Planning my BA-6A

[DaveP]

Matt,
I've attached the reality, judge for yourself!

Still did get to share a dressing room with the Bonzo Dog do dah band (they were the band at the end of the Beatles Magical Mystery Tour film.
best
Dave

 

[lassoharp]

a pint of beer over my 100W PA amp whilst we were playing in a pub, one hot Mullard EL34 went out with a blue flash, I was lucky it didn't get the other three.

Pint of beer probably cost more than the Mullard tube back then!  Well, maybe not but . . . my how things have changed!

 

[DaveP]

Alan,
I seem to remember it wiping out my share of the take that night which was £2~£3 or ~$8 at the time.  It probably cost me to play there!
best
Dave

 

[letterbeacon]

Bonzo Dog Do Dah are great -it must have been awesome sharing a dressing room with them!

I've finally finished wiring this beast -let the troubleshooting begin!

I have 2x 47r resistors in parallel on each anode of the 5R4GY, and for some reason I'm not having the same problem as before (the B+ dropping with less resistance).  I think it may be because previously I croc-clipped the parallel resistors instead of soldering them on?  Either way, the B+ hasn't lowered, but it's still too low.  I'm getting 256VDC instead of 285VDC.

Below are voltage readings I've taken, along with RCA's voltage readings in bold taken from the manual.

6SK7
V1 Pin 8 = 121vdc (135)
V2 Pin 8 = 128vdc (135)

V2 Pin 6 = 99.3vdc (103)
V2 Pin 6 = 96.9vdc (103)

V1 Pin 5 = 4.5vdc (5.15)
V2 Pin 5 = 5.27vdc (5.15)

6J7
V3 Pin 3 = 66.1vdc (71)
V4 Pin 3 = 42.6vdc (71)

V3 Pin 4 = 34vdc (42)
V4 Pin 4 = 34vdc (42)

V3 Pin 8 = 2.5vdc (1.18)
V4 Pin 8 = 2.61vdc (1.18)

6V6
V5 Pin 3 = 247vdc (275)
V6 Pin 3 = 247vdc (275)

V5 Pin 4 = 256vdc (285)
V6 Pin 4 = 256vdc (285)

V5 Pin 8 = 14vdc (13.1)
V6 Pin 8 = 13.5vdc (13.1)

6H6
V7 Pin 4 & 8 = 63vdc (66)

0D3
V8 Pin 5 = 146.4 (155)

5R4GY
V9 Pin 4 & 6 = 520vac (315vac)

It seems most of the voltages are in the ball park there, although if I could raise the B+ it looks like a lot of the low voltages would creep closer to the ideal.

Not sure why I've got such a difference on pin 3 between the two 6J7 tubes.  That could be because I'm using CC resistors with varying tolerances.  Does this worry anyone?

The big mystery is still the B+ though.  I measure 520VAC on pins 4 & 6 of the 5R4GY (much more than the RCA manual says should be there) and yet I'm only getting 256VDC B+.  Perhaps I should bring the anode resistors right down and see what happens? No, the anode resistors are only dropping 5V, so it can't be them.  Must be something drawing a lot of current.

If you can't get any better than that, then as Alan says, you can only reduce the current draw by increasing the  cathode resistors, say from 330 to 360 or 390.  It won't affect the perfomance at all.

There is another possibility; check the current through each output tube and make sure they are in reasonable balance.  If there is excessive current with no signal then there is an ultrasonic oscillation drawing extra current, check with scope.

hope that helps
best
Dave

I would quite like to keep the bias points of the 6V6 in the same place ideally so I might try that as a last resort.

Apologies if this is a stupid question, but I've looked online and can't find the answer:  how do I check the current draw of the 6V6s?

 

[letterbeacon]

I just measured the total current draw of the unit to see if it was pulling excessive current.  I measured in the same place as David Kulka did on a real BA-6A (see quote below) and I measured 105mA.  So 15mA less than a real one, and still the voltages a low.  Strange...

The BA-6A I tested had its power transformer set to the 115 volt line tap so I ran it plugged into a variac set for exactly 115 volts output. I interrupted the wire connected to the output of L1 and inserted a Fluke 8060A to measure current, and your estimate was right on -- DC current was 124 ma. I also measured the small AC current component at this point (before C117), it was about 2.5 ma.

DC voltage at the choke input was 340; at the choke output it was 318. Hope that helps, and good luck to all with this project! A BA-6A, working right, is a fantastic limiter.

 

[letterbeacon]

I've just run some signal through it and if I turn the input and output controls up to maximum then the signal comes through, although nothing shows up on the gain reduction meter.  I also notice I get a little blue flash in one of the 6V6 tubes when I select between A and B on the BAL-LIMIT switch and between the 'off' and 'GR' on the meter switch.

