Tube amp transformer

[Yazoo]

I would like to build Rob Robinette's Fender Deluxe micro and want to re-use a transformer from an old build I don't need. Rob recommends a 250-0-250v transformer for the solid state rectification version. The transformer I have is 190-0-190v. If  I ignore the centre tap and use a diode bridge,  I calculate I should get around 530V not taking into consideration voltage drop under load. I can then use a resistor to drop the voltage to the 390v B+ Rob recommends.

Are there any problems doing it this way?

[danfrank]

I'd use your transformer as originally intended, for a B+ of 260-270 volts. I'm assuming that this amp uses 6V6 tubes and since it's a "micro" that it's intended use is a small (relatively) low volume amp . 530 volts is too high even for JJ 6V6 tubes, and using a dropping resistor is going to cause major sag and heat. I think it will sound much better with the 260+ volts

[danfrank]

Looks like RR's site is down but I was able to look at an eBay auction and see that the output tube of this amp is a 12AU7. Definitely keep the B+ low, your transformer should work well as designed.
Or get the correct transformer. Even 390v is pushing a 12AU7

[Yazoo]

Thanks for your replies. I also had trouble accessing Rob's site but was able to use the Wayback Machine to get to the project. Rob says the following:
" For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode."

He describes experimenting with the voltages as follows:

"I hooked the Deluxe Micro's heaters to a dedicated 6.3V transformer then used a variac to control just the high voltage from a 375-0-375V transformer so I could vary the amp's B+1 voltage. I tried everything between 230 to 460V B+1. At 275V of B+1 I got 270V at the power tube plates and 103V at the V1 plates. With a low B+1 like this the amp looses some headroom and sparkle. The amp sounded great with a 460V B+1 with plenty of clean headroom, extra clarity and chime but pushing the power tube this hard will probably shorten its life."

This is why I was aiming at the higher voltage. Re. getting another transformer, I'm trying to exert some self-control and not go on another spending spree.

[Kevin Mitchell]

Hey I wanted to build a Micro Deluxe too! I had built a full Deluxe Reverb and sadly can't crank it often since I live in an apartment - may change if the downstairs neighbors keep acting sketchy 

Anyways, have you considered using an SMPS? I'm analog to the bone when it comes to signal processing but for something like this a full size PT is a bit much, especially when you're shunting the voltage for lower output.

[Marcos - Munky]

I'd just use your transformer instead of the one suggested by Rob, assuming it can output enough current for the tubes. You'll have less voltage and a bit less output power, but it should work.

Or go smps. In fact, if everything goes well tonight I'll be etching a board for a tube amp using a dual triode for the preamp and a 12AU7 with parallel triodes wiring in SE, and I'll be powering it using a smps. Just don't know when I'll be able to finish it tho  .

[Steben]

190V will still work, provided the needed current can be delivered.
The amp does not seem to be choosen for high power, so a little less voltage won't really hurt.
Remember you might have to adjust bias etc. I'm not that keen on looking at supply voltage only when talking about "sparkle and sound". It is all about bias.
I've biased an Epi Valve Jr on different anode impedances (different choice of OT tap) and all sounded great, as long as the bias was corrected with a correct cathode resistor.

[Yazoo]

Thanks for your replies.

Could you clear one thing up please? I thought a diode-rectified 250-0-250v supply would only give 250*1.412 volts DC, so approx. 353 volts with no load, yet Rob gives 390 volts, presumably under load. What have I misunderstood?

[Rob Strand]

Pretty much all transformer will measure higher than Vac * sqrt(2) on the DC side when there's no load.   Although for low output voltages the diode drops need to be considered.  The rated Vac for a transformer is specified under full load, a resistive load.    So the no load voltage is high then the voltage drops to the rated voltage under load.  That's called the regulation.  How much drop depends on the design and more often the size of the transformer.   A 4VA transformer might drop 20% to 25% whereas a 50VA transformer might only drop 10%, and a 100VA transformer might drop 7%.    Multi-tapped transformers for tube amps are often designed for 10% regulation.

