Possible wrong installation of diodes in place of a 6X4

[slidincharlie (Carlo P)]

Hello guys
in summer 2009 I started a thread where I got some useful advice from some of you amp geeks:
http://music-electronics-forum.com/t14482/
Among other things, following Bob Martinelli's advice I replaced the 6X4 rectifier tube in my Silvertone 1482 amp with two 1N4007 diodes plus a resistor in series.
The result of the mod addressed only partially one of the original issues with this amp, i.e. low (5.9 VAC) heater voltage.
Anyway... I have been using the amp until now and it worked just fine, only a tad more raucous and crunchy (gainy?) than before.
Now, while I have the chassis on my bench again for fixing a tremolo issue, I have realized that I had installed the diodes+resistor gizmo differently from Bob's advice.
The drawing below shows the current assemblage as opposed to original Bob's setup
My main concern is if the current (wrong) installation may cause any damage to the amp in the long run, or if it is anyway less than optimal and I need to put things right.
Attached is also the 1482 schematic diagram.
TIA, Carlo

Silvertone 1482.pdf

[J M Fahey]

No big deal.

You are using 270 ohms instead of 470 so you have less voltage drop/sag so you have somewhat higher +B voltage.

If it's still reasonable (post voltage values) leave it as is; if too high use the suggested 470 ohms.

Of course, measure voltages with all tubes installed and warm.

[slidincharlie (Carlo P)]

Voltage values are not good.
They are actually much lower than expected from schematics, and worse than I recalled from previous measurements.
Heaters: 5.3 VAC as measured between the two heater wires at any point in the circuit (e.g. between pins 2-7 in a 6V6 tube or between the pilot lamp pins).
B+ at the first filter cap node: 290 VDC instead of 350 VDC.
Plate voltage on the 6V6's: 280 VDC instead of 346 VDC.
What is going on here?
As for the amp tone, it sounds very crunchy and a bit mushy, which I didn't care very much because it is supposed to be more or less the typical 1482 tone.

P.S.: should I start a new thread in the Troubleshooting section telling the whole story from the beginning?

[g1]

What is your line voltage coming in to your amp (or at your wall outlet)?
Your voltages do seem lower than you previously reported (be sure to check your meter's battery).
Here for reference are the numbers you had when the amp was good:

The heater voltage is still 5.9 Vac all over the place: I expected a little bit more (especially after all the work involved with the double heater supply setup)...
The B+ is still low (around 318 V on the plate of the 6V6's) even with a 270 ohm resistor at the rectifier.
The power tubes are biased around 10.5W, which sounds fine for a cathode biased amp (don't know if it is class A), no? With the same tubes (but higher B+) this amp used to see 12W plate dissipation with the previous PT and tube rectifier.

[eschertron]

Voltage values are not good.
They are actually much lower than expected from schematics, and worse than I recalled from previous measurements.
Heaters: 5.3 VAC as measured between the two heater wires at any point in the circuit

+1 to checking wall voltage and batteries in the meter.

What does the heater voltage measure with all tubes out? If the voltage is well below 6.3vac then is it possible the PT primary is wired wrong?

The 6X4 tube is out of the amp for good, right?

[g1]

eschertron: the heater winding was not wound right so 5.9VAC loaded was as good as he ever got. He was able to get good sound when it was at 5.9V.

[slidincharlie (Carlo P)]

The battery in my DMM is fine - I tried two other batteries and the readings did not change.
My wall voltage is now around 217 VAC. Pretty low compared to the usual 230 VAC. This may be responsible for the low AC voltage coming out of the PT: around 288 VAC instead of 335 VAC.
With all tubes out the readings are a bit higher: 5.7 VAC for the heaters, 305 VAC from the PT secondary and a bold 420 VDC for the B+.
As g-one has noted, readings are now lower than five years ago.

[g1]

What are the chances that the PT is actually designed for 240V rather than 230V ?
That would explain your 5.9V heaters even with 230V line voltage.
I think right now you have a line voltage (power grid) issue. Once your line voltage gets back to the normal 230V, your heater and plate voltages should go back to what you had before (5.9VAC and 318VDC).
Do you have access to a variac?

[slidincharlie (Carlo P)]

g-one I see your point.
I have a variac... never used it though... I'll take it out and see if I can manage to use it.

[g1]

That's very fortunate. Most Variac's allow for a little increase in the line voltage. Use your meter rather than trusting the numbers on the variac, see if you can get up to 230V and re-check your voltages. Also if possible do it with 240V from variac and see if you can get better heater voltage.