 

[lassoharp]

Not sure why I've got such a difference on pin 3 between the two 6J7 tubes.  That could be because I'm using CC resistors with varying tolerances.  Does this worry anyone?

Sounds like normal differences between tubes.  Your resistors will match way better than any two tubes typically will. 

although if I could raise the B+ it looks like a lot of the low voltages would creep closer to the ideal.

They will go slightly lower if that's what you mean.

Must be something drawing a lot of current.

6V6 pair pulls the bulk of it.

how do I check the current draw of the 6V6s?

With single cathode resistor - you have voltage drop there.  Ohm's law gives you the figure you need.


Also note that just like the SJ7s and most any other tubes the 6V6 pairs will behave differently - ie they will give different voltage and current draw when subbed in the same circuit.  Sometimes you will see vastly different numbers - 30V or more difference.  The good news is that tube circuits are very tolerant of the differences in terms of general performance of the amp section.  Unless you are melting something with too high of a voltage or current or bottomed out at starvation voltages you tend not to hear too great a difference with the typical variations you see.  Plus, you NFB to 'erase' a number of differences and keep everything in tow.

If you want to compare your differences to what what have been expected in the original - use about +/- 15%.  You can check the manual and see what resistor tolerances they gave and use that figure to calculate.  They were probably using wider resistor tolerances than what you're using in there now.

Ah, and the rectifier - just as with the other tubes - one 5V4 will output maybe 5, maybe 10, maybe 15 or 20 volts higher than another.  Sometimes that's due to a slipping thru the cracks - you buy the tube as NOS but it may be slightly used and just gets passed along as new.

 

[DaveP]

Matt,

A few points:

I would try to find a better matched 6J7 pair, I would not want as much as 24V difference, that is 57% difference!

If you have got 520V rms ac across the rect tube anodes, with perfect diodes you would have 369V dc, but you have got a voltage drop of 114V which is way too much.  A good check would be to sub the 5R4GY with a couple of 1N4007 and see what you get.  If its nearer to 369V then you have been sold a clapped out rectifier tube.  Re check all the transformer rect/choke wiring just in case.

Don't worry about all of this hassle it is typical of jobs like this and because we have all been through similar stuff we are able to give our advice.

Re the Bonzo's: They hogged the open gas oven which was the only heating in there and would have told us to F off if we had been stupid enough to try to talk to them, so we just played it cool, in both senses of the word!  That was the reality of 1967 band life between the top act and the supporting band. :-[
best
DaveP

 

[letterbeacon]

Thanks for the replies!

They will go slightly lower if that's what you mean.

If the B+ raises by 30v, won't the rest of the voltages that are fed by the B+ via a dropping resistor also rise by 30v?

With single cathode resistor - you have voltage drop there.  Ohm's law gives you the figure you need.

Great, thank you.

If you have got 520V rms ac across the rect tube anodes, with perfect diodes you would have 369V dc, but you have got a voltage drop of 114V which is way too much.  A good check would be to sub the 5R4GY with a couple of 1N4007 and see what you get.  If its nearer to 369V then you have been sold a clapped out rectifier tube.  Re check all the transformer rect/choke wiring just in case.

Good idea, I'll see if Maplin has the 1N4007 in stock.  I bought the rectifier from Jim at Vacuum Tubes Inc, so I'd be surprised if it was a bad tube, but hopefully this will rule it out.

I suspect it's something to do with my wiring somewhere along the way.  The fact it only passes audio faintly, and a 6V6 flashes blue when I change the meter knob makes me think I've messed up along the way somewhere.  Troubleshooting is all part of the fun though, eh?

It seems I've lost the probes to my scope so I'll have to leave tracing the signal until next week when I can get some more.  So close now though!

 

[lassoharp]

If the B+ raises by 30v, won't the rest of the voltages that are fed by the B+ via a dropping resistor also rise by 30v?

Yes, momentarily.  Then the tube will draw more current because of the higher supply voltage which will, assuming the cathode resistance stays constant, create a larger voltage drop  How much depends on the inherent characteristics of the tube which can be estimated with the set of plate curves for a particular tube.


Do you have a couple spare sets of 6V6s lying around ?  If so, try subbing them for the current pair and see how much things change. 

Also try bumping up the value of the 6V6 cathode resistors if tube subbing fails to change the situation .  390r looks like next standard choice.

Are you using the original OTs?    Reason I ask is that they likely have relatively high DCR which may be a good bit higher than say the Sowter drop ins.  So there is some upstream dropping resistance in the original which causes the 6V6s to pull  a little less current than if you assumed the OT DCR negligible.

 

[letterbeacon]

Here are my calculations for the current draw of the 6v6s.

V5 Pin 8 = 14.42V.  The other end of the cathode resistor is ground, meaning the resistor drops 14.42V.
current = volts/ resistance
14.42 / 330 = 43mA

V6 Pin 8 = 13.85V
13.85 / 330 = 41mA

That does seem like they're pulling huge amounts of current to me.

