Question on a Tube Guitar Amp

[zayance]

Allright did the bias mod, and now i could easily get the voltage to -45VDC (it was at -55VDC before),
But when checking Cathode current, well it's about 12.5mA, wich seems pretty cold still.
Plate currents are at about 20mA. EDIT: I've used a 15K resistor to gnd on the bias pot.
Bias at Max was giving -30VDC for 18.6mA Cathode current.

 

[CJ]

check the DCR of the output transformer, maybe the plate windings are open and you are running on screen current, turn off the power for this test of course,

but if you got a ton of output while injecting 50 mv, then this theory will prove wrong.

at -35 you should be able to see the plates getting a bit red if you turn out the lights,

try the ma meter across the output transformer again.

how does it sound with -45 bias, any better?

make sure your batteries are not low in your meter, on some meters this can cause goofy readings,

 

[zayance]

Hi CJ,

The DCR of the Output transformer is:
Red to Brown 36R
Red to Blue 36R
Red to Yellow/Blue 5R
Red to Yellow/Brown 5R

The amp delivers fine with a proper guitar, and yes the tone improved, more agressive sound the more you bias high...
When bias at max (well the max here), the plates are fine, not a slight reddish color...
Don't know what to think about the bias, as seems like the values should be more around 30mA to 50mA, then this low..
But the amp works fine... Now is it having a problem that was there from the beginning? hmmm can't tell..., but it's true that on first look of the amp guts, i saw a black colored spot on one of the 6L6 sockets, and the resistors were changed compared to the carbon ones on the others...
So his amp was serviced before i guess, and maybe had some injury since that black spot...

As is i guess he has some margin with the pot beeing accessible at the back, if he wants more grind to it, without affecting
much the tubes i guess... I'll make him try the amp as is, with the bias at 12.5mA, and if he wants more (i liked it at 12.5mA,
more was too much for my ears) then will push up the bias at the back...

I'll try the mA across the output tranny with the Bias at max and will report back.

 

[zayance]

mA across the Output transformer:

12mA Bias

Red ----> Brown----->23mA
Red ----> Blue  ----->23mA

Bias pot at max (around 17.5mA)

Red ----> Brown----->31mA
Red ----> Blue  ----->31mA


Maybe would need to tweak the bias pot a little more or change it to another value, so the bias tweaking would be larger?

 

[zayance]

So would it be ok to change the bias pot to say 25K or even 50K for having a higher range?
Or the Circuit won't like that??


Thanks again for the help.

 

[CJ]

you could put a parallel resistor across it, that way the pot will not fry due to excess current,

do the tubes get hot when you drop the bias to -45?

you should start to smell burning paint at that voltage,

i would try another meter of possible, the readings across the cathode resistors do not seem to match the readings across the output xfmr,

 

[zayance]

For testing purpose i used a 25k instead of the 10k, a 1/2watt one,
And try to push it up a little more, got bias to about 27mA at max...
The tubes seemed to be fine, i just see the blue glows appearing at max,
So i left it at about 12mA, and so -45VDC
I'm using a fluke and the batteries are all ok...
My friend needs his amp now, so i mount it back again, and i want him to try it
Out as is.
I've seen some other post on a vintage guitar amp forum, and someone seem to have
The same issue with bias pot at full the value wasn't exceeding 19mA, but the responses weren't
Much helpful...
No paint small, the tubes are hot to touch but that's normal i guess??

Will see if the amp runs ok, as i don't have much more time to experiment,
I told him to let me know what's up and not to push it if he experience some weirdness...

Thanks a lot again for the help CJ.

 

[CJ]

maybe the tubes are really tired, if you get it back, try to find a used RCA 6L6 and pop it in there to see what the current is,

 

[zayance]

Well actually he tried the amp yesterday with me, and while the tone improved, the power was lost,
So i kept the amp, he doesn't care much to not get the amp afterall, as he has some other options, i care a little because i don't get what
could have happened to this amp, manipulating inside might have caused something wrong, but since all voltages are ok, i'll check with some other power tubes i have in stock, i think they are ok, but i'll also test the output transformer again, i see that it would be possible to just send a 1VAC with a variac on the secondary and so check if the primary is lifting that signal enough and equally, who knows  :-\...
EDIT: AND the preamp section, as PRR suggested, one bad tube or contact in there, could suck a lot of power too

Keep in touch...

 

[zayance]

Did the simple test of injecting 1VAC to the Secondary of the Output Transformer to see what's up.
Secondary: 1VAC
Primary: 11VAC

Both windings the same, Red to Blue & Red to Brown.

After reading a forum response and the basic calculation associated, and since the primary impedance of the Twin Reverb Output Transformer should be around 2000Ohms CT (reference on the Hammond datasheet i found) here i should then have something around 20VAC.

