RCA Ba2A Pre amp qestions & answers

[letterbeacon]

Great, thanks for the answer.  This is for my BA-2 clone, I just came across that schematic whilst browsing through Gearslutz.

 

[letterbeacon]

Me again!

What's the best type of capacitor to use as C8?  The treble boost bit of the circuit (so signal flows through it).  It's 330pf and I can find silver mica and polystyrene.  What was used in the original?

Thanks!

 

[emrr]

It'd be mica in the original.  I wouldn't install R11/C8 unless needed, since it's really transformer specific.  The values may need to be modified. 

 

[letterbeacon]

Great thanks Doug.  Hopefully I'll be using the original output transformer so I guess I will have to install it.

 

[emrr]

It compensates for the combo of input and output, it's not exclusive to one or the other.  So, you'll really need to measure it with and without to see.  It's not a lot of boost in the first place, and elimination will give you a little more total gain since R11 is part of the volume voltage divider with a cap bypass. 

 

[letterbeacon]

Ah I see, looking forward to experimenting with it.

As for a PT, I had my eyes on the Hammond 370x which has a secondary winding with 240v-0v-240v, is that going to be ok?  The original had a PT with 230v-0v-230v on the secondary, but I can't find one like that.

 

[emrr]

That sort of PT voltage will be fine.  Could add up to a bit more current through T2, can always increase size of R8-9 to compensate. 

Another trivia bit I am remembering.  I have seen two variations in the transformer for T2, depending on the era the part is from.  The schematic shows 2570 ohm DCR on the pri, there are also later transformers where it's 10K.  I thought that was a bad transformer, the first one I came across, but it worked correctly and I then found others that read the same. 

 

[emrr]

A better schematic, camera phone style.  Someday I'll have time to scan properly.  Amazing there's not a single better one on the web already.

http://www.groupdiy.com/index.php?topic=44740.msg571378#msg571378

 

[gary o]

Aha I had R11 & C8 instaled in my DIY BA2A.......I just bypassed them & it sounds better to me so I think I will leave them out then , could put a switch in their maybe & try some other values for fun.....Thanks Doug.

 

[letterbeacon]

That sort of PT voltage will be fine.  Could add up to a bit more current through T2, can always increase size of R8-9 to compensate.

Thanks Doug.  If there was too much current through T2, what would happen?  I'm trying to apply Ohm's Law to the schematic so I can work out how much current is flowing through with the correct transformer, but I can't find out anywhere what the voltage drop is across the 6X5 rectifier.

 

[emrr]

You might cook the transformer with a higher voltage, or a funky tube drawing too much current through it.  It's an unknown spec, so hard to say. 

 

[emrr]

V3 / 215AC-0-215AC according to voltage chart, differs from schematic 230-0-230 / 288VDC
V2 plate 232VDC cathode 8.8VDC
V1 plate 73VDC cathode 2.4VDC

 

[letterbeacon]

Hmmm, interesting.  If the voltage chart says 215ac-0-215ac (despite what the schematic says) then is my 240-0-240 PT going to be way too much?  Bit scared of cooking my output transformer!!

 

[lassoharp]

The schematic for the 230-0-230 version calls for dropping resistor values of 3.3K & 3.9K.  Use 5ma total draw as the ceiling.  You will be ~10V high with the 240-0-240.  Off the cuff I'd say a pair of 4.7Ks for R8 & R9 should get you in the ball park.  Tweak with standard value combos from there if needed.

 

[letterbeacon]

Thanks lassoharp.

V3 / 215AC-0-215AC according to voltage chart, differs from schematic 230-0-230 / 288VDC
V2 plate 232VDC cathode 8.8VDC
V1 plate 73VDC cathode 2.4VDC

Sorry if I'm being a bit slow, but do I need to tweak R8 & R9 to get 288VDC at the output transformer?

One more PT question (apologies, but this is my first undocumented project!).  Would the 370x in the following table have enough power?  It has a secondary of 240-0-240 and a total power of 41VA.  So that's 41/ 480 = 0.08 total supply amps.  Not much, but is it enough? I can't work it out from the schematic.  What would you recommend from the following table:

 

[lassoharp]

Yes, more or less.  It's really just a straightforward application of Ohm's law but for best results I would suggest using a breadboard set up to tweak the resistor values.  This is to take into account  the power transformer's relative sag or stiffness & the fact that different tubes (1620s) even if same brand will give slightly different voltages under the same circuit conditions.  Different 6X5s will also vary slightly in their output voltage.

If you have an RCA tube book handy, look up 6X5 or 6X4.  They will give you a chart that allows you to predict what the output voltage will be at the rectifier's output for a given total current draw.  You must pick a current draw - 4.5-5ma for one channel or double that for two - then work from there to determine the total resistance value needed to get to your target ~288VDC. 

 

[letterbeacon]

Right, I get it.  Is the 4.5-5ma current draw an educated guess?  If the maximum current draw of a channel is 5ma, then the 8ma ceiling of the 370x should be fine, I think.

Thanks very much - looks like I've got some maths to do!

 

[lassoharp]

It was a rough guess.  Using the factory numbers that emrr posted - 5.6ma would be the exact number to aim for.  Should be easy to get with modern 1-2% resistors.

I'm not sure what you mean by the 8ma figure for the 370X.  What you typically want to do is aim for running the PT at 50% of it's max current handling capability.  You'll be well within that.

 

[emrr]

The 370X will put out 240-0-240 with a 40 mA load.  It will give a higher voltage with 5-6 mA draw.  All of which can be mitigated by trimming R8/9. 

 

[letterbeacon]

Ok, so I may have got this totally wrong, but here goes...!

I build the circuit, and tweak R8 & R9 to bring the voltage at pin 1 of the output transformer down to 288VDC.  So to protect the transformer, can I replace it with a 2.2k 2W resistor so there is a dummy load across the power supply?

lassoharp - how did you get the 5.6ma figure?  I'm still getting to grips with Ohms Law, and no matter what figures I try out from the ones emrr gave earlier, I can never get 5.6ma.  I'm keen to learn so your workings out would be of interest.

Thanks!