OT, the bigger the better? And which one to choose?

[shocki]

Recently I played some small wattage boutique amps. For example a Valvetech and a Marble Amp (basicly those are Tweed Amps)
Both amps had very big OTs. Way bigger than the OT in my Deluxe Reverb which is really tiny.

The Valvetech one I played was actually a Deluxe Reverb "clone". But it stayed very tight in the bass when turned up high. Something which my Deluxe Reverb cannot deliver. Also the tweed one stayed very tight when turned up in a pleasent way.

So I am pretty sure that has to do with the big OT. I would like to build some kind of tweed clone and I think a bigger OT would be the key what I am looking for. Which is the sweet Deluxe tone but with a tight bass and a bit more headroom.

I have no idea which OT to use though. If I use 6V6 in the Tweed I would have to use one of these tiny OT with the correct primary impedance for the 6v6 I think.
Outputtransformer for: FenderŪ TWEED Deluxe 5E3, U - Output Transformers

I also found this OT. But it looks only slightly overwound from the picture. Still pretty tiny.
http://www.triodeelectronics.com/40-18090.html

Or could I just choose an OT from one of the bigger amps?

Thanks alot for your help.

[Steve Conner]

I agree, a bigger OT tends to make bigger tone. The only issue is that a big OT can let too much bass through and cause your speaker to fart at high volume. You might like to try one of the Hammond models: not the new guitar-specific line, but the old full-bandwidth ones.

[shocki]

I think I can handle the addtional bass with changing caps in the circuit so it works out fine. But I have to stick to the correct primary impedance for the 6V6 which would be 6600 Ohms? Ist that correct?

I checked out the Hammond site but could not find the transfomer you talked about.

[Alan0354]

I agree, a bigger OT tends to make bigger tone. The only issue is that a big OT can let too much bass through and cause your speaker to fart at high volume. You might like to try one of the Hammond models: not the new guitar-specific line, but the old full-bandwidth ones.

I just changed out the huge Bassman 100 OT to the one used in Bandmaster/Vibrolux/Pro Reverb.......Of cause I disconnect a pair of 6L6GC. The big transformer just don't compress and have too much bottom. It almost sounded like booming!!! It just does not sound as good. I just bought the cheap Magnetic Components for $46, no complain yet.

[shocki]

Ahh ok. Those ones I think?
Hammond Mfg.-"Classic" Push-Pull-Easy Wire-Tube Output Transformers - (1608A-1620A, 1645A & 1650A Series)

Still don't know exactly which primary load I should choose.

[Steve Conner]

Yes, those ones.

Re impedances, see this thread: http://music-electronics-forum.com/t8900/

The Tweed Deluxe used an 8k OT. Later Deluxes went to 6.6k, which might be a good compromise if you can't decide whether you want to use 6V6s or 6L6s.

[jpfamps]

The Tweed Deluxe transformer is straight wound (no interleaving) on a fairly small core. I don't know if grain orientated steel was used or not. However the take home message is that this was a cheap transformer.

Although the Tweed Deluxe is revered for it's tone, it's worth remembering that it was probably not considered a professional amp by Fender during it's production.

From my own investigations, all things being equal (which they probably aren't), then a larger transformer will be able to deliver more power, although my suspicion is that this is mostly due to reduced IsqR loss as thicker wire is used.

Subjectively, a smaller transformer seems to have a bit more top end. This could be either be due to loss of bass in the smaller transformer (although I've done AB comparisons with two transformers, both with 5 sections, that where specified to pass 50 Hz at full power and both run below rated power), or lower inter winding capacitance due to less wire in the smaller transformer.

One question I would be interested to hear peoples thoughts one regarding the perceived loss of bass in a smaller transformer, which effect predominates: lower primary inductance (my vote), or transformer saturation? Presumably, a lower primary inductance would make it harder to drive enough current into the primary winding to saturate the transformer?

[Alan0354]

What are the different winding patterns for OT and their characteristics?

[Enzo]

In this context, when you have a center tapped transformer winding, you have a couple choices. You can use two wires and wind the two halves of the winding at the same time. That results in the same wire lengths and the same DC resistance. Two ends are then connected together to form the center tap of the now united winding.


The other option is to simply wind half the turns, tap on a center tap wire, then finish winding. That results in still the same number of turns on either side of the CT, but the outer windings will have much longer wire length and thus greater DC resistance, but it costs less to make. The winding still works because transformers work on turns ratios, not DC resistance. But for guys trying to adjust idle current, the difference in resistance will cause a difference in idle current side to side.

[Alan0354]

In this context, when you have a center tapped transformer winding, you have a couple choices. You can use two wires and wind the two halves of the winding at the same time. That results in the same wire lengths and the same DC resistance. Two ends are then connected together to form the center tap of the now united winding.


The other option is to simply wind half the turns, tap on a center tap wire, then finish winding. That results in still the same number of turns on either side of the CT, but the outer windings will have much longer wire length and thus greater DC resistance, but it costs less to make. The winding still works because transformers work on turns ratios, not DC resistance. But for guys trying to adjust idle current, the difference in resistance will cause a difference in idle current side to side.

