Ampeg VL-1002 --> Hiwatt DR103 Conversion

[turbofeedus]

Alright, tackling a new project even with the clubman still warm from the iron.
I've always wanted a Hiwatt DR103 and I'm going to try and build one from the carcass of an Ampeg VL-1002.

Imgur album with schematics

Imgur album with some pics of the guttage

Some questions I have:

1) I was able to get a datasheet of sorts from TMI who made the OT. The DR103 used a high fidelity OT, with apparently a freq response of up to 112kHz? This OT is a much more typical 40-15kHz. How much is this going to affect the end product?

2) The ampeg is bridge rectified with the center tap connected between two caps, but the DR103 used a "standard" bridge rectifier setup. Is it worth it /necessary to hook up the HT center tap the ampeg way, or can I just lift it? What diodes are typically used here? 1N5408? UF4007?

3)Looking at the bias supply, If I wanted to make it adjustable, I would assume I'd replace the 47k resistor with something like a 10K, and use a 50k linear pot?

That's all for now. I'll keep updating this thread with progress.

[gingertube]

The Ampeg has a "Voltage Doubler" Power Supply for the high voltage.

Just keep that same arrangement. It will give you suitable voltage and current for the 100W version of the HiWatt Output Stage. Do use 2 x 220uF rather than 2 x 100uF in the original Ampeg. 350V rated shoul be fine.
Note that beecause of the voltage double nature the voltage share resistors across the caps seen on the HiWatt Schematic are not needed.

The HiWatts used Partridge Output Transformers which were indeed HifI specification. Get a re-issue if you can, else a Hammond 1650TA will be close enough (or you could also just build with the old Ampeg Output Tranny and see how it goes).

Use Ultrafast Soft Recovery diodes for power supply. The UF4007 are rated at 1A and should be OK but for "Brute Force and Ignorance" approach use the 3 Amp rated UF5048 or equivalent.
I am on a 1N5408 extermination hunt through all my amps - it has the worst diode switching noise I've ever seen. In a 2 x EL34 Amp those spikes were clearly seen on an oscilloscope hooked to the output transformer secondary.

The 2 x orange wires on the Ampeg Power tranny are the bias supply winding. Bridge rectify it and tie the +ve side to 0V to get a raw Bias -V supply. Then add a pot to get a suitable bias voltage range. I recommend 4 pots 1 for each output tube. UF style rectifier diodes fgor this too.

I know that I am skipping a deal of detail here but this should be enough to get you started thinking about it.

Cheers,
Ian

[Stevem]

Even in the late 80s production Hiwatt amps when they stopped using Partidge units the transformers where still massive!
The 50 watt PTs and OTs where the size of most 100 watt iron in 98% of most other brand amps!

The next closest amp I can think of in terms of overkill iron would be Sunn amps up to 1971 or so.

All the DR103s that I have ever worked on easily pumped out over 100 watts RMS with a strong testing set of output tubes and new filters.

If your gonna play it cranked out, then don't skimp on the wattage handling of the speakers!

With all this being said just note that the Ampge only has 465 volts to the output tube plates and the Hiwatt has some 520 , so your build will not have the real punch that the Hiwatt has and this means that preamp tone and feel wise the power supply dropping resistors in your build will need to be of lesser value then what the Hiwatt schematic shows!

By my math placeing a 11% resistor correction factor on the dropping resistors will put you right where you want to be at least for starters!

[turbofeedus]

Thanks Ian, let me see if I understand you correctly.

The Ampeg has a "Voltage Doubler" Power Supply for the high voltage.

Just keep that same arrangement. It will give you suitable voltage and current for the 100W version of the HiWatt Output Stage. Do use 2 x 220uF rather than 2 x 100uF in the original Ampeg. 350V rated shoul be fine.
Note that beecause of the voltage double nature the voltage share resistors across the caps seen on the HiWatt Schematic are not needed.

Ah I see, I didn't even know about voltage doublers. So If I were to just measure the AC from the HT windings to the center tap, it would be ~230VAC, but because of the way the bridge rectifier is hooked up it's doubling the voltage and rectifying it(~460VDC). Is it worth it to also have the shared capacitor network like the hiwatt? Is the power filtering in the hiwatt schematics sufficient?

