200W Steel String Singer Build--What PT?

[David Root]

I recently acquired a NOS 1976 vintage OT for Fender's 300 PS bass amp, from dreric, and I have a big (27 x 10 x 2 1/2, 1/8" aluminum) chassis that I have built successive different design prototypes on. Also a matching head cab. It is set up for octal preamp tubes and I would like to do it that way with 6SL7s, especially as it's not a high gain design.

The 300PS OT is nominal 3K primary, I measured mine at 2,940 ohms. Designed for 700Vp/340Vs and 300W with four 6550s. This 3K primary also fits four 6550/KT88 at about 625 Vp and 280 Vs, according to the plate curves, at about 200W.

So I was thinking of a 200W SSS, sort of a regular 150W SSS with headroom (!). "Brought to you by the Dept. of Redundancy Dept."

I can't readily find an available PT that fits the bill and I'm open to suggestions. I know that the 150W SSS is about 500 Vp and close to that on the screens, but I have reservations about real high Vs on current production 6550/KT88.

However, perhaps running that high screen voltage gives a better tone than a higher plate voltage with a lower screen voltage....????

What think ye?

[Cliff Schecht]

You're fucking nuts if you need that much power from an amplifier.

As far as the plate/screen voltage ratio goes, you will increase the headroom as you increase the screen voltage in relation to the plate voltage up until you actually increase the screen voltage higher than the plate. At this point, the screen starts stealing signal away from the plate and gives you a crude form of negative feedback by decreasing the gain, but I don't think it increases bandwidth as negative feedback typically does. Keeping the screen voltage a LOT lower than your B+ is best for linearity (which is why datasheets for tubes always are like this) but not necessarily what you want to do for a guitar amp. Something to consider (please correct me if I'm wrong anywhere in this post ).

[M Fowler]

I know Weber has some high powered PT and OT trannies on there site.

[talbany]

I recently acquired a NOS 1976 vintage OT for Fender's 300 PS bass amp, from dreric, and I have a big (27 x 10 x 2 1/2, 1/8" aluminum) chassis that I have built successive different design prototypes on. Also a matching head cab. It is set up for octal preamp tubes and I would like to do it that way with 6SL7s, especially as it's not a high gain design.

The 300PS OT is nominal 3K primary, I measured mine at 2,940 ohms. Designed for 700Vp/340Vs and 300W with four 6550s. This 3K primary also fits four 6550/KT88 at about 625 Vp and 280 Vs, according to the plate curves, at about 200W.

So I was thinking of a 200W SSS, sort of a regular 150W SSS with headroom (!). "Brought to you by the Dept. of Redundancy Dept."

I can't readily find an available PT that fits the bill and I'm open to suggestions. I know that the 150W SSS is about 500 Vp and close to that on the screens, but I have reservations about real high Vs on current production 6550/KT88.

However, perhaps running that high screen voltage gives a better tone than a higher plate voltage with a lower screen voltage....????

What think ye?

Dave..
I am not sure running screens higher is necessarily better for tone as cliff states.. I have not done that so I can't confirm.. Here is something to consider.. Whenever I run a pair of 6550's in the 600-650 range I'll add an OC3 to run the screens.. OC3's drop exactly 100v so in a way gives you a simulated ultralinear type voltages wo/ the tap which is good.. The added bonus is it's also a regulator so you generally don't need the screen resistors unless you want the added protection.. Hammond does this in there B-3 amps run 6550's.. I don't notice a tone difference however there is a slight difference in how the 50's distort.. with 4 in a guitar amp I doubt you will ever reach that point... The 100v drop on the screens will lengthen tube life and help linearity... OC3's are cheap and last forever.. Neon gas.. For safe measure I filter the screens = to the plates.. I've done this on several 6550 amps w 650v on the plates.. Jeff King who plays with Reba has one he likes for session work..It's funny they all think it's a rectifier...They are also cool looking as they flash purple..A rectifier that flashes purple...LOL!!..The wife faxed everything AGAIN!! to Fed Ex I'll check on it first thing morning.. Be Well!!

Tony

[David Root]

No, I don't NEED that much power, although I have a 4x12 with four Delta Pro 12As in it that can handle it flat out. I don't NEED a convertible with a 305HP small block 350 either but I have one ('98 Trans Am). It's a lot of fun to drive!

I just never built anything that big before, I have a bulletproof OT for it and the chassis is big enough to use octal pre's which do sound nice in a clean amp.

The Weber PT, thanx for the tip, I had forgotten about it, but it will only do about 550V quiescent (800V CT).

[David Root]

Tony, that's interesting. Thanx for sharing that, I have the space in the chassis to do that with the OC3. Plus the "purple haze" sounds cool! Would the simulation of UL operation also up the effective OT primary impedance though? Somehow I don't think so. I have to deal with a 3K primary.

