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voivodata 19th July 2007 08:40 PM

Guitar amp PT design questions
Hi !
I’m about to order guitar amp power transformer from a local shop. The guys told me that all they need are ratings for the secondary windings and they will calculate the number of windings , lamination sizes, etc.... So here are my questions:

1. How many amps for the 6.3 VAC winding - it will be used for two EL34 (JJ/Tesla in Push Pull ) + five ECC83 (JJ) + three relays (for channel switching)?
2. How many amps for the 330 VAC winding - B+ ?
3. How many amps for the 50 VAC, which will be used to bias the EL34s?
4. What wire gauge should be used for the shielding layer between the primary and secondary (one side of that is insulated and the other goes to chassis)?
5. Is similar shielding recommended between secondaries?
6. For hum reduction is it better to have a center tap for the 6.3 VAC winding or is it better to use a pot?
7. Have you any other suggestions for better design?

Sorry for these really beginner's questions…I’ve read few posts and theory about such calculations but I’m not quite sure I can manage myself, especially considering that in reality a good PT would probably require some reserve in power, size, etc…

PeteN 19th July 2007 09:06 PM

This is just a ballpark set of figures, so it's probably best to add a few percent to these.

EL34 - 1.5A (1500mA) each for the filaments, so you need 3A for these, minimum.
ECC83 - 320mA per is a safe bet (varies), times 5 means another 1.6A

Relays probably don't need more than 50mA, I suppose it depends on the size of the relay. I had a look at a 6Amp power relay which needed 44mA coil current at 12V, you can get away with much smaller (more 'delicate') relays for signal switching due to the tiny voltages and currents. Three relays thus are going to want about 150mA.

Total for 6.3V = 3+ 1.6 + 0.15 = 4.75A so allow 5A which will comfortably allow you to double the current the relays can have (100mA per), and still have 100mA left over for variations in tube current.

B+ I'm not so sure on, I've been trying to figure this out myself for 2xEL34 as well. Logic tells me for Class AB PP EL34s quiescent current would be less than 30mA, depending on your operating point (from looking at a set of curves, -30V operating point with 400V HT will put you at less than 10mA, even.
However, given a 60Vpk-pk signal input, current will swing up to about 250mA (only on one tube at a time, though), while the other goes into cutoff. Assume a full-volume sinewave input, RMS(250mA) is 176mA, so I'd think that B+ with 200mA 'oomph' should be sufficient for the EL34s?
ECC83s are going to want no more than 5mA each, which means 25mA for all of them, so 225mA so far.
Call it 230mA to be extra safe?
You probably need to consider power lost in rectification, smoothing and so on so these could be too low for all I know.

(Again, this is just logic, not experience, I'd urge you to totally ignore me and go with what someone more qualified to answer says).

From what I've read bias will hardly need any current sourcing ability at all. It depends on your biasing scheme though. The higher the grid resistors you feed the bias voltage through, the less current you'll need. You say 50Volts and EL34s, I say 680k grid resistors, that means 0.07mA bias current.
You'll probably want to consider grid leakage current here though as well, so again, I advise you ignore me and wait until someone who knows their stuff comes along. Looking at single manufacturer's site quickly, 100mA seems to be popular for 10-50V taps. I'm sure that's far more than sufficient for 5 amplifiers, but what do I know ;)

4. No idea.
5. No idea... Logic tells me it cant hurt, if you have the money
6, 7. No idea.

ThSpeakerDude88 20th July 2007 03:06 AM

Is this for a homebuilt amp or a replacement xfrmr for a production amp? If it is homebuilt, have a look at ax84's website. Their 50 watt push pull amp uses two EL34's and uses a Hammond 272JX power transformer.

Its specs are: 300-0-300 @250ma
5V @ 4a
6.3v@ 8a.

The Hammond 372JX should have a 50v bias tap, while the 272 does not and bias has to be obtained by a reverse biased diode and some dropping resistors off the B+ winding.

It is $100 from tuebsandmore.

Also , you might try this xfrmr :

That one gives you a little more b+ (325v instead of 300) and costs $20 less.

BudP 20th July 2007 04:14 AM

2 Attachment(s)

Here is a schematic of our typical 50 watt guitar amp power transformer. This has provision for a tube rectifier and you may not want that. The B+ and rectifier windings are on the inside of a full width .005 thick copper shield, not a wound shield. This is done for safety and noise. The folks you have wind your power should provide two turns of mylar tape on the B+ side of the shield and three turns on the low voltage side, with standard up wall lip up on the tape if they use a bobbin winding form. They will know what this means. The rectifier winding is with the B+ because the full B+ DC voltage is available in these windings. Tell them not to cheat here, your safety is at stake and while wound shields will do a good job on noise they will not provide safety shielding to ground like a solid copper shield will.

Assuming that the rectifier winding is not needed ask them to provide a 9 volt AC winding for 2 amps AC so you can rectify it down to 6 volts DC or 5 volts DC for your relays. The tap on the B+ winding can be for any voltage you care to use and the current will not exceed that shown. Go ahead and drop the AC voltage to 660 for both sides, especially if you are using solid state rectifiers.

Ask them to provide you with no less than 5% no load to full load regulation in the transformer and you will have a power house power supply that will not fade.

Enjoy your project too.


voivodata 21st July 2007 07:07 AM

Thank you all for your time. Now I have a better idea how to proceed

Another question came to mind after reading your replies:

Are there any benefits of having say a 330 – 0 – 330 VAC, 250mA B+ secondary as opposed to one 330 VAC winding. In case I go with the second option should that be 330 VAC, 500mA. ?

Thanks again and cheers!
PS: the amp will be a homebrew with power stage such as in most Fenders and a 4 stage preamp designed for insane amounts of crushing overdrive :).

BudP 21st July 2007 07:26 AM

2 Attachment(s)

So long as both FWCT and FWB have a choke after the lead capacitor there is no difference in power requirements from the transformer. Both will draw equivalent total VA, just as you have described. Nor will their be a difference in power availability. If you are going for super lead overdrive land, read the attached OPT voices description page. The SLO clone crowd has been gobbling up the Hi Def OPT.


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