So if you take 1.1*250*sqrt(2)  = 389V   you pretty much get Rob's values.     That's for no-load, or small-loads.  If you push a power amp then the voltages will start to drop under load.

[Marcos - Munky]

I just found this at Rob's page:

Using my Firefly's Hammond 269EX 190-0-190v power transformer I only got a B+1 of 285V and the V1 plates were at 110V which is a little low. The Deluxe Micro sounded good but not as good as with a B+1 closer to 400V. If you do use the Hammond 269EX 190-0-190V $47 power transformer, I recommend you use solid state rectification and reduce the B+2 27K ohm 2 watt voltage dropping resistor to 22K to keep the V1 preamp tube's plate voltage up near optimum.

[Yazoo]

Thanks again for your posts. The transformer I have which is quite beefy is in a power amp only version I did based on the Firefly. I'll measure the actual voltages on this. I think I will go ahead and try the 190-0-190 transformer. I can always upgrade it later.

[Steben]

Thanks again for your posts. The transformer I have which is quite beefy is in a power amp only version I did based on the Firefly. I'll measure the actual voltages on this. I think I will go ahead and try the 190-0-190 transformer. I can always upgrade it later.

What's the power / current rating?
As I said: I read "upgrading" as in higher output. But do you need that in a small power amp?

[merlinb]

Jeez somebody post the schematic already. 400V is crazy hot for a 12AU7 with a transformer load, I would expect arcing within a short time. As others have said, use the voltage you have already and adjust the biasing to obtain the Toan™, not the B+.

[Steben]

Ouch, parallel ...
Personally I'ld go for firefly style push pull...

[Yazoo]

The transformer is rated at 80mA for the high voltage. I measured the voltage at the first filter cap and I got almost exactly what I expected, 268 volts. I've ordered the parts for the Deluxe and I'll see how it works out.

I know there is no direct comparison between a 12au7 and a 6V6 but I found the Mojotone Deluxe schematic which gives the 6V6 plate voltage as 375V, whereas the datasheet states a maximum of 315V so I'm guessing the 12au7 is also being overdriven in the micro version.

[amptramp]

A lot of guitar amplifiers stress their output tubes to the maximum (or higher) voltages because they tend to be used for only a couple of hours a week even for a professional band.  If you look at public address amplifiers, they are derated to much more conservative values because in some uses, they were on 24/7, playing music with the occasional "cleanup on aisle 3" message thrown in.  I have a Muzak 920B PA amplifier which uses 6L6 outputs to get 15 watts.  There is a resistor you can short going from the centre tap of the B+ winding of the power transformer to ground.  If you short it, you get 20 watts and the voltages for both setups is given on the schematic on the inside of the bottom cover of the amp.

In a guitar amp, you would get 15 watts from a pair of 6BQ5's or in extreme cases, 6V6's.

A 12AU7 would not survive long at 400 volts.  If you want to get this kind of voltage, use a 12BH7 - it can take 450 volts and the 12BH7A can take 500 volts but this is a taller tube with twice the filament current but the same pinout.  But as other people have noted, at this level of power, push-pull outputs give advantages in that you can use the full range of magnetization in the output transformer core, not just magnetization in one direction.  Some more ambitious designs use the transformer in an H bridge configuration so they can use a single-ended winding and still get magnetization in both directions.  If you do this, you will need separate filament transformers for the upper tubes to avoid exceeding the heater to cathode voltage rating.

[Yazoo]

I am going to use a 12BH7. I've fretted and fretted about the transformer and ended up getting a 250v-0-250v transformer. I always do this. 

[Yazoo]

Just for information, I built the amp and measured 340v on the cathode pin of the 12bh7 using a 250v-0-250 transformer.

I had to use 0.47 ohm resistors on both of the heater taps to drop the heater voltage from 7.15 volts to 6.1 volts.

[PRR]

....measured 340v on the cathode pin of the 12bh7....

That doesn't sound right.

[mozz]

If it's a micro fender, why not use pentodes for the outputs? There are hundreds of 7 pin tv tubes that will give you a watt or 2 in push pull.