So, when you use 600-650 Vp, what PT and rectification arrangement do you use? I had been planning to take out the screen supply off the middle of the two series plate caps and take it down to 280 from about 310 or so.

I'll check the Fedex tracking number again in the morning.

[Chad]

You're fucking nuts if you need that much power from an amplifier.

Maybe you don't need that much power (I know I don't) But I say more POWER to ya David.

[Mill Creek Audio]

Check these folks out for some reasonably priced toroids, from one that works for a D'lator to something that will work for this project.
http://stores.ebay.com/Antek-inc
I got a few to try in my next 100W amps....

[David Root]

Thanx Millcreek, I checked out their website too, these are super well priced! Chinese origin presumably, but if their tech info is to be believed they are good quality. I only looked at PTs.

They only give AC power on secondary into a resistive load. I can do the math from req'd 200W at OT secondary back to PT secondary using cap input load. But I'm somewhat confused by their quoting their numbers with the two secondary coils in parallel. Is there some sleight of hand going on here?

[Mill Creek Audio]

Try the AN-4T360, it's720V CT @ 1.2a with two 6.3V windings @ 5amps each. I used a few to build 100W/ch stereo amps (4-6L6 per ch) for a customer and they turned out very nice. I also have some high watt projects down the road, several Super Twin OT's and some from the PS400!

[David Root]

Thanx, I was looking at the AN-4T450, to get ~625 plate volts.

Also 9 lbs in a toroid is about 15-18 EI equivalent so that is a good weight equivalent too.

At $50.00 that is an unbelievably good price. I would get the steel case too, adds $29.00 but still half the price of the Weber 300W PT.

[talbany]

Tony, that's interesting. Thanx for sharing that, I have the space in the chassis to do that with the OC3. Plus the "purple haze" sounds cool! Would the simulation of UL operation also up the effective OT primary impedance though? Somehow I don't think so. I have to deal with a 3K primary.

So, when you use 600-650 Vp, what PT and rectification arrangement do you use? I had been planning to take out the screen supply off the middle of the two series plate caps and take it down to 280 from about 310 or so.

I'll check the Fedex tracking number again in the morning.

Much of this depends on the tone your chasing.. If your looking clean and punchy I recommend the higher screen voltage of the OC3 over running them @ 1/2 MM method
You can do the OC3 on either tube or SS rectification I've done it with both... The Hammond schematic shows the OC3 regulating everything from the screens back.. I regulate screens only..

Tony

[tictac]

I built a Stereo SSS type amp running 4 6550's with a One Electron power transformer model number BFT-1. It has two B+ secondaries: a 385-0-385 @ 340mA, and a 260-0-260 @ 360mA. I use the 385-0-385 winding to operate the plates (530VDC) and the 260-0-260 winding for the screen supply (360VDC)

This PT is built for HiFi use so the specs are good, it also has two 6.3V windings and a 5V winding. Antique Electronics has them I believe. I got mine from the designer John Atwood and I believe the newer model BFT-1b has even more current capacity.

TT

[David Root]

tt, I have two of the BFT-1Bs. One of them is in the chassis I want to use for this build, running a pair of 8417s at 100W. It's a good PT alright but it won't support 200W I think.

How did you wire the screen taps? I had trouble with that.

Tony, thanx again, I am looking for beautiful clean SSS tone, with the filters for adding specific harmonics to individualize the basic tone. Maybe I should think about this more, as I know the real 150W SSS used basically 500 V on plates and screens, I just worry about modern 6550/KT88s being able to handle that screen voltage/current. Is that realistic or do I worry too much?

[talbany]

tt, I have two of the BFT-1Bs. One of them is in the chassis I want to use for this build, running a pair of 8417s at 100W. It's a good PT alright but it won't support 200W I think.

How did you wire the screen taps? I had trouble with that.

Tony, thanx again, I am looking for beautiful clean SSS tone, with the filters for adding specific harmonics to individualize the basic tone. Maybe I should think about this more, as I know the real 150W SSS used basically 500 V on plates and screens, I just worry about modern 6550/KT88s being able to handle that screen voltage/current. Is that realistic or do I worry too much?

I wouldn't sweat the 500v on either.. Especially the 88's.. The OC3 trick applies mainly to plate voltages running in the 6-650v range... If say you have a transformer you want to use to match the OPT voltages of the 300 or if your modding an amp with that high a plate voltage the OC3 works..If your starting from scratch find one in the 500v like the original's naturally..

FWIW.. I prefer NOS 6550's Like GE's over new production ones hands down!!..(including 88's).. If you can find some for a decent price get them.. If I were going with newer tubes I would use the 88's.. From my experience...

Tony