Transformers for 4-KT88 Bass Amp

[big_teee]

I plan on building a 4 KT88, or 4 6550 bass amp.
All the Transformers I find available, are 100w Specs.
So what Iron would you use for an amp of this size.
I have found some PTs that would work, big concern is the 100w OT?
I'm looking for new Transformer possibilities.
Thanks,
T

[bob p]

So you've decided that the build for your friend is going to be 4xKT88? Have you decided what type of power output you want and what sort of B+ voltages you want to use?

If you're starting off designing an amp around the tubes and the tube data sheets, and you haven't already made your mind up on a transformer set, then the first things that I'd want to know are the operating class, what kind of B+ you want to use and what kind of power output you want to achieve. Selecting one of the canned sets of specs from the tube data sheet would help a lot in determining what to look for in terms of transformer specs.

The other way to approach the problem is to design around commercially available iron for the application, using something from a production amp that's popular and available, rather than completely re-inventing the wheel.

As they say, there are more ways than one to skin a cat.

[pdf64]

There are a couple of big Hammonds, 1650TA and WA, both 1k9 p-p impedance.
Hammond Mfg.-"Classic" Push-Pull-Easy Wire-Tube Output Transformers - (1608A-1620A, 1645A & 1650A Series)
I seem to remember a thread here in which good results were had from this application.
But yes, more options would be nice.
Pete

[bob p]

Part of the problem with the Hammond iron is that when you get into the 100W range, the primaries aren't designed around 4 tube operation. At 100W, the best Z matches for a 1650R are at 5k, that's good for a pair of tubes operated at really high voltage, not such a good match for a quad. For really high power you have the 1650W, which has the primary Z at 1900, which is optimized for sextets of tubes rather than quads.

http://www.hammondmfg.com/1608.htm

As I recall from what Terry has said to me earlier, he wanted to avoid the high B+ in the area of 600-700V.

I don't think Hammond iron is the best for what he's trying to accomplish. Maybe something from one of the guitar amp manufacturers might work better?

[big_teee]

Thanks for all the input.
I wasn't aware that Hammond made the big OT transformers.
My goal was kind of the big marshall approach.
They ran 4 6550s in some of their amps like the JCM 800 2203, but they kept the bias low.
I have been looking at the Classictone project Iron.
ClassicTone 100W Project Style Universal Power Transformer
ClassicTone Project Style 100W Output Transformer, 2.2K to 4/8/16 Ohms
I put KT88s in the title, but with the Classictone, PT it has 2 HT sec. taps, I could run 4 6L6s on low tap, or 4 6550s, KT88s on the high tap.
Probably around 450-460v on low, and maybe up to 510-530 bridge diode rectified voltage on the high tap rated at 500ma.
The 2.2k OT is a larger sized marshall type OT rated at 100w.
I figure that is a conservative rating.
So that is where I am at this point, just trying to find Iron that would survive the 6550s, or KT88s ran at a conservative bias current.
I'm just trying to gather info on what all is available, and what others have built in this size?
Thanks,
T

[bob p]

Bass amp tone is so subjective, I'm surprised that we're talking about buying iron before we talk about what kind of tone your friend is looking for. Amps that use the design center values for power tubes tend to have very different sound/feel than amps that run the voltages at or beyond the tubes' specified limits and have that "rip your face off" type of tone.

Some things that I'd be asking if someone came to me to build their amp, would be things like:

What kind of bass amps sound good to you?
Do you like a Fender or Ampeg style preamp?
How many watts?
Do you want squishy, compressed tone, or exaggerated dynamics, or something in between?

Depending on how these questions get answered would tell me what kind of preamp and power amp need to be put together.

Based on your previous PM, I had the impression that you wanted *more* than 100W, not something with a 100W spec. The good news is that any amp up to and including 100W is going to be vastly less expensive to build than soemthing like a 200W Hiwatt, a Super Bassman, or an SVT, because the iron for those amps is going to be damned expensive. From a practical standpoint, I think of bass amp sizes as falling into 3 groups: Little ones like a B15, Medium ones like a Bassman or a V4-B, and Big-Ass ones. When it comes to the big-ass ones, I think it makes sense to buy a used amp from one of the big manufacturers. It's just too expensive to build the uber-amps out of new parts.