So the good expensive ones wind symmetrically both half at the same time so they are side by side on every turn. Actually when we wound transformer a high power switching supply, we had to do this because at over 50KHz square wave, even the waveform is imbalance between both half. I just do not realize it is important for audio frequency.

What do you call this kind of winding?

Do people do scattered winding to lower inter winding capacitance to increase frequency response? Or do they just wind from left to right, cover with paper, then right back to left, cover with paper..........

[Enzo]

I don't know that I consider it important at audio, but I suppose it means the people making the transformer were a bit more serious about it. I don't have a term for it, someone above said interleaved, which sounds good to me. I call the basic on "simple wound" myself.


Where it matters to me is when someone is troubleshooting and sees his Classic 30 output transformer measure 120 ohms on one side and 90 ohms on the other (or whatever) and they assume it must be bad. Nope, that is normal.

[Chuck H]

I just want to 2nd Steve and elaborate that the OT size is just one part of the equation that = the tone of the amp. Some amp designs do well with a smaller, even cheesier OT that rolls off some bottom end due to inductance, saturation, whatever. And some amps really come to life with a bigger OT that tightens thngs up.

I think a Deluxe type circuit with a spankin OT will probably sound boomy and fart the speaker unless some other circuit alterations are made. It'll still sound basically tweed like, but bigger.

If you want to run 6L6's in the amp you'll probably do well with the really typical 6.6k pri Z that either 6L6's or 6V6's will do fine with. At the Deluxe Vp it's kind of a tweener value for either tube. Of course, there's another issue! A higher plate voltage will definitely help a bigger OT shine by tightening the bottom end. If the tube is making a mushy bottom end due to lowish plate volts then the best a bigger OT can do is faithfully reproduce it. So, with your criteria in mind, you may want to up the Vp a little too. Now, If you go 6V6's only I think the 8K impedance at a slightly higher Vp will work pretty well. And I'll even suggest the Hammond 1608 (suffix "A" or not). It's rated for 10watts but don't worry. It's bigger than your Deluxe Reverb OT and can handle anything a 20W amp can throw at it. I have one in my personal 15W amp and it lives cranked. Produces fabulous full spectrum distortion with tight bottom end and the OT just gets warm. It's the only amp I've been using for over four years and no trouble yet. I run it into three different loads. A reactive/resistive load attenuator, a resistive dummy load and, of course, a normal guitar speaker. I've even accidentally fired the amp up without a load and blanged on the strings on two occasions without failure. It's tough and burly. A tech I spoke to at Hammond told me that when choosing an off the shelf (16XX) model for a guitar amp that it's fine and even preferable to half the watt rating.

Oh... An old member here, shea (that we don't hear from anymore. Unless he's using a different SN), used the Hammond 1608 in his amps and won a couple of biggish amp tone shootout type things. It's a good 'un.

[Steve Conner]

Alan, the RDH4 has a chapter on output transformer design that gives all the commonly used interleaving patterns. I doubt guitar amps ever got beyond the first two in the list.

Two wires side-by-side ("bifilar") winding carries some serious insulation challenges due to the high AC voltage between wires. As far as I know, only McIntosh hi-fi amps used it.

If you're going to make a highly interleaved OT for hi-fi, there are ways of distributing the wire to equalise the DC resistance in the two half-primaries, but for guitar amps it's not worth the bother.

PP OTs don't saturate, unless the output stage develops a DC imbalance when overdriven.

[Alan0354]

Alan, the RDH4 has a chapter on output transformer design that gives all the commonly used interleaving patterns. I doubt guitar amps ever got beyond the first two in the list.

Two wires side-by-side ("bifilar") winding carries some serious insulation challenges due to the high AC voltage between wires. As far as I know, only McIntosh hi-fi amps used it.

If you're going to make a highly interleaved OT for hi-fi, there are ways of distributing the wire to equalise the DC resistance in the two half-primaries, but for guitar amps it's not worth the bother.

PP OTs don't saturate, unless the output stage develops a DC imbalance when overdriven.

Thanks, I have not got to the transformer chapter. Still stuck on chapter 13 and too busy to complete it. It's not that I intentionally just like this, it is also because RDH4 have a bad habit of referring the chapter 12 and 13 in chapter 2. So you kind of force to jump to 12 and 13 whether you like it or not!!!

So most OT for guitar just wind one side, pull the center tap out and then continue to wind the second side on top?

Is interleaving just winding from left to right, put a layer of paper or something and then wind back from right to left? If most of the guitar OT are interleaving, there is not that big a difference between the expensive Mercury Magnetics vs other cheaper brands, maybe the core material difference?

For example, there are a few companies making the replacement OT for Fender Bandmaster/Vibrolux/Pro Reverb. They all the same size and same spec. Do they sound the same in the exact same amp?

[frus]

PP OTs don't saturate, unless the output stage develops a DC imbalance when overdriven.

could you please explain what does "DC imbalance" mean? does it occur when, for instance, the tubes are not well matched and one of them has higher output?

btw, I was wondering, in PP one tube is shut off for a part of its work cycle. does it mean that the OT would saturate many times per second, or does it cancel through time?