Bassman Micro EF80 PT: Still another idea

King Fan

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"Which PT?" planning questions abound here, and so do "Wrong PT!" build problems. Go figure. :) Hence, a question:

*For the EF80 version* of his Bassman Micro, our amazing friend @robrob suggests the $29 Antek 200V 130mA AS-05T200 toroidal power transformer; noting it requires a bridge rectifier (no HT CT). For a B+ target, he notes, "EF80's run fine in the mid and high 200's" and suggests plate voltage 220-300V ("design max is 300v for plate and screen"). With SS recification, a choke, 2xEF80 (a 2.5W tube) and 3 preamp tubes, I calculate this Antek would give me B+ ~268V. And this 200V PT also has a 180V tap in case you wanted even less B+. But what if you wanted a bit more?

Aha, also for $29, Antek makes a 240V version, the AS-05T240, *with an alternate tap at 220V.* If I ran the 220 tap, I calculate B+ around 294V. And if that was too close to 300, or came in higher than calculated, I could add a sag resistor before the first filter cap.

So that's the question: Aim for B+ 268V (with no ability to increase) or 294V (with the ability to add a dropping resistor)? What say you?
 
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Lingwendil

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easier to work with higher and drop down. Easy to tune the output with dropping resistors.

I have a 05T240, and after a bridge with 1N4007s I get just a bit over 300 volts under a 50mA load depending on wall voltage and how much heater power you draw.
 
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King Fan

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Thanks. For sure, the ability to tune is good. I haven't tried to figure the exact resistance, but I'm guessing you wouldn't need *too* big a dropping resistor.

I meant to tag both @dougsta and @2L man here; they both know a lot about toroidals (and everything).
 

dougsta

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Thanks. For sure, the ability to tune is good. I haven't tried to figure the exact resistance, but I'm guessing you wouldn't need *too* big a dropping resistor.

I meant to tag both @dougsta and @2L man here; they both know a lot about toroidals (and everything).
I cheat and roll my own iron!
Always a good idea to have a stock of beefy resistors handy though, try a few dropping options (series or parallel some for options) and work out the theoretical drop and measure against what you get when the valves start pulling load at idle or power chords.
 

King Fan

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I cheat and roll my own iron!
Always a good idea to have a stock of beefy resistors handy though, try a few dropping options (series or parallel some for options) and work out the theoretical drop and measure against what you get when the valves start pulling load at idle or power chords.
Good point. My guess / calculation is with either of these Anteks, 2xEF80, and 3x dual triodes, the HT current draw might be 40-50mA? I'm assuming diode rectification, and putting the EF80 @ 2.5W.

And if you had 300V B+ with 50mA current draw, you could drop 10, 20, or 30V with roughly a 200, 400, or 600Ω (3W) resistor? So if I'm right I could have a resistor posse to combine around that range and do some alligator try-outs...

For those keeping score at home, I like this B+ / current draw calculator.
 

frankenxtein

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That's a neat little transformer. AS-05T200
I was searching for info on a hybrid 6.3v tube rectifier and learned something from
this post.
Depending on how much voltage you want to drop , adding that resistance to a PT secondary is going to drop more voltage on top of a tube rectifier.
I wish I'd have paid more attention in school.

It stands to reason you can drop voltage before your bridge rectifier.

Here's another online calculator that I've found useful. http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html
 

Len058

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Edit!!!
(You might be at the right track. I'm running into problems with my 200V 85mA transformer, running @ european 230V.)

I made a mistake. The tail resistor was 68k instead of 6,8k, which screwed everything but the amp worked. I'll give my voltages with an Antek 200v equivalent, we use in Europe.

Specs: I run it at the 240 tap, grid voltage is around 234V
  • Pri: 0-120-230-240 V (black-yellow-purple-red)
  • Sec 1: 200 V @ 0,085 A (red)
  • Sec 2: 6,3 V @ 2 A (green)
Out of the diodes: 284V
Plate EF80: 283
B+3 230V before the 10K dropping resistor it's 281
169 at V1B NB I used a 82K and a 56 load resistor and I made 4 inputs, so I deviated from Rob's design. I've ordered some 68 and 47K resisters to try as V1 load resistors. V1 is still a bit of a struggle to get right.

The voltage drops too much in the circuit. V1 was at 90V. I can get it up to 110 but that's about it. The 85mA is not enough? Does the circuit draw more current?

I've got another EF80 amp with the same amount of tubes but it uses a normal PT @
Primary: 230 V AC
Sec 1 : 230 V, 0,12A
sec 3: 6,3 V 4 A
It has more than enough power. I had to drop the voltage for the EF80's to 286 with a 2k dropping resistor. It didn't effect the voltages at the pre amp tubes too much. It was originally an amp with EL84's instead of EF80's.
 
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Paul-T

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Hmmm. This is slightly worrying as I was planning on building a JTM45 micro using this transformer. But in any case, it's great that more people are building these and sharing their experience - thank you, @Len058 .
 

Len058

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Hmmm. This is slightly worrying as I was planning on building a JTM45 micro using this transformer. But in any case, it's great that more people are building these and sharing their experience - thank you, @Len058 .
I'm going to measure all resistors and voltages to make sure I didn't make a huge mistake somewhere. So stay tuned.
 

2L man

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You should measure PT output AC voltage and all DC B+ voltages and tube anode and cathode voltages and it is possible to find out where the voltage is lost.
 

King Fan

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Tiny house? Good one. Heh, better to have that second room and never use it than to find you need it when your housemate is binge-watching HGTV. Besides, if I read right, some folks have had trouble getting B+ in the high 200s with 190-200V HTs. And I’ve had good luck with a little sag resistor….
 

Hyakuya

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I use this little beauty in a Champ micro and a Baseman micro.


Both of them run into a hybrid bridge so that I can use EZ81 rectifiers. In the Champ I use a dropping resistor to get the B+ right and for the Baseman I put together a variable voltage control so I can adjust the B+.
 

Len058

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You should measure PT output AC voltage and all DC B+ voltages and tube anode and cathode voltages and it is possible to find out where the voltage is lost.
Thanks found a problem at the cathode of V1. Everything seems to be fine now.
 

2L man

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Hmmm. This is slightly worrying as I was planning on building a JTM45 micro using this transformer. But in any case, it's great that more people are building these and sharing their experience - thank you, @Len058 .
Math there is: When 200VAC is full wave rectified and filtered is turns to 280VDC and its 50mAAC current rating drop to about 35mADC. This is because filter capacitors get charged using rectified sine wave peaks filling the enpty space there come between rectified sine wave peaks. Filtered DC RMS increase 1,41 higher!

Square of 2 (~1.41) is often used as a multiplier or divider when sine wave calculations are done :)
 




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