mean the PT secondary?

To further muddy your waters, have you read Aiken's treatise regarding Vox AC 30 voltages?

http://aikenamps.com/

Go to "Tech Info", then "Is the Vox AC-30 really class A?"

Regarding your VVR experience, were you using global VVR or using it only on the power stage?

I'm a proponent of lowering the B+1 and adjusting the dropping string to maintain preamp voltages as in the "original" amp.

[dehughes]

By "B+ winding", do you mean the PT secondary?

I'm a proponent of lowering the B+1 and adjusting the dropping string to maintain preamp voltages as in the "original" amp.

Yes, I mean the PT secondary.

And yes, I too am considering dropping the B+1 voltages and adjusting the string to keep the preamp voltages in line with the original. That has worked for me in another amp, so I have high hopes for my AC30.

So, would 240v per side of the PT secondary be a bit too low, you think?

[chopstuck]

Hi Dave,

My set of Toneslut trannies had two taps one at about 300v and the other at 260v. The 260 AC gets me to about 300-320v on the plates. When going this route you could put a 3 way switch in place of a standby switch and have both low and hi voltage options from the front panel without wrecking the cosmetics.

If you want to use your original PT you could put on a zener to drop the V+ to your choice and make that switchable too. Various zener circuits are out there. I used one that cost me about $2-3 using a power mosfet and a small zener. I got a useful 50v drop out of my old PA iron.

Check out RJGuitar for new iron as Moose isn't slutting around right now.

[dehughes]

Thanks Marc. What Rk are you running with the 260v secondary in order to get your EL84s in the 300-320v range?

[chopstuck]

I'm using a shared 50R for all four el84 s.

The old tubes don't seem to mind.

Marc

[dehughes]

I'm using a shared 50R for all four el84 s.

The old tubes don't seem to mind.

Marc

Thanks. Okay, so that give you pretty close to their "max" dissipation, huh? Good deal. Maybe I'll shoot for 250-260v on the B+ winding instead, being as I have a 62 ohm Rk.

As for the Zener idea, yeah...I could do that, but part of me would rather just have a piece (the transformer) designed to run "naturally" instead of hamstringing it to work. I know, a Zener is reversible, easy to install, inexpensive....but part of me also wants to get used to designing my own transformers, and this would be another opportunity to do that.

[solderstain]

With all respect, all this stuff about changing PTs is too much work. There's a simpler way to drop voltages, and most of us here have probably used it at times - change the rectifier tube.

I have an original 1963 'copper panel' JMI AC30 with added TB that is my 'desert island' amp. These things were basically spec'd and built when wall voltage was lower than it is today. Here in Arizona, we can see wall voltage at 125 vac during the day when load on the electrical distribution system is low. That translates into some B+ voltages that will cook some of the yummy vintage EL84s (ask me how I know).

After I sprang the considerable coin for a replacement set of vintage tubes, I also started using my variac to drop wall voltage to 115 vac.

Read something at the Vintage Amps forum - pull the GZ34 rectifier and use a 5V4.

Presto! I did a little test to see how well it worked in the real world - pulled my AC30 chassis and put it on the bench. Measured voltages in all positions at modern wall voltage. Then measure the same voltages with a variac limiting wall voltage to 115 vac. THEN, replace the rectifier tube with a 5V4, took the variac out of the circuit and measured with regular wall voltage and the 5V4 rectifier - BINGO! B+ voltages with the 5V4 rectifier tube were back down in the range that the JMI folks expected when wall voltage was in the 115 vac range. Here is a sample:

B+ on EL84s with wall voltage at 122 vac and GZ34 rectifier: 347 vdc
B+ on EL84s with variac set at 115 vac and GZ34 rectifier: 328 vdc
B+ on EL84s with wall voltage at 122 vac and 5V4 rectifier: 330 vdc

The AC30 still 'runs hot', but that's the nature of the beast. But dropping the B+ with a rectifier change has reduced that a bit.

It's easier and cheaper and nearly instantly reversable. Don't change PTs. The rectifier change won't drop it as low as you seem to want, but this is a very good (and simple) step.

[dehughes]

You know, I have a few 5V4s...old RCA ones, NOS, actually. They're great...and that's a great idea, and I've tried it, but I'm shooting for even lower voltages than what a 5V4 would give me. As well, I've been really happy going through Heyboer and have something lined up with Alden already, and I'd like to see how my PT idea feels once I get it in the amp.

Good to see your voltage readings, though. Thanks for posting that...helpful info.

[Wayne]

I'm sure you guys already know, but it's an easy one to forget (for me, anyway) - if your b+ is too high due to modern wall voltage and you drop it with a different recto, check your heater voltage too. I know 7VAC is within spec for most 6.3V tubes, but it still makes me shudder.

W

[solderstain]

I'm sure you guys already know, but it's an easy one to forget (for me, anyway) - if your b+ is too high due to modern wall voltage and you drop it with a different recto, check your heater voltage too. I know 7VAC is within spec for most 6.3V tubes, but it still makes me shudder.

W

I'm not sure what your point is about the heater voltages? The rectifier tube used doesn't affect the heater string for the signal tubes. Am I mis-understanding you?

[Structo]

He is saying if the wall voltage is higher than the designed primary voltage then the heater voltage will be accordingly higher.

But keeping the wall voltage at the designed 115v, it keeps all the voltages correct for the amp.

What I don't get is when looking at current production power transformers, why do they still make a majority of them for 115v?

I can't think of anyplace in America that doesn't have 120v these days.
Sometimes higher as noted.

[Wayne]

That was exactly my point - high wall voltage causes two problems. A rectifier with a higher voltage drop only solves one of those problems.

W