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Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 13, 2019.
No I'm not. DC Volts.
If you are measuring the same thing and getting different results, then re-measure until you feel confident you have the right number. I don't think of this measurement as being hard to get a good number. ive seen the resistance of the OT give some weird results but not the voltage.
place one probe on pin 3 and the other on the OT centertap (usually red)
the number you are looking for is bigger than 0.3V and less than 8V.
It should be about 2 volts.
Could you share the actual schematic you built to, including any mods?
Its a hoffman board. Only mods are -a raw switch(per robrobs mods)- lifts the ground on the tone stack for boost
-switchable negative feedback- just a carling switch in the feedback wire.
Im starting with a 5U4, but ive got a GZ34 ill try too.
Great - thx!
From a troubleshooting perspective, just wanted to make sure we were all working with a common understanding of what you've got there - no assumptions
You know what they say when you assume...
So, the plot thickens a bit. Got home from work, swapped the 5U4 for a GZ34, and took some new voltage measurements:
B+ (OT Center tap) to ground- 445.7 V DC
Pin 3 of V5 to ground- 445.5 V DC
Pin 3 of V4 to ground- 445.7 V DC
I'd say that's no voltage drop across the OT. Again, it sounds OK (maybe not as much headroom as I'd expect), and the two sides of the OT have resistance (59.4 ohms / 54.5 ohms across the two legs). What should I check next?
What could cause this (no voltage drop across the OT, but measurable resistance)?
This may or may not help but I wanted to mention it in case it's causing your measurement problems. Anytime I see mv readings when probing for B+ that should be much higher, I know that the probe tips are not making solid enough contact for a proper reading. There can be an oxide layer or flux residue that prevents the tips from making a good electrical connection and it usually shows up as a very small reading on the meter. Good sharp tips or better yet, mini clips for the leads, will allow you to get a better reading on the socket.
Do you have the 1 ohm resistors on the cathodes of the power tubes? If so, check the cathode voltages and that should at least let you know the current through the tube (plate + screen). If the voltage is screwy, make sure those resistors are indeed 1 ohm.
Try directly measuring the plate to center tap voltage. Clip one probe to a plate and clip the other probe to the center tap.
Thanks to all for the advice- it's really appreciated. Hopefully some others are learning something from my stupidity. I'll check those things tonight.
Another general question, about connecting to the point labeled A on this layout:
There are several things connected to "A", as you all know (Center tap of OT, Standby switch, first filter cap- actually two 22 uF/500V electrolytics in parallel). This board has turrets, not eyelets. Questions:
Does it matter if I connect these three things to the turret actually labeled A or the turret directly to the left? As I see it, these two points are electrically equivalent, so it doesn't matter.
Do these three things need to connect directly to the turret marked A. Put another way, could I solder the wire from the standby switch and the OT center tap together, and then run a wire to the turret labeled A?
I'm not in front of the amp, I'm just trying to go through in my mind anything that might be different from the layout.
Nope - both spots are electrically the same so that doesn't matter.
What voltage would you expect to see there? I'm assuming this is pin 8 to ground measured in V DC, correct?
As @shortcircuit mentions, this will be the idle tube current, in mV:
V = I*R
This cleverly chosen value (likely for exactly this purpose!) leaves us with:
V = I
So if you measure 20 mV, this means 20 mA cathode current!
I agree with the above - just put those 1-ohm resistors in there and make it easy. There's a reason why everybody does this! It is a quick, easy, and safe way to measure the actual current in your output tubes. No muss, no fuss, no bother.
Agreed. However, one still needs to put probes on contacts in a live circuit to get the plate voltage so that one can understand exactly what is going on there....plate dissipation is the goal. Current draw is not the end of the game.
well yes, Wally makes a fair point. You do need to actually get down and dirty and measure some high voltage at some point. If you have built an amp I guess you know that.
Since this guy built the amp and posted on the "DIY build-your-own-amp" page, and mentioned several times that he had measured plate voltage and other B+ voltages at various locations, I guess I just jumped to the conclusion that he was aware of the relationship between plate voltage and current draw, and how that related to plate dissipation. But perhaps not!
Agreed. I was coming at it from a diagnostic perspective. It's convenient to have resistors in all the paths when trying to figure out what current is flowing where.
OK, I re soldered the bias connections and took some new measurements. This is with a GZ34 and with the 50 Kl bias pot all the way clockwise
Resistance from OT center tap to plate of 6L6: V4:54.5 ohm, V5: 59.4 ohm
Voltage drop from OT center tap to plate of 6L6: V4:-1.054 V DC, V5: -1.117 V DC
Plate current (V/R)- V4: 0.0193 amps V5: .0189 amps
Plate Voltage: V4: 416 V DC, V5: 417 V DC
Plate dissipation (Plate Current X Plate voltage): V4: 8.0 Watts, V5: 7.8 Watts
That would be ok if this was a deluxe, but for a bassman leaves a lot to be desired, Again, the Bias pot is all the way clockwise. Other measurements:
B+1: 420 V
B+2: 417 V
B+3: 373 V
B+ 4:307 V (all well below spec)
HV winding of the PT: 656 V AC
Cathode Voltage (pin 8 to ground): -20mV (0.02 V) DC
So I need to increase those voltage drop numbers to something more like 2.7 V DC or increase my B+ voltage (or some combination of the two. Not sure How to do that or if I can. I'm also worried that something is wrong with my OT. It's a classic Tone 40-18000 (basically a Tweed Bassman OT but with 2/4/8 ohm taps). I hope I didn't cook it or something.
Any thoughts? Can I address this by changing the 1K bias range resistor?
EDIT: Changed 6V6 to 6L6. Typo.
I thought this amp was running 6L6/5881 type tubes?
If there is a bias pot, why is there a -20 cathode voltage?