Edit: Nope I see this is wrong, it should be ~160VAC. That would yield (160*1.414) = ~230VDC through each cap, and ~465VDC output.

The HiWatts used Partridge Output Transformers which were indeed HifI specification. Get a re-issue if you can, else a Hammond 1650TA will be close enough (or you could also just build with the old Ampeg Output Tranny and see how it goes).

Yeah I'm gonna wing it with the ampeg OT, my goal is to not spend a ton of $$ like I did on the clubman.

Use Ultrafast Soft Recovery diodes for power supply. The UF4007 are rated at 1A and should be OK but for "Brute Force and Ignorance" approach use the 3 Amp rated UF5048 or equivalent.
I am on a 1N5408 extermination hunt through all my amps - it has the worst diode switching noise I've ever seen. In a 2 x EL34 Amp those spikes were clearly seen on an oscilloscope hooked to the output transformer secondary.

Gotcha, I'm of the "use overspecc'ed stuff when feasable" mindset as well. UF5048 it is.

The 2 x orange wires on the Ampeg Power tranny are the bias supply winding. Bridge rectify it and tie the +ve side to 0V to get a raw Bias -V supply. Then add a pot to get a suitable bias voltage range. I recommend 4 pots 1 for each output tube. UF style rectifier diodes fgor this too.

Makes sense, it looks like the typical setup for that is just all 4 pots in parallel, like what TW does here except two more pots/nodes. That look alright?

Even in the late 80s production Hiwatt amps when they stopped using Partidge units the transformers where still massive!
The 50 watt PTs and OTs where the size of most 100 watt iron in 98% of most other brand amps!

The next closest amp I can think of in terms of overkill iron would be Sunn amps up to 1971 or so.

All the DR103s that I have ever worked on easily pumped out over 100 watts RMS with a strong testing set of output tubes and new filters.

If your gonna play it cranked out, then don't skimp on the wattage handling of the speakers!

With all this being said just note that the Ampge only has 465 volts to the output tube plates and the Hiwatt has some 520 , so your build will not have the real punch that the Hiwatt has and this means that preamp tone and feel wise the power supply dropping resistors in your build will need to be of lesser value then what the Hiwatt schematic shows!

By my math placeing a 11% resistor correction factor on the dropping resistors will put you right where you want to be at least for starters!

Thanks Steve, yeah I know it won't be a perfect clone. I'm more interested in just re-purposing an amp that was collecting dust into something new and cool.
I have a 300W 4X12 marshall cab that should be suitable. I did plan on correcting the dropping resistors, I've been reading up on load lines, I think I have enough of a grasp to work out matching them up as best as possible.

- Joe

[Stevem]

Enjoy !

[turbofeedus]

Alright, here's my crudely drawn point to point layout.
This is the first time I've drawn point to point from just a schematic, so any corrections/suggestions are welcome.

p2pdrawnlayout-1.JPG

[Stevem]

I would break that daisy changed ground buss open in atleast 3 places and then run those 3 ground wires to one of the bolts on the PT.
One of the new ground points should be everything dealing with the PI section including the master volume pot.

All of the power supply filter ground nodes should have there own ground wire going back to a gounding point near the PT.
Use solder on rings, or heat the plastic off of crip on rings and then crimp and solder them.

It's far better when proving out the noise floor to have more separate ground wires/ sections then you think you may, as you then can always play around with the grounding scheme latter and then daisy chain things that make no difference to clean up your wiring!

[turbofeedus]

Yeah good call Steve, I've had good luck in the past though with keeping all of the preamp on the ground bus, and doing separate grounding for the transformers and filtering.
Maybe I'll try both and see which yields a difference, if any?

[Stevem]

On any unknown build it's always far better to snip shorter and buss back ground wires that turn out to not help with 120 HZ hum levels then have to add them in!

It takes time to play around with them , but in the end you can rest assured that it's as good as it's gonna get!