Smartest way to 200W?

[es345]

When you say "too big" do you mean the spreadsheet's power estimate is too high? What about plate and screen current?

yes, in the first version the output power estimation as well as the plate current was too high, because the DCR primaries wasn't taken into account. The screen current is an indication only as it is defined as a percentage of the plate current. This indeed is a clear simplification.

It seems like the crucial values to make note of are how low the valve can pull the anode and the DC resistance of the transformer primary. I have a small amp that I built as a teenager. I assumed it to be about an 18 watt amp and eventually when I measured it two years ago I was surprised to see it only does about 13 watts RMS sine, regardless of which output tubes I put in it (it's set up for any normal 8 pin output). The output transformer, though sized for at least 25 watts, has some of the highest DCR primaries I've seen. I guess that explains a lot.

Have you found that it corresponds well with your own builds?

One of the reason to modify the spreadsheet was that the resulting power of my 6xGU50 powerstage was lower than expected.

Here is the background:
I originally ordered a 4K to 4/8 Ohm OT for another configuration but decided to use that one for the "quick test" as a 2K to 2/4 Ohm in that powerstage. The DCR primary from mid to one anode is about 100Ohm. I know from other OTs it should be in the range of 50 Ohms. So I modified my spreadsheet taking into account the DCR primary. The result was now much more accurate.

Conclusion for myself: Now I have ordered a 2K2 to 4/8 Ohm. It will come during September. I hope that the resulting output power will change from currently 350W to ~400W.

Coming to the topic of the miminum voltage at the anode:
what I have seen normally that is somewhere between 50 to 100V depending on the tube type, Voltage at G2 and operating mode (AB versus AB2)

as an attachment for illustration you can see 2 scope captures of the GU50 powerstage: one just without grid current (about 250W) and one with grid current (350W) just at the border to clipping.
I have captured G1, Output voltage at load load resistor("Ausgang"), Anode and g2.

Hans- Georg

[imaradiostar]

Thanks, that's very helpful. I'm in a similar situation. I am attempting to use some junk scope transformers and a pair of KT120's with an Edcor transformer that was intended for use as a 6.6k:4/8/16 transformer as a 3.3k:2/4/8 with the thought that 3.3k will be a better match for the KT120's and a 2 ohm tap is useful when driving certain bass cabinet combinations. The primary measures about 120 ohms a side- more than I expected. I will likely find power output will fall short.

One of my power transformers will yield about 700 volts and the other about 600, both with a screen voltage around 400 volts. I don't trust the curves given for the KT120 so I really hadn't even bothered to plot a load line. My gut told me 3.3k should be good. I'm hoping that using a 6.6k transformer at a lower reflected impedance will allow me to get closer to 150 watts while retaining decent low frequency response.

I guess I'll find out.

I like the idea of a GU50 amp- I believe we talked about that before. I hope to build one some day.

Jamie

[Tom Phillips]

...I am attempting to use some junk scope transformers...

Jamie,
In the past I've investigated using junk scope power transformers for amp builds. I found that although they were large and sometimes had secondaries that would give the appropriate voltage, the secondary current rating was too low for a high power guitar amp. Make sure you verify the current specs before you build with those scope transformers.
Regards,
Tom

[imaradiostar]

Thanks Tom, that's good advice and I've been concerned about it. The scopes from which I acquired the transformers were monsters- both dual trace, both with a lot of tubes- a lot of unused filament taps and current there. Still, I'm concerned.

I have one scope PT that outputs 600 volts at 100ma DC and is down less than 10% at 700ma. That one is probably destined for a 4x 6550 or KT88 amp, probably something akin to a Hiwatt or Trace Elliot circuit. I'll have to scrounge an output transformer, of course. If it doesn't work out with four outputs I think it has more than enough power for a high power two tube amp. It also has windings that could make for a really nice +-300 volt driver section which is overkill but with a little work and some mosfets or big triodes could make a great AB2 driver section.

I've not yet tested the current output of my Heathkit io14 PT but it makes about 700 DC volts in basic testing. The scope it came from produced about 200ma DC from one of the high voltage windings. Best I can tell from voltage ratios and DCR all of the three HT windings use the same gauge wire, which bodes well for guitar amp use. Idle current for a 2xKT120 amp is likely to be well under 200ma so I'm hoping that the filter caps can compensate and help it deliver the required current peaks during high power excursions. The three different voltage windings allow me to generate different combinations of screen and plate voltage so I will likely use it with GU50's and my transformer at 6k6 if it doesn't work out with KT120's at 3k3. 100 watts should be easy that way and by using voltage doublers I could probably run a gu50 at around 900 volts with slightly lower current for well over 100 watts.

I figure for bass guitar use imperfect regulation won't be as big a deal as it might be with a high gain guitar amp. Maybe that's nuts but I'm willing to try. I'm also tempted to build a high voltage mosfet based series regulator if the supply sags too much. If I burn up a free PT I'm not too worried because I can always have a new one wound that'll fit the same hole and correct the lack of current.

Jamie

[Tom Phillips]

Jamie,
Sounds like you are considering all the options well. I've saved several PTs from old tube type Tektronix scopes. If not used as is, I think that the cores are good candidates for rewinding. They are really nice looking overall with the fancy cast end bells. All in all part of the collection of stuff I save because it looks too good to toss out.
Tom

[imaradiostar]

Hey, just a small update in case anyone cares. I wired up the smaller PT (Knight scope) to my breadboard with an assortment of different arrangements with both the KT120's and Sovtek 6550's. I tried the Edcor output transformer at both 6.6k and mismatched to 3.3k. When used ultralinear it's a wash- power output was similar but the 3.3k wiring drew a lot more current. Wire in pentode mode I could get more power at 3.3k but it drew way more power and the PT sagged to well under 650 volts. With ultralinear mode wired at 6k6 and KT120's I could get over a hundred watts at relatively low distortions and around 130 watts max with substantially more distortion. I'm using 1k screen resistors in an attempt to protect the tubes.

The best part- even with the output looking like a square wave, the ultralinear wired version looked good on the scope and the PT only sagged from 720 to 680. I'd say it's pretty usable and I can expect around 100 watts when used that way, 130 or so at full tilt. The transition from clean to dirty is very smooth and nice looking. I can imagine it'll make a nice loud bass amp. This is really what I wanted anyway! Too much power can be a bad thing. I was really hoping for a 2 ohm output tap, though.

I guess the take-home from that is to use higher plate impedances and ultralinear mode when trying to utilize a PT that may not have enough current for the job.

I'm seriously tempted (when funds allow) to order 5k and 4.2k transformers from Edcor for comparison sake. Perhaps it would make more sense to just have MCI or Heyboer wind something for me.

To answer the original question- the smartest way to 200 watts would be a Hiwatt 200 watt transformer set from Heyboer. It will work really well, I promise.

Jamie