250-0-250 for an AC30 PT?

[dehughes]

I've spoken with Ross at length about this issue, and his thought is 260v per side is a better starting point for the secondary voltages on the PT I'm having built by Heyboer for my AC30 project.

I wager he's right, ultimately, but I'm still drawn to low voltages, and would like the EL84 quad (rectified by a GZ34 and with a 62ohm shared cathode resistor) to hit about 320v MAX on the plates. I'm thinking 250v per side on the secondary would get me there, or even a bit under, based on my calculations.

Has anyone ever run a PT in their AC30 with such low voltages, and if so, what did you think of it?

Thanks!

[Andy Le Blanc]

the one I've been working is a pair EL84, plate volt on a cathode bias
are running around 335, pretty close to the RCA AB1 for the type, nice tone
despite other issues. Current draw at max plate dissipation for the pair
is around 80 ma. for a 150ma. rated pt. with a 260v ac taps on the sec.
Just plan ahead for the zero-signal current draw for the quad at Max dissipation.
Even if you never run them that hot.

[dehughes]

Right. I forgot about max current draw at idle...but then, I do have the PT rated for 250ma, so I think we're good, being as it'll only be running three 12AX7s, one EF86, and a quad of EL84s. I think the total current draw with those tubes was 200something-ish (can't remember exactly...), so I figured 250ma was enough.

So, if you're getting 335 on the plates with a 260-0-260 secondary, then I should be dandy with a 250v secondary, yeah?

[dartanion]

Just use a Wreck PT David.

[dehughes]

Just use a Wreck PT David.

? Explain...I know next to nothing about Wreck PT specs.... Are you thinking a Rocket PT?

[M Fowler]

As far as I know the AC30 is 290-0-290 so the Heyboer HTS-5199 would work well and has the 5v tap for tube rectifier. Unless you want to go with lower secondary voltage like 260 or so.

[Andy Le Blanc]

whats the ma .? putting in extra taps should raise the ma. a bit.

[dehughes]

So I'm getting way confused with transformer ratings and rectifier conversion specs. I have a 2 x EL84 Top Hat Club Royale which is measuring 275vac on the 5AR4 plates when on standby...off standby measures 266vac on the recto plates, 321vdc on the recto's cathode, and then puts 313vdc on the EL84 pair's plates (cathodes are biased with a 120 ohm resistor).

These figures put the conversion factor of the 5AR4 at about 1.167, which seems low (SS diodes are about 1.41, and I always figured 5AR4s would be somewhere in the 1.35 area). Maybe my values represent the difference between in-circuit measurements as opposed to just measuring the 5AR4 conversion without any other tubes in the circuit...

So, for my PT, if I run 250vac secondaries into a 5AR4, using the 1.167 figure I'd get about 291vdc on the recto cathode....which is a bit low. If I use the standard 1.31 conversion for a 5AR4, 250vac gets me 327vdc rectified, which is just about what I want, as that'd put less than 320vdc on the EL84 (quad) plates.

Let me know if I'm totally missing something, but my real-world measurements are causing me to doubt my pre-conceived rectifier conversion values...and I don't want to have a PT made that will be too low. I'm wondering if 260-0-260 would be the smarter bet....OR, if perhaps in my Club Royale measurements I'm seeing the voltage drop from a smaller, weaker PT being taxed by current draw at idle.

So Andy, you're running 260vac per side into what recto in order to get 335vdc on the EL84 plates with what Rk?

[paulster]

I've spoken with Ross at length about this issue, and his thought is 260v per side is a better starting point for the secondary voltages on the PT I'm having built by Heyboer for my AC30 project.

I wager he's right, ultimately, but I'm still drawn to low voltages, and would like the EL84 quad (rectified by a GZ34 and with a 62ohm shared cathode resistor) to hit about 320v MAX on the plates. I'm thinking 250v per side on the secondary would get me there, or even a bit under, based on my calculations.

Has anyone ever run a PT in their AC30 with such low voltages, and if so, what did you think of it?

Thanks!

My AC30 build uses a 280-0-280 from Heyboer and has 323V on the plates with a 50ohm cathode resistor, using a Mullard yellow label (military spec) GZ34.

[dartanion]

The HTS 5199 is 300-260-0-260-300 arrangement. Using the 260V taps on a Liverpool (ss recto and 50 ohm Rk) gets you in at around 330-335V on the plates of the EL84s. Using a GZ34/5AR4 should put in the 300-320V range with this PT. The 5199 is rated at 300ma, so it's pretty stout and is not overtaxed. The 300V taps might put you too high with a GZ34. From what I recall, the AC30 PTs were 290-0-290 at 200ma, so you can grab a replacement AC30 PT from CEdist, or where else has them, but I believe a 5199 would work just fine. IIRC, the one Rocket style amp I built that I used a GZ34 and 5199 came out with plate voltage of 315, using a 50 Rk. I favor SS recto in that circuit, so now my plate voltages are in the 330 range.

[dehughes]

Wonderful. Thanks so much. That provides me with just the information I need! You guys are great!

[dehughes]

Okay, I'm thinking that I'd like to do 250-0-250v at 250ma. Being as my current draw is about 226ma, I'm hoping that would let me get a good 320vdc given I have my EL84s biased a bit colder than the typical Rocket/AC30 (I'm using a 62 ohm Rk), so I wouldn't be drawing as much current, thus allowing my plate voltages to be higher than if I was using a 50 ohm Rk.

The PT I was using before was rated for 303-0-303v and put around 360v on the plates. This was too high for my tastes....so I'm thinking a good 50v less should get me where I want to go. We'll see...

[paulster]

If you're getting a transformer wound then have them put a couple of taps on it if you're not 100% sure. That way you can switch between them and see which tone you actually like better.

[M Fowler]

rjguitar already has transformers with the correct specs at the lowest prices around, just saying.

[rawnster]

Totally, RJ sells an Edcor EM0727

260-0-260 250mA
6.3V CT 4.8A
5V 3A

http://www.rjguitars.net/Transformers.html