Soooooo i hate to say it, but  does that mean that the Output Transformer is not fried, but has a fault??

EDIT: Or wait i'm getting it wrong, 2000ohms is the overall primary impedance so these numbers should be correct then, right?

http://www.hammondmfg.com/pdf/EDB1750W.pdf

 

[CJ]

you want to go across the whole pri to the 8 ohm tap to get turns ratio and Z ratio,

also, go from the UL taps to 8 ohms, make sure that ratio is different,

also do a DCR check on the pri, sec will be too low to measure, you will think you have a short,

first thing we did at the TV shop when we got in a new hunk of meat was tube swapping.

easy when you have a wall of tubes behind you, expensive nowadays, black plate RCA 6L6 was $3.50 our cost,

there is a form of tube swapping where you do not have to buy tubes,

pull 2 of the output tubes and measure the bias on the 2 tubes remaining,

then take those out and measure the other 2, kind of a poor man tube checker,

still do not understand why your cathode resistor bias readings do not match the transformer shunt method,

 

[walter]

The transformer shunt method of measuring current does not include the screen current. The cathode resistor method does. I used to do the shunt the transformer, with the amp on, good times. Now I use a modified octal plug and socket with the meter between pin/socket 8, and the rest 1 to 1.

 

[zayance]

The Output Transformer has only 4ohms output, it's not the one with multiple choices.

I did the tube swapping, the bias remains low as 5mA, either way, i had turned the bias down before, so maybe this means that tubes are all dead, all ok i guess since they were tottaly new from what my friend says...
DCR on Output transformer shows a slight mismatch between windings, and is higher than for the Hammond example, don't know if the little mismatch could be a prob, i mean there is tolerances in those things but i could be wrong...
Red to Brown : 35R
Red to Blue : 36.9R
Red to Yellow/Brown: 5R
Red to Yellow/Blue : 4.27R

 

[PRR]

> Output transformer shows a slight mismatch between windings

That's normal.

Anyway.... 2 ohms on a 2K(?) winding is approximately zero.

 

[zayance]

Yes of course....

Well i passed more time yesterday on it, and pinched all tube sockets pins,
cleaned contacts, redone some solder joints, but the amp is still behaving
the same, amplifies but not enough, bias still at that low value etc.. etc...
I don't understand what happened really, i would prefer a failure than this, like no sound,
or some weird voltage reading somewhere etc... EDIT: well i already got that part right 

Still at it when i have some time....

Thanks again for the help.

 

[walter]

Divide and conquer. Determine if the low output is a problem in the pre-amp, or power amp. Use another amp for this test. Take signal from another amps pre-amp or effect send and connect it to the phase invertor in, if the amp is good and loud, the issue is in the pre section. You can set the amps side by side and use a jumper wire with a 600v coupling cap to inject signal in various points in the amp in question. I do this with music going into my troubleshooting amp, both amps on, and touch several points until I narrow down my search. This is a fun and exciting way of troubleshooting that gets results fast. One hand behind your back, and the other hand steady.

 

[CJ]

those output transformer DCR readings look funny,

i have a UL transformer here with 40% screen taps,

14 ohms from red to blu/wht  32 ohms from red to blu,  so i have

14 + 18 = 32 ohms per half of the transformer primary,

unless Fender faked an Ultra linear transformer by having the taps only a few turns in,

how are the speakers?

 

[zayance]

Good idea to divide with another amp.
Unfortunatly the only Guitar amp i still have is also a non working one, it's a Fender Super 112 that was given to me as is long time ago,
burned parts and Filter Caps dead from what i remember...
I always tought of repairing it, and i'll do so one day haha, but maybe i could use its output Transformer, after measuring it of course, and try it in the
Twin, taking one pair of tubes out for keeping the watts down to 60, And so wire the Transformer on the remaining pair? That would give a clear answer i guess?
EDIT: I mean V7 and V10 Out for example or V8 / V9....
The Speakers measure good, i even tried with the Super 112's just to see what's up...

 

[zayance]

Sooo Do you think it would be a bad idea to use the Output Transformer of the Super 112 i have (making sure it looks ok)
in the Twin, so to make sure of those weird values...
I tought maybe trying to repair that Super 112, with all the little skills i have now about those amps, and use it for spotting the prob on the
Twin, but i don't want to be caught in another mess if it happens, so i tought using what's in there for now....

From what i understand it would need to lower the amp wattage by, as said previously, taking out two 6L6 Tubes out,
and it also would need rewiring the thing, with the CT (Red) connected as is, meaning after the Standby Switch.
Will this be possible, or the Super 112's Transformer won't like to be used there?

Thanks again, and sorry for bringing that topic up again, it should have been solved until now, but i have little time for it so :/

 

[CJ]

try the output with 2 tubes pulled, this will eliminate the OPT as the culprit,