[big_teee]

I just want to find iron that will stand 100w +.
I have most of the other stuff worked out.
I asked all those questions.
Now I'm looking for the parts that will survive it.
I would like to stick with the 100-100w+ parameters.
The main concern is the OT survivability.
The Hammond 1650t should take care of those concerns.
Another question is the 1650t rated at 120w continuous?
4x42x70%=118 Watts.
I would probably bias more in the 60% range.
Thanks,

[loudthud]

I used 4x6550 with the Hammond 1650T and 278CX. It does 150W. (Note the 278CX only has a 6A heater winding.) Weber had some big transformers but they are not on the website now. Hammond has some newer power transformers now that can supply plenty of current at voltages under 500V, but no new OTs with reasonable primary impedances.

[big_teee]

I used 4x6550 with the Hammond 1650T and 278CX. It does 150W. (Note the 278CX only has a 6A heater winding.) Weber had some big transformers but they are not on the website now. Hammond has some newer power transformers now that can supply plenty of current at voltages under 500V, but no new OTs with reasonable primary impedances.

On the 278CX specs, it says 800v ct.
400-0-400.
So what HT voltage are you getting, 550v or so?
The classictone has a 0-380v tap for bridge rectifier.
What HT would I get out of that, it has a 500ma rating, and a 10A 6v rating?
T

[loudthud]

On the 278CX specs, it says 800v ct.
400-0-400.
So what HT voltage are you getting, 550v or so?
The classictone has a 0-380v tap for bridge rectifier.
What HT would I get out of that, it has a 500ma rating, and a 10A 6v rating?
T

IIRC B+ runs 560V. 100uF - 193R choke - 235uF full Pi filter. Ripple 1V pk-pk at full power.

0-380 should give you +525 or so, is that 500mA a DC or AC rating?

[big_teee]

IIRC B+ runs 560V. 100uF - 193R choke - 235uF full Pi filter. Ripple 1V pk-pk at full power.

0-380 should give you +525 or so, is that 500mA a DC or AC rating?

That is a DC rating.
http://www.classictone.net/40-18069.pdf
On Dreamtone it shows 420ma or so required for 4 6550s
PT Current Draw
Thanks for the help.
T

[defaced]

The Classic Tone transformers you're listing originated out of a group buy we did over at the SLO Clone forum a couple years ago. I have a PT and OT from that buy that I can hook up and get measurements if it'll help you. I also have a Hammond 1650T which is in my prototyping rig with ~550v DC B+. That rig uses an Antek 400v AC, 400VA power toroid transformer.

Given the price/performance ratio (per the ratings, I've not used my transformers yet), I'd be real incline to go with the Classic Tone transformers. The PT is well featured with secondaries at good ratings for many configurations and the stack on the OT is the same size as the 1650T, but without the cost.

[Leo_Gnardo]

Similar to Loudthud I built an amp with Hammonds, 1650T OT and 278CX PT. Came out well with @ 525V on the HV rail and @ 140W at clip with GE6550A's and later KT88's. However, I added a 10A filament transformer so I wouldn't be taxing the 6A rating of the PT. Also, I was foxed at first by Hammond's secondary wiring on the OT. Once I understood it, put in a switch to select 4 or 8 ohms. Antique offers a switch so that 4, 8 or 16 can be selected. I bought one but the contacts looked sort of small to handle 100+ watts . . so reasoning that it's a bit unusual to find 16 ohm bass cabs, opted for just 4 and 8 ohms. The newer Hammond "easy-wire" OT's may eliminate the need for fancy output impedance switching.

One thing about that 278CX PT, it's a monster! Except for that vexing 6 amp filament rating, what a terrific transformer. All the hi voltage current you could ever want.

I borrowed the pre design from Fender white Bassman. Tone - well it was for a blues bass player and he wanted plenty of bass so he got it. Pants-shakin' foot-rattlin' eyeball-floatin' bass. Did I mention it throbs pretty good? And enough top end for plenty of note definition.

edit: And ever since I've wanted to build one for myself. One of these days . . .

[bob p]

Leo, did you mean to say 278cx?

[Leo_Gnardo]

Leo, did you mean to say 278cx?

Ah whoops yessir I did, both times. 12+ pounds of rockin' iron. Just wish it had say 10A of 6.3V. Will go back & fix. Thanks bob p!