[J M Fahey]

The clue appears rereading the old 2009 post.
The very poorly wound transformer was "understood" by the no clue winder, by seeing the schematic, as needing TWO 6.3V windings in series.
Read the schematic, not like somebody used to Tube amps, but like somebody who has never brushed shoulders with them.
The HV winding is labelled "335-0-335 VAC" which is clear enough; the filament one, with less space available, reads "6.3V AC" and shows two dotted lines going to each end, which also has a center tap.
Clearly he understood TWO 6.3VAC windings were indicated, connected in series ... which he did.

Quite close, with 0.2V difference between them, which is of no consequence if we use them in series.

By the way, most don't realize that transformer voltages are "digital" meaning that if wires start and end on the same side of the bobbin, you can have "steps" of at least 1 full turn ; as calculated by Steve Conner some 0.2V each .

For example it might be impossible to get exact 6.3V , you may be forced to choose between 6.2 and 6.4V
Source of eternal stress for anal guys and profit for the Psychiatrist.

WE "know" that "all filament windings are 6.3VAC, except direct heated rectifiers which are all 5VAC"

That is not automatically assumed by people outside our area, at all.
Why would they?.
Thus I have used one coil to supply the tubes (including the 6X4 rectifier) and left the other coil unhooked.

My problem: although I have about 6.5 Vac across the two wires unhooked, I get only 5.9 Vac when everything is conneed and the amp is on.
The DC voltages in every single point of the circuit are ok and close to specs, and the amp sounds fine (maybe a little bit crucnhier than before, but this IS a crunchy sounding amp).
I told the guy to allow for 3 A rating in the heater coils. <-- I'm certain he looked you funny and shrug his shoulders, then he used the (thinner) wire which actually fit in available space, doubly so because he had to fit TWO 6.3V windings there

So you have always been driving your filaments from a seriously underrated winding, which used to supply "a little more" without load but which drops too much .
Combine that with probable somewhat lower voltage at the wall and we arrive to your present situation.

I would put the other winding to good use, to relieve the overloaded one.

By simple Math, I would connect one 6V6 (the biggest eater) with both 12AX7 to one winding, and remaining 6V6, 6AU6 and (if used) 6X4 to the other.
I bet that will reasonably equalize loads.

Each filament pair should have its own artificial resistive center tap.

As of +V , it has been reported both too high and too low in different posts, measure it and fine tune through rectifier choice: 1N4007 (even without series resistors) if too low; 6X4 if too high.

That said, and if you ever rewind that (or another) transformer, follow the old established Italian tradition of offering user adjustable mains voltage settings, to make amps work properly everywhere.

Not kidding, I have worked over 40 years ago with killer Italian PA systems (Semprini the king of Voice amps, Meazzi, FBT, etc.) and all included a multistepped switchable "variac" in the rack, as standard Factory issue.

Look at the bottom unit, including AC line voltage meter and multi stepped voltage selector (around 4 or 5V per step):



they were designed to be able to travel to the smallest, up in the middle of nowhere peasant town or village, supplied with too long, too thin power wires, where wall voltage might go as down as 160VAC (or even less) and work properly.

Dear carlo, try rewinding this way and test HV too.

[g1]

I would put the other winding to good use, to relieve the overloaded one.

By simple Math, I would connect one 6V6 (the biggest eater) with both 12AX7 to one winding, and remaining 6V6, 6AU6 and (if used) 6X4 to the other.
I bet that will reasonably equalize loads.

Each filament pair should have its own artificial resistive center tap.

He did end up using both heater windings, in a little different arrangement:
http://music-electronics-forum.com/t14482-2/#post124101

[J M Fahey]

Just curious, do you have some space left (2 or 3 mm) between the bobbin and the iron sheet?

I mean something like on the top right one, where there's some space between the bobbin brown paper and the grey lamination iron.



Because if you have some space there, we can squeeze a few extra turns and accomplish 2 goals:

1) rise AC voltage from 5.9 to 6.3 or 6.4 which is already good.

2) as the cherry on the pudding (or the panna montata on top of the ice cream) we might equalize both windings and be able to parallel them.
So far it seems to have 1 turn difference between them

FWIW when I want to wind 120/240V primary transformers, I wind them bifilar, period.

Only way to guarantee equality between them.

[slidincharlie (Carlo P)]

Wow! To translate from Italian: lotta meat on the fire (or should I say on the barbecue)!
Let me read and digest the latest posts.
In the meantime I am gonna open a new thread about how to hook and use my variac, and I will also check again the chassis to confirm (or not) the actual heaters hookup.
P.S.: forgive me for my delayed replies - the Ampage email notification system does not work with me.

[slidincharlie (Carlo P)]

Before going on with the variac experiment and the rest, I wish to post some more info about this amp:
1) one heater winding passes through the 6X4 socket (the 6X4 is not there) and supplies the 6AU6 (vibrato tube) and both 6V6's
2) measuring the resistance between each end of the power supply secondary (disconnected from the circuit) and its central tap I read 288 ohms at one end and 305 ohms at the other end. Is this normal?