General Power Supply Question

[JoeTele]

I'm starting to work my way through some tube circuits mathematically. A future project I have in mind which is a sort of anchor for this study is an amp that is essentially a fusion of an AC-30 power section with an AB763 Blackface preamp, say a Super Reverb for the sake of argument. To do this, I would need higher voltages sent to the preamp tube to get the 270 volts on the plates (410 w/the 100k plate resistors) than would go to the power tubes (about 350 from what I've gathered). From what I've been able to calculate, about 360 volts are coming off of the GZ34 in the AC30. I'm willing to make changes in time, but from a theoretical point of view, is this scenario doable/desirable? Could you start with a higher voltage on the PT secondary than the 560 end-to-end on a standard AC-30 PT and run a resistance between the power rail and the plates to bring it down? Other options?

If I did decide to just go with the stock AC-30 transformer, is there a minimum resistance I would need to decouple the filter caps out to the preamp so I could achieve that (I assume it's necessary) with minimal voltage drop?

Thanks as always!

Joe

[roberto]

I don't know what do you consider as preamp and poweramp, anyway considering to split just before the PI, you need something around 1/5th of the swing of a AB763 to properly run the new PA.

You can live with the same supply without worries.
I would reduce the lows a bit.

[Leo_Gnardo]

I would need higher voltages sent to the preamp tube to get the 270 volts on the plates

Although that may hew to the Fender "spec" don't be afraid to raise or lower that voltage to your own taste. Lower voltages on the pre tube plates make for a smoother mellower tone, higher for a bright & taut "high tension" tone. Solder n listen, dial it in until your ears say "yeah that's IT!"

[Firestorm]

There's no one blackface preamp supply voltage: different models range from around 410 down to around 260. As Leo says, the lower voltages can sound mellower or browner or something because they have less headroom (and the swing may not be centered). But blackface Fenders don't need much headroom in the preamp anyway because the tonestack introduces so much insertion loss where it's located. Your life will be simpler if you just set the B+1 for your Voxish voltages then drop them modestly to supply the PI and preamp.

[JoeTele]

Thanks for the info! As far as the PI, I was figuring I'd probably want the voltage (and for that matter the tail resistor) to line up with Vox values, though I may tweak to taste, as I intent to put a good deal of listen, experiment, etc. I certainly appreciate the variance throughout the AB763 amps, but I really do tend to like the sound of higher voltages and am not so much into the brown sound so I thought I'd start with the Super. That said, it looks like I should still be able to squeeze 200+ volts for the actual preamp tube so that along with a non-cathode follower tone stack should give me enough blackface influence in the amp.

Thanks again!

Joe

[JazzGuitarGimp]

You could start with a transformer that will give you 400+V for the preamp, then VVR the PA and PI down to AC30 levels. Once you've got it dialed in, you can replace the VVR pot with two fixed resistors.

[Firestorm]

Of course the following isn't right. The voltage division part works as described, but you can't actually draw current from that junction because it has to go through the resistors. Oops.

You could also choose a transformer to give you your 410 volts rectified DC and connect that to a 100uF cap in series with a 47uF cap. The voltage at the 100uF cap will be 410 filtered by 32uF. The voltage at the 100/47 junction will be 279 filtered by the 47uF. Take your power amp connections to the center tap, choke, screen filter and maybe the PI from there and take your preamp connections and maybe the PI if you prefer from the 410. Unless you are using ideal capacitors add a 47K resistor across the 100uF and a 100K resistor across the 47uF to help reinforce the voltage division. The 100uF cap has to be rated for at least 131V and the 47uF cap for at least 279V.

[pdf64]

You could also choose a transformer to give you your 410 volts rectified DC and connect that to a 100uF cap in series with a 47uF cap. The voltage at the 100uF cap will be 410 filtered by 32uF. The voltage at the 100/47 junction will be 279 filtered by the 47uF. Take your power amp connections to the center tap, choke, screen filter and maybe the PI from there and take your preamp connections and maybe the PI if you prefer from the 410. Unless you are using ideal capacitors add a 47K resistor across the 100uF and a 100K resistor across the 47uF to help reinforce the voltage division. The 100uF cap has to be rated for at least 131V and the 47uF cap for at least 279V.

This seems nonsensical; how can dc be drawn through a cap?
You may be thinking of B+ supplies that use a FWB and a CT B+ winding, in which the CT can be used to provide VB+/2, eg http://bmamps.com/Schematics/fender/75_schem.pdf
In that case, the CT provides the path for dc.

[pdf64]

I'm starting to work my way through some tube circuits mathematically. A future project I have in mind which is a sort of anchor for this study is an amp that is essentially a fusion of an AC-30 power section with an AB763 Blackface preamp, say a Super Reverb for the sake of argument. To do this, I would need higher voltages sent to the preamp tube to get the 270 volts on the plates (410 w/the 100k plate resistors) than would go to the power tubes (about 350 from what I've gathered). From what I've been able to calculate, about 360 volts are coming off of the GZ34 in the AC30. I'm willing to make changes in time, but from a theoretical point of view, is this scenario doable/desirable?..

60s JMI AC30 operated in the UK at 240V 50Hz run at about 330VB+; much more than that and the EL84 run crazy hot.

Why should there be a significant or even noticeable difference between a pre-amp supply voltage of ~300V and ~400V?
A BF PR or DR may be seen define the classic BF tone just as much as an SR or TR, whilst operating on way lower pre-amp VB+, probably lower than would be obtained from a regular AC30 B+.
I think you may be obsessing over minutiae, eg an AC30 power amp will have a lot more treble and bass than a BF power amp, due to running open loop.
Stuff like that seems way more significant.

[martin manning]

EL84's are not as fragile as is commonly thought. A recent example here (Traynor) had them running at 450V with reasonable life:
https://tubeamparchive.com/viewtopic.ph ... highlight=

[pdf64]

My take is that any perceived tonal benefit from cathode bias may be seen to be lost and overtaken by the drawbacks when the VB+ is increased much past the point where hot class AB can be achieved.

[M Fowler]

The last Traynor YBA-2B I worked on PP EL84's ran a 310-0-310 PT.

I found an arc'd over power tube socket was main problem and I changed out the filter caps.

I thought that the plate voltage was very high compared to the other amps I worked on but these Traynors have lasted a long time with higher voltage.

[roberto]

obsessing over minutiae

I really like the sound of english words together with latin ones. I will reuse that one.
Another one is EG of course.

[JoeTele]

Again, all great info. My knowledge is under development so I'd like to revisit my earlier statement of about 360 volts at the rectifier of the AC-30. The current production transformers I've looked at have 560 on the HV secondary/280 to center tap. I know there is a bit more going on here, but the 280 x 1.414 gives about 396 volts, minus the 10-15 through the GZ34 is still at 380+ volts, which at this point I'm willing to believe would be more than enough for my preamp tubes. That leaves me wondering about the 350 or so volts on the plates of the EL84s in the AC30, given the are fed right from the junction of the rectifier and the first filter cap. How is this so? Internal resistance of the OT?

Thanks yet again!

Joe

[JoeTele]

Never mind. Forgot to take into account actual operating conditions.

Joe