Advice on build the classic 300W Ampeg SVT

[dpacek]

Now that my 100w version of a B15 is finally humming it's time to go
bigger.

Has anyone attempted to build an SVT before ? any advice ?

I'm looking at KOC Schematic in TUT3, but not lovin his simplified pre-amp
and his exclusive use of Hammond transformers seems to make this more expensive
than using stock SVT replacement transformers.

The Joe Piazza schematic from 1970 looks good but not sure what chassis to use.
The only chassis I can think of is the Hammond 17x10x3 and KOC recommends adding a stiffening panel to support the weight.

Dan

[dpacek]

oops, bad choice of words. my amp is not Humming ..its actually working fine.

[imaradiostar]

Maybe I'm nutty but I'd probably make something in line with a Reeves Custom 225 or 200 watt Hiwatt first. You can get Heyboer transformers similar to the ones used in the Reeves amp and make a solid 225 to 250 watts while being a little easier to move than an SVT. Reviewers seem to universally love that amp too. It doesn't stray far from the old Hiwatt design from the things I've read.

You might look at using multiple Antec Inc toroids for the power section and a single Hammond or Heyboer for the output. There are a few threads on the forum from people that have done this.

jamie

[Bruce / Mission Amps]

I with Jamie 100% .... over the years, I've worked on quite a few of those old SVTs, 6146 and 6550 versions and a 200 watt Hiwatt with four KT88s at real B+, is way easier and kicks some serious booty with good iron!!
I have a late 60's-early 70's Ampeg SVT power tranny here somewhere from a salvaged amp.... if you really need to go that route, but over counter trannys are available that will suit your needs very well.

[g1]

I'll agree with the others, maybe not the best choice for a high powered build. Not great to service and probably worse to build.
Aside from the Hiwatt option, there was a Traynor YBA-3A Super Custom Special. Over 250 watts from four 6KG6 tubes. Apparently 6P42S or EL509 are subs for 6KG6 and can be had fairly cheap.
Some claim Ampeg got the idea for the SVT from the Traynor. Unfortunately it weighed in at around 100 lbs.

[Steve Conner]

The only reason to make the SVT is if you want the crazy preamp EQ. But bear in mind you can mix the SVT preamp with any of the power sections mentioned above.

I've seen surplus OPTs from the Fender 300PS going cheap on Ebay, in fact I have one that I never got round to using. They have a weird impedance, 3.3k to 8 ohms, but you could run it into 4, or it might suit Bruce's high B+ approach as is.

[kg]

the svt power amp section provided some of the initial design for the BAGA.

as for output tranny i have nothing but good things to say about the hammond 1650w. it is more than beefy enough for this application and is arguably as good if not better than the original svt OT.

honestly, if your bank account (and back) can handle it, i would interstage transformer couple a small power amp to a large power amp consisting of multiple parallel output tubes. if you can't do that, then go with mosfet source follower drivers for the multiple parallel output tubes. either way, make sure you have a low DCR output tube grid driver so that you can 1. completely avoid grid blocking distortion, and 2. squeeze a bit more output power from a given b+.

the only thing about high plate voltage/low screen voltage operation of output tubes is that the "best" load impedance is higher, and output trannies in that vein are harder to source (and thus generally more expensive)... steven's ebay OT finds notwithstanding!

[Steve Conner]

i would interstage transformer couple a small power amp to a large power amp

I got the 300PS interstage transformer with it too!

[imaradiostar]

I got the 300PS interstage transformer with it too!

you is smart.

I wonder how much interstage driving power you'd need to drive 4xkt88's with a hammond 124 series interstage transformer? I suppose very little right up until it starts to draw grid current. The bandwidth of those little transformers does worry me though.

jamie

[kg]

bastard! now strap those suckers up!

[kg]

you is smart.

I wonder how much interstage driving power you'd need to drive 4xkt88's with a hammond 124 series interstage transformer? I suppose very little right up until it starts to draw grid current. The bandwidth of those little transformers does worry me though.

jamie

they are more money for sure, but lundahl has an excellent reputation in the hifi world:

Lundahl Transformers - Tube amplifier transformers

distributor in the us is kevin carter @ K+K

K & K Audio - Lundahl Transformers, audio DIY kits and more

[imaradiostar]

300 Watt Output Transformer Fender PS 300 use 6550 KT88 | eBay

Seems like a lot for what it is. A custom transformer is the same money or less!

jamie

[kg]

angela has the off the shelf hammond for 240

Hammond Output Transformer 1650W

plus you get UL taps (very highly suggested if your screens can handle your vp).

[Steve Conner]

300 Watt Output Transformer Fender PS 300 use 6550 KT88 | eBay

Seems like a lot for what it is. A custom transformer is the same money or less!

Yeah, that's too much. I paid $100 for mine.

[dpacek]

Finally getting back to this project - I have decided to go with the stock 70's SVT pre-amp
(complete with tapped coil for mid range selector) and Stock PSU (TUT3 Fig 12-1 )with Kevin O'connors 'simplified' power amp (see TUT3 Fig 12-5)
The simplified design has a different splitter design but has individual bias controls.

the coupling of the stock pre-amp and TUT3 power amp has me a little perplexed.
The stock pre-amp out is from a cathode follower staqe (cathode to capacitor with the capacitor connected to one end load resistor to ground and then the signal line runs to the first stage grid of the SVT PA thru a couple more
resistors) while the input to the splitter circuit diagramed in TUT3 fig 12-5 is a line from pre-am Anode followed by coupling cap then a resistor to grid.

so do go

preamp out cathode ->cap(&load resistor) --> resistor-->PA grid

OR

preamp out cathode ->cap(&load resistor) --coupling cap --Resistor -> PA grid ?

Hope you guys are still watching this thread


Dan