5F2-A Power Supply questions

[MikeJH]

Hi everyone,

This is my first post on this forum, and I would also add that I am very new to amp building; so I do hope you will excuse me if this topic has been covered before.

I have successfully built a small SE amp from a kit and am now planning a scratch build of a 5F2-A Princeton. I am having some problems establishing correct operating voltages for the tubes and am hoping some of you can confirm what they should be?

1. I believe the 6V6 needs to run with a plate voltage of around 350V to get ‘classic’ Princeton tone. Is this correct? I would also like to know if this is purely the voltage at the Anode, or is it the difference in voltage between Anode and Cathode?

2. I have no idea what the correct plate voltage should be for the 12AX7. Can anyone tell me please?

3. Here in the UK we don’t have the abundance of transformer suppliers that you have in the USA, but I have found a couple of potentially suitable Power Transformers that are readily available. One is a 275-0-275; the other is a 290-0-290. Bearing in mind I intend to use Solid State rectification, could someone please advise me on which of these two is the best to use in respect of getting the correct plate voltage on the 6V6?

Thanks in advance for any help that you can offer

Mike

[echuta13]

325-350v on the 6V6's plate should be in the realm (or close enough).
The 5F2 had somewhere around 300v on it's plate according to it's schematic.

You should aim for around 150v on the 12ax7's plates I'd say anywhere between 140-160v is pretty good.
Both of those transformers will most likely give you too much voltage.

I've been using a Hammond 269JX, and that's about perfect for me in the states. Don't know if that helps you or not...

[tubeswell]

With SS rectification, any PT that supplies 50-100mA and about 240-0-240 to 275-0-275 will get you there. The 240V one will get you about 340VDC B+ under load. The 275V one will get you about 380VDC B+ under load.

For a bit more tonal versatility, if you get a PT that is say 300-0-300 @ 120mA, then you can run a 6V6 with a string of (say 3 or 4) reverse-biased 5W 15V zeners in series off the Centre tap to ground, which you scan switch a short across when you want to plug in a 6L6CG (and un-short them when you want to put in a 6V6). - This raises and lowers the B+ respectively. (Along similar lines I built a tube-rectified one like this last year, that you could increase the B+ on by plugging in different rectifriers - but the ability to short-circuit the zeners would achieve the same result). You can use a DPDT switch for this and use the other pole to switch the cathode resistor from 470R for the 6V6 to 270R for the 6L6, so your ready to go with the flick of a switch.

The heater should be 6.3V 2A for this application (or 2.5A if you want to add another pre-amp tube one day and do a sort of multi-stage AX84 SE lead gueetar amp, or add a tremolo or one-tube reverb).

If you are getting an OT made up, you want it to be 8k primary for a 6V6, and get say 4R, 8R and 16R secondary taps. That way if you have (say) an 8R speaker and you want to run a 6L6, you can plug the speaker into the 16R tap to get a 4k load resistance.

[MikeJH]

Thanks Echuta13 & Tubeswell, your comments are most helpful.

Tubeswell – you have raised some interesting options here; bearing in mind the PT options I have available I shall look more closely at what you have suggested.




Tubeswell wrote

"If you are getting an OT made up, you want it to be 8k primary for a 6V6, and get say 4R, 8R and 16R secondary taps. That way if you have (say) an 8R speaker and you want to run a 6L6, you can plug the speaker into the 16R tap to get a 4k load resistance."

Question

In respect of the OT, you are suggesting an 8Kohm primary impedance. This fits with what I see on the 6V6 tube data sheets, in that they suggest 8.5K for a plate voltage of 315V or greater. Do you know of any suitable ‘off-the-shelf’ OT that is suitably rated? I had been hoping to use a Hammond 125SE series OT – do you know if this would work. They appear to be configurable such that you can get 2.5K, 5K, or 10K primary impedance. Would 10K be close enough, or should I try and find something that is actually 8K?

Thanks again

Mike

[tubeswell]

Anything from 5k-8k5 will work, but 5k won't be quite as champy as 8k, because you will have less power (the lower load resistance translates into slightly higher voltage and less current going into the speaker than a higher load resistance would, so the speaker doesn't get pushed as hard).

[MikeJH]

Any ideas on a manufacturer that can supply the necessary single ended OT with an 8K primary impedance? The only transformers I can find with an 8K primary appear to be designed for push-pull applications.

[tubeswell]

Hammond 125ESE

Most transformer winders will be able to wind you one if you specify that you want a 12-15W Single-ended OT with an 8k primary and 4R, 8R and 16R secondaries. They're usually pretty cheap to make

[MikeJH]

The Hammond 125ESE. From what I see on the Hammond website (http://www.hammondmfg.com/125SE.htm ) this transformer can be configured to have 2.5K, 5K & 10K Ohms input impedance. Is there some way of setting it up to get 8K, or am I being dumb and missing something obvious here?

[tubeswell]

No you're not dumb

These guys might be closer to home for you

http://www.vvttransformers.co.uk/vtp117100.htm

Or run this one at twice the rated secondary impedance for 8k4 primary zed (i.e. attach an 8R speaker to the 3-4R tap)

http://www.vvttransformers.co.uk/vtp115100.htm

[MWJB]

Mike JH,

Torres Engineering is not that far from you, in Pinner, they have a suitable OT at a reasonable price.

See torresamps.com

[MikeJH]

Tubeswell, MWJB,

Many thanks, your information is most helpful – looks like I’ll be able to get a suitable OT after all