Current Draw and the Dreamtone Calculator

[lord preset]

I am puzzled by what I am seeing using the Dreamtone PT Current Draw calculator http://www.dreamtone.org/Calculate_Current_Form.htm.

I started off pondering over an AC-30ish build and started thinking about why my Hayseed 30 clone has a PT the size of Rhode Island and many other 30-watt EL84 amps have much smaller transformers. The typical AC-30 replacement transformer is specced at 290-0-290 @ 260-280 ma. But when I put 300-0-300 PT voltage, 5u4B rectifier, 4 EL84s and 4 preamp tubes into the Dreamtone calculator I get 168.8 MA. If tube data sheets are to be believed, EL84s draw 48-50ma on the plates and 5ma on the screens so a quad of EL84s drawing less than ~220 ma does not seem possible.

So is the Dreamtone calculator just wrong or is it basing the answer on unstated assumptions that would make its answer valid? And are other 30-watters like, say the Peavey Classic 30 just running undersized PTs until they melt?

[matt h]

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[lord preset]

Thanks for the detailed answer. From what have read the early AC30 PTs were more like 160ma but they scaled up later. Not clear why. The 250-280ma figure comes from the ratings of replacement PTs on the market - I don't specifically know what the ratings are for stock Vox iron but I assume it's similat because AC30s are certainly very heavy.

[matt h]

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[lord preset]

Stupid maybe but I love the tone. The reason I brought this up is that I was wondering if I can achieve proper Ac30-ness with a smaller, lighter PT. My Hayseed is lighter than a Vox but mainly because it's 1-12.

[randalp3000]

I've built quite a few of these 4 cathode biased el84 type amps. That calculator doesn't really seem to work for figuring these out. The best luck I've had is with a traiwreck PT running 250-260Ac SS recto getting around 330V on the plates with 55R(2x110r parallel) cathode resistor. I seem to find them a bit sweeter sounding around 330VDC or under, 340 and up tends to get a bit on the harsh side. The output section is drawing close to 200mA by itself, a Marshall or fender will barely hit 100mA fixed bias.

Big current = big, hot, expensive, and heavy transformers. also make sure your OP trans can handle the current as well. Another reason Ac30 OP transformers are as big or bigger than a Marshall 50w. I also like to fun a computer fan on my transformers to keep them running cool and more consistent sounding.

edit, also the classic 30 is fixed bias and running much cooler so smaller PT.

[matt h]

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[lord preset]

As the fruit pertains to OP's quest: want to use smaller transformers and run less current? You can totally do it, you may just have to "work for it"-- and tweak your power supply and PI circuits a bit to dial in the operation of the amp for the conditions the power tubes are running. This is quite analogous to power-scaling (VVR) stuff in a way. if you used VVR to reduce your amp to 97% full power, it's unlikely you'll hear a difference when everything gets scaled together. (this ends up being equivalent to your wall voltage suddenly being about 2V lower one day. You might notice, but very, very unlikely. Relative humidity of the air would play a much greater role in acoustic variation from one day to the next.)

Or for another angle how about one of these? http://www.antekinc.com/as-1t250/. Combine the 250v taps in series for 250-0-250 with a SS rectifier and plenty of current @ <4lbs. Run the two 6.3v/3A taps in parallel for 6.3v/6A. What is the downside? I have no idea but I rarely hear of folks using Antek toroids in guitar amps. Maybe they cause cancer or warts?

[matt h]

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[Phil_S]

Look at the AS-1T200. You can wire it for 200-0-200. The HT winding will have capacity for about 310mA and I suspect that is conservatively rated. At ~$35 plus shipping, this is a bargain and will put your B+ solid state rectified at about 270-280V. I usually figure there will be 4% winding loss after applying the 1.4x multiplier. These toroid transformers may be more efficient; I don't know. In any case, if the circuit is demanding less than 310mA, the 4% loss factor may go out the window and you may see it go north of 280 by maybe 10V or so. Additionally, the spec is at 115VAC line voltage, so figure your increase based on actual line voltage/115, likely to be in the 5-6% range, making the real number closer to (assume line=122VAC) 122/115 x 1.4 x 200 = 297VDC. I wouldn't hesitate to try one of these. I think going for the 230V or 250V model is going to raise your B+ too high.

