high voltages on input tubes

[ElliottKoch]

Although this is my first amp build, I'm not a complete novice as far as electronics go, just new to tubes.

That said, I'm having issues with getting the correct voltage on the input 12AX7 tubes. Where I am supposed to be reading about 260V, I read 420V. The power tubes (4-6L6) all have the same input reading (420V) where the schematic suggests 460V.

Although this is an ab763 fender twin build, I was lured into using the GZ34 rectifier (possibly a mistake as the power supply doesn't have the appropriate connections (i.e. two 5V ac connections to pins 2 and 8, can I run the lamp wires in their place en route to the 100 Ohm load resistors after leaving the lamp?

Last thing, I just noticed that one of my 6L6 tubes has a floating glass bead inside. Foolishly I didn't inspect them once I received them and living in Australia, returning them is likely to be impossible. Do you think the tube is still functional?

[ElliottKoch]

a bit of an update...

traced the voltage issue back to the filter caps, but still can't find the problem.

I've wired the filter caps to the '65 twin schematic (three 20 uF in parallel to ground separated by the appropriate resistors and two 50 uF caps in series with the appropriate resistors in parallel), however all voltage readings to ground at the key connecting points are all 420V, where they should be 410V, 450V 458V and 460V (see schematic).

I believe that this is what is causing my high voltage at the preamp tube, I've triple checked my wiring and can't find where I've gone wrong.

[tubeswell]

Your reading of plates being the same as the B+ voltage indicates that the tubes aren't drawing current/forming the load. I infer from this that you haven't got the ground return side of the circuit connected to the ground. Check for continuity between your cathodes and the chassis ground with your R-meter. (You should get a reading of the cathode resistor value between each cathode and the chassis)

[sluckey]

Although this is an ab763 fender twin build, I was lured into using the GZ34 rectifier (possibly a mistake as the power supply doesn't have the appropriate connections (i.e. two 5V ac connections to pins 2 and 8, can I run the lamp wires in their place en route to the 100 Ohm load resistors after leaving the lamp?

A single GZ34 is not big enough to power four 6L6s. You really need two GZ34s connected in parallel and a hefty 5VAC PT winding.  Are you saying your power transformer has no 5VAC winding? And you want to run the GZ34 filaments from the 6.3VAC source that powers the lamp and other filaments? If that's the case, then STOP!!! That will be a costly mistake. Best to just use solid state diodes just like a real TR does. If I've misunderstood the statement I quoted, please explain.

Now, if you have B+ I guess you've already wired the GZ43. As for the high voltages, do you have any tubes plugged in (other than the GZ34)?

[ElliottKoch]

abandoned the gz34 and have wired six 1N4007 diodes as per the TR schematic, and the rectifier is doing its job though the resistance of the circuit is a bit high (about 7 MOhms from either input to output - higher than I would expect).

Voltages were without tubes, (want to limit the cost of my mistakes ;D ).

However, once I sorted the rectifier, I did plug in the tubes, with no real change.

Tested my resistance to ground as well as check the wiring (for a tenth time).

resistance measurements are as follow:
V1:
Pin1   100 kOhm
Pin2   ~30 kOhm
Pin3   1500 Ohm
Pin4/5 open
Pin6   100 kOhm
Pin7   ~1 Ohm
Pin8   1500 kOhm

V2: (same as V1 except Pin7 ~100 Ohm)

V3:
Pin1/6 1 MOhm
Pin2/7 1 MOhm
Pin3/8 1 kOhm
Pin4/5 open

V4:
Pin1 open
Pin2 1500 Ohm
Pin3 100 kOhm
Pin4/5 open
Pin6 100 kOhm (+/-)
Pin7 100 Ohm (+/-)
Pin8 1500 Ohm

V5/6: (tremolo circuit and not enabled anyway)
all either open or caps (resistance increases as cap charges)

Power tubes same story (caps prevent resistance measurements to ground).

The Hoffman schematic (above) is for a DR, will wiring the 10KL trim pot this way (different from the TR) be causing this?

[tubenit]

I mean this respectfully ...................

Does your voltmeter have a good battery?  I say that because I got really weird voltages once on an amp and it turned out that it was a bad battery in my voltmeter. (even though it read resistance OK).

AND because I have mentioned that a few times on the forum & others have admitted discoverying the same problem. Replacing the battery resolved the issue for them.

Just trying to rule out something simple.

With respect, Tubenit

[sluckey]

You must plug in all tubes before your voltage readings will be meaningful.

[ElliottKoch]

no disrespect received. I've freaked myself out by, after draining the filter caps with the ameter and forgetting to swap back the leads before reading resistances etc. however, this is not one of those times and the battery is new.

voltages have been taken with all tubes in place and all voltages read at places in TR schematic (above) all read the same as B+.
(give or take a couple of volts).

