Help with Duncans PSU designer

[dmartn149]

I'm building a small guitar amp from an old reel to reel tape recorder. I naively thought that I could just put the pieces together following the schematic. Now I realize that my power transformer is not the same as the one in the schematic I'm using. I'm trying to use Duncan's PSU designer, but now my ignorance is getting in the way.

On the PSU designer there is an ohms value for each capacitor. I don't know what to do with that. Also for R4 (load) should that be the value of all the plate resistors in parallel? What about the OT primary resistance? should that be factored in too? Anything else I'm missing?



Thanks

[nickb]

The resistance (ohms) for the capacitors is the Equivalent Series Resistance and you can find the value on the datasheet for the capacitors you plan to use. It's not critical so you can just make it 1 ohms and that will be close enough, at least for now .

The load resistance you can calculate as the "desired output voltage" / "expected load current"

[dmartn149]

Thanks nickb. I know what my desired output voltage is, but have no idea what the expected load current would be (or even what that means) Is that from the data sheet for the tube?

[nickb]

The load current is the amps drawn by the circuit and therefore you need intimate knowledge of it to figure that out with any accuracy. It would be best if can you post a schematic, then we can estimate it.

[Chuck H]

I do fine by inserting current taps and changing the final resistive load to a current tap. The current taps are valued at the expected current for each node. For example:
First node will feed a pair of 6l6's at what I figure will be 30mA each. So 60mA.
Second node, screens, probably 3.5-ish-mA each at idle. So 7mA.
Successive preamp nodes each supply a pair of triodes at around 1.2mA each. So 2.4mA for a node feeding a whole 12ax7.

If you have a reverb or some other circuit that will draw a different, but specific current, you need to know what that current is so you can plug it in. For example:

In a recent build I used a single 12dw7 for the reverb. The low mu triode is drawing about 9mA at idle because it's biased as a power tube to drive the reverb tank.

Doing things this way is where the "stepped" load feature of the program is really great because you can plug in expected current changes under load at a specific time interval of the test and see how an amp will respond, sag, recover, expected voltage drops, etc.

When looking for finer details like this is does become more important to know things like capacitor ESR, transformer winding resistance and accounting for the actual primary/secondary ratio of the transformer (I'm remembering when Hammond iron only came with a 115V primary wind).

I get results within 10% and usually closer using the program this way.

Hope this can help.

[dmartn149]

Thanks guys. The recorder that I'm converting had a 6aq5, 12ax7, and 5879, so I wanted to build a champ-ish amp and add a second input that would use the 5879 instead of the 12ax7.
I found a schematic for a "micro champ" and I figured I would use the #1 channel of a gibson ga40 LP as the schem for my 5879 channel. Here are the schematics.

Micro-Champ-R2B.pdf
ga40.pdf

Chuck, where do you get those figures. Do you just know from experience?

[nickb]

Thx for schemo. Very easy as all the voltages were marked PLUS it's class A. There are three power supply loads, I've called them RL1, RL2 and RL3. See the marked up schemo below for details.

EDIT: A bit extra info:

For PSD2 here is how to to it.



You'l need to plug in your transformer secondary RMS voltage and effective source resistance. You should be able to get this from the datasheet or if you already have the transformer you can measure it Reff= Rsec + Rpri *(Vsec/Vpri)^2 where Rsec is measured from the centre tap. Or use the calculator built in to PSUD2 that I belatedly just found.

..and here is the PSUD2 file

[dmartn149]

Yes I already have the transformer. It's the one that was in the recorder. I used the calculator to find the source resistance (333.7ohm) I still don't know the RMS voltage. The calculator in the PSU designer wants manufacturer supplied figures. Nominal output voltage, nominal output current, and regulation. I've measured 277v from ea. secondary to ct, but I'm pretty sure that's not the correct number for "T1" Also I'd still like to know where you get the load current for ea. tube. Is that from the datasheets? Is that plate current @ the specified voltage?
Thanks so much for your time and expertise.

[nickb]

I calculated the current for each tube by using the voltages and resistance values on the schematic and applying ohms law. For example, for V1A the voltages on either side of the 100K resistor R9 are 233V and 154V so current in mA = (233-154)/100 mA = 0.79mA.

The 277V you measure is the correct value for T1 as it is the off-load value.

If your source resistance is 334 ohms than a 5% (13.85V) drop in output voltage corresponds to a load of 41mA and I would say that is probably not too far off the the maximum your transformer can safely supply on B+. Of course 277V is a teeny bit on the high side e.g you'll get 277V (coincidently) on node "V1" instead of 245V but I don't think that will cause you a problem. It'll be louder

Don't overlook the heater supply. Add up the heater current drawn by the valves in the original and do the same for the Micro Champ and see how they compare.

When you have it up and running check the transformer isn't getting too hot. As a very rough guide you should be able to touch it and keep your hand there for a few seconds without distress.

Duh!... I just spotted that I had the plate current of the 6AQ5 at 32mA it should be 29mA. I forgot to subtract out the screen current . I need an emoticon for a smack on the side of the head.

EDIT: Hey, isn't Brian May rumoured to have got his tone by plugging in through a tape recorder?

[dmartn149]

I calculated the current for each tube by using the voltages and resistance values on the schematic and applying ohms law. For example, for V1A the voltages on either side of the 100K resistor R9 are 233V and 154V so current in mA = (233-154)/100 mA = 0.79mA.

Okay I think I've got it. I did the math on V1b and got the same results as you did .9mA (.908333)

Don't overlook the heater supply. Add up the heater current drawn by the valves in the original and do the same for the Micro Champ and see how they compare.

I haven't really given any thought to the heater supply. It's the same PT heating same tubes. The Micro Champ has an artificial center tap on the heater supply, but I was under the impression that that wouldn't affect the voltage or current.

[nickb]

Okay I think I've got it. I did the math on V1b and got the same results as you did .9mA (.908333)


I haven't really given any thought to the heater supply. It's the same PT heating same tubes. The Micro Champ has an artificial center tap on the heater supply, but I was under the impression that that wouldn't affect the voltage or current.

OK I see. You were asking about the transformer so I was assuming it came from elsewhere. The artificial center tap is probably worth doing and adds very little current.

Hope it all goes well

[dmartn149]

I was just a little concerned about the voltage going to the 6aq5. I had read that 250v was the max recommended plate v. I just read yesterday that Gibson ran them @ 300v in the ga17 scout, so I'm not so worried now. Thanks again for the help, and schooling.