General PT question

[mort]

Am I correct in thinking that PT's for guitar tube amps need only be selected for their voltage and current capabilities and don't play heavily into the nuances of the overall sound?

So does it really matter the output tube type used, as long as it will handle the current demands and deliver the appropriate voltages? For example, using a Twin Reverb PT for a 4x6v6 amp. Would you be concerned with anything besides the current and voltages?

[R.G.]

Everything involved in audio becomes a never-ending series of finer and finer approximations.

To a first approximation, no, only the voltages and currents matter, and even then only "enough" for the current.
Second approximation: Yes, the current ratings matter because players want the power voltage to sag a little with power output so the output sound becomes a little compressed.
Third approximation: the amount of "sag" has to be *just right*, which of course varies from player to player.
Fourth approximation: (well, maybe it might be the second one) the transformer should not emit magnetic field leakage and cause hum
Fifth through ten-thousandth approximations: the less technical the player/amp builder and the more mystically inclined, the more every part in the amp contributes to the universal harmony of the transformer with the celestial resonances of the quantum flux capacitors in the... uh, sorry. Got carried away.

[Dave H]

So does it really matter the output tube type used, as long as it will handle the current demands and deliver the appropriate voltages? For example, using a Twin Reverb PT for a 4x6v6 amp. Would you be concerned with anything besides the current and voltages?

The current will be "enough" but if it's only loaded with 4 6V6s I'd think the B+ would be too high (for 6V6). It's 460V with 4 6L6s it could be up to 500V with 6V6s

[mort]

The current will be "enough" but if it's only loaded with 4 6V6s I'd think the B+ would be too high (for 6V6). It's 460V with 4 6L6s it could be up to 500V with 6V6s

Thank you. That's the practical consideration that was escaping me.

So now the question is do I spend more money on the properly engineered transformer or do I buy a more affordable PT and deal with dropping voltage... Hmmm

[Tom Phillips]

...So now the question is do I spend more money on the properly engineered transformer or do I buy a more affordable PT and deal with dropping voltage...

If you need to buy a transformer anyway I'd recommend that you directly buy one with the specs needed for your build. "Properly engineered" (Good quality) should win out over "more affordable" (which could mean cheap unreliable, low quality).

Regarding "affordable" there are plenty of good quality fairly priced transformers available. Just avoid the companies that spew too much hype to support their high prices.

[Tom Phillips]

Something else to consider.
If you are planning to build your own amp and you are not trying to make an exact physical clone of a classic amp then you will have your choice of more off-the-shelf transformers. This is because you will not need to pick from available drop in replacement transformers. This gives you another cost saving opportunity.
Cheers,
Tom

[Leo_Gnardo]

So does it really matter the output tube type used, as long as it will handle the current demands and deliver the appropriate voltages? For example, using a Twin Reverb PT for a 4x6v6 amp. Would you be concerned with anything besides the current and voltages?

For a 4x6v6 amp I'd suggest PT115* from Ruby/Magic Parts, intended for Vibrolux/Pro/Vibroverb. If you don't have a vendor's account you may have to ask a friend that does to get one for you. (Ruby/Magic sells only to the trade, not individuals.) I've used many to replace PT's in all sorts of amps including Ampeg B-15. I think you'll be happy with the price. Are they reliable? Well consider one of their customers is a very well known amp builder whose name rhymes with rumble. Should be good enough for anybody. And I think you'll be VERY pleased with the price.

* 120V primary, 650VCT 180 mA + 6.3V 4A + 5v 3A secondaries. "Lay down" style as you encounter in most Fenders.

[Gingertube]

When designing from scratch then you have the option of choosing the best way of doing things rather than being "forced" into a particular circuit method.
So - choose a power transformer on which you can use a full bridge rectifier for B+, that means no centre tap. It's a last 5% thing and you trade that against the more likely availablity of a CT style secondary winding transformer.
Cheers,
Ian

[Chuck H]

choose a power transformer on which you can use a full bridge rectifier for B+, that means no centre tap. It's a last 5% thing and you trade that against the more likely availablity of a CT style secondary winding transformer.

Well... I may be demonstrating my ignorance now, but... Why is a bridge rectified HV wind preferable to a full wave rectified HV?

Incidentally, I looked into this thread precisely because there are some knowledgeable posters in on it, yourself included, and I always hope to glean any knowledge that doesn't go over my head.