I really appreciate your help and will be forever in your debt if you can help me get this running.

by any chance, could the tubes be bad?

checked the PT and OT, both appear to be fine, short of the PT provideing 420V at B+ and power tubes, not the 460V that's given in the schematic, but I figured being off by 10% was reasonable.

[tubenit]

Again, I mean this respectfully ................

Q1:
On your earlier post, you don't mention pin 9 on the 12A_7 tubes.  Can you confirm that one of the 6.3v heater wires is attached to pins 4&5  AND that the other heater wire is attached to pin 9 on each of those 12A_7 tubes, please?

Q2:
AND if they are wired correctly, are you getting 6.3v AC across the pins (4/5 & 9)?

Q3:
AND when you check the cathode resistors resistance, you actually touched the black lead to the chassis and not to the cathode resistor wire itself, correct?  

Q4:
The filter caps on the B+ rail are grounded, correct?

With respect, Tubenit

[ElliottKoch]

no stress, it takes a lot to offend me, and its late on this side of the World, so, my brain its a bit fuzzy.

Q1:
pin9 is not connected as per the Hoffman schematic. Ooohhh! That humble pie tastes soooo good. Actually, I did say, that though I'm not a complete novice (meaning, know electronics fairly well), this is my first circuitry with tubes. This is what is meant by the heaters. So, not everything is wired apparently. Do I daisy chain the connections then pass them to the 100 Ohm resistors then to ground (like the Hoffman schematic)?

Q2:
no

Q3:
yes

Q4:
yes with three separate paths to ground.

[stingray_65]

a typical heater circuit will follow like this:

2 green wires from PT will go to a 3 lug terminal. like this http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=BoardBuilding&ORDER_ID=!ORDERID!

Here is where I put the (2) 100 ohm resistors that form the artificial CT to ground.

I then run a twisted pair of 20ga wire to the lamp. Again from the terminal lugs I run an 18ga twisted pair (use 2 colors) to pins 7 and 2 of my first power tube.

Here is where the 2 colors come into play, pin 7 of the first power tube should connect to pin 7 of the next, the same for pins 2. we use twisted wires to reduce emf noise (remember the heaters are AC powered) and by keeping the heaters "phased" correctly any stray noise picked up should be cancelled because the two power tubes are inverse to each other.

From the power tubes pin 7&2, run a lighter 20 ga twisted pair, again 2 colors.

keeping phase again one color to pin 9 and the other color to pins 4&5.

Some guys pinch pins 4&5 together (adds a tiny bit of space on a small socket), I just strip the lead a bit long and thread it from one lug to the other.

To keep any stray EM noise from being picked up by some VERY sensitive inputs (pins 1&6 both near heaters) I run my heater wires vertically from the socket about 3/4"  and then over to the next socket.

[tubenit]

pin9 is not connected as per the Hoffman schematic

Hoffman drew up the layouts presuming people knew how to wire up heater wiring. And that is why it was not drawn out with more definition.

In the HOffman Library of Information there is a wealth of information including how to wire up heater wiring.

Fixing this may indeed fix your problem. The amp will NOT work without the heater wiring properly done.

With respect, Tubenit

[stingray_65]

pin9 is not connected as per the Hoffman schematic

Hoffman drew up the layouts presuming people knew how to wire up heater wiring. And that is why it was not drawn out with more definition.

In the HOffman Library of Information there is a wealth of information including how to wire up heater wiring.

Fixing this may indeed fix your problem. The amp will NOT work without the heater wiring properly done.

With respect, Tubenit

LOL Jeff !
That picture is worth 1000 of my words 

[ElliottKoch]

you guys are brilliant.

I'd also come to the same conclusion when Tubenit mentioned pin 9 needing have a voltage difference between pins 4/5, hence the "humble pie" remark.

thanks for all the help. I'm sure that it'll be working later today.

legends, all of you. some day songs will be sung to sing your praise.

cheers

[tubenit]

when Tubenit mentioned pin 9 needing have a voltage difference between pins 4/5

I do not know what that comment means?

With your voltmeter on VAC,  put your red and black probes on pins 4/5 (which are tied together sort of as one unit) and pin 9.  You should have 6.3 voltages (more or less) across that IF you have this wired correctly.

With respect, Tubenit

[ElliottKoch]

I merely meant that, your statement of having a 6.3 V(ac) reading between pin9 and pins4/5 lead to a realisation that I'd stuffed it. Then looking at the schematics again, was able to deduce I'd not wired the heaters.

(my brain - hence english - are a bit jumbled. I make perfectly good sense to me, though others have no idea what I'm saying).