Do you have a couple spare sets of 6V6s lying around ?  If so, try subbing them for the current pair and see how much things change.

Unforetunately I don't have an extra pair to try, so I might try raising the cathode resistors.

Are you using the original OTs?    Reason I ask is that they likely have relatively high DCR which may be a good bit higher than say the Sowter drop ins.  So there is some upstream dropping resistance in the original which causes the 6V6s to pull  a little less current than if you assumed the OT DCR negligible.

I'm using the original input and output TX, but the Sowter interstage.

I'm going to try Dave's idea of temporarily using 1N4007s as the rectifiers and will report back.

 

[letterbeacon]

If you have got 520V rms ac across the rect tube anodes, with perfect diodes you would have 369V dc, but you have got a voltage drop of 114V which is way too much.  A good check would be to sub the 5R4GY with a couple of 1N4007 and see what you get.  If its nearer to 369V then you have been sold a clapped out rectifier tube.  Re check all the transformer rect/choke wiring just in case.

I've just swapped out the rectifier tube for two 1N4007 and I measure 325V just before the choke, 312V just after it.  So the same problem then, meaning it can't be the tube rectifier.

Hmmmm, looks like I'm going to have to go through this wiring with a fine toothcomb...

 

[letterbeacon]

I've just tried running some tone through the unit and I lose the signal at the output of the input tx.  Is this normal?  I have a heralthy 1k sine wave on the primary, and nothing on the secondary...

 

[lassoharp]

That does seem like they're pulling huge amounts of current to me.

Looks normal for 6V6 at those conditions.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6V6GT.pdf

I've just tried running some tone through the unit and I lose the signal at the output of the input tx.  Is this normal?  I have a heralthy 1k sine wave on the primary, and nothing on the secondary...

No . . . . you should be getting clean signal there.  Double check connections.

Have you tried just running some music through the whole thing yet?

 

[letterbeacon]

I have tried music and I got a weak signal back with the input attenuator and the output attenuator both up to maximum.

My input transformer is actually from an 86A.  Looking at the schematic for the 86A it looks like it has pins 11 & 5 tied to ground which I haven't done on mine.  I'll solder them together and report back.

 

[lassoharp]

My input transformer is actually from an 86A.  Looking at the schematic for the 86A it looks like it has pins 11 & 5 tied to ground which I haven't done on mine.  I'll solder them together and report back.

This should have no major effect on signal integrity.    Having or not having 11 & 5 and 6 & 12 tied  together would should that be the case.

 

[DaveP]

Check the correct way of wiring up the Sowter interstage.  You have done one of a few things:

Earthed every part of the secondary or put two windings in antiphase or have failed to join parts of the secondary correctly.

I'm beginning to wonder if your power TX is up to the job of running a tube rectifier.  I take it that your RMS voltage across the diodes is now~455V?  Now the resistance of the rect tube has gone it is drawing more current from the TX and volts have dropped.  Are there any other choices of input voltage tappings?  Like 220/230/240V?  If so you could move to a lower one to boost the secondary voltage.
best
DaveP

 

[letterbeacon]

On hooking up the CT on the input TX I realised I hadn't connected pin 8 - the earth.  Having done that, the voltages are still low but... it works!  Everything appears to be working as it should.  I've just run a random drum loop through it and it sounds great!

It's also really really quiet.  I turned up the input and the output to max (with nothing running through it) and I could just hear a tiny amount of hiss.  But it really was a tiny amount and with everything on max, so nothing to worry about.

It would be nice to get the voltages up to where they should be though.

I'm beginning to wonder if your power TX is up to the job of running a tube rectifier.  I take it that your RMS voltage across the diodes is now~455V?  Now the resistance of the rect tube has gone it is drawing more current from the TX and volts have dropped.  Are there any other choices of input voltage tappings?  Like 220/230/240V?  If so you could move to a lower one to boost the secondary voltage.

Unfortunately I didn't measure the RMS when I had the diodes installed.

I'm using the Hammond 370FX for the PT.  It has 200/ 220/ 240 input taps and I'm currently using the 240 one.  The specs on the Hammond website say it's rated at 550V C.T.
@ 173ma, perhaps this isn't enough?

 

[lassoharp]

I turned up the input and the output to max (with nothing running through it) and I could just hear a tiny amount of hiss. 

I'd call that a success!  10W PP pentode output stage cranked to max and tiny amount of hiss is a high five situation. 

I've just run a random drum loop through it and it sounds great!

And it will over a fairly wide range of voltage variations from what the manual states. 

Congrats!

 

[DaveP]

Well done Matt,

All you need to do now is put the mains on the 220V tap and not the 240V and you should be nearer the mark, thats what those taps are for, fine tuning.
best
DaveP