[Gingertube]

The answer is basically in the first sentence here ( the required transformer is simpler and therefore should be cheaper) - There are some other benefits with lower diode reverse voltage rating required.
The Valve Wizard.

This is the bit on the " 2 phase / full wave" supply.
The Valve Wizard

And a new supply design example
The Valve Wizard

Cheers,
Ian

[Tom Phillips]

...the required transformer is simpler and therefore should be cheaper...

We have not talked about the other way to get a cheap PT which is to rewind a transformer salvaged from other equipment. To be clear, I'm not recommending that mort go down that path because to keep it affordable you need to donate many hours of free labor. I'm writing this because, when Ian said "simpler", it instantly reminded me of hand rewinding PTs for my first amps. I always wound them for use with a FWB rectifier because then I only needed half the turns of larger gauge wire for the secondary. That was back when I was a teenager with much more time than money. I have not rewound another PT in several decades. It's a straightforward process but a lot of work and a pain to get the proper supplies.
Cheers,
Tom

[eschertron]

The answer is basically in the first sentence here ( the required transformer is simpler and therefore should be cheaper) - There are some other benefits with lower diode reverse voltage rating required.
The Valve Wizard.

This is the bit on the " 2 phase / full wave" supply.
The Valve Wizard

And a new supply design example
The Valve Wizard

Cheers,
Ian

I have been lurking on some of these recent PT threads since I'm struggling with a current amp design, and am gleaning info about design from these threads and take each post as an opportunity to learn. Please take my comments in that light.

You had me going in circles for a while looking for the phrase "the required transformer is simpler and therefore should be cheaper" in this thread or in the link. Finally I realized the reference to Merlin's assertion that "it economises on transformer design"; which does not always translate into cheaper for the end user. With that in mind, I looked briefly into Hammond's catalog and dug up the 269AX/369AX and the 269JX/369JX which are 115ma@250V versus 69ma@500V respectively. The lower-voltage/higher current models are a few $US cheaper than the others; but there is a slight difference in total capacity, so they're not a perfect comparison.

I understand that a FWB design forces the use of a diode bridge (or network), so the design is going to be simpler than one using a valve rectifier. OK cost savings there. And looking at the Hammond data sheets, the 269AX is slightly smaller (lighter?) than the 269JX. Bonus there too. You have opened my awareness to the possibility of using a FWB design. I never really considered it, as all the Hammonds are CT transformers. I will have to look at other suppliers now to see what they offer.

Cheers back at ya!

[g1]

Well... I may be demonstrating my ignorance now, but... Why is a bridge rectified HV wind preferable to a full wave rectified HV?

I too was a little confused and now understand it has to do with economy. Otherwise, I was wondering, if bridge was preferable, why not just leave the CT disconnected .
Makes sense now, thanks.

[Gingertube]

Seems I opened a can of worms.

Yes its is a cost/size/efficiency thing which is mostlt applicable when designing/specifying a new transformer for an applicatiion.

Over at DIYAudio DF96 posted this:
QUOTE:
"In a given transformer there is a certain amount of space for the secondary winding(s). To use much more or less than this will upset the balance between copper and iron losses, and the balance between primary and secondary losses. For a given secondary voltage you can either do one winding (and use a bridge) or have a CT which means twice as many turns in total. Twice as many turns means only half the cross-sectional area per turn because the winding still has to fit in the same space. Half the area means twice the resistance, so for a given current flow you get twice the power loss. To get back to the same power you have to drop the current to 0.707 as much i.e. a drop of about 30%.

As others have said, this was an acceptable compromise in the days of valve rectifiers with common cathodes. It meant only one diode drop instead of two - when a diode drop could be 10-50V. It also avoided the need for common-anode rectifiers which would have more complicated innards than common-cathode. "

That expains the size/cost efficiency thing.
If you want to use a tube rectifier then stick with a centre tapped secondary so you only need 2 diodes (1 tube with a common cathode).
If you are designing from new and want minimum cost/size/weight go for a single secondary and use the bridge.

NOTE:
If you use a centre tapped tranny and use a bridge with the centre tap left open, then you get 2 x the voltage AND because both half windings are then delivering current on both half cycles (instead of alternating) the current rating is halved.

Cheers,
Ian

[trobbins]

The old centre-tapped secondary had the added intrinsic benefit for valve rectifiers of including a higher effective series resistance - which could mean an additional economy in not needing to add a series power resistor. That doesn't mean it should be used for a modern design - but assists in helping appreciate why it was used so generically in older days.