On the filament windings for these transformers, I think I'd wire one 3A winding for the power tubes and the other for the other tubes. I'm thinking with the line voltage 6% over spec that you'll get right around 6.6 or 6.7V not 6.3V, so you will benefit from 4x EL84 requiring 3.04A to nudge the voltage down a bit. On the other winding, you may find you need a dropping resistor on one leg (maybe between .25 and 1.0 ohm 5W) to tame the filament voltage. The advantage of only demanding about 1A of current is that you won't need such a large wattage dropping resistor if the voltage needs to be tamed and the amount of heat generated will be less.

[rock_mumbles]

I've used Antek PT's, they tend to give a bit lower rectified DC voltages than expected ...

For a plate voltage of about 310V with four 6P1P output tubes and three 6N2P-EV preamp tubes I used an AS-2T230

We have used the 100VA PT's for two 6V6's or EL84's ...
An AS-1T275 in an 18 watt Superlite gives plate voltages of around 325V

The 200VA PT is generally used for two 6L6's or EL34's ...

They are large and take up quite a bit of real estate, now I glue them down to the chassis so they can't move around ... I had one come loose on an amp traveling around the country.

They are economical, and we haven't seen one fail, but I still don't really like them much ...

[lord preset]

I've used Antek PT's, they tend to give a bit lower rectified DC voltages than expected ...

For a plate voltage of about 310V with four 6P1P output tubes and three 6N2P-EV preamp tubes I used an AS-2T230

We have used the 100VA PT's for two 6V6's or EL84's ...
An AS-1T275 in an 18 watt Superlite gives plate voltages of around 325V

The 200VA PT is generally used for two 6L6's or EL34's ...

They are large and take up quite a bit of real estate, now I glue them down to the chassis so they can't move around ... I had one come loose on an amp traveling around the country.

They are economical, and we haven't seen one fail, but I still don't really like them much ...

Thanks for the hands on report. What rectification are you using with these PTs? And besides being large what is it about them you don't like?

[rock_mumbles]

...

They are large and take up quite a bit of real estate, now I glue them down to the chassis so they can't move around ... I had one come loose on an amp traveling around the country.

They are economical, and we haven't seen one fail, but I still don't really like them much ...

Thanks for the hands on report. What rectification are you using with these PTs? And besides being large what is it about them you don't like?

We've always used ss diode rectification since they only have 6.3V heater taps ... and generally wire the two HV taps in series for a center tapped PT ...
The larger transformers can be noisy if you aren't careful how they are mounted ... there are huge "noise leaks" where the leads exit the toroid.
My brother and I each built push-pull EL34 amps using the two AS-3T325's a friend had bought and never used ... they had a lot of noise picked up by your guitar if you were within about 6 feet of the amp.
For the least noise, the leads need to exit on the bottom of the transformer next to the chassis ... then you need to be careful that the leads don't get squished between the toroid and the chassis ...
My brother and I also no longer use the stock mounting bolt for mounting, we use a fully threaded bolt up through the chassis with a lock nut so the bolt is held firmly to the chassis ... I generally make a spacer to raise up the toroid a bit and glue or bolt the spacer to the chassis and then glue the toroid to the spacer ...
A lot of (beginner) people mess up the wiring although it's not difficult if you understand what you are doing ...

[Phil_S]

It is good to know my math skills are ok, as I predicted the 200V model Antek would yield 200*1.4*.96 = 268 which I rounded to 270VDC. (.96 = 4% loss.) Now I'll apologize for my poor reasoning that it might be as much as 297 If the 230V model gives 310VDC, the 200V model gives 269.5VDC if all other factors are constant.

I'll second the thanks for good data on actual use.

[matt h]

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